2010A&A...524A..69D

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 2-10keV peak luminosities of L_X, peak_∼10^34–36erg/s, although the two confirmed neutron star low-mass X-ray binaries AX J1745.6-2901 and GRS 1741-2853 can also become brighter (L_X, peak_∼10^36–37erg/s). We discuss the observed long-term 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 2-10keV luminosity of L_X∼1x10³⁷ (D/7.2kpc)²erg/s. 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 (L_X∼10³²erg/s) and its maximum outburst luminosity of L_X∼10³⁶erg/s. CXOGC J174535.5-290124 and CXOGC J174538.0-290022, as well as three other very-faint X-ray transients that were detected by Swift monitoring observations in 2006, have very low time-averaged mass-accretion rates of <{dot}(M)>_long≲2x10^–12M_☉/yr. Despite having obtained two years of new data in 2008 and 2009, no new X-ray transients were detected.