Tag Archives: #blackhole

Whats The Importance Of Black Hole Repositioning? (Cosmology)

Based on a suite of smoothed particle hydrodynamics simulations with the swift code and a Bondi-Hoyle-Lyttleton subgrid gas accretion model, Yannick Bahe and colleagues investigated the impact of repositioning on SMBH growth and on other baryonic components through AGN feedback. They found that repositioning has direct physical consequences such as it promotes SMBH mergers and thus accelerates their initial growth. In addition, it raises the peak density of the ambient gas and reduces the SMBH velocity relative to it, giving a combined boost to the accretion rate that can reach many orders of magnitude. Their study recently appeared in Arxiv.

Energy feedback from active galactic nuclei (AGN) that are powered by supermassive black holes (SMBHs) at the centres of massive galaxies is an essential ingredient of galaxy formation simulations. The orbital evolution of SMBHs is affected by dynamical friction that cannot be predicted self-consistently by contemporary simulations of galaxy formation in representative volumes. Instead, such simulations typically use a simple “repositioning” of SMBHs, but the effects of this approach on SMBH and galaxy properties have not yet been investigated systematically.

Now, Yannick Bahe and colleagues investigated the impact of repositioning on SMBH growth by using hydrodynamical simulations and a Bondi-Hoyle-Lyttleton subgrid gas accretion model.

They showed that, across at least a factor ∼1000 in mass resolution, SMBH repositioning (or an equivalent approach) is a necessary prerequisite for AGN feedback; without it, black hole growth is negligible. They also showed that, limiting the effective repositioning speed to ≲ 10 km s¯1 delays the onset of AGN feedback and severely limits its impact on stellar mass growth in the centre of massive galaxies.

Projected SMBH positions (circles, colour-coded by 𝑚BH) are plotted over the corresponding gas density-temperature map (brightness representing surface density and hue temperature, increasing from pink to yellow) of the most massive 𝑧 = 0 halo (𝑀200c = 1.7 × 10¹³ M) in the 25 Mpc eagle-resolution simulations with the Default (left) and NoRepositioning (centre) models, respectively. © Yannick Bahe et al.

Finally, they shed light on three mechanisms through which repositioning affects SMBH growth, and hence AGN feedback. Firstly, it enables SMBH mergers – which lead to higher SMBH masses and hence increase the Bondi-Hoyle-Lyttleton accretion rate. Secondly, it moves SMBHs to regions of higher gas density, by up to several orders of magnitude. Thirdly, it (indirectly) slows SMBHs down by an order of magnitude with respect to their ambient gas.

“Our results suggest that a more sophisticated and/or better calibrated treatment of SMBH repositioning is a critical step towards more predictive galaxy formation simulations.”, they conclude.


Reference: Yannick M. Bahé, Joop Schaye, Matthieu Schaller, Richard G. Bower, Josh Borrow, Evgenii Chaikin, Folkert Nobels, Sylvia Ploeckinger, “The importance of black hole repositioning for galaxy formation simulations”, Arxiv, 2021. https://arxiv.org/abs/2109.01489


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What Is The Structure & Properties Of Accretion Disk Around A Rotating Black Hole Without Reflection Symmetry? (Cosmology)

By adopting a theory-agnostic approach and considering a recently proposed Kerr-like black hole model, Chen and Yang investigated the structure and properties of accretion disk around a rotating black hole without reflection symmetry. They showed that, in the absence of reflection symmetry, the accretion disk is curved surface in shape. Furthermore, they found that the parameter ϵ would shrink the size of the innermost stable circular orbits (ISCO), and enhance the efficiency of the black hole in converting rest-mass energy to radiation during accretion. Their study recently appeared in Arxiv.

Rotating black holes without equatorial reflection symmetry i.e. Z2 symmetry, can naturally arise in effective low-energy theories of fundamental quantum gravity, in particular, when parity-violating interactions are introduced. Due to the complexity of the theories, the rotating black hole solutions do not have analytic expressions and they can only be studied using numerical or perturbative approaches.

Now, Chen and Yang adopted a theory-agnostic approach and considered a relatively simple Kerr-like black hole model to investigate the astrophysical implications of Z2 asymmetry.

“The Kerr-like metric we considered is relatively simple in that its geodesic equations are designed to be completely separable. Therefore, the metric can be a good approximation of those complicated solutions in effective theories and can be very useful in studying the astrophysical implications of reflection asymmetry in a phenomenological manner.”

At first, they investigated the properties of accretion disk around the Kerr-like black hole and found that, in the absence of reflection symmetry, the accretion disk is curved surface in shape, rather than a flat disk lying on the equatorial plane.

