The 2013 outburst of a transient very faint X-ray binary, 23 arcsec from Sgr A*

We report observations using the Swift/XRT, NuSTAR, and Chandra X-ray telescopes of the transient X-ray source CXOGC J174540.0-290005 during its 2013 outburst. Due to its location in the field of multiple observing campaigns targeting Sgr A*, this is one of the best-studied outbursts of a very faint X-ray binary (VFXB; peak LX < 1036 erg s−1) yet recorded, with detections in 173 ks of X-ray observations over 50 d. VFXBs are of particular interest, due to their unusually low outburst luminosities and time-averaged mass transfer rates, which are hard to explain within standard accretion physics and binary evolution. The 2013 outburst of CXOGC J174540.0-290005 peaked at LX(2-10 keV) = 5.0 × 1035 erg s−1, and all data above 1034 erg s−1 were well fitted by an absorbed power law of photon index ∼1.7, extending from 2 keV out to ≳70 keV. We discuss the implications of these observations for the accretion state of CXOGC J174540.0-290005.