Searches for inspiral gravitational waves associated with short gamma-ray bursts in LIGOs fifth and Virgos first science run

Mergers of two compact objects, like two neutron stars or a neutron star and a black hole, are the probable progenitor of short gamma-ray bursts. These events are also promising sources of gravitational waves, that are currently motivating related searches by an international network of gravitational wave detectors. Here we describe a search for gravitational waves from the in-spiral phase of two coalescing compact objects, in coincidence with short GRBs occurred during during LIGO’s fifth science run and Virgo’s first science run. The search includes 22 GRBs for which data from more than one of the detectors in the LIGO/Virgo network were available. No statistically significant gravitational-wave candidate has been found, and a parametric test shows no excess of weak gravitational-wave signals in our sample of GRBs. The 90%~C.L. median exclusion distance for GRBs in our sample is of 6.7 Mpc, under the hypothesis of a neutron star - black hole progenitor model.

💡 Research Summary

The paper presents a targeted search for gravitational‑wave (GW) signals from the inspiral phase of compact binary coalescences that could be the progenitors of short gamma‑ray bursts (GRBs). Short GRBs, defined by durations less than about two seconds, are widely believed to arise from the merger of two neutron stars (NS‑NS) or a neutron star and a black hole (NS‑BH). Such mergers emit a characteristic “chirp” GW signal during the inspiral, which can be captured by ground‑based interferometers.

The authors used data from LIGO’s fifth science run (S5, 2005‑2007) and Virgo’s first science run (VSR1, 2007). They selected 22 short GRBs for which at least two detectors in the LIGO‑Virgo network were operating simultaneously, ensuring that a coincident GW search could be performed. For each GRB, a ±6 second “on‑source” window centered on the GRB trigger time was defined, while comparable “off‑source” windows from the same detectors were used to estimate the background false‑alarm rate.

The GW search employed a matched‑filter pipeline with a template bank based on post‑Newtonian approximations of the inspiral waveform. Templates covered component masses of 1–3 M⊙ for neutron stars and 5–10 M⊙ for black holes, neglecting spin effects to keep the bank tractable. Single‑detector signal‑to‑noise ratios (SNRs) were computed and then combined across detectors using a coincidence test that required consistent time delays and phases. An event was retained as a candidate only if its combined SNR corresponded to a false‑alarm rate (FAR) below 1 yr⁻¹.

Background estimation was performed by applying the same pipeline to the off‑source data, yielding a distribution of combined SNRs against which the on‑source maximum could be compared. For every GRB, the resulting p‑value exceeded 0.2, indicating no statistically significant excess over background. Consequently, no GW candidates associated with the 22 short GRBs were identified.

To quantify the search sensitivity, the authors injected simulated NS‑BH inspiral signals at various distances and sky locations into the real data and measured the recovery efficiency. Under the NS‑BH hypothesis, the median 90 % confidence exclusion distance for the sample was 6.7 Mpc. In other words, if a NS‑BH merger associated with any of these GRBs had occurred within roughly 7 Mpc, the pipeline would have detected it with at least 90 % probability. This distance is modest, reflecting the limited sensitivity of the S5/VSR1 detector configuration.

A parametric “weak‑signal” test was also carried out to search for a collective excess of sub‑threshold events across the GRB sample. The test returned null results, reinforcing the conclusion that the data are consistent with noise.

The paper concludes that, while no GW signal was found, the analysis validates the search methodology, provides a quantitative upper limit on the distance to potential NS‑BH progenitors of short GRBs, and sets a benchmark for future observing runs. With the advent of advanced detectors (LIGO A+, Virgo+, KAGRA), the horizon for inspiral detections will expand to several hundred megaparsecs, dramatically increasing the chances of observing a coincident GW‑GRB event and thereby confirming the merger origin of short GRBs.