Prompt and afterglow X-ray emission from the X-Ray Flash of 2002 April 27

¹ Istituto di Astrofisica Spaziale e Fisica cosmica – Sezione di Bologna, CNR, via Gobetti 101, 40129 Bologna, Italy e-mail: amati@bo.iasf.cnr.it
² Dipartimento di Fisica, Università di Ferrara, via Paradiso 12, 44100 Ferrara, Italy
³ SRON National Institute for Space Research, Sorbonnelaan 2, 3584 CA Utrecht, The Netherlands
⁴ Astronomical Institute, Utrecht University, PO Box 80000, 3508 TA Utrecht, The Netherlands
⁵ ASI Science Data Center c/o ESRIN, via G. Galilei, 00044 Frascati (RM), Italy
⁶ Istituto di Astrofisica Spaziale e Fisica cosmica, CNR, via Fosso del Cavaliere, 00133 Roma, Italy
⁷ Osservatorio Astronomico di Roma, via Frascati 33, 00040 Monteporzio Catone (RM), Italy
⁸ Istituto di Astrofisica Spaziale e Fisica cosmica – Sezione di Palermo, CNR, via La Malfa 153, 90146 Palermo, Italy

Received: 26 January 2004
Accepted: 27 June 2004

Abstract

We report on the X-ray observations of the X-ray flash (XRF) which occurred on 2002 April 27, three days before BeppoSAX was switched off. The event was detected with the BeppoSAX Wide Field Cameras but not with the Gamma ray Burst Monitor. A follow-up observation with the BeppoSAX Narrow Field Instruments was soon performed and a candidate afterglow source was discovered. We present the results obtained. We also include the results obtained from the observations of the XRF field with the Chandra X-ray satellite. The spectral analysis of the prompt emission shows that the peak energy of the  spectrum is lower than 5.5 keV, with negligible spectral evolution. The X-ray afterglow spectrum is consistent with a power law model with photon index of ~2, while the 2–10 keV flux fades as a power law with a decay index -1.33. Both these indices are typical of GRBs. A very marginal excess at ~4.5–5 keV is found in the afterglow spectrum measured by BeppoSAX. As for many GRBs, the extrapolation of the 2–10 keV fading law back to the time of the prompt emission is consistent with the X-ray flux measured during the second part of the event. We estimate a possible range of values of the redshift and discuss our results in the light of current models of XRFs.