Two Rapidly Variable Galactic X-ray Transients Observed with Chandra, XMM-Newton, and Suzaku

Abstract

We have identified two moderately bright, rapidly variable transients in new and archival X-ray data near the Galactic center. Both objects show strong, flaring variability on timescales of tens to thousands of seconds, evidence of N_H variability, and hard spectra. XMMU J174445.5-295044 is seen at 2-10 keV fluxes of 3 × 10^-11 to <10^-12 erg cm^-2 s^-1, with N_H at or above 5 × 10²² cm^-2, by XMM-Newton, Chandra, and Suzaku. A likely Two Micron All Sky Survey (2MASS) counterpart with K_S = 10.2 shows colors indicative of a late-type star. CXOU J174042.0-280724 is a likely counterpart to the fast hard transient IGR J17407-2808. Chandra observations find F_X (2-10 keV) ~10^-12 erg cm^-2 s^-1, with large N_H variations (from 2 × 10²² to >2 × 10²³ cm^-2). No 2MASS counterpart is visible, to K_S >13. XMMU J174445.5-295044 seems likely to be a new symbiotic star or symbiotic X-ray binary, while CXOU J174042.0-280724 is more mysterious, likely an unusual low-mass X-ray binary.