A four-year baseline Swift study of enigmatic X-ray transients located near the Galactic center

Abstract

We report on continued monitoring observations of the Galactic center carried out by the X-ray telescope aboard the Swift satellite in 2008 and 2009. This campaign revealed activity of the five known X-ray transients AX J1745.6-2901, CXOGC J174535.5-290124, GRS 1741-2853, XMM J174457-2850.3 and CXOGC J174538.0-290022. All these sources are known to undergo very faint X-ray outbursts with 2-10 keV peak luminosities of LX,peak ∼ 10 34-36 erg s-1, although the two confirmed neutron star low-mass X-ray binaries AX J1745.6-2901 and GRS 1741-2853 can also become brighter (LX,peak ∼ 1036-37 erg s-1). We discuss the observed longterm lightcurves and X-ray spectra of these five enigmatic transients. In 2008, AX J1745.6-2901 returned to quiescence following an unusually long accretion outburst of more than 1.5 years. GRS 1741-2853 was active in 2009 and displayed the brightest outburst ever recorded for this source, reaching up to a 2-10 keV luminosity of LX ∼ 1× 1037 (D/7.2 kpc)2 erg s-1. This system appears to undergo recurrent accretion outbursts approximately every 2 years. Furthermore, we find that the unclassified transient XMM J174457-2850.3 becomes bright only during short episodes (days) and is often found active in between quiescence (LX ∼ 1032 erg s-1) and its maximum outburst luminosity of LX ∼ 1036 erg s-1. CXOGC J174535.5-290124 and CXOGC J174538.0-290022, as well as three other veryfaint X-ray transients that were detected by Swift monitoring observations in 2006, have very low time-averaged mass-accretion rates of Mlong