They also explored the astrophysical implications of Z2 asymmetry on the accretion disk properties around a Kerr-like black hole. In particular, they found that, in a toy model with a specific choice of the deviation function, the parameter ϵ would shrink the size of the innermost stable circular orbits (ISCO), and enhance the efficiency of the black hole in converting rest-mass energy to radiation during accretion.

Finally, they investigated the gravitational redshift effect and computed the g-factor associated with the emission coming from the ISCO in the Kerr-like spacetime. They suggested that the spin measurements based on the redshift g-factor observations should be analyzed with great care and assuming the Kerr hypothesis, such measurements could overestimate the true spin value of the black hole if the black hole is actually the Kerr-like one with a large deviation parameter |ϵ| (at a level of ∼ 16% if |ϵ| ∼ 17).

“There are other important observables of the accretion disk that we have not explored in this study, such as the disk temperature and luminosity. In the cases in which the spacetime possesses equatorial reflection symmetry, one can adopt the thin-disk model and the calculations of these observables can be quite straightforward. However, in the absence of reflection symmetry, the disk is a curved surface and one has to rebuild the corresponding curved thin-disk model. This is beyond the scope of the present paper. In addition, it will be interesting to investigate the detailed motions of particles after they cross the ISCO and enter the plunging phase. All the above explorations would give further insights into the fundamental differences between Kerr and Kerr-like black holes, and possibly their observable signatures. We will leave these interesting topics to future works.”, they conclude.


Reference: Che-Yu Chen, Hsiang-Yi Karen Yang, “Curved accretion disks around rotating black holes without reflection symmetry”, Arxiv, pp. 1-20, 2021. https://arxiv.org/abs/2109.00564


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V404 Cygni: Huge Rings Around A Black Hole (Cosmology)

  • Astronomers spotted an unusual set of rings in X-rays around a black hole with a companion star.
  • These rings are created by light echoes, a phenomenon similar to echoes on Earth from sound waves bouncing off hard surfaces.
  • NASA’s Chandra X-ray Observatory and Neil Gehrels Swift Observatory were used to detect X-rays ricocheting off dust clouds.
  • The rings provide information about the black hole, its companion, and the intervening dust clouds.

This image features a spectacular set of rings around a black hole, captured using NASA’s Chandra X-ray Observatory and Neil Gehrels Swift Observatory. The X-ray images of the giant rings reveal information about dust located in our galaxy, using a similar principle to the X-rays performed in doctor’s offices and airports.

The black hole is part of a binary system called V404 Cygni, located about 7,800 light years away from Earth. The black hole is actively pulling material away from a companion star — with about half the mass of the Sun — into a disk around the invisible object. This material glows in X-rays, so astronomers refer to these systems as “X-ray binaries.”

On June 5, 2015, Swift discovered a burst of X-rays from V404 Cygni. The burst created the high-energy rings from a phenomenon known as light echoes. Instead of sound waves bouncing off a canyon wall, the light echoes around V404 Cygni were produced when a burst of X-rays from the black hole system bounced off of dust clouds between V404 Cygni and Earth. Cosmic dust is not like household dust but is more like smoke, and consists of tiny, solid particles.

In this composite image, X-rays from Chandra (light blue) were combined with optical data from the Pan-STARRS telescope in Hawaii that show the stars in the field of view. The image contains eight separate concentric rings. Each ring is created by X-rays from V404 Cygni flares observed in 2015 that reflect off different dust clouds. (An artist’s illustration explains how the rings seen by Chandra and Swift were produced. To simplify the graphic, the illustration shows only four rings instead of eight.)

V404 Cygni Rings
V404 Cygni Rings (Credit: X-ray: NASA/CXC/U.Wisc-Madison/S. Heinz et al.; Optical/IR: Pan-STARRS)

A team of researchers led by Sebastian Heinz of the University of Wisconsin in Madison analyzed 50 Swift observations of the system made in 2015 between June 30 and August 25, and Chandra observations made on July 11 and 25, 2015. It was such a bright event that the operators of Chandra purposely placed V404 Cygni in between the detectors so that another bright burst would not damage the instrument.

The rings tell astronomers not only about the black hole’s behavior, but also about the landscape between V404 Cygni and Earth. For example, the diameter of the rings in X-rays reveals the distances to the intervening dust clouds the light ricocheted off. If the cloud is closer to Earth, the ring appears to be larger, and vice versa. The light echoes appear as narrow rings rather than wide rings or haloes because the X-ray burst lasted only a relatively short period of time.

