From AAS NOVA : “How Often Do Black Holes Get Kicked Out of Star Clusters?”

From AAS NOVA

19 November 2021
Kerry Hensley

Regions where stars are densely packed, like the Milky Way’s nuclear star cluster shown here, are potential sites of black-hole mergers. Credit: Stefan Gillessen, Reinhard Genzel, and Frank Eisenhauer.

A leading theory for how massive (and supermassive) black holes form is hierarchical merging — the idea that massive black holes are built from a series of collisions between lighter black holes. Densely packed environments like globular clusters and nuclear star clusters at the centers of galaxies are expected to be prime sites for black-hole mergers, but gravitational-wave “kicks” might get in the way.

Get a Kick Out of This

When black holes merge, they emit gravitational waves that carry away energy and momentum.

Gravitational waves. Credit: W.Benger-Zib. MPG Institute for Gravitational Physics [MPG Institut für Gravitationsphysik] (Albert Einstein Institute) (DE)

Artist’s by now iconic conception of two merging black holes similar to those detected by LIGO. Credit: Aurore Simonnet /Caltech MIT Advanced aLIGO(US)/Sonoma State University (US).

The waves aren’t emitted equally in all directions, so the act of merging imparts a “kick” to the merger product. It’s far from a gentle nudge — typical kick velocities are estimated to be hundreds or thousands of kilometers per second. If a black hole gets a big enough kick, it can leave the dense cluster it formed in and emerge into rarefied space, where it’s far less likely to undergo further mergers. How typical is it for black holes to get ejected from their nascent star clusters, and what does it mean for the theory of hierarchical merging?

Probabilities of five merger products having a given kick velocity. [Adapted from Mahapatra et al. 2021]

Current gravitational-wave detectors aren’t sensitive enough to detect the ejection of a black hole directly, but we can infer the kick velocity from the gravitational waves we observe. A team led by Parthapratim Mahapatra (Chennai Mathematical Institute (IN)) applied mathematical models to data from 47 black-hole mergers detected by LIGO and Virgo to investigate the likelihood of black holes being kicked out of different types of star clusters. The authors estimated each event’s kick velocity, which depends on the ratio of the masses of the merging black holes, their spin rates, and other factors.

Retention probability as a function of cluster escape velocity for several gravitational-wave events in the catalog. The grey lines correspond to five of the six merger events that showed signs of previous hierarchical mergers; the red dotted line is the sixth. [Mahapatra et al. 2021]

Globular vs Nuclear

Mahapatra and coauthors used their estimates to determine the probability of a merger product remaining in the cluster in which it formed as a function of the cluster’s escape velocity, finding a wide range of retention likelihoods. The authors pay special attention to the six events in the catalog that show signs of being the products of previous hierarchical mergers, like GW190521 — the most massive black-hole merger detected so far. They estimate that a merger like GW190521 would have a 50% chance of remaining bound to a star cluster that had an escape velocity of 700 km/s. Since this escape velocity is well within the expected range for nuclear star clusters, this means that if GW190521 were housed in such a dense environment, its massive product may well merge again in the future.

Ultimately, it’s clear that some star clusters are better at retaining post-merger black holes than others. Given their low escape velocities, globular clusters are unlikely to be the sites of repeated mergers; of the 40 cataloged mergers with the best data, the authors estimate that 17 would be retained by nuclear star clusters and only 2 would be retained by globular clusters. Without knowing more about the environments in which these mergers took place, it’s impossible to say whether an individual merger product will escape from its surroundings, but the authors’ findings have important implications for where hierarchical merging is likely to take place generally.

Citation

“Remnant Black Hole Kicks and Implications for Hierarchical Mergers,” Parthapratim Mahapatra et al 2021 ApJL 918 L31.
https://iopscience.iop.org/article/10.3847/2041-8213/ac20db

See the full article here .

five-ways-keep-your-child-safe-school-shootings

Please help promote STEM in your local schools.

Stem Education Coalition

AAS Mission and Vision Statement

The mission of the American Astronomical Society is to enhance and share humanity’s scientific understanding of the Universe.

The Society, through its publications, disseminates and archives the results of astronomical research. The Society also communicates and explains our understanding of the universe to the public.
The Society facilitates and strengthens the interactions among members through professional meetings and other means. The Society supports member divisions representing specialized research and astronomical interests.
The Society represents the goals of its community of members to the nation and the world. The Society also works with other scientific and educational societies to promote the advancement of science.
The Society, through its members, trains, mentors and supports the next generation of astronomers. The Society supports and promotes increased participation of historically underrepresented groups in astronomy.
The Society assists its members to develop their skills in the fields of education and public outreach at all levels. The Society promotes broad interest in astronomy, which enhances science literacy and leads many to careers in science and engineering.

Adopted June 7, 2009

The society was founded in 1899 through the efforts of George Ellery Hale. The constitution of the group was written by Hale, George Comstock, Edward Morley, Simon Newcomb and Edward Charles Pickering. These men, plus four others, were the first Executive Council of the society; Newcomb was the first president. The initial membership was 114. The AAS name of the society was not finally decided until 1915, previously it was the “Astronomical and Astrophysical Society of America”. One proposed name that preceded this interim name was “American Astrophysical Society”.

The AAS today has over 7,000 members and six divisions – the Division for Planetary Sciences (1968); the Division on Dynamical Astronomy (1969); the High Energy Astrophysics Division (1969); the Solar Physics Division (1969); the Historical Astronomy Division (1980); and the Laboratory Astrophysics Division (2012). The membership includes physicists, mathematicians, geologists, engineers and others whose research interests lie within the broad spectrum of subjects now comprising contemporary astronomy.

In 2019 three AAS members were selected into the tenth anniversary class of TED Fellows.

The AAS established the AAS Fellows program in 2019 to “confer recognition upon AAS members for achievement and extraordinary service to the field of astronomy and the American Astronomical Society.” The inaugural class was designated by the AAS Board of Trustees and includes an initial group of 232 Legacy Fellows.

From AAS NOVA : ” Merging Black Holes vs. Gas and Stars”

From AAS NOVA

11 October 2021
Kerry Hensley

This simulated image shows a massive black hole at the center of a galaxy. Some massive black holes may be the result of mergers between the black holes hosted by two or more galaxies. Credit: D. Coe, J. Anderson,The National Aeronautics and Space Agency (US), The European Space Agency [Agence spatiale européenne][Europäische Weltraumorganisation](EU), and and R. van der Marel (Space Telescope Science Institute (US))]

When galaxies merge, what happens to the massive black holes at their centers? Today’s article explores the math behind the merger.

When galaxies merge, it shakes up star formation and sets the stage for a massive black hole merger. [NASA, ESA, the Hubble Heritage (STScI/The Association of Universities for Research in Astronomy (AURA)(US))-ESA/Hubble Collaboration, and A. Evans (The University of Virginia (US), Charlottesville/National Radio Astronomy Observatory (US)/Stony Brook University-SUNY (US))]

An Emerging Question

Two galaxies, adrift in the universe, pass near one another. If they become gravitationally entangled, the billion-year process of merging begins as they gradually coalesce into a single galaxy. As part of this process, the massive black holes at the centers of the colliding galaxies undergo a merger of their own.

As these massive black holes begin their death spiral, they encounter other galactic material like stars and gas. While simulations have shown that interacting with nearby stars causes the black-hole binary to spiral inward more quickly, the results aren’t as clear when it comes to gaseous material. Some studies have found that the presence of gas hastens the merger, while others suggest that it delays the merger instead.

The rate at which massive black holes merge has implications for upcoming gravitational-wave observatories, like the Laser Interferometer Space Antenna (LISA).

