Red-skewed Kα iron lines in GX 13+1

Abstract

Context. Broad, asymmetric, and red-skewed Fe K α emission lines have been observed in the spectra of low-mass X-ray binaries hosting neutron stars (NSs) as a compact object. Because more than one model is able to describe these features, the explanation of where and how the red-skewed Fe lines are produced is still a matter of discussion. It is broadly accepted that the shape of the Fe K α line is strongly determined by the special and general relativistic effects occurring in the innermost part of the accretion disk. In this relativistic framework, the Fe fluorescent lines are produced in the innermost part of the accretion disk by reflection of hard X-ray photons coming from the central source (corona and/or NS surface). We developed an alternative and nonrelativistic model, called the windline model, that is capable to describe the Fe line features. In this nonrelativistic framework, the line photons are produced at the bottom of a partly ionized outflow (wind) shell as a result of illumination by the continuum photons coming from the central source.In this model the red-skewness of the line profile is explained by repeated electron scattering of the photons in a diverging outflow. Aims. Examining the asymmetry of the fluorescent Fe K emission line evident in the XMM-Newton EPIC-pn spectra of the NS source GX 13+1, we aim to distinguish between the two line models. Because GX 13+1 is a well-known disk-wind source, it is a perfect target for testing the windline model and compare the spectral fits between the relativistic and nonrelativistic line models. Methods. We used two XMM-Newton EPIC-pn observations in which the Fe line profiles were previously reported in the literature. These observations are not strongly affected by pile-up, and the Fe emission lines appear very strong and red-skewed. In order to access the goodness of the fit and distinguish between the two line models, we used the run-test statistical method in addition to the canonical χ 2 statistical method. Results. The diskline and windline models both fit the asymmetric Fe line well that is present in the XMM-Newton energy spectra of GX 13+1. From a statistical point of view, for the two observations we analyzed, the run-test was not able to distinguish between the two Fe line models, at 5% significance level.

Relativistic X-ray reflection and photoionized absorption in the neutron star low-mass X-ray binary GX 13+1

“…When we fitted the narrow residuals with Gaussian emission lines (Model 2), we found that the relativistic Fe-line complex could only be explained if the source inclination was low, i ࣠ 30 • . A low inclination is inconsistent with previous spectral results obtained with other instruments with better energy resolution than NuSTAR (D íaz Trigo et al 2012 ;Maiolino et al 2019 ). Additionally, a high inclination is necessary to explain the presence of dips in the light curves of GX 13 + 1 (Iaria et al 2014 ).…”

“…In Model 3, we considered a similar continuum model as Model 2, but we used multiplicative Gaussian absorption profiles, gabs , to fit MNRAS 522, 3367-3377 (2023) the narrow residuals that we identified as Fe XXVI K α at ∼6.7 keV, and either Ni XXVII and/or Ni XXVIII at ∼8 keV. Interestingly, our best-fitting Model 3 yielded an inclination of approximately ∼70 • , consistent with previous observations (D íaz Trigo et al 2012 ;Maiolino et al 2019 ).…”

“…Therefore, we will not consider this scenario further. Moreo v er, previous studies using other satellites have shown the presence of ionized absorbing material (D íaz Trigo et al 2012 ;Maiolino et al 2019 ).…”

“…They concluded that the presence of a disc wind and/or a warm atmosphere may explain the observations of GX 13 + 1, leading to a system inclination of 60-80 • . In addition, Maiolino et al ( 2019 ) analysed the same XMM -Newton observations, fitting a diskline type model to an Fe K α asymmetric emission line, finding that the inclination was also ∼60 • .…”

Testing Comptonization as the origin of the continuum in nonmagnetic Cataclysmic Variables. The photon index of X-ray emission

“…On the other hand, they noticed that spectral features expected by the reflection scenario such as the K α fluorescent lines are either very weak or not observed during the outburst state in U Gem. It is important to stress, however, that reflection scenario is not the only one capable to explain the K α fluorescent line production (see, e.g., Maiolino et al 2019). Mukai et al (2003) showed that the Chandra spectrum of the DN U Gem (as well as SS Cyg and V603 Aql) can be described by the multi-temperature bremsstrahlung cooling-flow model mkcflow with kT max = 20 keV.…”