CXOGBSJ174444.7-260330: a new long orbital period cataclysmic variable in a low state

TL;DR

This study characterizes a newly discovered long orbital period cataclysmic variable star in a low accretion state, using spectroscopy and photometry to determine its properties and demonstrate the effectiveness of the Galactic Bulge Survey.

Contribution

It provides detailed measurements of the system's orbital period, component masses, and inclination, and shows that CVs in low states can mimic quiescent neutron star or black hole spectra.

Findings

Orbital period P ≈ 5.69 hours.

White dwarf primary with M1 ≈ 0.8 Msun.

Evidence of variability on timescales of months or faster.

Abstract

We present phase-resolved spectroscopy and photometry of a source discovered with the Chandra Galactic Bulge Survey (GBS), CXOGBSJ174444.7-260330 (aka CX93 and CX153 in the previously published GBS list). We find two possible values for the orbital period P, differing from each other by 13 seconds. The most likely solution is P =5.69014(6) hours. The optical lightcurves show ellipsoidal modulations, whose modeling provides an inclination of 32+-1 degrees for the most likely P. The spectra are dominated by a K5V companion star (the disc veiling is <~5%). Broad and structured emission from the Balmer lines is also detected, as well as fainter emission from HeI. From the absorption lines we measure K2 =117+-8km/s and v sin i = 69+-7km/s. By solving the system mass function we find M1=0.8+-0.2Msun for the favored P and i, consistent with a white dwarf accretor, and M2=0.6+-0.2Msun. We estimate a distance in the range 400-700 pc. Although in a low accretion state, both spectroscopy and photometry provide evidence of variability on a timescale of months or faster. Besides finding a new, long orbital period cataclysmic variable in a low accretion state, this work shows that the design of the GBS works efficiently to find accreting X-ray binaries in quiescence, highlighting that the spectra of CVs in a low-accretion state can at times appear suggestive of a quiescent neutron star or a black hole system.