The rise and fall of the high-energy afterglow emission of GRB 180720B

Abstract

The Gamma Ray Burst (GRB) 180720B is one of the brightest events detected by the Fermi satellite and the first GRB detected by the H.E.S.S. telescope above 100 GeV. We analyse the Fermi (GBM and LAT) and Swift (XRT and BAT) data and describe the evolution of the burst spectral energy distribution in the 0.5 keV - 10 GeV energy range over the first 500 seconds of emission. We reveal a smooth transition from the prompt phase, dominated by synchrotron emission in a moderately fast cooling regime, to the afterglow phase whose emission has been observed from the radio to the GeV energy range. The LAT (0.1 - 100 GeV) light curve initially rises ($F_{\mathrm{LAT}}\propto t^{2.4}$), peaks at $\sim$78 s, and falls steeply ($F_{\mathrm{LAT}}\propto t^{-2.2}$) afterwards. The peak, which we interpret as the onset of the fireball deceleration, allows us to estimate the bulk Lorentz factor ${\Gamma }_0\sim 150(300)$ under the assumption of a wind-like (homogeneous) circum-burst medium density. We derive a flux upper limit in the LAT energy range at the time of H.E.S.S. detection, but this does not allow us to unveil the nature of the high energy component observed by H.E.S.S. We fit the prompt spectrum with a physical model of synchrotron emission from a non-thermal population of electrons. The 0 - 35 s spectrum after its $EF(E)$ peak (at 1 - 2 MeV) is a steep power law extending to hundreds of MeV. We derive a steep slope of the injected electron energy distribution $N(\gamma )\propto {\gamma }^{-5}$. Our fit parameters point towards a very low magnetic field ($B^{\prime }\sim 1$ G) in the emission region.

arXiv comment10 pages, 6 figures, submitted to A&A

References in corpus (57)

