A possible bright blue supernova in the afterglow of GRB 020305

Astronomy & Astrophysics manuscript no. (will be inserted by hand later) arXiv:astro-ph/0504050v1 3 Apr 2005 A possible bright blue SN in the afterglow of GRB 020305 ⋆ J. Gorosabel 1,2 , J.P.U. Fynbo 3 , A. Fruchter 2 , A. Levan 4 , J. Hjorth 3 , P. Nugent 5 , A.J. Castro-Tirado 1 , J.M. Castro Cer´on 2,3 , J. Rhoads 2 , D. Bersier 2 , and I. Burud 2 Instituto de Astrof´isica de Andaluc´ia (IAA-CSIC), P.O. Box 03004, E-18080 Granada, Spain Space Telescope Science Institute, 3700 San Martin Drive, Baltimore, MD 21218, USA Niels Bohr Institute, University of Copenhagen, Juliane Maries Vej 30, DK-2100 Kobenhavn O, Denmark Department of Physics and Astronomy, University of Leicester, University Road, Leicester, LE1 7RH, UK Lawrence Berkeley National Laboratory, MS 50-F, 1 Cyclotron Road, Berkeley, CA 94720, USA Received ; accepted Abstract. We report on ground-based and HST(+STIS) imaging of the afterglow and host galaxy of the Gamma-Ray Burst (GRB) of March 5 2002. The GRB occurred in a R = 25.17 ± 0.14 galaxy, which apparently is part of an interacting system. The lightcurve of the optical afterglow shows a rebrightening, or at least a plateau, 12–16 days after the gamma-ray event. U BV RIK ′ multi-band imaging of the afterglow ∼12 days after the GRB reveals a blue spectral energy distribution (SED). The SED is consistent with a power-law with a spectral index of β = −0.63 ± 0.16, but there is tentative evidence for deviations away from a power-law. Unfortunately, a spectroscopic redshift has not been secured for GRB 020305. From the SED we impose a redshift upper limit of z . 2.8, hence excluding the pseudo redshift of 4.6 reported for this burst. We discuss the possibilities for explaining the lightcurve, SED and host galaxy properties for GRB 020305. The most natural interpretation of the lightcurve and the SED is an associated supernova (SN). Our data can not precisely determine the redshift of the GRB. The most favoured explanation is a low redshift (z ∼ 0.2) SN, but a higher redshift (z & 0.5) SN can not be excluded. We also discuss less likely scenarios not based on SNe, like a burst occurring in a z = 2.5 galaxy with an extinction curve similar to that of the Milky Way. Key words. gamma rays: bursts – techniques: photometric 1. Introduction For long duration GRBs the relation with supernovae (SNe) became firmly established with the discovery of the type Ic su- pernova SN 2003dh associated with GRB 030329 (Stanek et al. 2003; Hjorth et al. 2003). This result lends strong support to the collapsar model (Woosley 1993), but a SN is also an ingredient in other models (e.g. Dado et al. 2003; Fryer & Heger 2004). However, the associated SNe follow a broad distribution of op- tical luminosities (Zeh et al. 2004). Furthermore the connection of GRBs with SNe of other types than Ic can not be excluded, motivated by the two possible associations of GRBs and II type SNe (SN 1997cy, Germany et al. 2000; SN 1999E, Rigon et al. 2003). Therefore, the afterglow lightcurves and SEDs around the SN peak are far from being described by an universal SN template. In this study we present ground and space-based opti- cal observations of GRB 020305 carried out from 11.5 to 321.2 days after the burst. Send offprint requests to: J. Gorosabel Based on observations made with the Nordic Optical Telescope, operated on the island of La Palma jointly by Denmark, Finland, Iceland, Norway, and Sweden. Correspondence to: jgu@iaa.es GRB 020305 was localised by the HETE-II satellite on March 5.4968 UT (Ricker et al. 2002). The high-energy emis- sion as seen by the Interplanetary network (IPN) consisted in two broad pulses, with a total GRB duration of ∼280s (Hurley et al. 2002), placing it in the long-soft burst category. Price et al. (2002) reported the presence of a transient optical source in the HETE-II/IPN error box in images taken ∼20 hours after the GRB. Further imaging confirmed the fading behaviour of the candidate (Lee et al. 2002; Ohyama et al. 2002). The paper is structured as follows: Sect. 2 details the ob- servations and the data reduction, Sect. 3 reports the results on the SED, lightcurve and host galaxy, Sect. 4 discusses several interpretations of the results, and finally Sect. 5 draws the con- clusions of this study. 2. Observations and data reduction 2.1. NOT observations We observed the field of GRB 020305 from the ground with the 2.56-m Nordic Optical Telescope (NOT) on 2002 March 16.95–22.18 UT, i.e. 11.45–16.68 days after the GRB. The in- strument used was the Andaluc´ia Faint Object Spectrograph (ALFOSC) equipped with a 2048 2 pixel Loral CCD having a