Rapid-response mode VLT/UVES spectroscopy of GRB 060418^*

Conclusive evidence for UV pumping from the time evolution of Fe II and Ni II excited- and metastable-level populations

¹ European Southern Observatory, Alonso de Córdova 3107, Casilla 19001, Santiago 19, Chile e-mail: pvreeswi@eso.org
² Departamento de Astronomía, Universidad de Chile, Casilla 36-D, Santiago, Chile
³ Department of Physics and Astronomy, University of Victoria, Victoria, BC, Canada
⁴ Institute of Theoretical Astrophysics, University of Oslo, PO Box 1029 Blindern, 0315 Oslo, Norway
⁵ Astrophysikalisches Institut Potsdam, An der Sternwarte 16, 14482 Potsdam, Germany
⁶ Space Telescope Science Institute, 3700 San Martin Drive, Baltimore, MD 21218, USA
⁷ Dark Cosmology Centre, Niels Bohr Institute, University of Copenhagen, 2100 Copenhagen, Denmark
⁸ European Southern Observatory, Karl-Schwarzschild-Str. 2, 85748 Garching bei München, Germany
⁹ Institut d'Astrophysique de Paris – UMR 7095 CNRS & Université Pierre et Marie Curie, 98bis Boulevard Arago, 75014 Paris, France
¹⁰ LERMA, Observatoire de Paris, 61 Avenue de l'Observatoire, 75014 Paris, France
¹¹ Max-Planck-Institut für Extraterrestrische Physik, Giessenbachstrasse, 85748 Garching bei München, Germany
¹² Astronomical Institute “Anton Pannekoek”, University of Amsterdam & Center for High Energy Astrophysics, Kruislaan 403, 1098 SJ Amsterdam, The Netherlands

Received: 20 November 2006
Accepted: 11 March 2007

Abstract

We present high-resolution spectroscopic observations of GRB 060418, 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 GRB 060418 to be started only 10 min after the initial Swift satellite trigger. A sequence of spectra covering 330-670 nm were acquired at 11, 16, 25, 41 and 71 minutes (mid-exposure) after the 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 allowed transitions involving Fe II fine-structure levels (⁶D_7/2, ⁶D_5/2, ⁶D_3/2, and ⁶D_1/2), and metastable levels of both Fe II (⁴F_9/2 and ⁴D_7/2) and Ni II (⁴F_9/2), at the host-galaxy redshift . 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) = , log N(Ni II,  kpc, and  km s^-1. The success of our UV pumping modeling implies that no significant amount of Fe II or Ni II is present at distances smaller than ~1.7 kpc, 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 = , and the total rest-frame UV flux that irradiated the cloud since the GRB trigger, constraining the magnitude of a possible UV flash.