Article

On the Afterglow of the X‐Ray Flash of 2003 July 23: Photometric Evidence for an Off‐Axis Gamma‐Ray Burst with an Associated Supernova?

J. P. U. FynboDepartment of Physics and Astronomy, University of Aarhus, Ny Munkegade, DK-8000 Århus C, DenmarkJ. SollermanStockholm Observatory, Department of Astronomy, AlbaNova, S-106 91 Stockholm, SwedenJ. HjorthNiels Bohr Institute, Astronomical Observatory, University of Copenhagen, Juliane Maries Vej 30, DK-2100 Copenhagen Ø, DenmarkF. GrundahlDepartment of Physics and Astronomy, University of Aarhus, Ny Munkegade, DK-8000 Århus C, DenmarkJ. GorosabelInstituto de Astrofísica de Andalucía (CSIC), c. Camino Bajo de Huétor, 24, E-18.008 Granada, SpainM. WeidingerDepartment of Physics and Astronomy, University of Aarhus, Ny Munkegade, DK-8000 Århus C, DenmarkP. MøllerEuropean Southern Observatory, Karl Schwarzschild-Strasse 2, D-85748 Garching, GermanyB. L. JensenNiels Bohr Institute, Astronomical Observatory, University of Copenhagen, Juliane Maries Vej 30, DK-2100 Copenhagen Ø, DenmarkP. M. VreeswijkEuropean Southern Observatory, Alonso de Córdova 3107, Casilla 19001, Santiago 19, ChileClaes FranssonStockholm Observatory, Department of Astronomy, AlbaNova, S-106 91 Stockholm, SwedenE. Ramírez-RuizSchool of Natural Sciences, Institute for Advanced Study, Einstein Drive, Princeton, NJ 08540; Chandra FellowP. JakobssonNiels Bohr Institute, Astronomical Observatory, University of Copenhagen, Juliane Maries Vej 30, DK-2100 Copenhagen Ø, DenmarkS. F. JørgensenNiels Bohr Institute, Astronomical Observatory, University of Copenhagen, Juliane Maries Vej 30, DK-2100 Copenhagen Ø, DenmarkC. VinterNiels Bohr Institute, Astronomical Observatory, University of Copenhagen, Juliane Maries Vej 30, DK-2100 Copenhagen Ø, DenmarkM. I. AndersenAstrophysikalisches Institut Potsdam, An der Sternwarte 16, D-14482 Potsdam, GermanyJ. M. Castro CerónSpace Telescope Science Institute, 3700 San Martin Drive, Baltimore, MD 21218A. J. Castro‐TiradoInstituto de Astrofísica de Andalucía (CSIC), c. Camino Bajo de Huétor, 24, E-18.008 Granada, SpainA. S. FruchterSpace Telescope Science Institute, 3700 San Martin Drive, Baltimore, MD 21218J. GreinerMax-Planck-Institute for Extraterrestrial Physics, D-85741 Garching, GermanyC. KouveliotouUniversities Research Association, Marshall Space Flight Center (NASA), Huntsville, AL 35812A. J. LevanX-ray Astronomy Group, Department of Physics and Astronomy, Leicester University, Leicester LE1 7RH, UKS. KloseThüringer Landessternwarte Tautenburg, D-07778 Tautenburg, GermanyN. MasettiIASF/CNR, Sezione di Bologna, Via Gobetti 101, I-40129 Bologna, ItalyH. PedersenNiels Bohr Institute, Astronomical Observatory, University of Copenhagen, Juliane Maries Vej 30, DK-2100 Copenhagen Ø, DenmarkE. PalazziINAF, Osservatorio Astronomico di Trieste, via Tiepolo, I-34131 Trieste, ItalyE. PianJames E. RhoadsSpace Telescope Science Institute, 3700 San Martin Drive, Baltimore, MD 21218E. RolIASF/CNR, Sezione di Bologna, Via Gobetti 101, I-40129 Bologna, ItalyTomohiko SekiguchiSolar Terrestrial Environment Laboratory, Nagoya University, Nagoya 464-8601, JapanN. R. TanvirDepartment of Physical Sciences, University of Hertfordshire, College Lane, Hatfield, Hertfordshire AL10 9AB, UKP. J. TristramA. de Ugarte PostigoSpace Telescope Science Institute, 3700 San Martin Drive, Baltimore, MD 21218R. A. M. J. WijersIASF/CNR, Sezione di Bologna, Via Gobetti 101, I-40129 Bologna, ItalyEdward van den HeuvelIASF/CNR, Sezione di Bologna, Via Gobetti 101, I-40129 Bologna, Italy

2004en

ABI

Abstract

We present optical and near-infrared follow-up observations of the X-Ray Flash (XRF) of July 23 2003. Our observations in the R-band cover the temporal range from 4.2 h to 64 days after the high energy event. We also present the results of multicolor imaging extending to the K-band on three epochs. The lightcurve of the R-band afterglow the first week after the burst is similar to the lightcurve for long duration Gamma-Ray Bursts (GRBs), i.e., a broken power-law with a late time slope of alpha=2.0 (F_nu propto t^-alpha). Furthermore, the spectral energy distribution (SED) has a power-law (F_nu propto nu^beta) shape with slope beta=-1.0. However, the decay slope at t<1 day is shallow, consistent with zero. This is in qualitative agreement with the prediction that XRFs are off-axis classical GRBs. After the first week there is a strong bump in the lightcurve, which peaks at around 16 days. The SED after the peak becomes significantly redder. We discuss the possible interpretations of this bump, and conclude that an underlying supernova is the most likely explanation since no other model appears consistent with the evolution of the SED. Finally, we present deep spectroscopy of the burst both in the afterglow and in the bump phase. A firm upper limit of z=2.3 is placed on the redshift of XRF030723 from the lack of Ly-alpha forest lines in the spectrum of the afterglow. The lack of significant absorption and emission lines in either of the two spectra excludes a spectroscopic redshift determination.

Identifiers

DOI: 10.1086/421260

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