2007A&A...468...83V


C.D.S. - SIMBAD4 rel 1.7 - 2019.10.20CEST17:59:47

2007A&A...468...83V - Astronomy and Astrophysics, volume 468, 83-96 (2007/6-2)

Rapid-response mode VLT/UVES spectroscopy of GRB060418. Conclusive evidence for UV pumping from the time evolution of Fe II and Ni II excited- and metastable-level populations.

VREESWIJK P.M., LEDOUX C., SMETTE A., ELLISON S.L., JAUNSEN A.O., ANDERSEN M.I., FRUCHTER A.S., FYNBO J.P.U., HJORTH J., KAUFER A., MOLLER P., PETITJEAN P., SAVAGLIO S. and WIJERS R.A.M.J.

Abstract (from CDS):

We present high-resolution spectroscopic observations of GRB060418, obtained with VLT/UVES. These observations were triggered using the VLT Rapid-Response Mode (RRM), which allows for automated observations of transient phenomena, without any human intervention. This resulted in the first UVES exposure of GRB060418 to be started only 10min after the initial Swift satellite trigger. A sequence of spectra covering 330-670nm were acquired at 11, 16, 25, 41 and 71 minutes (mid-exposure) after the trigger, with a resolving power of 7km/s, and a signal-to-noise ratio of 10-15. This time-series clearly shows evidence for time variability of allowed transitions involving Fe II fine-structure levels (6D7/2, 6D5/2, 6D3/2, and 6D1/2), and metastable levels of both Fe II (4F9/2 and 4D7/2) and Ni II (4F9/2), at the host-galaxy redshift z=1.490. This is the first report of absorption lines arising from metastable levels of Fe II and Ni II along any GRB sightline. 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 with high confidence. The UV pumping model, in which the GRB afterglow UV photons excite a cloud of atoms with a column density N, distance d, and Doppler broadening parameter b, provides an excellent fit, with best-fit values: log N(Fe II)=14.75+0.06–0.04, log N(Ni II)=13.84±0.02, d=1.7±0.2kpc, and b=25±3km/s. The success of our UV pumping modeling implies that no significant amount of Fe II or Ni II is present at distances smaller than ∼1.7kpc, most likely because it is ionized by the GRB X-ray/UV flash. Because neutral hydrogen is more easily ionized than Fe II and Ni II, this minimum distance also applies to any H I present. Therefore the majority of very large H I column densities typically observed along GRB sightlines may not be located in the immediate environment of the GRB. The UV pumping fit also constrains two GRB afterglow parameters: the spectral slope, β=-0.5+0.8–1.0, and the total rest-frame UV flux that irradiated the cloud since the GRB trigger, constraining the magnitude of a possible UV flash.

Abstract Copyright:

Journal keyword(s): gamma rays: bursts - galaxies: abundances - galaxies: ISM - galaxies: distances and redshifts - galaxies: quasars: absorption lines

Errata: + corrigendum vol. 532, p. C3 (2011)

Simbad objects: 10

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Number of rows : 10

N Identifier Otype ICRS (J2000)
RA
ICRS (J2000)
DEC
Mag U Mag B Mag V Mag R Mag I Sp type #ref
1850 - 2019
#notes
1 GRB 021004 gB 00 26 54.68 +18 55 41.6           ~ 507 1
2 * bet Pic PM* 05 47 17.0876901 -51 03 59.441135 4.13 4.03 3.86 3.74 3.58 A6V 1597 1
3 * eta Car Em* 10 45 03.5455050 -59 41 03.951060 6.37 7.034 6.21 4.90 4.41 OBepec 2148 0
4 GRB 030323 gB 11 06 09.4 -21 46 13           ~ 122 1
5 GRB 050730 gB 14 08 17.090 -03 46 18.90   20.32 17.68     ~ 276 1
6 GRB 060418 gB 15 45 42.600 -03 38 20.00 15.73 16.19 14.98     ~ 317 1
7 GRB 020813A gB 19 46 41.874 -19 36 04.81           ~ 281 1
8 GRB 050922C gB 21 09 33.000 -08 45 30.10 15.15 15.86 14.69     ~ 244 0
9 GRB 060607 gB 21 58 50.400 -22 29 46.70 18.49 16.67 15.09     ~ 249 1
10 GRB 051111A gB 23 12 33.170 +18 22 28.80 19.77 20.08 19.50     ~ 183 1

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