V404 Cygni Rings
Illustration showing how the rings seen by Chandra & Swift were produced © chandra

The researchers also used the rings to probe the properties of the dust clouds themselves. They compared the X-ray spectra — that is, the brightness of X-rays over a range of wavelengths — to computer models of dust with different compositions. Different compositions of dust will result in different amounts of the lower energy X-rays being absorbed and prevented from being detected with Chandra. This is a similar principle to how different parts of our body or our luggage absorb different amounts of X-rays, giving information about their structure and composition.

The team determined that the dust most likely contains mixtures of graphite and silicate grains. In addition, by analyzing the inner rings with Chandra, they found that the densities of the dust clouds are not uniform in all directions. Previous studies have assumed that they did not.

A paper describing the V404 Cygni results was published in the July 1, 2016, issue of The Astrophysical Journal (preprint). The authors of the study are Sebastian Heinz, Lia Corrales (University of Michigan); Randall Smith (Center for Astrophysics | Harvard & Smithsonian); Niel Brandt (The Pennsylvania State University); Peter Jonker (Netherlands Institute for Space Research); Richard Plotkin (University of Nevada, Reno); and Joey Neilson (Villanova University).

This result is related to a similar finding of the X-ray binary Circinus X-1, which contains a neutron star rather than a black hole, published in a paper in the June 20, 2015, issue of The Astrophysical Journal, titled, “Lord of the Rings: A Kinematic Distance to Circinus X-1 from a Giant X-Ray Light Echo” (preprint). This study was also led by Sebastian Heinz.

There have been multiple papers published every year reporting studies of the V404 Cygni outburst in 2015 that caused these rings. Previous outbursts were recorded in 1938, 1956 and 1989, so astronomers may still have many years to continue analyzing the 2015 one.

NASA’s Marshall Space Flight Center manages the Chandra program. The Smithsonian Astrophysical Observatory’s Chandra X-ray Center controls science from Cambridge, Massachusetts, and flight operations from Burlington, Massachusetts.


Reference: S. Heinz et al, A Joint Chandra and Swift View of the 2015 X-Ray Dust Scattering Echo of V404 Cygni, The Astrophysical Journal (2016). DOI: 10.3847/0004-637X/825/1/15 , arxiv.org/abs/1605.01648


Provided by Chandra X-ray Center

Astronomers Discover How To Feed A Black Hole (Cosmology)

The black holes at the centres of galaxies are the most mysterious objects in the Universe, not only because of the huge quantities of material within them, millions of times the mass of the Sun, but because of the incredibly dense concentration of matter in a volume no bigger than that of our Solar System. When they capture matter from their surroundings they become active, and can send out enormous quantities of energy from the capture process, although it is not easy to detect the black hole during these capture episodes, which are not frequent.

However, a study led by the researcher Almudena Prieto, of the Instituto de Astrofísica de Canarias (IAC), has discovered long narrow dust filaments which surround and feed these black holes in the centres of galaxies, and which could be the natural cause of the darkening of the centres of many galaxies when their nuclear black holes are active. The results of this study have recently been published in the journal Monthly Notices of the Royal Astronomical Society (MNRAS).

Using images from the Hubble Space Telescope, the Very Large Telescope (VLT) at the European Southern Observatory (ESO), and the Atacama Large Millimetre Array (ALMA) in Chile, the scientists have been able to obtain a direct visualization of the process of nuclear feeding of a black hole in the galaxy NGC 1566 by these filaments. The combined images show a snapshot in which one can see how the dust filaments separate, and then go directly towards the centre of the galaxy, where they circulate and rotate in a spiral around the black hole before being swallowed by it.

“This group of telescopes has given us a completely new perspective of a supermassive black hole, thanks to the imaging at high angular resolution and the panoramic visualization of its surroundings, because it lets us follow the disappearance of the dust filaments as they fall into the black hole”, explains Almudena Prieto, the first author on the paper.

The study is the result of the long-term PARSEC project of the IAC, which aims to understand how supermassive black holes wake up from their long lives of hibernation, and after a process in which they accrete material from their surroundings, they become the most powerful objects in the Universe.

Part of this work was carried out within the Master’s thesis in Astrophysics of the University of La Laguna of Jakub Nadolny, carried out at the IAC within the PARSEC project. Researchers Mar Mezcua and Juan A. Fernández Ontiveros were also advisers to this work, while they had PARSEC postdoctoral contracts at the IAC.

Featured image: The image shows the process of nuclear feeding of a black hole in the galaxy NGC 1566, and how the dust filaments, which surround the active nucleus, are trapped and rotate in a spiral around the black hole until it swallows them. Credit: ESO.


