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

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 s after the BAT trigger, a flux variation of in a very short time was observed. Its measured fluence in the 0.2-10 keV band was ~ 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, erg), a soft spectrum (photon index 2.65), no significant flux above ~50 keV and a peak energy 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.