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

Abstract

For the past decades a rare subclass of cataclysmic variables (CV), with magnetized white dwarfs (WD) as accretors has been studied and called intermediate polars (IP). They have been discussed as the main contributor to the diffuse X-ray emission due to unresolved point sources close to the Galactic center (GC) and in the Galactic bulge (GB). In an ongoing X-ray survey (0.5-10 keV energy band) of about 3 deg x 3 deg around the GC with the XMM-Newton observatory we conducted a systematic search for transient X-ray sources. Promising systems were analyzed for spectral, timing, and multi-wavelength properties to constrain their nature. We discovered a new highly variable (factor $\mathrm{\gtrsim{20}}$) X-ray source about 1.25 deg south of the GC. We found evidence making the newly discovered system a candidate IP. The X-ray light curve shows a period of $\mathrm{511\pm10 s}$ 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 $\mathrm{13.9\pm2.5 keV}$, suggesting a WD mass of $\mathrm{0.4-0.5 M_\odot}$. Among many candidates we could not identify a blue optical counterpart as would be expected for IPs. 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 especially high peak X-ray luminosity and therefore a very faint X-ray transient (VFXT).