Photoionization Emission Models for the Cyg X-3 X-Ray Spectrum

Abstract

We present model fits to the X-ray line spectrum of the well known High Mass X-ray binary Cyg X-3. The primary observational dataset is a spectrum taken with the Chandra X-ray Observatory High Energy Transmission Grating (HETG) in 2006, though we compare it to all the other observations of this source taken so far by this instrument. We show that the density must be ≥ 10 12 cm −3 in the region responsible for most of the emission. We discuss the influence of the dust scattering halo on the broad band spectrum and we argue that dust scattering and extinction is not the most likely origin for the narrow feature seen near the Si K edge. We identify the features of a wind in the profiles of the strong resonance lines and we show that the wind is more apparent in the lines from the lighter elements. We argue that this wind is most likely associated with the companion star. We show that the intensities of most lines can be fitted, crudely, by a single component photoionized model. However, the iron K lines do not fit with this model. We show that the iron K line variability as a function of orbital phase is different from the lower energy lines, which indicates that the lines arise in physically distinct regions. We discuss the interpretation of these results in the context of what is known about the system and similar systems.

The obscured X-ray binaries V404 Cyg, Cyg X–3, V4641 Sgr, and GRS 1915+105

“…All the spectra show a strong iron line with a line width above 200 eV. This has been resolved as a combination of a neutral iron Kα line (6.4 keV) and ionized iron lines, with the strongest component arising from the Fe XXV Kα line (6.7 keV) and possibly from Fe XXVI Lyα line (7.0 keV) with Chandra in the case of Cyg X-3 (Paerels et al 2000;Kallman et al 2019) and V404 Cyg (King et al 2015). The energy band from ∼7 keV to ∼10 keV appears to be affected by the absorption of the abovementioned species of iron in all sources.…”

“…The interstellar absorption for Cyg X-3 is relatively high as a result of its location in the plane of the Galaxy, and likely because it is located behind two spiral arms and the Cygnus X star-forming region (McCollough et al 2016). We fixed the lower limit of the hydrogen column to 3.5 × 10 22 atoms cm −2 , which is approximately the value found in studies where instruments with softer X-ray response were used (Koljonen et al 2018;Kallman et al 2019). Because the reprocessing matter in the model is neutral, we added a smeared edge in model B to account for the absorption of highly ionized iron.…”

“…This probably is a scattering cloud of decreasing ionization parameter. Similar modeling with two ionized absorbers was successfully used in Kallman et al (2019) to fit the X-ray emission lines from Chandra data with a medium-ionization Notes. Parameters without errors are kept frozen at the value shown.…”

“…Highly ionized iron lines are resolved with Chandra and reveal a distinct component of gas at much higher ionization, in addition to a component from fluorescence by neutral or near-neutral material. Attempts to unify the iron emission with that of lower-Z elements implied a need for an additional absorption component, possibly associated with a disk wind (Kallman et al 2019).…”

Multiple photoionization cross sections for Fe2+ K shell

“…The ionization balance is regulated by photoionization and recombination processes in the photoionized plasmas. The photoionized plasmas were identified in stellar gas, supernova remnants, around compact objects such as black holes and neutron stars (see, e.g., Watanabe et al 2006;Lee et al 2009;Kallman et al 2019). Furthermore, laboratorygenerated photoionized plasmas were studied using power lasers and fast magnetic pinch machines (see, e.g., Foord et al 2006;Hall et al 2014;White et al 2018).…”