Gravity is talking. Lisa will listen. Dialogos of Eide.

European Space Agency(EU)/National Aeronautics and Space Administration (US) eLISA space based, the future of gravitational wave research.

Massive black-hole mergers at the centers of colliding galaxies are expected to be the loudest source of low-frequency gravitational waves in upcoming surveys — but if some process prevents these mergers, there may be nothing to listen to.

Artist’s by now iconic conception of two merging black holes similar to those detected by LIGO. Credit: Aurore Simonnet /Caltech MIT Advanced aLIGO(US)/Sonoma State University (US).

Black Holes on Paper

Elisa Bortolas (The University of Milano-Bicocca [Università degli Studi di Milano-Bicocca](IT)) and collaborators used a mathematical model of a black-hole merger to understand how interactions with stars and the presence of gas affect the inspiraling of the binary. Unlike most previous work, the set of differential equations developed by Bortolas and coauthors allowed for the effects of stars and gas to be considered simultaneously rather than separately.

The authors find that stars and gas tend to compete with one another as the black holes merge. If the black-hole pair accretes only a little mass from the surrounding material, gravitational interactions with nearby stars cause the black-hole pair to tighten inward. If the accretion rate is higher, the presence of a gaseous disk works to expand the binary pair, delaying the merger. Eventually, though, the stars win out, and the binary pair draws close enough to shed massive amounts of energy in the form of gravitational waves, sending the black holes on a collision course.

Looking Ahead to Future Detections

The results from Bortolas and coauthors showed that while the presence of gas can delay a merger, it won’t prevent it altogether. Under the conditions the authors explored, the presence of gas increased the time to the merger by a factor of a few, but all mergers occurred within a few hundred million years.

This is good news for LISA and other gravitational-wave detectors, and there are implications for the non-gravitational-wave detections of these events as well; the presence of gas in the black holes’ surroundings seems to make them pause with just a few light-years between them, increasing the chance that a survey might detect them in this phase.

Citation

“The Competing Effect of Gas and Stars in the Evolution of Massive Black Hole Binaries,” Elisa Bortolas et al 2021 ApJL 918 L15.

https://iopscience.iop.org/article/10.3847/2041-8213/ac1c0c

See the full article here .

five-ways-keep-your-child-safe-school-shootings

Please help promote STEM in your local schools.

Stem Education Coalition

AAS Mission and Vision Statement

The mission of the American Astronomical Society is to enhance and share humanity’s scientific understanding of the Universe.

The Society, through its publications, disseminates and archives the results of astronomical research. The Society also communicates and explains our understanding of the universe to the public.
The Society facilitates and strengthens the interactions among members through professional meetings and other means. The Society supports member divisions representing specialized research and astronomical interests.
The Society represents the goals of its community of members to the nation and the world. The Society also works with other scientific and educational societies to promote the advancement of science.
The Society, through its members, trains, mentors and supports the next generation of astronomers. The Society supports and promotes increased participation of historically underrepresented groups in astronomy.
The Society assists its members to develop their skills in the fields of education and public outreach at all levels. The Society promotes broad interest in astronomy, which enhances science literacy and leads many to careers in science and engineering.

Adopted June 7, 2009

The society was founded in 1899 through the efforts of George Ellery Hale. The constitution of the group was written by Hale, George Comstock, Edward Morley, Simon Newcomb and Edward Charles Pickering. These men, plus four others, were the first Executive Council of the society; Newcomb was the first president. The initial membership was 114. The AAS name of the society was not finally decided until 1915, previously it was the “Astronomical and Astrophysical Society of America”. One proposed name that preceded this interim name was “American Astrophysical Society”.

The AAS today has over 7,000 members and six divisions – the Division for Planetary Sciences (1968); the Division on Dynamical Astronomy (1969); the High Energy Astrophysics Division (1969); the Solar Physics Division (1969); the Historical Astronomy Division (1980); and the Laboratory Astrophysics Division (2012). The membership includes physicists, mathematicians, geologists, engineers and others whose research interests lie within the broad spectrum of subjects now comprising contemporary astronomy.

In 2019 three AAS members were selected into the tenth anniversary class of TED Fellows.

The AAS established the AAS Fellows program in 2019 to “confer recognition upon AAS members for achievement and extraordinary service to the field of astronomy and the American Astronomical Society.” The inaugural class was designated by the AAS Board of Trustees and includes an initial group of 232 Legacy Fellows.

From AAS NOVA : “Addressing a Gap in Our Knowledge of Black Holes”

From AAS NOVA

This now iconic computer-simulated image of a supermassive black hole at the core of a galaxy. Credit D. Coe, J. Anderson. National Aeronautics Space Agency (US), European Space Agency [Agence spatiale européenne][Europäische Weltraumorganisation](EU)

Artist’s by now iconic conception of two merging black holes similar to those detected by LIGO. Credit: Aurore Simonnet /Caltech MIT Advanced aLIGO(US)/Sonoma State University (US).

One way for black holes to form is in supernovae, or the deaths of massive stars. However, our current knowledge of stellar evolution and supernovae suggests that black holes with masses between 55 and 120 solar masses can’t be produced via supernovae. Gravitational-wave signals from black hole mergers offer us an observational test of this “gap” in black hole masses.

Black Hole Boundaries

You need a massive star to go supernova to produce a black hole. Unfortunately, extremely massive stars explode so violently they leave nothing behind! This scenario can occur with pair-instability supernovae, which happens in stars with core masses between 40 and 135 solar masses. The “pair” in “pair-instability” refers to the electron–positron pairs that are produced by gamma rays interacting with nuclei in the star’s core. Energy is lost in this process, meaning that there’s less resistance to gravitational collapse.

As the star collapses further, two things can happen. If the star is sufficiently massive, its core ignites in an explosion that tears the star apart, leaving no remnant. If the star is less massive, the core ignition causes the star to pulse and shed mass till it leaves the pair-production stage and its core collapses normally into black hole. The most massive black hole that can be produced in this scenario is roughly 55 solar masses, forming the lower end of the black hole mass gap.

On the other side of the mass gap, it’s theoretically possible for certain massive stars to collapse normally without entering the pair-production state, thus evolving into black holes with masses greater than 120 solar masses. The unique thing about these massive stars is that they are low metallicity, containing practically no elements that are heavier than helium.

So the bottom line is that we’re unlikely to observe any black holes with masses between 55 and 120 solar masses. But how can we test this prediction? Gravitational-wave signals are an option! Properties of merging black holes are coded into the gravitational waves produced by the merger, including the black hole masses. So, a recent study led by Bruce Edelman (University of Oregon (US)) looked at our current catalog of black hole merger signals to see if the mass gap would emerge from the data.

Mind the Gap, If There Is a Gap

Edelman and collaborators used two established model distributions of black hole masses to approach the problem. They also altered the models so the gap was explicitly allowed and so higher black hole masses could be explored without artificially inflating the rate of mergers above the gap. Edelman and collaborators then fit their models to data from 46 binary black hole mergers observed by the Laser Interferometer Gravitational-Wave Observatory and the Virgo interferometer.

Masses in the Stellar Graveyard GWTC-2 plot v1.0 BY LIGO-Virgo. Credit: Frank Elavsky and Aaron Geller at Northwestern University(US)

Caltech/MIT Advanced aLigo at Hanford, WA(US), Livingston, LA(US) and VIRGO Gravitational Wave interferometer, near Pisa(IT).

Interestingly, the existence of the gap is rather ambiguous! One factor is the inclusion of the merger associated with the signal GW190521, which was likely a high mass merger whose component black holes straddle the mass gap. If the gap doesn’t exist, it’s possible that the unexpected black holes are formed by the merging of smaller black holes. On the whole, this result points to many avenues of study when it comes to pair-instability supernovae and black hole formation!