Article: M. Almudena Prieto, Jakub Nadolny, Juan A. Fernández-Ontiveros, Mar Mezcua. “Dust in the central parsecs of unobscured AGN: more challenges to the torus”. Monthly Notices of the Royal Astronomical Society, July 8, 2021. DOI: https://doi.org/10.1093/mnras/stab1704


Provided by IAC

Flashes Of Light From The Hidden Side Of The Black Hole (Cosmology)

Observing the supermassive black hole at the heart of I Zwicky 1 – a galaxy with an active core 876 million light-years from us – a team of astronomers led by Dan Wilkins of Stanford University has noticed X-photon reverberations from all regions. of the accretion disk, including those “behind” the black hole. We talk about it with Elisa Costantini, astrophysicist at the Dutch space research institute Sron and co-author of the article published today in Nature

Looking the other side of the coin? Now it also applies to black holes, at least if we want to explain some of the behaviors of light in their accretion disk . The analysis of the X-ray emission coming from the supermassive black hole in the galaxy I Zwicky 1 highlighted the reflection of photons – visible as flashes of light – from the “hidden” side of the black hole. The variation in energy of these photons makes their origin evident in relation to space. Photons reflected from the opposite side of the accretion disk are “bent” around the black hole and magnified by the intense gravitational field. The study , published today in Nature, was led by a team of scientists from the universities of Stanford and Penn State in the US, Saint Mary in Canada and Sron, the Netherlands’ space research institute. Elisa Costantini , born in Rimini, graduated in Bologna and with a PhD in extraterrestrial physics, obtained in 2004 at the Max Planck Institute, in Germany , talks about it . After a post doc at the University of Utrecht, in the Netherlands, since 2008 she has been an astronomer associated with Sron.

First of all, let us guide us in the geography of a black hole: event horizon, accretion disk, crown.

“A black hole is an object about a hundred million times the mass of the Sun, compressed into a relatively small space. Suffice it to say that a black hole that has the same mass as the Sun would have a diameter of six kilometers. The event horizon is what defines the virtual surface of the black hole (the black hole does not have a solid surface) and is in fact the horizon beyond which not even light can escape its gravity – hence the black hole name. If the black hole is active, then it is constantly absorbing gas from its surroundings. This gas spirals around the black hole forming a disk. This phase of growth lasts a long time, about ten million years. In all this time the matter continues to grow, compact, heat up enormously, especially near the black hole. Such temperatures are reached – we are talking about millions of Kelvin degrees – that they ionize the gas, therefore they separate the electrons from the atom. These free electrons form a structure, which we call the corona, above the disk. We can sum it up by saying that there is an accretion disk around the black hole and a corona of electrons hovering above ».

Now that we know how to proceed, can you explain to us what your study revealed and what is the importance of the results achieved?

“In this study we have interpreted the change in brightness, as a function of time, of an active galactic nucleus, thus a galaxy hosting a supermassive black hole. The fact that an accretion disk varies in brightness is not an unusual fact, indeed, and in most cases it is not possible to give a certain explanation for these phenomena. So let’s say they are random, unpredictable. But this time we saw flashes of light in the X-band followed by others, weaker, detected at different energies. And this thing repeated itself twice: we had a flash and immediately after another with the same substructures, and it cannot be accidental. So, using a theoretical model that explains how photons behave in the presence of a black hole, we came to the conclusion that the secondary flashes were nothing more than photons reflected from behind the black hole. The light curves, therefore the variations in brightness as a function of time,

X-rays are emitted by a corona of energetic particles around the black hole. Some of these rays reach the observer directly, but others (red lines) illuminate the internal regions of the accretion disk (in orange) and their reflection from the disk (blue lines) is observed. The strong bending of light in the gravitational field around the black hole focuses the rays towards the black hole and on the inner regions of the disk. The rays reflected from the back of the disk can be bent around the black hole (in rotation), allowing X-rays to re-emerge from regions of the disk that would otherwise be hidden behind the black hole. Credits: DR Wilkins et al., Nature, 2021

How do you distinguish the photons arriving from one side of the disk and the other?

“The model tells us. The photons come from an emission line – the so-called iron line – which is produced at a certain energy and is tracer of the disc. Applying classical mechanics, the Doppler effect and general relativity to the model, we see that as a function of time the emission line can have a certain profile that can be found in the light curve ».

And the observation of these photons slightly “bent” around the black hole is in agreement with general relativity?

