## From Astrobites : “The Origin of the Origin of the Universe”

From Astrobites

2.4.23
Katherine Lee

Authors: J. C. Mather, E. S. Cheng, D. A. Cottingham, R. E. Eplee Jr., D. J. Fixsen, T. Hewagama, R. B. Isaacman, K. A. Jensen, S. S. Meyer, P. D. Noerdlinger, S. M. Read, L. P. Rosen, R. A. Shafer, E. L. Wright, C. L. Bennett, N. W. Boggess, M. G. Hauser, T. Kelsall, S. H. Moseley Jr., R. F. Silverberg, G. F. Smoot, R. Weiss, and D. T. Wilkinson

First Author’s Institution: NASA Goddard Space Flight Center, Greenbelt, Maryland, USA

Status: published in ApJ [open access]

Back in the mid-20th century, there were two competing theories about the origin of the Universe. Scientists, including Edwin Hubble and Georges Lemaître, had already established that space was expanding.

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Edwin Hubble

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Some argued that if you run this expansion back in time, it implies a beginning when everything must have been compressed into a hot, dense singularity, exploding outward from that point in a “Big Bang”. Other astronomers, however, were uncomfortable with the idea that the Universe even had an origin at all. These scientists, most notably Fred Hoyle, argued instead for a cosmology in which the Universe had always existed and had always been expanding, with new galaxies springing up periodically to fill in the gaps. This picture of our Universe is referred to as the “Steady State Theory”.

These two theories predict fundamentally different things about the background temperature of the Universe. If matter in the Universe does not originate from a single point, as in the Steady State picture, then we would expect the background radiation to be chaotic in nature; there would be no reason for different unconnected regions of spacetime to look the same as each other.

However, if everything in the Universe comes from the same initial conditions, then everything should be roughly the same temperature. This can also be expressed as the idea that the Universe should be in thermodynamic equilibrium on large scales, and that if you measure the intensity of background radiation at all frequencies, you should see a blackbody spectrum—the characteristic spectrum of an object in equilibrium, dependent only on the object’s temperature. Thus, a key prediction of the Big Bang theory is that the temperature should be nearly constant over the entire sky, with the differences (called anisotropies) from this constant average temperature being extremely small—around one part in 100,000!

COBE comes to the rescue

Big Bang cosmologists in the 1960s believed that the peak of the Universe’s blackbody spectrum should be in the microwave frequency range, defined as between 300 MHz and 300 GHz. This would be expected from a massive explosion of energy at the Big Bang, the light from which would have been redshifted into the microwave range as it traveled through the expanding universe. So, if the Big Bang theory is true, we should expect to see a constant source of background radiation coming from all directions in the microwave sky: a so-called Cosmic Microwave Background, or CMB.

The detection of this CMB radiation in 1965 by Arno Penzias and Robert Woodrow Wilson, as well as the cosmological interpretation of that detection by Robert Dicke, Jim Peebles, Peter Roll, and David Wilkinson, laid the groundwork for modern cosmology, and was the beginning of the end for the idea that the Universe had no origin.

However, Penzias and Wilson’s discovery was not an accurate measurement of the CMB’s temperature or spectrum. No anisotropies had been detected, and there was still debate over whether or not the CMB spectrum was truly a blackbody. The goal of the Cosmic Background Explorer (COBE) satellite, launched by NASA in 1989, was to answer these lingering questions.

COBE was split into three instruments: the Differential Microwave Radiometer (DMR), the Far-InfraRed Absolute Spectrophotometer (FIRAS), and the Diffuse Infrared Background Experiment (DIRBE). DMR measured the CMB anisotropies, while DIRBE mapped infrared radiation from foreground dust.

igure 1: A diagram of the FIRAS instrument, taken from Figure 1a of Mather et. al. (1999).

FIRAS, meanwhile, was designed to measure the CMB spectrum. It scanned the entire sky multiple times in order to minimize errors, and measured the temperature over a wide range of frequencies between 30 and FIRAS, meanwhile, was designed to measure the CMB spectrum. It scanned the entire sky multiple times in order to minimize errors and measured the temperature over a wide range of frequencies between 30 and nearly 3000 GHz. After eliminating known sources of interference such as cosmic rays, as well as subtracting the effects of light from the Milky Way galaxy and of the Doppler shift caused by the movement of the Earth through space, these scans were then averaged together to create direct measurements of the CMB intensity at various frequencies.

Figure 2: The cosmic microwave background spectrum, as measured by FIRAS. It shows a near-perfect blackbody, with any deviations from total thermodynamic equilibrium being much too small to see. This plot is taken from Figure 4 of Fixsen et al. (1996), which notes that “uncertainties are a small fraction of the line thickness.”line thickness.”

The authors found that the background radiation in our universe is in fact extremely close to being a perfect bThe authors of today’s paper found that the background radiation in our Universe is in fact extremely close to being a perfect blackbody! The final temperature found by FIRAS was reported by Mather et al. (1999) to be 2.725 K, with an uncertainty of just 0.002 K! This is an incredibly high-precision measurement and represents the final nail in the coffin for cosmologies other than the Big Bang. John C. Mather received the Nobel Prize in 2006 for his work as FIRAS’s project lead.

Figure 3: A comparison of the abilities of the COBE [above], WMAP, and Planck satellites to resolve tiny fluctuations in the CMB temperature, called anisotropies. Image: NASA/JPL-Caltech/ESA (Wikimedia Commons)

Today, cosmologists use the CMB and its anisotropies to characterize the early history of the universe, find galaxy clusters in the later universe, and even look for new physics! The COBE measurements represented the dawn of a new era in cosmology, and laid the groundwork for modern CMB measurements. The science we do toToday, cosmologists use the CMB and its anisotropies to characterize the early history of the Universe, find galaxy clusters in the later Universe, and even look for new physics! Later full-sky measurements taken by the Wilkinson Microwave Anisotropy Probe (WMAP) and the Planck satellite added never-before-seen levels of precision to our ability to study the structure and content of the Universe, and future missions like LiteBIRD will continue to improve our ability to study the CMB even more closely, building on COBE’s groundbreaking data. These experiments still rely upon the CMB temperature established by FIRAS, which remains the definitive result even 23 years after its publication.

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Inflation

In physical cosmology, cosmic inflation, cosmological inflation is a theory of exponential expansion of space in the early universe. The inflationary epoch lasted from 10^−36 seconds after the conjectured Big Bang singularity to some time between 10^−33 and 10^−32 seconds after the singularity. Following the inflationary period, the universe continued to expand, but at a slower rate. The acceleration of this expansion due to dark energy began after the universe was already over 7.7 billion years old (5.4 billion years ago).

