Detection of Low-energy Breaks in Gamma-Ray Burst Prompt Emission Spectra

Abstract

The radiative process responsible for gamma-Ray Burst (GRB) prompt emission has not been identified yet. If dominated by fast-cooling synchrotron radiation, the part of the spectrum immediately below the $\nu F_{\nu }$ peak energy should display a power-law behavior with slope ${\alpha }_2=-3/2$, which breaks to a higher value ${\alpha }_1=-2/3$ (i.e. to a harder spectral shape) at lower energies. Prompt emission spectral data (usually available down to $\sim 10-20$keV) are consistent with one single power-law behavior below the peak, with typical slope $⟨\alpha ⟩=-1$, higher than (and then inconsistent with) the expected value ${\alpha }_2=-3/2$. To better characterize the spectral shape at low energy, we analyzed 14 GRBs for which the Swift X-ray Telescope started observations during the prompt. When available, Fermi-GBM observations have been included in the analysis. For 67% of the spectra, models that usually give a satisfactory description of the prompt (e.g., the Band model) fail in reproducing the $0.5-1000$keV spectra: low-energy data outline the presence of a spectral break around a few keV.We then introduce an empirical fitting function that includes a low-energy power law ${\alpha }_1$, a break energy $E_{\mathrm{break}}$, a second power law ${\alpha }_2$, and a peak energy $E_{\mathrm{peak}}$. We find $⟨{\alpha }_1⟩=-0.66$ ($\sigma =0.35$), $⟨\log (E_{\mathrm{break}}/\mathrm{keV})⟩=0.63$ ($\sigma =0.20$), $⟨{\alpha }_2⟩=-1.46$ ($\sigma =0.31$), and $⟨\log (E_{\mathrm{peak}}/\mathrm{keV})⟩=2.1$ ($\sigma =0.56$).The values $⟨{\alpha }_1⟩$ and $⟨{\alpha }_2⟩$ are very close to expectations from synchrotron radiation. In this context, $E_{\mathrm{break}}$ corresponds to the cooling break frequency.

Published in ApJ

References in corpus (14)

• The Physics of Gamma-Ray Bursts and Relativistic Jets
• Detection of a Thermal Spectral Component in the Prompt Emission of GRB 100724B
• Klein-Nishina Effects on Optically Thin Synchrotron and Synchrotron Self-Compton Spectrum
• Quasi-blackbody component and radiative efficiency of the prompt emission of gamma-ray bursts
• X-ray Hardness Evolution in GRB Afterglows and Flares: Late Time GRB Activity Without N_H Variations
• A general scheme for modeling gamma-ray burst prompt emission
• Towards a Better Understanding of the GRB Phenomenon: a New Model for GRB Prompt Emission and its effects on the New Non-Thermal L${}_{\mathrm{i}}^{\mathrm{NT}}$-E${}_{\mathrm{peak},\mathrm{i}}^{\mathrm{rest},\mathrm{NT}}$ relation
• Slow Heating Model of Gamma-Ray Burst: Photon Spectrum and Delayed Emission
• The width of gamma-ray burst spectra
• Synchrotron emission in the fast cooling regime: which spectra can be explained?
• Photosphere emission in the X-Ray Flares of Swift Gamma-Ray Bursts and Implications for the Fireball Properties
• GRB 060607A: A GRB with Bright Asynchronous Early $X$-ray and Optical Emissions
• Swift captures the spectrally evolving prompt emission of GRB 070616
• Swift multi-wavelength observations of the bright flaring burst GRB051117A

Cited by in corpus (43)

• Fermi-GBM Discovery of GRB 221009A: An Extraordinarily Bright GRB from Onset to Afterglow
• The case for a minute-long merger-driven gamma-ray burst from fast-cooling synchrotron emission
• Evidence of two spectral breaks in the prompt emission of gamma ray bursts
• Gamma-ray bursts afterglow physics and the VHE domain
• GRB 190114C: from prompt to afterglow?
• Synchrotron Radiation Dominates the Extremely Bright GRB 221009A
• Neutrino signal dependence on gamma-ray burst emission mechanism
• Synchrotron radiation in $\gamma$-ray burst prompt emission
• Pre-merger alert to detect the very-high-energy prompt emission from binary neutron-star mergers: Einstein Telescope and Cherenkov Telescope Array synergy
• Pitch-Angle Anisotropy Imprinted by Relativistic Magnetic Reconnection
• Temporal Evolution of Prompt GRB Polarization
• On the $\alpha$-Intensity Correlation in Gamma-Ray Bursts: Subphotospheric Heating with Varying Entropy
• MAGIC detection of GRB 201216C at $z=1.1$
• Synchrotron-Self-Compton radiation from magnetically-dominated turbulent plasmas in relativistic jets
• Gamma Ray Burst studies with THESEUS
• The slope of the low energy spectrum of Gamma-Ray Burst prompt emission
• Electromagnetic Counterparts of Compact Binary Mergers
• The power of the rings: the GRB 221009A soft X-ray emission from its dust-scattering halo
• A marginally fast-cooling proton-synchrotron model for prompt GRBs
• GRB Spectrum from Gradual Dissipation in a Magnetized Outflow
• Dissecting the Energy Budget of a Gamma-Ray Burst Fireball
• Evolution Patterns of the Peak Energy in the GRB Prompt Emission
• Spectral index-flux relation for investigating the origins of steep decay in $\gamma$-ray bursts
• Multi-wavelength radiation models for low-luminosity GRBs, and the implications for UHECRs
• Constrains on the physics of the prompt emission from a distant and energetic gamma-ray burst GRB 220101A
• A new fitting function for GRB MeV spectra based on the internal shock synchrotron model
• One Fits All: A Unified Synchrotron Model Explains GRBs with FRED-Shaped Pulses
• Insights into the physics of GRBs from the high-energy extension of their prompt emission spectra
• Inverse Compton Scattering Spectra of Gamma-Ray Burst Prompt Emission
• Onset of particle acceleration during the prompt phase in gamma-ray bursts as revealed by synchrotron emission in GRB160821A
• Prompt Gamma-Ray Burst Emission from Internal Shocks -- New Insights
• Study of prompt emission of short gamma ray bursts using multi-color blackbody: a clue to the viewing angle
• A detailed time-resolved and energy-resolved spectro-polarimetric study of bright GRBs detected by AstroSat CZTI in its first year of operation
• Gamma-ray Burst Spectrum with a Time-dependent Injection Rate of High-energy Electrons
• Multiple Components in the Broadband $\gamma$-ray Emission of the Short GRB 160709A
• The Low-Energy Spectral Index of Gamma-Ray Burst Prompt Emission from Internal Shocks
• A comprehensive view of blackbody components in the X-ray spectra of GRBs
• Gamma-Ray Burst prompt emission from the synchrotron radiation of relativistic electrons in a rapidly decaying magnetic field
• Prospects for detection rate of very-high-energy γ-ray emissions from short γ-ray bursts with the HADAR experiment
• A Comprehensive Consistency Check between Synchrotron radiation and the Observed Gamma-ray Burst Spectra
• Hadronic supercriticality in spherically expanding sources: application to GRB prompt emission
• Numerical simulations of internal shocks in spherical geometry: hydrodynamics and prompt emission
• Broad-band Spectral Modeling of Prompt Emission from Gamma-Ray Bursts Observed by the Transiting Exoplanet Survey Satellite