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1. The Fastest Path to Discovering the Second Electromagnetic Counterpart to a Gravitational Wave Event

   https://doi.org/10.1088/1538-3873/adb076  

   Shah, Ved G; Foley, Ryan J; Narayan, Gautham (February 2025, Publications of the Astronomical Society of the Pacific)

   Abstract The discovery of a second electromagnetic counterpart to a gravitational wave event represents a critical goal in the field of multi-messenger astronomy. In order to determine the optimal strategy for achieving this goal, we perform comprehensive simulations comparing two potential paths forward: continuing the current LIGO-Virgo-KAGRA (LVK) observing run, O4, versus temporarily shutting down the detectors for upgrades before beginning the next observing run, O5. Our simulations incorporate current O4 instrument sensitivities and duty cycles, as well as projected configurations for O5, while accounting for variables such as binary neutron star merger rates, system properties, viewing angles, dust extinction, and kilonova (KN) observables. Our results indicate that a KN discovery would occur $125_{-125}^{+253}$days (middle 50% interval) sooner in O5 compared to O4, suggesting that extending O4 would lead to faster discovery if the shutdown period between runs is  >4 months. Moreover, for 88% of our simulations, continuing O4 results in earlier KN discovery when compared to the expected two-year shutdown between O4 and O5. Given these findings and the critical importance of avoiding a  >10 yr gap between first and second electromagnetic counterpart discoveries, we suggest LVK consider extending O4 operations for as long as feasible prior to shutting down for critical upgrades. 

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2. Data-driven Expectations for Electromagnetic Counterpart Searches Based on LIGO/Virgo Public Alerts

   https://doi.org/10.3847/1538-4357/ac366d  

   Petrov, Polina; Singer, Leo P.; Coughlin, Michael W.; Kumar, Vishwesh; Almualla, Mouza; Anand, Shreya; Bulla, Mattia; Dietrich, Tim; Foucart, Francois; Guessoum, Nidhal (January 2022, The Astrophysical Journal)

   Abstract Searches for electromagnetic counterparts of gravitational-wave signals have redoubled since the first detection in 2017 of a binary neutron star merger with a gamma-ray burst, optical/infrared kilonova, and panchromatic afterglow. Yet, one LIGO/Virgo observing run later, there has not yet been a second, secure identification of an electromagnetic counterpart. This is not surprising given that the localization uncertainties of events in LIGO and Virgo’s third observing run, O3, were much larger than predicted. We explain this by showing that improvements in data analysis that now allow LIGO/Virgo to detect weaker and hence more poorly localized events have increased the overall number of detections, of which well-localized,gold-platedevents make up a smaller proportion overall. We present simulations of the next two LIGO/Virgo/KAGRA observing runs, O4 and O5, that are grounded in the statistics of O3 public alerts. To illustrate the significant impact that the updated predictions can have, we study the follow-up strategy for the Zwicky Transient Facility. Realistic and timely forecasting of gravitational-wave localization accuracy is paramount given the large commitments of telescope time and the need to prioritize which events are followed up. We include a data release of our simulated localizations as a public proposal planning resource for astronomers. 

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3. Properties of “Lite” Intermediate-mass Black Hole Candidates in LIGO-Virgo’s Third Observing Run

   https://doi.org/10.3847/2041-8213/adc5f8  

   Ruiz-Rocha, Krystal; Yelikar, Anjali B; Lange, Jacob; Gabella, William; Weller, Robert A; O’Shaughnessy, Richard; Holley-Bockelmann, Kelly; Jani, Karan (May 2025, The Astrophysical Journal Letters)

   Abstract Over a hundred gravitational-wave (GW) detections and candidates have been reported from the first three observing runs of the Advanced LIGO-Virgo-KAGRA (LVK) detectors. Among these, the most intriguing events are binary black hole mergers that result in a “lite” intermediate-mass black hole (IMBH) of ∼10²M_⊙, such as GW170502 and GW190521. In this study, we investigate 11 GW candidates from LVK’s third observing run with total detector-frame masses in the lite IMBH range. Using the Bayesian inference algorithmRIFT, we systematically analyze these candidates with three state-of-the-art waveform models that incorporate higher harmonics, which are crucial for resolving lite IMBHs in LVK data. For each candidate, we infer the premerger and postmerger black hole masses in the source frame, along with black hole spin projections across all three models. Under the assumption that these are binary black hole mergers, our analysis finds that five have a postmerger lite IMBH with masses ranging from 110 to 350M_⊙with over 90% confidence interval. Additionally, we note that one of their premerger black holes is within the pair-instability supernova mass gap (60–120M_⊙), and two premerger black holes are above the mass gap. Furthermore, we report discrepancies among the three waveform models in intrinsic parameters, with at least three GW candidates showing deviations beyond accepted statistical limits. While the astrophysical certainty of these candidates cannot be established, our study provides a foundation to probe the lite IMBH population that emerge within the low-frequency noise spectrum of LVK detectors. 

