Similar Fermi-GBM sGRBs to GW/sGRB 170817A in MeV-GeV energies

TL;DR

This work searches the Fermi-GBM sGRB catalog for bursts analog to the gravitational-wave–associated sGRB 170817A by exploiting a two-component emission signature. Using Zone-A Comptonized and Zone-B blackbody spectral fits, hardness ratios HR1/HR2, Mahalanobis outlier rejection, and K-means clustering, they identify eight candidate sGRBs resembling 170817A and analyze their light curves and spectra in detail. With pseudo-redshift estimates via an Amati-like relation, they estimate luminosity distances and compute the expected GW+sGRB event rates across LVK observing runs O1–O5, finding consistency with O2 observations and predicting several multimessenger detections in future runs. The results support a population of off-axis, low-luminosity sGRBs that can inform the rates and beaming of NS–NS mergers and guide multimessenger search strategies.

Abstract

The rate of observed gravitational waves (GWs) from neutron star-neutron star (NS-NS) mergers detected by the Laser Interferometer Gravitational-Wave Observatory (LIGO) indicates the existence of more than one short gamma-ray bursts (sGRBs) similar to GW/sGRB 170817A within the total gamma-ray bursts (GRBs) recorded by satellite detectors such as BATSE, Fermi-Gamma-ray Burst Monitor (Fermi-GBM), and Swift-Burst Alert Telescope (Swift-BAT). We investigated sGRBs in the Fermi-GBM dataset based on their MeV-GeV $γ$-ray emission features, to identify sGRBs similar to sGRB 170817A. Any addition of such events can impact the rate of NS-NS CBC events observed by LIGO. SGRB 170817A exhibits two distinct emission components: a non-thermal peak and a thermal component. We adopted a multifaceted approach to identify analogous sGRBs, which involved computing the hardness ratios $HR_{1}$ and $HR_{2}$ and then clustering them via the K-means algorithm. Our further studies reveal the presence of eight such events in Fermi-GBM data, which will enable us to calculate the rate of electromagnetic (EM) counterparts associated with LIGO GW events (GW+sGRB events) across all observing runs. Giving an estimation, by the end of the $O_4$ LIGO run, there could be nearly 5 GW+sGRB events. Deviation from this number may raise concerns about our understanding of the evolution of such events over distance.

Figures (6)

• Figure 3: Contour based clustering ($HR_{1}$ v/s $HR_{2}$) at k = 6, each cluster represented by a distinct color. Contour lines indicate the density distribution of points within each cluster, which visually represents the data's structure, highlighting regions with higher concentrations of data points. Stars (red color) are centroids, the diamond ( blue color) is sGRB 170817A.
• Figure 4: Workflow illustrating our process of finding out the twin sGRBs alike sGRB 170817A. F refers to the various filtering phases followed.
• Figure 5: Variation of the inertia with the number of clusters. The black dashed line is the kneedle point representing the maximum curvature, corresponding to the optimal K number
• Figure 6: sGRBs similar to sGRB 170817A. The order of the light curves is the following: sGRB 170817A, sGRB 150805, sGRB 150101, sGRB 131128, sGRB 131004, sGRB 130808, sGRB 120524, sGRB 090108, and sGRB 081122. All the light curves are produced in the three energy ranges 50-350 keV, 10-50 keV and 10-350 keV with the binning size of 0.032 sec and 0.004 sec. The first and second peak time zones are shown with a solid (red) line and a dot-dashed (green) line, respectively. The light curve is the addition of the counts from the NAI detectors used for analysis. The horizontal solid (black) line is the background.
• Figure 7: Correlation of $E_{iso}$ with $E_{peak}$ of Fermi-GBM sGRBs with known redshift till 17 August 2017.