A new model for the X-ray continuum of the magnetized accreting pulsars

Abstract

Context. Accreting highly magnetized pulsars in binary systems are among the brightest X-ray emitters in our Galaxy. Although a number of high-quality broad-band (0.1-100 keV) X-ray observations are available, the spectral energy distribution of these sources is usually investigated by adopting pure phenomenological models rather than models linked to the physics of accretion. Aims. In this paper, a detailed spectral study of the X-ray emission recorded from the high-mass X-ray binary pulsars Cen X-3, 4U 0115+63, and Her X-1 is carried out by using BeppoSAX and joined Suzaku +NuStar data, together with an advanced version of the compmag model, which provides a physical description of the high-energy emission from accreting pulsars, including the thermal and bulk Comptonization of cyclotron and bremsstrahlung seed photons along the neutron star accretion column.Methods. The compmag model is based on an iterative method for solving second-order partial differential equations, whose convergence algorithm has been improved and consolidated during the preparation of this paper. Results. Our analysis shows that the broad-band X-ray continuum of all considered sources can be self-consistently described by the compmag model. The cyclotron absorption features (not included in the model) can be accounted for by using Gaussian components. From the fits of the compmag model to the data we inferred the physical properties of the accretion columns in all sources, finding values reasonably close to those theoretically expected according to our current understanding of accretion in highly magnetized neutron stars.Conclusions. The updated version of the compmag model has been tailored to the physical processes that are known to occur in the columns of highly magnetized accreting neutron stars and it can thus provide a better understanding of the high-energy radiation from these sources. The availability of broad-band high-quality X-ray data, such as those provided by BeppoSAX in the past and currently from NuStar and other facilities, is crucial to fully exploit the potentialities of the model. The advent of the Astro-H mission, endowed with an unprecedented combination of high sensitivity and X-ray broad-band coverage, provides good perspectives to improve our understanding of accretion onto highly magnetized neutron stars through physical models like the one adopted here.

Looking at A 0535+26 at low luminosities with NuSTAR

“…It is also close the application of their model to the spectrum of Her X-1 (Wolff et al 2016). Farinelli et al (2016) applied an advanced version of the compmag model to data of Cen X-3, 4U 0115+63, and Her X-1 and found smaller electron temperatures of 0.8-3 keV which they explained being due to the inclusion of second order bulk Comptonization in the RTE. Comparing our compmag fits to the successful application of the same version of the model to data of the accreting pulsar RX J0440.9+4431 by Ferrigno et al (2013), however, the resulting Compton-y values are consistently much smaller than unity in both cases (although we find higher optical depths and lower electron temperatures than these authors).…”

New Constraints on the Geometry and Kinematics of Matter Surrounding the Accretion Flow in X-Ray Binaries from Chandra High-energy Transmission Grating X-Ray Spectroscopy

“…In the last few years, a number of papers have presented analyses for some of these objects using broadband X-ray data from missions such as NuSTAR or Suzaku. Thus Jaisawal & Naik (2015) analyze Suzaku data for 4U1700−37; Sasano et al (2014) analyze Suzaku data for 4U1822−371; Naik et al (2011) analyze Suzaku, andFarinelli et al (2016) Suzaku and NuSTAR data for Cen X−3; Tomsick et al (2014) and Parker et al (2015) analyze Suzaku and NuSTAR data for Cyg X−1, while Walton et al (2016) NuSTAR-only data for the same object; Suchy et al (2012) analyze Suzaku data for GX 301−2; Yoshida et al (2017) analyze Suzaku data for GX 1+4; Fürst et al (2013) and Wolff et al (2016) analyze NuSTAR, andFarinelli et al (2016) Suzaku and NuS-TAR data for Her X−1; Shtykovsky et al (2017) analyze NuSTAR, and Hung et al (2010) Suzaku data for LMCX−4; Jaisawal & Naik (2014) and Pradhan et al (2014) analyze Suzaku data for OAO 1657−415;and Maitra & Paul (2013) analyze Suzaku data for Vela X−1.…”

Glancing through the accretion column of EXO 2030+375

“…A number of sophisticated spectral models have been developed to describe the X-ray energy distribution of highly magnetized accreting X-ray pulsars (e.g., Becker & Wolff 2007;Farinelli et al 2012;Farinelli et al 2016). They include a fairly detailed description of the physics of the NS accretion column, where the bulk of the X-ray emission is produced (see Sect.…”