A Peculiarly Short-duration Gamma-Ray Burst from Massive Star Core Collapse

Abstract: Gamma-ray bursts (GRBs) have been phenomenologically classified into long and short populations based on the observed bimodal distribution of duration. Multi-wavelength and multi-messenger observations in recent years have revealed that in general long GRBs originate from massive star core collapse events, whereas short GRBs originate from binary neutron star mergers. It has been known that the duration criterion is sometimes unreliable, and multi-wavelength criteria are needed to identify the physical origin of a particular GRB. Some apparently long GRBs have been suggested to have a neutron star merger origin, whereas some apparently short GRBs have been attributed to genuinely long GRBs whose short, bright emission is slightly above the detector's sensitivity threshold. Here we report the comprehensive analysis of the multi-wavelength data of a bright short GRB 200826A. Characterized by a sharp pulse, this burst shows a duration of 1 second and no evidence of an underlying longer-duration event. Its other observational properties such as its spectral behaviors, total energy, and host galaxy offset, are, however, inconsistent with those of other short GRBs believed to originate from binary neutron star mergers. Rather, these properties resemble those of long GRBs. This burst confirms the existence of short duration GRBs with stellar core-collapse origin, and presents some challenges to the existing models.

Analysis of a Peculiarly Short-Duration Gamma-Ray Burst from Massive Star Core Collapse

The paper under review presents a comprehensive investigation into the phenomenology associated with a specific gamma-ray burst (GRB 200826A) that challenges conventional categorization by duration. Traditionally, gamma-ray bursts have been classified into long and short-duration bursts, with the majority deriving long GRBs from the collapse of massive stars and short GRBs from neutron star mergers. However, the short-duration GRB 200826A, closely scrutinized in this study, exhibits characteristics that are typically associated with long GRBs. This anomaly suggests a need for refinement in the categorization scheme currently employed for GRBs.

GRB 200826A, identified by the Fermi Gamma-ray Burst Monitor (GBM), possesses a remarkably short duration of approximately 1 second. Despite fitting the duration profile of short GRBs, its associated isotropic-equivalent energy output ((E_{\gamma, iso} \approx 7.09 \times 10^{51}) erg) is reminiscent of long GRBs, as it considerably exceeds the typical energy values observed among short GRBs. Additionally, the spectral and temporal features—such as the photon index and the spectral lag, combined with host galaxy properties like high star formation—also align more consistently with long GRBs.

The paper crucially employs various analytical methods to examine the origin of GRB 200826A. Notably, it utilizes the spectral hardness ratio and the Amati relation (a notable empirical correlation between the peak energy and isotropic energy of GRBs) to explore if this short burst might belong to a subclass of core-collapse-origin short GRBs. The GRB's location in the (E_{\rm p,z}-E_{\gamma, iso}) space aligns with that typically occupied by long GRBs as defined by the Amati relation. Furthermore, characteristics such as the excessive amplitude parameter (f) ((\approx 7.58\pm 1.23)) suggest that this GRB is indeed genuinely short in duration, dismissing possibilities for reclassification under hidden extended emissions.

In seeking to account for the unique characteristics articulated, the study explores several theoretical scenarios. One possibility discussed is that of a supernova preceding the GRB, a scenario known as supranova. This concept posits that a neutron star, formed through previous star collapse, evolves and collapses itself, facilitating the observed GRB. However, the inclusion of evolved observation technology to further refine spectroscopic techniques and elucidate additional energy correlates might challenge this or any other standard GRB models.

The implications of this research extend beyond simple classification; they redefine criteria for identifying GRB progenitors more accurately. The findings underscore the complexity in GRB classification systems and encourage further exploration into a sub-class of core-collapse-related short-duration GRBs. Increased observation and more refined simulations can bolster our understanding of such entities.

In conclusion, the analysis and outcomes presented in this paper advocate for a more nuanced approach toward GRB classification. Consideration of refined observables, such as distinct host galaxy environments and anomalous energy outputs, is essential. Future work in this burgeoning intersection of observational astronomy and theoretical modeling may evolve to encompass these variations, providing a more comprehensive understanding of GRB phenomena and the diverse astrophysical processes that drive them.