ACHANDRASURVEY OF FLUORESCENCE Fe LINES IN X-RAY BINARIES AT HIGH RESOLUTION

Abstract

Fe K line fluorescence is commonly observed in the X-ray spectra of many X-ray binaries (XRBs) and represents a fundamental tool to investigate the material surrounding the X-ray source. In this paper, we present a comprehensive survey of 41 XRBs (10 HMXBs and 31 LMXBs) with Chandra with specific emphasis on the Fe K region and the narrow Fe Kα line, at the highest resolution possible. We find that (1) the Fe Kα line is always centered at λ = 1.9387 ± 0.0016 Å, compatible with Fe i up to Fe x; we detect no shifts to higher ionization states nor any difference between high mass X-ray binaries (HMXBs) and low mass X-ray binaries (LMXBs). (2) The line is very narrow, with FWHM < 5 mÅ, normally not resolved by Chandra which means that the reprocessing material is not rotating at high speeds. (3) Fe Kα fluorescence is present in all the HMXBs in the survey. In contrast, such emissions are astonishingly rare (∼10%) among LMXBs where only a few out of a large number showed Fe K fluorescence. However, the line and edge properties of these few are very similar to their high mass cousins. (4) The lack of Fe line emission is always accompanied by the lack of any detectable K edge. ( 5) We obtain the empirical curve of growth of the equivalent width of the Fe Kα line versus the density column of the reprocessing material, i.e., EW Kα versus N H , and show that it is consistent with a reprocessing region spherically distributed around the compact object. (6) We show that fluorescence in XRBs follows the X-ray Baldwin effect as previously only found in the X-ray spectra of active galactic nuclei. We interpret this finding as evidence of decreasing neutral Fe abundance with increasing X-ray illumination and use it to explain some spectral states of Cyg X-1 as a possible cause of the lack of narrow Fe line emission in LMXBs. ( 7) Finally, we study anomalous morphologies such as Compton shoulders and asymmetric line profiles associated with the line fluorescence. Specifically, we present the first evidence of a Compton shoulder in the HMXB X1908+075. Also, the Fe Kα lines of 4U1700−37 and LMC X-4 present asymmetric wings, suggesting the presence of highly structured stellar winds in these systems.

The accretion environment of supergiant fast X-ray transients probed withXMM-Newton

“…The fits to the spectra of a few sources also required the addition of a thin Gaussian line with a centroid energy compatible with that expected from neutral iron at 6.4 keV. Similar lines in HMXBs are known to occur due to the fluorescence of the X-ray photons released by the NS onto the surrounding stellar wind material (see, e.g., Torrejón et al 2010;Giménez-García et al 2016). We refer the reader to the following sub-sections dedicated to each source for further details.…”

The X-ray emission of local luminous infrared galaxies

“…In the latter its origin is associated with X-ray binaries and SNRs. The 6.4 keV Fe Kα line is observed in Galactic X-ray binaries (White et al 1983;Torrejón et al 2010). Using high-resolution X-ray spectroscopy, Torrejón et al (2010) studied a sample of 41 X-ray binaries.…”

“…For this reason and because the X-ray binary population in starbursts is dominated by HMXBs we only consider HMXBs in the following. We estimated the median EW of the Fe Kα line produced in X-ray binaries using the data of Torrejón et al (2010), EW = 0.07 ± 0.04 keV. Assuming that the spectra of the HMXBs in the 2-10 keV energy range can be represented as a power-law with photon index Γ = 1.2 (Persic & Rephaeli 2002), we obtain log L FeKα /L 2−10 keV = −2.1 ± 0.6.…”

THE XMM-NEWTON AND INTEGRAL OBSERVATIONS OF THE SUPERGIANT FAST X-RAY TRANSIENT IGR J16328-4726∗ †

“…The iron fluorescence lines show an interesting evolution: the centroid is at ∼6.8 keV when the source is in the JKL state while shifts up to ∼7.1 keV at higher fluxes. As the iron line centroid is correlated with 2-10 keV unabsorbed flux (see Table 1 and Table 2), line emissions at ∼ 6.8 keV and ∼ 7.1 keV could come from highly ionised iron ions: the ionization level is higher than Fe XXV and Fe XX for the state A 2 and JKL, respectively (Kallman et al 2010). Since the theory predicts that the iron line intensity ratio I K β /I Kα is ∼ 0.13 ph cm −2 s −1 (Kallman et al 2010), the lack of a strong iron line at 6.4 keV during time intervals A 2 and JKL exclude that the observed iron line at ∼ 6.8-7.1 keV can be fluorescence iron line K β , not respecting this iron line intensity ratio.…”