Starship Asterisk*

Vaibhavi Gawas wrote:
Type Ia supernovae (SNIae), observed to be caused by the detonation of a white dwarf star, are generally a very consistent type of stellar explosion that make up a significant part of the distance ladder. Their consistency even led us to discover dark energy! However, more and more exotic examples of have been discovered, and this has led researchers to question our understanding of the physics behind SNIae. In today’s paper, the authors look at how the presence of a magnetic field could impact the types of SNIa explosions that we see.

White dwarf stars accompanied by a binary partner tend to accrete mass over a period of time. As white dwarfs grow in mass, they experience a stronger gravitational force that drives the star towards collapse. We know that the white dwarfs are supported against the gravitational collapse by the electron degeneracy pressure. However, electron degeneracy can battle against only a limited amount of gravitational pressure. ...

what if some exotic phenomenon could change the Chandrasekhar mass limit? We have observed SNIae with greater than expected luminosities (the superluminous SNIae). These superluminous SNIae, need a progenitor white dwarf of 2.1-2.8 solar mass in order to match observations. Research into the nature of highly magnetized white dwarfs has led to the discovery that some white dwarfs could indeed hold mass beyond 1.4 solar mass, even up to 2.8 solar mass sometimes. These advances question the standard candles we have used until now and also provide a possible explanation for the progenitors of superluminous SNIae. ...

• Physical Review Letters 110(7):1102 (15 Feb 2013) DOI: 10.1103/PhysRevLett.110.071102
• arXiv.org > astro-ph > arXiv:1301.5965 > 25 Jan 2013

Alex Pizzuto wrote:
The precision engineering required for gravitational wave astronomy is simply astounding: a giant system of mirrors and lasers spanning a distance of multiple kilometers, that are sensitive enough to detect a ripple in spacetime that is 10,000 times smaller than the size of a proton. If this doesn’t sound impressive enough to you, now imagine building it on the moon.

That’s exactly what the authors of today’s paper propose: a gravitational wave (GW) detector on the moon. Before discussing exactly why a lunar GW detector would be beneficial, let’s go over some of the basics of GW observation in general. ...

• arXiv.org > gr-qc > arXiv:2007.08550 > 16 Jul 2020

Keir Birchall wrote:
Dwarf galaxies are believed by some to be time capsules, but instead of old records, they are thought to preserve the seeds of black holes formed in the early Universe. This is because most dwarf galaxies detected in the nearby Universe don’t show signs of interacting with their galactic neighbours, leaving these relatively low mass collections of gas, dust and stars to evolve in isolation. Without contamination from other galaxies, astronomers can treat these dwarf galaxies as pristine pockets of the Universe’s past. So by analysing the distribution and masses of the black holes in these dwarf galaxies astronomers can hope to shed some light on how they formed.

Two formation mechanisms dominate discussion: either black holes formed from the collapse of early generations of stars, known as Pop III’s, or they formed from the direct collapse of gas and dust. If the former mechanism dominates then we would expect to find large numbers of low mass black holes, while the latter mechanism is predicted to produce a much smaller number of higher mass seeds. Unfortunately, dwarf galaxies are much fainter than their higher-mass counterparts so are difficult to detect. The often-invisible black holes within provide an even greater challenge.

An easier way to detect them is waiting for the black hole to accrete material and emit huge amounts of radiation, a phenomenon known as an active galactic nucleus (AGN). Over the past decade, there has been a huge increase in the number of AGN detected in dwarf galaxies. Today’s authors aim to place some of these AGN on a well-known mass & velocity dispersion relation to try and gain insight into how black holes may have formed in the early Universe. ...

• Astrophysical Journal Letters 898(1):L3 (2020 Jul 20) DOI: 10.3847/2041-8213/aba0c1
• arXiv.org > astro-ph > arXiv:2006.15150 > 26 Jun 2020

Xing-Ye Zhu wrote:
For most astronomers, it is just common sense that dark matter accounts for approximately 85% of the matter in the universe. However, as long as the constituents of dark matter remain a mystery, some astronomers remain skeptical about our conventional understanding of dark matter. Recently, astronomer Alexandre Deur suggested that the theory of relativity itself may explain a phenomenon widely regarded as evidence for dark matter. ...

