From AAS NOVA: “Featured Image: A Supernova Remnant in the Large Magellanic Cloud”

From AAS NOVA

10 December 2018
Susanna Kohler

N49 is a supernova remnant located about 160,000 light-years away in the Large Magellanic Cloud. As the dramatic three-color Chandra X-ray image of N49 shows above, the interactions between the supernova shock wave and the surrounding interstellar medium have led to the formation of complex structure. In a new publication led by Yumiko Yamane and Hidetoshi Sano (Nagoya University), a team of scientists details a study using radio-continuum observations from a host of telescopes (Mopra, ASTE, ALMA, and ATCA) to complement prior X-ray observations of N49.

CSIRO ATNF Mopra Telescope located near the town of Coonabarabran in north-west New South Wales.

NAOJ Atacama Submillimeter Telescope Experiment (ASTE) deployed to its site on Pampa La Bola, near Cerro Chajnantor and the Llano de Chajnantor Observatory in northern Chile

ESO/NRAO/NAOJ ALMA Array in Chile in the Atacama at Chajnantor plateau, at 5,000 metres

CSIRO Australia Compact Array, six radio telescopes at the Paul Wild Observatory, is an array of six 22-m antennas located about twenty five kilometres (16 mi) west of the town of Narrabri in Australia.

The 1.42-GHz observations are shown in contours overlaid on the Chandra image above. The study reveals clumps of carbon monoxide on the outer edge of the N49 bubble, providing evidence for dynamical interactions between the gas and the supernova remnant shock wave. To read more about what the authors found, check out the paper below.

Citation

“ALMA Observations of Supernova Remnant N49 in the LMC. I. Discovery of CO Clumps Associated with X-Ray and Radio Continuum Shells,” Y. Yamane et al 2018 ApJ 863 55.
http://iopscience.iop.org/article/10.3847/1538-4357/aacfff/meta

Related Journal Articles

Discovery of Molecular and Atomic Clouds Associated with the Magellanic Superbubble 30 Doradus C doi: 10.3847/1538-4357/aa73e0
Non-thermal X-Rays and Interstellar Gas Toward the γ-Ray Supernova Remnant RX J1713.7–3946: Evidence for X-Ray Enhancement around CO and H I Clumps doi: 10.1088/0004-637X/778/1/59
Dense Molecular Clumps Associated with the Large Magellanic Cloud Supergiant Shells LMC 4 and LMC 5 doi: 10.1088/0004-637X/796/2/123
A Detailed Study of the Interstellar Protons toward the TeV γ-Ray SNR RX J0852.0–4622 (G266.2–1.2, Vela Jr.): The Third Case of the γ-Ray and ISM Spatial Correspondence doi: 10.3847/1538-4357/aa9219
The Two Molecular Clouds in RCW 38: Evidence for the Formation of the Youngest Super Star Cluster in the Milky Way Triggered by Cloud–Cloud Collision doi: 10.3847/0004-637X/820/1/26
Cloud–Cloud Collision as a Trigger of the High-mass Star Formation: a Molecular Line Study in RCW120 doi: 10.1088/0004-637X/806/1/7

See the full article here .

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Please help promote STEM in your local schools.

Stem Education Coalition

AAS Mission and Vision Statement

The mission of the American Astronomical Societyis 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

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From AAS NOVA: “Using Machine Learning to Find Planets”

From AAS NOVA

7 December 2018
Susanna Kohler

The first science image taken by TESS. Can computers help us to classify transit light curves from missions like TESS? [NASA/MIT/TESS]

How do we find the signals of exoplanets lurking in the vast quantity of data that comes out of a mission like Kepler or the Transiting Exoplanet Survey Satellite (TESS)?

NASA/MIT TESS

A new study has some suggestions for how best to get computers to do the heavy lifting for us.

Two common false positives — grazing eclipsing binaries (left) and background eclipsing binaries (right) — can mimic the signal of a transiting planet. [NASA/Ames Research Center]

Managing a Mess of Data

Recent years have seen a boom in exoplanet research — in large part due to the enormous data sets produced by transiting exoplanet missions like Kepler and, now, TESS. But the >3,000 confirmed Kepler planets weren’t all just magically apparent in the data! Instead, the discovery of planets is the result of careful classification of transit-like signals amid a sea of false positives from things like stellar eclipses and instrumental noise.

Given the number of light curves that need classifying, we can use any automated help we can get. Enter machine learning, a process by which computers can be trained to identify patterns and make decisions. Using a tool called deep learning, scientists have already shown that machines can do a pretty good job of automatically classifying Kepler transit signals as either exoplanets or false positives. But can we do even better?

