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

Abstract

Swift J2218.4+1925, a hard X-ray source detected by Swift 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 SDSS, 2M ASS and W ISE 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% 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 o − 50 o and a magnetic colatitude β ∼ 55 o − 64 o are estimated. The source appears to accrete over a large area ∼ 24 o wide. A multi-temperature 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 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 WD mass of 0.97 M ⊙ and a distance of 230 − 250 pc.

Hard X-ray cataclysmic variables

“…3). Three of the four low-luminosity IPs have indeed P Ω < 2 h. The 13 confirmed hard X-ray polars detected so far (see Bernardini et al, 2014;Gabdeev et al, 2017;Mukai, 2017;Bernardini et al, 2017Bernardini et al, , 2019b are found at 10 33 erg s −1 , overlapping the low-luminosity IPs. Among them, those few with determined magnetic field strengths (up to ∼ 40 × 10 6 G), not be expected to be strong in hard X-rays, challenge our knowledge of the emission properties in mCVs.…”

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

“…The spin modulation has a highly structured shape that resembles those observed in IPs rather than in polars, which show instead a typical on/off behaviour due to self occultation of the accreting pole and thus alternate bright and faint phases (see Warner 1995;Matt et al 2000 for the prototypical polar; Ramsay et al 2009, Traulsen et al 2010and Bernardini et al 2014 for more recent studies). The modulation has a large amplitude (about 30%) and the detection of the second harmonic suggests that accretion occurs onto two polar regions located at the opposite sides of the WD surface, and both visible.…”

2PBC J0658.0–1746: a hard X-ray eclipsing polar in the orbital period gap

“…J0658 is then the second hard X-ray discovered polar in the orbital period gap, together with SWIFT J2218.4+1925 (2.16 h) (Bernardini et al 2014). So far the total number of polars in the gap amounts to 34 systems (Ritter & Kolb 2003, updated 2016, ∼ 26% of the whole polar population.…”