Revealing the characteristics of the dark GRB 150309A: Dust extinguished or high-z?^⋆

¹ Instituto de Astrofísica de Andalucía (IAA-CSIC), Glorieta de la Astronomía s/n, 18008 Granada, Spain
e-mail: ajct@iaa.es
² Departamento de Ingeniería de Sistemas y Automática, Escuela de Ingenierías, Universidad de Málaga, C. Dr. Ortiz Ramos sn, 29071 Málaga, Spain
³ Aryabhatta Research Institute of Observational Sciences (ARIES), Manora Peak, Nainital 263002, India
e-mail: rahulbhu.c157@gmail.com
⁴ Department of Physics, Deen Dayal Upadhyaya Gorakhpur University, Gorakhpur 273009, India
⁵ Thũringer Landessternwarte Tautenburg, Sternwarte 5, 07778 Tautenburg, Germany
⁶ Department of Astronomy, University of Florida, Bryant Space Science Center, Gainesville, FL 32611, USA
⁷ Grantecan, Santa Cruz de la Palma, Tenerife, Spain
⁸ Special Astrophysical Observatory of Russian Academy of Sciences, Nizhniy Arkhyz 369167, Russia
⁹ Crimean Astrophysical Observatory, Russian Academy of Sciences, Nauchnyi 298409, Russia
¹⁰ Institute for Science and Technology in Space, SungKyunKwan University, Suwon 16419, Korea
¹¹ School of Physics and Astronomy, University of Birmingham, Birmingham B15 2TT, UK
¹² School of Astronomy and Space Science, Nanjing University, Nanjing 210093, PR China
¹³ Key Laboratory of Modern Astronomy and Astrophysics, Nanjing University, Ministry of Education, Nanjing 210093, PR China
¹⁴ INAF, Istituto di Astrofisica e Planetologia Spaziali, Via Fosso del Cavaliere 100, 00133 Rome, Italy
¹⁵ School of Physics, O’Brien Centre for Science North, University College Dublin, Belfield, Dublin 4, Ireland
¹⁶ Astronomical Institute of the Czech Academy of Sciences (ASU-CAS), Fričova 298, 251 65 Ondřejov, Czech Republic
¹⁷ Universidad de Granada, Facultad de Ciencias Campus Fuentenueva s/n, 18071 Granada, Spain
¹⁸ Nikolaev National University, Nikolska 24, Nikolaev 54030, Ukraine
¹⁹ Nikolaev Astronomical Observatory, Nikolaev 54030, Ukraine
²⁰ Department of Physics, University of Helsinki, Gustav Hällströmin katu 2, 00014 Helsinki, Finland
²¹ Institute of Physics and Technology, Ural Federal University, Mira Street 19, 620002 Ekaterinburg, Russia
²² European Space Astronomy Centre (ESA-ESAC), Camino bajo del Castillo, s/n, Villafranca del Castillo, 28692 Villanueva de la Cañada, Madrid, Spain
²³ Instituto de Astrofísica de Canarias, Via Láctea s/n, La Laguna, Tenerife, Spain

Received: 31 January 2023
Accepted: 8 August 2023

Abstract

Context. Dark gamma-ray bursts (GRBs) constitute a significant fraction of the GRB population. In this paper, we present a multi-wavelength analysis (both prompt emission and afterglow) of an intense (3.98  ×  10⁻⁵ erg cm⁻² using Fermi-Gamma-Ray Burst Monitor) two-episodic GRB 150309A observed early on until ∼114 days post burst. Despite the strong gamma-ray emission, no optical afterglow was detected for this burst. However, we discovered near-infrared (NIR) afterglow (K_S-band), ∼5.2 h post burst, with the CIRCE instrument mounted at the 10.4 m Gran Telescopio Canarias (hereafter, GTC).

Aims. We aim to examine the characteristics of GRB 150309A as a dark burst and to constrain other properties using multi-wavelength observations.

Methods. We used Fermi observations of GRB 150309A to understand the prompt emission mechanisms and jet composition. We performed early optical observations using the BOOTES robotic telescope and late-time afterglow observations using the GTC. A potential faint host galaxy was also detected in the optical wavelength using the GTC. We modelled the potential host galaxy of GRB 150309A in order to explore the environment of the burst.

Results. The time-resolved spectral analysis of Fermi data indicates a hybrid jet composition consisting of a matter-dominated fireball and magnetic-dominated Poynting flux. The GTC observations of the afterglow revealed that the counterpart of GRB 150309A was very red, with H − K_S > 2.1 mag (95% confidence). The red counterpart was not discovered in any bluer filters of Swift UVOT/BOOTES, which would be indicative of a high redshift origin. Therefore we discarded this possibility based on multiple arguments, such as spectral analysis of the X-ray afterglow constrain z < 4.15 and a moderate redshift value obtained using the spectral energy distribution (SED) modelling of the potential galaxy. The broadband (X-ray to NIR bands) afterglow SED implies a very dusty host galaxy with a deeply embedded GRB (suggesting A_V ≳ 35 mag).

Conclusions. The environment of GRB 150309A demands a high extinction towards the line of sight. Demanding dust obscuration is the most probable origin of optical darkness as well as the very red afterglow of GRB 150309A. This result establishes GRB 150309A as the most extinguished GRB known to date.