The dark nature of GRB 130528A and its host galaxy

¹ Instituto de Astrofísica de Andalucía (IAA-CSIC), Glorieta de la Astronomía s/n, 18008 Granada, Spain
e-mail: sjeong@iaa.es
² Unidad Asociada Departamento de Ingeniería de Sistemas y Automática, E.T.S. de Ingenieros Industriales, Universidad de Málaga, Spain
³ Institute de Radioastronomie Millimétrique (IRAM), 300 rue de la Piscine, 38406 Saint Martin d’Hères, France
⁴ Unidad Asociada Grupo Ciencias Planetarias UPV/EHU-IAA/CSIC, Departamento de Física Aplicada I, E.T.S., Ingeniería, Universidad del País Vasco UPV/EHU, Bilbao, Spain
⁵ Ikerbasque, Basque Foundation for Science, Bilbao, Spain
⁶ Nikolaev National University, Nikolska 24, 54030 Nikolaev, Ukraine
⁷ Aryabhatta Research Institute of Observational Sciences, Manora Peak, 263 002 Nainital, India
⁸ Terskol Branch of Institute of Astronomy Russian Academy of Sciences, Elbrus ave., 81/33, Tyrnyauz, 361623 Kabardino-Balkaria Republic, Russia
⁹ Saint Petersburg State University, Universitetskiy ave 28, Petrodvorets, 198504 Saint Petersburg, Russia
¹⁰ Petrozavodsk State University, Lenin Str., 33, 185910 Petrozavodsk, Russia
¹¹ Open joint stock company, Scientific Production Corporation System of Precision Instrument Making, Aviamotornaya, 53, Moscow, Russia
¹² Special Astrophysical Observatory, Nizhniy Arkhyz, Zelenchukskiy region, 369167 Karachai-Cherkessian Respublic, Russia
¹³ Universidad de Jaén, Campus Las Lagunillas, s/n, Jaén, Spain
¹⁴ Yunnan Observatories, Chinese Academy of Sciences, 650011 Kunming, PR China
¹⁵ Key Laboratory for the Structure and Evolution of Celestial Objects, Chinese Academy of Sciences, 650011 Kunming, PR China
¹⁶ University of Chinese Academy of Sciences, 100049 Beijing, PR China
¹⁷ Department of Physics, Sungkyunkwan University, Suwon, Korea

Received: 11 April 2014
Accepted: 20 August 2014

Abstract

Aims. We study the dark nature of GRB 130528A through multi-wavelength observations and conclude that the main reason for the optical darkness is local extinction inside of the host galaxy.

Methods. Automatic observations were performed at the Burst Optical Observer and Transient Exploring System (BOOTES)-4/MET robotic telescope. We also triggered target of opportunity (ToO) observations at Observatorio de Sierra Nevada (OSN), IRAM Plateau de Bure Interferometer (PdBI) and Gran Telescopio Canarias (GTC + OSIRIS). The host galaxy photometric observations in optical to near-infrared (nIR) wavelengths were achieved through large ground-based aperture telescopes, such as 10.4 m Gran Telescopio Canarias (GTC), 4.2 m William Herschel Telescope (WHT), 6 m Bolshoi Teleskop Alt-azimutalnyi (BTA) telescope, and 2 m Liverpool Telescope (LT). Based on these observations, spectral energy distributions (SED) for the host galaxy and afterglow were constructed.

Results. Thanks to millimetre (mm) observations at PdBI, we confirm the presence of a mm source within the XRT error circle that faded over the course of our observations and identify the host galaxy. However, we do not find any credible optical source within early observations with BOOTES-4/MET and 1.5 m OSN telescopes. Spectroscopic observation of this galaxy by GTC showed a single faint emission line that likely corresponds to [OII] 3727 Å at a redshift of 1.250 ± 0.001, implying a star formation rate (M_⊙/yr) > 6.18 M_⊙/yr without correcting for dust extinction. The probable line-of-sight extinction towards GRB 130528A is revealed through analysis of the afterglow SED, resulting in a value of ${\mathit{A}}_{\mathit{V}}^{\mathrm{GRB}}$≥ 0.9 at the rest frame; this is comparable to extinction levels found among other dark GRBs. The SED of the host galaxy is explained well (χ²/d.o.f. = 0.564) by a luminous (M_B = −21.16), low-extinction (A_V = 0, rest frame), and aged (2.6 Gyr) stellar population. We can explain this apparent contradiction in global and line-of-sight extinction if the GRB birth place happened to lie in a local dense environment. In light of having relatively small specific star formation rate ~5.3 M_⊙/yr (L/L^⋆)^-1, this also could explain the age of the old stellar population of host galaxy.