Broadband Observations of the Afterglow of GRB 000926: Observing the Effect of Inverse Compton Scattering

Abstract

GRB 000926 has one of the best-studied afterglows to date, with multiple X-ray observations, as well as extensive multifrequency optical and radio coverage. Broadband afterglow observations, spanning from X-ray to radio frequencies, provide a probe of the density structure of the circumburst medium, as well as of the ejecta energetics, geometry, and physical parameters of the relativistic blast wave resulting from the explosion. We present an analysis of Chandra X-Ray Observatory observations of this event, along with Hubble Space T elescope and radio monitoring data. We combine these data with groundbased optical and IR observations and Ðt the synthesized afterglow light curve using models where collimated ejecta expand into a surrounding medium. We Ðnd that we can explain the broadband light curve with reasonable physical parameters if the cooling is dominated by inverse Compton scattering. For this model, an excess due to inverse Compton scattering appears above the best-Ðt synchrotron spectrum in the X-ray band. No previous bursts have exhibited this component, and its observation would imply that the GRB exploded in a moderately dense (n D 30 cm~3) medium, consistent with a di †use interstellar cloud environment.

Testing the Standard Fireball Model of Gamma‐Ray Bursts Using Late X‐Ray Afterglows Measured bySwift

“…The XMM-Newton observatory also detected X-ray afterglows. In particular, GRB 030329 confirmed the hypernova connection (Tiengo et al 2003;Stanek et al 2003;Hjorth et al 2003), and multiwavelength observations and analysis of this bright afterglow and similar events ( Harrison et al 2001;Panaitescu & Kumar 2001, 2002, 2003Willingale et al 2004) established that afterglows were broadly consistent with the expected synchrotron spectrum and temporal evolution.…”

Hubble Space Telescopeand Ground‐based Optical and Ultraviolet Observations of GRB 010222

“…We do not favor an interpretation in which the break is due to a transition of the blast wave to the nonrelativistic phase (in 't Zand et al 2001;Masetti et al 2001). The same claim has been made for GRB 000926 (Piro et al 2001), but the high ambient density causes the self-absorption frequency to lie far above the radio passband (Harrison et al 2001), resulting in an undetectable radio afterglow, contrary to observations. The jet model with continuous energy injection (Bjö rnsson et al 2002) is an interesting alternative, but adequate testing of this model will require a full broadband data set.…”

BATSE Observations of Gamma‐Ray Burst Tails

“…While they expect the onset and importance of the inverse Compton emission to vary from one burst to another, the hardness of the summed rates from all the long ([2 s) bursts between 1000 and 2000 s after their aligned peaks might be indicative of the onset of the IC frequency entering the observable energy range of BATSE and the domination of this IC component over the fading synchrotron tail. At later times in the afterglow, IC emission has been reported by Harrison et al (2001) as contributing to the X-ray afterglow spectrum of GRB 000926 2 and 10 days after the GRB, but no other afterglows have so far been observed with this bump in their afterglow spectrum.…”