Inflation theory was developed in the late 1970s and early 80s, with notable contributions by several theoretical physicists, including Alexei Starobinsky at Landau Institute for Theoretical Physics, Alan Guth at Cornell University, and Andrei Linde at Lebedev Physical Institute. Alexei Starobinsky, Alan Guth, and Andrei Linde won the 2014 Kavli Prize “for pioneering the theory of cosmic inflation.” It was developed further in the early 1980s. It explains the origin of the large-scale structure of the cosmos. Quantum fluctuations in the microscopic inflationary region, magnified to cosmic size, become the seeds for the growth of structure in the Universe. Many physicists also believe that inflation explains why the universe appears to be the same in all directions (isotropic), why the cosmic microwave background radiation is distributed evenly, why the universe is flat, and why no magnetic monopoles have been observed.

The detailed particle physics mechanism responsible for inflation is unknown. The basic inflationary paradigm is accepted by most physicists, as a number of inflation model predictions have been confirmed by observation; however, a substantial minority of scientists dissent from this position. The hypothetical field thought to be responsible for inflation is called the inflaton.

In 2002 three of the original architects of the theory were recognized for their major contributions; physicists Alan Guth of M.I.T., Andrei Linde of Stanford, and Paul Steinhardt of Princeton shared the prestigious Dirac Prize “for development of the concept of inflation in cosmology”. In 2012 Guth and Linde were awarded the Breakthrough Prize in Fundamental Physics for their invention and development of inflationary cosmology.

Alan Guth, from M.I.T., who first proposed Cosmic Inflation.

Alan Guth’s notes:
Alan Guth’s original notes on inflation.
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Nobel Prize in Physics for 2011 Expansion of the Universe

4 October 2011

The Royal Swedish Academy of Sciences has decided to award the Nobel Prize in Physics for 2011

with one half to

and the other half jointly to

and

Written in the stars

“Some say the world will end in fire, some say in ice…” *

What will be the final destiny of the Universe? Probably it will end in ice, if we are to believe this year’s Nobel Laureates in Physics. They have studied several dozen exploding stars, called supernovae, and discovered that the Universe is expanding at an ever-accelerating rate. The discovery came as a complete surprise even to the Laureates themselves.

In 1998, cosmology was shaken at its foundations as two research teams presented their findings. Headed by Saul Perlmutter, one of the teams had set to work in 1988. Brian Schmidt headed another team, launched at the end of 1994, where Adam Riess was to play a crucial role.

The research teams raced to map the Universe by locating the most distant supernovae. More sophisticated telescopes on the ground and in space, as well as more powerful computers and new digital imaging sensors (CCD, Nobel Prize in Physics in 2009), opened the possibility in the 1990s to add more pieces to the cosmological puzzle.

The teams used a particular kind of supernova, called Type 1a supernova. It is an explosion of an old compact star that is as heavy as the Sun but as small as the Earth. A single such supernova can emit as much light as a whole galaxy. All in all, the two research teams found over 50 distant supernovae whose light was weaker than expected – this was a sign that the expansion of the Universe was accelerating. The potential pitfalls had been numerous, and the scientists found reassurance in the fact that both groups had reached the same astonishing conclusion.

For almost a century, the Universe has been known to be expanding as a consequence of the Big Bang about 14 billion years ago. However, the discovery that this expansion is accelerating is astounding. If the expansion will continue to speed up the Universe will end in ice.

The acceleration is thought to be driven by dark energy, but what that dark energy is remains an enigma – perhaps the greatest in physics today. What is known is that dark energy constitutes about three quarters of the Universe. Therefore the findings of the 2011 Nobel Laureates in Physics have helped to unveil a Universe that to a large extent is unknown to science. And everything is possible again.

*Robert Frost, Fire and Ice, 1920
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What do we do?

Astrobites is a daily astrophysical literature journal written by graduate students in astronomy. Our goal is to present one interesting paper per day in a brief format that is accessible to undergraduate students in the physical sciences who are interested in active research.

Reading a technical paper from an unfamiliar subfield is intimidating. It may not be obvious how the techniques used by the researchers really work or what role the new research plays in answering the bigger questions motivating that field, not to mention the obscure jargon! For most people, it takes years for scientific papers to become meaningful.

Our goal is to solve this problem, one paper at a time. In 5 minutes a day reading Astrobites, you should not only learn about one interesting piece of current work, but also get a peek at the broader picture of research in a new area of astronomy.

## From Astrobites And The NASA/ESA/CSA James Webb Space Telescope: “It’s Full of Stars – The Mysterious Heart of the Phantom Galaxy”

From Astrobites

And

The NASA/ESA/CSA James Webb Space Telescope

1.19.23
H Perry Hatchfield

Authors: Nils Hoyer, Francesca Pinna, Albrecht W. H. Kamlah, Francisco Nogueras-Lara, Ania Feldmeier-Krause, Nadine Neumayer, Mattia C. Sormani, Médéric Boquien, Eric Emsellem, Anil C. Seth, Ralf S. Klessen, Thomas G. Williams, Eva Schinnerer, Ashley T. Barnes, Adam K. Leroy, Silvia Bonoli, J. M. Diederik Kruijssen, Justus Neumann, Patricia Sánchez-Blázquez, Daniel A. Dale, Elizabeth J. Watkins, David A. Thilker, Erik Rosolowsky, Frank Bigiel, Kathryn Grasha, Oleg V. Egorov, Daizhong Liu, Karin M. Sandstrom, Kristen L. Larson, Guillermo A. Blanc, and Hamid Hassani

First Author’s Institution: Donostia International Physics Center, Spain

Status: accepted for publication in ApJL [open access]

What lies at the center of a spiral galaxy? Turns out it’s not so easy to tell! Galaxy centers are messy, crowded, brilliant places where astronomers have to confront heavily obscured stellar populations, extremely dense and chaotic clouds of dust and gas, and the complexities of powerful gravitational and magnetic forces. Many spiral galaxies, like the subject of today’s paper (NGC 628, shown in Figure 1), have star-forming rings around their centers and a mysterious empty gap in the innermost few tens of parsecs. Well, not entirely empty – it’s full of stars! In fact, if we lived on a planet near the Milky Way’s Galactic Center, we would be able to see more than a million times as many stars in our night sky.

Figure 1. A zoom-in on the beautiful grand-design spiral structure of NGC 628, observed (in the right panel) by early JWST observations. The intricate webs of dust filaments and brilliant star-forming regions revealed by JWST’s MIRI instrument highlight the complexity of galactic structure on smaller scales. The different colors correspond to varying MIRI bands (red: MIRI F2100W, orange: MIRI F1130W, cyan: MIRI F770W, overall grayscale brightness: MIRI F1000W). The image on the left is credited to and observed by Hal Heaton, and the image on the right is credited to Judy Schmidt (NASA, ESA, CSA).

