Swift J174540.7-290015: a new accreting binary in the Galactic Center

TL;DR

This paper reports the discovery and multi-wavelength analysis of Swift J174540.7-290015, a new low mass X-ray binary near the Galactic Center, exhibiting spectral state changes and typical accretion signatures.

Contribution

It presents the identification and detailed spectral and positional analysis of a new accreting binary in the Galactic Center region, including its spectral states and environmental properties.

Findings

Detected the transient near the Galactic Center with spectral state changes.

Observed a soft X-ray thermal spectrum with a high energy tail.

Found high neutral absorption and low iron abundance, suggesting dust depletion.

Abstract

We report on the identification of the new Galactic Center (GC) transient Swift J174540.7-290015 as a likely low mass X-ray binary (LMXB) located at only 16 arcsec from Sgr A*. This transient was detected on 2016 February 6th during the Swift GC monitoring, and it showed long-term spectral variations compatible with a hard to soft state transition. We observed the field with XMM-Newton on February 26th for 35 ks, detecting the source in the soft state, characterised by a low level of variability and a soft X-ray thermal spectrum with a high energy tail (detected by INTEGRAL up to ~50 keV), typical of either accreting neutron stars or black holes. We observed: i) a high column density of neutral absorbing material, suggesting that Swift J174540.7-290015 is located near or beyond the GC and; ii) a sub-Solar Iron abundance, therefore we argue that Iron is depleted into dust grains. The lack of detection of FeK absorption lines, eclipses or dipping suggests that the accretion disc is observed at a low inclination angle. Radio (VLA) observations did not detect any radio counterpart to Swift J174540.7-290015. No evidence for X-ray or radio periodicity is found. The location of the transient was observed also in the near-IR with GROND at MPG/ESO La Silla 2.2m telescope and VLT/NaCo pre- and post-outburst. Within the Chandra error region we find multiple objects that display no significant variations.