An Optical Search for Outbursting X-Ray Binaries

Transient Low-Mass X-ray binaries (LMXBs) have historically been discovered largely by X-ray and gamma-ray all-sky monitors. However, the X-ray outburst is also accompanied by an optical brightening, which empirically can precede detection of X-rays. Newly sensitive optical synoptic surveys offer a complementary pathway for discovery, and potential for insight into the initial onset and propagation of the thermal instability that leads to the ionization of the accretion disk. We use the Zwicky Transient Facility (ZTF) alert stream, which detects, packages, and streams hundreds of thousands of variable and transient sources per night in near real time to perform a comprehensive live search at optical wavelengths for outbursting XRBs. Our pipeline first crossmatches the positions of the alerts to cataloged X-ray sources, and then analyzes the 30-day lightcurve of matched alerts by thresholding on differences with an 8-day exponentially weighted moving average. In addition to an 18 month long live search for XRBs, we ran our pipeline over four years of ZTF archival data. We recovered 4 known XRBs in our archival search. We also independently detected an outburst of MAXI J1957+032, and an accreting millisecond pulsar, in the live search and found the first outburst of Swift J1943.4+0228, an unclassified X-ray transient, in 10 years.