The nuclei of many galaxies are home to Nuclear Star Clusters. These clusters are made up of hundreds of millions, even billions of stars, contained within a few to tens of parsecs (relatively tiny, given the immense number of stars). While the origin and the nature of these high-density star clusters remain largely mysterious, we can study them to learn a lot about the star formation properties and histories of their host galaxies. Our own Galactic Center’s nuclear star cluster has been a great observational challenge due to the pesky fact that we’re embedded in the Galaxy and have to stare through most of the disk to figure out what’s going on in there. Other galaxy centers are hard to observe because, well, they’re very far away! Resolving the details of the nuclear stellar populations of galaxies would require, say, an immensely powerful modern infrared space telescope…

Today’s authors are a part of the PHANGS-JWST collaboration, a survey project aiming to observe a large sample of galaxies in the nearby universe at high resolution with a variety of our most cutting-edge telescopes, including ALMA (the Atacama Large Millimeter/submillimeter Array), The Hubble Space Telescope, and, most recently, JWST![included herein].

With the first batches of science-ready data beaming down from JWST’s orbit, the authors have started observing the core of the nearby grand-design spiral galaxy (well, relatively nearby for another galaxy- about 30 million lightyears away) named NGC 628 (also called Messier 74, or “the Phantom Galaxy”).

Figure 2. New data from JWST’s NIRCam instrument [below] (top row) and MIRI [below] (bottom row) towards the nuclear region (the inner few hundred parsecs) of NGC 268. The dust and gas (visible in the bottom panels) at the center forms a ring-like structure around a mysterious central void, while the stellar cluster’s range is more visible in the top panels. Image credit: adapted from Figure 1 in today’s paper.

Figure 3: A comparison of the stellar mass properties of various nuclear star clusters, including those belonging to NGC 628 (examined in today’s paper) and the Milky Way. The total galaxy’s mass of stars is shown on the x-axis, and the ratio of the star mass of the nuclear star cluster to the total star mass of the galaxy is shown on the y-axis. Image credit: adapted from Figure 9 in today’s paper.

The authors use a variety of methods to narrow in on a consistent mass for the nuclear star cluster in NGC 628, finding a relatively consistent stellar mass around 107 solar masses (not so different from the estimated value for our own Milky Way’s nuclear star cluster!). For some yet-unknown reason, the ratio of the stellar mass of the nuclear star cluster to the total stellar mass of NGC 628 (and the Milky Way!) is quite low in comparison to other galaxies, as shown in Figure 3. The stars in NGC 628’s central cluster are nearly all very old and metal rich– a signpost that very little recent star formation has occurred in the central few parsecs of this galaxy, seemingly in line with the lack of gas and dust in the central cavity. The authors speculate on the cause for this central void and lack of recent star formation, suggesting that it might be due to the extreme gravitational dynamics of the region or some sort of feedback process from the galactic nucleus, but its origin remains a mystery.

The authors also use their NIRCam and MIRI data from JWST in combination with data from the Hubble Space Telescope to model the cluster properties and its spectral energy distribution (i.e. the flux of light from this star cluster as a function of all the different wavelengths available). However, the mid-infrared data from JWST show an excess of emission that seems, according to their modeling, inconsistent with the light from other wavelength ranges, appearing larger and brighter than expected in the mid-infrared. The authors propose a range of different explanations for this excess, from the influence of a massive black hole, to the presence of an infalling star cluster, or even a subtle background galaxy hiding in plain sight! While JWST is giving us an unprecedented look at the nature of its nuclear star cluster, it seems that, for now, the Phantom Galaxy will keep some secrets close to its heart…

Comments are invited and will be appreciated, especially if the reader finds any errors which I can correct. Use “Reply”

The NASA/ESA/CSA James Webb Space Telescope is a large infrared telescope with a 6.5-meter primary mirror. Webb was finally launched December 25, 2021, ten years late. Webb will be the premier observatory of the next decade, serving thousands of astronomers worldwide. It will study every phase in the history of our Universe, ranging from the first luminous glows after the Big Bang, to the formation of solar systems capable of supporting life on planets like Earth, to the evolution of our own Solar System.

Webb is the world’s largest, most powerful, and most complex space science telescope ever built. Webb will solve mysteries in our solar system, look beyond to distant worlds around other stars, and probe the mysterious structures and origins of our universe and our place in it.

Webb was formerly known as the “Next Generation Space Telescope” (NGST); it was renamed in Sept. 2002 after a former NASA administrator, James Webb.

Webb is an international collaboration between National Aeronautics and Space Administration, the European Space Agency (ESA), and the Canadian Space Agency (CSA). The NASA Goddard Space Flight Center managed the development effort. The main industrial partner is Northrop Grumman; the Space Telescope Science Institute operates Webb.

Several innovative technologies have been developed for Webb. These include a folding, segmented primary mirror, adjusted to shape after launch; ultra-lightweight beryllium optics; detectors able to record extremely weak signals, microshutters that enable programmable object selection for the spectrograph; and a cryocooler for cooling the mid-IR detectors to 7K.

There are four science instruments on Webb: The Near InfraRed Camera (NIRCam), The Near InfraRed Spectrograph (NIRspec), The Mid-InfraRed Instrument (MIRI), and The Fine Guidance Sensor/ Near InfraRed Imager and Slitless Spectrograph (FGS-NIRISS).

Webb’s instruments are designed to work primarily in the infrared range of the electromagnetic spectrum, with some capability in the visible range. It will be sensitive to light from 0.6 to 28 micrometers in wavelength.
National Aeronautics Space Agency Webb NIRCam.

Webb has four main science themes: The End of the Dark Ages: First Light and Reionization, The Assembly of Galaxies, The Birth of Stars and Protoplanetary Systems, and Planetary Systems and the Origins of Life.

Launch was December 25, 2021, ten years late, on an Ariane 5 rocket. The launch was from Arianespace’s ELA-3 launch complex at European Spaceport located near Kourou, French Guiana. Webb is located at the second Lagrange point, about a million miles from the Earth.

What do we do?

Astrobites is a daily astrophysical literature journal written by graduate students in astronomy. Our goal is to present one interesting paper per day in a brief format that is accessible to undergraduate students in the physical sciences who are interested in active research.

Reading a technical paper from an unfamiliar subfield is intimidating. It may not be obvious how the techniques used by the researchers really work or what role the new research plays in answering the bigger questions motivating that field, not to mention the obscure jargon! For most people, it takes years for scientific papers to become meaningful.

Our goal is to solve this problem, one paper at a time. In 5 minutes a day reading Astrobites, you should not only learn about one interesting piece of current work, but also get a peek at the broader picture of research in a new area of astronomy.

## From Astrobites : “The Silence of the Lambda – Tracking the Disruption of Satellites around Milky Way-Mass Galaxies in Cosmological Simulations”

From Astrobites

1.11.23

Nguyễn Bình (Binh Nguyen). Credit: University of Arizona.

Many Milky Way-mass galaxies have been observed to be surrounded by faint stellar halos that extend beyond the main disk. These are the remnants of smaller satellite galaxies that once orbited the larger galaxy, but were disrupted by its tidal forces. Using the FOGGIE cosmological hydrodynamic simulations, we track the evolution of a Milky Way-mass galaxy and its satellites over cosmic time to understand when the tidal disruption of the satellites begins, and whether it leaves any detectable evidence in the present-day stellar halo of the host galaxy. Our analysis shows that while nearly all early-infalling (before z = 1) satellites are completely disrupted by the present, those that fall in after z = 1 may retain a large fraction of their stellar populations, along with a dark matter subhalo that is identifiable by the halo-finder at the present. We also discover a match in time between the peaks in mass loss from the satellites and the peaks in tidal force experienced by these satellites from the central Milky Way-mass galaxy (Figure 1). This suggests that most of the satellites are not undergoing significant pre-processing in other groups prior to falling in.