Local (left) and global (right) views of the light curves (cyan) and centroids (maroon) for an example confirmed planet (top) and background eclipsing binary (bottom). Click for a closer look. [Ansdell et al. 2018]

The recent 2018 NASA Frontier Development Lab provided an excellent opportunity to find out. This eight-week research incubator was aimed at applying cutting-edge machine-learning algorithms to challenges in the space sciences. As part of this lab, two machine-learning experts were paired with two space-science researchers to try to improve machine-learning models for exoplanet transit classification. The results are presented in a new publication led by scientist Megan Ansdell (Center for Integrative Planetary Science, UC Berkeley).

Insider Knowledge

Ansdell and collaborators started with a basic machine-learning model that classified signals based on straightforward local and global views of the light curves. To improve upon it, they added scientific domain knowledge — information or insight that might not be generally known, but can be provided by a domain expert.

Recall (top; the fraction of true planets recovered) and precision (bottom; the fraction of classifications that are correct) of the Exonet model, as a function of MES, a measure of the signal-to-noise of candidate transits. [Ansdell et al. 2018]

More Planets to Come

How did Ansdell and collaborators do? Using their modified model, “Exonet”, a computer can classify a Kepler data set with 97.5% accuracy and 98% average precision. That means that 97.5% of its classifications — exoplanet or false-positive — are correct, and an average of 98% of transits classified as planets are true planets. Not bad, for a machine!

One of the added benefits of the authors’ model is that it is ideal for generalization — for example, from Kepler to TESS data. The authors are currently working on a study using Exonet to classify simulated TESS data. And yesterday’s first public data release from TESS has provided plenty of fresh data to work with in the future!

Citation

“Scientific Domain Knowledge Improves Exoplanet Transit Classification with Deep Learning,” Megan Ansdell et al 2018 ApJL 869 L7.
http://iopscience.iop.org/article/10.3847/2041-8213/aaf23b/meta

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 Societyis 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

From AAS NOVA: ” Speeding Electrons in a Solar Jet”

From AAS NOVA

5 December 2018
Susanna Kohler

Recent observations of the Sun have revealed new details about magnetic reconnection. Above, the Sun is pictured in the extreme ultraviolet. [NASA/SDO/AIA]

NASA/SDO

How is energy released in explosive events like flares and jets? One of the most likely culprits is magnetic reconnection — but we still have a lot of questions about how this process works. In a recent study, radio observations of the Sun provide us with a closer look.

Observations of radio bursts allow the authors to trace the trajectories of electron beams (colored points and tracks) back to their common reconnection sites (marked by stars). Two different groups of beams are shown in (a) and (b). Background is the SDO/AIA 193 Å EUV image. [Adapted from Chen et al. 2018]

A Nearby Laboratory

Fast magnetic reconnection is a plasma process in which magnetic field lines with opposite directions approach each other and then abruptly reconfigure.

NASA Magnetic reconnection, Credit: M. Aschwanden et al. (LMSAL), TRACE, NASA

According to theory, the suddenly released magnetic energy can then be converted, heating the surrounding plasma and accelerating particles like electrons to semirelativistic speeds.

Unfortunately, testing this model against observations poses a challenge: most astrophysical jets — like those at the centers of active galaxies — lie vast distances away from us, preventing us from exploring the process of reconnection in detail. Thus, questions like where and how electrons get energized are difficult to answer, since we can’t easily observe the process.

There is one convenient nearby laboratory in which we can study reconnection, however: the Sun. In a new study, a team of scientists led by Bin Chen (New Jersey Institute of Technology) have used high-resolution radio observations of the Sun to pinpoint the location of magnetic reconnection and particle acceleration with greater accuracy than ever before.

Pinpointing Acceleration

Chen and collaborators used the unique capabilities of the Karl G. Jansky Very Large Array (VLA) to observe bursts of radio emission associated with a solar jet in November 2014.

NRAO/Karl V Jansky Expanded Very Large Array, on the Plains of San Agustin fifty miles west of Socorro, NM, USA, at an elevation of 6970 ft (2124 m)

Radio bursts like these are emitted from groups of electrons that travel along tubes of magnetic flux at incredible speeds — between one tenth and one half the speed of light! By observing these radio bursts, the authors hoped to answer a fundamental question: where exactly did the emitting electrons first get accelerated?

Three-dimensional magnetic model of the jet eruption — during pre-eruption (left), rise (center), and eruption (right) phase — from the perspective of an observer at Earth (top) and as viewed from the side (bottom). The stars indicate the origins of the electron beams and sites of reconnection. [Chen et al. 2018]

By combining their radio observations with extreme ultraviolet imaging from the Solar Dynamics Observatory (SDO), X-ray data from the Reuven Ramaty High Energy Solar Spectroscopic Imager (RHESSI), and three-dimensional magnetic modeling, the authors are able to trace the origin of each group of electrons back to an extremely compact region in the low solar corona.

