Today we have a guest blogger, Uroš Kostić, who is currently a researcher at the Faculty of Mathematics and Physics at the University of Ljubljana in Slovenia. Here, he describes theoretical work on the destruction of asteroids by supermassive black holes, involving a collaboration between University of Ljubljana, Slovenia, and Astronomical Observatory in Padova, Italy, from 2005 - 2008. During the project, Uroš was preparing his PhD at University of Ljubljana under the supervision of Professor Andrej Čadež.

These results by UroŇ° were referenced by Kastytis Zubovas in the paper that we publicized last month (http://chandra.si.edu/photo/2012/sgra/). We were unable to mention the work by Uroš and his team in the release, but we felt that the originality of their work, published before the Zubovas et al. study, warranted a blog posting. Academic research invariably builds on the work done by others and to paraphrase Isaac Newton: "If I have seen further it is by standing on the shoulders of giants."



The movie shows the appearance of an asteroid of 50-60 km in size approaching a supermassive black hole (like the one at the center of our Galaxy), as it would be seen by a distant observer 5 degrees above the orbital plane. Due to the strong gravitational field, the mounting tidal forces eventually destroy the asteroid, which becomes elongated into a spiral structure. During this process, tidal energy of a few percent of mc^2 is released and part of it is emitted in the form of infrared light and X-rays.

The discovery in 2001 of infrared and X-ray flares coming from the Galactic center and the study of stellar orbits gave additional support to the presence of a black hole of several million Solar masses in the center of our Galaxy. At that time we were already modeling tidal disruption of stars by supermassive black holes; our models predicted that disrupted stars would produce strong but short flares of electromagnetic radiation. However, we soon realized that flares from our Galactic center were not energetic enough to be produced in such a way. Instead, a body with smaller mass would need to be disrupted by the black hole.

We made a detailed model of tidal disruption of a small, compact object, such as a comet or an asteroid, and predictions of the appearance and light curve of such events. We took into account gravitational lensing by the black hole, aberration of light and Doppler effects.

The luminosity curves of such tidal disruption events bear marks of this strong gravitational field: the short-lasting flares are due to these highly relativistic effects, while the overall increase and decrease in amplitude is a consequence of the asteroid being squeezed and elongated in a long spiral shape by the changing tidal force. With this model we were able to reproduce and fit some X-ray and infrared flares (Čadež et al. 2006, 2008, 2010; Kostić et al. 2009).

Although the correct physical mechanism for melting and evaporating asteroids is still under debate (e.g. Zubovas et al. 2012 propose friction with surrounding medium), the underlying idea remains the same: strong tidal forces (Gomboc & Čadež 2005) are necessary to destroy the asteroids, and in this process X-ray and IR flares are produced. Our work differs from Zubovas et al. (2012) as we take into account general relativistic effects near the black hole.

The work by Zubovas et al. (2012) confirms our ideas; moreover, using statistics, they also confirm that such events are frequent enough to produce the observed number and rate of flares coming from the Galactic center.

Perhaps we are finally approaching the physical mechanism that is responsible for the intriguing X-ray and infrared flares from Sgr A*.

More information can be found at

http://astro.ago.uni-lj.si/bh/pmwiki.php?n=Main.XRF

These results are part of the research concerning tidal interaction around black holes that was carried out in a long-term international project led by Andrej Čadež at the University of Ljubljana, Slovenia and also included Massimo Calvani of the Astronomical Observatory in Padova and Andreja Gomboc from the University of Ljubljana.

-Uroš Kostić

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