Figure 1: Plot showing the total mass of an infalling satellite galaxy before (solid blue line) and after (dotted blue line) entering the more massive host galaxy. The pink peaked line show the tidal force experienced by the satellite. You can see that each peak corresponds with a big drop in satellite mass! Image Credit: Binh et al.

This work will be presented as iPoster 407.06 on Thursday at AAS241!

Comments are invited and will be appreciated, especially if the reader finds any errors which I can correct. Use “Reply”

What do we do?

Astrobites is a daily astrophysical literature journal written by graduate students in astronomy. Our goal is to present one interesting paper per day in a brief format that is accessible to undergraduate students in the physical sciences who are interested in active research.

Reading a technical paper from an unfamiliar subfield is intimidating. It may not be obvious how the techniques used by the researchers really work or what role the new research plays in answering the bigger questions motivating that field, not to mention the obscure jargon! For most people, it takes years for scientific papers to become meaningful.

Our goal is to solve this problem, one paper at a time. In 5 minutes a day reading Astrobites, you should not only learn about one interesting piece of current work, but also get a peek at the broader picture of research in a new area of astronomy.

## From Astrobites : “A glimpse into the Very (High Energy) bright future”

From Astrobites

1.7.23
Jessie Thwaites

Authors: Biswajit Banerjee, Gor Oganesyan, Marica Branchesi, Ulyana Dupletsa, Felix Aharonian, Francesco Brighenti, Boris Goncharov, Jan Harms, Michela Mapelli, Samuele Ronchini, and Filippo Santoliquido

Corresponding Author Affiliation: Gran Sasso Science Institute, INFN – Laboratori Nazionali del Gran Sasso, and INAF – Osservatorio Astronomico d’Abruzzo, Italy

Status: ArXiv open access

Multi-messenger astronomy is a powerful tool allowing us to study astrophysical sources by looking at them in different “messengers” – different signals that could come from the same source, which include photons, neutrinos, gravitational waves, and cosmic rays. By studying these different messengers, we can get new insight into the astrophysical sources that produce these signals.

One of the first multi-messenger success stories was a binary neutron star merger observed on August 17th, 2017 by the LIGO and Virgo gravitational wave detectors, which was named GW170817. 1.7 seconds after the merger, the Fermi and INTEGRAL satellites observed a short gamma-ray burst, GRB 170817A, which was associated with the gravitational wave signal.

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LIGO-VIRGO-KAGRA-GEO 600-LIGO-India-ESA/NASA LISA

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(For more information about GRBs, check out the Astrobites Guide to Transient Astronomy!) This discovery was hugely exciting in the multi-messenger astronomy community – it cemented neutron star mergers as the source of short gamma-ray bursts (GRBs), and paved the way for further investigation of neutron star mergers in both gravitational waves and light.

Since then, more investigation into short GRBs has taken place, but there are still open questions to solve. Next generation very high energy (VHE) telescopes and next generation gravitational wave detectors may give us more insight into the properties of neutron star mergers, which is the topic of today’s paper.

Promptly searching for prompt emissions

We separate gamma-ray bursts (GRBs) into two types based on the length of time they last, called short (lasting less than 2 seconds) and long (lasting more than 2 seconds). Short GRBs are those associated with binary neutron star mergers, which is the focus of today’s paper.

Figure 1: A schematic of the gravitational waves observed from a binary neutron star merger, and the prompt emission of photons after the merger. The light blue line shows the gravitational wave, which increases rapidly in frequency and amplitude before the merge. After a short delay, the short gamma-ray burst can be seen (dark red color) which is powered by relativistic jets after the merge of the two neutron stars. (Figure 2 in the paper.)

There are often two phases of GRB emission: the prompt or initial burst phase, and an afterglow or later emission. In this paper, the authors target the prompt phase of the emission, which is believed to be powered by relativistic jets, a high energy beam of photons that is produced when the neutron stars merge. By looking at this initial burst of light, astronomers can gain insight into what is happening inside the jet, and what is powering the high energy photons we see.

Although we’ve seen very high energy (energies greater than 30 GeV) emission from the afterglow phase of GRBs before, we have not yet observed VHE emission from the prompt phase. Observing this emission is crucial to our understanding of the relativistic jet which produces the GRB, but observation of the prompt emission has its challenges.

Today’s paper analyses the prospects for detection of VHE gamma-rays from binary neutron star mergers with the next generation of gravitational wave detectors in the Einstein Telescope (ET) and Cosmic Explorer (CE) and the next generation of VHE gamma-ray telescopes using the Cherenkov Telescope Array (CTA).

The challenges and the solutions

Two of the main challenges to observing a short GRB from a gravitational wave event are the localizations of the events and the time it takes to reposition the telescope after an event occurs.

Gravitational wave event locations are typically difficult to pinpoint, leading to large localization areas on the sky (often up to 100s or 1000s of square degrees) while most VHE telescopes have much smaller fields of view (10s of degrees). Both the ET and CE gravitational wave detectors are expected to improve the localization of the gravitational wave event, even before the merge happens.

These telescopes will also be able to detect the inspiral-the period right before the merge when the neutron stars get very close together-of the two neutron stars which will give early warning to telescopes up to 15 minutes before the merge happens.

Figure 2: Number of gravitational wave events expected to be detected per year using the next generation of gravitational wave detectors, with an early warning (time before the merger) of 15 minutes (left panel) or 5 minutes (right panel). These are shown versus their sky localization on the x-axis in degrees squared. The θv<10 degrees in the y-axis means that we are positioned to see the relativistic jets from these mergers, which means that we could observe a short GRB from these events. (Figure 5 bottom left two panels in the paper).

Figure 3: Number of CTA VHE prompt photon detections expected from these binary neutron star mergers from Figure 2 (above), assuming either 15 minutes (left) or 5 minutes (right) early warning time. There are more gravitational wave events with a 5 minute early warning time than 15 minutes, leading to more detections, but as the sky localization in either case gets large the field of view of the telescopes can no longer cover the entire region where the gravitational wave event may be. (Figure 7 first column in the paper).

This early warning will be a huge advantage to telescopes trying to observe the burst of light expected from the merge. They will be able to reposition and be ready for the burst before it happens, which is critical since a short GRB lasts only up to 2 seconds.

The Čerenkov Telescope Array (CTA) also brings large improvements to these issues on the VHE photon side. CTA’s Medium Size Telescopes have a field of view of 44 square degrees and 90 second slew time (the slew time is the time it takes to re-point the telescopes), and its Large Size Telescopes have a field of view of 13 square degrees and 20 second slew time. Using this, and the early warning from the gravitational wave detectors, CTA could possibly provide the first detection of this VHE prompt photon emission from a binary neutron star merger.

