MeV absorption in gamma-ray bursts as a probe of their progenitor winds

TL;DR

This work investigates MeV-scale absorption of prompt GRB photons caused by backscattered X-rays in a dense circumburst wind, a process that loads the external medium with leptons and alters the prompt spectrum via $γ$-$γ$ absorption. The authors develop a semi-analytical model that couples Thomson scattering, $γ$-$γ$ absorption, and pair-loading in a wind-like external medium, predicting a distinctive saddle-shaped absorption for a hard low-energy spectrum ($α>-1$). Applying the model to GRB 190114C’s early prompt emission, they achieve a significantly better fit than the Band function alone and infer wind densities of order $\hat{A}_⋆\sim10^{4}$ cm$^{-3}$ and an absorption radius $R_0\sim2.5\times10^{16}$ cm, implying high progenitor mass-loss rates. The results suggest that external MeV absorption can account for common GRB spectral properties, provides a direct probe of the immediate circumburst environment, and could serve as a redshift indicator through the rest-frame absorption threshold around $2.5\,m_e c^2$.

Abstract

A small fraction of X-ray photons from gamma-ray bursts (GRBs), after escaping the relativistic jet, are scattered by electrons in the circumburst medium. Subsequent photon-photon absorption between the incoming MeV $γ$-rays and the back-scattered X-rays generate electron-positron pairs, enriching the surrounding medium with leptons. We investigate how these back-scattered photons modify the prompt GRB spectrum through $γ-γ$ absorption. In a dense and pair-loaded wind environment, the emerging spectra exhibit a broad attenuation structure, whose morphology is sensitive to the low-energy spectral index $α$. In particular, spectra with $α> -1$ develop a pronounced, saddle-shaped absorption between 1 and 100 MeV (rest frame). Such external MeV absorption could account for the spectral curvature seen in some bright GRBs, and may point to enhanced mass loss from their progenitor stars, consistent with early observations of core-collapse supernovae.

Figures (5)

• Figure 1: Sketch of the process that leads to the absorption feature that we discuss in this work. (a) The radiation from the GRB jet illuminates some external material located at a radius $R_0$. (b) Some of the photons in the incident GRB radiation undergo Thomson scattering off the electrons in the external medium. Because of the Klein-Nishina suppression of the Thomson cross section, the spectrum of the scattered photon spectrum cuts off at a photon energy $E_\mathrm{sc}\sim m_\mathrm{e}c^2$. (c) Incident photons above a threshold energy $E\gtrsim 2.5 m_\mathrm{e}c^2$ can annihilate with the scattered photons, leaving an absorption feature in the transmitted spectrum. If the low-energy photon index in the incident spectrum satisfies $\alpha>-1$, then the feature is saddle-shaped.
• Figure 2: Comparison between the Band model (left-hand panel) and the absorbed Band model (right-hand panel) applied to the early prompt emission spectrum of GRB 190114C (0-4.15 s).
• Figure 3: Exact and approximate $\gamma-\gamma$ absorption mean free path. Solid lines show the exact $\gamma-\gamma$ absorption mean free path (Eq. \ref{['lmbd_exact_solution']}) of a photon with energy $\varepsilon_\mathrm{sc}$ scattered at a radius $R_0=10^{16}\,\mathrm{cm}$ to a cosine angle $\mu$ (different colors refer to different values of $\mu$ as given in the legend) moving through an incident radiation with $L=10^{53}\,\mathrm{erg/s}$, $E_\mathrm{p}=1\,\mathrm{MeV}$, $\alpha=-0.6$ and $\beta=-2.2$. Dashed lines show the corresponding approximate mean free path (Eq. \ref{['lmbd_appr_solution']}) setting $\eta=0.35$. Each dotted line shows the maximum energy of a scattered photon whose scattering angle $\mu$ is given by the corresponding color in the legend.
• Figure 4: Scattered photon survival time and corresponding spectrum. We have assumed $t_0=1$ s, $\alpha=-2/3$, $\beta=-2.2$, $E_{p}=1$ MeV, $L=10^{53}$ erg/s and $R=10^{16}\,\mathrm{cm}$. The solid lines in the top panel show the survival time of scattered photons with three different scattering angles (given in the legend) before they completely annihilate with incident photons. Lines with the same colors in the bottom panel show the corresponding spectrum of these scattered photons, expressed as the product of the survival time and the specific scattering rate. The grey dashed line shows the incident spectrum, arbitrarily normalized, for comparison.
• Figure 5: Example incident and absorbed spectra and corresponding optical depth for incident photons. Incident spectra (grey solid lines) are characterised by $\beta=-2.2$, $E_\mathrm{p}=1$ MeV, $L=10^{53}$ erg/s, and a low-energy photon index $\alpha=-2/3$ (left-hand panels, representative of synchrotron in marginally fast cooling) or $\alpha=-3/2$ (right-hand panels, for synchrotron in fast cooling). The absorbed spectra, assuming $t=1$ s (which implies $R_0=7.9\times 10^{15}\,\mathrm{cm}$) and $A_\star = 10^3\,\mathrm{cm^{-3}}$, are shown with red lines in the top panels. The corresponding optical depth, which includes both Thomson scattering and $\gamma-\gamma$ absorption, is shown in the bottom panels. For $\alpha=-2/3$, the absorption feature is saddle-shaped, while for $\alpha=-3/2$ it produces a spectral cut-off.