Astronomy & Astrophysics

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Issue		A&A Volume 408, Number 3, September IV 2003
Page(s)		941 - 947
Section		Galactic structure and dynamics
DOI		10.1051/0004-6361:20031044

A&A 408, 941-947 (2003)
DOI: 10.1051/0004-6361:20031044

The afterglow and the host galaxy of GRB 011211

P. Jakobsson^{1, 2}, J. Hjorth¹, J. P. U. Fynbo^{1, 3}, J. Gorosabel^{4, 5}, K. Pedersen¹, I. Burud⁵, A. Levan^{2, 5}, C. Kouveliotou⁶, N. Tanvir⁷, A. Fruchter⁵, J. Rhoads⁵, T. Grav⁸, M. W. Hansen⁸, R. Michelsen¹, M. I. Andersen⁹, B. L. Jensen¹, H. Pedersen¹, B. Thomsen³, M. Weidinger³, S. G. Bhargavi¹⁰, R. Cowsik¹⁰ and S. B. Pandey¹¹

¹ Astronomical Observatory, University of Copenhagen, Juliane Maries Vej 30, 2100 Copenhagen Ø, Denmark
² Department of Physics and Astronomy, University of Leicester, University Road, Leicester, LE1 7RH, UK
³ Department of Physics and Astronomy, University of Aarhus, Ny Munkegade, 8000 Århus C, Denmark
⁴ IAA-CSIC, PO Box 03004, 18080 Granada, Spain
⁵ Space Telescope Science Institute, 3700 San Martin Drive, Baltimore, MD 21218, USA
⁶ NSSTC, SD-50, 320 Sparkman Drive, Huntsville, Alabama 35805, USA
⁷ Department of Physical Sciences, University of Hertfordshire, College Lane, Hatfield, Herts AL10 9AB, UK
⁸ Institute of Theoretical Astrophysics, University of Oslo, PB 1029 Blindern, 05315 Oslo, Norway
⁹ Astrophysikalisches Institut Potsdam, An der Sternwarte 16, 14482 Potsdam, Germany
¹⁰ Indian Institute of Astrophysics, Sarjapur Road, Bangalore 560 034, India
¹¹ State Observatory, Manora Peak, Nainital 263 129, India

(Received 4 November 2002 / Accepted 4 July 2003)

Abstract
We present optical, near-infrared, and X-ray observations of the optical afterglow (OA) of the X-ray rich, long-duration gamma-ray burst GRB 011211. Hubble Space Telescope (HST) data obtained 14, 26, 32, and 59 days after the burst, show the host galaxy to have a morphology that is fairly typical of blue galaxies at high redshift. We measure its magnitude to be $R = 24.95 \pm 0.11$ . We detect a break in the OA R-band light curve which is naturally accounted for by a collimated outflow geometry. By fitting a broken power-law to the data we find a best fit with a break $1.56 \pm 0.02$ days after the burst, a pre-break slope of $\alpha_1 = -0.95 \pm 0.02$ , and a post-break slope of $\alpha_2 = -2.11 \pm 0.07$ . The UV-optical spectral energy distribution (SED) around 14 hours after the burst is best fit with a power-law with index $\beta = -0.56 \pm 0.19$ reddened by an SMC-like extinction law with a modest $A_V = 0.08 \pm 0.08$ mag. By comparison, from the XMM-Newton X-ray data at around the same time, we find a decay index of $\alpha_{{\rm X}} = -1.62 \pm 0.36$ and a spectral index of $\beta_{\rm X} = -1.21^_$ . Interpolating between the UV-optical and X-ray implies that the cooling frequency is located close to ~10 ¹⁶ Hz in the observer frame at the time of the observations. We argue, using the various temporal and spectral indices above, that the most likely afterglow model is that of a jet expanding into an external environment that has a constant mean density rather than a wind-fed density structure. We estimate the electron energy index for this burst to be $p \sim2.3$ .

Key words: cosmology: observations -- gamma rays: bursts -- stars: supernovae: general -- ISM: dust, extinction

Offprint request: P. Jakobsson, pallja@astro.ku.dk

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