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Register a new userThe X-ray afterglow of GRB030329 at early and late times
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A. Tiengo
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Thanks to its extraordinary brightness, the X-ray afterglow of GRB030329 could be studied by XMM-Newton up to two months after the prompt Gamma-ray emission. We present the results of two XMM-Newton observations performed on May 5 and 29, as well as an analysis of the Rossi-XTE data of the early part of the afterglow, discussing in particular the stability of the X-ray spectrum and presenting upper limits on the presence of X-ray emission lines.
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We report on XMM-Newton and Rossi-XTE observations of the bright (fluence $sim$ 10$^{-4}$ erg cm$^{-2}$) and nearby (z=0.1685) Gamma-Ray Burst GRB030329 associated to SN2003dh. The first Rossi-XTE observation, 5 hours after the burst, shows a flux decreasing with time as a power law with index 0.9$pm$0.3. Such a decay law is only marginally consistent with a further Rossi-XTE measurement (at t-t$_{GRB}sim$30 hr). Late time observations of this bright afterglow at X-ray wavelengths have the advantage, compared to optical observations, of not being affected by contributions from the supernova and host galaxy. A first XMM-Newton observation, at t-t$_{GRB}sim$37 days, shows a flux of 4$times10^{-14}$ erg cm$^{-2}$ s$^{-1}$ (0.2-10 keV). The spectrum is a power law with photon index $Gamma$=1.9 and absorption $<2.5times10^{20}$ cm$^{-2}$, consistent with the Galactic value. A further XMM-Newton pointing at t-t$_{GRB}sim$61 days shows a flux fainter by a factor $sim$2. The combined Rossi-XTE and XMM-Newton measurements require a break at t$sim$0.5 days in the afterglow decay, with a power law index increasing from 0.9 to 1.9, similar to what is observed in the early part of the optical afterglow. The extrapolation of the XMM-Newton spectra to optical frequencies lies a factor of $sim10$ below simultaneous measurements. This is likely due to the presence of SN2003dh.
Extensive X-ray, optical and radio observations of the bright afterglow of the Gamma Ray Burst GRB 030329 are used to construct the multi-frequency evolution of the event. The data are fitted using the standard fireball shock model to provide estimates of the initial energy, epsilon = 6.8 x 10^52 ergs sr^-1, the density of the ambient medium, n_0 = 1 cm^-3, the electron and magnetic energy density fractions, epsilon_e = 0.24 & epsilon_B = 0.0017, the power law index of the relativistic electron spectrum, p = 2.25, and the opening angle of the jet, theta_j = 3 degrees. Deviations from the standard model seen in the optical and radio are most likely attributable to the concurrent hypernova SN2003dh. Peaks at 0.23 and 1.7 days in the R-band are much brighter than expected from a standard SN, and there is a large radio excess over the expected afterglow flux for t>2 days. No deviation from the best-fit afterglow model is seen in the X-ray decline, indicating that the excess optical and radio flux from 1-5 days arises from a later injection of slower electrons by the central engine.
The X-ray afterglow of the Gamma-Ray Burst GRB 030329, associated to SN2003dh at z=0.1685, has been observed with XMM-Newton 258 days after the burst explosion. A source with flux of (6.2 +/- 2.3) 10^{-16} erg cm^{-2} s^{-1} (0.5-2 keV) has been detected at the GRB position. This measurement, together with a re-analysis of the previous X-ray observations, indicates a flattening of the X-ray light curve ~40 days after the burst. This is in remarkable agreement with the scenario invoking the presence of two jets with different opening angles. The wider jet should be responsible for the observed flattening due to its transition into the non-relativistic Sedov-Taylor phase.
We report on 5 Chandra observations of the X-ray afterglow of the Gamma-Ray Burst GRB 060729 performed between 2007 March and 2008 May. In all five observations the afterglow is clearly detected. The last Chandra pointing was performed on 2008-May-04, 642 days after the burst - the latest detection of a GRB X-ray afterglow ever. A reanalysis of the Swift XRT light curve together with the three detections by Chandra in 2007 reveals a break at about 1.0 Ms after the burst with a slight steepening of the decay slope from alpha = 1.32 to 1.61. This break coincides with a significant hardening of the X-ray spectrum, consistent with a cooling break in the wind medium scenario, in which the cooling frequency of the afterglow crosses the X-ray band. The last two Chandra observations in 2007 December and 2008 May provide evidence for another break at about one year after the burst. If interpreted as a jet break, this late-time break implies a jet half opening angle of about 14 degrees for a wind medium. Alternatively, this final break may have a spectral origin, in which case no jet break has been observed and the half-opening angle of the jet of GRB 060729 must be larger than about 15 degrees for a wind medium. We compare the X-ray afterglow of GRB 060729 in a wind environment with other bright X-ray afterglows, in particular GRBs 061121 and 080319B, and discuss why the X-ray afterglow of GRB 060729 is such an exceptionally long-lasting event.
We present analytic flux prescriptions for broadband spectra of self-absorbed and optically thin synchrotron radiation from gamma-ray burst afterglows, based on one-dimensional relativistic hydrodynamic simulations. By treating the evolution of critical spectrum parameters as a power-law break between the ultrarelativistic and non-relativistic asymptotic solutions, we generalize the prescriptions to any observer time. Our aim is to provide a set of formulas that constitutes a useful tool for accurate fitting of model-parameters to observational data, regardless of the dynamical phase of the outflow. The applicability range is not confined to gamma-ray burst afterglows, but includes all spherical outflows (also jets before the jet-break) that produce synchrotron radiation as they adiabatically decelerate in a cold, power-law medium. We test the accuracy of the prescriptions and show that numerical evidence suggests that typical relative errors in the derivation of physical quantities are about 10 per cent. A software implementation of the presented flux prescriptions combined with a fitting code is freely available on request and on-line. Together they can be used in order to directly fit model parameters to data.
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