NASA Reuven Ramaty High Energy Solar Spectroscopic Imager, launched on 5 February 2002, ceased science operations on 11 April 2018

This unprecedented localization of the electrons’ source to an area of just ~600 km2 allows Chen and collaborators to conclude that this location is a magnetic reconnection null point — a central location where different magnetic flux tubes are brought together, reconfigure, and release magnetic energy, accelerating electrons during a brief (less than 50 milliseconds!) reconnection event behind the erupting jet spire.

Chen and collaborators demonstrate that these unprecedented observations provide new constraints for magnetic reconnection models, bringing us one step closer to understanding the explosive releases of energy from magnetic structures in our universe.

Citation

“Magnetic Reconnection Null Points as the Origin of Semirelativistic Electron Beams in a Solar Jet,” Bin Chen et al 2018 ApJ 866 62.
http://iopscience.iop.org/article/10.3847/1538-4357/aadb89/meta

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 Societyis 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

From AAS NOVA: “When Is the Next Glitch on Pulsar J0537-6910?”

From AAS NOVA

4 December 2018
Lisa Drummond

Pulsars emit radiation that sweeps over the Earth like a lighthouse. We observe this radiation as a sequence of pulses. [Bill Saxton/NRAO/AUI/NSF]

Title: Predicting the Starquakes in PSR J0537-6910
Authors: John Middleditch, Francis E. Marshall, Q. Daniel Wang, Eric V. Gotthelf, William Zhang
First Author’s Institution: Los Alamos National Laboratory

Status: Published in ApJ

Artist’s illustration of a pulsar, a fast-spinning, magnetised neutron star. [NASA]

Pulsars (rotating, magnetised neutron stars) emit radiation that sweeps periodically over the Earth (like the beam of a lighthouse sweeping across the ocean). We detect this radiation as a sequence of pulses, with the frequency of the pulse corresponding to the frequency of rotation of the star. Pulsars will typically spin down over their lifetime due to electromagnetic braking, but this is a fairly slow process. Occasionally, in some pulsars, we will detect a sudden increase in the frequency of the pulses. This is called a pulsar glitch. Essentially, the mismatch in the rotation of the fluid inside the star and the solid crust on the outside of the star causes a catastrophic event that we see as an increase in the frequency of the pulses.

Women in STEM – Dame Susan Jocelyn Bell Burnell

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

Dame Susan Jocelyn Bell Burnell 2009

Dame Susan Jocelyn Bell Burnell (1943 – ), still working from http://www. famousirishscientists.weebly.com

The question that the paper we’re exploring today — originally published in 2006 — seeks to answer is: can you predict the next glitch in a pulsar? In general, this is a challenging task, with different pulsars exhibiting different glitching behaviours that need to be captured in your model. However, for one particular pulsar, PSR J0537-6910, this can be accomplished fairly straightforwardly, due to the strong correlation between the size of each glitch and the waiting time until the next glitch. The authors of today’s paper exploit this correlation to develop a method to predict the next starquake on PSR J0537-6910.

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 Societyis 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

From AAS NOVA: “Exploring the Escaping Atmosphere of HAT-P-11b”

From AAS NOVA

3 December 2018
Susanna Kohler

Artist’s impression of the exoplanet HAT-P-11b and its host star. [Harvard Center for Astrophysics/D. Aguilar]

The atmospheres of planets close to their host stars live a tenuous existence. New observations from the Hubble Space Telescope show signs of a Neptune-like exoplanet’s atmosphere being eroded away.

Evaporation at Work

Small planets observed to orbit closely around their host star fall into two main populations:

those with radii smaller than 1.5 Earth radii, thought to be primarily rocky cores with little or no remaining atmosphere, and
those with radii larger than 2 Earth radii, thought to retain some of their hydrogen and helium atmospheres.

What causes the difference between these two populations? We think that all close-in exoplanets are sculpted by the energetic radiation of their host stars. This radiation can erode away the primordial atmospheres — and for the smallest planets, this will leave only their rocky cores behind.

As we work to understand the detailed physics of this photoevaporation, it would be helpful to be able to directly watch a planet’s atmosphere escaping in this way. In a new study, scientist Megan Mansfield (University of Chicago) and collaborators present just the thing: observations of the escaping atmosphere of the exoplanet HAT-P-11b.

