Discovery and characterization of ZTF J0112+5827: An 80.9-minute polar with strong cyclotron features

Abstract

Context. A new X-ray Cataclysmic variable (CV) candidate exhibits distinct light-curve characteristics in the ZTF’s g, r, and i bands. The paper includes the optical identification and multiwavelength analysis of this CV candidate.Aims. This work aims to determine if a previously identified CV candidate, ZTF J0112+5827, is a polar system by examining its X-ray and cyclotron radiation characteristics.Methods. We characterized the X-ray emission of ZTF J0112+5827 using the ROSAT observations. The gri-band optical light curves were obtained from the Zwicky Transient Facility. After two nights of time-domain spectroscopic observations with the Palomar 200-inch telescope, we mapped the accretion structures using Doppler tomography.Results. ZTF J0112+5827 exhibits an orbital period of 80.9 minutes, determined from the ZTF light curves, and an average X-ray flux of (68.4 ± 15.7)×10^{−14 erg s−1 cm−2 in the 0.1–2.4 keV range. It shows an ellipsoidal-like variability curve in the g band, with two prominent humps around phases of ∼0.0 and ∼0.7 in the i and r bands. In the spectra corresponding to these phases, a redward-increasing power-law continuum appears, which is accompanied by prominent features of cyclotron emission humps. Emission lines of He II and Balmer series were observed. The magnetic field strength of ZTF J0112+5827 was determined from the cyclotron harmonics. Its tomography map revealed the presence of accretion streams, but there was no evidence of an accretion disk structure. The line-of-sight velocity of the Balmer emission was measured at about 500 km s−1, the majority of which was contributed by accretion streams and accretion spots. Our result confirms that ZTF J0112+5827 is a polar system. It contains a magnetic white dwarf with a magnetic field strength of MG.}