'Long' gamma-ray bursts (GRBs) are commonly accepted to originate in the explosion of particularly massive stars, which give rise to highly relativistic jets. Inhomogeneities in the expanding flow result in internal shock waves that are believed to produce the gamma-rays we see. As the jet travels further outward into the surrounding circumstellar medium, 'external' shocks create the afterglow emission seen in the X-ray, optical and radio bands. Here we report observations of the early phases of the X-ray emission of five GRBs. Their X-ray light curves are characterised by a surprisingly rapid fall-off for the first few hundred seconds, followed by a less rapid decline lasting several hours. This steep decline, together with detailed spectral properties of two particular bursts, shows that violent shock interactions take place in the early jet outflows.

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 peak ∼ 210 s after the BAT trigger, a flux variation of δF/F ∼ 6 in a very short time δt/t peak 1 was observed. Its measured fluence in the 0.2−10 keV band was ∼1.4 × 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 iso ∼ 10 51 erg), a soft spectrum (photon index 2.65), no significant flux above ∼50 keV and a peak energy E 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.

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 peak ∼ 210 s after the BAT trigger, a flux variation of δF/F ∼ 6 in a very short time δt/t peak ≪ 1 was observed. Its measured fluence in the 0.2-10 keV band was ∼ 1.4 × 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 iso ∼ 10 51 erg), a soft spectrum (photon index 2.65), no significant flux above ∼ 50 keV and a peak energy E 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.

We present Swift and XMM-Newton observations of the bright gamma-ray burst GRB 050326, detected by the Swift Burst Alert Telescope. The Swift X-Ray Telescope (XRT) and XMM-Newton discovered the X-ray afterglow beginning 54 min and 8.5 h after the burst, respectively. The prompt GRB 050326 fluence was (7.7 ± 0.9) × 10 −6 erg cm −2 (20-150 keV), and its spectrum was hard, with a power law photon index Γ = 1.25 ± 0.03. The X-ray afterglow was quite bright, with a flux of 7 × 10 −11 erg cm −2 s −1 (0.3-8 keV), 1 h after the burst. Its light curve did not show any break nor flares between ∼1 h and ∼6 d after the burst, and decayed with a slope α = 1.70 ± 0.05. The afterglow spectrum is well fitted by a power-law model, suffering absorption both in the Milky Way and in the host galaxy. The rest-frame hydrogen column density is significant, N H,z > ∼ 4 × 10 21 cm −2 , and the redshift of the absorber was constrained to be z > 1.5. There was good agreement between the spatial, temporal, and spectral parameters as derived by Swift-XRT and XMM-Newton. By comparing the prompt and afterglow fluxes, we found that an early break probably occurred before the beginning of the XRT observation, similarly to many other cases observed by Swift. However, the properties of the GRB 050326 afterglow are well described by a spherical fireball expanding in a uniform external medium, so a further steepening is expected at later times. The lack of such a break allowed us to constrain the jet half-opening angle ϑ j > ∼ 7• . Using the redshift constraints provided by the X-ray analysis, we also estimated that the beaming-corrected gamma-ray energy was larger than 3 × 10 51 erg, at the high end of GRB energies. Despite the brightness in X rays, only deep limits could be placed by Swift-UVOT at optical and ultraviolet wavelengths. Thus, this GRB was a "truly dark" event, with the optical-to-X-ray spectrum violating the synchrotron limit. The optical and X-ray observations are therefore consistent either with an absorbed event or with a high-redshift one. To obey the Ghirlanda relation, a moderate/large redshift z > ∼ 4.5 is required.

No abstract