Observations of a Hot Neptune

Artist’s illustration of WASP-107b, the first planet for which Hubble discovered helium escaping from its atmosphere. [ESA/Hubble, NASA, M. Kornmesser]

HAT-P-11b is a Neptune-sized exoplanet that orbits very close to its host star in a system that’s located approximately 120 light-years from Earth. Using Hubble, Mansfield and collaborators discovered the subtle signature of helium escaping from the atmosphere of HAT-P-11b — making this the second planet for which this signature has been discovered by Hubble (the first was WASP 107-b) and one of only a handful of planets for which we’ve seen signs of atmospheric escape.

By comparing these observations to models, Mansfield and collaborators estimate that HAT-P-11b is losing mass at a rate of roughly 10^9–10^11 g/s. This rate, while high, is still low enough that the planet has only lost a few percent of its mass over its history, leaving its bulk composition largely unaffected. This is consistent with what we would expect for a planet of its size: since it’s larger than 2 Earth radii, it should retain some of its hydrogen and helium atmosphere.

Narrowband spectrum of HAT-P-11b (blue and gray points) compared to three 1D models of hydrodynamic escape (red, green, and orange lines). [Mansfield et al. 2018]

A New Approach

Why are these escaping-helium detections important? Observations like this one represent a new method for exploring exoplanet atmospheres! The helium signature detected from HAT-P-11b had long been theorized as a way to study escaping atmospheres, but until Hubble’s recent observations of helium in the atmosphere of WASP 107-b, the potential of this approach remained untapped.

Now two planets have been observed with this particular signal — and the signal from HAT-P-11b has been additionally confirmed with CARMENES instrument in Spain, marking the first time the same signature of photoevaporation has been detected by both ground- and space-based facilities.

CARMENES spectrograph, mounted on the Calar Alto 3.5 meter Telescope, located in Almería province in Spain on Calar Alto, a 2,168-meter-high (7,113 ft) mountain in Sierra de Los Filabres

Calar Alto 3.5 meter Telescope, located in Almería province in Spain on Calar Alto, a 2,168-meter-high (7,113 ft) mountain in Sierra de Los Filabres

Calar Alto Observatory located in Almería province in Spain on Calar Alto, a 2,168-meter-high (7,113 ft) mountain in Sierra de Los Filabres

Future observations like these — from both existing instruments and upcoming observatories like the James Webb Space Telescope — will hopefully continue to shed light on how atmospheres evaporate from small, close-in exoplanets.

Citation

“Detection of Helium in the Atmosphere of the Exo-Neptune HAT-P-11b,” Megan Mansfield et al 2018 ApJL 868 L34.
http://iopscience.iop.org/article/10.3847/2041-8213/aaf166/meta

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 Societyis 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

From AAS NOVA: “Simulating WFIRST’s Search for Supernovae”

From AAS NOVA

NASA/WFIRST

New exoplanets, distant galaxies, unexpected transients — the successful discoveries of major astronomical missions get splashed across news headlines. What generally isn’t seen, however, is the often decades-long development process that led to these successful missions — a process that includes not only technology and engineering feats, but also the meticulous planning necessary to optimize the use of an observatory with a limited lifetime.

Want a closer look? A recently published study provides an insider’s view of these complex planning stages for a proposed upcoming mission, the Wide Field Infrared Survey Telescope (WFIRST).

A New Eye in the Sky

In 2010, the astronomy community selected WFIRST as the highest-ranked large space-based mission for the next decade. Like most major missions, WFIRST has been through its share of ups, downs, and funding scares in the planning process — but as of this writing, it’s on the books for a planned launch in the mid 2020s.

WFIRST will use a telescope the size of Hubble’s (i.e., a 2.37-m mirror) that was donated in 2012 by the National Reconnaissance Office. It will host two main instruments: a coronagraph that will be used for exoplanet and planetary disk studies, and a wide-field instrument that will be used to probe dark-energy models. The wide-field instrument will have two components: a wide-field channel imager, and an integral field channel spectrometer.

Vying for Time

Looking at just the dark-energy science objective, we can already see timing challenges emerge. WFIRST seeks to constrain the nature of dark energy by discovering and measuring the distance to Type Ia supernovae, thereby measuring the evolution of dark energy over time.

But though WFIRST’s proposed mission duration is five years, only a total of 6 months of observing time can be devoted to the supernova survey. Should this time be primarily spent on wide-field imaging to detect as many supernovae as possible? Or should we employ a targeted strategy, using the spectrometer to better determine redshifts of the supernovae discovered? What areas should the survey cover, at what depth? How frequently should we look at the same patches of sky?

