Relativistic Astrophysics

 Tidal effects on small bodies by massive black holes

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 The compact radio source Sagittarius A* (Sgr A*) at the center of our Galaxy harbours a super massive black hole whose mass (∼ 3.7x10^6Msun) has been measured from stellar orbital motions. Sgr A* is therefore the nearest laboratory where supermassive black hole astrophysics can be tested, and the environment of black holes can be investigated. Since it is not an active galactic nucleus, it also offers the possibility to observe the capture of small objects that may orbit the central black hole. We studied the effects of the strong gravitational field of the black hole on small objects, like a comet or an asteroid. We also explore the idea that the flares detected in Sgr A* might be produced by the final accretion of single dense objects with mass of the order of 1020 g, and that their timing is not a characteristic of the sources, but rather of the space-time of the central galactic black hole they are moving in. The problem is studied numerically, using ray tracing techniques, in a Schwarzschild background. We find that tidal effects are strong enough to melt the solid object, and present calculated temporal evolution of the light curve of the infalling objects as a function of various parameters. Our modelling of tidal disruption suggests that during tidal squeezing the conditions for synchrotron radiation can be met. We show that the light curve of a flare can be deduced from dynamical properties of geodesic orbits around black holes and that it only weakly depends on physical properties of the source.

People: M. Calvani

Collaboration: A. Čadež, A. Gomboc, U. Kostić (Ljubljana Univ., Slovenia) 

Publications: Kostić et al. (2009), A&A in press; Čadež et al. (2008), A&A 487,527; Gomboc et al. (2008), ralc.conf. 141; Čadež et al. (2008), cosp 37,435

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