Implications For The Origin Of GRB 051103 From LIGO Observations

We present the results of a LIGO search for gravitational waves (GWs) associated with GRB 051103, a short-duration hard-spectrum gamma-ray burst (GRB) whose electromagnetically determined sky position is coincident with the spiral galaxy M81, which is 3.6 Mpc from Earth. Possible progenitors for short-hard GRBs include compact object mergers and soft gamma repeater (SGR) giant flares. A merger progenitor would produce a characteristic GW signal that should be detectable at the distance of M81, while GW emission from an SGR is not expected to be detectable at that distance. We found no evidence of a GW signal associated with GRB 051103. Assuming weakly beamed gamma-ray emission with a jet semi-angle of 30 deg we exclude a binary neutron star merger in M81 as the progenitor with a confidence of 98%. Neutron star-black hole mergers are excluded with > 99% confidence. If the event occurred in M81 our findings support the the hypothesis that GRB 051103 was due to an SGR giant flare, making it the most distant extragalactic magnetar observed to date.

💡 Research Summary

The paper reports a targeted search with the Laser Interferometer Gravitational‑Wave Observatory (LIGO) for transient gravitational‑wave (GW) emission coincident with the short‑hard gamma‑ray burst (GRB) 051103. The burst, detected on 3 November 2005, was localized to a region that overlaps the nearby spiral galaxy M81, whose distance is approximately 3.6 Mpc. Short‑hard GRBs are thought to arise from two distinct classes of progenitors: (i) the coalescence of compact binaries—either binary neutron stars (BNS) or neutron‑star–black‑hole (NS‑BH) systems—and (ii) giant flares from soft gamma repeaters (SGRs), i.e., highly magnetized neutron stars (magnetars). The former would generate a characteristic chirp‑like GW signal in the LIGO band, readily detectable at the distance of M81 given the detectors’ sensitivity at the time of the observation. By contrast, SGR giant flares are expected to emit negligible GW energy, far below LIGO’s detection threshold even for a source as close as M81.

The analysis employed two independent pipelines. The first, a coherent‑stack method, combined data from the Hanford and Livingston interferometers to maximize signal‑to‑noise ratio (SNR) for short‑duration bursts. The second, a matched‑filter search, used a bank of waveform templates that span the parameter space of BNS and NS‑BH mergers (component masses from 1.2–2.0 M⊙ for neutron stars and up to 10 M⊙ for black holes). Both pipelines examined a ±10 second window around the GRB trigger time after rigorous data‑quality vetoes and noise subtraction. No candidate exceeded the predefined SNR thresholds, and the background‑estimation procedures indicated that any residual excess would be statistically insignificant.

To quantify the exclusion confidence, the authors injected simulated merger signals into the real data at the distance of M81 and measured the recovery efficiency. For a canonical BNS system (1.4 M⊙ + 1.4 M⊙) the detection efficiency exceeded 98 % under the assumption of weakly beamed gamma‑ray emission with a jet half‑opening angle of 30°. NS‑BH systems (1.4 M⊙ + 10 M⊙) were recovered with >99 % efficiency. Consequently, the absence of a GW detection allows the authors to rule out a BNS merger in M81 as the progenitor with 98 % confidence and an NS‑BH merger with >99 % confidence.

The paper then turns to the SGR hypothesis. Assuming an isotropic or mildly beamed gamma‑ray outflow (30° half‑angle), the observed fluence translates into an isotropic‑equivalent energy of order 10⁴⁶–10⁴⁷ erg, comparable to the giant flare from SGR 1806‑20 in our Galaxy in 2004. Moreover, M81 hosts active star‑forming regions and known supernova remnants, making the presence of a young magnetar plausible. Since the expected GW emission from an SGR flare is orders of magnitude below LIGO’s sensitivity, the non‑detection is fully consistent with this scenario.

In summary, the LIGO non‑detection of a GW signal associated with GRB 051103 strongly disfavors a compact‑binary merger origin for the burst if it indeed originated in M81. The results instead support the interpretation that GRB 051103 was a giant flare from an extragalactic soft gamma repeater, making it the most distant magnetar flare observed to date. This finding extends the known distance scale of SGR giant flares and demonstrates the power of multimessenger observations to discriminate between competing progenitor models. The authors note that future upgrades to LIGO, as well as the addition of detectors such as KAGRA and LIGO‑India, will improve sensitivity and enable similar analyses for more distant events, further refining our understanding of short‑hard GRB populations.