The authors use a simulation of both the gravitational wave detectors and the CTA medium size telescopes to calculate the number of binary neutron star mergers that CTA could observe VHE photons from the prompt phase of a GRB. With only 5 minutes of early warning from ET and CE, CTA would be able to detect VHE prompt photons from many GRBs, even those with large (more than 1000 square degrees) localizations!

The takeaways

With the next generation of both gravitational wave detectors and VHE gamma-ray detectors, searches for VHE photon emission from binary neutron star mergers will be within reach for the first time. This will allow astronomers and astrophysicists to probe the fundamental properties of relativistic jets formed during the merger of the two neutron stars.

Comments are invited and will be appreciated, especially if the reader finds any errors which I can correct. Use “Reply”

What do we do?

Astrobites is a daily astrophysical literature journal written by graduate students in astronomy. Our goal is to present one interesting paper per day in a brief format that is accessible to undergraduate students in the physical sciences who are interested in active research.

Reading a technical paper from an unfamiliar subfield is intimidating. It may not be obvious how the techniques used by the researchers really work or what role the new research plays in answering the bigger questions motivating that field, not to mention the obscure jargon! For most people, it takes years for scientific papers to become meaningful.

Our goal is to solve this problem, one paper at a time. In 5 minutes a day reading Astrobites, you should not only learn about one interesting piece of current work, but also get a peek at the broader picture of research in a new area of astronomy.

## From Astrobites : “The Fires Within – Investigating the Atmospheres of Inflated Hot Jupiters”

From Astrobites

1.3.23
Lili Alderson

Authors: Thaddeus D. Komacek, Peter Gao, Daniel P. Thorngren, Erin M. May, Xianyu Tan

First Author’s Institution: Department of Astronomy, University of Maryland

Status: Published in ApJ Letters (open access)

One of the defining and most puzzling features of hot Jupiter exoplanets is that they often have much larger radii than expected. These giants are thought to be created by strong stellar flux from their host stars heating the deep interiors of the planets and inflating them. There’s evidence of this theory in observations too, with links between hot Jupiters with the largest radii often being highly irradiated by their host stars. Because the most inflated exoplanets also have puffy atmospheres, they typically make great targets for characterization since larger atmospheres produce bigger signals. Therefore, understanding the impacts of hot interiors on the circulation patterns and structure of an atmosphere could be an important step to figuring out exactly what makes hot Jupiters tick.

Fire Up the Models!

To study the impacts of internal heat on exoplanet atmospheres, the authors produce two variations of General Circulation Models (GCMs). The first, a “fixed flux” model, uses an interior temperature comparable to those typically used in previous studies. The second, a “hot interior” model, better matches the expected deep temperatures from evolutionary models of hot Jupiters given the strong heating they receive from their host stars. For each version of the GCM, various simulations are produced of exoplanets at different orbital radii and surface gravities, with the atmosphere in each scenario allowed to equilibrate for the equivalent of 3500 Earth days. In total, the various setups resulted in a grid of 28 GCM simulations.

Figure 1: Maps of the hottest exoplanet in the model grid in the final 500 Earth days of the GCM simulation. The gradient colouring highlights the local temperature across the latitudes and longitudes of the atmosphere, while the arrows illustrate the circulation patterns in the atmosphere. Each row shows the temperature map at a different pressure depth within the atmosphere, with the deep interior at the top of the plot and the upper atmosphere at the bottom. On the left, the GCM results for the fixed flux version are shown, while the GCM results for the hot interior version are shown in the centre. On the right, the difference between the two setups at each pressure depth is shown. Figure 1 in the paper.

Figure 1 shows a comparison between the final atmosphere resulting from the fixed flux and the hot interior GCMs for the hottest exoplanet in the model grid. Here, the difference between each GCM is shown for various pressure depths within the atmosphere in the righthand column, with the highest pressures deeper in the atmosphere. These results show that a hot interior leads to differences in both the wind speed and the temperature, with changes in temperature of up to hundreds of Kelvin. These changes in atmospheric dynamics are seen at all depths in the atmosphere, but the changes are not necessarily consistent throughout the atmosphere. At pressure depths of 1 millibar (those probed by the transmission spectroscopy technique often used to study exoplanet atmospheres), the temperature differences are very localised, with the largest differences occurring in “chevron” shaped features. The changes in wind speed also impact the region studied in transmission spectroscopy. The differences in wind speeds at the limb of the atmosphere (the region studied in transmission) at these pressures are comparable to the typical uncertainties being achieved in ground-based high-resolution observations.

What Does This Mean for Other Exoplanets?

Expanding their studies across the whole model grid, the authors find that similar patterns in the atmospheric dynamics are seen for all the orbital radii and surface gravities considered. There are, however, some differences between the impacts at low and high gravities. The hot interior GCM leads to differences in atmospheric temperature of up to 10% compared to the fixed flux GCM for the lowest gravity case, while the high gravity case sometimes leads to temperature differences of over 20%.

With all the potential changes seen when considering a hot interior, particularly with differences occurring in the region probed by transmission spectroscopy, might the current standard “fixed flux” models make it harder to interpret these and similar observations? By observing exoplanets throughout their entire orbit in a phase curve, JWST is expected to constrain the pressure-temperature profiles of hot Jupiter atmospheres to 10s of Kelvin. Given that the hot interior GCM results differed in places by up to hundreds of Kelvin, it does indeed seem possible that such assumptions could be problematic.

Comments are invited and will be appreciated, especially if the reader finds any errors which I can correct. Use “Reply”

What do we do?

Astrobites is a daily astrophysical literature journal written by graduate students in astronomy. Our goal is to present one interesting paper per day in a brief format that is accessible to undergraduate students in the physical sciences who are interested in active research.

Reading a technical paper from an unfamiliar subfield is intimidating. It may not be obvious how the techniques used by the researchers really work or what role the new research plays in answering the bigger questions motivating that field, not to mention the obscure jargon! For most people, it takes years for scientific papers to become meaningful.

Our goal is to solve this problem, one paper at a time. In 5 minutes a day reading Astrobites, you should not only learn about one interesting piece of current work, but also get a peek at the broader picture of research in a new area of astronomy.

## From Astrobites : “The First Exoplanet Discovery (Around a Very Non-Sunlike Star)”

From Astrobites

1.2.23
Macy Huston

Authors: A. Wolszczan & D. Frail

First Author’s Institution: Arecibo Observatory at the time, Penn State Astronomy & Astrophysics now

Status: Published in Nature (closed access)

The number of discovered exoplanets reached an amazing milestone of 5,000 recently, but it was only 30 years ago that the first discovery was made. The 2019 Nobel Prize in Physics was awarded in part to Michel Mayor and Didier Queloz for the 1995 discovery of the first known exoplanet around a Sun-like star. But, in 1992, Aleksander Wolszczan and Dale Frail discovered a pair of planets around a star very unlike the Sun: a pulsar

Finding Pulsar Planets

This first exoplanet system was discovered using an interesting technique: Pulsar Timing.