These are just some of the many questions survey designers must wrestle with in order to optimize the WFIRST mission and give the project the best chance of answering our questions. To aid decision-making, a team of scientists led by Rebekah Hounsell (University of California, Santa Cruz and University of Illinois at Urbana-Champaign) has now conducted a series of simulations to explore different supernova survey strategies for WFIRST.

An Optimized Reference

Hounsell and collaborators realistically simulated supernova light curves and spectra as viewed by WFIRST’s instruments. They then explored 11 survey strategies with different time allocations between the imager and the spectrometer, taking into account various uncertainties. Their results suggest an imaging-focused strategy would be the most successful at increasing our understanding of the dark-energy equation of state.

Though we won’t know exactly which strategy is the most optimal until we’ve determined some of the specific systematic uncertainties of the mission, Hounsell and collaborators’ study has laid the groundwork for future planning of the mission. What’s more, their results confirm that WFIRST will have the potential to significantly advance our understanding of dark energy — so keep an eye on this project in the future!

Citation

“Simulations of the WFIRST Supernova Survey and Forecasts of Cosmological Constraints,” R. Hounsell et al 2018 ApJ 867 23.
http://iopscience.iop.org/article/10.3847/1538-4357/aac08b/meta

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 Societyis 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

From AAS NOVA: “The Shape of Cosmic Dawn”

From AAS NOVA

Artist’s illustration of the first stars in the universe. A new study suggests that the majority of the first stars may have been located in the rarest, most massive galaxy halos. [N.R. Fuller, National Science Foundation]

In March of this year, a team of scientists announced an unprecedented radio detection: a signal from the first stars that formed in the universe. But the shape of this signal was not quite what we predicted — and theorists are now exploring what this means about the dawn of the universe.

National Science Foundation (NASA, JPL, Keck Foundation, Moore Foundation, related) — Funded BICEP2 Program; modifications by E. Siegel.

A Cosmic Timeline

Our models of cosmic history tell us that after the Big Bang, the expanding universe consisted of a hot, opaque, ionized soup of gas. Perhaps 370,000 years later, recombination of these electrons and protons into neutral hydrogen atoms allowed light to travel freely through the universe — releasing the radiation we see today as the cosmic microwave background (CMB).

CMB per ESA/Planck

At this stage in the universe’s history, there were not yet any stars or galaxies. With no sources of light, the universe continued on in the “cosmic dark ages” until redshifts of around z ~ 20, perhaps 100–200 million years after the Big Bang. By then, dark matter and gas had clumped into bound objects dense enough to ignite nuclear fusion — lighting up the first stars of our universe.

Dark Ages Universe ESO

The shape of the best-fit EDGES signal is shown in the red dashed line; the corresponding signal from the authors’ model, in which the first stars are preferentially clustered in halos with mass above 10^9.4 solar masses, is shown with a blue line. The authors’ goal here is only to reproduce the sharpness of the drop on the low-frequency (right) side of the signal. [Adapted from Kaurov et al. 2018]

Fingerprint of the First Stars

These first stars are very distant and faint, so we don’t yet have the technology to detect them directly. But the ultraviolet radiation that these hot, young stars emitted would have heated the gas around them. According to models, this hot gas would then have absorbed some of the background radiation, causing a small dip in the intensity of the CMB at the radio wavelengths of 21 cm.

It is this dip — this subtle fingerprint of the first stars — that the EDGES team detected. But the shape of the signal wasn’t exactly as we were expecting: it’s both significantly deeper and has much sharper boundaries than predicted.

Testing EDGES at MIT Haystack Observatory

EDGES telescope in a radio quiet zone at the Murchison Radio-astronomy Observatory in Western Australia.

While many studies have since been published attempting to explain the surprising depth of the signal, a team of scientists led by Alexander Kaurov (Princeton’s Institute for Advanced Study) has instead opted to focus on the other puzzle: that of the signal’s sharp boundaries.

Inhomogeneous brightness temperature (left) and kinetic gas temperature (right) in the authors’ simulation, at six characteristic epochs. At z ~ 24 no sources have formed yet; around z ~ 22, evidence of the first sources is seen; by z ~ 19.7, many more are present. [Kaurov et al. 2018]

Seeing the Light

In a recent publication, Kaurov and collaborators examined possible scenarios that could lead to the sharpness seen on the low-frequency side of the EDGES signal. Physically, this feature tells us that as the first stars turned on, the universe was flooded with ultraviolet photons much more quickly than we expected.

Kaurov and collaborators show that this suddenness can be naturally explained if the sources of these photons — the first stars — were not distributed evenly throughout the universe’s structure, but were instead initially concentrated only in the rarest and most massive halos — those weighing more than a billion solar masses.

