Spectral analysis of the low-mass X-ray pulsar 4U 1822-371: Reflection component in a high-inclination system
Authors: Anitra A. , Di Salvo T. , Iaria R. , Burderi L. , Gambino A. F. , Mazzola S. M. , Marino A. , Sanna A. , Riggio A. 
Published: September 2021

Context. The X-ray source 4U 1822-371 is an eclipsing low-mass X-ray binary and X-ray pulsar, hosting a NS that shows periodic pulsations in the X-ray band with a period of 0.59 s. The inclination angle of the system is so high (80–85°) that in principle, it should be hard to observe both the direct thermal emission of the central object and the reflection component of the spectrum because they are hidden by the outer edge of the accretion disc. Despite the number of studies carried out on this source, many aspects such as the geometry of the system, its luminosity, and its spectral features are still debated.

Aims. Assuming that the source accretes at the Eddington limit, the analysis performed in this paper aims to investigate the presence of a reflection component. No such component has been observed before in a high-inclination accretion-disc corona source such as 4U 1822-371. To do this, we use large-area instruments with sensitivity in a broad energy range.

Methods. We analysed non-simultaneous XMM-Newton and NuSTAR observations of 4U 1822-371 and studied the average broad-band spectrum. We first reproduced the results reported in the literature, then focused on the research of reflection features. We modelled the spectral emission of the source using two different reflection models, DISKLINE plus PEXRIV or, alternatively, the self-consistent reflection model RFXCONV. We also included six Gaussian components ascribable to emission lines at low energies.

Results. In our analysis, we find significant evidence of a reflection component in the spectrum, in addition to two narrow (Gaussian) lines at 6.4 and 7.1 keV associated with neutral (or mildly ionised) iron, Fe Kα, and Kβ transitions, respectively. The continuum spectrum is well fitted by a saturated Comptonisation model with an electron temperature of 4.9 keV and a thermal black-body-like component that might be emitted by the accretion disc at a lower temperature (∼0.2 keV). We identify emission lines from O VIII, Ne IX, Mg XI, and Si XIV. We also added two new eclipse times related to NuSTAR and Swift observations to the most recent ephemeris reported in literature, updating thus the ephemeris and finding a Porb = 5.57063023(34) h and a Ṗorb value of 1.51(5) × 10−10 s s−1.

Conclusions. In our proposed scenario, 4U 1822-371 is accreting at the Eddington limit with an intrinsic luminosity of ∼1038 erg s−2, while the observed luminosity is two orders of magnitude lower because of the high inclination angle of the system. Despite this high inclination, we find that a reflection component is required to fit residuals at the Fe line range and to model the hard excess observed in the NuSTAR spectrum. The inclination inferred from the reflection component is in agreement with values previously reported in literature for this source, while the best-fit value of the inner disc radius is still uncertain and model dependent. More observations are therefore needed to confirm these results, which can give important information on the central emitting region in this enigmatic and peculiar source.