Sgr A* flares: tidal disruption of asteroids and planets?

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.Comment: 11 pages. MNRAS submitte

Echoes of multiple outbursts of Sagittarius A⋆revealed byChandra

“…Characterizing the frequency of such events along with their durations and their intensities will be fundamental to identifying the physical processes responsible for these past changes in the luminosity of Sgr A . They could be due to catastrophic events such as, a transient jet induced by a partial stellar capture (Yu et al 2011), a capture of planets (Zubovas et al 2012), or the accretion of debris produced by tidal interaction of stars (Sazonov et al 2012), but they can also be explained by stochastic variations of the accretion rate due to the emission of clumps by the winds of massive stars orbiting Sgr A (Cuadra et al 2008). …”

A systematicChandrastudy of Sgr A⋆– I. X-ray flare detection

“…One is the production of hot spots or episodic mass ejections in the accretion flow and/or jets, due to the magnetic reconnection or other magnetohydrodynamical process (e.g., Markoff et al 2001;Yuan, Quataert & Narayan 2004;Liu, Petrosian & Melia 2004;Yuan et al 2009;Dodds-Eden et al 2010). The other is the tidal disruptions of approaching planetesimals by the SMBH (e.g., Kostić et al 2009;Zubovas, Nayakshin & Markoff 2012). In particular, the comparison of the flare statistics with the theoretical expectation of the so-called self-organized criticality (SOC) system (Katz 1986;Bak, Tang & Wiesenfeld 1987) suggests that the spatial dimension responsible for the production of the flares is S = 3, which is similar to that for solar flares and further implies a magnetic reconnection origin of the X-ray flares (Wang et al 2015;Li et al 2015).…”

The Milky Way’s Supermassive Black Hole: How Good a Case Is It?

“…The existence of smaller dust grains at the center is probably limited due to intense UV and X-ray radiation field and they plausibly completely vanish on the scale of tens of gravitational radii in the hot accretion flow. On the other hand, the continuous formation of dust in stellar winds, in particular of AGB stars (Yusef-Zadeh et al, 2017), as well as the speculative asteroid or planet infall towards Sgr A* (Zubovas et al, 2012) could in principle replenish the dust content even very close to the black hole.…”