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-ray Monitor (WXM) and French Gamma Telescope observations of XRF 030723 along with observations of the XRF afterglow made using the 6.5 m Magellan Clay telescope and Chandra. The observed peak energy E-pk(obs) of the nu F-nu burst spectrum is found to lie within (or below) the WXM2 - 25 keV passband at 98.5% confidence, and no counts are detected above 30 keV. Our best-fit value is E-pk(obs) = 8.4(-3.4)(+3.5) keV. The ratio of X-ray to gamma-ray flux for the burst follows a correlation found for GRBs observed with HETE-2, and the duration of the burst is similar to that typical of long-duration GRBs. If we require that the burst isotropic equivalent energy E-iso and E-pk satisfy the relation discovered by Amati et al. (2002), a redshift of z = 0.38(-0.18)(+0.36) can be determined, in agreement with constraints determined from optical observations. We are able to fit the X-ray afterglow spectrum and to measure its temporal fade. Although the best-fit fade is shallower than the concurrent fade in the optical, the spectral similarity between the two bands indicates that 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 a supernova bump. We interpret the prompt and afterglow X-ray emission as arising from a jetted GRB observed off-axis and possibly viewed through a complex circumburst medium that is due to a progenitor wind.