Early in the universe, the number of these rare halos grew rapidly. The authors use simulations to confirm that this sudden explosion of massive halos hosting bright, hot stars could have produced the flood of ultraviolet photons necessary to explain the EDGES signal.

If this scenario is correct, it has interesting implications for future observations. If the majority of the first stars were, indeed, located within the few rarest of halos, these halos would be especially bright. Though they’re scarce, these sources might be observable with the upcoming James Webb Space Telescope — providing us with another window into cosmic dawn.

Citation

“Implication of the Shape of the EDGES Signal for the 21 cm Power Spectrum,” Alexander A. Kaurov et al 2018 ApJL 864 L15. http://iopscience.iop.org/article/10.3847/2041-8213/aada4c/meta

Related journal articles
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See the full article for further references with links.

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 Societyis 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

From AAS NOVA: “A Black Hole X-Ray Binary Rises”

From AAS NOVA

14 November 2018
Susanna Kohler

Artist’s impression of an X-ray binary, a binary system consisting of a black hole accreting matter from a donor star. [ESA/NASA/Felix Mirabel]

New observations have captured a feeding black hole in our galaxy as it bursts onto the scene.

This representation of an X-ray binary shows the accretion disk that surrounds the black hole. According to models, instabilities in this accretion disk can lead to the binary going into outburst. [NASA/R. Hynes]

Bursting Binaries

Some of the most easily discoverable stellar-mass black holes in our own galaxy are those in X-ray binaries: binary star systems that consist of a star in orbit with a compact object like a black hole or a neutron star. In such a system, mass is siphoned off the donor star, forming an accretion disk around the feeding compact object. The accreting material emits at X-ray wavelengths, providing these systems with their signature emission.

X-ray binaries come in two main types, depending on the size of the donor star: low-mass and high-mass. In a low-mass X-ray binary (LMXB), the donor star typically weighs less than one solar mass. One type of LMXB, known as a transient/outbursting LMXB, has a peculiar quirk: though they often go undetected in their dim, quiescent accretion state, these sources exhibit sudden outbursts in which the brightness of the system increases by several orders of magnitude in less than a month.

Where do theses outbursts begin? What causes the sudden eruption? What else can we learn about these odd sources? Though theorists have built detailed models of transient LMXBs, we need observations that can confirm our understanding. In particular, most observations only capture transient LMXBs after they’ve transitioned into an outbursting state. But a sneaky telescope has now caught one source in the process of waking up.

ASAS-SN Off-the-shelf equipment helps keep the price tag low for ASAS-SN’s hardware. Mark Elphick-Los Cumbres Observatory

The All Sky Automated Survey for SuperNovae or ASAS-SN is an automated program to search for new supernovae and other astronomical transients. It has robotic telescopes in both the northern and southern hemispheres. Currently, it can survey the entire sky approximately once every day.

Initially, there were four ASAS-SN telescopes at Haleakala and another four at Cerro Tololo, an LCOGT site. Twelve more telescopes were deployed in 2017 in Chile, South Africa and Texas, with funds from the Moore Foundation, the Ohio State University, the Mount Cuba Astronomical Foundation, China, Chile, Denmark, and Germany. All the telescopes (Nikon telephoto f400/2.8 lenses) have a diameter of 14 cm and ProLine PL230 CCD cameras. The pixels in the cameras span 7.8 arc seconds, so follow up observations on other telescopes are usually required to get a more accurate location.

The main goal of the project is to look for bright supernovae, including the most powerful supernova ever discovered, ASASSN-15lh. However other transient objects are frequently discovered, including nearby tidal disruption events, Galactic novae (e.g., ASASSN-16kt, ASASSN-16ma, and ASASSN-18fv), cataclysmic variables, and stellar flares, including several of the largest flares ever seen. In July 2017 ASAS-SN has discovered its first comet, ASASSN1. It can detect new objects with magnitudes between 18 and 8.

Objects discovered receive designations starting with ASASSN followed by a dash, a two digit year and letters, for example ASASSN-15lh.

Sudden Discovery

The All-Sky Automated Survey for SuperNovae (ASAS-SN, pronounced “assassin”) regularly scans the sky hunting for transient sources. In March of this year, it spotted a new object: ASASSN-18ey, a system roughly 10,000 light-years away that shows all the signs of being a new black-hole LMXB.

ASASSN-18ey’s initial discovery prompted a flurry of follow-up observations by astronomers around the world. As of 1 October 2018, the tally had hit more than 360,000 observations — giving ASASSN-18ey the potential to be the best-studied black-hole LMXB outburst to date.

In a recent publication led by Michael Tucker (Institute for Astronomy, University of Hawai’i), a team of scientists details what we know about ASASSN-18ey so far, and what it can tell us about how LMXBs behave.