Modified Newtonian Dynamics, or MOND, for example, is the most discussed out of all the gravitation corrections to explain the missing mass problem (see this astrobite for further discussion of MOND vs. dark matter). It modifies the Newtonian gravitation law at low accelerations to enhance the effective gravitational attraction. Similarly, most of the other corrections require new descriptions of gravitation. But recently, as Deur proposes in this work, the effect of general relativity may account for the missing mass, without introducing any new corrections. ...

• arXiv.org > astro-ph > arXiv:2004.05905 > 10 Apr 2020

Ishan Mishra wrote:
Much of our knowledge about the atmospheric properties of exoplanets comes from transmission spectroscopy. An exoplanet’s apparent size (inferred from the amount of starlight it blocks out) varies with wavelength as molecules (plus atoms, ions, clouds or hazes) in the upper layer of the exoplanet’s atmosphere absorb different wavelengths of the star’s light. Clouds are especially important, as they affect atmospheric spectra and inhibit our ability to learn about the fundamental atmospheric properties for the majority of exoplanets (one example of this is shown in Figure 1). Not only are atmospheric clouds ubiquitous in our solar system, but many exoplanets show strong evidence for clouds (for example, GJ 1214b and HD 209458b)!

Typical transmission spectra analysis methods, like atmospheric retrievals, assume a 1D atmosphere that only changes radially, because working with detailed 2D/3D models is computationally challenging. However, as you might have guessed, planets are 3D! The transmission spectra we collect in our telescopes are a combination of multiple spectra from different locations in the atmosphere. Atmospheric composition and temperature can vary in 3D, and the distribution of clouds on a planet can also be wildly inhomogeneous, i.e., non-uniform.

A category of exoplanets called hot Jupiters (Jupiter like gas-giants orbiting very close to their host stars) are especially likely to have non-uniform cloud distributions. Because hot Jupiters are tidally locked, their daysides and nightsides have huge temperature contrasts. Cloud properties are highly sensitive to how the temperature of the atmosphere changes with height, longitude and latitude (referred to as the atmosphere’s “local thermal structure”). So, we expect that a hot Jupiter will have clouds with diverse properties (for example, on Earth, water clouds form where it is cold enough for water to condense). In particular, models show that for many hot Jupiters, the thermal structure on the east limb is substantially hotter than the temperature on the west limb (see Figure 2). Since various gases condense to form clouds at different temperatures, this leads to clouds with very different properties forming on the east limb versus the west limb.

We have evidence for non-uniform clouds through phase curve observations of hot Jupiters (and brown dwarfs), where we observe how the reflected starlight from the planet changes as the planet orbits its host star. However, various difficulties with obtaining phase curve measurements make this method of probing cloud cover difficult to generalize to the vast majority of exoplanets. One promising alternative is transmission spectroscopy. Today’s paper explores if transit measurements of hot Jupiters with the James Webb Space Telescope (JWST) can provide a strong signature of non-uniform clouds. ...

• Astrophysical Journal 887(2):170 (2019 Dec 20) DOI: 10.3847/1538-4357/ab55d9
• arXiv.org > astro-ph > arXiv:1910.07527 > 16 Oct 2019

Yujia Wei wrote:
Fast radio bursts (FRBs) are transient radio pulses that last a few milliseconds and have large dispersion measures (DMs). The DM is a measure of the time delay of radio signals at different frequencies (the lower the frequency, the later the arrival time) caused by passing through free electrons. Several observed FRBs are observed to repeat; they are called repeating FRBs. ...

Previous models of FRBs from BNS mergers have focused on the period prior to merging, where the resulting FRBs are one-time events. Here the author presents a new hypothesis for producing repeating FRBs to explain all these above observational features. Decades to hundreds of years before merging, the magnetospheres of two neutron stars would interact endlessly with each other in binary neutron star (BNS) systems. When the magnetic field lines from the magnetospheres of the individual neutron stars become close to each other, they will be disconnected or merged with other magnetic field lines and realigned, releasing a large amount of energy. This process is called magnetic reconnection and is shown in Figure 2. The magnetic reconnection process converts the energy stored in the magnetic field into heat and kinetic energy, allowing particles to flow out along the magnetic field lines. The energy-carrying particles interact with the surrounding medium to produce beams of radiation. FRBs would be detected when these bright beams of radiation are pointed at the earth. ...

