on behalf of the Swift-XRT teamWe present a comprehensive statistical analysis of Swift X-ray light-curves of Gamma-Ray Bursts (GRBs), with more than 650 GRBs. Two questions drive this effort: (1) Does the X-ray emission retain any kind of memory of the prompt phase? (2) Where is the dividing line between long and short GRBs? We show that short GRBs decay faster, are less luminous and less energetic than long GRBs, but are interestingly characterized by very similar intrinsic absorption. Our analysis reveal the existence of a number of relations that link the X-ray to prompt parameters in long GRBs; short GRBs are outliers of the majority of these 2-parameter relations. Here we concentrate on a 3-parameter (E pk − E γ,iso − E X,iso ) scaling that is shared by the GRB class as a whole (short GRBs, long GRBs and X-ray Flashes -XRFs): interpreted in terms of emission efficiency, this scaling may imply that GRBs with high E pk are more efficient during their prompt emission.
Context. Data from cosmic microwave background radiation (CMB), baryon acoustic oscillations (BAO), and supernovae Ia (SNe-Ia) support a constant dark energy equation of state with w 0 ∼ −1. Measuring the evolution of w along the redshift is one of the most demanding challenges for observational cosmology. Aims. We discuss the existence of a close relation for gamma-ray bursts (GRBs), named Combo-relation, based on characteristic parameters of GRB phenomenology such as the prompt intrinsic peak energy E p,i , the X-ray afterglow initial luminosity L 0 and the rest-frame duration τ of the shallow phase, and the index of the late power-law decay α X . We use it to measure Ω m and the evolution of the dark energy equation of state. We also propose a new calibration method for the same relation, which reduces the dependence on SNe Ia systematics. Methods. We have selected a sample of GRBs with 1) a measured redshift z; 2) a determined intrinsic prompt peak energy E p,i ; and 3) a good coverage of the observed (0.3-10) keV afterglow light curves. The fitting technique of the rest-frame (0.3-10) keV luminosity light curves represents the core of the Combo-relation. We separate the early steep decay, considered a part of the prompt emission, from the X-ray afterglow additional component. Data with the largest positive residual, identified as flares, are automatically eliminated until the p-value of the fit becomes greater than 0.3. Results. We strongly minimize the dependency of the Combo-GRB calibration on SNe Ia. We also measure a small extra-Poissonian scatter of the Combo-relation, which allows us to infer from GRBs alone Ω M = 0.29 Conclusions. In view of the increasing size of the GRB database, thanks to future missions, the Combo-relation is a promising tool for measuring Ω m with an accuracy comparable to that exhibited by SNe Ia, and to investigate the dark energy contribution and evolution up to z ∼ 10.
We present the first systematic study of X-ray flare candidates in short gamma-ray bursts (SGRBs) exploiting the large 6-year Swift data base with the aim to constrain the physical nature of such fluctuations. We find that flare candidates appear in different types of SGRB host galaxy environments and show no clear correlation with the X-ray afterglow lifetime; flare candidates are detected both in SGRBs with a bright extended emission in the soft γ -rays and in SGRBs which do not show such component. We furthermore show that SGRB X-ray flare candidates only partially share the set of observational properties of long GRB (LGRB) flares. In particular, the main parameter driving the duration evolution of X-ray variability episodes in both classes is found to be the elapsed time from the explosion, with very limited dependence on the different progenitors, environments, central engine lifetimes, prompt variability timescales and energy budgets. On the contrary, SGRB flare candidates significantly differ from LGRB flares in terms of peak luminosity, isotropic energy, flare-to-prompt luminosity ratio and relative variability flux. However, these differences disappear when the central engine time-scales and energy budget are accounted for, suggesting that (i) flare candidates and prompt pulses in SGRBs likely have a common origin; (ii) similar dissipation and/or emission mechanisms are responsible for the prompt and flare emission in LGRBs and SGRBs, with SGRBs being less energetic albeit faster evolving versions of the long class. Finally, we show that in strict analogy to the SGRBprompt emission, flares candidates fall off the lag–luminosity relation defined by LGRBs, thus strengthening the SGRB flare–prompt pulse connection
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