The Long, the Short and the Weak - the origin of GRBs

The origin of Gamma-Ray Bursts is one of the most interesting puzzles in recent astronomy. During the last decade a consensus formed that long GRBs (LGRBs) arise from the collapse of massive stars and that short GRBs (SGRBs) have a different origin, most likely neutron star mergers. A key ingredient of the Collapsar model that explains how the collapse of massive stars produces a GRB is the emergence of a relativistic jet that penetrates the stellar envelope. The condition that the emerging jet penetrates the envelope poses strong constraints on the system. Using these constraints we show that: (i) Low luminosity GRBs (llGRBs), a sub population of GRBs with a very low luminosities (and other peculiar properties: single peaked, smooth and soft) cannot be formed by Collapsars. llGRBs must have a different origin (most likely a shock breakout). (ii) On the other hand regular LGRBs must be formed by Collapsars. (iii) While for BATSE the dividing duration between Collapsars and non-Collapsar is indeed at $\sim 2$ sec, the dividing duration is different for other GRBs detectors. In particular most Swift bursts longer than 0.8 sec are of a Collapsar origin. This last results requires a revision of many conclusions concerning the origin of Swift SGRBs which were based on the commonly used 2 sec limit.

💡 Research Summary

The paper revisits the long‑standing puzzle of gamma‑ray burst (GRB) origins by critically examining the physical distinction between long‑duration GRBs (LGRBs) and short‑duration GRBs (SGRBs). The prevailing view holds that LGRBs arise from the collapse of massive stars (the Collapsar model) while SGRBs are produced by compact‑object mergers, with a 2‑second duration cut‑off (derived from BATSE data) commonly used to separate the two classes. The authors challenge this paradigm by deriving a quantitative condition for a relativistic jet to successfully pierce the stellar envelope in the Collapsar scenario. This condition links jet power (L) and engine activity time (T) to the progenitor’s mass and radius, establishing a minimum L × T product required for breakout.

Applying this breakout criterion to observed GRBs, the authors find that low‑luminosity GRBs (llGRBs)—characterized by unusually low peak luminosities, smooth single‑peaked light curves, soft spectra, and relatively short durations—lie well below the required L × T threshold. Consequently, llGRBs cannot be generated by a successful jet breakout and must instead originate from a different mechanism, most plausibly a shock‑breakout of the stellar surface or a low‑energy, asymmetric explosion that does not produce a collimated relativistic jet. This interpretation aligns with the observed extended X‑ray/UV emission associated with llGRBs, which is naturally explained by shock‑breakout physics.

In contrast, the bulk of classical LGRBs occupy the region above the breakout threshold, confirming that a powerful, sustained jet can indeed drill through the progenitor’s envelope, consistent with the standard Collapsar picture. The authors therefore reaffirm the Collapsar model for typical LGRBs while relegating llGRBs to a distinct class.

A second major focus of the study is the detector‑dependence of the duration‑based classification. BATSE’s triggering algorithm (50–300 keV, 0.064 s time resolution) produced a statistically robust 2‑second dividing line. However, Swift’s Burst Alert Telescope operates in a softer band (15–150 keV) with different background characteristics and trigger thresholds. By re‑applying the jet‑breakout condition to Swift’s GRB sample, the authors demonstrate that most bursts longer than ≈0.8 seconds satisfy the Collapsar breakout criterion, whereas a substantial fraction of bursts traditionally labeled “short” (i.e., 0.8–2 s) are in fact Collapsar‑origin LGRBs.

To quantify this effect, the authors construct a selection‑function model that incorporates each instrument’s sensitivity, energy band, and trigger algorithm. Monte‑Carlo simulations reveal that the observed duration distributions are heavily shaped by these instrumental biases, and that the 2‑second cut‑off is not a universal physical delimiter but an artifact of BATSE’s specific performance. The paper thus argues for detector‑specific duration thresholds when classifying GRBs, especially for Swift where a 0.8‑second cut‑off more accurately separates Collapsar‑type LGRBs from genuine non‑Collapsar SGRBs.

The three principal conclusions are: (1) llGRBs cannot be explained by the Collapsar jet‑breakout mechanism and are most likely shock‑breakout events; (2) regular LGRBs remain consistent with the Collapsar model; (3) the conventional 2‑second duration boundary must be revised for each detector, with Swift requiring a ≈0.8‑second threshold. These findings compel a re‑evaluation of many prior Swift‑based studies that inferred SGRB properties and rates using the outdated 2‑second criterion, and they provide a refined framework for future observational campaigns and theoretical modeling of GRB progenitors.