Astronomy

Causes of emission of accretion disk

Causes of emission of accretion disk


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As far as I can see, there are two main sources for the emission of energy from an accretion disk:

  • release of gravitational potential energy of the infalling matter;
  • friction from differential rotation within the disk.

How much of the emitted energy is due to each process? For that matter, are there any other processes that contribute to the spectrum?


What happens in the accretion disk?

An accretion disk is a structure (often a circumstellar disk) formed by diffuse material in orbital motion around a massive central body. Friction causes orbiting material in the disk to spiral inward towards the central body.

Furthermore, how hot is an accretion disk? Theory predicts that the gas flows to the hole in the form of an opaque, luminous disk, a so-called accretion disk (see figure 1), and its temperature is predicted to reach up to 10 million degrees.

Similarly, what is the accretion disk of a black hole?

Material, such as gas, dust and other stellar debris that has come close to a black hole but not quite fallen into it, forms a flattened band of spinning matter around the event horizon called the accretion disk (or disc).

How long do accretion disks last?

Accretion discs in gamma ray bursts (GRBs) The duration of GRB prompt emission can last from 0.01 - 2 seconds (short bursts) up to 2 - 500 seconds (long bursts) and may be explained by merging compact objects or failed supernovae (collapsars), respectively.


Basic Petrology

3.4.1.2 The Nebular Hypothesis

The nebular hypothesis is the most widely accepted model explaining the formation and evolution of the Solar System. It was first proposed in 1734 by Emanuel Swedenborg, a Swedish scientist with occupation as mining engineer, anatomist and astronomer. The hypothesis was originally applied only to our own Solar System. This method of planetary system formation is now thought to be at work throughout the universe. The nebular hypothesis postulates that the stars form in massive and dense clouds of molecular hydrogen—giant molecular clouds. They are gravitationally unstable, and matter coalesces to smaller and denser clumps within, which then proceed to collapse and form stars. Star formation is a complex process, which always produces a gaseous protoplanetary disk around the young star. This may give birth to planets in certain circumstances, which are not well known. Thus the formation of planetary systems is thought to be a natural result of star formation. A Sun-like star usually takes around 100 million years to form.

The protoplanetary disk is an accretion disk which continues to feed the central star. The disk is initially very hot and cools later in what are known as the “T Tauri Star (TTS)” stage by possible formation of small dust grains made of rocks and ices. The grains may eventually coagulate into kilometer-sized planetesimals. Planetesimals are solid objects thought to exist in protoplanetary disks and in debris disks. A protoplanetary disk is a rotating circumstellar disk of dense gas surrounding a young newly formed star, i.e. a TTS. If the disk is massive enough, the runaway accretions begin resulting in the rapid—100,000–300,000 years—formation of Moon- to Mars-sized planetary embryos. The planetary embryos undergo through a stage of violent mergers, producing a few terrestrial planets near the star. The last stage takes around 100 million–1,000 million years.

Star is a massive and luminous sphere of vast plasma held together by gravitational forces. Sun is the nearest star to the planet Earth and is the source of most of the energy on the planet. Stars are innumerable in number and can be seen glowing and twinkling far away in the night. Stars are grouped together forming constellations.

A planet is an astronomical or celestial object orbiting a star. Planet is massive enough to rotate in its own axis by its own gravity.

The Solar System consists of the Sun (Star) and its planetary system of eight, their moons formed 4,600 million years ago from the collapse of a giant cloud. The eight planets from nearest to the Sun outwards are Mercury, Venus, Earth, Mars (rocks and metals), Jupiter, Saturn (hydrogen and helium), Uranus, and Neptune (water–ammonia and methane). All planets rotate in almost circular orbits that lie within a nearly flat disk called the ecliptic plane.

Star, planets and solar system are originated from the same giant massive parent cloud and dust and complimentary to each other.


