Column-density variability of excited levels of FeII and NiII in GRB 060418

Details Column-density variability of excited levels of FeII and NiII in GRB 060418 Column-density variability of excited levels of FeII and NiII in GRB 060418

May 2008

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2008aipc.1000..496v&link_type=abstract

GAMMA-RAY BURSTS 2007: Proceedings of the Santa Fe Conference. AIP Conference Proceedings, Volume 1000, pp. 496-500 (2008).

Physics

Gamma-Ray Sources, Gamma-Ray Bursts, Quasars, Active Or Peculiar Galaxies, Objects, And Systems, Interstellar Medium And Nebulae In External Galaxies, Distances, Redshifts, Radial Velocities, Spatial Distribution Of Galaxies, Quasars

Scientific paper

We present high-resolution spectroscopic observations of GRB 060418, obtained with VLT/UVES. A sequence of spectra covering 330-670 nm were acquired at 11, 16, 25, 41 and 71 minutes (mid-exposure) after the Swift trigger, with a resolving power of 7 km s-1, and a signal-to-noise ratio of 10-15. This time-series clearly shows evidence for time variability of permitted transitions involving FeII fine-structure levels and metastable levels of both FeII and NiII, at the host-galaxy redshift z = 1.490. We model the observed evolution of the level populations with three different excitation mechanisms: collisions, excitation by infra-red photons, and fluorescence following excitation by ultraviolet photons. Our data allow us to reject the collisional and IR excitation scenarios. The UV pumping model, in which the GRB afterglow UV photons excite a cloud of atoms with column densities log N(FeII) = 14.75+/-0.06, log N(NiII) = 13.84+/-0.02, at a distance of 1.7+/-0.2 kpc provides an excellent fit. The success of our UV pumping modeling implies that no significant amount of neutral material (FeII, NiII or HI) is present at distances smaller than roughly 1.7 kpc. Therefore, the neutral gas responsible for the large HI column densities typically observed along GRB sightlines may not be located in the immediate environment of the GRB.

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