• Astrophysical Journal Letters 890(2):L24 (2020 Feb 20) DOI: 10.3847/2041-8213/ab7244
• arXiv.org > astro-ph > arXiv:2002.00335 > 02 Feb 2020 (v1), 22 Feb 2020 (v2

Sumeet Kulkarni wrote:
The answer: not much at all – they can be nearly perfect spheres! A new paper by the LIGO-Virgo collaboration presents the latest results of the search for continuous gravitational waves from their third observing run (O3).

Neutron stars represent one of matter’s weirdest manifestations. With a mass little more than that of the Sun packed into a big city, getting to know their size, shape and structure can unlock the most fundamental questions in atomic physics. What makes up a neutron star? Are they rigid or squishy? Are they perfectly spherical? If they have deformities, what is the tallest ‘mountain’ they can support? ...

Neutron stars can exist in pairs and do the tango like the binaries mentioned above, but the cool thing is that they can also radiate gravitational waves while being single!

Any physical deformation, like a ‘mountain’ on the neutron star crust will give rise to a large quadrupole moment since they rotate extremely fast. The particular kind of neutron stars studied here are called millisecond pulsars: entire stars which complete one rotation within a few tens of milliseconds, much less than the blink of an eye. Even if the pulsar were perfectly spherical on the outside, it may have internal deformities in its core – something very little is known about. Or, it may be slightly elliptical in shape and wobble asymmetrically as it spins, giving rise to gravitational wave radiation.

All of the above mechanisms of lone neutron star gravitational waves have a tantalizing characteristic: their frequency is almost entirely constant. This is because it is determined by the frequency of rotation of the neutron star. These gravitational waves are hence known as ‘continuous’ waves, distinguishing them from the transient, ‘chirping’ waves given out by colliding binaries. ...

• LIGO Scientific Collaboration, Virgo Collaboration: R. Abbott et al
• arXiv.org > astro-ph > arXiv:2007.14251 > 28 Jul 2020

Sumeet Kulkarni of astrobites and/or the LIGO Scientific Collaboration and the Virgo Collaboration wrote:

When the upper limit measurements (colored vertical lines) of the quadrupole moment (or ellipticity) lie to the left of the black lines, it means that the spin-down limit has been surpassed.

Jamie Sullivan wrote:
The Hubble constant (H0) has quickly become a star in the world of forced astronomical acronyms (H0LiCOW, SH0ES), and for good reason. Measuring H0 quantifies the expansion of the universe, and is how cosmologists converged on the idea that we live in a universe that is accelerating in its expansion due to dark energy. Discrepant measurements of H0, which has been dubbed the Hubble tension, has been a big deal lately, not only in the cosmological community, but also on this site! Don’t take my word for it – Each word links a different bite!

There is a lot of great material in those bites, so we’ll just settle for the one-sentence version of the Hubble tension here. Measurements of H0 at late times using type Ia supernovae (74 km/s/Mpc by the SH0ES team) and at early times using the Cosmic Microwave Background (67 km/s/Mpc by the Planck team) are different enough from each other (at >3σ) that cosmologists had to come up with a name for the problem – the Hubble tension. Many (though not all) cosmologists have been racking their brains for years now over what might be causing this tension, and today’s paper addresses the prospects of a particular model that purports to solve it. ...

• arXiv.org > astro-ph > arXiv:2006.11235 > 19 Jun 2020

Brent Shapiro-Albert wrote:
Pulsars are rapidly rotating neutron stars that emit radio waves like a light house as they rotate. They come in many flavors, like millisecond pulsars (MSPs), pulsars that complete a rotation in less than 30 milliseconds, many of which belong to a family of “spider” pulsars. These spider pulsars are so named because they blow away or accrete the mass of their binary companion, similar to how some spiders kill their male partners. The two (or three) types of spider pulsars are known as black widows, which have accreted most of the mass from their binary companion star, and redbacks, which are currently accreting mass from their companions.

Spider MSPs are particularly hard to find when searching in the radio regime due to excess gas from their companions obscuring the pulsed emission. However, since the Fermi Gamma-Ray Space Telescope started publishing catalogs of unassociated gamma-ray sources, many have been found to be spider MSPs in follow up radio observations. But today’s paper has, for the first time, confirmed the redback MSP nature of the gamma-ray source 3FGL J2039.6−5618 by detecting gamma-ray pulses before finding the radio pulses! ...