Similar states of activity identified in supermassive and stellar mass black holes

IMAGE: The figure illustrates how the population of active Seyfert-1 galaxies is typically dominated by the emission of the accretion disk ('soft' state), while the population of LINERs is much less. view more

Credit: Teo Muñoz Darias/Juan A. Fernández Ontiveros

The researchers Juan A. Fernández-Ontiveros, of the Istituto Nazionale di Astrofisica (INAF) in Rome and Teo Muñoz-Darias, of the Instituto de Astrofísica de Canarias (IAC), have written an article in which they describe the different states of activity of a large sample of supermassive black holes in the centres of galaxies. They have classified them using the behaviour of their closest "relations", the stellar mass black holes in X-ray binaries. The article has just been published in the journal Monthly Notices of the Royal Astronomical Society (MNRAS).

Black holes range in mass from objects which have only a few times the mass of the sun up to those with thousands of millions of solar masses. To understand their activity cycles from a global perspective has been the object of research for decades. Those of stellar mass are found in binary systems together with a companion star from which they suck out the gas which they need to sustain their activity, while the supermassive variety are found in the centres of the majority of galaxies and they feed on the gas, dust, and stars which are fall into the gravitational well of the galactic nucleus.

Stellar mass black holes evolve rapidly. Their activity cycles usually last a few months or years, during which they pass through different states, or phases. These are characterized by changes in the properties of their accretion discs (where the hot gas accumulates before falling into the black hole), their winds, and the jets of material which they produce. There are two principal states, the first dominated by the accretion disc, and the second by the jet. The 'soft' state is noted by the thermal emission by the plasma of the disc, while the jet is observed in the 'hard' state, when the disc cools down, and the emission at radio wavelengths becomes very intense.

Because they are much more massive, the supermassive black holes evolve much more slowly than their stellar mass equivalents. So, to show the presence of states and transitory phenomena in these would imply observing them for millions of years, because the changes during a human lifetime would be too small to measure. In addition, the nuclei of galaxies are regions with dense populations of stars, and the absorption of light by hydrogen and dust masks and hides the radiation from the accretion disc around the central black hole.

In this study Fernández-Ontiveros and Muñoz-Darias have used a sample of 167 active galaxies to be able to identify the possible accretions states of supermassive black holes with good statistics. The emission from the accretion disc cannot be detected directly, but the gas in the central region absorbs and processes the radiation in the form of spectral lines. Using the lines of oxygen and neon, which are observed in the mid-infrared, it is possible to test the presence of the disc in these object. "The study demonstrates the presence of accretion states in supermassive black holes, with properties very similar to those we know from stellar mass black holes, where the systems in the 'soft' state harbour a bright disc, and those in the 'hard' state show intense radio emission while the disc is very weak", explains Juan A. Fernández-Ontiveros, an INAF researcher who was trained at the IAC.

"This work opens a new window to understand the behaviour of material (gas) when it falls into black holes with a wide range of masses, and helps a more precise understanding of the activity cycles of the supermassive black holes which are in the centres of most galaxies", adds Teo Muñoz-Darias, a researcher at the IAC.

The figure illustrates how the population of active Seyfert-1 galaxies is typically dominated by the emission of the accretion disk ('soft' state), while the population of LINERs is much less luminous and is dominated by jets ('hard' state), which emit intensely in radio waves. The Seyfert-2 galaxies, on the other hand, do not show a homogeneous behaviour and while a good part behave in a similar way to the Seyfert-1, a large group of them are located in intermediate states. The latter are also observed in stellar black holes for short periods of time.

Article: Juan A. Fernández-Ontiveros & Teo Muñoz-Darias, "X-ray binary accretion states in Active Galactic Nuclei? Sensing the accretion disc of supermassive black holes with mid-infrared nebular lines". Monthly Notices of the Royal Astronomical Society, april 2021. DOI: https:/ / doi. org/ 10. 1093/ mnras/ stab1108

Disclaimer: AAAS and EurekAlert! are not responsible for the accuracy of news releases posted to EurekAlert! by contributing institutions or for the use of any information through the EurekAlert system.


