High‐Energy Observations of XRF 030723: Evidence for an Off‐Axis Gamma‐Ray Burst?

We report High Energy Transient Explorer 2 (HETE-2) Wide Field X-rayMonitor/French Gamma Telescope observations of XRF030723 along withobservations of the XRF afterglow made using the 6.5m Magellan Clay telescopeand the Chandra X-ray Observatory. The observed peak energy E_pk_obs of the nuF_nu burst spectrum is found to lie within (or below) the WXM 2-25 keV passbandat 98.5% confidence, and no counts are detected above 30 keV. Our best fitvalue is E_pk_obs=8.4+3.5/-3.4 keV. The ratio of X-ray to Gamma-ray flux forthe burst follows a correlation found for GRBs observed with HETE-2, and theduration of the burst is similar to that typical of long-duration GRBs. If werequire that the burst isotropic equivalent energy E_iso and E_pk_rest satisfythe relation discovered by Amati et al. (2002), a redshift of z=0.38+0.36/-0.18can be determined, in agreement with constraints determined from opticalobservations. We are able to fit the X-ray afterglow spectrum and to measureits temporal fade. Although the best-fit fade is shallower than the concurrentfade in the optical, the spectral similarity between the two bands indicatesthat the X-ray fade may actually trace the optical fade. If this is the case,the late time rebrightening observed in the optical cannot be due to asupernova bump. We interpret the prompt and afterglow X-ray emission as arisingfrom a jetted GRB observed off-axis and possibly viewed through a complexcircumburst medium due to a progenitor wind.Comment: 14 pages, 9 figures, to appear in Ap