The exceptionally extended flaring activity in the X-ray afterglow of GRB 050730 observed with Swift and XMM-Newton

Astronomy and Astrophysics (Impact Factor: 4.38). 04/2007; 471. DOI: 10.1051/0004-6361:20066227
Source: arXiv


We present the results of a detailed spectral and temporal analysis of Swift and XMM-Newton observations of the high redshift (z=3.969) GRB 050730. The X-ray afterglow of GRB 050730 was found to decline with time with superimposed intense flaring activity that extended over more than two orders of magnitude in time. Seven distinct re-brightening events starting from 236 s up to 41.2 ks after the burst were observed. The underlying decay of the afterglow was well described by a double broken power-law model with breaks at t_1= 237 +/- 20 s and t_2 = 10.1 (-2.2) (+4.6) ks. The temporal decay slopes before, between and after these breaks were alpha_1 = 2.1 +/- 0.3, alpha_2 = 0.44 (-0.08) (+0.14) and alpha_3 = 2.40 (+0.07) (-0.09), respectively. The spectrum of the X-ray afterglow was well described by a photoelectrically absorbed power-law with an absorbing column density N_H=(1.28 +/- 0.26) 10^22 cm^-2 in the host galaxy. Strong X-ray spectral evolution during the flaring activity was present. In the majority of the flares (6/7) the ratio Delta_t/t_p between the duration of the event and the time when the flare peaks was nearly constant and about 0.6-0.7. We showed that the observed spectral and temporal properties of the first three flares are consistent with being due both to high-latitude emission, as expected if the flares were produced by late internal shocks, or to refreshed shocks, i.e. late time energy injections into the main afterglow shock by slow moving shells ejected from the central engine during the prompt phase. The event fully satisfies the E_p-E_iso Amati relation while is not consistent with the E_p-E_jet Ghirlanda relation. Comment: 12 pages, 5 figures, accepted for publication in A&A

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Available from: Eleonora Troja, Nov 28, 2012
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    ABSTRACT: Scattering of the forward-shock synchrotron emission by a relativistic outflow located behind the leading blast-wave may produce an X-ray emission brighter than that coming directly from the forward-shock and may explain four features displayed by Swift X-ray afterglows: flares, plateaus (slow decays), chromatic light-curve breaks, and fast post-plateau decays. For a cold scattering outflow, the reflected flux overshines the primary one if the scattering outflow is nearly baryon-free and highly relativistic. These two requirements can be relaxed if the scattering outflow is energized by weak internal shocks, so that the incident forward-shock photons are also inverse-Compton scattered, in addition to bulk-scattering. Sweeping-up of the photons left behind by the forward shock naturally yields short X-ray flares. Owing to the boost in photon energy produced by bulk-scattering scattering, the reflected emission is more likely to overshine that coming directly from the forward shock at higher photon energies, yielding light-curve plateaus and breaks that appear only in the X-ray. The brightness, shape, and decay of the X-ray light-curve plateau depend on the radial distribution of the scatterer's Lorentz factor and mass-flux. Chromatic X-ray light-curve breaks and sharp post-plateau decays cannot be accommodated by the direct forward-shock emission and argue in favour of the scattering-outflow model proposed here. On the other hand, the X-ray afterglows without plateaus, those with achromatic breaks, and those with very long-lived power-law decays are more naturally accommodated by the standard forward-shock model. Thus the diversity of X-ray light-curves arises from the interplay of the scattered and direct forward-shock emissions. Comment: to appear in MNRAS, 12 pages, 7 figures
    Monthly Notices of the Royal Astronomical Society 08/2007; 383(3). DOI:10.1111/j.1365-2966.2007.12607.x · 5.11 Impact Factor
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    ABSTRACT: In this paper we study synchrotron and synchrotron self Compton (SSC) emission from internal shocks (IS) during the prompt and X-ray flare phases of Gamma-Ray Bursts (GRBs). The aim is to test the IS model for the flare emission and for whether GRBs can be GeV sources. We determine the parameters for which the IS model can account for the observed prompt and X-ray flares emission, and study the detectability of the high energy SSC emission by the AGILE and GLAST satellites. We find that the detectability of the SSC emission during the prompt phase of GRBs improves for higher values of the fireball Lorentz factor and of the temporal variability. If IS is the mechanism responsible of the flare emission, and the Lorentz factor of the shells producing the flare is of the order of 100, the flare light curves are expected to present some substructures with temporal variability of 10-100 ms which are much smaller than the average duration of flares, and similar to those observed during the prompt phase of GRBs. If one assumes lower Lorentz factors, such as 10-25, then a larger temporal variaibility of 40 s can also account for the observed flare properties. However in this case we predict that X-ray flares do not have a counterpart at very high energies (MeV-GeV). An investigation on the substructures of the X-ray flares light curves, and simultaneous X-ray and high energy observations, will allow us to corroborate the hypothesis that late IS are responsible of the X-ray flares. Comment: 10 pages, 11 figures, accepted for the pubblication in Astronomy & Astrophysics. Referee comments implemented
    Astronomy and Astrophysics 09/2007; 480(1). DOI:10.1051/0004-6361:20078518 · 4.38 Impact Factor
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