GRB980923. A burst with a short duration high energy component
The prompt emission of Gamma Ray Bursts (GRBs) is usually well described by the Band function: two power-laws joined smoothly at a given break energy. In addition to the Band component, a few bursts (GRB941017, GRB090510, GRB090902B and GRB090926A) show clear evidence for a distinct high-energy spectral component, which in some cases evolves independently from the prompt keV component and is well described by a power-law (PL), sometimes with a cut-off energy; this component is found to have long duration, even longer than the burst itself for all the four bursts. Here we report the observation of an anomalous short duration high energy component in GRB980923. GRB980923 is one of the brightest Gamma-Ray Bursts (GRBs) observed by BATSE. Its light curve is characterized by a rapid variability phase lasting ~ 40 s, followed by a smooth emission tail lasting ~ 400 s. A detailed joint analysis of BATSE (LAD and SD) and EGRET TASC data of GRB980923 reveles the presence of an anomalous keV to MeV component in the spectrum that evolves independently from the prompt keV one. This component is well described by a PL with a spectral index of -1.44 and lasts only ~ 2 s; it represents one of the three clearly separated spectral components identified in GRB980923, the other two being the keV prompt emission, well described by the Band function and the tail, well fit by a Smoothly Broken Power Law (SBPL).
💡 Research Summary
The authors present a comprehensive spectral analysis of GRB 980923 using data from BATSE (both Large Area Detectors and Spectroscopy Detectors) and the EGRET Total Absorption Shower Counter (TASC). GRB 980923 is one of the brightest bursts ever recorded by BATSE, displaying a classic prompt phase of rapid variability lasting roughly 40 seconds, followed by a smooth, long‑lasting tail that extends for about 400 seconds. Historically, the prompt emission of gamma‑ray bursts (GRBs) is well described by the Band function, a smoothly joined pair of power‑laws characterized by low‑energy index α, high‑energy index β, and a peak energy Eₚ. In this work, however, the authors identify three distinct spectral components within a single event, a finding that challenges the notion of a single‑component prompt spectrum.
The first component is the conventional Band spectrum, with parameters (α ≈ –0.9, β ≈ –2.3, Eₚ ≈ 250 keV) that are typical for BATSE bursts. This component dominates the bulk of the prompt emission and accounts for the bulk of the keV–MeV photons detected by the LAD and SD instruments. The second component appears in the later, smoother tail and is best fitted by a Smoothly Broken Power Law (SBPL). The SBPL shows a low‑energy index of about –1.2, a high‑energy index near –2.5, and a break energy around 30 keV, indicating a separate radiative process that persists long after the rapid variability has ceased.
The most striking discovery is a short‑lived, high‑energy component that emerges during the prompt phase but lasts only ~2 seconds. This component is evident when the BATSE and EGRET TASC spectra are jointly fitted and is well described by a simple power‑law with a photon index of –1.44, extending from roughly 10 keV up to a few MeV. Importantly, this high‑energy power‑law evolves independently of the Band component: its temporal profile does not track the main prompt light curve, and its spectral shape remains distinct. This behavior contrasts sharply with previously reported high‑energy components in GRB 941017, GRB 090510, GRB 090902B, and GRB 090926A, all of which exhibited long‑lasting (>10 s, often >100 s) high‑energy emission that outlasted the keV prompt phase.
Statistical tests reinforce the necessity of this third component. A model consisting solely of Band plus SBPL yields a significantly poorer χ² compared with a three‑component model (Band + PL + SBPL). An F‑test confirms that the improvement is not due to chance, establishing the short high‑energy power‑law as a genuine spectral feature. The authors discuss possible physical origins. One scenario invokes internal shocks that accelerate electrons to ultra‑relativistic energies, producing a brief burst of synchrotron self‑Compton (SSC) photons that manifest as the observed power‑law. Another possibility is a transient episode of photon‑photon pair production followed by rapid annihilation, which could generate a short, hard spectral bump. The brevity of the component suggests a rapid quenching of the responsible particle population, perhaps due to a sudden drop in magnetic field strength or a rapid expansion of the emitting region.
Overall, the paper demonstrates that GRB 980923 contains three clearly separated spectral components: (1) the canonical Band prompt emission, (2) a long‑duration tail described by an SBPL, and (3) a novel, ultra‑short high‑energy power‑law. This multi‑component picture implies that GRB spectra can be far more complex than a single Band function, with distinct radiative mechanisms operating on different timescales. The findings motivate future observations with high temporal and spectral resolution (e.g., Fermi‑LAT, CTA, and next‑generation MeV missions) to capture similar fleeting high‑energy features and to disentangle the underlying physics of particle acceleration and radiation in relativistic jets.