• arXiv.org > astro-ph > arXiv:2007.14849 > 29 Jul 2020

Ali Crisp wrote:
Stellar metallicity refers to the amount of a star made up of elements heavier than hydrogen or helium, or “metals” to astronomers (chemists would like to have a word with us). These heavier elements are formed by nuclear fusion in the stellar core as the star evolves. Metallicity is referred to in terms of the iron-to-hydrogen ratio or the alpha-to-iron ratio – where alpha refers to the alpha process elements such as carbon, oxygen, and neon. In the 1940’s, Walter Baade proposed a stellar classification system based on metallicity, splitting stars into Population I (metal-rich) and Population II (metal-poor). Since then, astronomers have created a third classification: Population III. This population contains extremely metal-poor stars which are thought to be artifacts of the early universe, before heavier elements were present in the interstellar medium. However, it’s important to note that Population III stars have not been observed and are currently purely hypothetical.

Metal-poor stars are of interest because – since they are so old – they can tell us important information about the early evolution of the universe. The number of known metal-poor stars has been growing rapidly since the 1980’s, and more recent surveys such as the Sloan Digital Sky Survey (SDSS) and the Radial Velocity Experiment (RAVE) have contributed many stars to the sample. However, many of the candidates still need high resolution spectra taken to find out more about these stars. ...

• Astronomy & Astrophysics (accepted 08 July 2020) DOI: 10.1051/0004-6361/202038028
• arXiv.org > astro-ph > arXiv:2007.03994 > 08 Jul 2020 (v1), 16 Jul 2020 (v2)

Jenny Calahan wrote:
As the famous astronomer and science communicator Carl Sagan said: “The nitrogen in our DNA, the calcium in our teeth, the iron in our blood, the carbon in our apple pies were made in the interiors of collapsing stars. We are made of star stuff” This idea that the ingredients for our apple pies originated from the interiors of stars is pretty impressive, but it begs the question: how did stuff leftover from stars get onto planets like Earth and form humans and life (and pies!)? Astronomers are actively answering that question by studying protoplanetary disks.

Stars form out of clouds of dust and gas, enhanced with the leftovers of dead stars. As the cloud collapses, a disk starts to form around the newly forming star. This disk is called a protoplanetary disk, and is the precursor to planetary systems. They are three main components to a protoplanetary disk: gas, small dust (~micron sized, or 1/50 the width of a human hair), and large dust (~mm sized, or about the size of a pencil tip) (see Figure 1). Each of these components come together to form planets. In particular, the large dust will coagulate to form planet bébés a.k.a. planetesimals. Thus, tracking the large dust and where it is located in a disk over time will shed insight into the beginnings of planet formation.

In today’s Astrobite, the authors present observations of 12 different protoplanetary disks, looking particularly at wavelengths in which large dust grains emit. They specifically probed edge-on disks, meaning that we are looking at them on their sides. The authors chose to look at edge-on disks in order to measure how high large dust grains extend above the midplane (the middle of the disk). There are two phenomena that describe the large-scale movement of large dust grains. One is ‘vertically settling,’ where grains simply fall down into the midplane, and the other is ‘radially drift,’ where large grains move from large radii towards the central star. These observations are a direct probe of these two phenomena that impact planet formation as well our general understanding of the composition of protoplanetary disks. ...

• Astronomy & Astrophysics (accepted 05 Aug 2020) DOI: 10.1051/0004-6361/202038087
• arXiv.org > astro-ph > arXiv:2008.06518 > 14 Aug 2020

bystander wrote: ↑Wed Aug 26, 2020 6:59 pm H0w Low Can You Go? Limiting Early
Dark Energy with Large-scale Structure
astrobites | Daily Paper Summaries | 2020 Aug 21

Jamie Sullivan wrote:
The Hubble constant (H0) has quickly become a star in the world of forced astronomical acronyms (H0LiCOW, SH0ES), and for good reason. Measuring H0 quantifies the expansion of the universe, and is how cosmologists converged on the idea that we live in a universe that is accelerating in its expansion due to dark energy. Discrepant measurements of H0, which has been dubbed the Hubble tension, has been a big deal lately, not only in the cosmological community, but also on this site! Don’t take my word for it – Each word links a different bite!

