An Ordinary Short Gamma-Ray Burst with Extraordinary Implications: Fermi-GBM Detection of GRB 170817A

TL;DR

The paper analyzes the Fermi GBM detection of GRB 170817A, the first gamma-ray burst coincident with a gravitational-wave event from a binary neutron star merger GW170817, establishing multimessenger astronomy. It details GBM instrument capabilities, trigger/localization procedures, and standard analyses (duration, spectrum, and catalog comparisons), then extends to a deep dive into spectral components, lag, variability, and limits on other gamma-ray emission. The main emission is well described by a Comptonized spectrum with $E_{peak}$ around a few hundred keV, while a later thermal-like tail near $kT\approx10$ keV suggests cocoon photospheric emission; overall the burst is short, softer than typical short GRBs, and consistent with a compact binary merger progenitor. The study also quantifies GBM’s detectability and situates GRB 170817A within the short-GRB population, highlighting implications for jet structure, emission mechanisms, and multimessenger astronomy.

Abstract

On August 17, 2017 at 12:41:06 UTC the Fermi Gamma-ray Burst Monitor (GBM) detected and triggered on the short gamma-ray burst GRB 170817A. Approximately 1.7 s prior to this GRB, the Laser Interferometer Gravitational-Wave Observatory (LIGO) triggered on a binary compact merger candidate associated with the GRB. This is the first unambiguous coincident observation of gravitational waves and electromagnetic radiation from a single astrophysical source and marks the start of gravitational-wave multi-messenger astronomy. We report the GBM observations and analysis of this ordinary short GRB, which extraordinarily confirms that at least some short GRBs are produced by binary compact mergers.

Figures (10)

• Figure 1: The 256 ms binned lightcurve of GRB 170817A in the 50--300 keV band for NaIs 1, 2, and 5. The red band is the un-binned Poisson maximum likelihood estimate of the background.
• Figure 2: The position of Fermi at trigger time (red dot) and its orbital path moving from West to East. The maximum latitudinal extent of Fermi's orbit is shown by the dashed orange lines and the hatched region is the polygon that defines the South Atlantic Anomaly region for GBM, inside of which the GBM detectors are turned off.
• Figure 3: The GBM and HLV initial and final localizations. The original GBM human-in-the-loop localization (50% and 90% regions) is shown with purple dashed contours, and the original BAYESTAR skymap (90% region) is shown with green dashed contour. The targeted search localization and the LALInference HLV skymap are the corresponding solid contours. The inset shows a close-up of the GBM localization and the position of the the optical transient candidate (black star). The Earth as seen from Fermi is shown in blue, the $3\sigma$ IPN annulus is shown as the gray band, and the directions of the three closest NaI detectors are shown in light brown.
• Figure 4: Distributions comparing measured and spectral fit parameters of GRB 170817A to those of short (black) and long (grey) GBM GRBs. The value obtained for GRB 170817A is indicated by the orange line, and the $1\sigma$ uncertainty is both shaded and delineated in green. Both the fluences (top left) and 64 ms peak fluxes (top right) were calculated in the 10--1000 keV range. The time-integrated spectral parameters (middle left and right) and the peak spectral parameters (bottom left and right) were computed using the Comptonized spectrum.
• Figure 5: [Left] The GBM $\rm T_{90}$ distribution fit with two log normal distributions. The $1\sigma$ confidence interval for GRB 170817A is shaded below the summed curve. The red region is the probability that the event belongs to the short class, while the light blue is the probability that it belongs to the long class. [Right] The duration ($\rm T_{90}$) vs the hardness ratio, an analog for the spectral hardness of the burst. Assuming exactly two distinct populations the data are fit with two-dimensional log-normal distributions. Red dots are those most likely to belong to the long class, and blue dots to the short class. The black cross is the centroid and $1\sigma$ uncertainty for GRB 170817A.