A search for the afterglows, kilonovae, and host galaxies of two short GRBs: GRB 211106A and GRB 211227A

¹ Università degli Studi dell’Insubria, Dipartimento di Scienza e Alta Tecnologia, Via Valleggio 11, 22100 Como, Italy
² INAF-Osservatorio Astronomico di Brera, Via E. Bianchi 46, 23807 Merate, LC, Italy
e-mail: matteo.ferro@inaf.it
³ INAF-Osservatorio di Astrofisica e Scienza dello Spazio, Via Piero Gobetti 93/3, 40129 Bologna, Italy
⁴ INAF-Osservatorio Astronomico di Capodimonte, Salita Moiariello 16, 80131 Napoli, Italy
⁵ DARK, Niels Bohr Institute, University of Copenhagen, Jagtvej 128, 2200 Copenhagen N, Denmark
⁶ Cosmic Dawn Center (DAWN), Niels Bohr Institute, University of Copenhagen, Jagtvej 128, 2200 N Copenhagen, Denmark
⁷ Niels Bohr Institute, University of Copenhagen, Jagtvej 128, 2200 Copenhagen N, Denmark
⁸ Università degli Studi di Milano, Dipartimento di Fisica, Via Celoria 16, 20133 Milano, Italy
⁹ Université Paris-Saclay, CNRS/IN2P3, IJCLab, 15 Rue Georges Clémenceau, 91405 Orsay, France
¹⁰ An-Najah National University, Omar Ibn Al-Khattab St., PO Box 7 Nablus, Palestine
¹¹ Department of Astrophysics/IMAPP, Radboud University, Heyendaalseweg 135, PO Box 9010, 6500 GL Nijmegen, The Netherlands
¹² Department of Physics, University of Warwick, Gibbet Hill Road, Coventry CV4 7AL, UK
¹³ INAF-Osservatorio Astronomico di Roma, Via di Frascati 33, 00040 Monte Porzio Catone, RM, Italy
¹⁴ ASI, Italian Space Agency, Space Science Data Centre, Via del Politecnico snc, 00133 Rome, Italy
¹⁵ University of Messina, Mathematics, Informatics, Physics and Earth Science Department, Via F.D. D’Alcontres 31, Polo Papardo, 98166 Messina, Italy
¹⁶ Institute for Gravitational Wave Astronomy and School of Physics and Astronomy, University of Birmingham, Edgbaston, Birmingham B15 2TT, UK
¹⁷ Department of Physics and Astronomy, Clemson University, Kinard Lab of Physics, 140 Delta Epsilon Ct., Clemson, SC 29634-0978, USA
¹⁸ Centre for Astrophysics and Cosmology, Science Institute, University of Iceland, Dunhagi 5, 107 Reykjavik, Iceland
¹⁹ The George Washington University, Physics Department, 725 21st street NW, Washington, DC 20052, USA
²⁰ School of Physics and Centre for Space Research, University College Dublin, Belfield, D04 V1W8 Dublin, Ireland
²¹ Thüringer Landessternwarte Tautenburg, Sternwarte 5, 07778 Tautenburg, Germany
²² Astronomical Institute Anton Pannekoek, University of Amsterdam, Science Park 904, 1098 XH Amsterdam, PO Box 94249, 1090 GE Amsterdam, The Netherlands
²³ INAF-IASF Milano, Via Corti 12, 20133 Milano, Italy
²⁴ Physics Department, University of Calabria, Via P. Bucci, 87036 Arcavacata di Rende, CS, Italy
²⁵ INFN-Laboratori Nazionali di Frascati, Via Enrico Fermi 54, 00044 Frascati, RM, Italy
²⁶ School of Physics and Astronomy, University of Leicester, University Road, Leicester LE1 7RH, UK
²⁷ Anton Pannekoek Institute for Astronomy, University of Amsterdam, Science Park 904, 1098 XH Amsterdam, The Netherlands
²⁸ School of Mathematical and Physical Sciences, Macquarie University, Macquarie Park, NSW 2109, Australia

Received: 6 June 2023
Accepted: 8 August 2023

Abstract

Context. GRB 211106A and GRB 211227A are two recent gamma-ray bursts (GRBs) whose initial X-ray position enabled us to possibly associate them with bright, low-redshift galaxies (z < 0.7). The prompt emission properties suggest that GRB 211106A is a genuine short-duration GRB and GRB 211227A is a short GRB with extended emission. Therefore, they are likely to be produced by a compact binary merger. However, a classification based solely on the prompt emission properties can be misleading.

Aims. The possibility of having two short GRBs occurring in the local Universe makes them ideal targets for the search of associated kilonova (KN) emission and for detailed studies of the host galaxy properties.

Methods. We carried out deep optical and near-infrared (NIR) follow-up with the ESO-VLT FORS2, HAWK-I, and MUSE instruments for GRB 211106A and with ESO-VLT FORS2 and X-shooter for GRB 211227A, starting from hours after the X-ray afterglow discovery up to days later. We performed photometric analysis to look for afterglow and KN emissions associated with the bursts, together with imaging and spectroscopic observations of the host galaxy candidates. We compared the results obtained from the optical/NIR observations with the available Swift X-Ray Telescope (XRT) and others high-energy data of both events.

Results. For both GRBs we placed deep limits to the optical/NIR afterglow and KN emission. We identified their associated host galaxies, GRB 211106A at a photometric redshift z = 0.64, GRB 211227A at a spectroscopic z = 0.228. From MUSE and X-shooter spectra we derived the host galaxy properties, which turned out to be consistent with short GRBs typical hosts. We also compared the properties of GRB 211106A and GRB 211227A with those of the short GRBs belonging to the S-BAT4 sample, here extended up to December 2021, in order to further investigate the nature of these two bursts.

Conclusions. Our study of the prompt and afterglow phase of the two GRBs, together with the analysis of their associated host galaxies, allows us to confirm the classification of GRB 211106A as a short GRB, and GRB 211227A as a short GRB with extended emission. The absence of an optical/NIR counterpart down to deep magnitude limits is likely due to high local extinction for GRB 211106A and a peculiarly faint kilonova for GRB 211227A.