Abstract
It is theoretically expected that a supermassive black hole (SMBH) in the centre of a typical nearby galaxy disrupts a Solar-type star every ~ 10^5 years, resulting in a bright flare lasting for months. Sgr A*, the resident SMBH of the Milky Way, produces (by comparison) tiny flares that last only hours but occur daily. Here we explore the possibility that these flares could be produced by disruption of smaller bodies - asteroids. We show that asteroids passing within an AU of Sgr A* could be split into smaller fragments which then vaporise by bodily friction with the tenuous quiescent gas accretion flow onto Sgr A*. The ensuing shocks and plasma instabilities may create a transient population of very hot electrons invoked in several currently popular models for Sgr A* flares, thus producing the required spectra. We estimate that asteroids larger than ~ 10 km in size are needed to power the observed flares, with the maximum possible luminosity of the order 10^39 erg s^-1. Assuming that the asteroid population per parent star in the central parsec of the Milky Way is not too dissimilar from that around stars in the Solar neighbourhood, we estimate the asteroid disruption rates, and the distribution of the expected luminosities, finding a reasonable agreement with the observations. We also note that planets may be tidally disrupted by Sgr A* as well, also very infrequently. We speculate that one such disruption may explain the putative increase in Sgr A* luminosity ~ 300 yr ago.
Highlights
Most of the nearby supermassive black holes (SMBHs) are rather dim (e.g. Ho 2008), suggesting that little gas is supplied to them at the current epoch
The bound streams precess and self-intersect on the return passage past the black hole, resulting in very strong shocks. The result of these shocks should be a small-scale accretion disc around the SMBH, and power a spectacularly bright L ∼ 1044–1046 erg s−1 flare, lasting a few months (Lodato, King & Pringle 2009). Such candidate events have been observed in nearby galaxies (Esquej et al 2008), and there are some recent observations of gamma-ray sources that are best explained by jets resulting from stellar tidal disruption events (Bloom et al 2011; Campana et al 2011)
While it is superficially similar to the one presented in this paper, these authors only considered the tidal disruption of asteroids, without investigating the interaction of the remnants with the gaseous accretion flow around Sgr A∗, which forms a significant part of our investigation
Summary
Most of the nearby supermassive black holes (SMBHs) are rather dim (e.g. Ho 2008), suggesting that little gas is supplied to them at the current epoch. Short-time-scale magnetic reconnection event models seem to be more promising than transient density variation models (Markoff et al 2001; Dodds-Eden et al 2010) Another class of flare models envisages a transient feature in the accretion flow around Sgr A∗. Nayakshin, Cuadra & Sunyaev (2004) suggested that stars orbiting Sgr A∗ strike an optically thick disc and that the resulting shocks produce the observed X-ray flares. This model is firmly disfavoured by the constraints on the NIR flaring region size of 10 au, as it would require unphysically large stellar densities in the innermost region. While it is superficially similar to the one presented in this paper, these authors only considered the tidal disruption of asteroids, without investigating the interaction of the remnants with the gaseous accretion flow around Sgr A∗, which forms a significant part of our investigation (see Section 3 below)
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