GRB 171205A/SN 2017iuk: A local low-luminosity gamma-ray burst

¹ Space Science Data Center - Agenzia Spaziale Italiana, via del Politecnico, s.n.c, 00133 Roma, Italy
e-mail: valerio.delia@asdc.asi.it
² INAF – Osservatorio Astronomico di Brera, via Bianchi 46, 23807 Merate, Italy
³ INAF – IASF Palermo, via Ugo La Malfa, 153, 90146 Palermo, Italy
⁴ Department of Astronomy and Space Sciences, Istanbul University, 34119 Beyazit, Istanbul, Turkey
⁵ Mullard Space Science Laboratory, University College London, Holmbury St Mary, RH5 6NT Dorking, Surrey, UK
⁶ Ioffe Physical-Technical Institute, Politekhnicheskaya 26, 194021 St. Petersburg, Russia
⁷ Center for Research and Exploration in Space Science and Technology (CRESST), NASA Goddard Space Flight Center, 20771 Greenbelt, MD, USA
⁸ Department of Physics, University of Maryland, 1000 Hilltop Circle, 21250 Baltimore County, Baltimore, MD, USA
⁹ Department of Physics and Astronomy, University of Leicester, University Road, LE7 1RH Leicester, UK
¹⁰ Department of Astronomy & Astrophysics, The Pennsylvania State University, University Park, 16802, PA, USA
¹¹ Department of Physics, University of Warwick, CV4 7AL Coventry, UK
¹² George P. and Cynthia Woods Mitchell Institute for Fundamental Physics & Astronomy, Texas A. & M. University, Department of Physics and Astronomy, 4242 TAMU, College Station, 77843, TX, USA
¹³ Astrophysics Science Division, NASA Goddard Space Flight Center, Mail Code 661, 20771 Greenbelt, MD, USA
¹⁴ Joint Space-Science Institute, University of Maryland, 20742 College Park, MD, USA

Received: 13 July 2018
Accepted: 19 August 2018

Abstract

Context. Gamma-ray bursts (GRBs) occurring in the local Universe constitute an interesting sub-class of the GRB family, since their luminosity is on average lower than that of their cosmological analogs. Attempts to understand in a global way this peculiar behaviour is still not possible, since the sample of low redshift GRBs is small, and the properties of individual objects are too different from each other. In addition, their closeness (and consequently high fluxes) make these sources ideal targets for extensive follow-up even with small telescopes, considering also that these GRBs are conclusively associated with supernova (SN) explosions.

Aims. We aim to contribute to the study of local bursts by reporting the case of GRB 171205A. This source was discovered by Swift Burst Alert Telescope (BAT) on 2017, December 5 and soon associated with a low redshift host galaxy (z = 0.037), and an emerging SN (SN 2017iuk).

Methods. We analyzed the full Swift dataset, comprising the UV-Optical Telescope (UVOT), X-ray Telescope (XRT) and BAT data. In addition, we employed the Konus-Wind high energy data as a valuable extension at γ-ray energies.

Results. The photometric SN signature is clearly visible in the UVOT u, b and ν filters. The maximum emission is reached at ∼13 (rest frame) days, and the whole bump resembles that of SN 2006aj, but lower in magnitude and with a shift in time of +2 d. A prebump in the ν-band is also clearly visible, and this is the first time that such a feature is not observed achromatically in GRB–SNe. Its physical origin cannot be easily explained. The X-ray spectrum shows an intrinsic Hydrogen column density N_H,int = 7.4^+4.1_−3.6 × 10²⁰ cm⁻², which is at the low end of the N _H, int, even considering just low redshift GRBs. The spectrum also features a thermal component, which is quite common in GRBs associated with SNe, but whose origin is still a matter of debate. Finally, the isotropic energy in the γ-ray band, E_iso = 2.18^+0.63_−5.0 × 10⁴⁹ erg, is lower than those of cosmological GRBs. Combining this value with the peak energy in the same band, E_p = 125⁺¹⁴¹₋₃₇ keV, implies that GRB 171205A is an outlier of the Amati relation, as are some other low redshift GRBs, and its emission mechanism should be different from that of canonical, farther away GRBs.