The Hot--blackbody Spectral Excess in Low--luminosity High--Mass X--Ray Binaries

Abstract

We report on the main results obtained thanks to an observation campaign with XMM-Newton of four persistent, low-luminosity (L_{X} ∼ 10^{34} erg s^{-1}) 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 (kT_{BB} > 1 keV) and small area (R_{BB} < 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.