Outbursting compact binary candidates from the Zwicky Transient Facility

Compact binaries consist of a “normal” star accreting material onto the surface of a white dwarf, neutron star, or black hole. The white dwarf case constitutes cataclysmic variables (CVs), which are often distinguishable from the others by their characteristic optical light curve variability. CVs make up the vast majority of compact binaries since more stars end their lives as white dwarfs than the more massive alternatives. Compact binaries with a neutron star or black hole primary are collectively known as X-ray binaries (XRBs). Identification of XRBs, particularly those at low accretion levels, is more difficult, typically requiring multi-wavelength follow-up observations. In all cases, instabilities in the accretion disk can lead to changes in brightness. The Zwicky Transient Facility (ZTF) provides a photometric variability catalog of the Northern sky to a depth of 20.5 mag. This provides considerable overlap with the ROSAT all-sky survey source catalog, which identified X-ray sources in the soft 0.1−2.4 keV energy band. We perform a cross match between these two catalogs and investigate well-observed ZTF counterparts that are unclassified in the SIMBAD Astronomical Database. We implement a step function model-fitting algorithm to identify light curves that exhibit a consistent sustained increase or decrease in magnitude, which is a possible signature of an accretion state-change. We present several candidate compact binaries, with analysis of follow-up spectra and archival multi-wavelength observations.