An Ordinary Short Gamma-Ray Burst with Extraordinary Implications: Fermi-GBM Detection of GRB 170817A (1710.05446v1)

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.

• The paper demonstrates that GRB 170817A's detection confirms a binary neutron star merger as the source of both gravitational waves and a gamma-ray burst.
• It details Fermi-GBM's robust analysis methodology, including time-resolved spectral and temporal analysis with a T90 duration of 2.0 ± 0.5 seconds.
• The study highlights how the observed alignment of GRB localization with LIGO/Virgo and optical counterparts paves the way for advancements in multi-messenger astronomy.

Overview of the Fermi-GBM Detection of GRB 170817A

The paper "An Ordinary Short Gamma-Ray Burst with Extraordinary Implications: Fermi-GBM Detection of GRB 170817A" details the observations and analyses of the short gamma-ray burst (GRB) GRB 170817A by the Fermi Gamma-ray Burst Monitor (GBM). This event is pivotal as it marks the first confirmed coincident detection of both gravitational waves (GW) and electromagnetic (EM) radiation from a single astrophysical source, a binary neutron star merger, showcasing the commencement of gravitational-wave multi-messenger astronomy.

Highlights and Results

1. Detection and Significance: The GRB 170817A was detected by Fermi-GBM approximately 1.7 seconds after the Laser Interferometer Gravitational-Wave Observatory (LIGO) flagged a binary compact merger candidate. This not only signifies the first unambiguous joint observation of GW and EM radiation but also confirms that some short GRBs are associated with binary neutron star mergers.
2. Instrumentation and Detection Mechanisms: The Fermi-GBM utilizes 14 detectors across a broad energy spectrum (~8 keV to 40 MeV) to detect and analyze GRBs. The paper provides a comprehensive description of GBM's capabilities, highlighting its effective sky coverage and ability to detect EM counterparts to GW signals, proving essential for multi-messenger astrophysics.
3. Localization and Observations: Initial localization of the GRB by GBM showed alignment with the GW localization provided by LIGO and Virgo. The paper details subsequent optical transient detections consistent with the GRB's location and distance, strengthening the association between this GRB and a compact binary merger.
4. Spectral and Temporal Analysis: The GBM detected GRB 170817A, although ordinary in initial detection characteristics, showed intriguing features upon detailed spectral and temporal analysis. The duration, spectrum, and peak flux are analyzed to situate the GRB within the short-hard GRB population, yet its extended soft emission is particularly noteworthy.
5. Numerical and Methodological Results: The paper reports a time-resolved analysis with a recorded $T_{90}$ duration of 2.0 ± 0.5 s, placing this GRB among the shorter events. The influence of pulse shape and variability, as well as spectral lag measurements, provide deeper insights into the emission mechanisms and source characteristics.

Implications and Future Directions

The observations of GRB 170817A emphasize the capability of the Fermi-GBM, combined with GW detections, to further elucidate the progenitors of GRBs. The confirmed connection between short GRBs and binary neutron star mergers opens new avenues for understanding the physics behind these phenomena and for studying the lifecycle of such systems.

In terms of theoretical implications, joint detections contribute valuable constraints to the models of GRB emissions and their progenitor environments. Practically, this observation marks a significant step toward refining localization techniques in real-time and improving the efficiency and accuracy of multi-messenger alerts, which are paramount for directing further follow-up observations across the EM spectrum.

Looking forward, continuous advancements in instrumentation and analysis methods will likely enhance the sensitivity and response time of multi-messenger detections. Such improvements will lead to an increased number of joint detections, offering broader statistical insights into GRB populations and their physical origins. The synergy of GW and EM observations is poised to revolutionize our understanding of violent cosmic events, unlocking further mysteries of the universe.