Citation

“Poking Holes: Looking for Gaps in LIGO/Virgo’s Black Hole Population,” Bruce Edelman et al 2021 ApJL 913 L23.
https://iopscience.iop.org/article/10.3847/2041-8213/abfdb3

See the full article here .

five-ways-keep-your-child-safe-school-shootings

Please help promote STEM in your local schools.

Stem Education Coalition

AAS Mission and Vision Statement

The mission of the American Astronomical Society is to enhance and share humanity’s scientific understanding of the Universe.

The Society, through its publications, disseminates and archives the results of astronomical research. The Society also communicates and explains our understanding of the universe to the public.
The Society facilitates and strengthens the interactions among members through professional meetings and other means. The Society supports member divisions representing specialized research and astronomical interests.
The Society represents the goals of its community of members to the nation and the world. The Society also works with other scientific and educational societies to promote the advancement of science.
The Society, through its members, trains, mentors and supports the next generation of astronomers. The Society supports and promotes increased participation of historically underrepresented groups in astronomy.
The Society assists its members to develop their skills in the fields of education and public outreach at all levels. The Society promotes broad interest in astronomy, which enhances science literacy and leads many to careers in science and engineering.

Adopted June 7, 2009

The society was founded in 1899 through the efforts of George Ellery Hale. The constitution of the group was written by Hale, George Comstock, Edward Morley, Simon Newcomb and Edward Charles Pickering. These men, plus four others, were the first Executive Council of the society; Newcomb was the first president. The initial membership was 114. The AAS name of the society was not finally decided until 1915, previously it was the “Astronomical and Astrophysical Society of America”. One proposed name that preceded this interim name was “American Astrophysical Society”.

The AAS today has over 7,000 members and six divisions – the Division for Planetary Sciences (1968); the Division on Dynamical Astronomy (1969); the High Energy Astrophysics Division (1969); the Solar Physics Division (1969); the Historical Astronomy Division (1980); and the Laboratory Astrophysics Division (2012). The membership includes physicists, mathematicians, geologists, engineers and others whose research interests lie within the broad spectrum of subjects now comprising contemporary astronomy.

In 2019 three AAS members were selected into the tenth anniversary class of TED Fellows.

The AAS established the AAS Fellows program in 2019 to “confer recognition upon AAS members for achievement and extraordinary service to the field of astronomy and the American Astronomical Society.” The inaugural class was designated by the AAS Board of Trustees and includes an initial group of 232 Legacy Fellows.

From AAS NOVA : “Reweighing A Heavy Neutron Star”

From AAS NOVA

16 August 2021
Susanna Kohler

Artist’s impression of how the pulses emitted by the pulsar PSR J0740+6620 are affected by the gravity of its white-dwarf companion. Credit: B. Saxton/National Radio Astronomy Observatory (US)/Associated Universities Inc (US)

What does the inside of a neutron star — the incredibly dense remnant of an evolved star — look like? New observations of one of the most massive neutron stars provide some clues.

Mysterious Interior

With the mass of multiple Suns packed into the rough size of a city, neutron stars represent one of the most dense, exotic environments in the universe. We can’t create an equivalent environment on Earth, so we rely on theoretical models — constrained by observations — to understand how matter behaves under these extreme circumstances.

Different theoretical models predict different interior structures for neutron stars, each described by an equation of state. In turn, each equation of state predicts a different maximum mass that a neutron star can reach before the overwhelming crush of gravity causes it to collapse into a black hole.

The heaviest neutron stars we spot in the universe, then, can help us to set upper limits and rule out some equations of state, narrowing down which models of neutron star interiors are most likely.

The catch? Measuring the precise masses of objects located thousands of light-years away is difficult! Luckily, the universe occasionally offers up clever tricks for doing so.

A Delay from Gravity

Some highly magnetized neutron stars emit beams of light that regularly pulse across our line of sight as they rotate. If these incredibly precise cosmic clocks — pulsars — have a binary companion, and if we view that binary edge-on, then we have a unique opportunity for some mass measurements.

Dame Susan Jocelyn Bell Burnell, discovered pulsars with radio astronomy. Jocelyn Bell at the Mullard Radio Astronomy Observatory, University of Cambridge(UK), taken for the Daily Herald newspaper in 1968. Denied the Nobel.

In such a system, the distortion of spacetime caused by the gravity of the companion object can affect the signal of the pulsar, such that the pulses arrive at Earth at slightly offset times. This effect, known as the Shapiro time delay, allows us to precisely measure the companion’s mass — which can then be used with the binary orbit to establish the pulsar’s mass.

In a recent study, a team of scientists led by Emmanuel Fonseca (McGill University (CA); West Virginia University (US)) have now used this approach with new observations of the pulsar PSR J0740+6620 to place the tightest constraints on its mass yet — and it’s a doozy.

Tipping the Scales

Fonseca and collaborators use observations from the 100-m Green Bank Telescope and the Canadian Hydrogen Intensity Mapping Experiment (CHIME) telescope to carefully model the Shapiro delay and measure the properties of PSR J0740+6620 and its companion, significantly improving upon previous measurements.

Green Bank Radio Telescope, West Virginia, USA, now the center piece of the Green Bank Observatory(US), being cut loose by the National Science Foundation(US), supported by Breakthrough Listen Project, West Virginia University, and operated by the nonprofit Associated Universities, Inc..

CHIME Canadian Hydrogen Intensity Mapping Experiment-A partnership between The University of British Columbia (CA), The University of Toronto (CA), McGill University (CA), Yale University (US), and The National Research Council Canada [Conseil national de recherches (CA) at The Canada NRCC Dominion Radio Astrophysical Observatory in Penticton, British Columbia(CA) Altitude 545 m (1,788 ft).

The authors show that PSR J0740+6620 weighs in at 2.01–2.15 solar masses — confirming its status as the heaviest precisely measured neutron star currently known. They also confirm that the binary lies ~3,700 light-years away, and that the companion is an unusually cold white dwarf of just 0.25 solar mass.

Even more precise constraints — both on PSR J0740+6620 and other high-mass neutron stars — will be enabled by ongoing observations with currently technology, and by future studies using next-generation telescopes. Each improvement brings us a little closer to understanding the matter in these extreme objects.

Citation

“Refined Mass and Geometric Measurements of the High-mass PSR J0740+6620,” E. Fonseca et al 2021 ApJL 915 L12.

https://iopscience.iop.org/article/10.3847/2041-8213/ac03b8

See the full article here .

five-ways-keep-your-child-safe-school-shootings

Please help promote STEM in your local schools.

Stem Education Coalition

AAS Mission and Vision Statement

The mission of the American Astronomical Society is to enhance and share humanity’s scientific understanding of the Universe.

The Society, through its publications, disseminates and archives the results of astronomical research. The Society also communicates and explains our understanding of the universe to the public.
The Society facilitates and strengthens the interactions among members through professional meetings and other means. The Society supports member divisions representing specialized research and astronomical interests.
The Society represents the goals of its community of members to the nation and the world. The Society also works with other scientific and educational societies to promote the advancement of science.
The Society, through its members, trains, mentors and supports the next generation of astronomers. The Society supports and promotes increased participation of historically underrepresented groups in astronomy.
The Society assists its members to develop their skills in the fields of education and public outreach at all levels. The Society promotes broad interest in astronomy, which enhances science literacy and leads many to careers in science and engineering.