There is a lot of great material in those bites, so we’ll just settle for the one-sentence version of the Hubble tension here. Measurements of H0 at late times using type Ia supernovae (74 km/s/Mpc by the SH0ES team) and at early times using the Cosmic Microwave Background (67 km/s/Mpc by the Planck team) are different enough from each other (at >3σ) that cosmologists had to come up with a name for the problem – the Hubble tension. Many (though not all) cosmologists have been racking their brains for years now over what might be causing this tension, and today’s paper addresses the prospects of a particular model that purports to solve it. ...

Constraining Early Dark Energy with Large-Scale Structure ~ Mikhail M. Ivanov et al

• arXiv.org > astro-ph > arXiv:2006.11235 > 19 Jun 2020

Kaitlyn Shin wrote:
A field of study perfect for the radio astronomy and mystery-loving aficionado, fast radio bursts (FRBs) are an extremely energetic astrophysical phenomena — each burst of radio emission lasting just a few milliseconds — with causes that are still unknown. Most FRBs are extragalactic: of the ~100+ verified FRBs detected to date, nine FRBs have been localized to host galaxies outside our own, although very recently, an FRB was detected from within the Milky Way. The FRBs detected thus far have been due to instruments such as the Canadian Hydrogen Intensity Mapping Experiment (CHIME) FRB project and the Parkes Observatory. However, we would need to detect many more FRBs if we want to use FRBs to address cosmological science questions. In order to increase the number of FRBs needed to answer these questions, we will likely need a new generation of advanced radio telescopes.

One of the most promising — and ambitious — proposed next-generation radio telescopes is the Square Kilometre Array (SKA). The SKA has a design of thousands of antennas and dishes observing the sky with over a square kilometer of collecting area over a frequency range of 50 MHz to 14 GHz. Its sensitivity, angular resolution, field of view, and sky mapping speed would all be better than those of current radio telescopes. Today’s paper adopts the sensitivity of Phase 2 of the SKA (SKA2) and tries to answer the question: How many FRBs could the SKA detect? ...

• Monthly Notices of the RAS 497(3):4107 (Oct 2020) DOI: 10.1093/mnras/staa2238
• arXiv.org > astro-ph > arXiv:2008.00007 > 31 Jul 2020

Ashley Piccone wrote:
We know that as they age, galaxies transition from blue, star-forming disks to red, quiescent ellipticals, but the stages of evolution and the process of stopping star formation (often called quenching) are still mysterious. One clue to answering these questions may be post-starburst galaxies, or galaxies that recently experienced a period of intense star formation and are now calm and quiet. The authors of today’s paper explore the properties of the stars and gas in a post-starburst galaxy to explain what mechanisms may have stopped the star formation. ...

• Astrophysical Journal Letters 860(2):L18 (2018 Jun 20) DOI: 10.3847/2041-8213/aaca9a
  arXiv.org > astro-ph > arXiv:1804.10216 > 26 Apr 2018

Jason Hinkle wrote:
Of all the galaxies in the universe, only a small fraction are active, meaning that they have a source of energy that is not related to stellar processes. The centers of these active galaxies are known as active galactic nuclei (AGN) and are powered by the accretion of gas onto a supermassive black hole (SMBH). AGN are fundamentally multi-wavelength objects, emitting light over the entire electromagnetic spectrum. They are commonly classified according to their optical emission.

AGN are classified as Type 1 or Type 2 based on the width of spectral lines in their optical spectrum. Type 1 AGN have both narrow and broad emission lines. Broad lines are created by Doppler shifts due to gas orbiting close to the central SMBH in the broad line region (BLR). For Type 2 AGN though, only narrow emission lines are seen. The canonical AGN unification model suggests that this is due to a dusty torus blocking our view of the BLR.

There is a small subset of objects, known as changing-look AGN (CL-AGN), that change from Type 1 to Type 2 or vice versa. In recent years the number of new CL-AGN discoveries has increased due to expanded spectroscopic surveys. Some proposed explanations include changing obscuration of the BLR, variable accretion onto the SMBH, or even tidal disruption events where a star is ripped apart and accretes onto the SMBH. Whatever the cause, CL-AGN provide a unique insight into the complexities of AGN activity. In today’s paper, the authors use integral field spectroscopy from MaNGA to study the host galaxies of these CL-AGN to understand what role (if any) their environment plays. ...

