Abstract for Proposal 092341
Gamma-Ray bursts as probes to study interstellar dust, and vice versa
We propose a ToO observation of a GRB sufficiently bright to produce
detectable X-ray expanding rings due to dust scattering with Galactic
clouds along its line of sight. By studying these rings we can obtain
accurate and model-independent measures of the distances to the dust
clouds and probe the properties of their dust grains (such as the size
distribution, which is poorly constrained by observations at other
wavelengths). Furthermore, the GRB soft X-ray spectrum, usually not
observable by GRB instruments, can be reconstructed from the analysis of
the dust-scattered emission. Proposals with the same goal, but different
trigger criteria, were accepted in previous AOs and allowed us to discover
6 rings around GRB160623A.
Details on Observing Strategy and Trigger Criteria
The trigger criterium is (F x P) > 2 x t^1.5, where F is the GRB fluence
in the 15-150 keV band (in units of 1e-4 erg cm-2), P is the highest
peak of the Lallement et al. (2022) extinction map in the direction of
the GRB (in units of 1e-3 mag/pc) and t is the average time (in days) of
the XMM observation to be scheduled. Since the X-ray ring flux declines
with time, the observation should be performed as soon as possible,
but a late time (up to several days) observation is still possible if
(F x P) is large enough (>2 x t^1.5) and the ring is expected to be
still inside the EPIC field of view. The ring radius (in arcmin) can be
computed as 4.5(t/d)^0.5, where d is the distance (in kpc) of the dust
cloud, identified as a peak in the extinction map.