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Register a new userLow frequency view of GRB 190114C reveals time varying shock micro-physics
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David Alexander Kann
Publication date
2019
fields
Physics
and research's language is
English
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We present radio and optical afterglow observations of the TeV-bright long Gamma Ray Burst (GRB) 190114C at a redshift of $z=0.425$, which was detected by the MAGIC telescope. Our observations with ALMA, ATCA, and uGMRT were obtained by our low frequency observing campaign and range from $sim1$ to $sim140$ days after the burst and the optical observations were done with three optical telescopes spanning up to $sim25$ days after the burst. Long term radio/mm observations reveal the complex nature of the afterglow, which does not follow the spectral and temporal closure relations expected from the standard afterglow model. We find that the microphysical parameters of the external forward shock, representing the share of shock-created energy in the non-thermal electron population and magnetic field, are evolving with time. The inferred kinetic energy in the blast-wave depends strongly on the assumed ambient medium density profile, with a constant density medium demanding almost an order of magnitude higher energy than in the prompt emission, while a stellar wind-driven medium requires approximately the same amount energy as in prompt emission.
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We present ALMA 97.5 GHz total intensity and linear polarization observations of the mm-band afterglow of GRB 190114C spanning 2.2 to 5.2 hours after the burst. We detect linear polarization at the $approx 5,sigma$ level, decreasing from $Pi=(0.87pm0.13)%$ to $(0.60pm0.19)%$, and evolving in polarization position angle from $(10pm5)^circ$ to $(-44pm12)^circ$ during the course of the observations. This represents the first detection of polarized millimeter emission in a $gamma$-ray burst. We show that the optical and X-ray observations between $0.03$ days and $sim0.3$ days are consistent with a fast cooling forward shock expanding into a wind environment. However, the optical observations at $lesssim0.03$ days, as well as the radio and millimeter observations arise from a separate component, which we interpret as emission from the reverse-shocked ejecta. Using the measured linear polarization, we constrain the coherence scale of tangled magnetic fields in the ejecta to an angular size of $theta_{rm B} approx10^{-3}$ radian, while the rotation of the polarization angle rules out the presence of large scale, ordered axisymmetric magnetic fields, and in particular a large scale toroidal field, in the jet.
GRB 190114C is the first gamma-ray burst detected at Very High Energies (VHE, i.e. >300 GeV) by the MAGIC Cherenkov telescope. The analysis of the emission detected by the Fermi satellite at lower energies, in the 10 keV -- 100 GeV energy range, up to ~ 50 seconds (i.e. before the MAGIC detection) can hold valuable information. We analyze the spectral evolution of the emission of GRB 190114C as detected by the Fermi Gamma-Ray Burst Monitor (GBM) in the 10 keV -- 40 MeV energy range up to ~60 sec. The first 4 s of the burst feature a typical prompt emission spectrum, which can be fit by a smoothly broken power-law function with typical parameters. Starting on ~4 s post-trigger, we find an additional nonthermal component, which can be fit by a power law. This component rises and decays quickly. The 10 keV -- 40 MeV flux of the power-law component peaks at ~ 6 s; it reaches a value of 1.7e-5 erg cm-2 s-1. The time of the peak coincides with the emission peak detected by the Large Area Telescope (LAT) on board Fermi. The power-law spectral slope that we find in the GBM data is remarkably similar to that of the LAT spectrum, and the GBM+LAT spectral energy distribution seems to be consistent with a single component. This suggests that the LAT emission and the power-law component that we find in the GBM data belong to the same emission component, which we interpret as due to the afterglow of the burst. The onset time allows us to estimate the initial jet bulk Lorentz factor Gamma_0 is about 500, depending on the assumed circum-burst density.
The short gamma-ray burst (GRB) 170817A was the first GRB associated with a gravitational-wave event. Due to the exceptionally low luminosity of the prompt $gamma$-ray and the afterglow emission, the origin of both radiation components is highly debated. The most discussed models for the burst and the afterglow include a regular GRB jet seen off-axis and the emission from the cocoon encompassing a choked jet. Here, we report low radio-frequency observations at 610 and 1390~MHz obtained with the Giant Metrewave Radio Telescope (GMRT). Our observations span a range of $sim7$ to $sim152$ days after the burst. The afterglow started to emerge at these low frequencies about 60~days after the burst. The $1390$~MHz light curve barely evolved between 60 and 150 days, but its evolution is also marginally consistent with a $F_ upropto t^{0.8}$ rise seen in higher frequencies. We model the radio data and archival X-ray, optical and high-frequency radio data with models of top-hat and Gaussian structured GRB jets. We performed a Markov Chain Monte Carlo analysis of the structured-jet parameter space. Though highly degenerate, useful bounds on the posterior probability distributions can be obtained. Our bounds of the viewing angle are consistent with that inferred from the gravitational wave signal. We estimate the energy budget in prompt emission to be an order of magnitude lower than that in the afterglow blast-wave.
We use high--quality, multi-band observations of Swift GRB120404A, from gamma-ray to radio frequencies, together with the new hydrodynamics code of van Eerten et al. (2012) to test the standard synchrotron shock model. The evolution of the radio and optical afterglow, with its prominent optical rebrightening at t_rest 260-2600 s, is remarkably well modelled by a decelerating jet viewed close to the jet edge, combined with some early re-energization of the shock. We thus constrain the geometry of the jet with half-opening and viewing angles of 23 and 21 deg respectively and suggest that wide jets viewed off-axis are more common in GRBs than previously thought. We also derive the fireball microphysics parameters epsilon_B=2.4e-4 and epsilon_e=9.3e-2 and a circumburst density of n=240 cm^-3. The ability to self-consistently model the microphysics parameters and jet geometry in this way offers an alternative to trying to identify elusive canonical jet breaks at late times. The mismatch between the observed and model-predicted X-ray fluxes is explained by the local rather than the global cooling approximation in the synchrotron radiation model, constraining the microphysics of particle acceleration taking place in a relativistic shock and, in turn, emphasising the need for a more realistic treatment of cooling in future developments of theoretical models. Finally, our interpretation of the optical peak as due to the passage of the forward shock synchrotron frequency highlights the importance of high quality multi-band data to prevent some optical peaks from being erroneously attributed to the onset of fireball deceleration.
Gamma-ray bursts (GRBs) of the long-duration class are the most luminous sources of electromagnetic radiation known in the Universe. They are generated by outflows of plasma ejected at near the speed of light by newly formed neutron stars or black holes of stellar mass at cosmological distances. Prompt flashes of MeV gamma rays are followed by longer-lasting afterglow emission from radio waves to GeV gamma rays, due to synchrotron radiation by energetic electrons in accompanying shock waves. Although emission of gamma rays at even higher, TeV energies by other radiation mechanisms had been theoretically predicted, it had never been detected previously. Here we report the clear detection of GRB 190114C in the TeV band, achieved after many years of dedicated searches for TeV emission from GRBs. Gamma rays in the energy range 0.2--1 TeV are observed from about 1 minute after the burst (at more than 50 standard deviations in the first 20 minutes). This unambiguously reveals a new emission component in the afterglow of a GRB, whose power is comparable to that of the synchrotron component. The observed similarity in the radiated power and temporal behaviour of the TeV and X-ray bands points to processes such as inverse Compton radiation as the mechanism of the TeV emission, while processes such as synchrotron emission by ultrahigh-energy protons are disfavoured due to their low radiative efficiency.
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