«Yes, because we started from the prescriptions given by general relativity. So we assumed that the environment around the black hole behaved this way. The data are not inconsistent with the theory. However, this is only a first step. As mentioned, this new methodology can be applied to many cases, extending and confirming the results ».

Where is the galaxy you studied, I Zwicky 1, located? And why did you choose to study his de black hole?

“It’s in the constellation of Pisces, and it’s 876 million light-years away from us. This galaxy is interesting from various points of view. Over the past fifteen years my collaborators and I have observed it three times with dedicated pointings of the ESA’s X-telescope, Xmm-Newton, and have discovered that its accretion disk emits a wind of gas traveling at more than 1000 km / if it behaves as a function of time in a peculiar way, unlike other objects we know. It is a galaxy that we were exploring even before the study started ».

Did the phenomenon you observed surprise you or was it your purpose from the start?

“The first author, Dan Wilkins of Stanford University, had started with a classical approach to studying the temporal characteristics of this black hole. As mentioned before, the fact of being able to interpret a phenomenon that in general we consider to be random or in any case an overlap of so many phenomena that it cannot be fully understood, was a real surprise ».

Featured image: Elisa Costantini, astronomer at Sron, the Netherlands space research institute, co-author of the study published today in Nature © INAF


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Astrophysicists Detected Light From Behind A Black Hole (Cosmology)

Watching X-rays flung out into the universe by the supermassive black hole at the center of a galaxy 800 million light-years away, Stanford University astrophysicist Dan Wilkins noticed an intriguing pattern. He observed a series of bright flares of X-rays—exciting, but not unprecedented—and then, the telescopes recorded something unexpected: additional flashes of X-rays that were smaller, later and of different “colors” than the bright flares.

According to theory, these luminous echoes were consistent with X-rays reflected from behind the black hole—but even a basic understanding of black holes tells us that is a strange place for light to come from.

“Any light that goes into that black hole doesn’t come out, so we shouldn’t be able to see anything that’s behind the black hole,” said Wilkins, who is a research scientist at the Kavli Institute for Particle Astrophysics and Cosmology at Stanford and SLAC National Accelerator Laboratory. It is another strange characteristic of the black hole, however, that makes this observation possible. “The reason we can see that is because that black hole is warping space, bending light and twisting magnetic fields around itself,” Wilkins explained.

The strange discovery, detailed in a paper published July 28 in Nature, is the first direct observation of light from behind a black hole—a scenario that was predicted by Einstein’s theory of general relativity but never confirmed, until now.

“Fifty years ago, when astrophysicists starting speculating about how the magnetic field might behave close to a black hole, they had no idea that one day we might have the techniques to observe this directly and see Einstein’s general theory of relativity in action,” said Roger Blandford, a co-author of the paper who is the Luke Blossom Professor in the School of Humanities and Sciences and Stanford and SLAC professor of physics and particle physics.

How to see a black hole

The original motivation behind this research was to learn more about a mysterious feature of certain black holes, called a corona. Material falling into a supermassive black hole powers the brightest continuous sources of light in the universe, and as it does so, forms a corona around the black hole. This light—which is X-ray light—can be analyzed to map and characterize a black hole.

The leading theory for what a corona is starts with gas sliding into the black hole where it superheats to millions of degrees. At that temperature, electrons separate from atoms, creating a magnetized plasma. Caught up in the powerful spin of the black hole, the magnetic field arcs so high above the black hole, and twirls about itself so much, that it eventually breaks altogether—a situation so reminiscent of what happens around our own Sun that it borrowed the name “corona.”

“This magnetic field getting tied up and then snapping close to the black hole heats everything around it and produces these high energy electrons that then go on to produce the X-rays,” said Wilkins.

As Wilkins took a closer look to investigate the origin of the flares, he saw a series of smaller flashes. These, the researchers determined, are the same X-ray flares but reflected from the back of the disk—a first glimpse at the far side of a black hole.

“I’ve been building theoretical predictions of how these echoes appear to us for a few years,” said Wilkins. “I’d already seen them in the theory I’ve been developing, so once I saw them in the telescope observations, I could figure out the connection.”

Future observations

The mission to characterize and understand coronas continues and will require more observation. Part of that future will be the European Space Agency’s X-ray observatory, Athena (Advanced Telescope for High-ENergy Astrophysics). As a member of the lab of Steve Allen, professor of physics at Stanford and of particle physics and astrophysics at SLAC, Wilkins is helping to develop part of the Wide Field Imager detector for Athena.