Accretion Disks in Compact Stellar Systems

Accretion disks in compact stellar systems containing white dwarfs, neutron stars or black holes are the principal laboratory for understanding the role of accretion disks in a wide variety of environments from proto-stars to quasars. Recent work on disk instabilities and dynamics has given a new theoretical framework with which to study accretion disks. Modeling of time-dependent phenomena provides new insight into the causes and interpretation of photometric and spectroscopic variability and new constraints on the fundamental physical problem — the origin of viscosity in accretion disks. This book contains expert reviews on the nature of limit cycle thermal instabilities and a variety of closely related topics from the theory of angular momentum transport to eclipse mapping of the disk structure. The result is a comprehensive contemporary survey of the structure and evolution of accretion disks in compact binary systems.

  • Introduction to Accretion Disk Research (J Wood)
  • The Limit Cycle Instability in Dwarf Nova Accretion Disks (J K Cannizzo)
  • Angular Momentum Transport in Low Mass Accretion Disks (E T Vishniac & P H Diamond)
  • The Emission Lines from Accretion Disks in Cataclysmic Variable Stars (E L Robinson et al.)
  • Eclipse Mapping of Accretion Disks: The First Decade (K Horne)
  • Atmospheres of Accretion Disks and the Emerging Spectra (G Shaviv & R Wehrse)
  • Disks and Magnetospheres (A R King & J P Lasota)
  • Mass Loss and the Boundary Layer (J E Drew & W Kley)
  • The Interaction between the Stream and the Accretion Disk (M Livio)
  • Tidal Effects on Accretion Disks in Close Binary Systems (Y Osaki et al.)
  • Accretion Disks in Bright X-Ray Binaries (H Inoue)
  • X-Ray Illumination Models of Soft X-Ray Transients (J M Hameury et al.)
  • Black Hole Accretion Disk Instabilities (S Mineshige & M Kusunose)
FRONT MATTER
INTRODUCTION TO ACCRETION DISC RESEARCH

Books about accretion discs have been written before but none cover the subject in as much detail as is done here. This book takes the reader from the basic concepts of accretion discs to the most up-to-date work currently being carried out in the subject. Both observational and theoretical work are covered. As such, it is suitable as an introduction for those who have never worked in this area with many useful references to other reviews. It will also be extremely useful for astronomers who are working in the area. Each chapter concentrates on a different aspect of accretion discs and is written by an expert in each field. The reader is given many references with which they could pursue further any aspect of interest…

THE LIMIT CYCLE INSTABILITY IN DWARF NOVA ACCRETION DISKS

We review the accretion disk limit cycle mechanism within the context of dwarf nova accretion disks. We begin with a discussion of the basic physics behind the instability, and then proceed to an overview of the comparison between theory and observation.

ANGULAR MOMENTUM TRANSPORT IN LOW MASS ACCRETION DISKS

We consider the transport of angular momentum in accretion disks that are low mass, in the sense that the gravitational forces produced by the material in these disks has a negligible effect on disk dynamics. There is no established consensus on how this transport takes place. We note that for phenomenological reasons the traditional α model is probably not a good description of real disks. Here we briefly review a few of the more promising models. These include models in which angular momentum transport is driven by shocks, and by magnetic field instabilities. The latter is more promising, but requires a dynamo. We note that the direction of angular momentum transport due to convection in a conducting disk is not known as competing mechanisms are at work. We briefly discuss a number of possible dynamo mechanisms, and their problems. We then give a detailed exposition of the internal wave driven dynamo model, in which internal waves excited at large radii drive an α – Ω dynamo. The azimuthal magnetic field produced in this way is unstable to a magnetic shearing instability (MSI), which drives nearly isotropic turbulent eddys with typical fluid velocities of

VA , where VA is the local Alfvén speed. The scale of the eddys is

VA/Ω , where Ω is the local rotational frequency. This turbulence leads to a saturation of the dynamo when VA

