Some of the most energetic processes seen in the Universe arise close to a super-massive black hole such as relativistic jets and winds. These are now known to play a key role in determining the growth of galaxies across cosmic time, but the mechanisms by which they are launched remain unclear. Continua a leggere The Extremes of Black Hole Accretion
E’ noto che ogni galassia ospita nel suo nucleo un buco nero supermassiccio spesso circondato da un disco di accrescimento super brillante composto di gas a temperature elevate che dà luogo alla fenomenologia tipica dei quasar. Ora, un gruppo di ricercatori della Penn State University hanno individuato con grande sorpresa una nuova classe di quasar distanti la cui esistenza non è prevista dagli attuali modelli che descrivono le proprietà e la fenomenologia dei nuclei galattici attivi.
“The gas in this new type of quasar is moving in two directions: some is moving toward Earth but most of it is moving at high velocities away from us, possibly toward the quasar’s black hole“, said study co-author Niel Brandt, Distinguished Professor of Astronomy and Astrophysics at Penn State. “Just as you can use the Doppler shift for sound to tell if an airplane is moving away from you or toward you, we used the Doppler shift for light to tell whether the gas in these quasars is moving away from Earth or toward these distant black holes, which have a mass from millions to billions of times that of the Sun“. Matter around these black holes forms a quasar disc that is bigger than Earth’s orbit around the Sun and hotter than the surface of the Sun. These quasars generate enough light to be seen across the observable Universe. The international research team, led by Patrick Hall of York University in Toronto, discovered the unusual quasars with data from a large sky survey, the Sloan Digital Sky Survey (SDSS-III). “Matter falling into black holes may not sound surprising“, said Hall, “but what we found is, in fact, quite mysterious and was not predicted by current theories“. Such gas is found in only about 1 out of 10,000 quasars, and only 17 cases now are known. “The gas in the disc must eventually fall into the black hole to power the quasar, but what is often seen instead is gas blown away from the black hole by the heat and light of the quasar, heading toward us at velocities up to 20 per cent of the speed of light“, Hall said. “If the gas is falling into the black hole, then we don’t understand why it’s so rare to see infalling gas. There’s nothing else unusual about these quasars. If gas can be seen falling into them, why not in other quasars?” Hall noted there is one other possible explanation for these objects. “It could be that the gas moving away from us is not falling into the black hole but is orbiting around it, just above the disc of hot gas and is very gradually being pushed away from the black hole“, he said. “A wind like that will show gas moving both toward us and away from us. To make an analogy, imagine an ant on a spinning merry-go-round, crawling from the center to the edge. You will see the ant moving toward you about half the time and away from you about half the time. The same idea could apply to the gas in these quasars. In either case, the gas in these quasars is moving in an unusual fashion“. Models of quasars and their winds will have to be revised to account for these objects. To help understand what revision is needed, the research team is observing these quasars further using Canadian and American access to the Gemini-North telescope in Hawaii.
The greatest challenge in black hole astrophysics lies in the attempt to unify Galactic, intermediate and supermassive black holes under the same physical scheme to gain a more profound understanding of different classes of objects. In such grand-unification models, the observed behaviours and manifestations of accreting black holes are driven by only a few fundamental parameters, such as mass, accretion rate and spin. The conference tackles the issues related to accretion physics, production of jets/winds/outflows and their time evolution by putting a particular emphasis on the role of the net accretion rate onto black holes of all sizes. We are going to explore to which extent grand-unification schemes of black hole accretion are possible and also where they possibly fail. Participants are encouraged to present theoretical and/or observational results and prospects, across the entire electromagnetic spectrum, and to discuss them in the light of grand-unification paradigms of black hole accretion.
Topics suggested for the conference :
- Accretion / ejection flows around black holes: theory and observational predictions
- Disks, winds and jets at different accretion rates: observations and phenomenology
- XRBs and AGN grand-unification and scaling relations – what do they mean and where do they fail?
- Ultraluminous X-ray sources and intermediate mass black holes – the missing link?
- Time scales and variability in different accretion states
- Spin evolution and black hole mergers
- Feeding and feedback across the mass scale
During the past decade, massive black holes have become central objects of study in areas of astrophysics that were traditionally not connected. Along with traditional studies of black holes as high energy astrophysical sources, massive black holes have become pivotal to the understanding of galaxy formation and evolution. Similarly, massive black hole binaries have become the main targets of the future generation of gravitational wave experiments, motivating new research on the orbital decay and merging of black holes. Finally, studies of our own Galactic Center have also undergone tremendous progress and are expected be able to probe general relativistic effects induced by the central supermassive black hole. With this conference, we will bring together experts from the diverse groups involved in the study of massive black holes, producing a novel summary of the status of knowledge and fostering a productive interaction between various research communities that normally operate separately.
Themes that we will focus on will include
(1) Formation mechanisms of massive black hole seeds, confronting weaknesses and strengths of different models and placing them in the context of cosmic structure formation.
