Identifying a new intermediate-polar using XMM-Newton and INTEGRAL

TL;DR

This paper identifies 2XMMi J180438.7-145647 as a bright intermediate polar using X-ray data, correcting previous optical classification, and estimates its properties including distance, white dwarf mass, and accretion behavior.

Contribution

The study provides the first X-ray based identification of the source as an intermediate polar, clarifies its distance, and analyzes its spectral and timing properties to understand its accretion mechanism.

Findings

Identified the source as an intermediate polar with a ~24 min white dwarf spin period.

Estimated the white dwarf mass to be approximately 0.60 solar masses.

Suggested the source is a stream-fed IP at a distance less than 2.5 kpc.

Abstract

The bright X-ray source, 2XMMi J180438.7-145647 is fortunate to have long baseline observations in INTEGRAL that compliment observations taken by other missions. Optical spectroscopy of this object has suggested a distance of ~7 kpc and an identification with a low mass X-ray binary. We instead use the X-ray data from 0.3-40 keV to identify the source as a bright intermediate polar (IP) with an estimate for the white dwarf mass of~0.60 M_solar. This identification is supported by the presence of an iron triplet, the component lines of which are some of the strongest seen in IPs; and the signature of the spin period of the white dwarf at ~24 mins. We note that the lack of broad-band variability may suggest that this object is a stream-fed IP, similar in many respects to the well studied IP, V2400 Oph. Phase-binning has allowed us to create spectra corresponding to the peaks and troughs of the lightcurve from which we determine that the spectra appear harder in the troughs, consistent with the behaviour of other IPs binned on their spin periods. This work strongly suggests a mis-identification in the optical due to the presence of large columns of enshrouding material. We instead propose a distance to the source of <2.5 kpc to be consistent with the luminosities of other IPs in the dim, hard state. The relatively high flux of the source together with the strength of the iron lines may, in future, allow the source to be used to diagnose the properties of the shock heated plasma and the reflected component of the emission.