• Monthly Notices of the RAS (online 29 Aug 2020) DOI: 10.1093/mnras/staa2627
• arXiv.org > astro-ph > arXiv:2008.11336 > 26 Aug 2020

Andreas Christ Sølvsten Jørgensen wrote:
How would you go about determining the properties of carbon in fusion reactions? While the answers to such questions can be detrimental to our understanding of stellar evolution, tight constraints on nuclear reaction rates can often be hard to come by. Indeed, it might not be feasible to measure the relevant cross-sections under laboratory conditions. This is, for instance, the case for the reaction that underlies the formation of oxygen through the fusion of carbon with helium. The fact that we do not have tight constraints on some nuclear reaction rates translates directly into ambiguities in predictions of numerical stellar models, posing a problem for theoretical astrophysicists.

Today’s authors present a creative approach to address this issue by investigating the remnants of massive stars: they search for answers by looking at stellar black holes. More specifically, they turn the problem on its head. Rather than using existing constraints on nuclear reaction rates to infer constraints on stellar evolution, they demonstrate how our knowledge on the final fate of stars can shed light on nuclear reaction rates. To do so, today’s authors draw on both detailed stellar evolution calculations and gravitational wave measurements. ...

• arXiv.org > astro-ph > arXiv:2006.06678 > 11 Jun 2020

Ellis Avallone wrote:
Nearly 70 years ago, Eugene Parker, a young professor at the University of Chicago, discovered something that would change out understanding of all stars, including our own Sun. The solar wind is a continuous stream of particles from the Sun that is part of the solar corona, or atmosphere. Prior to Parker’s 1958 discovery, other scientists had surmised that such a stream of particles could exist through observations of comet tails, but none had done the calculations to rigorously show that the solar wind must exist.

Today we’re going to take a closer look at the physics that led to this monumental discovery to understand the methods behind an idea that once seemed like madness. ...

• Astrophysical Journal 128:664 (Nov 1958) DOI: 10.1086/146579

bystander wrote: ↑Thu Sep 03, 2020 10:24 pm Parker’s Solar Wind
astrobites | Daily Paper Summaries | 2020 Sep 03

Ellis Avallone wrote:
Nearly 70 years ago, Eugene Parker, a young professor at the University of Chicago, discovered something that would change out understanding of all stars, including our own Sun. The solar wind is a continuous stream of particles from the Sun that is part of the solar corona, or atmosphere. Prior to Parker’s 1958 discovery, other scientists had surmised that such a stream of particles could exist through observations of comet tails, but none had done the calculations to rigorously show that the solar wind must exist.

Today we’re going to take a closer look at the physics that led to this monumental discovery to understand the methods behind an idea that once seemed like madness. ...

Dynamics of the Interplanetary Gas and Magnetic Fields ~ E. N. Parker

• Astrophysical Journal 128:664 (Nov 1958) DOI: 10.1086/146579

Michael Foley wrote:
Turbulence, or chaotic changes in the pressure and velocity of a fluid, is one of the great mysteries of classical physics. Much of the gas in galaxies is known to be turbulent, but the mechanisms that developed and maintain this turbulence remain areas of active research. While we still don’t know all the details of the physics behind turbulence, a lot of time and effort has gone into identifying statistics that can tell us whether gas is turbulent or not. In other words, we know what turbulence looks like even if we don’t know all the details of how it works (see this Youtube video for a great introduction to turbulence and the power spectrum, a statistic used in today’s paper). Today’s weather forecast calls for strong winds blowing in from the arXiv as we explore a new paper studying how stellar winds from star clusters can drive such turbulence.

Stellar winds, particularly those from massive stars like O or B types, blow bubbles in the surrounding cold gas by pushing it outwards and leaving a cavity behind. These are analogous to the bubbles we see on Earth that are created by air pushing into some other medium. In the case of a stellar-wind bubble, the “air” is hot stellar wind material. When massive stars are found in a star cluster, their bubbles tend to overlap and form a “superbubble”. One incredible example of this is the Orion Nebula Cluster (Fig 1). The authors of today’s paper run simulations that roughly mimic the stellar profile of the Orion Nebula Cluster, and they too find the creation of large superbubble. ...

