2XMMi J225036.9+573154 - a new eclipsing AM Her binary discovered using XMM-Newton

We report the discovery of an eclipsing polar, 2XMMi J225036.9+573154, using XMM-Newton. It was discovered by searching the light curves in the 2XMMi catalogue for objects showing X-ray variability. Its X-ray light curve shows a total eclipse of the white dwarf by the secondary star every 174 mins. An extended pre-eclipse absorption dip is observed in soft X-rays at phi=0.8-0.9, with evidence for a further dip in the soft X-ray light curve at phi~0.4. Further, X-rays are seen from all orbital phases (apart from the eclipse) which makes it unusual amongst eclipsing polars. We have identified the optical counterpart, which is faint (r=21), and shows a deep eclipse (>3.5 mag in white light). Its X-ray spectrum does not show a distinct soft X-ray component which is seen in many, but not all, polars. Its optical spectrum shows Halpha in emission for a fraction of the orbital period.

💡 Research Summary

The authors report the discovery of a new eclipsing polar, designated 2XMMi J225036.9+573154 (hereafter XMM J2250+5731), identified through a systematic search of the 2XMMi catalogue for X‑ray sources exhibiting significant variability. By applying a χ²‑based variability test to all sources with more than 500 EPIC counts, they isolated roughly 400 candidates, which were then inspected manually for periodic signatures. One object displayed a clear, repeating dip in its X‑ray light curve with a period of 0.1210 days (174.2 minutes), corresponding to a total eclipse of the white dwarf by its companion.

The X‑ray light curve, constructed from combined EPIC pn and MOS data, shows several noteworthy features. A deep, total eclipse occurs at phase φ = 0.0 in all energy bands. Prior to eclipse, a pronounced pre‑eclipse absorption dip is evident in the soft band (0.2–1 keV) spanning φ ≈ 0.8–0.9; this dip is much broader than in typical polars, suggesting that the accretion stream rises out of the orbital plane over a wide range of azimuths and obscures the accretion region for an extended interval. Additionally, a second, weaker dip appears around φ ≈ 0.4 in the soft band, possibly caused by a secondary stream component or by the rotation of the accretion region out of view. At higher energies (≥2 keV) the light curve is smoother, lacking obvious dips, which is consistent with the stream being more transparent at those energies.

Spectral analysis was limited by modest signal‑to‑noise, so the authors adopted a simple absorbed thermal bremsstrahlung model with a fixed temperature of 20 keV. The best‑fit absorption column is NH ≈ 2 × 10²⁰ cm⁻². The observed 0.2–10 keV fluxes range from 2.1 to 3.5 × 10⁻¹³ erg s⁻¹ cm⁻², and the unabsorbed bolometric fluxes from 4.8 to 8.1 × 10⁻¹³ erg s⁻¹ cm⁻². No distinct soft blackbody component (kT ≈ 40 eV), commonly seen in many polars, is required by the data. Simulations show that a soft component with kT ≲ 20 eV could remain hidden beneath the modest absorption and the UVW2 non‑detection (upper limit ≈ 2.4 × 10⁻¹⁷ erg s⁻¹ cm⁻² Å⁻¹). Thus the system belongs to the subset of polars that lack a prominent soft X‑ray excess, possibly due to low mass‑transfer rates or geometric beaming effects.

Optical identification was achieved with fast white‑light photometry from the Nordic Optical Telescope (NOT) and g‑band imaging from the Isaac Newton Telescope (INT). The counterpart is very faint (r ≈ 21 mag) and exhibits a deep (>3.5 mag) eclipse lasting about 12 minutes, matching the X‑ray ephemeris. Archival IPHAS photometry shows the source to be blue (r − i ≈ 0.2) with a modest Hα excess, placing it in the colour space occupied by cataclysmic variables. Follow‑up spectroscopy with the WHT/ISIS instrument detected Hα emission in nine out of sixteen exposures, primarily outside the pre‑eclipse dip phase, confirming the presence of an accretion stream but also indicating that the emission can be suppressed when the stream blocks the line of sight.

Using the unabsorbed bolometric flux and assuming the source was in a high accretion state during the XMM‑Newton observation, the authors estimate an X‑ray luminosity of ≈ 8 × 10²⁹ d₁₀₀² erg s⁻¹. Comparing with the typical high‑state polar luminosity (~2 × 10³² erg s⁻¹) implies a distance of roughly 1.5–2 kpc. At Galactic coordinates l = 107.2°, b = −1.6°, this places the system near the Perseus spiral arm.

In discussion, the authors interpret the extended pre‑eclipse dip as evidence for a stream that is lifted well above the orbital plane, perhaps due to a magnetic axis that is tilted and leading the line of centres by several tens of degrees. The lack of a soft X‑ray component, together with the presence of X‑ray emission at all orbital phases (except during eclipse), makes XMM J2250+5731 unusual among eclipsing polars, which often show strong soft excesses and may be X‑ray faint outside the bright phase. The observed optical variability, with the source appearing up to a magnitude brighter in earlier IPHAS data, suggests that the system undergoes high/low accretion state transitions, a common behaviour in polars.

Overall, this work demonstrates the power of systematic variability searches in large X‑ray catalogues to uncover rare eclipsing magnetic cataclysmic variables. The detailed timing and spectral characteristics of XMM J2250+5731 provide valuable constraints on accretion geometry, magnetic field orientation, and mass‑transfer dynamics in polars. Future high‑resolution X‑ray timing, phase‑resolved spectroscopy, and polarimetric observations will be essential to map the magnetic topology and to understand why some polars lack a detectable soft X‑ray component.