Pulsars are rapidly rotating neutron stars that may periodically emit intense beams of radiation toward Earth with very regular and precise timing. When a planet is in orbit with a star, the star also orbits the system’s center of mass. This “wobble” can cause periodic variations in the timing between observed pulses.

This orbital motion is also exploited in the radial velocity exoplanet detection method.

PSR1257+12

PSR1257+12 (also named Lich, by the International Astronomical Union) is a “millisecond pulsar”, a rapidly rotating neutron star with a period of just 6.2 milliseconds! The authors measured the arrival time of 4,040 pulses from the star, noting a slight periodic variation in the arrival times. They found that this variation aligned with a combination of 2 periods: 66.6 and 98.2 days. The authors were able to rule out the typical causes for pulsar period variations: neutron star seismology, eclipsing binary pulsars, spin axis precession, and magnetospheric phenomena. Only one plausible explanation remained: the pulsar has two low-mass companions perturbing its orbital motion.

The authors also found possible evidence for a third planet on a roughly year-long orbit but could not confirm it at the time. The two discovered planets are on orbits similar to Mercury’s and have estimated masses of a few times Earth’s.

Millisecond pulsars, like PSR1257+12, are thought to form when a neutron star accretes matter from a binary stellar companion. It seems unlikely that a planet in such a system would survive, so the authors propose that the planets are “second generation” planets which formed in the neutron star’s accretion disk.

The prospect of planets being able to form and exist around neutron stars and other extreme environments has made the search for exoplanets a lot more interesting! The field is not limited to living stars; it extends even to dead and undead stars which should have destroyed everything around them.

Pulsar Planets Today

In current naming conventions, these two planets are called PSR B1257+12 c and PSR B1257+12 d, though perhaps their IAU names, Poltergeist and Phobetor, are more fun. In 1994 a follow up study discovered a third planet in the system, PSR B1257+12 b (or Draugr), a moon-mass planet on a 25.3 day orbit.

Artist’s depiction of PSR B1257+12 and its planets. (Image credit: NASA/JPL-Caltech)

Before these pulsar planets were discovered, a 1991 paper claimed the discovery of the first pulsar planet around PSR 1829-10, but it was later retracted. The discovery of PSR B1257+12’s planets was followed by the discovery of the first exoplanet around a Sunlike star in 1995, and the vast majority of exoplanet science since has focused on those orbiting main sequence stars.

As of November, 2022, the NASA Exoplanet archive documents 7 confirmed planets (including these first two) discovered via pulsar timing variations, out of 5,190 total confirmed exoplanets. Pulsar planets are rare, presumably because of the incredibly harsh environments created by the stellar evolution processes that result in pulsars.

So, how did these rare planets end up where they did? The PSR B1257+12 planets likely formed from the debris disk left behind by a merging companion white dwarf. Another pulsar planet, PSR J1719-1438b, is thought to be a remnant (diamond!) core of a white dwarf companion whose outer layers were ripped away by the pulsar. One pulsar planet, PSR B1620-26b, is actually circumbinary, orbiting a binary pulsar-white dwarf system. The planet is theorized to have originally formed around the white dwarf’s main sequence progenitor before being captured by the pulsar.

A circumstellar disk was discovered around pulsar 4U 0142+61, which is also a highly magnetized neutron star, or magnetar. This demonstrates that the material left behind from supernovae may allow companionless pulsars to form new planets!

Ultimately, pulsar planets are rare, but as radio telescope resolution and sensitivity improve, we may find more of these disks, as well as pulsar timing variation planets. What terrifying worlds will we find next? And might they even be habitable?

Comments are invited and will be appreciated, especially if the reader finds any errors which I can correct. Use “Reply”

What do we do?

Astrobites is a daily astrophysical literature journal written by graduate students in astronomy. Our goal is to present one interesting paper per day in a brief format that is accessible to undergraduate students in the physical sciences who are interested in active research.

Reading a technical paper from an unfamiliar subfield is intimidating. It may not be obvious how the techniques used by the researchers really work or what role the new research plays in answering the bigger questions motivating that field, not to mention the obscure jargon! For most people, it takes years for scientific papers to become meaningful.

Our goal is to solve this problem, one paper at a time. In 5 minutes a day reading Astrobites, you should not only learn about one interesting piece of current work, but also get a peek at the broader picture of research in a new area of astronomy.

## From Astrobites : “History lessons from chemical elements”

From Astrobites

12.26.22
Sabina Sagynbayeva

Authors: Henrique Reggiani, Kevin C. Schlaufman, Brian F. Healy, Joshua D. Lothringer, and David K. Sing

First Author’s Institution: The Observatories of the Carnegie Institution for Science

Paper Status: Accepted for publication in The Astronomical Journal [open access]

H2O vs CO

There are many ways to study planet formation, including looking at a planet’s interaction with its protoplanetary disk, a planet’s interaction with other planets and/or stars that pass near the planet, or a planet’s chemistry. Today’s authors focus on planetary chemistry. By examining the abundance of various chemical elements in a giant planet’s atmosphere, it is possible to determine where the planet formed in its parent protoplanetary disk. Most astronomers investigate water (H2O) and carbon/oxygen (C/O) abundance. There is a region in a protoplanetary disk where water ice accumulates called the H2O ice line (or the snow line). CO2 is, on the other hand, solid ice beyond the snow line. C/O abundance ratios could be attributed to either the formation of a planet along the CO2 or CO ice lines, or to the planet accreting carbon-rich grains inside the H2O ice line. Today’s paper focuses on analyzing the oxygen and carbon abundances for the planet WASP-77 A b in an effort to study its formation.

If you want to know more about a planet, you need to learn about its star first. That’s why an important part of the paper discusses how the authors investigated the stellar parameters for WASP-77 A using photometric, astrometric, and spectroscopic data. Those parameters include its mass, radius, effective temperature, surface gravity, and iron abundance. The authors infer the stellar parameters using different techniques. Luckily, they didn’t have to reinvent the wheel, because other astronomers have created a number of open software packages that make all of this inference and analysis easier (usually, paper authors acknowledge such packages at the end of the paper). As you might have guessed, the authors of today’s papers were not the first to infer stellar parameters for WASP-77 A. However, you, just like the authors of today’s paper, would want to get the stellar parameters from the data yourself, and compare them with other studies. Furthermore, the authors argue that the inferred stellar parameters are good for planet atmospheric characterization only if the produced oxygen and carbon abundances can describe stellar mass and radius, as well as the planet parameters. In Figure 1, you can see their inferred chemical abundances for the star WASP-77 A compared to other similar stars from the GALAH survey.

The white circles show where the star is located in the abundance space, and darker colors indicate that a chemical is more abundant. The authors find an “excellent agreement” between their parameters and those inferred in other papers [PASP (below)].

