XMM-Newton and NuSTAR discovery of a likely IP candidate XMMU J173029.8--330920 in the Galactic disc

Abstract

We aim to characterise the population of low-luminosity X-ray sources in the Galactic plane by studying their X-ray spectra and periodic signals in the light curves. We are performing an X-ray survey of the Galactic disc using and the source XMMU J173029.8--330920 was serendipitously discovered in our campaign. We performed a follow-up observation of the source using our pre-approved target of opportunity time. We used various phenomenological models in xspec for the X-ray spectral modelling. We also computed the Lomb-Scargle periodogram to search for X-ray periodicity. A Monte Carlo method was used to simulate 1000 artificial light curves in order to estimate the significance of the detected period. We also searched for X-ray, optical, and infrared counterparts of the source in various catalogues. The spectral modelling indicates the presence of an intervening cloud with $N_ H $ that partially absorbs the incoming X-ray photons. The X-ray spectra are best fit by a model representing emission from a collisionally ionised diffuse gas with a plasma temperature of $kT=26^ $ keV. Furthermore, an Fe $K_ alpha $ line at $6.47^ $ keV was detected with an equivalent width of the line of $312 eV. We discovered a coherent pulsation with a period of $521.7 s. The 3--10 keV pulsed fraction of the source is around sim 50--60. The hard X-ray emission with plasma temperature $kT=26^ $ keV, iron $K_ alpha $ emission at 6.4 keV, and a periodic behaviour of $521.7 s suggest XMMU J173029.8--33092 to be an intermediate polar. We estimated the mass of the central white dwarf to be $0.94-1.4\ odot $ by assuming a distance to the source of $ kpc.