Optical and near-infrared observations of the GRB020405 afterglow^*

¹ Istituto di Astrofisica Spaziale e Fisica Cosmica – Sezione di Bologna, CNR, via Gobetti 101, 40129 Bologna, Italy
² INAF – Osservatorio Astronomico di Trieste, via G.B. Tiepolo 11, 34131 Trieste, Italy
³ Dipartimento di Astronomia, Università di Bologna, via Ranzani 1, 40126 Bologna, Italy
⁴ Istituto di Radioastronomia – Sezione di Firenze, CNR, largo E. Fermi 5, 50125 Florence, Italy
⁵ Department of Physics and Astronomy, University of Leicester, University Road, Leicester, LE1 7RH, UK
⁶ Astrophysikalisches Institut, 14482 Potsdam, Germany
⁷ Institute of Astronomy “Anton Pannekoek", University of Amsterdam, Kruislaan 403, 1098 SJ Amsterdam, The Netherlands
⁸ The Johns Hopkins University, 3400 North Charles Street, Bartimore, MD 21218, USA
⁹ INAF – Osservatorio Astronomico di Roma, via Frascati 33, 00040 Monteporzio Catone, Italy
¹⁰ INAF – Osservatorio Astronomico di Padova, vicolo dell'Osservatorio 5, 35122 Padua, Italy
¹¹ Instituto de Astrofísica de Andalucía (IAA-CSIC), PO Box 03004, 18080 Granada, Spain
¹² Astronomical Observatory, University of Copenhagen, Juliane Maries Vej 30, 2100 Copenhagen Ø, Denmark
¹³ Observatoire de Paris-Meudon - LESIA, 5 Place Jules Janssen, 92195 Meudon, France
¹⁴ Max-Planck-Institut für Extraterrestrische Physik, 85741 Garching, Germany
¹⁵ State Observatory, Manora Peak, NainiTal, 263129 Uttaranchal, India
¹⁶ Space Telescope Science Institute, 3700 San Martin Drive, Baltimore, MD 21218, USA
¹⁷ Real Instituto y Observatorio de la Armada, Sección de Astronomía, 11 110 San Fernando-Naval (Cádiz), Spain
¹⁸ Dipartimento di Fisica, Università di Ferrara, via Paradiso 12, 44 100 Ferrara, Italy
¹⁹ Department of Physics and Astronomy, University of Århus, Ny Munkegade, 8000 Århus C, Denmark
²⁰ Thüringer Landessternwarte Tautenburg, 07778 Tautenburg, Germany
²¹ NASA MSFC, SD-50, Huntsville, AL 35812, USA
²² Istituto di Astrofisica Spaziale e Fisica Cosmica – Sezione di Palermo, CNR, via La Malfa 153, 90146 Palermo, Italy
²³ Department of Physical Sciences, University of Hertfordshire, College Lane, Hatfield, Herts AL10 9AB, UK
²⁴ European Southern Observatory, Casilla 19001, Santiago 19, Chile

Corresponding author: N. Masetti, masetti@bo.iasf.cnr.it

Received: 19 February 2003
Accepted: 27 March 2003

Abstract

We report on photometric, spectroscopic and polarimetric monitoring of the optical and near-infrared (NIR) afterglow of GRB020405. Ground-based optical observations, performed with 8 different telescopes, started about 1 day after the high-energy prompt event and spanned a period of ~10 days; the addition of archival HST data extended the coverage up to ~150 days after the GRB. We report the first detection of the afterglow in NIR bands. The detection of Balmer and oxygen emission lines in the optical spectrum of the host galaxy indicates that the GRB is located at redshift . Fe ii and Mg ii absorption systems are detected at and at in the afterglow optical spectrum. The latter system is likely caused by absorbing clouds in the galaxy complex located ~2'' southwest of the GRB020405 host. Hence, for the first time, the galaxy responsible for an intervening absorption line system in the spectrum of a GRB afterglow is spectroscopically identified. Optical and NIR photometry of the afterglow indicates that, between 1 and 10 days after the GRB, the decay in all bands is consistent with a single power law of index . The late-epoch VLT J-band and HST optical points lie above the extrapolation of this power law, so that a plateau (or “bump") is apparent in the VRIJ light curves at 10–20 days after the GRB. The light curves at epochs later than day ~20 after the GRB are consistent with a power-law decay with index . While other authors have proposed to reproduce the bump with the template of the supernova (SN) 1998bw, considered the prototypical “hypernova”, we suggest that it can also be modeled with a SN having the same temporal profile as the other proposed hypernova SN2002ap, but 1.3 mag brighter at peak, and located at the GRB redshift. Alternatively, a shock re-energization may be responsible for the rebrightening. A single polarimetric R-band measurement shows that the afterglow is polarized, with % and polarization angle . Broad-band optical-NIR spectral flux distributions show, in the first days after the GRB, a change of slope across the J band which we interpret as due to the presence of the electron cooling frequency . The analysis of the multiwavelength spectrum within the standard fireball model suggests that a population of relativistic electrons with index  produces the optical-NIR emission via synchrotron radiation in an adiabatically expanding blastwave, with negligible host galaxy extinction, and the X–rays via Inverse Compton scattering off lower-frequency afterglow photons.