Adopted June 7, 2009

The society was founded in 1899 through the efforts of George Ellery Hale. The constitution of the group was written by Hale, George Comstock, Edward Morley, Simon Newcomb and Edward Charles Pickering. These men, plus four others, were the first Executive Council of the society; Newcomb was the first president. The initial membership was 114. The AAS name of the society was not finally decided until 1915, previously it was the “Astronomical and Astrophysical Society of America”. One proposed name that preceded this interim name was “American Astrophysical Society”.

The AAS today has over 7,000 members and six divisions – the Division for Planetary Sciences (1968); the Division on Dynamical Astronomy (1969); the High Energy Astrophysics Division (1969); the Solar Physics Division (1969); the Historical Astronomy Division (1980); and the Laboratory Astrophysics Division (2012). The membership includes physicists, mathematicians, geologists, engineers and others whose research interests lie within the broad spectrum of subjects now comprising contemporary astronomy.

In 2019 three AAS members were selected into the tenth anniversary class of TED Fellows.

The AAS established the AAS Fellows program in 2019 to “confer recognition upon AAS members for achievement and extraordinary service to the field of astronomy and the American Astronomical Society.” The inaugural class was designated by the AAS Board of Trustees and includes an initial group of 232 Legacy Fellows.

From AAS NOVA : “Connecting the Pieces of a Black Hole Temper Tantrum”

From AAS NOVA

7 July 2021
Susanna Kohler

Artist’s illustration of a black hole X-ray binary, in which a black hole accretes matter from a companion star. Credit: Gabriel Pérez, Solar Maximum Mission (SMM) | High Altitude Observatory (Center for Astrophysics in La Palma | Instituto de Astrofísica de Canarias • IAC (ES)]

Accreting, stellar-mass black holes are anything but predictable. A new study explores what’s happening as these feeding monsters erupt in violent outbursts and then settle down again.

This illustration shows some of the possible transient behavior of the gas flowing onto and away from a black hole in an X-ray binary. [NASA/JPL-Caltech (US)]

An Unsteady Existence

Black hole X-ray binaries (BHXBs) consist of a stellar-mass black hole that siphons material from an ordinary companion star. As this material flows between the objects, it forms an accretion disk around the black hole. BHXBs shine in X-rays from the hot material of this disk, and from a mysterious corona — ultra-hot gas that exists in some unknown form above the disk.

BHXBs may accrete quietly and steadily much of the time, but on occasion, they undergo sudden outbursts, substantially brightening in X-rays. Unlike supermassive black holes, which evolve on extremely long timescales, stellar-mass black holes can change over just days or weeks — short enough for us to watch!

In a new study led by Jingyi Wang (MIT Kavli Institute for Astrophysics and Space Research (US)), a team of scientists presents observations by NICER — an X-ray telescope installed on the International Space Station — of a BHXB throwing such a temper tantrum.

MIT Kavli Institute for Astrophysics and Space Research.

NICER on the ISS

Reflections of a Transition

During an outburst, a BHXB undergoes state transitions, displaying changes in the X-ray luminosities and energies as either the corona or the disk takes over to dominate the emission. In addition to the X-ray changes, persistent radio emission from a slow and steady jet can be suddenly replaced by a short-lived radio flare that then subsides.

Despite many observations, we lack the details of what’s happening on small scales, close around the black hole. What form does the corona take? Does its size or extent change over time? What drives the state transitions? And how are the different components of this system — disk, corona, and jet — related, if at all?

The top plot (a) shows the inferred corona height (black line) over the span of the outburst. The radio emission, including the flare, can be seen in red. The bottom two diagrams show the authors’ picture of the geometry of the black hole, disk, and corona at two points during the state changes: during the rise from quiescence (b), and at the end of the outburst when the jet base is ejected (c). [Adapted from Wang et al. 2021.]

Wang and collaborators used NICER data of the BHXB system MAXI J1820+070 during a 2018 outburst to track the lag caused by light travel time between X-rays that arrived directly from the corona and light from the corona that was reflected by the disk before reaching us. By modeling the changes in this reverberation lag as the system underwent state transitions, the team could infer the geometry on the small scales we can’t observe, helping us to understand the tantrum.

A Connected Picture

Wang and collaborators show that the best explanation of NICER’s observations is that the height of the X-ray corona changes during the BHXB’s state transitions. They argue that the corona first contracts, and then rapidly expands during the outburst, preceding a radio flare by ~5 days.

Under the authors’ interpretation, these signs point to a neat picture of BHXB outbursts: a quietly accreting black hole has a disk and a steady jet, and the corona makes up the base of that jet. When the BHXB goes into outburst, it ejects that jet base as a bright knot in its final moments of outburst, before fading back to quiescence.

While this model isn’t yet definitive, this latest evidence points to a clear connection between the disk, jet, and corona of a BHXB. We’re sure to gain more insight ahead!

Citation

“Disk, Corona, Jet Connection in the Intermediate State of MAXI J1820+070 Revealed by NICER Spectral-timing Analysis,” Jingyi Wang et al 2021 ApJL 910 L3.
https://iopscience.iop.org/article/10.3847/2041-8213/abec79

See the full article here .

five-ways-keep-your-child-safe-school-shootings

Please help promote STEM in your local schools.

Stem Education Coalition

AAS Mission and Vision Statement

The mission of the American Astronomical Society is to enhance and share humanity’s scientific understanding of the Universe.

The Society, through its publications, disseminates and archives the results of astronomical research. The Society also communicates and explains our understanding of the universe to the public.
The Society facilitates and strengthens the interactions among members through professional meetings and other means. The Society supports member divisions representing specialized research and astronomical interests.
The Society represents the goals of its community of members to the nation and the world. The Society also works with other scientific and educational societies to promote the advancement of science.
The Society, through its members, trains, mentors and supports the next generation of astronomers. The Society supports and promotes increased participation of historically underrepresented groups in astronomy.
The Society assists its members to develop their skills in the fields of education and public outreach at all levels. The Society promotes broad interest in astronomy, which enhances science literacy and leads many to careers in science and engineering.

Adopted June 7, 2009

The society was founded in 1899 through the efforts of George Ellery Hale. The constitution of the group was written by Hale, George Comstock, Edward Morley, Simon Newcomb and Edward Charles Pickering. These men, plus four others, were the first Executive Council of the society; Newcomb was the first president. The initial membership was 114. The AAS name of the society was not finally decided until 1915, previously it was the “Astronomical and Astrophysical Society of America”. One proposed name that preceded this interim name was “American Astrophysical Society”.

The AAS today has over 7,000 members and six divisions – the Division for Planetary Sciences (1968); the Division on Dynamical Astronomy (1969); the High Energy Astrophysics Division (1969); the Solar Physics Division (1969); the Historical Astronomy Division (1980); and the Laboratory Astrophysics Division (2012). The membership includes physicists, mathematicians, geologists, engineers and others whose research interests lie within the broad spectrum of subjects now comprising contemporary astronomy.

In 2019 three AAS members were selected into the tenth anniversary class of TED Fellows.

The AAS established the AAS Fellows program in 2019 to “confer recognition upon AAS members for achievement and extraordinary service to the field of astronomy and the American Astronomical Society.” The inaugural class was designated by the AAS Board of Trustees and includes an initial group of 232 Legacy Fellows.

From AAS NOVA : “Observing ‘The Accident’- an Enigmatic Brown Dwarf”

From AAS NOVA

2 July 2021
Susanna Kohler

Artist’s illustration of a Y-type brown dwarf. [NASA/JPL-Caltech (US)]

It’s human nature to try to categorize the things we observe in the universe. But what happens when something doesn’t fit into the neat categories we’ve established? A new study explores one such object: an especially perplexing brown dwarf.

Brown dwarfs are intermediate in size between the largest planets and the smallest stars. [NASA/JPL-Caltech/UCB]

A Hidden Population

Brown dwarfs — substellar objects that aren’t massive enough to fuse hydrogen in their cores — are an intriguing population. Not quite stars and not quite planets, these objects occupy an uneasy in-between space that’s worth studying further.

