GRB 090902B: Afterglow Observations and Implications

The Astrophysical Journal (Impact Factor: 6.73). 04/2010; 714(1):799. DOI: 10.1088/0004-637X/714/1/799
Source: arXiv

ABSTRACT The optical-infrared afterglow of the Large Area Telescope (LAT)-detected long-duration burst, GRB 090902B, has been observed by several instruments. The earliest detection by ROTSE-IIIa occurred 80 minutes after detection by the Gamma-ray Burst Monitor instrument on board the Fermi Gamma-Ray Space Telescope, revealing a bright afterglow and a decay slope suggestive of a reverse shock origin. Subsequent optical-IR observations followed the light curve for 6.5 days. The temporal and spectral behavior at optical-infrared frequencies is consistent with synchrotron fireball model predictions; the cooling break lies between optical and XRT frequencies ~1.9 days after the burst. The inferred electron energy index is p = 1.8 ± 0.2, which would however imply an X-ray decay slope flatter than observed. The XRT and LAT data have similar spectral indices and the observed steeper value of the LAT temporal index is marginally consistent with the predicted temporal decay in the radiative regime of the forward shock model. Absence of a jet break during the first 6 days implies a collimation-corrected γ-ray energy E γ > 2.2 × 1052 erg, one of the highest ever seen in a long-duration gamma-ray bursts. More events combining GeV photon emission with multiwavelength observations will be required to constrain the nature of the central engine powering these energetic explosions and to explore the correlations between energetic quanta and afterglow emission.

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    ABSTRACT: We analyze the >100-MeV data for three gamma-ray bursts (GRBs) detected by the Fermi satellite (GRBs 080916C, 090510, 090902B) and find that these photons were generated via synchrotron emission in the external forward shock. We arrive at this conclusion by four different methods as follows. (1) We check the light curve and spectral behaviour of the >100 MeV data, and late-time X-ray and optical data, and find them consistent with the so-called closure relations for the external forward shock radiation. (2) We calculate the expected external forward shock synchrotron flux at 100 MeV, which is essentially a function of the total energy in the burst alone, and it matches the observed flux value. (3) We determine the external forward shock model parameters using the >100 MeV data (a very large phase space of parameters is allowed by the high-energy data alone), and for each point in the allowed parameter space we calculate the expected X-ray and optical fluxes at late times (hours to days after the burst) and find these to be in good agreement with the observed data for the entire parameter space allowed by the >100 MeV data. (4) We calculate the external forward shock model parameters using only the late-time X-ray, optical and radio data and from these estimate the expected flux at >100 MeV at the end of the sub-MeV burst (and at subsequent times) and find that to be entirely consistent with the high-energy data obtained by Fermi/LAT. The ability of a simple external forward shock, with two empirical parameters (total burst energy and energy in electrons) and two free parameters (circumstellar density and energy in magnetic fields), to fit the entire data from the end of the burst (1–50 s) to about a week, covering more than eight decades in photon frequency –>102 MeV, X-ray, optical and radio – provides compelling confirmation of the external forward shock synchrotron origin of the >100 MeV radiation from these Fermi GRBs. Moreover, the parameters determined in points (3) and (4) show that the magnetic field required in these GRBs is consistent with shock-compressed magnetic field in the circumstellar medium with pre-shocked values of a few tens of μG.
    Monthly Notices of the Royal Astronomical Society 11/2010; 409(1):226 - 236. · 5.52 Impact Factor
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    ABSTRACT: GRB 090902B, detected by the Fermi Large Array Telescope (Fermi/LAT), shows extended high-energy emission (>100 MeV) up to 103 s after the burst, and decays with time in a power law as t –1.5. It has also been observed by several follow-up low-energy instruments, including an early optical detection around 5000 s after the burst. The optical emission at early time decays faster than t –1.6, and has been suspected to originate from the reverse shock. We here explore the models that can possibly explain the broadband afterglow emission of GRB 090902B. We find that the reverse-shock model for the early optical emission would overpredict the radio afterglow flux that is inconsistent with observations. A partially radiative blast wave model, which though able to produce a sufficiently steep decay slope, cannot explain the broadband data of GRB 090902B. The two-component jet model, which consists of a narrow and bright jet component in the core and a surrounding wider and less energetic jet component, is shown to be able to explain the broadband afterglow data, including the LAT high-energy data after ~50 s and low-energy (radio, optical, and X-ray) afterglow data. The early-time high-energy emission detected by LAT before ~50 s is likely due to an internal origin similar to that of the sub-MeV emission. The highest energy (33 GeV) photon of GRB 090902B detected at 80 s can marginally be accommodated within the forward-shock emission under the optimistic condition that electrons are accelerated by the Bohm diffusive shock.
    The Astrophysical Journal 02/2011; 730(1):1. · 6.73 Impact Factor
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    ABSTRACT: Many previous studies have determined that the long lasting emission at X-ray, optical and radio wavelengths from gamma-ray bursts (GRBs), called the afterglow, is likely produced by the external forward shock model. In this model, the GRB jet interacts with the circum-stellar medium and drives a shock that heats the medium, which radiates via synchrotron emission. In this work, we carried out a detailed analysis of the late time afterglow data of GRB 090902B using a very careful accounting of the Inverse Compton losses. We find that in the context of the external forward shock model, the only viable option to explain the X-ray and optical data of GRB 090920B is to have the electron energy distribution deviate from a power-law shape and exhibit some slight curvature immediately downstream of the shock front (we explored other models that rely on a single power-law assumption, but they all fail to explain the observations). We find the fraction of the energy of shocked plasma in magnetic field to be ∼10−6 using late time afterglow data, which is consistent with the value obtained using early gamma-ray data. Studies like the present one might be able to provide a link between GRB afterglow modelling and numerical simulations of particle acceleration in collisionless shocks. We also provide detailed calculations for the early (≲ 103 s) high-energy (>100 MeV) emission and confirm that it is consistent with origin in the external forward shock. We investigated the possibility that the ∼10 keV excess observed in the spectrum during the prompt phase also has its origin in the external shock and found the answer to be negative.
    Monthly Notices of the Royal Astronomical Society 08/2011; 417(2):1584 - 1600. · 5.52 Impact Factor

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