Synchrotron cooling in energetic gamma-ray bursts observed by the Fermi Gamma-Ray Burst Monitor ^⋆

¹ Max-Planck-Institut für extraterrestrische Physik, Giessenbachstraße 1, 85748 Garching, Germany
e-mail: sptfung@mpe.mpg.de
² Excellence Cluster Universe, Technische Universität München, Boltzmannstraße 2, 85748 Garching, Germany
³ The Oskar Klein Centre for Cosmoparticle Physics, AlbaNova, 106 91 Stockholm, Sweden
⁴ Department of Physics, KTH Royal Institute of Technology, AlbaNova, 106 91 Stockholm, Sweden
⁵ Center for Space Plasma and Aeronomic Research (CSPAR), University of Alabama in Huntsville, 320 Sparkman Drive, Huntsville, AL 35805, USA
⁶ Astrophysics Office, ZP12, NASA/Marshall Space Flight Center, Huntsville, AL 35812, USA
⁷ Planetarium Südtirol, Gummer 5, 39053 Karneid, Italy
⁸ Universities Space Research Association, 320 Sparkman Drive, Huntsville, AL 35805, USA
⁹ Department of Space Science, University of Alabama in Huntsville, 320 Sparkman Drive, Huntsville, AL 35899, USA
¹⁰ School of Physics, University College Dublin, Belfield, 4 Dublin, Ireland

Received: 25 August 2014
Accepted: 27 October 2014

Abstract

Context. We study the time-resolved spectral properties of energetic gamma-ray bursts (GRBs) with good high-energy photon statistics observed by the Gamma-Ray Burst Monitor (GBM) onboard the Fermi Gamma-Ray Space Telescope.

Aims. We aim to constrain in detail the spectral properties of GRB prompt emission on a time-resolved basis and to discuss the theoretical implications of the fitting results in the context of various prompt emission models.

Methods. Our sample comprises eight GRBs observed by the Fermi GBM in its first five years of mission, with 1 keV–1 MeV fluence f> 1.0 × 10^-4 erg cm^-2 and a signal-to-noise ratio level of S/N ≥ 10.0 above 900 keV. We performed a time-resolved spectral analysis using a variable temporal binning technique according to optimal S/N criteria, resulting in a total of 299 time-resolved spectra. We performed Band function fits to all spectra and obtained the distributions for the low-energy power-law index α, the high-energy power-law index β, the peak energy in the observed νF_ν spectrum E_p, and the difference between the low- and high-energy power-law indices Δs = α − β. We also applied a physically motivated synchrotron model, which is a triple power-law with constrained power-law indices and a blackbody component, to test the prompt emission for consistency with a synchrotron origin and obtain the distributions for the two break energies E_b,1 and E_b,2, the middle segment power-law index β, and the Planck function temperature kT.

Results. The Band function parameter distributions are α = -0.73^+0.16_-0.21, β =ي-2.13^+0.28_-0.56, Ep = 374.4^+307.3_-187.7 , , keV (log₁₀Ep = 2.57^+0.26_-0.30), and Δs = 1.38^+0.54_-0.31 , with average errors σ_α ~ 0.1, σ_β ~ 0.2, and σ_Ep ~ 0.1E_p. Using the distributions of Δs and β, the electron population index p is found to be consistent with the “moderately fast” scenario, in which fast- and slow-cooling scenarios cannot be distinguished. The physically motivated synchrotron-fitting function parameter distributions are E_b,1 = 129.6^+132.2_-32.4 keV, E_b,2 = 631.4^+582.6_-309.6 keV, β = -1.72^+0.48_-0.25 , and kT = 10.4^+4.9_-3.7 keV, with average errors σ_β ~ 0.2, σ_Eb,1 ~ 0.1E_b,1, σ_Eb,2 ~ 0.4E_b,2, and σ_kT ~ 0.1kT. This synchrotron function requires the synchrotron injection and cooling break (i.e., E_min and E_cool) to be close to each other within a factor of ten, often in addition to a Planck function.

Conclusions. A synchrotron model is found that is consistent with most of the time-resolved spectra for eight energetic Fermi GBM bursts with good high-energy photon statistics as long as both the cooling and injection break are included and the leftmost spectral slope is lifted either by including a thermal component or when an evolving magnetic field is accounted for.