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Issue A&A
Volume 374, Number 2, August I 2001
Page(s) 382 - 393
Section Cosmology
DOI http://dx.doi.org/10.1051/0004-6361:20010740



A&A 374, 382-393 (2001)
DOI: 10.1051/0004-6361:20010740

GRB010222: Afterglow emission from a rapidly decelerating shock

N. Masetti1, E. Palazzi1, E. Pian2, 1, F. Mannucci3, L. A. Antonelli4, A. Di Paola4, P. Saracco5, S. Savaglio4, L. Amati1, C. Bartolini6, S. Bernabei7, 8, D. Bettoni9, S. Covino5, S. Cristiani10, S. Desidera11, S. Di Serego Alighieri12, R. Falomo9, F. Frontera1, 13, F. Ghinassi14, A. Guarnieri6, A. Magazzù14, R. Maiolino12, M. Mignoli7, L. Nicastro15, M. Pedani14, A. Piccioni6, B. M. Poggianti9, V. Testa4, G. Valentini16 and A. Zacchei14

1  Istituto Tecnologie e Studio delle Radiazioni Extraterrestri, CNR, Via Gobetti 101, 40129 Bologna, Italy
2  Osservatorio Astronomico di Trieste, Via G.B. Tiepolo 11, 34131 Trieste, Italy
3  Centro per l'Astronomia Infrarossa e lo Studio del Mezzo Interstellare, CNR, Largo E. Fermi 5, 50125 Florence, Italy
4  Osservatorio Astronomico di Roma, via di Frascati 33, 00040 Monteporzio Catone, Italy
5  Osservatorio Astronomico di Brera, via Bianchi 46, 23807 Merate, Italy
6  Dipartimento di Astronomia, Università di Bologna, Via Ranzani 1, 40127 Bologna, Italy
7  Osservatorio Astronomico di Bologna, Via Ranzani 1, 40127 Bologna, Italy
8  Instituto de Astrofísica de Canarias, C/ Vía Láctea s/n, 38200, La Laguna, Tenerife, Spain
9  Osservatorio Astronomico di Padova, Vicolo dell'Osservatorio 5, 35122 Padua, Italy
10  Dipartimento di Astronomia, Università di Padova, Vicolo dell'Osservatorio 5, 35122 Padua, Italy
11  Osservatorio Astronomico di Asiago, Via dell'Osservatorio 8, 36012, Asiago, Italy
12  Osservatorio Astrofisico di Arcetri, Largo E. Fermi 5, 50125 Florence, Italy
13  Dipartimento di Fisica, Università di Ferrara, via Paradiso 12, 44100 Ferrara, Italy
14  Telescopio Nazionale Galileo, Roque de Los Muchachos Astronomical Observatory, PO Box 565, 38700 Santa Cruz de La Palma, Spain
15  Istituto di Fisica Cosmica ed Applicazioni all'Informatica, CNR, via ugo La Malfa 153, 90146 Palermo, Italy
16  Osservatorio Astronomico di Collurania-Teramo, Via Maggini, 64100 Teramo, Italy

(Received 20 March 2001 / Accepted 2 May 2001)

Abstract
The GRB010222 optical and near-infrared (NIR) afterglow was monitored at the TNG and other Italian telescopes starting ~1 day after the high-energy prompt event. The BVR light curves, which are the best sampled, are continuously steepening and can be described by two power laws, $f(t) \propto t^{-\alpha}$, of indices $\alpha_1 \sim$ 0.7 and $\alpha_2 \sim$ 1.3 before and after a break occurring at about 0.5 days after the GRB start time, respectively. This model accounts well also for the flux in the U, I and J bands, which are less well monitored. The temporal break appears to be achromatic. The two K-band points are not consistent with the above behaviour, and rather suggest a constant trend. A low-resolution optical spectrum has also been taken with TNG. In the optical spectrum we found three absorption systems at different redshifts (0.927, 1.155 and 1.475), the highest of which represents a lower limit to, and probably coincides with, the redshift of the GRB. The broad-band optical spectral energy distributions do not appear to vary with time, consistently with the achromatic behaviour of the light curves. We compare our measurements with different afterglow evolution scenarios and we find that they favor a transition from relativistic to non-relativistic conditions in the shock propagation.


Key words: gamma rays: bursts -- radiation mechanisms: non-thermal -- line: identification -- cosmology: observations

Offprint request: N. Masetti, masetti@tesre.bo.cnr.it

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