• Astrophysical Journal Letters 899(2):L30 (2020 Aug 20) DOI: 10.3847/2041-8213/ababae
• arXiv.org > astro-ph > arXiv:2006.14626 > 25 Jun 2020

Laila Linke wrote:
Black holes come in many different sizes. Stellar black holes, which originate from the supernovae of massive stars, weigh a few times the mass of the sun (M_☉ ). In contrast, the supermassive black holes (SMBH) in the centres of galaxies can be millions or even billions of times as massive as the sun. For example, the black hole in the centre of the Milky Way has a mass of 4 x 10^₆ M_☉ and the black hole in the galaxy M87, whose image was taken last year, weighs 6.5 x 10^₉ M_☉. But can black holes grow arbitrarily large or is there a natural size limit? To answer this question, the authors of today’s paper consider Stupendously Large Black Holes (SLABs) with masses above 10^₁₁ M_☉. They discuss how SLABs could form, what observable effects they would cause and even how they could shed light on the nature of dark matter. ...

• arXiv.org > astro-ph > arXiv:2008.08077 > 18 Aug 2020 (v1), 09 Sep 2020 (v2)

Luna Zagorac wrote:
Though it makes up ~85% of the matter mass of the universe, dark matter has proven a tricky puzzle to solve. This is in part because, while we know the total mass of dark matter necessary to match astronomical predictions to observations, we have no indication of the individual masses of its constituent particles. While probably the most popular candidates are multitudes of small particles (like WIMPs or axions), another explanation could be a handful of very large objects, which we refer to collectively as MAssive Compact Halo Objects (MACHOs).

The draw of MACHOs is that they aren’t necessarily composed of exotic new bodies or phenomena: they could be anything with a lot of mass and low luminosity, such as brown dwarfs or neutron stars. A few years ago, a special type of black hole called primordial black holes (PBH) were a popular MACHO dark matter candidate. They became popular after the Laser Interforometer Gravitational Observatory (LIGO) began detecting mergers of black holes hypothesized to be primordial, but were soon ruled out by the same observations. In today’s paper, the authors revisit these calculations to breathe new life into the possibility of PBH and MACHOs as dark matter. ...

• arXiv.org > astro-ph > arXiv:2008.10743 > 24 Aug 2020

James Negus wrote:
Fueled by the accretion of gas and dust onto compact supermassive black holes, active galactic nuclei (AGN) shine like distant lighthouses on the Universe’s dark shores. At a closer glance, a “corona” of hot gas (analogous to that of our own Sun) encircles the inner accretion disk of these energetic objects and produces X-rays. Astronomers believe this emission be intricately connected to the violent swirl of matter at the galactic center. However, the true nature of these coronal regions remains uncertain.

To better understand the dynamics of AGN coronal regions and their dependency on accretion, today’s authors analyze quasar observations using the Chandra X-Ray Observatory. To ensure they are probing coronal activity that is dependent on accretion flow, the researchers exclude radio-loud AGN, which often experience X-ray variations from jets, and AGN with broad absorption lines, which tend to feature X-ray variability due to changing column densities (the X-rays are absorbed). They net a final sample of 462 AGN, which were collectively observed ~ 1,600 times for nearly ~ 12 years. ...

• arXiv.org > astro-ph > arXiv:2008.12778 > 28 Aug 2020

Ishan Mishra wrote:
Comet 2I/Borisov (the ‘I’ stands for interstellar and ‘2’ denotes that it’s the second such object to be discovered) was detected on 2019 August 30 by amateur astronomer G. Borisov (using a 0.65-meter telescope he designed and built himself!). Its trajectory through our solar system and a hyperbolic orbit clearly tell us that it’s coming from outside the solar system. 2I/Borisov is the second interstellar interloper to our solar system, after 1I/’Oumuamua. But unlike 1I/’Oumuamua, 2I/Borisov was observed to be actively outgassing material (when passing close to the Sun, a comet warms and begins to release gases) and hence labelled as the first interstellar comet. 1I/’Oumuamua on the other hand was discovered (back in October 2017) when it was already leaving the Solar System, making detailed studies difficult. Specifically, there were no gas spectroscopic detections made for 1I/’Oumuamua. 2I/Borisov on the other hand, which has looked a lot like comets in our solar system with dust and volatile gases blowing off it, provided opportunities for astronomers to learn about its chemical composition.

