Exploring the Galactic foreground of the gamma ray burst GRB070508

© Denes et al. 2019

When studying the afterglow of gamma ray bursts (GRBs) it is important to account for the Galactic foreground. This is typically done by using data from the Leiden/Argentina/Bon Survey (LAB), which has a spatial resolution of 36’, and assuming that all the gas is optically thin. This is a less than ideal approach, considering that the afterglow data usually has a 1000 times better resolution and that the optical depth is also often not negligible. Today’s image illustrates this problem. 

The right hand side shows the Galactic neutral hydrogen (HI) foreground of GRB070508. The bottom image is from the Galactic All Sky Survey (GASS) and has a resolution of 16’. The top image is a zoom in, combining data from the Australia Telescope Compact Array (ATCA) and GASS with a resolution of ~3’. The cross marks the position of the GRB and the dots mark the nearby continuum sources. We can see, that the foreground of the GRB is clearly higher in the high resolution image. 

The right hand side shows the brightness temperature (Tb) spectrum at the position of the GRB (black) and the closest continuum source (J204442-782027, orange); the optical depth towards the continuum source; and the column density (N(HI)) with (blue) and without (orange) optical depth correction. In this case, accounting for the optical depth raises the foreground column density towards the GRB by 60%. 

For more details see Denes et al. 2019 ( https://arxiv.org/abs/1909.00622 )