The hot-blackbody spectral excess in low-luminosity High-Mass X-Ray Binaries

TL;DR

This study reveals a common high-temperature blackbody spectral excess in low-luminosity Be accreting pulsars, likely originating from neutron star polar caps, based on XMM-Newton observations.

Contribution

It is the first to identify and characterize a high-temperature blackbody excess as a common feature in low-luminosity X-ray binaries, suggesting a polar cap origin.

Findings

Spectral excess characterized by high temperature (>1 keV) blackbody component.

Blackbody parameters are consistent across multiple sources.

The excess likely originates from neutron star polar caps.

Abstract

We report on the main results obtained thanks to an observation campaign with XMM-Newton of four persistent, low-luminosity (Lx ~ 10^34 erg/s) and long-period (P > 200 s) Be accreting pulsars. We found that all sources considered here are characterized by a spectral excess that can be described with a blackbody component of high temperature (kTbb > 1 keV) and small area (Rbb < 0.5 km). We show that: 1) this feature is a common property of several low-luminosity X-ray binaries; 2) for most sources the blackbody parameters (radius and temperature) are within a narrow range of values; 3) it can be interpreted as emission from the NS polar caps.