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 \Gamma = 1.72 \pm 0.21 and temporal decay index \alpha = 1.87 \pm 0.1 , with a marginally significant excess column density of cold gas N _ { H } = ( 4.7 \pm 3.7 ) \times 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 \mathrel { \hbox { \raise 2.15 pt \hbox { $ < $ } \hbox to 0.0 pt { \lower 2.15 pt \hbox { $ \sim% $ } } } } 5 \times 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 .
Alternative 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 GRB afterglows .