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4. MWA rapid follow-up of gravitational wave transients: Prospects for detecting prompt radio counterparts

   https://doi.org/10.1017/pasa.2023.49  

   Tian, J; Anderson, G E; Cooper, A J; Gourdji, K; Sokolowski, M; Rowlinson, A; Williams, A; Sleap, G; Dobie, D; Kaplan, D L; et al (January 2023, Publications of the Astronomical Society of Australia)

   Abstract We present and evaluate the prospects for detecting coherent radio counterparts to gravitational wave (GW) events using Murchison Widefield Array (MWA) triggered observations. The MWA rapid-response system, combined with its buffering mode ($$\sim$$4 min negative latency), enables us to catch any radio signals produced from seconds prior to hours after a binary neutron star (BNS) merger. The large field of view of the MWA ($$\sim$$$$1\,000\,\textrm{deg}^2$$at 120 MHz) and its location under the high sensitivity sky region of the LIGO-Virgo-KAGRA (LVK) detector network, forecast a high chance of being on-target for a GW event. We consider three observing configurations for the MWA to follow up GW BNS merger events, including a single dipole per tile, the full array, and four sub-arrays. We then perform a population synthesis of BNS systems to predict the radio detectable fraction of GW events using these configurations. We find that the configuration with four sub-arrays is the best compromise between sky coverage and sensitivity as it is capable of placing meaningful constraints on the radio emission from 12.6% of GW BNS detections. Based on the timescales of four BNS merger coherent radio emission models, we propose an observing strategy that involves triggering the buffering mode to target coherent signals emitted prior to, during or shortly following the merger, which is then followed by continued recording for up to three hours to target later time post-merger emission. We expect MWA to trigger on$$\sim$$$$5-22$$BNS merger events during the LVK O4 observing run, which could potentially result in two detections of predicted coherent emission. 

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5. A dark standard siren measurement of the Hubble constant following LIGO/Virgo/KAGRA O4a and previous runs

   https://doi.org/10.1093/mnras/stae2390  

   Bom, C_R; Alfradique, V.; Palmese, A.; Teixeira, G.; Santana-Silva, L.; Santos, A.; Darc, P. (October 2024, Monthly Notices of the Royal Astronomical Society)

   ABSTRACT We present a new constraint on the Hubble constant ($$H_0$$) from the standard dark siren method using a sample of five well-covered gravitational wave (GW) alerts reported during the first part of the fourth observing run of the Laser Interferometer Gravitational-Wave Observatory (LIGO), the Virgo and Kamioka Gravitational Wave Detector (KAGRA) collaborations (LVK) and with three updated standard dark sirens from third observation run in combination with the previous constraints from the first three runs. Our methodology relies on the galaxy catalogue method alone. We use a deep learning method to derive the full probability density estimation of photometric redshifts using the Legacy Survey catalogues. We add the constraints from well localized binary black hole mergers to the sample of standard dark sirens analysed in our previous work. We combine the $$H_0$$ posterior for 5 new standard sirens with other 10 previous events (using the most recent available data for the five novel events and updated three previous posteriors from O3), finding $$H_0 = 70.4^{+13.6}_{-11.7}~{\rm km~s^{-1}~Mpc^{-1}}$$ (68 per cent confidence interval) with the catalogue method only. This result represents an improvement of $$\sim 23~{{\ \rm per\ cent}}$$ comparing the new 15 dark siren constraints with the previous 10 dark siren constraints and a reduction in uncertainty of $$\sim 40~{{\ \rm per\ cent}}$$ from the combination of 15 dark and bright sirens compared with the GW170817 bright siren alone. The combination of dark and bright siren GW170817 with recent jet constraints yields $$H_0$$ of $$68.0^{+4.4}_{-3.8}~{\rm km~s^{-1}~Mpc^{-1}}$$, a $$\sim 6~{{\ \rm per\ cent}}$$ precision from standard sirens, reducing the previous constraint uncertainty by $$\sim 10~{{\ \rm per\ cent}}$$. 

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