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Issue A&A
Volume 450, Number 1, April IV 2006
Page(s) 59 - 68
Section Extragalactic astronomy
DOI http://dx.doi.org/10.1051/0004-6361:20054172

A&A 450, 59-68 (2006)
DOI: 10.1051/0004-6361:20054172

X-ray flare in XRF 050406: evidence for prolonged engine activity

P. Romano1, A. Moretti1, P. L. Banat1, D. N. Burrows2, S. Campana1, G. Chincarini1, S. Covino1, D. Malesani3, G. Tagliaferri1, S. Kobayashi2, 4, B. Zhang5, A. D. Falcone2, L. Angelini6, 7, S. Barthelmy6, A. P. Beardmore8, M. Capalbi9, G. Cusumano10, P. Giommi11, M. R. Goad8, O. Godet8, D. Grupe2, J. E. Hill6, 12, J. A. Kennea2, V. La Parola10, V. Mangano10, P. Mészáros2, D. C. Morris2, J. A. Nousek2, P. T. O'Brien8, J. P. Osborne8, A. Parsons6, M. Perri9, C. Pagani2, 1, K. L. Page8, A. A. Wells8 and N. Gehrels6

1  INAF - Osservatorio Astronomico di Brera, via E. Bianchi 46, 23807 Merate (LC), Italy
    e-mail: romano@merate.mi.astro.it
2  Department of Astronomy & Astrophysics, Pennsylvania State University, 525 Davey Lab, University Park, PA 16802, USA
3  International School for Advanced Studies (SISSA-ISAS), via Beirut 2-4, 34014 Trieste, Italy
4  Center for Gravitational Wave Physics, Pennsylvania State University, 104 Davey Lab, University Park, PA 16802, USA
5  Department of Physics, University of Nevada, Las Vegas, NV 89154-4002, USA
6  NASA/Goddard Space Flight Center, Greenbelt, MD 20771, USA
7  Department of Physics and Astronomy, The Johns Hopkins University, 3400 North Charles Street, Baltimore MD 21218 USA
8  X-Ray and Observational Astronomy Group, Department of Physics & Astronomy, University of Leicester, LE1 7RH, UK
9  ASI Science Data Center, via G. Galilei, 00044 Frascati, Roma, Italy
10  INAF - Istituto di Astrofisica Spaziale e Fisica Cosmica Sezione di Palermo, via U. La Malfa 153, 90146 Palermo, Italy
11  Agenzia Spaziale Italiana, Unità Osservazione dell'Universo, Viale Liegi 26, 00198 Roma, Italy
12  Universities Space Research Association, 10211 Wincopin Circle, Suite 500, Columbia, MD, 21044-3432, USA

(Received 7 September 2005 / Accepted 3 January 2006)

Abstract
We present observations of XRF 050406 , the first burst detected by Swift showing a flare in its X-ray light curve. During this flare, which peaks at $t_{\rm peak} \sim 210$ s after the BAT trigger, a flux variation of $\delta F / F \sim 6$ in a very short time $\delta t / t_{\rm peak} \ll 1$ was observed. Its measured fluence in the 0.2-10 keV band was ~ $1.4 \times 10^{-8}$ erg cm-2, which corresponds to 1-15% of the prompt fluence. We present indications of spectral variations during the flare. We argue that the producing mechanism is late internal shocks, which implies that the central engine is still active at 210 s, though with a reduced power with respect to the prompt emission. The X-ray light curve flattens to a very shallow slope with decay index of ~0.5 after ~4400 s, which also supports continued central engine activity at late times. This burst is classified as an X-ray flash, with a relatively low fluence (~10-7 erg cm-2 in the 15-350 keV band, $E_{\rm iso} \sim 10^{51}$ erg), a soft spectrum (photon index 2.65), no significant flux above ~50 keV and a peak energy $E_{\rm p} < 15$ keV. XRF 050406 is one of the first examples of a well-studied X-ray light curve of an XRF. We show that the main afterglow characteristics are qualitatively similar to those of normal GRBs. In particular, X-ray flares superimposed on a power-law light curve have now been seen in both XRFs and GRBs. This indicates that a similar mechanism may be at work for both kinds of events.


Key words: gamma rays: bursts -- X-rays: bursts -- X-rays: individuals: XRF 050406

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