But the coldest brown dwarfs — which fall into spectral category Y and have effective temperatures below ~450 K — are a challenging population to study! These chilly objects don’t emit much light, and what little they do radiate is concentrated in the infrared near 5 µm. These objects are therefore difficult to observe from the ground, so we rely on space-based missions to discover the faint light from these objects.

An Accidental Find

Thus far, we’ve managed to detect around 50 of these cold Y dwarfs. To better categorize them, we plot them on color–color and color–magnitude plots to compare their brightnesses at different wavelengths. A recent discovery, however, isn’t behaving as expected.

WISE 1534–1043 nicknamed “The Accident” when it was found serendipitously in a field imaged by the Wide-field Infrared Survey Explorer, is a lone brown dwarf speeding across the sky.

National Aeronautics and Space Administration(US) Wise/NEOWISE Telescope.

An article led by J. Davy Kirkpatrick (California Institute of Technology (US)) now presents new, follow-up observations of this puzzling object collected with the Hubble Space Telescope and with the Keck Observatory in Hawaii.

National Aeronautics Space Agency(USA) European Space Agency [Agence spatiale européenne][Europäische Weltraumorganisation](EU)/ Hubble Wide Field Camera 3

National Aeronautics and Space Administration(US)/European Space Agency [Agence spatiale européenne] [Europäische Weltraumorganisation] (EU) Hubble Space Telescope

Keck/MOSFIRE on Keck 1, Mauna Kea, Hawaii, USA.

W.M. Keck Observatory two ten meter telescopes operated by California Institute of Technology(US) and the University of California(US), Maunakea Hawaii USA, altitude 4,207 m (13,802 ft). Credit: Caltech.

Color–color (top) and color–magnitude (bottom) plots exploring WISE 1534–1043’s photometric properties (red data points, plotted using two different models) show that its behavior is unique among known, nearby, cold brown dwarfs. [Adapted from Kirkpatrick et al. 2021]

Defying Categorization

These new observations pinpoint WISE 1534–1043’s location — just ~50 light-years away — and confirm its bizarre observational properties. Looking at the color–color and color–magnitude plots to the right, it’s clear that WISE 1534–1043 lies in a quadrant entirely on its own.

Measurements of The Accident’s absolute brightness at different wavelengths are all in line with the coldest known Y dwarfs. But its relative colors (as shown by the W1 – W2, ch1 – ch2, and J – ch2 measurements in the plots) and magnitudes fall entirely outside of the range of known brown dwarfs.

Identity Options

What could be the explanation for these enigmatic properties? Kirkpatrick and collaborators use their observations to consider four possible identities for The Accident:

An extremely low-metallicity, old, cold brown dwarf
An extremely low-mass, low-gravity, young brown dwarf
An ejected exoplanet
An ultracold stellar remnant (like a white dwarf or an exotic ablated stellar core)

Of these options, the authors determine that the first is the most likely. If WISE 1534–1043 is old and remarkably low-metallicity, then the outer layers of its atmosphere would have decreased opacity, allowing us to see deeper into it and potentially explaining the unusual photometric properties. This could mean that The Accident represents the first known Y-type subdwarf — a brand new category of star.

“Verification, refutation, or further befuddlement” should be possible in the future with observations from the upcoming James Webb Space Telescope, suggest the authors. In the meantime, the refusal of objects like The Accident to fit neatly into boxes continues to keep us on our toes!

Citation

“The Enigmatic Brown Dwarf WISEA J153429.75-104303.3 (a.k.a. “The Accident”),” J. Davy Kirkpatrick et al 2021 ApJL 915 L6.
https://iopscience.iop.org/article/10.3847/2041-8213/ac0437

See the full article here .

five-ways-keep-your-child-safe-school-shootings

Please help promote STEM in your local schools.

Stem Education Coalition

AAS Mission and Vision Statement

The mission of the American Astronomical Society is to enhance and share humanity’s scientific understanding of the Universe.

The Society, through its publications, disseminates and archives the results of astronomical research. The Society also communicates and explains our understanding of the universe to the public.
The Society facilitates and strengthens the interactions among members through professional meetings and other means. The Society supports member divisions representing specialized research and astronomical interests.
The Society represents the goals of its community of members to the nation and the world. The Society also works with other scientific and educational societies to promote the advancement of science.
The Society, through its members, trains, mentors and supports the next generation of astronomers. The Society supports and promotes increased participation of historically underrepresented groups in astronomy.
The Society assists its members to develop their skills in the fields of education and public outreach at all levels. The Society promotes broad interest in astronomy, which enhances science literacy and leads many to careers in science and engineering.

Adopted June 7, 2009

The society was founded in 1899 through the efforts of George Ellery Hale. The constitution of the group was written by Hale, George Comstock, Edward Morley, Simon Newcomb and Edward Charles Pickering. These men, plus four others, were the first Executive Council of the society; Newcomb was the first president. The initial membership was 114. The AAS name of the society was not finally decided until 1915, previously it was the “Astronomical and Astrophysical Society of America”. One proposed name that preceded this interim name was “American Astrophysical Society”.

The AAS today has over 7,000 members and six divisions – the Division for Planetary Sciences (1968); the Division on Dynamical Astronomy (1969); the High Energy Astrophysics Division (1969); the Solar Physics Division (1969); the Historical Astronomy Division (1980); and the Laboratory Astrophysics Division (2012). The membership includes physicists, mathematicians, geologists, engineers and others whose research interests lie within the broad spectrum of subjects now comprising contemporary astronomy.

In 2019 three AAS members were selected into the tenth anniversary class of TED Fellows.

The AAS established the AAS Fellows program in 2019 to “confer recognition upon AAS members for achievement and extraordinary service to the field of astronomy and the American Astronomical Society.” The inaugural class was designated by the AAS Board of Trustees and includes an initial group of 232 Legacy Fellows.

From AAS NOVA : “Searching for Spots with Interferometry”

From AAS NOVA

23 June 2021
Tarini Konchady

Credit: http://www.chara.gsu.edu/instrumentation/chara-array

One of the telescopes in the CHARA array as seen at sunrise. The pipe directs light to the central Beam Synthesis Facility, which combines the light from all six telescopes. Credit: Steve Golden/CHARA.

One of the six telescopes that are part of the CHARA array at Mount Wilson Observatory, Los Angeles County, California.

Starspots are regions on the surface of a star that are cooler than their surroundings. Temperature affects brightness, so starspots can significantly alter the overall appearance of a star even when individual starspots can’t be distinguished. But if we characterize starspots in detail, we should be able to account for their effects.

Old and New Ways of Finding Starspots

Starspots are thought to be caused by stellar magnetic activity, so as much as they can obscure true stellar properties, they can also help us learn about the interiors of stars. We’ve also noticed that our Sun’s starspots behave very differently than starspots on other stars, adding another motivation to examine these phenomena on other stars.

Until recently, starspots have been studied through indirect methods like light curve modeling and Doppler imaging, which measures changes in stellar spectra caused by magnetic fields. These techniques have broadened our understanding of starspots, but they are also hamstrung by requiring certain assumptions about the stars being observed.

Direct imaging of starspots has been made possible through a relatively new technique called long-baseline optical/near-infrared interferometry (LBI). A recent study led by James Parks (Georgia State University (US)) uses this technique to observe starspots on λ Andromedae, a giant star in a binary with a less massive companion.

Stellar surface images based on observations taken in 2010. The top row is the starspot model images, the middle row is the observations, and the bottom row is the simulated images. The white dot is the resolution of the CHARA array, which was used to take the observations. The starspot models were constructed independently of the simulated images and vice-versa[Parks et al. 2021].

