The Chandra Dust-scattering Halo of Galactic Center Transient Swift J174540.7–290015

Abstract

We report the detection of a dust-scattering halo around a recently discovered X-ray transient, Swift J174540.7–290015, which in early 2016 February underwent one of the brightest outbursts ( F X ≈ 5 × 10 −10 erg cm −2 s −1 ) observed from a compact object in the Galactic Center field. We analyze four Chandra images that were taken as follow-up observations to Swift discoveries of new Galactic Center transients. After adjusting our spectral extraction for the effects of detector pile-up, we construct a point-spread function for each observation and compare it to the GC field before the outburst. We find residual surface brightness around Swift J174540.7–290015, which has a shape and temporal evolution consistent with the behavior expected from X-rays scattered by foreground dust. We examine the spectral properties of the source, which shows evidence that the object transitioned from a soft to hard spectral state as it faded below L X  ∼ 10 36 erg s −1 . This behavior is consistent with the hypothesis that the object is a low-mass X-ray binary in the Galactic Center.