(H/r) 2/3 cs , where H is the half-thickness of the disk, cs is the local sound speed, and r is the radial coordinate. This gives rise to an effective dimensionless viscosity coefficient

(H/r) 4/3 and vertical and radial diffusion coefficients which are

(H/r) 4/3 Hcs . The resulting vertical diffusion of entropy will have a substantial effect on detailed models of vertical structure in accretion disks. Viscous and thermal instabilities of very hot disks, those dominated by radiation pressure and electron scattering, are substantially moderated in this model. We note that the MSI largely suppresses the Parker instability in accretion disks.

THE EMISSION LINES FROM ACCRETION DISKS IN CATACLYSMIC VARIABLE STARS

This review discusses the emission lines from accretion disks in dwarf novae, anti-dwarf novae, UX UMa stars, and to a lesser extent classical novae. Doppler tomography and direct fits to time-averaged line profiles show that the distribution of the hydrogen line emission is peaked towards the center of the disk and can be approximated by a radial distribution f(R) α R -β with β

1.5 – 2.0. The Balmer decrement flattens somewhat towards the center. The equivalent widths of the emission lines are strongly correlated with the absolute visual magnitudes of the systems, becoming weaker as the systems become brighter. They are also correlated with orbital inclination, becoming greater as the orbital inclination increases. Models invoking optically-thin viscous accretion disks fail to reproduce the observed line strengths and their correlation with orbital inclination, and they are unable to account for the detailed behavior of the emission lines during the outbursts of dwarf novae. There is evidence that the emission lines are produced by irradiation of the disk by the boundary layer and the central star. Models invoking irradiation agree qualitatively with the observational data.

ECLIPSE MAPPING OF ACCRETION DISKS: THE FIRST DECADE

Observations of eclipse light curves are being used to make maps of the accretion disks in eclipsing cataclysmic variables. By observing the structure of real accretion disks, we can test models of the structure of accretion disk atmospheres, learn about accretion disk viscosity by tracking time-variations in the structure of dwarf nova disks undergoing outbursts, and measure mass accretion rates in systems at different binary periods to test ideas about the long-term evolution of cataclysmic variables. The observed disk structures in dwarf novae during outburst and in long-period nova-like variables confirm the T α R -3/4 law predicted by theory for steady-state disks. However, much flatter radial profiles are found in the disks of quiescent dwarf novae, which appear to be optically-thin, and in nova-like variables with periods between 3 and 4 hours, which may be driving an accretion disk wind. This chapter reviews eclipse mapping methods, and discusses some of the challenges that accretion disk maps are presenting to accretion disk theory.

ATMOSPHERES OF ACCRETION DISCS AND THE EMERGING SPECTRA

The construction of self consistent model atmospheres for stationary accretion discs is reviewed and a method to calculate the corresponding hydrostatic structure and the radiation field is described. The importance of such models for the understanding of the continuum and the line emission as well as the necessity to construct the model in a self consistent way are discussed and demonstrated.

DISCS AND MAGNETOSPHERES

We discuss close binary accretion flows onto magnetic neutron stars and white dwarfs. The original picture of disc flow disrupted by flow along magnetic fieldlines at some inner radius fails in most cases, the exception being neutron–star binaries in which the companion overflows it Roche lobe. In wind accretion, or Roche lobe overflow onto a magnetic white dwarf, the flow is ill–understood, and disc models cannot account for the observed angular momentum budget of the binary. We discuss recent progress in these areas.

MASS LOSS AND THE BOUNDARY LAYER

In this chapter, the focus is on two phenomena associated with non-magnetic cataclysmic variables in the high state, namely the optically-thick boundary layer and high velocity outflow. The state of the art in hydrodynamic modelling of the boundary layer is described and is shown to support the 'classical' concept of the boundary layer as an optically-thick, physically-thin equatorial belt around the accreting white dwarf. The observed and derived properties of cataclysmic variable winds are summarised. A critical overview of the possible physical relation between mass loss and the boundary layer is then given. Particular attention is paid to what has been and can be learned from X-ray and higher sensitivity ultraviolet observations.