Telescopes in Tandem

Interferometry is an imaging technique that combines the input of multiple telescopes as though they were a single and much larger telescope. LBI refers to interferometry conducted with telescopes that are spread out across a fairly large distance. For instance, the Event Horizon Telescope image of Messier 87’s central black hole-Messier 87*- was taken by eight radio telescopes spread out across an entire hemisphere!

To image the starspots on λ Andromedae, Parks and collaborators used observations taken by the Center for High Angular Resolution Astronomy (CHARA) array, which consists of six 1-meter optical telescopes arranged in a Y-shape. The highest resolution allowed by these observations was 0.4 milliarcseconds (for context, the Moon spans roughly 0.5 degrees, or nearly 2 million milliarcseconds). Data was also taken on the 0.4-meter telescope at Fairborn Observatory to obtain light curves of λ Andromedae that were roughly concurrent to the LBI observations.

Two Different Ways to Model Starspots

Parks and collaborators used the CHARA observations to model λ Andromedae’s starspots in two different ways. The first method was to model the star’s surface while allowing for starspots, where the resulting models were informed by the CHARA observations. The second way was image reconstruction, which uses the observing conditions during which images were taken to determine the underlying astrophysical components. The advantage of using image reconstruction over surface modeling is that image reconstruction requires fewer assumptions about the object in question. However, false artifacts can be generated during the reconstruction process.

Both methods found between one and four starspots on λ Andromedae at any given time. The starspots also pointed to a rotation period that matched the period determined from the concurrent light curves. Overall, the study was a successful demonstration of using LBI to image starspots on λ Andromedae and, excitingly, more detailed studies are to follow!

Citation

“Interferometric Imaging of λ Andromedae: Evidence of Starspots and Rotation,” J. R. Parks et al 2021 ApJ 913 54.

https://iopscience.iop.org/article/10.3847/1538-4357/abb670

See the full article here .

five-ways-keep-your-child-safe-school-shootings

Please help promote STEM in your local schools.

Stem Education Coalition

AAS Mission and Vision Statement

The mission of the American Astronomical Society is to enhance and share humanity’s scientific understanding of the Universe.

The Society, through its publications, disseminates and archives the results of astronomical research. The Society also communicates and explains our understanding of the universe to the public.
The Society facilitates and strengthens the interactions among members through professional meetings and other means. The Society supports member divisions representing specialized research and astronomical interests.
The Society represents the goals of its community of members to the nation and the world. The Society also works with other scientific and educational societies to promote the advancement of science.
The Society, through its members, trains, mentors and supports the next generation of astronomers. The Society supports and promotes increased participation of historically underrepresented groups in astronomy.
The Society assists its members to develop their skills in the fields of education and public outreach at all levels. The Society promotes broad interest in astronomy, which enhances science literacy and leads many to careers in science and engineering.

Adopted June 7, 2009

The society was founded in 1899 through the efforts of George Ellery Hale. The constitution of the group was written by Hale, George Comstock, Edward Morley, Simon Newcomb and Edward Charles Pickering. These men, plus four others, were the first Executive Council of the society; Newcomb was the first president. The initial membership was 114. The AAS name of the society was not finally decided until 1915, previously it was the “Astronomical and Astrophysical Society of America”. One proposed name that preceded this interim name was “American Astrophysical Society”.

The AAS today has over 7,000 members and six divisions – the Division for Planetary Sciences (1968); the Division on Dynamical Astronomy (1969); the High Energy Astrophysics Division (1969); the Solar Physics Division (1969); the Historical Astronomy Division (1980); and the Laboratory Astrophysics Division (2012). The membership includes physicists, mathematicians, geologists, engineers and others whose research interests lie within the broad spectrum of subjects now comprising contemporary astronomy.

In 2019 three AAS members were selected into the tenth anniversary class of TED Fellows.

The AAS established the AAS Fellows program in 2019 to “confer recognition upon AAS members for achievement and extraordinary service to the field of astronomy and the American Astronomical Society.” The inaugural class was designated by the AAS Board of Trustees and includes an initial group of 232 Legacy Fellows.

From AAS NOVA : “Probing Dust Close to Supermassive Black Holes”

From AAS NOVA

16 June 2021
Susanna Kohler

Artist’s illustration of the surroundings of a supermassive black hole at the heart of an active galaxy. Credit: M. Kornmesser/ European Southern Observatory [Observatoire européen austral][Europäische Südsternwarte] (EU) (CL)]

What’s going on deep in the centers of active galaxies, close around the supermassive black holes feeding off of their surroundings? A new study uses infrared observations to explore this inner region in one active galaxy.

Artist’s impression of the active galactic nucleus (DESY, Science Communication Lab)

A Unified Picture?

We know that active galactic nuclei (AGN) consist of a supermassive black hole accreting surrounding material and shining brightly across the electromagnetic spectrum. But the structure of the gas and dust close around the black hole, and the causes of the different emission we see, have remained a topic of debate.

Decades ago, scientists proposed that Type 1 and Type 2 AGN — two different categories of active galaxies with different observational properties — might be the same objects viewed from different angles. This unification scheme relies on the presence of a dusty torus — a puffed-up, donut-like dust structure close to the black hole. In this model, the torus obscures the inner, emission-line-producing gas from some viewing angles, changing the appearance of the AGN based on its orientation.

But recent infrared observations have challenged this view. With powerful mid-infrared telescopes, we’ve taken a closer look at the inner few hundred light-years of nearby active galaxies — and instead of revealing an obscuring torus of dust, these observations have shown polar dust structures.

European Southern Observatory(EU) , Very Large Telescope at Cerro Paranal in the Atacama Desert •ANTU (UT1; The Sun ) •KUEYEN (UT2; The Moon ) •MELIPAL (UT3; The Southern Cross ), and •YEPUN (UT4; Venus – as evening star). Elevation 2,635 m (8,645 ft) from above Credit J.L. Dauvergne & G. Hüdepohl atacama photo.

A Search for Distant Dust

How can we explain these observations? Theorists have a solution: in the disk–wind model, the dust close to the black hole is arranged in a hot, equatorial disk rather than a torus. Radiation pressure then blows some of this dust off into a cooler wind from the poles, producing the polar structures we’ve seen in mid-infrared observations. Obscuration comes from the disk and the launch region of the wind.

The equatorial disk in this model should lie on scales too small to have been previously observed in mid-infrared — but there’s a new tool on the scene! GRAVITY, an interferometric instrument on the Very Large Telescope Interferometer in Chile, operates in the near-infrared.

ESO VLTI GRAVITY

European Southern Observatory(EU) VLTI Interferometer image, Cerro Paranal, with an elevation of 2,635 metres (8,645 ft) above sea level, •ANTU (UT1; The Sun ),
•KUEYEN (UT2; The Moon ),
•MELIPAL (UT3; The Southern Cross ), and
•YEPUN (UT4; Venus – as evening star).

This makes it the perfect instrument to search for the very hot dust that would lie in a disk at the heart of an AGN.

In a new study led by James Leftley (University of Southampton (UK); University of Côte d’Azur [Université Côte d’Azur](FR); ESO, Chile), a team of scientists has now used GRAVITY to obtain near-infrared observations of the center of ESO 323-G77, a local active galactic nucleus.

Getting to the Heart of the Matter

Through careful analysis and modeling, Leftley and collaborators interpret their observations on scales of less than a light-year (for an object that’s hundreds of millions of light-years away!). The result? The near-infrared observations are consistent with an extended, equatorially aligned hot dust disk. The scale of this disk neatly matches the size predicted in disk–wind models.

