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Multiwavelength Studies of Galactic Black Holes

Final Report Summary - BLACK HOLE UNIVERSE (Multiwavelength Studies of Galactic Black Holes)


Executive Summary:

Black holes (BHs) are one of the most exotic objects of modern physics. They warp spacetime around them, preventing any signals from escaping. But BHs are real astrophysical objects, which have important effects on their environment. In the early universe, BHs were formed during the earliest collapse of matter into galaxies. One of the big mysteries is the fact that these BHs grow together with their galaxies. This fact may not sound surprising, but when one considers that the size of the "reach" of the BH is orders of magnitude smaller than the greater galaxy, it is difficult to understand how this could be achieved. The answer is related to work done in this ITN, and is relevant for our understanding of galaxy evolution. When a BH "feeds" off local material, in a process called accretion, the matter moves towards it in a disk. Such disks are observed in the optical through X-ray bands. Near the BH, a combination of strong gravity and magnetic fields leads to the expulsion of plasma and electromagnetic fields at near light speeds, a jet, which can carry enormous amounts of power.

Jets can heat up the gas within and surrounding their host galaxy and alter its growth. In doing so, they also eventually cut off their own matter supply, engendering a feedback cycle that regulates the BH and galaxy growth together.

Most galaxies with accreting BHs in their centers (Active Galactic Nuclei, or AGN) do not show strong jets. A big question is whether these are distinct classes, or if AGN with jets evolve into ones without jets over many millions of years. It is impossible to catch this evolution in real time, but studying small BHs, remnants of massive stars in our Galaxy, yields some clues. When accreting from a companion star, these Galactic BHs behave similar to their supermassive counterparts. Over the course of weeks to a year, they traverse a series of distinct states, some associated with jet production, some not. One of the primary goals of the ITN is to understand the physics of Galactic BHs via a variety of studies, and to attempt to extend our knowledge to the supermassive BHs in AGN.

Because BHs radiate across the entire electromagnetic spectrum, a variety of space- and ground-based instruments are used. One of the goals of the ITN is to train a new generation of astronomers in the use of these methods. This training also prepares researchers for careers in the use of space technology, an area crucial for Europe's technological leadership with currently a severe dearth of trained scientists. The Network is therefore committed to teaching the ESRs and ERs about other areas of opportunity, and giving them the appropriate skill sets to fulfill the needs of this market. A secondments program also ensures an early start to job mobility.

The network has achieved almost all of its milestones. Its scientific results and achievements are fully summarized on the network's www-pages, http://www.black-hole.eu . They include:

* The training of 10 Early Stage Researchers and 2 Experienced Researchers from 11 countries.
* The organization of two summer schools on multiwavelength astronomy (total of 80 participants), a workshop on Black Hole Astrophysics with 113 participants, and a conference with 85 participants from all over the world.
* The publication of 48 papers in the refereed literature, and 22 further non-refereed publications with strong ER/ESR participation.
* The publication of another 81 refereed papers which are a direct consequence of the networking activities within the ITN (participation by the network's advisors and collaboration of other researchers at the ITN host institutions).
* The publication of another 58 fast publications on astrophysical phenomena in so-called Astronomer's Telegrams.

Scientific highlights from the four years of the network include

* The discovery of a clear dependency between the variability of a black hole and its luminosity, led by ER Teo Munoz-Darias (INAF, Italy).
* Successful application of sophisticated jet models to black hole spectra led by ESR Pieter van Oers (University of Southampton, United Kingdom).
* The discovery of a source at the center of the globular cluster NGC 6388 with a mass less than 1500 solar masses and properties consistent with those expected for a black hole led by ESR David Cseh (CEA Saclay, France).
* The discovery that the black hole in Cygnus X-1 is maximally rotating led by ESR Refiz Duro (FAU Erlangen-Nuremberg, Germany).
* The study, by ESR Elise Egron (Cagliari), of relativistic broadening and interaction of hard X-rays with an accretion disk, in neutron star X-ray binaries which are thus behaving similar to black hole X-ray binaries.
* The most precise model showing how geometrical effects in the accretion flow influence the timing behavior by black holes (Pablo Cassatella, Southampton).
* The discovery of polarization in the hard X-ray tail of Cygnus X-1, making this source only the second persistent source where hard X-ray polarization has been discovered, by ITN advisors Jerome Rodriguez (CEA Saclay, France) and Joern Wilms (FAU Erlangen-Nuremberg, Germany) Please see http://www.black-hole.eu for further information, including a description of all research projects for the general public, links to all network publications, teaching materials from all network schools, the presentations from all network conferences, and more.