The role of XMM-Newton in the investigation of persistent BeXRBs

TL;DR

This paper reviews how XMM-Newton observations have advanced understanding of persistent BeXRBs, highlighting their spectral and timing properties, and reports new findings on the source 4U 0728-25.

Contribution

It provides an overview of XMM-Newton's role in characterizing persistent BeXRBs and presents new observational results for 4U 0728-25.

Findings

Most persistent BeXRBs share common spectral and timing properties.

A hot blackbody component contributes significantly to the flux.

Recent observations of 4U 0728-25 offer new insights.

Abstract

The persistent BeXRBs are a class of High-Mass X-ray Binaries (HMXRBs), which are characterized by persistent low X-ray luminosities ($L_{\rm X} \sim 10^{34}$ erg s$^{-1}$) and wide ($P_{\rm orb} >$ 30 d), almost circular orbits. In these sources the NS is slowly rotating (with $P_{\rm spin}$ well above 100 s) and accretes matter directly from the wind of the companion Be star, without the formation of an accretion disk. Since the '90s, when the first four members of this class were identified, several other sources of the same type have been discovered and investigated. Thanks to follow-up XMM-Newton observations, we have verified that most of them share common spectral and timing properties, such as a pulsed fraction that does not vary with the photon energy and a hot (kT = 1-2 keV) blackbody spectral component which contributes for 20-40 % to the total flux and has a size consistent with the NS polar cap. Here we provide an overview of how XMM-Newton contributed to constrain the observational properties and the current understanding of this type of sources. We also report about the first results obtained with a very recent XMM-Newton observation of the poorly known BeXRB 4U 0728-25.