Though the data are still too sparse and noisy to rule out the torus model in favor of the disk–wind model, these observations represent an important step in understanding how dust may be distributed in the heart of active galaxies.

Citation

“Resolving the Hot Dust Disk of ESO323-G77,” James H. Leftley et al 2021 ApJ 912 96 6
https://iopscience.iop.org/article/10.3847/1538-4357/abee80

See the full article here .

five-ways-keep-your-child-safe-school-shootings

Please help promote STEM in your local schools.

Stem Education Coalition

AAS Mission and Vision Statement

The mission of the American Astronomical Society is to enhance and share humanity’s scientific understanding of the Universe.

The Society, through its publications, disseminates and archives the results of astronomical research. The Society also communicates and explains our understanding of the universe to the public.
The Society facilitates and strengthens the interactions among members through professional meetings and other means. The Society supports member divisions representing specialized research and astronomical interests.
The Society represents the goals of its community of members to the nation and the world. The Society also works with other scientific and educational societies to promote the advancement of science.
The Society, through its members, trains, mentors and supports the next generation of astronomers. The Society supports and promotes increased participation of historically underrepresented groups in astronomy.
The Society assists its members to develop their skills in the fields of education and public outreach at all levels. The Society promotes broad interest in astronomy, which enhances science literacy and leads many to careers in science and engineering.

Adopted June 7, 2009

The society was founded in 1899 through the efforts of George Ellery Hale. The constitution of the group was written by Hale, George Comstock, Edward Morley, Simon Newcomb and Edward Charles Pickering. These men, plus four others, were the first Executive Council of the society; Newcomb was the first president. The initial membership was 114. The AAS name of the society was not finally decided until 1915, previously it was the “Astronomical and Astrophysical Society of America”. One proposed name that preceded this interim name was “American Astrophysical Society”.

The AAS today has over 7,000 members and six divisions – the Division for Planetary Sciences (1968); the Division on Dynamical Astronomy (1969); the High Energy Astrophysics Division (1969); the Solar Physics Division (1969); the Historical Astronomy Division (1980); and the Laboratory Astrophysics Division (2012). The membership includes physicists, mathematicians, geologists, engineers and others whose research interests lie within the broad spectrum of subjects now comprising contemporary astronomy.

In 2019 three AAS members were selected into the tenth anniversary class of TED Fellows.

The AAS established the AAS Fellows program in 2019 to “confer recognition upon AAS members for achievement and extraordinary service to the field of astronomy and the American Astronomical Society.” The inaugural class was designated by the AAS Board of Trustees and includes an initial group of 232 Legacy Fellows.

From AAS NOVA : “New Insights from LIGO/Virgo’s Merging Black Holes”

From AAS NOVA

Still from a simulation showing how black holes might interact in the chaotic cores of globular clusters. [Carl Rodriguez/Northwestern University (US) Visualization]

Since the first merger of two black holes detected in 2015, the LIGO/Virgo gravitational-wave detectors have observed a total of 47 confident collisions of black holes and neutron stars through the end of September 2019. What’s the big picture behind these events? The second gravitational-wave catalog is officially out — and the population statistics are in!

A New Catalog

In recent years, the Advanced LIGO detectors in Hanford, WA and Livingston, LA and the Advanced Virgo detector in Europe have kept a watchful vigil for ripples in spacetime that let us know that a pair of compact objects — black holes or neutron stars — has spiraled in and merged.

Caltech/MIT Advanced aLigo at Hanford, WA(US), Livingston, LA(US) and VIRGO Gravitational Wave interferometer, near Pisa(IT).

Masses in the Stellar Graveyard GWTC-2 plot v1.0 BY LIGO-Virgo. Credit: Frank Elavsky and Aaron Geller at Northwestern University(US)

During LIGO’s first two observational runs (O1 in 2015–16 and O2 in 2016–17) the two LIGO detectors discovered 11 merger events. After a series of upgrades to the detectors, the system came back online in April 2019 for its third run (O3). In just the first 26 weeks of the run (O3a), LIGO/Virgo jointly found another 36 mergers!

In a new publication recently accepted to Physical Review X, the collaboration has released its second catalog of gravitational-wave events (GWTC–2), which includes data from O1, O2, and O3a. And in a companion publication in The Astrophysical Journal Letters, the team has now analyzed the broader set of all 47 mergers in the catalog, using population models to gain deeper insight into the binary properties and how these systems form and evolve.

Learning About Collisions

So what have we learned from the GWTC–2 population?

Black-hole mass is more complicated than we originally thought.
The merging black holes in O1 and O2 all had primary masses below 45 solar masses, consistent with the theory that black holes of ~50–120 solar masses shouldn’t be able to form. But O3a included several primaries above 45 solar masses, so we can’t model the primary mass distribution as a single power law with a sharp cutoff at 45 solar masses anymore. This may suggest that we’re looking at different populations of black holes that formed in different ways.
Some black holes have spins that are misaligned with the angular momentum of the binary.
Nine of the recent detections exhibit misaligned spins, which is another clue about their formation. Black hole binaries that form and evolve in isolated pairs are expected to have aligned spins, whereas black hole binaries that form dynamically — due, for instance, to interactions in clusters of stars or in the disk of an active galactic nucleus — should have isotropically distributed spins. The authors show that the spinning GWTC–2 population is consistent with 25–93% of black holes forming dynamically. That’s a large range, but what’s important is that this also indicates there’s more than one formation channel at work!
The black hole merger rate probably increases with redshift.
Updated estimates suggest that binary black holes merge at a rate of 15–38 Gpc–3 yr-1 and binary neutron stars at a rate of 80–810 Gpc-3 yr-1. The merger rate appears to be higher at higher redshift, but this increase doesn’t quite parallel the known increase in star formation rate with redshift. An intriguing mystery!

The modeled median merger rate density (solid curve) as a function of redshift suggests that the merger rate increases with redshift. Still, the increase is not as steep as the increase in the star formation rate (dashed line). [Abbott et al. 2021]

A Smashing Good Time Ahead

These takeaways clearly represent a dramatic increase in our understanding of how and where black hole binaries form and evolve — but we still have so much left to learn! Luckily, there’s plenty more data ahead: the collaboration is now analyzing the remaining 5 months of data from O3, and the detectors are currently undergoing upgrades in preparation for O4, which is slated to begin in mid-2022.

Citation

“Population Properties of Compact Objects from the Second LIGO–Virgo Gravitational-Wave Transient Catalog,” R. Abbott et al 2021 ApJL 913 L7.

https://iopscience.iop.org/article/10.3847/2041-8213/abe949

See the full article here .

five-ways-keep-your-child-safe-school-shootings

Please help promote STEM in your local schools.

Stem Education Coalition

AAS Mission and Vision Statement

The mission of the American Astronomical Society is to enhance and share humanity’s scientific understanding of the Universe.

The Society, through its publications, disseminates and archives the results of astronomical research. The Society also communicates and explains our understanding of the universe to the public.
The Society facilitates and strengthens the interactions among members through professional meetings and other means. The Society supports member divisions representing specialized research and astronomical interests.
The Society represents the goals of its community of members to the nation and the world. The Society also works with other scientific and educational societies to promote the advancement of science.
The Society, through its members, trains, mentors and supports the next generation of astronomers. The Society supports and promotes increased participation of historically underrepresented groups in astronomy.
The Society assists its members to develop their skills in the fields of education and public outreach at all levels. The Society promotes broad interest in astronomy, which enhances science literacy and leads many to careers in science and engineering.

