Fermi and Swift Observations of GRB 190114C: Tracing the Evolution of High-energy Emission from Prompt to Afterglow

Abstract

We report on the observations of gamma-ray burst (GRB) 190114C by the Fermi Gamma-ray Space Telescope and the Neil Gehrels Swift Observatory. The prompt gamma-ray emission was detected by the Fermi GRB Monitor (GBM), the Fermi Large Area Telescope (LAT), and the Swift Burst Alert Telescope (BAT) and the long-lived afterglow emission was subsequently observed by the GBM, LAT, Swift X-ray Telescope (XRT), and Swift UV Optical Telescope. The early-time observations reveal multiple emission components that evolve independently, with a delayed power-law component that exhibits significant spectral attenuation above 40 MeV in the first few seconds of the burst. This power-law component transitions to a harder spectrum that is consistent with the afterglow emission observed by the XRT at later times. This afterglow component is clearly identifiable in the GBM and BAT light curves as a slowly fading emission component on which the rest of the prompt emission is superimposed. As a result, we are able to observe the transition from internal-shock- to external-shock-dominated emission. We find that the temporal and spectral evolution of the broadband afterglow emission can be well modeled as synchrotron emission from a forward shock propagating into a wind-like circumstellar environment. We estimate the initial bulk Lorentz factor using the observed high-energy spectral cutoff. Considering the onset of the afterglow component, we constrain the deceleration radius at which this forward shock begins to radiate in order to estimate the maximum synchrotron energy as a function of time. We find that even in the LAT energy range, there exist high-energy photons that are in tension with the theoretical maximum energy that can be achieved through synchrotron emission from a shock. These violations of the maximum synchrotron energy are further compounded by the detection of very high-energy (VHE) emission above 300 GeV by MAGIC concurrent with our observations. We conclude that the observations of VHE photons from GRB 190114C necessitates either an additional emission mechanism at very high energies that is hidden in the synchrotron component in the LAT energy range, an acceleration mechanism that imparts energy to the particles at a rate that is faster than the electron synchrotron energy-loss rate, or revisions of the fundamental assumptions used in estimating the maximum photon energy attainable through the synchrotron process.

East Asia VLBI Network observations of the TeV Gamma-Ray Burst 190114C

“…Other than being the first GRB ever to be detected by ground-based Cherenkov telescopes, GRB 190114C is a notable target of particular attraction for a number of other reasons. First, GRB 190114C shows evidence of a very early afterglow onset [14], allowing for an estimate of its initial bulk Lorentz factor (see also [15]). Second, GRB 190114C was observed and detected across 16 orders of magnitude in frequency, from γ-rays to radio, within several hours after the burst (see above paragraph), allowing for a study of the multi-band evolution of the afterglow starting early after the trigger (e.g., for a notable example of how important broadband afterglow modelling can be for the interpretation of this kind of events [16]).…”

Observation of inverse Compton emission from a long γ-ray burst

“…1), with total duration ∼ 25 s (see dashed line in Fig. 1) and total radiated energy E γ,iso = (2.5 ± 0.1) ×10 53 ergs (isotropic equivalent, in the energy range 1−10 4 keV 13 ). During the time of inter-burst quiescence at t ∼ [5 − 15] seconds and after the end of the last prompt pulse at t 25 s, the flux decays smoothly, following a power law in time F ∝ t α , with α 10−1000keV = −1.10 ± 0.01 13 .…”

“…The temporal and spectral characteristics of this smoothly varying component support an interpretation in terms of afterglow synchrotron radiation, making this one of the few clear cases of afterglow emission detected in the band 10 − 4 keV during the prompt emission phase. The onset of the afterglow component is then estimated to occur around t ∼ − 10 s 13,14 , implying an initial bulk Lorentz factor between 300 and 700 (Methods).…”

“…Nominally, the T 90 , i.e. the time interval during which a fraction between 5% and 95% of the total emission is observed, is much longer (> 100 s, depending on the instrument13 ), but is clearly contaminated by the afterglow component (Fig. 1) and does not provide a good measure of the actual duration of the prompt emission.…”

“…1) and does not provide a good measure of the actual duration of the prompt emission. A more detailed study of the prompt emission phase is reported in13 .AGILE (TheAstrorivelatore Gamma ad Immagini LEggero 38 ) could observe GRB 190114C until T 0 +330 s, before it became occulted by the Earth. GRB 190114C triggered the Mini-CALorimeter (MCAL) from T0−0.95 s to T0+10.95 s. The MCAL light flux curve in Fig.…”