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Register a new userContinuous optical monitoring during the prompt emission of GRB 060111B
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We present the time-resolved optical emission of GRB 060111B during its prompt phase, measured with the TAROT robotic observatory. This is the first time that the optical emission from a gamma-ray burst has been continuously monitored with a temporal resolution of a few seconds during the prompt gamma-ray phase. The temporal evolution of the prompt optical emission at the level of several seconds is used to provide a clue to the origin of this emission. The optical emission was found to decay steadily from our first measure, 28s after the trigger, in contrast to the gamma-ray emission, which exhibits strong variability at the same time. This behaviour strongly suggests that the optical emission is due to the reverse shock.
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In this work, we present the results obtained from a multi-wavelength campaign, as well as from the public Swift/BAT, XRT, and UVOT data of GRB 060111B for which a bright optical emission was measured with good temporal resolution during the prompt phase. We identified the host galaxy at R~25 mag; its featureless spectral continuum and brightness, as well as the non-detection of any associated supernova 16 days after the trigger and other independent redshift estimates, converge to z~1-2. From the analysis of the early afterglow SED, we find that non-negligible host galaxy dust extinction, in addition to the Galactic one, affects the observed flux in the optical regime. The extinction-corrected optical-to-gamma-ray spectral energy distribution during the prompt emission shows a flux density ratio $F_{gamma}/F_{opt}$=0.01-0.0001 with spectral index $beta_{gamma,opt}> beta_{gamma}$, strongly suggesting a separate origin of the optical and gamma-ray components. This result is supported by the lack of correlated behavior in the prompt emission light curves observed in the two energy domains. The properties of the prompt optical emission observed during GRB 060111B favor interpretation of this optical light as radiation from the reverse shock in a thick shell limit and in the slow cooling regime. The expected peak flux is consistent with the observed one corrected for the host extinction, likely indicating that the starting time of the TAROT observations is very near to or coincident with the peak time. The estimated fireball initial Lorentz factor is >260-360 at z=1-2, similar to the Lorentz factors obtained from other GRBs. GRB 060111B is a rare, good test case of the reverse shock emission mechanism in both the X-ray and optical energy ranges.
Gamma-ray burst GRB 140430A was detected by the Swift satellite and observed promptly with the imaging polarimeter RINGO3 mounted on the Liverpool Telescope, with observations beginning while the prompt $gamma$-ray emission was still ongoing. In this paper, we present densely sampled (10-second temporal resolution) early optical light curves in 3 optical bands and limits to the degree of optical polarization. We compare optical, X-ray and gamma-ray properties and present an analysis of the optical emission during a period of high-energy flaring. The complex optical light curve cannot be explained merely with a combination of forward and reverse shock emission from a standard external shock, implying additional contribution of emission from internal shock dissipation. We estimate an upper limit for time averaged optical polarization during the prompt phase to be as low as P < 12% (1$sigma$). This suggests that the optical flares and early afterglow emission in this GRB are not highly polarized. Alternatively, time averaging could mask the presence of otherwise polarized components of distinct origin at different polarization position angles.
After more than 40 years from their discovery, the long-lasting tension between predictions and observations of GRBs prompt emission spectra starts to be solved. We found that the observed spectra can be produced by the synchrotron process, if the emitting particles do not completely cool. Evidence for incomplete cooling was recently found in Swift GRBs spectra with prompt observations down to 0.5 keV (Oganesyan et al. 2017, 2018), characterized by an additional low-energy break. In order to search for this break at higher energies, we analysed the 10 long and 10 short brightest GRBs detected by the Fermi satellite in over 10 years of activity. We found that in 8/10 long GRBs there is compelling evidence of a low energy break (below the peak energy) and the photon indices below and above that break are remarkably consistent with the values predicted by the synchrotron spectrum (-2/3 and -3/2, respectively). None of the ten short GRBs analysed shows a break, but the low energy spectral slope is consistent with -2/3. Within the framework of the GRB standard model, these results imply a very low magnetic field in the emission region, at odds with expectations. I also present the spectral evolution of GRB 190114C, the first GRB detected with high significance by the MAGIC Telescopes, which shows the compresence (in the keV-MeV energy range) of the prompt and of the afterglow emission, the latter rising and dominating the high energy part of the spectral energy range.
The extremely bright optical flash that accompanied GRB 080319B suggested, at first glance, that the prompt $gamma$-rays in this burst were produced by Synchrotron self Compton (SSC). We analyze here the observed optical and $gamma$ spectrum. We find that the very strong optical emission poses, due to self absorption, very strong constraints on the emission processes and put the origin of the optical emission at a very large radius, almost inconsistent with internal shock. Alternatively it requires a very large random Lorentz factor for the electrons. We find that SSC could not have produced the prompt $gamma$-rays. We also show that the optical emission and the $gamma$ rays could not have been produced by synchrotron emission from two populations of electron within the same emitting region. Thus we must conclude that the optical and the $gamma$-rays were produced in different physical regions. A possible interpretation of the observations is that the $gamma$-rays arose from internal shocks but the optical flash resulted from external shock emission. This would have been consistent with the few seconds delay observed between the optical and $gamma$-rays signals.
We report the detection of a faint optical flash by the 2-m Faulkes Telescope North simultaneously with the second of two prompt gamma-ray pulses in INTEGRAL gamma-ray burst (GRB) 080603A, beginning at t_rest = 37 s after the onset of the GRB. This optical flash appears to be distinct from the subsequent emerging afterglow emission, for which we present comprehensive broadband radio to X-ray light curves to 13 days post-burst and rigorously test the standard fireball model. The intrinsic extinction toward GRB 080603A is high (A_V,z = 0.8 mag), and the well-sampled X-ray-to-near-infrared spectral energy distribution is interesting in requiring an LMC2 extinction profile, in contrast to the majority of GRBs. Comparison of the gamma-ray and extinction-corrected optical flux densities of the flash rules out an inverse-Compton origin for the prompt gamma-rays; instead, we suggest that the optical flash could originate from the inhomogeneity of the relativistic flow. In this scenario, a large velocity irregularity in the flow produces the prompt gamma-rays, followed by a milder internal shock at a larger radius that would cause the optical flash. Flat gamma-ray spectra, roughly F propto nu^-0.1, are observed in many GRBs. If the flat spectrum extends down to the optical band in GRB 080603A, the optical flare could be explained as the low-energy tail of the gamma-ray emission. If this is indeed the case, it provides an important clue to understanding the nature of the emission process in the prompt phase of GRBs and highlights the importance of deep (R> 20 mag), rapid follow-up observations capable of detecting faint, prompt optical emission.
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