Multiwavelength study of RX J2015.6+3711: a magnetic cataclysmic variable with a 2-hr spin period

Abstract: The X-ray source RX J2015.6+3711 was discovered by ROSAT in 1996 and recently proposed to be a cataclysmic variable (CV). Here we report on an XMM-Newton observation of RX J2015.6+3711 performed in 2014, where we detected a coherent X-ray modulation at a period of 7196+/-11 s, and discovered other significant (>6sigma) small-amplitude periodicities which we interpret as the CV spin period and the sidebands of a possible ~12 hr periodicity, respectively. The 0.3-10 keV spectrum can be described by a power law (Gamma = 1.15+/-0.04) with a complex absorption pattern, a broad emission feature at 6.60+/-0.01 keV, and an unabsorbed flux of (3.16+/-0.05)x10^{-12} erg/s/cm^2. We observed a significant spectral variability along the spin phase, which can be ascribed mainly to changes in the density of a partial absorber and the power law normalization. Archival X-ray observations carried out by the Chandra satellite, and two simultaneous X-ray and UV/optical pointings with Swift, revealed a gradual fading of the source in the soft X-rays over the last 13 years, and a rather stable X-ray-to-optical flux ratio (F_X/F_V ~1.4-1.7). Based on all these properties, we identify this source with a magnetic CV, most probably of the intermediate polar type. The 2 hr spin period makes RX J2015.6+3711 the second slowest rotator of the class, after RX J0524+4244 ("Paloma", P_spin~2.3 hr). Although we cannot unambiguously establish the true orbital period with these observations, RX J2015.6+3711 appears to be a key system in the evolution of magnetic CVs.