Figure 1: Chemical abundances for the star WASP-77 A as inferred by the authors of the paper, compared to other similar stars from the GALAH survey. The white circle in each image is the star of interest. A darker color indicates that a chemical is more abundant. The different panels show the different chemical elements ratios. Figure 3 in the paper.

Spectroscopy is the interaction of electromagnetic radiation with different substances. In the process of this interaction, we receive information both about the light itself and about the substance. By analyzing the spectral lines from a substance, one can discover what atoms and molecules it consists of and how these atoms and molecules interact with each other. The simplest case is the spectra of atoms and ions (for example, atomic hydrogen). These spectra are relatively easy to analyze and represent a set of narrow spectral lines. If you are studying the chemical abundances in astronomical objects, you are most certainly using spectroscopic data, just like the authors of this paper! Using, the atomic data, they can do the inference on the chemical abundances of different chemical elements (e.g. oxygen, carbon, etc.). Their inference on oxygen and carbon abundances reports even better confidence levels than in the previous study [Nature (below)] .

So what can we say about the planet?

It is not simple to relate a planet’s atmospheric carbon and oxygen abundances to where it formed in its parent protoplanetary disk. However, here is where the star comes in! If we know the chemical abundance of a star, then we also have an idea about the chemical composition of its planet. As the authors discovered, WASP-77 A b has significantly more carbon and oxygen than its host star; therefore it has a higher C/O abundance ratio, so the authors deduce that the planet formed outside of its parent protoplanetary disk’s H2O ice line. The authors also predict that WASP-77 A b underwent pebble accretion followed by the migration after the protoplanetary disk dissipated. So, this way chemistry becomes a tool to learn the history of the planetary system!

Science papers:
PASP 2013
See the above science paper for instructive material with images.
Nature 2021

Comments are invited and will be appreciated, especially if the reader finds any errors which I can correct. Use “Reply”

What do we do?

Astrobites is a daily astrophysical literature journal written by graduate students in astronomy. Our goal is to present one interesting paper per day in a brief format that is accessible to undergraduate students in the physical sciences who are interested in active research.

Reading a technical paper from an unfamiliar subfield is intimidating. It may not be obvious how the techniques used by the researchers really work or what role the new research plays in answering the bigger questions motivating that field, not to mention the obscure jargon! For most people, it takes years for scientific papers to become meaningful.

Our goal is to solve this problem, one paper at a time. In 5 minutes a day reading Astrobites, you should not only learn about one interesting piece of current work, but also get a peek at the broader picture of research in a new area of astronomy.

## From Astrobites : “One Plot to Rule Them All – A Review of the Skumanich Law”

From Astrobites

12.24.22
Maryum Sayeed

Author: Andrew Skumanich

First Author’s Institution: High Altitude Observatory, National Center for Atmospheric Research

Status: Published in The Astrophysical Journal [open access]

Introduction

Gyrochronology — the study of deriving stellar ages from stellar rotation periods — is a relatively new subfield in astronomy, but has already greatly progressed our knowledge of stellar physics. This bite focuses on the first observations of this idea, published by Andrew Skumanich in 1972. Since then, the paper has garnered over 1500 citations, and even has a dedicated conference for the subfield. Summarized below are the paper’s three main findings.

Figure 1: This plot shows the lithium abundance (x’s, top-most solid curve), rotational velocity (triangles, middle line), and Calcium emission (closed circles, bottom line) as a function of stellar ages (in Gyr). The three data points are from Hyades, the Sun and field stars in Ursa Major. The plot demonstrates that all three measurements decrease as a function of stellar age, with the relationship indicated in text on the plot.

The first conclusion of this paper is in regards to lithium in stars. The lithium abundance of the stellar surface originates from the lithium content of the molecular cloud from which the star formed. The outer convective layer is cooler than the inner radiative layer, so the temperature in the outer layers isn’t hot enough to destroy the lithium in that region (see Figure 2 below for a visual of the stellar structure). However, during the first dredge-up phase (which happens when a main-sequence star evolves to a red giant), the convective layer expands to other layers, thereby mixing with the inner, hotter, radiative core and exposing the lithium to high enough temperatures to destroy the lithium. Therefore, as a star evolves into a red giant, we would expect the star to decrease in lithium abundance. Skumanich observed this in his sample, where the oldest object had the least amount of lithium. This relationship between lithium and age is modeled as Li \propto e^{-\tau/1.1} and shown as the top curve in Figure 1.

Rotational Velocity

The second conclusion of the paper describes the rotational velocity of stars over time. All stars are born rotating due to the angular momentum of the protoplanetary disk in which they are formed. Once fusion turns on, stars spend most of their life on the main sequence burning hydrogen into helium in their cores. When the star begins to run out of hydrogen in its core, it expands to become a red giant. Throughout these stars’ lifetimes, they experience spin down, where their rate of rotation slows down when their magnetic field interacts with their stellar winds, causing the star to lose mass and slowing its rotation; the magnetic field therefore acts as a brake. This phenomenon is referred to as magnetic braking. In the paper, Skumanich observed evidence of this by measuring the rotational velocity of three objects — the Hyades, the Sun, and field stars in Ursa Major — with different ages. This is shown by the middle curve in Figure 1. He then derived a relationship between the rotational velocity ($\latex v_r$) and age (\tau) to be v_r \propto \tau^{-0.51}.

Calcium+ Emission

The final finding of this paper pertained to Calcium emission in stars. Due to the small size of stars, stellar surfaces are spatially unresolvable, making it difficult to observe chromospheric activity (i.e. activity in the lower regions of the Sun’s atmosphere, just outside the photosphere; see Figure 2). However, by studying the surface of the Sun, we see two intense lines in the violet part of the visible spectrum referred to as the Calcium H & K lines. These strong lines, which can be observed with ground based telescopes, are indicative of strong stellar magnetic activity; in fact, the Wilson H&K project sought out to measure Calcium H & K lines in solar-like stars to study stellar activity. Skumanich’s paper analyzed this activity as a function of stellar age, and noticed that Calcium emission, and hence magnetic activity, decreases as a star ages. This relationship between Ca and age \tau is modeled as \tau^{-0.54} and is shown by the bottom line in Figure 1.

Figure 2: a diagram with labeled regions of a main sequence star showing the core (innermost region), the radiative zone (middle layer) where radiative energy is transported via photons, and the convective zone (outermost layer) where energy is transported via the movement of hot stellar material. Credit: Wikipedia.

Discussion

Thanks to this one paper, Skumanich provided a relationship to connect stellar age to the lithium content, rotational velocity and Calcium emission in stars. In summary,

Li \propto e^{-\tau/1.1}
v_r \propto \tau^{-0.51}
Ca \propto \tau^{-0.54}

It is worth noting that since Skumanich’s paper, these relations have been greatly revised to include more data, and incorporate other complicated stellar physics.

Stellar Ages

This paper’s findings had an incredible impact on stellar physics, especially in determining stellar ages. The age of a star is difficult to calculate. If the star is part of a cluster, one could plot stars on a colour magnitude diagram, fit isochrones to the distribution, and empirically determine the age. But this method requires that the star be part of a cluster rather than be a field star. However, Skumanich’s paper gave us another tool to determine stellar age by using the star’s rotation period.

