An Optical Search for New Outbursting Low Mass X-Ray Binaries

Transient low-mass X-ray binaries (LMXBs) are discovered largely by X-ray and gamma-ray all-sky monitors. The X-ray outburst is also accompanied by an optical brightening, which empirically can precede the detection of X-rays. Newly sensitive optical synoptic surveys may 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 to perform a comprehensive search at optical wavelengths for previously undiscovered outbursting LMXBs. 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 a 19 month long live search, we ran our pipeline over 4 yr of ZTF archival data, recovering four known LMXBs. We also independently detected an outburst of MAXI J1957+032 in the live search and found the first outburst of Swift J1943.4+0228, an unclassified X-ray transient, in 10 yr. Using Monte Carlo simulations of the Galactic LMXB population, we estimate that 29% of outbursting LMXBs are detectable by ZTF and that 4.4% of LMXBs would be present in the crossmatched X-ray catalogs, giving an estimated Galactic population of ${3390}_{-1930}^{+3980}$ . We estimate that our current pipeline can detect 1.3% of all outbursting LMXBs, including those previously unknown, but that Rubin Observatory's Legacy Survey of Space and Time will be able to detect 43% of outbursting LMXBs.