U Hawaii Institute for Astronomy

ASASSN-18ey’s position on an X-ray vs. optical luminosity diagram, shown with orange and yellow markers as it evolves through its outburst, place it within the region dominated by black hole LMXBs (blue markers) throughout the outburst. [Tucker et al. 2018]

Confirming Models

What makes ASASSN-18ey unique is its discovery in optical wavelengths before X-ray. Tucker and collaborators use the various observations of this source to determine that there was a ~7.2-day lag between the flux rises in the optical and the X-ray light curves.

The full light curves of ASASSN-18ey from ASAS-SN, ATLAS, and Swift show the source’s rapid rise to outburst state. Click to enlarge. [Adapted from Tucker et al. 2018]

This week-long delay in the two flux increases is predicted by theoretical models in which the LMXB outbursts arise from an instability in the accretion disk surrounding the compact object. Being able to measure this lag for ASASSN-18ey even allowed Tucker and collaborators to determine precisely where in the disk the instability first arose: at a radius of maybe 10,000 km from the black hole — also consistent with models.

Further observations of ASASSN-18ey as it continues to evolve will undoubtedly shed more light on the state transitions and behavior of LMXBs. In the meantime, we can enjoy this sneaky look at a new black hole LMXB system on the rise.

Citation

ASASSN-18ey: The Rise of a New Black Hole X-Ray Binary, M. A. Tucker et al 2018 ApJL 867 L9.
http://iopscience.iop.org/article/10.3847/2041-8213/aae88a/meta

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 Societyis 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

From AAS NOVA: “Clues from an Unexpected Glitch”

From AAS NOVA

9 November 2018
Susanna Kohler

This false-color X-ray image shows Puppis A, a supernova remnant that hosts a central compact object (CCO). Astronomers have recently observed the first CCO to have a glitch in its rotation. [NASA/CXC/IAFE/G. Dubner et al & ESA/XMM-Newton]

NASA/Chandra X-ray Telescope

ESA/XMM Newton

Compact objects — the extremely dense remnants left behind after the death of massive stars — continually surprise us with their wide variety of properties and behaviors. Now one compact object known for its stability and predictability has thrown a further hitch into our understanding.

What’s Left Behind

Artist’s impression of the strong magnetic fields and beams of radiation emitted by a typical pulsar. [NASA]

When massive stars explode in spectacular supernovae, material collapses onto their cores to leave behind a dense neutron star or black hole. These objects can take a variety of forms, however — from quiet, unexciting bodies emitting little radiation, to pulsars: strongly magnetized neutron stars that bright shine pulses of radiation across us as they rotate.

One particularly puzzling type of body is something we — perhaps unoriginally — termed central compact objects, or CCOs. CCOs lie at the heart of supernova remnants, and they’re detected by their surface X-ray emission. Unlike typical pulsars, however, CCOs are not detectable in other wavelengths, and they do not have strong surface magnetic fields.

Pulse-phase residuals for pulsar 1E 1207.4–5209 reveal a glitch in its rotation at the end of September 2015. [Gotthelf & Halpern 2018]

Pulsar Hiccups

An additional feature distinguishing CCOs from pulsars is their stable and slow spin-down rate. As spinning neutron stars age, they lose energy, spinning slower and slower. In typical pulsars, this steady spin-down can be interrupted by hiccups known as glitches, during which the spin frequency of the pulsar suddenly jumps back up — perhaps due to surface starquakes, or motions of the neutron-star interior.

CCOs, however, are very stable rotators that lose their energy more gradually than typical pulsars. Until now, no one has observed glitches in neutron stars that have spin-down rates as small as those measured in CCOs. But new observations of the CCO 1E 1207.4–5209, presented in a recent publication by Columbia Astrophysics Laboratory researchers Eric Gotthelf and Jules Halpern, have now changed this.

Buried Fields?

Using XMM-Newton and Chandra X-ray data, Gotthelf and Halpern identify a major glitch in 1E 1207.4–5209’s rotation that occurred at the end of September 2015. This unexpected hiccup provides further evidence of the relation between CCOs and more typical pulsars, and it lets us examine the mechanisms that may be at work in the evolution of spinning neutron stars.

Pre- and post-glitch spectra of pulsar 1E 1207.4–5209 from XMM-Newton show no significant change in line centroids — and therefore magnetic field strength — following the glitch. [Gotthelf & Halpern 2018]

Gotthelf and Halpern compare the spectrum of 1E 1207.4–5209 before and after the glitch and show that there was no change in this object’s weak surface magnetic field. A magnetic-field cause for the glitch isn’t off the table yet, though: one model for CCO formation proposes that CCOs are born like normal pulsars with strong magnetic fields, but these fields are buried and hidden when material from the supernova falls back onto them. This resulting strong internal field could gradually diffuse to the surface, eventually causing a glitch like the one we observed.

