Swift J2218.4+1925: a new hard-X-ray-selected polar observed with XMM-Newton

Swift J2218.4+1925, a hard-X-ray source detected by Swift Burst Alert Telescope (BAT), has been proposed as a candidate magnetic cataclysmic variable of the polar type from optical spectroscopy. Using XMM-Newton we perform detailed timing and spectral analysis with simultaneous X-ray (0.3-10 keV) and optical B-band data. We complement the spectral study with archival hard-X-ray (14-70 keV) spectra collected by Swift BAT as well as with optical, near and mid-infrared photometry from Sloan Digital Sky Survey, Two-Micron All Sky Survey and Wide-field Infrared Survey Explorer archive, respectively. A strong periodic X-ray signal at 2.16 h, fully consistent with the recently determined spectroscopic orbital period, adds Swift J2218.4+1925 to the small group of hard-X-ray polars and locates it at the low edge of the orbital period gap. The X-ray pulse profile shows the typical bright and faint phases seen in polars, that last ∼70 and ∼30 per cent of the orbit, respectively. A pronounced dip centred on the bright phase is also detected. It is stronger at lower energies and is mainly produced by photoelectric absorption. A binary inclination i ∼ 40°-50° and a magnetic colatitude β ∼ 55°-64° are estimated. The source appears to accrete over a large area ∼24° wide. A multitemperature optically thin emission with complex absorption well describes the broad-band (0.3-70 keV) spectrum, with no signs of a soft X-ray blackbody component. The spectral shape strongly varies with the source rotation reaching plasma temperatures up to 55 keV, hardening at the dip and being softer during the faint phase (∼7 keV). We also find the first indication of an absorption edge due to a warm absorber in a polar. Indication of overabundance of neon is found in the Reflection Grating Spectrometer (RGS) spectra. The UV to mid-IR spectral energy distribution reveals an excess in the near and mid-IR, likely due to low cyclotron harmonics. We further estimate a white dwarf mass of 0.97 M_⊙ and a distance of 230-250 pc.