Rapid Response Mode observations of GRB 160203A: Looking for fine-structure line variability at z = 3.52

¹ Astronomical Institute Anton Pannekoek, University of Amsterdam, 1090 GE Amsterdam, The Netherlands
² GEPI, Observatoire de Paris, Université PSL, CNRS, 5 place Jule Janssen, 92190 Meudon, France
³ Space Science Data Center (SSDC) – Agenzia Spaziale Italiana (ASI), 00133 Roma, Italy
⁴ INAF – Osservatorio Astronomico di Roma, Via Frascati 33, 00040 Monte Porzio Catone, Italy
⁵ Institut d’Astrophysique de Paris, UMR 7095, CNRS-SU, 98 bis boulevard Arago, 75014 Paris, France
⁶ INAF – Osservatorio Astronomico di Brera, Via E. Bianchi 46, 23807 Merate (LC), Italy
⁷ Cosmic Dawn Center (DAWN), Rådmandsgade 64, 2200 København N, Denmark
⁸ Niels Bohr Institute, University of Copenhagen, Jagtvej 128, 2200 Copenhagen N, Denmark
⁹ Department of physics, University of Calabria, Via P. Bucci, Arcavacata di Rende (CS), Italy
¹⁰ INAF – Osservatorio di Astrofisica e Scienza dello Spazio, Via Piero Gobetti 93/3, 40129 Bologna, Italy
¹¹ INFN – Laboratori Nazionali di Frascati, Frascati, Italy
¹² Artemis, Observatoire de la Côte d’Azur, Université Côte d’Azur, CNRS, 06304 Nice, France
¹³ Department of Physics & Astronomy, Clemson University, Clemson, SC 29634, USA
¹⁴ European Southern Observatory, Karl-Schwarzschild Str. 2, 85748 Garching bei München, Germany
¹⁵ Physics Department, Lancaster University, Lancaster, LA1 4YB, UK
¹⁶ Department of Astrophysics/IMAPP, Radboud University, 6525 AJ Nijmegen, The Netherlands
¹⁷ University of Messina, Department of Mathematics, Informatics, Physics and Earth Sciences, Polo Papardo, Via Stagno d’Alcontres 31, 98166 Messina, Italy
¹⁸ Hessian Research Cluster ELEMENTS, Giersch Science Center, Max-von-Laue-Strasse 12, Goethe University Frankfurt, Campus Riedberg, 60438 Frankfurt am Main, Germany
¹⁹ Physics Department, George Washington University, 725 21st Street NW, Washington, DC 20052, USA
²⁰ Astronomy, Physics, and Statistics Institute of Sciences (APSIS), George Washington University, Washington, DC 20052, USA
²¹ Department of Physics and Astronomy, University of Leicester, University Road, Leicester LE1 7RH, UK
²² Astronomical Institute, Czech Academy of Sciences, Fričova 298, Ondřejov, Czech Republic
²³ Key Laboratory of Space Astronomy, National Astronomical Observatories, Chinese Sciences Academy, Beijing 100101, China

Received: 23 May 2024
Accepted: 24 June 2024

Abstract

Context. Gamma-ray bursts are the most energetic known explosions. Although they fade rapidly, they give us the opportunity to measure redshift and important properties of their host galaxies. We report the photometric and spectroscopic study of the Swift GRB 160203A at z = 3.518, and its host galaxy. Fine-structure absorption lines, detected in the afterglow at different epochs, allow us to investigate variability due to the strong fading background source.

Aims. We obtained two optical to near-infrared spectra of the GRB afterglow with X-shooter on ESO/VLT, 18 minutes and 5.7 hours after the burst, allowing us to investigate temporal changes of fine-structure absorption lines.

Methods. We measured H I column density log N(HI/cm^–2) = 21.75 ± 0.10, and several heavy-element ions along the GRB sightline in the host galaxy, among which Si II, Al II, Al III, C II, Ni II, Si IV, C IV, Zn II and Fe II, and Fe II^∗ and Si II^∗ fine-structure transitions from energetic levels excited by the afterglow, at the common redshift z = 3.518. We measured [M/H]_TOT = –0.78 ± 0.13 and a [Zn/Fe]_FIT = 0.69 ± 0.15, representing the total (dust corrected) metallicity and dust depletion, respectively. We detected additional intervening systems along the line of sight at ɀ = 1.03, ɀ = 1.26, ɀ = 1.98, ɀ = 1.99, ɀ = 2.20, and ɀ = 2.83. We could not measure significant variability in the strength of the fine-structure lines throughout all the observations and determined an upper limit for the GRB distance from the absorber of d < 300 pc, adopting the canonical UV pumping scenario. However, we note that the quality of our data is not sufficient to conclusively rule out collisions as an alternative mechanism.

Results. GRB 160203A belongs to a growing sample of GRBs with medium resolution spectroscopy, provided by the Swift/X-shooter legacy programme, which enables a detailed investigation of the interstellar medium in high-redshift GRB host galaxies. In particular, this host galaxy shows relatively high metal enrichment and dust depletion already in place when the universe was only 1.8 Gyr old.