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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. Jetted and Turbulent Stellar Deaths: New LVK-detectable Gravitational-wave Sources

   https://doi.org/10.3847/2041-8213/ace03a  

   Gottlieb, Ore; Nagakura, Hiroki; Tchekhovskoy, Alexander; Natarajan, Priyamvada; Ramirez-Ruiz, Enrico; Banagiri, Sharan; Jacquemin-Ide, Jonatan; Kaaz, Nick; Kalogera, Vicky (July 2023, The Astrophysical Journal Letters)

   Abstract Upcoming LIGO–Virgo–KAGRA (LVK) observing runs are expected to detect a variety of inspiralling gravitational-wave (GW) events that come from black hole and neutron star binary mergers. Detection of noninspiral GW sources is also anticipated. We report the discovery of a new class of noninspiral GW sources—the end states of massive stars—that can produce the brightest simulated stochastic GW burst signal in the LVK bands known to date, and could be detectable in LVK run A+. Some dying massive stars launch bipolar relativistic jets, which inflate a turbulent energetic bubble—cocoon—inside of the star. We simulate such a system using state-of-the-art 3D general relativistic magnetohydrodynamic simulations and show that these cocoons emit quasi-isotropic GW emission in the LVK band, ∼10–100 Hz, over a characteristic jet activity timescale ∼10–100 s. Our first-principles simulations show that jets exhibit a wobbling behavior, in which case cocoon-powered GWs might be detected already in LVK run A+, but it is more likely that these GWs will be detected by the third-generation GW detectors with an estimated rate of ∼10 events yr −1 . The detection rate drops to ∼1% of that value if all jets were to feature a traditional axisymmetric structure instead of a wobble. Accompanied by electromagnetic emission from the energetic core-collapse supernova and the cocoon, we predict that collapsars are powerful multimessenger events. 

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3. Constraining the LVK AGN channel with black hole spins

   https://doi.org/10.1093/mnras/stae1351  

   McKernan, B.; Ford, K_E_S (May 2024, Monthly Notices of the Royal Astronomical Society)

   ABSTRACT Merging black holes (BHs) are expected to produce remnants with large dimensionless spin parameters (aspin ∼ 0.7). However, gravitational wave (GW) observations with LIGO–Virgo–Kagra (LVK) suggest that merging BHs are consistent with modestly positive but not high spin (aspin ∼ 0.2), causing tension with models suggesting that high-mass mergers are produced by hierarchical merger channels. Some BHs also show evidence for strong in-plane spin components. Here, we point out that spin-down of BHs due to eccentric prograde post-merger orbits within the gas of an active galactic nucleus (AGN) disc can yield BHs with masses in the upper mass gap, but only modestly positive aspin, and thus observations of BHs with low spin do not rule out hierarchical models. We also point out that the fraction of binary black hole (BBH) mergers with significant in-plane spin components is a strong test of interactions between disc BBHs and nuclear spheroid orbiters. Spin magnitude and spin tilt constraints from LVK observations of BBHs are an excellent test of dynamics of BHs in AGN discs, disc properties, and the nuclear clusters interacting with AGNs. 

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4. Spin it as you like: The (lack of a) measurement of the spin tilt distribution with LIGO-Virgo-KAGRA binary black holes

   https://doi.org/10.1051/0004-6361/202245084  

   Vitale, Salvatore; Biscoveanu, Sylvia; Talbot, Colm (December 2022, Astronomy & Astrophysics)

   Context. The growing set of gravitational-wave sources is being used to measure the properties of the underlying astrophysical populations of compact objects, black holes, and neutron stars. Most of the detected systems are black hole binaries. While much has been learned about black holes by analyzing the latest LIGO-Virgo-KAGRA (LVK) catalog, GWTC-3, a measurement of the astrophysical distribution of the black hole spin orientations remains elusive. This is usually probed by measuring the cosine of the tilt angle (cos τ ) between each black hole spin and the orbital angular momentum, with cos τ  = +1 being perfect alignment. Aims. The LVK Collaboration has modeled the cos τ distribution as a mixture of an isotropic component and a Gaussian component with mean fixed at +1 and width measured from the data. We want to verify if the data require the existence of such a peak at cos τ  = +1. Methods. We used various alternative models for the astrophysical tilt distribution and measured their parameters using the LVK GWTC-3 catalog. Results. We find that (a) augmenting the LVK model, such that the mean μ of the Gaussian is not fixed at +1, returns results that strongly depend on priors. If we allow μ  >  +1, then the resulting astrophysical cos τ distribution peaks at +1 and looks linear, rather than Gaussian. If we constrain −1 ≤  μ  ≤ +1, the Gaussian component peaks at μ  = 0.48 −0.99 +0.46 (median and 90% symmetric credible interval). Two other two-component mixture models yield cos τ distributions that either have a broad peak centered at 0.19 −0.18 +0.22 or a plateau that spans the range [ − 0.5, +1], without a clear peak at +1. (b) All of the models we considered agree as to there being no excess of black hole tilts at around −1. (c) While yielding quite different posteriors, the models considered in this work have Bayesian evidences that are the same within error bars. Conclusions. We conclude that the current dataset is not sufficiently informative to draw any model-independent conclusions on the astrophysical distribution of spin tilts, except that there is no excess of spins with negatively aligned tilts. 

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5. Technical noise, data quality, and calibration requirements for next-generation gravitational-wave science

   https://doi.org/10.1088/1361-6382/ad694d  

   Capote, E.; Dartez, L.; Davis, D. (August 2024, Classical and Quantum Gravity)

   Abstract The next generation of ground-based gravitational-wave interferometers is expected to generate a bounty of new astrophysical discoveries, with sensitivities and bandwidths greatly improved compared to current-generation detectors. These detectors will allow us to make exceptional advancements in our understanding of fundamental physics, the dynamics of dense matter, and the cosmic history of compact objects. The fundamental design aspects of these planned interferometers will enable these new discoveries; however, challenges in technical noise, data quality, and calibration have the potential to limit the scientific reach of these instruments. In this work, we evaluate the requirements of these elements for next-generation gravitational-wave science, focusing on how these areas may impact the proposed Cosmic Explorer observatory. We highlight multiple aspects of these fields where additional research and development is required to ensure Cosmic Explorer reaches its full potential. 

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