Late-epoch optical and near-infrared observations of the GRB 000911 afterglow and its host galaxy

¹ INAF - Istituto di Astrofisica Spaziale e Fisica Cosmica, Sezione di Bologna, via Gobetti 101, 40129 Bologna, Italy (formerly IASF/CNR, Bologna) e-mail: masetti@bo.iasf.cnr.it;
² INAF - Osservatorio Astronomico di Trieste, via GB Tiepolo 11, 34131 Trieste, Italy;
³ INAF - Istituto di Radioastronomia, Sezione di Firenze, largo E. Fermi 5, 50125, Florence, Italy;
⁴ Niels Bohr Institute, University of Copenhagen, Juliane Maries Vej 30, 2100 Copenhagen Ø, Denmark;
⁵ Instituto de Astrofísica de Andalucía (IAA-CSIC), Apartado de Correos 03004, 18080 Granada, Spain;
⁶ Thüringer Landessternwarte Tautenburg, 07778 Tautenburg, Germany;
⁷ INAF - Osservatorio Astronomico di Padova, vicolo dell'Osservatorio 5, 35122 Padua, Italy;
⁸ Astrophysikalisches Institut Potsdam, An der Sternwarte 16, 14482 Potsdam, Germany;
⁹ Space Telescope Science Institute, 3700 San Martin Drive, Baltimore, MD 21218, USA;
¹⁰ Dipartimento di Fisica, Università di Ferrara, via Paradiso 12, 44100 Ferrara, Italy;
¹¹ Max-Planck-Institut für Extraterrestrische Physik, Giessenbachstrasse, 85748 Garching, Germany;
¹² Institute of Astronomy “Anton Pannekoek”, University of Amsterdam, Kruislaan 403, 1098 SJ Amsterdam, The Netherlands;
¹³ NASA Marshall Space Flight Center, SD-50, Huntsville, AL 35812, USA;
¹⁴ Department of Physics and Astronomy, University of Leicester, University Road, Leicester, LE1 7RH, UK;
¹⁵ INAF - Telescopio Nazionale Galileo, Roque de Los Muchachos Astronomical Observatory, PO Box 565, 38700 Santa Cruz de La Palma, Spain;
¹⁶ European Southern Observatory, Karl Schwarzschild-Strasse 2, 85748 Garching, Germany;
¹⁷ INAF - Istituto di Astrofisica Spaziale e Fisica Cosmica, Sezione di Palermo, via La Malfa 153, 90146 Palermo, Italy (formerly IASF/CNR, Palermo);
¹⁸ Centre for Astrophysics Research, University of Hertfordshire, College Lane, Hatfield, Herts AL10 9AB, UK;
¹⁹ European Southern Observatory, Casilla 19001, Santiago 19, Chile

Received: 25 January 2005
Accepted: 18 April 2005

Abstract

We present the results of an optical and near-infrared (NIR) monitoring campaign of the counterpart of Gamma-Ray Burst (GRB) 000911, located at redshift , from 5 days to more than 13 months after explosion. Our extensive dataset is a factor of 2 larger and spans a time interval ~4 times longer than the ones considered previously for this GRB afterglow; this allows a more thorough analysis of its light curve and of the GRB host galaxy properties. The afterglow light curves show a single power-law temporal decline, modified at late times by light from a host galaxy with moderate intrinsic extinction, and possibly by an emerging supernova (SN). The afterglow evolution is interpreted within the classical “fireball” scenario as a weakly collimated adiabatic shock propagating in the interstellar medium. The presence of a SN light curve superimposed on the non-thermal afterglow emission is investigated: while in the optical bands no significant contribution to the total light is found from a SN, the NIR J-band data show an excess which is consistent with a SN as bright as the known hypernova SN1998bw. If the SN interpretation is true, this would be the farthest GRB-associated SN, as well as the farthest core-collapse SN, discovered to date. However, other possible explanations of this NIR excess are also investigated. Finally, we studied the photometric properties of the host, and found that it is likely to be a slightly reddened, subluminous, extreme starburst compact galaxy, with luminosity ~0.1 , an age of ~0.5 Gyr and a specific Star Formation Rate (SFR) of ≈30  yr^-1 ()^-1. This is the highest specific SFR value for a GRB host inferred from optical/NIR data.