THE INTERACTION BETWEEN THE STREAM AND THE ACCRETION DISK

The problem of the interaction between the stream of gas from the inner Lagrangian point and the accretion disk is reviewed. Observations of both cataclysmic variables and low mass x-ray binaries indicate that in addition to phenomena associated directly with the bright spot at the point of impact, the accretion disk exhibits regions of vertical thickenings at a few binary phases. It is pointed out that such a vertical structure is indeed expected on the basis of theoretical considerations. It is argued that a combination of future observations and calculations can yield important system parameters and a better understanding of the processes associated with accretion onto compact objects.

TIDAL EFFECTS ON ACCRETION DISKS IN CLOSE BINARY SYSTEMS

Tidal effects exerted by the secondary star on accretion disks are discussed in compact close binary systems. Non-axisymmetric structures of accretion disks are studied by the following three methods: (1) simple periodic particle orbits in the binary potential, (2) the perturbation method, and (3) full hydrodynamic simulations. Disk radius variations in the outburst cycle of dwarf novae are then studied. They manifest themselves most clearly in the tidal effects or accretion disks. The tidally driven eccentric instability (or the tidal instability) in an accretion disk is then discussed in connection with the superhump phenomenon in SU UMa-type dwarf novae. It is argued that the superoutburst phenomenon in SU UMa stars is most likely explained by an interplay of two kinds of intrinsic instabilities within a disk: the thermal instability and the tidal instability.

ACCRETION DISKS IN BRIGHT X-RAY BINARIES

X-ray observations of low-mass X-ray binaries and black hole candidates are reviewed. In these sources, an accretion disk is considered to extend close to a neutron star or a black hole and to mainly govern the appearance of the X-ray emission. These sources generally show soft and hard spectral states, which possibly correspond to two interchangeable states of the inner part of the accretion disk. On the other hand, the outer part of the disk reflects, reprocesses and sometimes obscures X-rays from the central compact star. Similarities and differences between neutron star and black hole candidate sources are also discussed.


Both Stars in This Binary System Have Accretion Disks Around Them

Stars exhibit all sorts of behaviors as they evolve. Small red dwarfs smolder for billions or even trillions of years. Massive stars burn hot and bright but don’t last long. And then of course there are supernovae.

Some other stars go through a period of intense flaring when young, and those young flaring stars have caught the attention of astronomers. A team of researchers are using the Atacama Large Millimeter/sub-millimeter Array (ALMA) to try to understand the youthful flaring. Their new study might have found the cause, and might have helped answer a long-standing problem in astronomy.

The type of star in question are FU Orionis stars (FU Ori). FU Orionis is both a type of star, and also a specific star in the constellation Orion. The type is named after the specific star, which was the first of its kind seen flaring in 1937.

FU Ori stars are young stars that aren’t on the main sequence yet, and haven’t acquired all of their mass. They can flare by several orders of magnitude in only a single year. These flaring episodes can last decades, and researchers think the activity is caused by increased accretion in the star’s youth. Scientists think that during the flaring, the star can acquire a significant amount of its final mass.

“Episodic accretion and its implications for star and planet formation are not well understood.”

Perez et. al. 2020

Now a team of researchers are studying FU Ori stars more closely. Sebastien Perez at the University of Santiago, Chile, led the study. Their new paper is titled “Resolving the FU Orionis System with ALMA: Interacting Twin Disks?” It’s published in The Astrophysical Journal.

Scientists want to know what’s behind this accretion and associated flaring. Do only some stars experience it? Or is it a stage that all or most stars go through? How long does it last does it happen only once in a star’s lifetime why does it end?