While age is one of many properties of a star (such as mass or metallicity), it is the only property that allows us to study stellar evolution – and on a larger scale galaxy evolution – as a function of time. Without knowing a star’s age, it is difficult to fully understand the lifetime of a star.

We’re often taught that it’s rude to ask people their age. But when it comes to stars, astronomers are desperate to talk about ages.

Comments are invited and will be appreciated, especially if the reader finds any errors which I can correct. Use “Reply”

What do we do?

Astrobites is a daily astrophysical literature journal written by graduate students in astronomy. Our goal is to present one interesting paper per day in a brief format that is accessible to undergraduate students in the physical sciences who are interested in active research.

Reading a technical paper from an unfamiliar subfield is intimidating. It may not be obvious how the techniques used by the researchers really work or what role the new research plays in answering the bigger questions motivating that field, not to mention the obscure jargon! For most people, it takes years for scientific papers to become meaningful.

Our goal is to solve this problem, one paper at a time. In 5 minutes a day reading Astrobites, you should not only learn about one interesting piece of current work, but also get a peek at the broader picture of research in a new area of astronomy.

## From Astrobites : “Shifting our View on Neutron Stars with Gravitational Wave Parallax”

From Astrobites

12.22.22
Macy Huston

Authors: Magdalena Sieniawska, David Ian Jones, Andrew Lawrence Miller

First Author’s Institution: Centre for Cosmology, Particle Physics and Phenomenology (CP3), Université catholique de Louvain, Chemin du Cyclotron 2, B-1348 Louvain-la-Neuve, Belgium

Status: To be submitted to MNRAS (closed access)

Astronomers have long used parallax measurements to determine the distance to celestial objects, most notably with the Gaia space telescope.

Today’s paper proposes using a similar method to measure the distances to neutron stars with gravitational wave parallax.

Continuous Gravitational Waves

All of our gravitational wave detections so far have been compact binary coalescence, which is the inspiral and collision of two incredibly dense objects (which may be neutron stars and/or black holes). But, this isn’t the only process that can produce gravitational waves. Spinning compact objects themselves can produce continuous gravitational waves (CGWs), which are detectable over large timescales rather than as a single cataclysmic event.

It is important to note that a perfectly spherical neutron star’s rotation will not produce CGWs. Only a neutron star with long-lasting asymmetry (asymmetry that is misaligned from its rotation axis, at that) will produce CGWs. These can be caused by elastic and/or magnetic deformations or “mountains” on a neutron star’s surface. An example morphology is shown in Figure 1 below.

Figure 1: Simple model of a possible asymmetric neutron star shape. The star is elongated along the I_2 axis, which is misaligned from the I_3 rotation axis, which means that its rotation will produce continuous gravitational waves. (Image credit: Sieniawska & Bejger, 2019)

A Pair of Parallaxes

Geometric parallax is calculated by measuring the extremely small change in a star’s apparent position on the sky when Earth moves to the opposite side of the Sun in its orbit. With that angle (which is 1 AU divided by the distance) and our knowledge of Earth’s orbital distance, we can use trigonometry to calculate how far away the star is. Gravitational wave parallax follows a similar idea, though the execution is more difficult.

With gravitational wave detectors it’s not so easy to just take a picture and find a precise location. However, gravitational waves will appear to change slightly in frequency due to the Doppler effect as Earth orbits the Sun. This allows us to measure the source’s ecliptic latitude and the apparent change in the source’s ecliptic latitude as Earth moves, as shown in Figure 2 below. Just like with Gaia, this gives us the angle we need for the trigonometric calculation of distance.

Figure 2: Parallax geometry, showing the source’s ecliptic latitude \beta and apparent change \delta \beta. SSB is the Solar System barycenter, and Earth (\oplus) is shown at its orbital distance R_{orb} on each side of the Sun. (Figure 1 in the paper)

So, can we actually detect this effect?

This gravitational wave parallax is only detectable if we can measure that angle. The ecliptic latitude resolution for the CGW measurement is comparable to or finer than the parallax angle. This depends on the distance to the source, observation time, the signal-to-noise ratio of the CGW detection, the true frequency of the signal, and the true ecliptic latitude of the source.

The authors examine detectability for two gravitational wave detectors: advanced LIGO (the end goal of current upgrades underway on LIGO, a.k.a. aLIGO) and the Einstein Telescope (the proposed next generation ground-based gravitational wave detector, a.k.a. ET).
___________________________________________________________________

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

This graphic shows the masses of all of LIGO’s announced gravitational wave detections, as well as black holes and neutron stars previously obtained through electromagnetic observations.
LIGO-Virgo-Kagra/Aaron Geller/Northwestern.
___________________________________________________________________

Results for aLIGO are shown in Figure 3. In general, parallax can be detected to higher distances for CGW detections at higher frequencies, higher observation times, and high signal-to-noise ratios.

Figure 3: Parallax and distance resolution across CGW parameters for aLIGO. The four panels show observation times from 1-10 years. The x and y axes cover a range of frequencies and signal-to-noise ratios, respectively. The colorbar quantifies the resolution such a measurement would provide for ecliptic latitude and the maximum distance at which parallax could be detectable. The epsilon parameter indicated by dotted lines quantifies the star’s deviation from spherical shape (or ellipticity) that would be required to produce such a signal. (Figure 2 in the paper.)

To give an example from the paper’s conclusion, parallax would be detectable with a year of aLIGO observation for a neutron star out to a distance of about 100 pc with an initial 400 Hz frequency and signal-to-noise ratio of 100. For reference, the nearest known neutron star is ~130 pc away; neutron stars may be spun up to ~600 Hz as a result of accretion from a binary companion; and a signal-to-noise ratio of 70 is LIGO’s standard threshold for detection.

The Future of CGW Parallax

Gravitational wave parallax provides an opportunity to measure distances to compact objects without electromagnetic wave detections. Additionally, an accurate measure of the distance to such an object can help put limits on its deviation from spherical shape. With gravitational wave detector upgrades on the horizon, the authors suggest adding considerations for finite source distances to CGW models to perform better searches. For now, the search for the first detectable CGWs continues.

Comments are invited and will be appreciated, especially if the reader finds any errors which I can correct. Use “Reply”

What do we do?

Astrobites is a daily astrophysical literature journal written by graduate students in astronomy. Our goal is to present one interesting paper per day in a brief format that is accessible to undergraduate students in the physical sciences who are interested in active research.

Reading a technical paper from an unfamiliar subfield is intimidating. It may not be obvious how the techniques used by the researchers really work or what role the new research plays in answering the bigger questions motivating that field, not to mention the obscure jargon! For most people, it takes years for scientific papers to become meaningful.

Our goal is to solve this problem, one paper at a time. In 5 minutes a day reading Astrobites, you should not only learn about one interesting piece of current work, but also get a peek at the broader picture of research in a new area of astronomy.

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