The Swift Bulge Survey: optical and near-IR follow-up featuring a likely symbiotic X-ray binary & a focused wind CV. (arXiv:2001.03683v1 [astro-ph.HE])

The Swift Bulge Survey: optical and near-IR follow-up featuring a likely symbiotic X-ray binary & a focused wind CV. (arXiv:2001.03683v1 [astro-ph.HE])
<a href="http://arxiv.org/find/astro-ph/1/au:+Shaw_A/0/1/0/all/0/1">A. W. Shaw</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Heinke_C/0/1/0/all/0/1">C. O. Heinke</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Maccarone_T/0/1/0/all/0/1">T. J. Maccarone</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Sivakoff_G/0/1/0/all/0/1">G. R. Sivakoff</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Strader_J/0/1/0/all/0/1">J. Strader</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Bahramian_A/0/1/0/all/0/1">A. Bahramian</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Degenaar_N/0/1/0/all/0/1">N. Degenaar</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Kennen_J/0/1/0/all/0/1">J. A. Kennen</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Kuulkers_E/0/1/0/all/0/1">E. Kuulkers</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Rau_A/0/1/0/all/0/1">A. Rau</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Sandoval_L/0/1/0/all/0/1">L. E. Rivera Sandoval</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Shishkovsky_L/0/1/0/all/0/1">L. Shishkovsky</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Swihart_S/0/1/0/all/0/1">S. J. Swihart</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Tetarenko_A/0/1/0/all/0/1">A. J. Tetarenko</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Wijnands_R/0/1/0/all/0/1">R. Wijnands</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Zand_J/0/1/0/all/0/1">J. J. M. in &#x27;t Zand</a>

The nature of very faint X-ray transients (VFXTs) – transient X-ray sources
that peak at luminosities $L_Xlesssim10^{36} {rm erg s^{-1}}$ – is poorly
understood. The faint and often short-lived 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 $sim$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.

The nature of very faint X-ray transients (VFXTs) – transient X-ray sources
that peak at luminosities $L_Xlesssim10^{36} {rm erg s^{-1}}$ – is poorly
understood. The faint and often short-lived 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 $sim$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.

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