Zeeman lines and a single cyclotron line in the low-accretion rate polar 1RXS J012851.9-233931 (RBS0206)

TL;DR

This paper reports the discovery of Zeeman and cyclotron features in a low-accretion polar, revealing a magnetic white dwarf with a field of about 36-45 MG, and discusses its potential for further magnetic field studies.

Contribution

First spectroscopic and photometric analysis of 1RXS J012851.9-233931 showing Zeeman and cyclotron features in a low-accretion polar, indicating a strong magnetic field and low accretion rate.

Findings

White dwarf magnetic field of 36 ± 1 MG from Zeeman features

Cyclotron hump indicating a plasma magnetic field of 45 ± 1 MG

Orbital period likely around 90 or 146 minutes

Abstract

We present a low-resolution discovery spectrum and CCD-photometry of the bright X-ray source 1RXS J012851.9-233931 found in the ROSAT All-Sky Survey. These first observations suggest that the source is an AM Herculis star (polar) accreting at a low rate. The optical spectrum is dominated by Zeeman absorption features from the white dwarf, indicating a mean photospheric magnetic field of 36 +- 1 MG. Only weak Balmer line emission was observed. In the near infra-red, a single intense cyclotron hump was observed. The inferred magnetic field strength in the accretion plama is 45 +- 1 MG, the temperature in the plasma is below 2 keV. Likely orbital periods are ~90 min or ~146 min, the latter inside the cataclysmic variable period gap. The system is an ideal target for further detailed investigations of the field structure on a magnetic white dwarf by phase-resolved spectropolarimetry.