Gaia19bxc: possible polar below the period minimum

Abstract

Gaia19bxc is a transient detected on 2019 May 9 by the Gaia Photometric Science Alerts Team. I analyzed the past public Zwicky Transient Facility (ZTF) data and found that Gaia19bxc has a period of 0.04473647(3) d and two different maxima in one cycle. This object also showed high and low states in the ZTF data. Based on the high amplitude (2.0 mag) of the short-term variations, short period, almost zero color indices between the different ZTF bands and the absence of a longer period, I classified it to be a likely polar. There has been no established polar below the period minimum of cataclysmic variables (CVs) and Gaia19bxc could be the first such an object. CVs below the period minimum usually have a secondary star with a stripped evolved core and Gaia19bxc is expected to have a similar secondary. If this is indeed the case, Gaia19bxc could become a highly magnetized exotic ultracompact binary during its secular evolution.