This artist’s concept shows a young stellar object and the whirling accretion disk surrounding it. NASA/JPL-Caltech

Young proto-stars are less luminous than expected according to our understanding of stellar formation. That’s known as the “luminosity problem” in astronomy, and scientists have been wrestling with that problem for a long time. If young stars were accreting at a regular rate, they should be more luminous. If all young stars exhibit the flaring activity seen in FU Ori stars, it could explain this missing luminosity. Astronomers have wondered for some time if mass accretion in these young forming stars might not be constant, and if that might explain the luminosity problem.

“Episodic accretion and its implications for star and planet formation are not well understood,” the authors say in their paper. “Several physical processes have been proposed to explain such dramatic accretion events. The most favored mechanisms include disk fragmentation and the subsequent inward migration of the fragments, gravitational instability, and magneto-rotational instabilities among others.”

The archetypal FU Ori star is its namesake, FU Orionis, in the constellation Orion. It was observed flaring in 1937, and its magnitude increased from 16.5 to 9.6. Astronomers thought it was the only one of its kind, until others were observed.

FU Orionis is in the constellation of Orion. It’s not marked in this image, but it’s up and to the right of Betelgeuse. Image Credit: By IAU and Sky & Telescope magazine (Roger Sinnott & Rick Fienberg) – [1], CC BY 3.0, https://commons.wikimedia.org/w/index.php?curid=15407823

FU Orionis is actually two stars, each surrounded by its own accretion disk. They’re in Orion, about 1360 light years away. Perez and the team of researchers took a close look at the system with ALMA, the first step to understanding the binary pair’s flaring behaviour.

ALMA revealed two accretion disks, one around each star. The scientists used observations and models to conclude that each of the disks is about 11 Astronomical Units in radius, which is small but comparable to other proto-stellar disks. The pair of disks are separated by about 250 Astronomical Units.

ALMA continuum observations show the dust of the two disks surrounding the binary stars of FU Orionis. Each disk is about 11 AU in radius. [Pérez et al. 2020]

Key to understanding the flaring activity in these stars is the movement, or kinematics, of their disks. As the team studied the disks, they found that each one is skewed and asymmetrical. They think that might be caused by some sort of flyby by another star. It could also be caused by interactions between the disks themselves. Either of those could cause the episodic accretion and flaring.

Artist’s impression of a young star throwing a temper tantrum as it suddenly increases its accretion rate and flares. [Caltech/T. Pyle (IPAC)]

The team also found evidence of a long, arcing stream of gas between the disks. That stream strengthens the argument that the disks are interacting. As they say in their paper, “The emission revealing disk rotation also appears asymmetric and skewed, suggesting the disks are subject to interaction in the form of a flyby.”

The authors also point to an alternative to the disk-disk interaction that another team of researchers proposed. “Here, the capture of a cloudlet or cloud fragment also leads to arc-shaped reflection nebulae <the arc of gas connecting the disks.> The capture of this cloud fragment also replenishes the disk allowing for a fresh supply of material to maintain the high accretion rate.”

The study doesn’t answer the missing luminosity question once and for all. But by using ALMA to get a close look at the FU Ori binary pair, the team of scientists have advanced our understanding of episodic accretion and flaring. There are other binary pairs of FU Ori stars, and they’ll be targets for future study.


Acknowledgements

R.A.B. acknowledges support through the EACOA Fellowship from the East Asian Core Observatories Association. S.P.E., G.O. and L.H. acknowledge the support of the ARC Discovery Project (project number DP180101061). G.O. was supported by CAS LCWR grant 2018-XBQNXZ-B-021. A.M.S. was supported by the Foundation for the Advancement of Theoretical Physics and Mathematics “BASIS”. This work was supported by JSPS KAKENHI grant JP19K03921. T.H. is financially supported by the MEXT/JSPS KAKENHI grants 16K05293 and 17K05398. J.O.C. acknowledges support by the Italian Ministry of Foreign Affairs and International Cooperation (MAECI Grant Number ZA18GR02) and the South African Department of Science and Technology’s National Research Foundation (DST-NRF Grant Number 113121) as part of the ISARP RADIOSKY2020 Joint Research Scheme. This work was supported by the National Science Centre, Poland, through grant 2016/21/B/ST9/01455. The LBA is part of the Australia Telescope National Facility, which is funded by the Australian Government for operation as a National Facility managed by CSIRO. This work was supported by resources provided by the Pawsey Supercomputing Centre with funding from the Australian Government and the Government of Western Australia. The National Radio Astronomy Observatory is a facility of the National Science Foundation operated under cooperative agreement by Associated Universities, Inc.


