Time evolution of the spectral break in the high-energy extra component of GRB 090926A

Abstract

We report on a new time-resolved analysis of the GRB 090926A broad-band spectrum during its prompt phase, and on its interpretation in the framework of prompt emission models. We characterize the high-energy extra component of GRB 090926A using Pass 8 data from the Fermi Large Area Telescope (LAT), which offer a greater sensitivity than any data set used in previous studies of this burst, particularly in the 30-100 MeV energy band. Then, we combine the LAT data with the Fermi Gamma-ray Burst Monitor (GBM) in joint spectral fits in order to characterize the time evolution of the broad-band spectrum from keV to GeV energies. Careful attention is paid to the LAT systematic effects. Finally, we perform a temporal analysis of the light curves and we compute the variability time scales during and after the bright spike at ~10 s post-trigger. Our analysis confirms and better constrains the spectral break in the high-energy extra component, which has been previously reported during the bright spike. Furthermore, it reveals that the spectral attenuation persists at later times, with an increase of the break characteristic energy up to the GeV domain until the end of the prompt phase. We interpret the high-energy spectral break as caused by photon opacity to pair creation. Requiring that all emissions are produced above the photosphere of GRB 090926A, we compute the bulk Lorentz factor of the outflow, $\Gamma$. The latter decreases from 230 during the spike to 100 at the end of the prompt emission. Assuming, instead, that the spectral break reflects the natural curvature of the inverse Compton spectrum, lower limits corresponding to larger values of $\Gamma$ are also derived. Combined with the extreme temporal variability of GRB 090926A, these Lorentz factors lead to emission radii $R\sim 10^{14}$ cm which are consistent with an internal origin of both the keV-MeV and GeV prompt emissions.

17 pages, 12 figures, accepted for publication in Astronomy and Astrophysics

References in corpus (5)

• The Internal-Collision-Induced Magnetic Reconnection and Turbulence (ICMART) Model of Gamma-Ray Bursts
• Multiwavelength observations of GRB 110731A: GeV emission from onset to afterglow
• Spectral evolution in gamma-ray bursts: predictions of the internal shock model and comparison to observations
• A leptonic-hadronic model for the afterglow of gamma-ray burst 090510
• GeV Emission from Prompt and Afterglow Phases of Gamma-Ray Bursts

Cited by in corpus (9)

• GRB 190114C: from prompt to afterglow?
• Closure relations of Gamma Ray Bursts in high energy emission
• Insights into the physics of GRBs from the high-energy extension of their prompt emission spectra
• A new fitting function for GRB MeV spectra based on the internal shock synchrotron model
• Multiple Components in the Broadband $\gamma$-ray Emission of the Short GRB 160709A
• Gamma-Ray Burst prompt emission from the synchrotron radiation of relativistic electrons in a rapidly decaying magnetic field
• Searching for an additional high-energy component in Fermi-LAT GRB afterglows
• Jitter radiation: towards TeV-photons of gamma-ray bursts
• Evidence for Rapid Variability at High Energies in GRBs