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

¹ School of Physics and Astronomy, Sun Yat-sen University, Zhuhai 519082, China
² CSST Science Center for the Guangdong-Hongkong-Macau Greater Bay Area, Sun Yat-sen University, Zhuhai 519082, China
³ School of Electrical and Electronic Engineering, Anhui Science and Technology University, Bengbu, Anhui 233030, China
⁴ INAF-Osservatorio Astronomico di Padova, Vicolo dell’Osservatorio 5, 35122 Padova, Italy
⁵ Institute of Space Sciences (ICE), Consejo Superior de Investigaciones CientÃífıcas (CSIC), Barcelona, Spain
⁶ School of Physics and Astronomy, Beijing Normal University, Beijing 100875, China
⁷ Yunnan Observatories, Chinese Academy of Sciences, Kunming 650216, China
⁸ International Center of Supernovae, Yunnan Key Laboratory, Kunming 650216, China
⁹ Physics Department and Tsinghua Center for Astrophysics, Tsinghua University, Beijing 100084, China
¹⁰ Beijing Planetarium, Beijing Academy of Science and Technology Beijing 100044, China
¹¹ University of Chinese Academy of Sciences, Beijing 100049, China
¹² National Astronomical Observatories, Chinese Academy of Sciences, Beijing 100012, China

^⋆ Corresponding authors; linjm66@mail2.sysu.edu.cn, nancy.elias@inaf.it

Received: 9 September 2024
Accepted: 1 January 2025

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 38.7_−1.1^+1.3 MG.