“It’s got a much bigger mirror than we’ve ever had on an X-ray telescope and it’s going to let us get higher resolution looks in much shorter observation times,” said Wilkins. “So, the picture we are starting to get from the data at the moment is going to become much clearer with these new observatories.”

Co-authors of this research are from Saint Mary’s University (Canada), Netherlands Institute for Space Research (SRON), University of Amsterdam and The Pennsylvania State University.


Reference: Light bending and X-ray echoes from behind a supermassive black hole, Nature (2021). DOI: 10.1038/s41586-021-03667-0 , www.nature.com/articles/s41586-021-03667-0


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Watching the Milky Way’s Supermassive Black Hole Feed (Cosmology)

The supermassive black hole at the center of our Milky Way galaxy, Sagittarius A* (“SgrA*), is by far the closest such object to us, about 27 thousand light-years away. Although it is not nearly so active or luminous as other galactic nuclei with supermassive black holes, its relative proximity makes it appear much brighter to us than other similar sources and provides astronomers with a unique opportunity to probe what happens when gas clouds or other objects get close to the “edge” of a black hole.

Sgr A* has been monitored at radio wavelengths since its discovery in the 1950’s; variability was first reported in the radio in 1984. Astronomers model that on average Sgr A* is accreting material at a few hundredths of an Earth-mass per year, a relatively very low rate. Subsequent infrared, submillimeter, and X-ray observations confirmed this variability but also discovered that the object often flares, with the brightness thereby increasing by as much as a factor of one hundred in X-rays. Most of the steady emission is thought to be produced by electrons spiraling at close to the speed of light (called relativistic motion) around magnetic fields in a small region only about an astronomical unit in radius around the source, but there is no agreement on the mechanism(s) powering the flares.

CfA astronomers Giovanni Fazio, Mark Gurwell, Joe Hora, Howard Smith, and Steve Willner were members of a large consortium that in July 2019 obtained simultaneous near infrared observations with the IRAC camera on Spitzer, with the GRAVITY interferometer at the European Southern Observatory, and with NASA’s Chandra and NuStar X-ray observatories (scheduled simultaneous observations with the Submillimeter Array were prevented by the Mauna Kea closure). SgrA* serendipitously underwent a major flaring event during these observations, enabling theoreticians for the first time to model a flare in considerable detail.

Relativistic electrons moving in magnetic fields emit photons by a process known as synchrotron radiation (the most conventional scenario) but there is also a second process possible in which photons (produced either by synchrotron emission or by other sources like dust emission) are scattered off the electrons and thereby acquire additional energy, becoming X-ray photons. Modeling which combination of effects was operative in the small region around SgrA* during the flaring event offers insights into the densities of the gas, the fields, and the origin of the flare’s intensity, timing, and shape. The scientists considered a variety of possibilities and concluded that the most probable scenario is the one in which the infrared flare was produced by the first process but with the X-ray flare produced by the second process. This conclusion has several implications for the activity around this supermassive black hole, including that the electron densities and magnetic fields are comparable in magnitude to those under average conditions but that sustained particle acceleration is required to maintain the observed flare. Although the models successfully match many aspects of the flare emission, the measurements are not able to constrain the detailed physics behind the particle acceleration; these are left to future research.

Featured image: A three-color image of the central regions of the Milky Way showing the location of Sagittarius A*, the galactic center’s supermassive blackhole; X-ray in blue, optical in yellow, and infrared in red. Astronomers have obtained simultaneous mulit-band observations of a bright flare from SgrA* and modeled the mult-band radiation to estimate properties of the accretion around the black hole.X-ray: NASA/CXC/UMass/D. Wang et al.; Optical: NASA/ESA/STScI/D.Wang et al.; IR: NASA/JPL-Caltech/SSC/S.Stolovy


Reference: “Constraining Particle Acceleration in Sgr A* With Simultaneous GRAVITY, Spitzer, NuSTAR and Chandra Observations,” GRAVITY Collaboration: R. Abuter et al. Astronomy & Astrophysics.


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Which Formation Mechanism Show Strong Clustering of Primordial Black Holes? (Cosmology)

Masahiro Kawasaki and colleagues investigated the clustering of primordial black holes (PBHs) formed by Affleck-Dine (AD) baryogenesis. They found that formed PBHs showed strong clustering due to stochastic dynamics of the AD field. Their study recently appeared in Arxiv.

In recent years, the LIGO-Virgo collaboration has detected gravitational waves emitted by merging binary black holes, which revealed the existence of black holes with masses ∼ 10−100 M. Interestingly, many of observed black holes have heavy masses around 30M. The origin of these massive black holes is still unknown. One fascinating candidate is the primordial origin.