(2) Co-evolution of galaxies and massive black holes, in particular the role of black hole feedback on galaxy formation.
(3) Evolution of massive black hole binaries, from the Newtonian to the relativistic regime, including predictions for gravitational wave experiments.
(4) Modeling of accretion discs, especially the latest generation of three-dimensional numerical simulations, addressing the state-of-the art in the field and discussing how to transfer the acquired knowledge to sub-grid models of black hole accretion during galaxy formation.
We expect the conference will generate the most up-to-date synthesis of our current knowledge on massive black holes.
Una serie di simulazioni numeriche mostra per la prima volta che la presenza di instabilità nel nucleo delle stelle di neutroni può determinare la formazione di campi magnetici giganteschi che possono a loro volta causare violente e drammatiche esplosioni stellari mai osservate nell’Universo.
An ultra-dense (“hypermassive”) neutron star is formed when two neutron stars in a binary system finally merge. Its short life ends with the catastrophic collapse to a black hole, possibly powering a short gamma-ray burst, one of the brightest explosions observed in the Universe. Short gamma-ray bursts as observed with satellites like XMM Newton, Fermi or Swift release within a second the same amount of energy as our Galaxy in one year. It has been speculated for a long time that enormous magnetic field strengths, possibly higher than what has been observed in any known astrophysical system, are a key ingredient in explaining such emission.
Scientists at the Max Planck Institute for Gravitational Physics (Albert Einstein Institute/AEI) have now succeeded in simulating a mechanism which could produce such strong magnetic fields prior to the collapse to a black hole.
How can such ultra-high magnetic fields, stronger than ten or hundred million billion times the Earth’s magnetic field, be generated from the much lower initial neutron star magnetic fields? This could be explained by a phenomenon that can be triggered in a differentially rotating plasma in the presence of magnetic fields: neighbouring plasma layers, which rotate at different speeds, “rub against each other”, eventually setting the plasma into turbulent motion. In this process called magnetorotational instability magnetic fields can be strongly amplified. This mechanism is known to play an important role in many astrophysical systems such as accretion disks and core-collapse supernovae. It had been speculated for a long time that magnetohydrodynamic instabilities in the interior of hypermassive neutron stars could bring about the necessary magnetic field amplification. The actual demonstration that this is possible has only now been achieved with the present numerical simulations. The scientists of the Gravitational Wave Modelling Group at the AEI simulated a hypermassive neutron star with an initially ordered (“poloidal”) magnetic field, whose structure is subsequently made more complex by the star’s rotation. Since the star is dynamically unstable, it eventually collapses to a black hole surrounded by a cloud of matter, until the latter is swallowed by the black hole.
These simulations have unambiguously shown the presence of an exponentially rapid amplification mechanism in the stellar interior, the magnetorotational instability. This mechanism has so far remained essentially unexplored under the extreme conditions of ultra-strong gravity as found in the interior of hypermassive neutron stars.
This is because the physical conditions in the interior of these stars are extremely challenging. The discovery is interesting for at least two reasons. First, it shows for the first time unambiguously the development of the magnetorotational instability in the framework of Einstein’s theory of general relativity, in which there exist no analytical criteria to date to predict the instability. Second, this discovery can have a profound astrophysical impact, supporting the idea that ultra strong magnetic fields can be the key ingredient in explaining the huge amount of energy released by short gamma-ray bursts.
Grazie ad uno studio guidato da alcuni astronomi della Johns Hopkins University e della Rochester Institute of Technology, è stato possibile trovare nuovi indizi che confermano una ipotesi a lungo dibattuta su come i buchi neri di massa stellare producono radiazione di alta energia.
“We’re accurately representing the real object and calculating the light an astronomer would actually see”, says Scott Noble, associate research scientist in RIT’s Center for Computational Relativity and Gravitation. “This is a first-of-a-kind calculation where we actually carry out all the pieces together. We start with the equations we expect the system to follow, and we solve those full equations on a supercomputer. That gives us the data with which we can then make the predictions of the X-ray spectrum”. Lead researcher Jeremy Schnittman, an astrophysicist at NASA’s Goddard Space Flight Center, says the study looks at one of the most extreme physical environments in the Universe: “Our work traces the complex motions, particle interactions and turbulent magnetic fields in billion-degree gas on the threshold of a black hole”.
By analyzing a supercomputer simulation of gas flowing into a black hole, the team finds they can reproduce a range of important X-ray features long observed in active black holes.
“We’ve predicted and come to the same evidence that the observers have”, Noble says. “This is very encouraging because it says we actually understand what’s going on. If we made all the correct steps and we saw a totally different answer, we’d have to rethink what our model is”. Gas falling toward a black hole initially orbits around it and then accumulates into a flattened disk. The gas stored in this disk gradually spirals inward and becomes compressed and heated as it nears the center. Ultimately reaching temperatures up to 20 million degrees Fahrenheit (12 million C), some 2,000 times hotter than the Sun’s surface, the gas shines brightly in low-energy, or soft, X-rays. For more than 40 years, however, observations show that black holes also produce considerable amounts of “hard” X-rays, light with energy 10 to hundreds of times greater than soft X-rays. This higher-energy light implies the presence of correspondingly hotter gas, with temperatures reaching billions of degrees.