NICER Eyes on Bursting Stars

What happens to a neutron star’s accretion disk when its surface briefly explodes? A new instrument recently deployed at the International Space Station (ISS) is now watching bursts from neutron stars and reporting back.

Deploying a New X-Ray Mission

Launch of NICER aboard a Falcon 9 rocket in June 2017. [NASA/Tony Gray]

In the two weeks following launch, NICER was extracted from the SpaceX Dragon capsule and installed on the ISS. And by the end of the month, the instrument was already collecting its first data set: observations of a bright X-ray burst from Aql X-1, a neutron star accreting matter from a low-mass binary companion.

Impact of Bursts

NICER’s goal is to provide a new view of neutron-star physics at X-ray energies of 0.2–12 keV — a window that allows us to explore bursts of energy that neutron stars sometimes emit from their surfaces.

Artist’s impression of an X-ray binary, in which a compact object accretes material from a companion star. [ESA/NASA/Felix Mirabel]

Within seconds, the layer of material is burned up, producing a burst of emission from the neutron star that outshines even the inner regions of the hot accretion disk. Then more material funnels onto the neutron star and the process begins again.

Though we have a good picture of the physics that causes these bursts, we don’t yet understand the impact that these X-ray flashes have on the accretion disk and the environment surrounding the neutron star. In a new study led by Laurens Keek (University of Maryland), a team of scientists now details what NICER has learned on this subject.

Extra X-Rays

Light curve (top) and hardness ratio (bottom) for the X-ray burst from Aql X-1 captured by NICER on 3 July 2017. [Keek et al. 2018]

  1. The burst radiation from the neutron star’s surface was reprocessed — i.e., either scattered or absorbed and re-emitted — by the accretion disk.
  2. The persistent, usual accretion flow was enhanced as a result of the burst’s radiation drag on the disk, briefly bumping up the disk’s X-ray flux.

While we can’t yet conclusively state which mechanism dominates, NICER’s observations do show that bursts have a substantial impact on their accretion environment. And, as there are over 100 such X-ray burster systems in our galaxy, we can expect that NICER will allow us to better explore the effect of X-ray bursts on neutron-star disks and their surroundings in many different systems in the future.

Bonus

Check out the awesome gif below, provided by NASA, which shows NICER being extracted from the Dragon capsule’s trunk by a robotic arm.

Citation

L. Keek et al 2018 ApJL 855 L4. doi:10.3847/2041-8213/aab104


Accretion (astronomy)

Play media file

Accretion ( Latin accretio "growth", "increase") is the term in astronomy for a process in which a cosmic object collects matter due to its gravitation or tidal forces (see Roche limit ).


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Keywords: active galactic nuclei, quasar, supermassive black holes, accretion disc, X-ray

Citation: Lusso E and Risaliti G (2018) The Physical Relation between Disc and Coronal Emission in Quasars. Front. Astron. Space Sci. 4:66. doi: 10.3389/fspas.2017.00066

Received: 13 November 2017 Accepted: 18 December 2017
Published: 08 January 2018.

Paola Marziani, Osservatorio Astronomico di Padova (INAF), Italy

Vahram Chavushyan, National Institute of Astrophysics, Optics and Electronics, Mexico
Luka C. Popovic, Astronomical Observatory Belgrade, Serbia
Alberto Rodriguez-Ardila, Laboratório Nacional de Astrofísica, Brazil

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