HIGH-RESOLUTION GRATING SPECTROSCOPY OF GRB 020405 WITH THE CHANDRA LOW ENERGY TRANSMISSION GRATING SPECTROMETER

摘要

We present high-resolution X-ray spectroscopy of GRB 020405 obtained with the Low Energy Transmission Grating Spectrometer (LETGS) on board the Chandra X-Ray Observatory starting 1.68 days after the burst. The spectrum appears featureless, with no evidence for emission lines, absorption edges, or narrow radiative recombination continua. The continuum can be fitted by a power law of photon index Γ = 1.72 +- 0.21 and temporal decay index α = 1.87 +- 1.10, with a marginally significant excess column density of cold gas N_H = (4.7 +- 3.7) x 10~(21) cm~(-2) at the redshift of the host galaxy. The absence of iron lines indicates that the density of nearby surrounding material was unlikely to be very dense (n approx< 5 x 10~(12) cm~(―3)) at the time of the Chandra observation. In the case of recombination following photoionization in an optically thin medium, most ionic species would be completely stripped at lower gas densities than this. In the case of a power-law spectrum reflecting off a "cold," opaque medium of low density, negligible emission features would be produced. Alternatively to these possible explanations for the lack of emission features, any X-ray line emission taking place in a dense medium in a " nearby reprocessor " scenario might have been overwhelmed by the bright afterglow continuum. Although the absence of discrete features does not unambiguously test for a connection between GRB 020405 and nucleosynthesis, it emphasizes the need for high-resolution X-ray spectroscopy to determine the exact emission mechanism responsible for the reported discrete lines in other gamma-ray burst afterglows.