Free access article

A&A 429, L33-L36 (2005)
DOI: 10.1051/0004-6361:200400101

Letter

Using close stars as probes of hot accretion flow in Sgr A ^*

S. Nayakshin

Max-Planck-Institut für Astrophysik, Karl-Schwarzschild-Str.1, 85741 Garching, Germany
    e-mail: serg@mpa-garching.mpg.de

(Received 20 October 2004 / Accepted 6 November 2004 )

Abstract
The extremely hot and tenuous accretion flow in the immediate vicinity of Sgr A ^* is believed to be invisible (too dim) in the X-ray band, except for short X-ray flares. Here we point out that during pericenter passages, close brightest stars irradiate the inner region of the accretion flow, providing a plenty of optical/UV photons. These seed photons are Compton up-scattered by the hot electrons of the accretion flow to higher frequencies, some into the X-ray band, potentially making the innermost accretion flow much brighter in X-rays than usual. We propose to use coordinated near infra-red and X-ray observations of close star passages to put constraints onto Sgr A ^* accretion theories. The absence of a noticeable change in the steady emission of Sgr A ^* as observed by Chandra in the year 2002, when the star named S2 passed through a pericenter of its orbit, already rules out the hotter of the "standard" Advection-Dominated Accretion Flows. The less dense accretion flows, in particular the model of Yuan et al. (2003), passes the test and is constrained to accretion rates no larger than ~ few    10  year ^-1.

Key words: accretion, accretion disks -- black hole physics -- Galaxy: center -- radiation mechanisms: general

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