Neglected X-ray discovered polars

I. Giant flares in V358 Aquarii

¹ Georg-August-Universität Göttingen, Institut für Astrophysik, Friedrich-Hund-Platz 1, 37077 Göttingen, Germany
e-mail: k.beuermann@t-online.de
² MPI für extraterrestrische Physik, Giessenbachstr. 6, 85740 Garching, Germany
³ Leibniz-Institut für Astrophysik Potsdam (AIP), An der Sternwarte 16, 14482 Potsdam, Germany
⁴ MPI für Astrophysik, Karl-Schwarzschild-Str. 1, 85740 Garching, Germany

Received: 16 March 2017
Accepted: 13 April 2017

Abstract

We report photometric and spectroscopic observations of the polar V358 Aqr (=RX J2316-0527) collected over 25 yr. It was discovered as a bright very soft X-ray source in the ROSAT All Sky Survey, but had lapsed into a low state when reobserved in 1993. We have obtained an optical photometric ephemeris, which is free of cycle count errors and allows to correctly phase events around the orbit even for observations that lie decades apart. V358 Aqr possesses an accreting pole in the upper hemisphere of the white dwarf that is visible over the entire orbital period of 209.45 min. The magnetic field strength derived from cyclotron line emission is 31.8 MG. The orbital motion of the cyclotron lines yields an inclination of 60° and an offset of the magnetic pole from the rotational axis of about 10°. The secondary star is of spectral type dM4.0 ± 0.5 and the distance is 540 ± 100 pc. V358 Aqr is peculiar in showing giant optical outbursts that bear all of the characteristics of stellar flares. With two flares observed in 72 h on source, the flare frequency may be high. The total energy radiated in the flare of 28 November 2010 exceeded 10³⁶ erg. While the flares clearly occur on the secondary star, it seems that they may or may not be connected with coronal mass ejection and subsequent accretion onto the white dwarf.