The Swift Bulge Survey: optical and near-IR follow-up featuring a likely symbiotic X-ray binary and a focused wind CV

Abstract

The nature of very faint X-ray transients (VFXTs) -transient X-ray sources that peak at luminosities L X 10 36 erg s −1 -is poorly understood. The faint and often shortlived outbursts make characterising VFXTs and their multi-wavelength counterparts difficult. In 2017 April we initiated the Swift Bulge Survey, a shallow X-ray survey of ∼16 square degrees around the Galactic centre with the Neil Gehrels Swift Observatory. The survey has been designed to detect new and known VFXTs, with follow-up programmes arranged to study their multi-wavelength counterparts. Here we detail the optical and near-infrared follow-up of four sources detected in the first year of the Swift Bulge Survey. The known neutron star binary IGR J17445-2747 has a K4III donor, indicating a potential symbiotic X-ray binary nature and the first such source to show X-ray bursts. We also find one nearby M-dwarf (1SXPS J174215.0-291453) and one system without a clear near-IR counterpart (Swift J175233.9-290952). Finally, 3XMM J174417.2-293944 has a subgiant donor, an 8.7 d orbital period, and a likely white dwarf accretor; we argue that this is the first detection of a white dwarf accreting from a gravitationally focused wind. A key finding of our follow-up campaign is that binaries containing (sub)giant stars may make a substantial contribution to the VFXT population.

A period-dependent spatial scatter of Galactic black hole transients

“…As a final note, our discussion is not meant to favour solely one or the other scenario. A combination is not ruled out, neither is an origin in the Bulge, where studies are ongoing (Shaw et al 2020). P. Atri.…”

Time domain astronomy with the THESEUS satellite

“…THESEUS will detect many VFXTs and enable follow-up observations with Athena and ground-based observatories at different wavelengths (e.g. [178]). It will help to investigate the repeating behavior observed in several VFXTs, which will ultimately allow us to obtain a better understanding of their mass-transfer rates.…”

Time Domain Astronomy with the THESEUS Satellite

“…THESEUS will detect many VFXTs and enable follow-up observations with Athena and ground-based observatories at different wavelengths (e.g. [175]). It will help to investigate the repeating behavior observed in several VFXTs, which will ultimately allow us to obtain a better understanding of their mass-transfer rates.…”