High-Resolution Grating Spectroscopy of Gamma-Ray Bursts 030328 and 041006 with Chandra LETGS

We present high-resolution X-ray spectroscopy of two recent γ-ray burst (GRB) afterglows observed with the Low Energy Transmission Grating Spectrometer on Chandra. The afterglows to GRBs 030328 and 041006 are detected beginning 15.33 and 16.8 hr after each burst, respectively, and are observed to fade in time during each ~90 ks observation. We fit for the continuum emission in each full data set and for the data sliced into half and quarter time sections. For both afterglows, the continuum emission is well described by an absorbed power-law model, and the model parameters describing the absorption and spectral slope do not appear to evolve in time. We perform a careful search for deviations from the model continua for the full and time-sliced data and find no evidence for significant (≳3 σ) narrow emission/absorption lines or edges. The lack of detections implies that line emission—if it is a general feature in GRB X-ray afterglows—occurs early (t ~ 0.3 days in the source frame) and/or is short-lived (dt ≲ 10 ks). We also comment on synchrotron fireball models for the X-ray and optical data.