Compton Echoes from Gamma‐Ray Bursts

Abstract

Recent observations of gamma-ray bursts (GRBs) have provided growing evidence for collimated outflows and emission, and strengthened the connection between GRBs and supernovae. If massive stars are the progenitors of GRBs, the hard photon pulse will propagate in the pre-burst, dense environment. Circumstellar material will Compton scatter the prompt GRB radiation and give rise to a reflection echo. We calculate luminosities, spectra, and light curves of such Compton echoes in a variety of emission geometries and ambient gas distributions, and show that the delayed hard X-ray flash from a pulse propagating into a red supergiant wind could be detectable by Swift out to z ∼ 0.2. Independently of the γ-ray spectrum of the prompt burst, reflection echoes will typically show a high-energy cutoff between m e c 2 /2 and m e c 2 because of Compton downscattering. At fixed burst energy per steradian, the luminosity of the reflected echo is proportional to the beaming solid angle, Ω b , of the prompt pulse, while the number of bright echoes detectable in the sky above a fixed limiting flux increases as Ω 1/2 b , i.e. it is smaller in the case of more collimated jets. The lack of an X-ray echo at ∼ one month delay from the explosion poses severe constraints on the possible existence of a lateral GRB jet in SN 1987A. The late r-band afterglow observed in GRB990123 is fainter than the optical echo expected in a dense red supergiant environment from a isotropic prompt optical flash. Significant MeV delayed emission may be produced through the bulk Compton (or Compton drag) effect resulting from the interaction of the decelerating fireball with the scattered X-ray radiation.

Interstellar gas in the Galaxy and the X-ray luminosity of Sgr A*in the recent past

“…supernova lightcurves (Morrison & Sartori 1969), scattering by interstellar dust (Alcock & Hatchett 1978), scattering of an isotropic (Sunyaev 1982) or anisotropic radio source (Gilfanov et al 1987) by the thermal gas in a cluster of galaxies, reverberation mapping in AGN (e.g. Peterson 1993) and Compton echoes from gamma-ray bursts (Madau et al 2000) to name a few. C. K. Cramphorn and R. A. Sunyaev: The X-ray luminosity of Sgr A * in the recent past 255…”

Pair Loading in Gamma‐Ray Burst Fireballs and Prompt Emission from Pair‐rich Reverse Shocks

“…One case of e AE pair production in GRBs is that the GRB photons are backscattered by the ambient medium and interact with other outgoing highenergy GRB photons, leading to e AE pair production and then deposition of momentum into the external medium. Such a case has been considered by , , Dermer & Bö ttcher (2000), Madau, Blandford, & Rees (2000), Mészáros, Ramirez-Ruiz, & Rees (2001), Beloborodov (2002), and Ramirez-Ruiz, MacFadyen, & Lazzati (2002), which could be called an '' external case.'' In this paper, we consider the '' internal case,'' in which pairs are produced within the relativistic fireball due to -absorption between high-energy photons, and the formed pairs remain mixing in the fireball matter.…”

Compton tails in long-duration gamma-ray bursts

“…It has been proposed (Panaitescu & Kumar 2001; Lazzati & Perna 2002) that the X‐ray emission properties of the GRB afterglow might give quantitative indications of the physical properties of this material (for example, composition, density and distribution). Nevertheless, the prompt gamma‐ray photons might also interact with such material and produce observable effects still in the prompt phase (Madau, Blandford & Rees 2000; Ramirez‐Ruiz et al 2001; Beloborodov 2002).…”