GRB observations with Swift produced the initially surprising result that many bursts have large, late-time X-ray flares. The flares were sometimes intense, had rapid rise and decay phases, and occurred late relative to the prompt phase. Many GRBs have had several flares, which were sometimes overlapping. The origin of the flares can be investigated by comparing the spectra during the flares to those of the afterglow and the initial prompt emission. In this work we have analyzed all significant X-ray flares from the first 110 GRBs observed by Swift. Significant X-ray flares (>3 ) were found in 33 of these GRBs, with 77 flares detected. A variety of spectral models have been fit to each flare. We find that the spectral fits sometimes favor a Band function model, which is more akin to the prompt emission than to that of the afterglow. While some flares are approximately as energetic as the prompt GRB emission, we find that the average fluence of the flares is approximately 10 times below the average prompt GRB fluence. We also find that the peak energy of the observed flares is typically in the soft X-ray band, as one might expect due to the X-ray selection of the sample. These results, when combined with those presented in the companion paper on temporal properties of flares, support the hypothesis that many X-ray flares are from late-time activity of the internal engine that spawned the initial GRB, not from an afterglow-related effect.

This paper discusses Swift observations of the γ-ray burst GRB 050315 (z=1.949) from 80 s to 10 days after the onset of the burst. The X-ray light curve displayed a steep early decay (t −5 ) for ∼ 200 s and several breaks. However, both the prompt hard X-ray/γ-ray emission (observed by the BAT) and the first ∼ 300 s of X-ray emission (observed by the XRT) can be explained by exponential decays, with similar decay constants. Extrapolating the BAT light curve into the XRT band suggests the rapidly decaying, early X-ray emission was simply a continuation of the fading prompt emission; this strong similarity between the prompt γ-ray and early X-ray emission may be related to the simple temporal and spectral character of this X-ray rich GRB. The prompt (BAT) spectrum was a steep down to ∼ 15 keV, and appeared to continue through the XRT bandpass, implying a low peak energy, inconsistent with the Amati relation. Following the initial steep decline the X-ray afterglow did not fade for ∼ 1.2×10 4 s, after which time it decayed with a temporal index of α ≈ 0.7, followed by a second break at ∼ 2.5 × 10 5 s to a slope of α ∼ 2. The apparent 'plateau' in the X-ray light curve, after the early rapid decay, makes this one of the most extreme examples of the steep-flat-steep X-ray light curves revealed by Swift. If the second afterglow break is identified with a jet break then the jet opening angle was θ 0 ∼ 5 • , and implying E γ ∼ > 10 50 erg.

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.

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The long gamma-ray burst GRB 060714 was observed to exhibit a series of five X-ray flares beginning $70 s after the burst trigger T 0 and continuing until $T 0 þ 200 s. The first two flares were detected by the Burst Alert Telescope (BAT) on the Swift satellite, before Swift had slewed to the burst location, while the last three flares were strongly detected by the X-Ray Telescope (XRT) but only weakly detected by the BAT. This burst provides an unusual opportunity to track a complete sequence of flares over a wide energy range. The flares were very similar in their light curve morphology, showing power-law rise and fall components, and in most cases significant substructure. The flares also showed strong evolution with time, both spectrally and temporally. The small timescale and large amplitude variability observed are incompatible with an external shock origin for the flares, and support instead late-time sporadic activity either of the central source or of localized dissipation events within the outflow. We show that the flares in GRB 060714 cannot be the result of internal shocks in which the contrast in the Lorentz factor of the colliding shells is very small, and that this mechanism faces serious difficulties in most Swift GRBs. The morphological similarity of the flares and the prompt emission and the gradual and continual evolution of the flares with time makes it difficult and arbitrary to draw a dividing line between the prompt emission and the flares.