Gotthelf and Halpern stress that we should continue to watch 1E 1207.4–5209 in the future, particularly to see if the glitch might have caused it the CCO to transition into a more typical radio-bright pulsar. In the meantime, we have a few more mysteries to puzzle over as we ponder these odd stellar remnants.

Citation

“The First Glitch in a Central Compact Object Pulsar: 1E 1207.4–5209,” E. V. Gotthelf and J. P. Halpern 2018 ApJ 866 154. http://iopscience.iop.org/article/10.3847/1538-4357/aae152/meta

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 Societyis 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

From AAS NOVA: “Electrons of the Interstellar Medium”

From AAS NOVA

5 November 2018
Kerry Hensley

This false-color infrared and ultraviolet image of the Helix Nebula (NGC 7293) highlights the glowing shells of gas surrounding the planetary nebula’s intensely hot stellar core. Today’s paper explores the electron energy distributions within planetary nebulae and H II regions. [NASA/JPL-Caltech]

The behavior of electrons in tenuous interstellar nebulae is up for discussion. What is the best way to describe the energies of electrons in these environments?

The gas of the Orion Nebula (M42) is ionized by the young high-mass stars at its center. H II regions like the Orion Nebula may host non-Maxwellian electron energy distributions. [NASA, ESA, M. Robberto (Space Telescope Science Institute/ESA) and the Hubble Space Telescope Orion Treasury Project Team]

Energetic Electrons

H II regions and planetary nebulae are bubbles of ionized gas surrounding young high-mass stars and dying low- to intermediate-mass stars, respectively. We can calculate the density, temperature, and composition of these nebulae by measuring the strengths of their emission lines, but we rely on assumptions about the plasma to interpret the observed line strengths.

Typically, we assume that the electrons in the diffuse, highly irradiated environments of H II regions and planetary nebulae adhere to a Maxwell-Boltzmann distribution, which describes the velocities of a system of particles in thermodynamic equilibrium. However, the observed line strengths don’t always match their theoretically predicted values, causing some astronomers to wonder if this assumption is correct.

A proposed alternative to the Maxwell-Boltzmann distribution is the κ-distribution, which has more particles with high velocities and has been used to describe electron populations in the hot, tenuous solar wind. Which distribution is a better fit for H II regions and planetary nebulae?

The calculated steady-state solution for an H II region. The steady-state solution only deviates significantly from a Maxwellian distribution above ~13 eV. [Draine & Kreisch 2018]

May the Best Distribution Win

Bruce Draine and Christina Kreisch of Princeton University approached this problem by deriving the steady-state electron energy distribution in H II regions and planetary nebulae from first principles.

The authors show that the steady-state electron energy distribution is very nearly Maxwellian. While there is a lingering high-energy tail, it contains only ~0.000005% of the electrons in the planetary nebula case and even fewer in the H II region case — not enough to cause the observed departure from theoretical line ratios.

However, it’s not enough to show that the steady-state solution is consistent with the expected Maxwellian distribution. The conditions in the plasma must allow the system to reach the steady-state solution within a reasonable amount of time. To explore this, the authors modeled the time evolution of a population of electrons with a highly nonthermal distribution.

Time evolution of the electron energy distribution (left) and the evolution of the distribution relative to a Maxwellian (right). Click to enlarge. [Draine & Kreisch 2018]

Going Steady

Assuming typical values for H II regions and planetary nebulae, Draine and Kreisch find that the distribution quickly relaxes to the steady-state solution. For Orion-Nebula-like conditions — ~3,000 electrons per cubic centimeter — the relaxation time is only 30 seconds. For the more highly irradiated environs of planetary nebula NGC 7293 (the Helix Nebula), the relaxation time is longer, but still short enough to reasonably assume that the steady-state solution will be achieved.

These results show that the Maxwellian distribution is still the best way to describe electrons in H II regions and planetary nebulae. What is causing the unexpected emission line ratios, then? The authors point out that our models assume that the emission arises from plasma with only one temperature — but in reality, the electron temperature likely varies spatially over the region from which we observe the emission.

Citation

“Electron Energy Distributions in H II Regions and Planetary Nebulae: κ-distributions Do Not Apply,” B. T. Draine and C. D. Kreisch 2018 ApJ 862 30.
http://iopscience.iop.org/article/10.3847/1538-4357/aac891/meta

Related journal articles
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See the full article for further references with links.

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