Iron Kα Emission from X-Ray Reflection: Predictions for Gamma-Ray Burst Models

Abstract

Recent observations of several g-ray burst (GRB) afterglows have shown evidence for a large amount of X-ray line-emitting material, possibly arising from ionized iron. A significant detection of an X-ray spectral feature, such as that found in the Chandra observation of GRB 991216, may provide important constraints on the immediate environment of the burst and hence on progenitor models. The large Fe Ka equivalent widths inferred from the X-ray observations favor models in which the line is produced when the primary X-ray emission from the source strikes Thomson-thick material and Compton scatters into our line of sight. We present such reflection spectra here, computed in a fully self-consistent manner, and discuss the range of ionization parameters that may be relevant to different models of GRBs. We argue that the presence of a strong hydrogen-like Ka line is unlikely, because Fe xxvi photons would be trapped resonantly and removed from the line core by Compton scattering. In contrast, a strong narrow emission line from He-like Fe xxv is prominent in the model spectra. We briefly discuss how these constraints may affect the line energy determination in GRB 991216.

High‐Resolution Grating Spectroscopy of Gamma‐Ray Bursts 030328 and 041006 withChandraLETGS

“…In the ''nearby reprocessor'' scenario, a long-lived central engine (Rees & Mészáros 2000) or a plasma bubble GRB (Mészáros & Rees 2001) ionizes material on the sides of a funnel carved out of the progenitor star by the GRB (at R $ 10 13 cm). Ballantyne & Ramirez-Ruiz (2001) study the equivalent widths of Fe lines, for each scenario, as a function of the continuum luminosity. Roughly, they find that the equivalent widths peak near 1 keV in each scenario for solar abundances and incident X-ray continuum luminosities of order 10 46 ergs s À1 .…”

“…Claims of low-to moderatesignificance emission lines have been made on the basis of data from several missions: Fe lines have been detected in afterglow data from ASCA (Advanced Satellite for Cosmology and Astrophysics; Yoshida et al 1999), BeppoSAX ( Piro et al 1999;Antonelli et al 2000), and the Chandra X-Ray Observatory (Piro et al 2000); and lines from highly ionized light, multiple-elements such as Mg, Si, S, Ar, and Ca have been detected in afterglow data from XMM-Newton (Reeves et al 2002;Watson et al 2003) and Chandra (Butler et al 2003). The detections are challenging to theorists because they typically imply large, concentrated masses of metals in the circumburst material (see, e.g., Lazzati et al 1999) and a very efficient reprocessing of the nonthermal afterglow continuum into line radiation (see Ballantyne & Ramirez-Ruiz 2001;Lazzati et al 2002). For observers, the challenge is to obtain significant and unambiguous detections or firm upper limits.…”

X-ray spectral diagnostics of the immediate environment of GRB 991216

“…We find that iron enrichment is not necessary to reproduce the line strength, although if the true ionization parameter is substantially different than the best-fit value ξ ≈ 10 3 , then a moderately super-solar abundance will help bring the line to the required power (Ballantyne & Ramirez-Ruiz 2001;Ballantyne et al 2002). In addition, even if at a non-statistically significant level, the model seems to underestimate the line strength in the grating spectrum, and a supersolar metallicity may cure this.…”

Reconsidering the origin of the X-ray emission lines in GRB 011211

“…Ballantyne & Ramirez-Ruiz 2001). Low values of the ionization parameter (ξ < 100) will produce a reflection component with evident lines.…”

“…Woosley & Weaver 1995). Reeves et al (2003) also considered the possibility that the spectrum is produced through the reflection by an ionized slab (Ross & Fabian 1993;Ballantyne & Ramirez-Ruiz 2001), but concluded that this model (either "pure" or with an underlying power-law) cannot reproduce the data. However (as explicitly noted by the authors), most of the discrepancy between the model and the data is localized in the region around 1 keV, where lines from elements not included in the ionized reflection model, such as S, Ar and Ca, are expected.…”