Letter to the Editor

New transient Galactic bulge intermediate polar candidate XMMU J175035.2-293557

¹ Max-Planck-Institut für extraterrestrische Physik, Giessenbachstraße, 85748 Garching, Germany
e-mail: fhofmann@mpe.mpg.de
² Univ. Grenoble Alpes, CNRS, Institut de Planétologie et d’Astrophysique de Grenoble (IPAG), 38000 Grenoble, France

Received: 26 February 2018
Accepted: 14 June 2018

Abstract

Context. For the past decades, a rare subclass of cataclysmic variables (CV), with magnetised white dwarfs (WD) as accretors, has been studied. They are called intermediate polars (IP) and have been suggested to be the main contributors to the diffuse, hard X-ray emission close to the Galactic center (GC) and in the Galactic bulge (GB), because most of them are unresolved there.

Aims. In an ongoing X-ray survey (0.5–10 keV energy band) of 3° × 3° around the GC with the XMM-Newton observatory, we conducted a systematic search for transient X-ray sources.

Methods. Promising systems were analyzed for spectral, timing, and multiwavelength properties to constrain their nature.

Results. We discovered a new highly variable (factor ≳20) X-ray source about 1.25° south of the GC. We found evidence that makes the newly discovered system a candidate IP. The X-ray light curve shows a period of 511 ± 10 s, which can be interpreted as the spin period of the WD. The X-ray spectrum is well fit by a bremsstrahlung model with a temperature of 13.9 ± 2.5 keV, suggesting a WD mass of 0.4−0.5 M_⊙. We were unable to identify a blue optical counterpart in any of the candidate sources, as would be expected for IPs.

Conclusions. The high X-ray absorption and absence of a clear optical counterpart suggest that the source is most likely located in the GB. This would make the system a transient IP (GK Per class) with an especially high peak X-ray luminosity, which means that it is a very faint X-ray transient.