Adopted June 7, 2009

The society was founded in 1899 through the efforts of George Ellery Hale. The constitution of the group was written by Hale, George Comstock, Edward Morley, Simon Newcomb and Edward Charles Pickering. These men, plus four others, were the first Executive Council of the society; Newcomb was the first president. The initial membership was 114. The AAS name of the society was not finally decided until 1915, previously it was the “Astronomical and Astrophysical Society of America”. One proposed name that preceded this interim name was “American Astrophysical Society”.

The AAS today has over 7,000 members and six divisions – the Division for Planetary Sciences (1968); the Division on Dynamical Astronomy (1969); the High Energy Astrophysics Division (1969); the Solar Physics Division (1969); the Historical Astronomy Division (1980); and the Laboratory Astrophysics Division (2012). The membership includes physicists, mathematicians, geologists, engineers and others whose research interests lie within the broad spectrum of subjects now comprising contemporary astronomy.

In 2019 three AAS members were selected into the tenth anniversary class of TED Fellows.

The AAS established the AAS Fellows program in 2019 to “confer recognition upon AAS members for achievement and extraordinary service to the field of astronomy and the American Astronomical Society.” The inaugural class was designated by the AAS Board of Trustees and includes an initial group of 232 Legacy Fellows.

From AAS NOVA : “Finding Just the Right Type of Detonation”

From AAS NOVA

26 May 2021
Tarini Konchady

Image of a type Ia supernova in the galaxy NGC 4526. The supernova is the bright white object just below the galaxy’s dark disk. [National Aeronautics Space Agency (US)/European Space Agency [Agence spatiale européenne][Europäische Weltraumorganisation](EU)]

Type Ia supernovae are a cornerstone of extragalactic distance measurements, so it’s important that we understand them very well. Right now, we’re fairly certain that Type Ia supernovae are the result of white dwarfs exploding. But how they explode is still an open question.

To Explode a White Dwarf

A creation mechanism for Type Ia supernovae where a white dwarf accretes mass from a companion (upper panel) till it explodes as a supernova (lower panel). [M. Weiss/NASA/Chandra X-ray Center (US)]

White dwarfs are remnants of relatively low-mass stars like our Sun. They are essentially exposed stellar cores, typically dominated by carbon and oxygen with an outer layer of helium. White dwarfs don’t produce energy of their own. Instead, they just cool off, slowly radiating away residual energy from when they were part of a star.

So how do you make something like a white dwarf explode? Just add mass! If a white dwarf accretes enough matter from a nearby companion, it can approach the Chandrasekhar limit of 1.4 solar masses and explode. This process seems fairly straightforward, but it turns out there are several potential ways to explode a white dwarf.

One scenario involves “double detonation”, where the helium shell of a white dwarf detonates and causes the carbon core to detonate in turn. Another scenario considers white dwarfs in a binary, with one white dwarf accreting material and exploding to knock the other one away.

Interestingly, observations suggest that a combination of these two scenarios — double detonation in white dwarf binaries — may be the likely progenitor of many Type Ia supernovae. One important constraint in this model is that the exploding white dwarf’s mass remains just below the Chandrasekhar limit.

With this in mind, a group of researchers led by Ken Shen (University of California-Berkeley (US)) considered sub-Chandrasekhar-mass explosion scenarios with a tricky but realistic assumption: that local thermodynamic equilibrium (LTE) does not hold.

Model and observed supernova light curves in different bands. The solid lines and the colored circles represent the model, while the unfilled shapes are observed supernovae. The model colors correspond to different white dwarf masses. The white dwarf’s carbon/oxygen ratio was assumed to be 50:50. [Shen et al. 2021]

Explosions Not in Equilibrium

When a system is in LTE, the energies and ionization levels of particles in the system are in some fixed relation with each other while temperature remains consistent across the system. There are astrophysical scenarios where LTE is a safe assumption, like in stars, but LTE certainly doesn’t hold in an event like a supernova.

To model explosions with non-LTE assumptions, Shen and collabors used two different modeling codes. A major difference between the two codes was computation time, and running the same explosion scenarios through both codes allowed Shen and collaborators to determine if the more time-efficient code could stand up to the other. The model outputs included spectra of the ensuing supernovae as well as their light curves across different filters.

Model Matches

A diagram showing the Phillips relation, with peak B-band brightness plotted against the decrease in B-band magnitude 15 days after the peak. Crosses correspond to observations of Type Ia supernovae. The colored shapes correspond to the models, with color representing white dwarf mass, shape representing the carbon/oxygen ratio, and shape outline representing the modeling code used. [Shen et al. 2021]

Shen and collaborators found that the model light curves were excellent matches to observed supernovae out to 15 days after the brightest point in the B-band light curve (“B-band maximum”). This means that the codes also successfully model an observed relation called the Phillips relation — the brighter a supernova’s peak B-band magnitude, the more slowly it will evolve past that peak. This can be seen by plotting peak B-band brightness against B-band brightness 15 days after the peak.

The model spectra are also good matches to observations, sometimes even out to 30 days after the peak. They are especially accurate near the peak, save for spectral features from “intermediate mass elements”, which generally include elements heavier than carbon up to calcium.

All in all, these initial non-LTE models of sub-Chandrasekhar detonations are an excellent match to a broad range of observed Type Ia supernovae near peak brightness! Future models will have to account for more conditions, but non-LTE seems to be the way to go.
Citation

“Non-local Thermodynamic Equilibrium Radiative Transfer Simulations of Sub-Chandrasekhar-mass White Dwarf Detonations,” Ken J. Shen et al 2021 ApJL 909 L18.

https://iopscience.iop.org/article/10.3847/2041-8213/abe69b

See the full article here .

five-ways-keep-your-child-safe-school-shootings

Please help promote STEM in your local schools.

Stem Education Coalition

AAS Mission and Vision Statement

The mission of the American Astronomical Society is to enhance and share humanity’s scientific understanding of the Universe.

The Society, through its publications, disseminates and archives the results of astronomical research. The Society also communicates and explains our understanding of the universe to the public.
The Society facilitates and strengthens the interactions among members through professional meetings and other means. The Society supports member divisions representing specialized research and astronomical interests.
The Society represents the goals of its community of members to the nation and the world. The Society also works with other scientific and educational societies to promote the advancement of science.
The Society, through its members, trains, mentors and supports the next generation of astronomers. The Society supports and promotes increased participation of historically underrepresented groups in astronomy.
The Society assists its members to develop their skills in the fields of education and public outreach at all levels. The Society promotes broad interest in astronomy, which enhances science literacy and leads many to careers in science and engineering.

Adopted June 7, 2009

The society was founded in 1899 through the efforts of George Ellery Hale. The constitution of the group was written by Hale, George Comstock, Edward Morley, Simon Newcomb and Edward Charles Pickering. These men, plus four others, were the first Executive Council of the society; Newcomb was the first president. The initial membership was 114. The AAS name of the society was not finally decided until 1915, previously it was the “Astronomical and Astrophysical Society of America”. One proposed name that preceded this interim name was “American Astrophysical Society”.

The AAS today has over 7,000 members and six divisions – the Division for Planetary Sciences (1968); the Division on Dynamical Astronomy (1969); the High Energy Astrophysics Division (1969); the Solar Physics Division (1969); the Historical Astronomy Division (1980); and the Laboratory Astrophysics Division (2012). The membership includes physicists, mathematicians, geologists, engineers and others whose research interests lie within the broad spectrum of subjects now comprising contemporary astronomy.

In 2019 three AAS members were selected into the tenth anniversary class of TED Fellows.

The AAS established the AAS Fellows program in 2019 to “confer recognition upon AAS members for achievement and extraordinary service to the field of astronomy and the American Astronomical Society.” The inaugural class was designated by the AAS Board of Trustees and includes an initial group of 232 Legacy Fellows.