EDP Sciences
Free access
Issue A&A
Volume 408, Number 3, September IV 2003
Page(s) 941 - 947
Section Galactic structure and dynamics
DOI http://dx.doi.org/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. Jakobsson1, 2, J. Hjorth1, J. P. U. Fynbo1, 3, J. Gorosabel4, 5, K. Pedersen1, I. Burud5, A. Levan2, 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)

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  $\beta_{\rm X} = -1.21^{+0.10}_{-0.15}$. 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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Editor-in-Chief: T. Forveille
Letters Editor-in-Chief: J. Alves
Managing Editor: C. Bertout

ISSN: 0004-6361 ; e-ISSN: 1432-0746
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