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

Aims. 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. Methods. We are performing an X-ray survey of the Galactic disc using XMM-Newton, 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 NuSTAR 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. Results. The spectral modelling indicates the presence of an intervening cloud with N_H ∼ (1.5 ‑ 2.3)×10²³ cm^‑2 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_‑5⁺¹¹ keV. Furthermore, an Fe K_α line at 6.47_‑0.06^+0.13 keV was detected with an equivalent width of the line of 312 ± 104 eV. We discovered a coherent pulsation with a period of 521.7 ± 0.8 s. The 3–10 keV pulsed fraction of the source is around ∼50–60%. Conclusions. The hard X-ray emission with plasma temperature kT = 26_‑5⁺¹¹ keV, iron K_α emission at 6.4 keV, and a periodic behaviour of 521.7 ± 0.8 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 M_⊙ by assuming a distance to the source of ∼1.4 ‑ 5 kpc.