• The Large Area Telescope on the Fermi Gamma-ray Space Telescope Mission
• The Physics of Gamma-Ray Bursts
• The Fermi Gamma-Ray Burst Monitor
• Unsteady Outflow Models for Cosmological Gamma-Ray Bursts
• Physical processes shaping GRB X-ray afterglow lightcurves: theoretical implications from the Swift XRT observations
• The Physics of Gamma-Ray Bursts and Relativistic Jets
• Relativistic Reconnection: an Efficient Source of Non-Thermal Particles
• Can internal shocks produce the variability in GRBs?
• Gamma-ray bursts from internal shocks in a relativistic wind: temporal and spectral properties
• Efficient acceleration and radiation in Poynting flux powered GRB outflows
• Particle Acceleration in Relativistic Magnetized Collisionless Electron-Ion Shocks
• The Complete Spectral Catalog of Bright BATSE Gamma-Ray Bursts
• The Fermi GBM Gamma-Ray Burst Spectral Catalog: Four Years Of Data
• The Synchrotron Shock Model Confronts a `Line of Death' in the BATSE Gamma-Ray Burst Data
• Relativistic Jets Shine through Shocks or Magnetic Reconnection?
• GRB after-glow: Supporting the cosmological fireball model, constraining parameters, and making predictions
• Large scale magnetic fields and their dissipation in GRB fireballs
• The First Fermi LAT Gamma-Ray Burst Catalog
• A very-high-energy component deep in the Gamma-ray Burst afterglow
• A decade of Gamma-Ray Bursts observed by $Fermi$-LAT: The 2${}^{nd}$ GRB catalog
• Panchromatic study of GRB 060124: from precursor to afterglow
• On the generation of high energy photons detected by the Fermi Satellite from gamma-ray bursts
• Spectral Properties of Blast Wave Models of Gamma-Ray Burst Sources
• External forward shock origin of high energy emission for three GRBs detected by Fermi
• Relativistic Shocks: Particle Acceleration and Magnetization
• GeV emission from Gamma Ray Bursts: a radiative fireball?
• Reconciling observed GRB prompt spectra with synchrotron radiation ?
• Short versus Long Gamma-Ray Bursts: spectra, energetics, and luminosities
• Constraints on the emission mechanisms of gamma-ray bursts
• Spectral properties of 438 GRBs detected by Fermi/GBM
• Cosmological GRBs: Internal vs. External Shocks
• Energies of GRB blast waves and prompt efficiencies as implied by modeling of X-ray and GeV afterglows
• Bulk Lorentz factors of Gamma-Ray Bursts
• Afterglow emission in Gamma-Ray Bursts: I. Pair-enriched ambient medium and radiative blast waves
• Gamma-ray bursts as cool synchrotron sources
• Synchrotron Origin of the Typical GRB Band Function - A Case Study of GRB 130606B
• The onset of the GeV afterglow of GRB 090510
• A general scheme for modeling gamma-ray burst prompt emission
• Relativistic Magnetic Reconnection in Electron-Positron-Proton Plasmas: Implications for Jets of Active Galactic Nuclei
• Detection of Low-energy Breaks in Gamma-Ray Burst Prompt Emission Spectra
• Evidence of two spectral breaks in the prompt emission of gamma ray bursts
• Consistency with synchrotron emission in the bright GRB 160625B observed by Fermi
• Constraints on the Synchrotron Emission Mechanism in GRBs
• Synchrotron self-Compton emission from external shocks as the origin of the sub-TeV emission in GRB 180720B and GRB 190114C
• Prompt optical emission as a signature of synchrotron radiation in gamma-ray bursts
• Characterization of Gamma-Ray Burst prompt emission spectra down to soft X-rays
• Diverse Temporal Properties of GRB Afterglow
• Synchrotron self-Compton as a likely mechanism of photons beyond the synchrotron limit in GRB 190114C
• The physical conditions of the afterglow implied by MAGIC's sub-TeV observations of GRB 190114C
• Panchromatic observations of the textbook GRB 110205A: constraining physical mechanisms of prompt emission and afterglow
• Proton-synchrotron as the radiation mechanism of the prompt emission of GRBs?
• High-energy emission from Gamma-Ray Bursts
• GRB 190114C: from prompt to afterglow?
• The Early Afterglows
• The 7.2/3 Years Collection of Well-Monitored Fermi-LAT GRB Afterglows
• Swift XRT and VLT Observations of the Afterglow of GRB 041223
• Gamma-Ray Bursts and Related Phenomena

Cited by in corpus (17)

• The case for a minute-long merger-driven gamma-ray burst from fast-cooling synchrotron emission
• Proton-synchrotron as the radiation mechanism of the prompt emission of GRBs?
• Modelling synchrotron and synchrotron self-Compton emission of gamma-ray burst afterglows from radio to very-high energies
• A blast from the infant Universe: the very high-z GRB 210905A
• Synchrotron Self-Compton Emission in the Two-Component Jet Model for Gamma-Ray Bursts
• Gamma Ray Burst studies with THESEUS
• The slope of the low energy spectrum of Gamma-Ray Burst prompt emission
• GRB-SN Association within the Binary-Driven Hypernova Model
• Evolution Patterns of the Peak Energy in the GRB Prompt Emission
• Fires in the deep: The luminosity distribution of early-time gamma-ray-burst afterglows in light of the Gamow Explorer sensitivity requirements
• A new fitting function for GRB MeV spectra based on the internal shock synchrotron model
• Insights into the physics of GRBs from the high-energy extension of their prompt emission spectra
• On the X-ray, optical and radio afterglows of the BdHN I GRB 180720B generated by the synchrotron emission
• Extended Emission of Cosmic Gamma-Ray Bursts Detected in the SPI-ACS/INTEGRAL Experiment
• Very high-energy gamma rays from GRB 180720B and GRB 190829A with external Compton emission
• Modelling multiwavelength afterglows of the VHE-GRB population
• Jets in a Gamma-Ray Burst During its Prompt Emission: Evolution of Lorentz Factor