We have already discussed number of possible mechanisms which give rise to primordial black holes such as, by solo-multi bumps, by first-order phase transition, by fall of inflation, by fifth force etc. One of the PBHs formation scenario we haven’t discussed yet is Affleck-Dine (AD) baryogenesis. In this scenario, the IR mode of the AD field diffuses by quantum fluctuations during inflation and has multiple vacua just after inflation. Then, while the origin of the AD field becomes the true vacuum, the false vacuum has a non-zero field value. The inhomogeneity of the field value results in the inhomogeneous baryogenesis, which forms baryon-rich bubbles. At the QCD phase transition, baryons in the bubbles form massive nucleons and generate density fluctuations. If the bubbles are large enough, the density fluctuations grow sufficiently and then collapse into PBHs at the horizon reentry. PBHs generated in this scenario can have masses larger than 10 M and are expected to explain the origin of LIGO-Virgo events.

Kawasaki and colleagues investigated the clustering of PBH’s formed by this scenario. They have studied the stochastic dynamics of the AD field during inflation and derived the PBH formation rate. They have also estimated the two-point correlation function of PBHs, which characterized the clustering of PBH’s.

They found that, formed PBHs show strong clustering due to stochastic dynamics of the AD field. They have also obtained a reduced PBH correlation function given below:

They used this reduced PBH correlation function to investigate the effect of the clustering on two phenomena related to PBHs; isocurvature fluctuations and the merger rate distribution.

First, PBHs induce isocurvature fluctuations due to their Poisson fluctuations, and the clustering also sources isocurvature fluctuations. They have estimated the power spectrum of density fluctuation of PBHs (as shown in fig. 2 below) and have put the upper bound on the PBH abundance and the significance of clustering by using the current isocurvature constraints from the Planck satellite as shown in Fig. 3 below.

Second, the clustering of PBHs can drastically change the binary formation rate of PBHs, and the resultant merger rate density. They have found that the merger rate increases with the clustering for a small PBH abundance due to the enhanced binary formation rate, while it decreases for a large PBH abundance since the three-body problem occurs more frequently for the clustered PBHs. As a result, it was found that it is difficult for their model to explain the LIGO-Virgo event rate of binary mergers when they conservatively neglect the binary merger in three-body systems.

“In future work, we will see whether our model can explain the merger rate observed by LIGO-VIRGO collaboration by correctly including PBH mergers in three-body systems.”

— concluded authors of the study

All images credit except featured: Masahiro Kawasaki et al.


Reference: Masahiro Kawasaki, Kai Murai, Hiromasa Nakatsuka, “Strong clustering of primordial black holes from Affleck-Dine mechanism”, Arxiv, pp. 1-18, 2021. e-Print: ArXiv: 2107.03580


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The Fantastic Story of M87’s Black Hole Photo (Astronomy)

Radboud University astrophysicist Heino Falcke, among the protagonists of the Event Horizon Telescope project and the first image of a black hole, has reconstructed the milestones of that historic milestone in the best-seller “The enigma of black holes. Discovering the universe and human nature ”, now also available in Italian bookstores. Corrado Ruscica interviewed him for us

On the morning of April 10, 2019, millions of people from all over the world followed the press conference live in which Heino Falcke , professor of astrophysics at Radboud University in Nijmegen and guest-professor at the Max Planck Institute for Radio Astronomy in Bonn, presented the first “Photo” of a black hole , the one at the center of the galaxy M87 – an image that has now become iconic. A grandiose scientific result made possible thanks to a titanic enterprise, born from an idea of ​​Falcke himself, which made it possible to coordinate eight observatories by virtually creating a single radio telescope the size of the Earth: Eht, the Event Horizon Telescope. A historic milestone whose fundamental stages are told by Falcke in the book The enigma of black holes. Discovering the universe and human nature , a best-seller now also available in Italy, published by Mondadori.

Traveling on an adventurous journey through space and time, from the study of black holes to the still unsolved mysteries of the universe, the author delves into an elusive and exciting enigma, questioning the doubts and certainties of science without neglecting a reference to the wedding ring. A surprising virtual itinerary, in which scientific rigor is combined with simple and fluent writing, vibrant with wonder and poetry. A path that is at the same time the dream of a man with his gaze turned towards the firmament and that makes us rediscover the fundamental questions with which we have always “knocked hard on the gates of heaven”. Media Inaf interviewed him.

Professor Falcke, what was your reaction when you first saw the image of the ring of light?