The new study bridges the gap between theory and observation, demonstrating that both hard and soft X-rays inevitably arise from gas spiraling toward a black hole.
Working with Noble and Julian Krolik, a professor at Johns Hopkins, Schnittman developed a process for modeling the inner region of a black hole’s accretion disk, tracking the emission and movement of X-rays, and comparing the results to observations of real black holes. Noble developed a computer simulation solving all of the equations governing the complex motion of inflowing gas and its associated magnetic fields near an accreting black hole. The rising temperature, density and speed of the infalling gas dramatically amplify magnetic fields threading through the disk, which then exert additional influence on the gas. The result is a turbulent froth orbiting the black hole at speeds approaching the speed of light. The calculations simultaneously tracked the fluid, electrical and magnetic properties of the gas while also taking into account Einstein’s theory of relativity. Running on the Ranger supercomputer at the Texas Advanced Computing Center located at the University of Texas in Austin, Noble’s simulation used 960 of Ranger’s nearly 63,000 central processing units and took 27 days to complete. Over the years, improved X-ray observations provided mounting evidence that hard X-rays originated in a hot, tenuous corona above the disk, a structure analogous to the hot corona that surrounds the Sun. “Astronomers also expected that the disk supported strong magnetic fields and hoped that these fields might bubble up out of it, creating the corona”, Noble says. “But no one knew for sure if this really happened and, if it did, whether the X-rays produced would match what we observe”. Using the data generated by Noble’s simulation, Schnittman and Krolik developed tools to track how X-rays were emitted, absorbed and scattered throughout both the accretion disk and the corona region.
Combined, they demonstrate for the first time a direct connection between magnetic turbulence in the disk, the formation of a billion-degree corona, and the production of hard X-rays around an actively “feeding” black hole.
In the corona, electrons and other particles move at appreciable fractions of the speed of light. When a low-energy X-ray from the disk travels through this region, it may collide with one of the fast-moving particles. The impact greatly increases the X-ray’s energy through a process known as inverse Compton scattering. “Black holes are truly exotic, with extraordinarily high temperatures, incredibly rapid motions and gravity exhibiting the full weirdness of general relativity”, Krolik says. “But our calculations show we can understand a lot about them using only standard physics principles”. The study was based on a non-rotating black hole. The researchers are extending the results to spinning black holes, where rotation pulls the inner edge of the disk further inward and conditions become even more extreme. They also plan a detailed comparison of their results to the wealth of X-ray observations now archived by NASA and other institutions. Black holes are the densest objects known. Stellar-mass black holes form when massive stars run out of fuel and collapse, crushing up to 20 times the Sun’s mass into compact objects less than 75 miles (120 kilometers) wide.The following video animation of supercomputer data takes you to the inner zone of the accretion disk of a stellar-mass black hole. Gas heated to 20 million degrees Fahrenheit as it spirals toward the black hole glows in low-energy, or soft, X-rays. Just before the gas plunges to the center, its orbital motion is approaching the speed of light. X-rays up to hundreds of times more powerful (“harder”) than those in the disk arise from the corona, a region of tenuous and much hotter gas around the disk. Coronal temperatures reach billions of degrees. Credit: NASA’s Goddard Space Flight Center
The study of relativistic jets in active galactic nuclei and other sources involving the accretion onto compact objects, like gamma-ray bursts and microquasars, has gained added interest thanks to the recent simultaneous multi spectral-range observations and the advances in the theoretical and numerical modeling. During June 10-14th, 2013, the Instituto de Astrofísica de Andalucía-CSIC in Granada, Spain, will host a meeting (poster) aimed to discuss the recent results in the study of relativistic jets. The meeting will focus on the study of the innermost regions of AGN jets to obtain a better understanding of the jet formation mechanisms and determine the origin and location of the high energy emission, as well as the role played by the magnetic field.
Topics to be discussed include:
- Jet formation
- Black hole, accretion disk, jet connection
- Multi-spectral-range emission
- Magnetic fields and polarization
- Jet dynamics and stability
- Unification models, microphysics, particle acceleration
- Relativistic stellar jets
“Spectral/timing properties of accreting objects: from X-ray binaries to AGN” is a 3-day workshop to be held at the European Space Astronomy Centre (ESAC) in April 2013. The aim of the workshop is to review the current understanding of the physics of accretion onto compact objects across the full range of masses, by bringing together young and active experts of the field. In order to encourage discussions and maximize collaborations, the attendance to the workshop will be limited to about 70 people, that will be selected on the basis of the proposed contribution after the registration deadline. There will be no registration fee.
The main scientific topics covered will be:
– Accretion modes at different scales;
– States and state transitions;
– Inflow/outflow connections;
– Accretion/ejection mechanisms;
– Unification schemes.