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Abstract We examine the role of LIGO-India in facilitating multimessenger astronomy in the era of next-generation observatories. A network with two L-shaped Cosmic Explorer (CE) detectors and one triangular Einstein Telescope (ET) would precisely localize nearly the entire annual binary neutron star (NS) merger population up to a redshift of 0.5—over 10,000 events would be localized within 10 deg², including approximately 150 events within 0.1 deg². Luminosity distance would be measured to within 10% for over 9000 events and within 1% for ∼100 events. Surprisingly, replacing the 20 km CE detector with LIGO-India operating at A^♯sensitivity (I^♯) yields a nearly identical performance. The factor-of-5 shorter arms are offset by a fourfold increase in baseline relative to a second CE in the US, preserving localization accuracy, with over 9000 events within 10 deg²and ∼90 events within 0.1 deg². This configuration detects ∼6000 events with luminosity distance uncertainties under 10%, including ∼50 with under 1%. Both networks provide $\mathcal{O}\left(100\right)$early-warning detections up to 10 minutes before merger, with localization areas ≤10 deg². WhileI^♯enables excellent localization and early warnings, its shorter arms and narrower sensitivity band would limit its reach for other science goals, such as detecting Population III binary black hole mergers atz≳ 10, NS mergers atz∼ 2, or constraining cosmological parameters. 

Abstract This study investigates the origins of GW230529, delving into its formation from massive stars within isolated binary systems. Utilizing population-synthesis models, we present compelling evidence that the neutron star component forms second. However, the event’s low signal-to-noise ratio introduces complexities in identifying the underlying physical mechanisms driving its formation. Augmenting our analysis with insights from numerical relativity, we estimate the final black hole mass and spin to be approximately 5.3M_⊙and 0.53, respectively. Furthermore, we employ the obtained posterior samples to calculate the ejecta mass and kilonova light curves resulting fromr-process nucleosynthesis. We find the ejecta mass to be within 0–0.06M_⊙, contingent on the neutron star equation of state. The peak brightness of the kilonova light curves indicates that targeted follow-up observations with a Rubin-like observatory may have detected this emission. 

Abstract GW230529 is the first compact binary coalescence detected by the LIGO–Virgo–KAGRA collaboration with at least one component mass confidently in the lower mass gap, corresponding to the range 3–5M_⊙. If interpreted as a neutron star–black hole merger, this event has the most symmetric mass ratio detected so far and therefore has a relatively high probability of producing electromagnetic (EM) emission. However, no EM counterpart has been reported. At the merger timet₀, Swift-BAT and Fermi-GBM together covered 100% of the sky. Performing a targeted search in a time window [t₀− 20 s,t₀+ 20 s], we report no detection by the Swift-BAT and Fermi-GBM instruments. Combining the position-dependentγ-ray flux upper limits and the gravitational-wave posterior distribution of luminosity distance, sky localization, and inclination angle of the binary, we derive constraints on the characteristic luminosity and structure of the jet possibly launched during the merger. Assuming atop-hatjet structure, we exclude at 90% credibility the presence of a jet that has at the same time an on-axis isotropic luminosity ≳10⁴⁸erg s⁻¹in the bolometric band 1 keV–10 MeV and a jet opening angle ≳15°. Similar constraints are derived by testing other assumptions about the jet structure profile. Excluding GRB 170817A, the luminosity upper limits derived here are below the luminosity of any GRB observed so far.