“It was a moment of awe, wonder and fear. Amazement, because we were looking at an exotic world never seen before until a few moments ago. Wonder and gratitude that it really worked and fear that we could have been misled. It sounded too good to be true. The image was surprisingly familiar. A bit like the photo of your biggest, secret love that you’ve never seen in person but always had in mind. And now you are in front of “her” for the first time and you see that she is more beautiful than you imagined ».

What have we learned from this image?

“That these dark, supermassive monsters residing at the center of galaxies are indeed black holes, or at least they show up as our theories predict. We see the light literally disappearing into the darkness of the event horizon. We can also test general relativity with higher accuracy than we are able to do within the solar system. Einstein’s theory works very well when applied to stellar-sized black holes , which we can “hear” through gravitational wave detectors such as Ligo and Virgo, but also to supermassive black holes., which have a mass 100 million times higher than the solar one and which we can observe with Eht. This is what we call the scale invariance of general relativity, which holds over a truly impressive wide range of values ​​».

Did you expect to find something different?

“Indeed, yes. The black hole’s shadow could be much smaller, the gas could hide the shadow, and even the predictions of general relativity could be wrong. We have never observed this region of space-time in detail. Eventually, hopefully things may turn out differently, because you want to be surprised. But to be the start, it’s comforting that things went as planned. Apparently, we did everything well but we will continue to look for any deviations from what we expect ».

Astrophysicist Heino Falcke with the English edition of his book, “Light in the Darkness: Black Holes, the Universe, and Us” © INAF

Eht was his idea. How did you manage to set up this great group of researchers?

“I was the first to point out that we could have seen the shadow of a black hole at certain radio frequencies by virtually building a telescope the size of the Earth. This goes back 25 years. Ultimately, it was a joint effort by several parties, growing over the years. But to do this you have to convince others that it is a good idea, then you have to create a group that wants to reach the goal together, wait for the radio telescopes to be built, have funds for the experiment and finally be sure not to stay. cut off. Science is also competition. Americans have a lot of financial resources, telescopes and excellent researchers. They definitely pursue their goals and require leadership. As Europeans, I believe we need to be able to have our say. For this project we have had excellent collaboration from excellent radio astronomy institutes in Germany, Italy and other European countries, and in addition we have received great support from the European Research Council. This time we acted as a team ».

Why do you think the world stopped that day and millions of people were thrilled to see the image of the black hole?

“Black holes go beyond science. They are modern, mythological objects that arouse mystery. They create a morbid fascination of death and destruction. They confront us with the limits of knowledge. And then they represent an exotic world so different from ours. “Seeing them” for the first time is like discovering a new continent ».

Why is it so important to study black holes?

“I believe the future of physics will be decided on the far edge of black holes. The problem of reconciling the two major theories of the universe, on the one hand quantum physics which describes infinitely small objects, and general relativity, which describes the largest objects and the entire universe, is still a mystery today. It is precisely in the vicinity of black holes that the discrepancy between the two theories becomes more evident ».

He said physics is on the verge of a paradigm shift. What exactly do you mean?

“The past century was the century of particle physics. The century we live in could be that of the physics of space-time. For the first time, we have experiments – gravitational wave detectors and the Event Horizon Telescope – that allow us to study physical phenomena at the extreme edge of the event horizon. We now have real data. While Einstein’s theory will not be overturned in some time, the ability to evaluate new theories alone represents enormous creative potential for scholars. Perhaps all of this is inspiring a new Einstein somewhere in the world, or a new Einstein has already been born when we showed the image of the black hole. “

Do you think Einstein and his theory will triumph forever?

“Of course, Einstein’s theory will still stand, but it will have to be changed in some regimes. This is similar to Newton’s theory of gravity, which describes physical phenomena within the solar system very well. But when real precision measurements are made, such as the orbit of Mercury, only then do we discover that a better description is needed. The same thing will happen with Einstein’s theory. If we go into the early epochs of cosmic history, if we look at the expansion of the universe or if we explore the event horizon of black holes, we will eventually need a more complete theory. “

What do you expect for the near future?

“We took the first step and showed the feasibility of the experiment, but science is made up of long paths. If we can make more images, we can reduce uncertainties by a factor of 3-10 over the next decade. To do this, it will take hard work and a lot of patience. It is necessary to optimize the set of radio telescopes and build one or more of them in Africa. Then, the next step will be to go into space to virtually make an antenna larger than the size of the Earth. It is not a crazy idea and we are working on it ».

Featured image: “The enigma of black holes. Discovering the universe and human nature “. By Heino Falcke and Jorg Romer. Mondadori, 2021. 336 pages, 22 euros © INAF


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