5° International Fermi Symposium

The two Fermi instruments have been surveying the high-energy sky since August 2008. The Large Area Telescope (LAT) has discovered more than a thousand new sources and many new source classes, bringing the importance of gamma-ray astrophysics to an ever-broadening community.

Pubblicità

Fast outflows in massive stars

Massive Stars are among the dominant sources of light in galaxies. Their high surface temperature, maintained over several evolutionary phases, implies that most of this light is emitted as ultraviolet photons. Through line scattering and the corresponding momentum transfer, this intense radiation field is able to accelerate fast outflows. 

Astrophysical Black Holes

Black holes are very fascinating and peculiar objects. According to General Relativity, uncharged black holes are completely specified by only two parameters, the mass M and the spin angular momentum J. In the last 40 years, we have discovered at least two classes of astrophysical black hole candidates: stellar-mass objects in X-ray binary systems and super-massive black hole candidates at the center of every normal galaxy. The mass of these objects can be inferred by robust dynamical measurements, by studying the orbital motion of gas or individual stars around them. The determination of the spin J is much more difficult and it is a hot topic of contemporary astrophysics. Then, there are still many open questions: What is the origin of the jets produced in the region around these objects? How could super-massive black holes become so heavy? Are these objects the Kerr black holes predicted by General Relativity? All our open questions may be addressed by studying the properties of the electromagnetic radiation emitted in the accretion process and, hopefully in a not distant future, by observing the gravitational waves emitted by these systems.

Read more: Fudan Winter School on Astrophysical Black Holes

Massive Black Holes: Birth, Growth and Impact

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.

Ultra strong magnetic fields in neutron stars

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.

MPI: The largest magnetic fields in the universe

arXiv: Magnetorotational instability in relativistic hypermassive neutron stars

New type of pulsating star

Un gruppo internazionale di astronomi hanno osservato i resti di una collisione stellare trovando che la luminosità della stella varia in modo insolito. L’analisi delle curve di luce sta fornendo agli astronomi preziosi indizi che permettono di comprendere cosa accade quando due stelle collidono.

Stars like our Sun expand and cool to become red giant stars when the hydrogen that fuels the nuclear fusion in their cores starts to run out. Many stars are born in binary systems so an expanding red giant star will sometimes collide with an orbiting companion star. As much as 90% of the red giant star’s mass can be stripped off in a stellar collision, but the details of this process are not well understood. Only a few stars that have recently emerged from a stellar collision are known, so it has been difficult to study the connection between stellar collisions and the various exotic stellar systems they produce. When an eclipsing binary system containing one such star turned up as a by-product of a search for extrasolar planets, Pierre Maxted and his colleagues decided to use the high-speed camera ULTRACAM to study the eclipses of the star in detail.

These new high-speed brightness measurements show that the remnant of the stripped red giant is a new type of pulsating star.

Many stars, including our own Sun, vary in brightness because of pulsations caused by sound waves bouncing around inside the star. For both the Sun and the new variable star, each pulsation cycle takes about 5 minutes. These pulsations can be used to study the properties of a star below its visible surface. Computer models produced by the discovery team show that the sound waves probe all the way to the centre of the new pulsating star. Further observations of this star are now planned to work out how long it will be before the star starts to cool and fade to produce a stellar corpse (“white dwarf’”) of abnormally low mass. Pierre Maxted from Keele University, who led the study, said  “We have been able to find out a lot about these stars, such as how much they weigh, because they are in a binary system. This will really help us to interpret the pulsation signal and so figure out how these stars survived the collision and what will become of them over the next few billion years”.

See animation here.

Keele University: Survivor of stellar collision is new type of pulsating star

Nature: Multi-periodic pulsations of a stripped red-giant star in an eclipsing binary system

arXiv: Discovery of a stripped red giant core in a bright eclipsing binary star