XMM-Newton observation of the highly magnetised accreting pulsar Swift J045106.8-694803: Evidence of a hot thermal excess

TL;DR

This study reports XMM-Newton observations of the highly magnetized accreting pulsar Swift J045106.8-694803, revealing a hot thermal excess likely originating from neutron star polar caps, with detailed spectral and timing analysis.

Contribution

It provides the first detection of a blackbody component with specific temperature and radius in this pulsar, and models its pulse profile to infer system geometry.

Findings

Detection of a 1.8 keV blackbody component with a 0.5 km radius.

Pulsed fraction decreases with energy, indicating phase-shifted emission components.

Blackbody pulsations are out of phase with the power-law component.

Abstract

Several persistent, low luminosity (L_X 10^{34} erg s^{-1}), long spin period (P>100 s) High Mass X-ray Binaries have been reported with blackbody components with temperatures >1 keV. These hot thermal excesses have correspondingly small emitting regions (<1 km^2) and are attributed to the neutron star polar caps. We present a recent XMM-Newton target of opportunity observation of the newest member of this class, Swift J045106.8-694803. The period was determined to be 168.5+/-0.2 s as of 17 July 2012 (MJD = 56125.0). At $L_X 10^{36} erg s^{-1}, Swift J045106.8-694803 is the brightest member of this new class, as well as the one with the shortest period. The spectral analysis reveals for the first time the presence of a blackbody with temperature kT_{BB}=1.8^{+0.2}_{-0.3} keV and radius R_{BB}=0.5+/-0.2 km. The pulsed fraction decreases with increasing energy and the ratio between the hard (>2 keV) and soft (<2 keV) light curves is anticorrelated with the pulse profile. Simulations of the spectrum suggest that this is caused by the pulsations of the blackbody being pi out of phase with those of the power law component. Using a simple model for emission from hot spots on the neutron star surface, we fit the pulse profile of the blackbody component to obtain an indication of the geometry of the system.