Comets spend most of their lives at large distances from any star, during which time their interior compositions remain relatively unaltered. Cometary observations can therefore provide direct insight into the chemistry that occurred during their birth at the time of planet formation. In the case of 2I/Borisov, it provides an opportunity to probe the primordial composition of an entirely different stellar system. 2I/Borisov was observed by two premier astronomical observatories, the Hubble Space Telescope (HST) and the Atacama Large Millimeter/submillimeter Array (ALMA), in the Fall/Winter of 2019, when the comet was near the perihelion of its trajectory around our Sun and hence its outgassing activity due to heating was highest. While HST observed the comet in the UV using its Cosmic Origins Spectrograph (COS), ALMA observed it in the millimeter wavelength range. The goal was to look for spectroscopic evidence for a suite of molecules commonly found in solar system comets, like H₂O, CO, HCN, etc. and understand their relative amounts in this comet, which helps placing it in context of our solar system comets.

While most of 2I/Borisov’s properties are similar to solar system comets, the result that stood out from the HST and ALMA observations and their subsequent analysis was the concentration of carbon monoxide or CO. Both HST and ALMA detected strong spectral emission lines of CO, whose strengths were used to deduce the amount of CO outgassing from the comet. The results show an extraordinarily high abundance ratio of CO to H₂O of around 35-155%, much higher than the average value of 4% in comets of our solar system (Figure 1). Even Oort Cloud comets, which come from a much colder and volatile gas rich part of the outer solar system, only have CO/H₂O ratios in the range 10% to 24% (Figure 2). This makes 2I/Borisov one of the most CO-rich comets ever observed. ...

• Nature Astronomy 4(9):867 (01 Sep 2020) DOI: 10.1038/s41550-020-1095-2
• arXiv.org > astro-ph > arXiv:2004.08972 > 19 Apr 2020

• Nature Astronomy 4(9):861 (01 Sep 2020) DOI: 10.1038/s41550-020-1087-2
• arXiv.org > astro-ph > arXiv:2004.09586 > 20 Apr 2020 (v1), 27 Apr 2020 (v2)

Mitchell Cavanagh wrote:
Galaxy clusters are among the largest gravitationally bound structures in the Universe. One of their defining characteristics is that they tend to be embedded within a large reservoir of superheated gas, known as the intracluster medium (ICM). With temperatures up to 10^₈ Kelvin, the ICM is a strong emitter of X-ray radiation. The resulting spectra is dominated by thermal bremßtrahlung radiation: radiation emitted when charged particles are decelerated. Characterising this thermal emission provides useful insights into the physical processes within the cluster, such as galaxy merging and AGN activity, as well as various physical parameters including temperature and metallicity. In order to obtain these parameters, one must first fit the observed spectra. However, the ICM is not necessarily uniform. Different regions are often characterised by multiple thermal components, hence requiring a mix of temperatures rather than a single temperature model to reproduce the observed spectra. The authors of today’s bite propose a new machine learning (ML) method to systematically estimate the different underlying thermal components in ICM spectra. As this approach is not reliant on any particular physical model, it is both efficient and portable.

The authors’ machine learning approach features two key techniques; principal component analysis (PCA) and random forests. The idea of PCA is to break large, multi-dimensional datasets into their principal components (amo, amare, amavi, amatum); these are a series of orthonormal basis vectors such that each vector points in a direction of maximal variance. This is analogous to solving for eigenvectors, and the data processing can be thought of as a change of basis. PCA is extremely useful for machine learning because it structures the data in a way that best highlights relevant features (while discarding those that are redundant/irrelevant). This improves the learning capability and efficiency of ML techniques. The authors use a random forest of decision tree classifiers to classify the processed data (i.e. the data after having been transformed via PCA). In a decision tree, the dataset is recursively partitioned until each subset corresponds to a specific class or category. Since decision trees are quite unwieldy and prone to overfitting, it is often beneficial to train several thousand at once (i.e. a random forest). Given an input corresponding to a region of X-ray emission, the goal is to output the number of unique thermal components. The authors create the training data using synthetic X-ray spectra based on observations taken from the Chandra observatory. ...

• arXiv.org > astro-ph > arXiv:2009.00643 > 01 Sep 2020