Letter to the Editor

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

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 [CITE], passes the test and is constrained to accretion rates no larger than ~ few   10 year^-1.