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Abstract We present a study aimed at quantifying the detectability of radio counterparts of binary neutron star (BNS) mergers with total masses ≲3M_⊙, which may form neutron star remnants. We focus on mergers localized by gravitational-wave (GW) observations to sky areas ≲10 deg², a precision that greatly facilitates optical counterpart identification and enables radio discovery even without detections at other wavelengths. Widely separated GW detectors are essential for building samples of well-localized BNS mergers accessible to radio telescopes, with minimum yearly detection rates (assuming the smallest values of the BNS local merger rate) ranging from a few with current GW detectors to hundreds with next-generation GW instruments. Current GW networks limit well-localized detections toz ≲ 0.2, while next-generation GW detectors extend the reach toz ≲ 0.8, encompassing the median redshift of short gamma-ray bursts (GRBs). With next-generation radio arrays operating at several tens of GHz and providing an order of magnitude improvement in sensitivity compared to the most sensitive ones available today, short GRB-like jet afterglows can be detected for a large fraction of the considered BNS mergers. At lower radio frequencies, detections with current radio interferometric arrays are feasible, though subject to synchrotron self-absorption effects. The enhanced sensitivity and survey speed of future radio interferometers operating at a few GHz, combined with the higher detection rate of well-localized BNSs enabled by next-generation GW observatories, are key to probing disk-wind and dynamical ejecta afterglows, as well as remnant diversity. 

The multimessenger detection of the binary neutron star (NS) merger GW170817 has revolutionized the field of gravitational wave (GW) astronomy. However, several important questions remain to be answered. One of these is the nature of the compact remnant leftover by GW170817 (short- or long-lived NS versus black hole). A key goal going forward is to understand the diversity of NS-NS merger remnants, and how such diversity maps onto their viability as gamma-ray burst (GRB) central engines. Here, we present a study aimed at assessing the sensitivity of triggered searches for intermediate-duration, postmerger GWs powered by long-lived GRB remnants using networks of current and future ground-based GW detectors and the cross-orrelation algorithm (CoCoA). We develop a Python-based framework to efficiently estimate CoCoA distance horizons for a broad range of postmerger secular bar-mode waveforms and for different GW detector networks. This framework can be used to identify the most promising regions of parameter space in which to concentrate search efforts, helping to design future search strategies to optimally balance search sensitivity and related parameter space gridding schema against computational cost. 

Abstract Hydrogen/helium-poor stripped-envelope core-collapse supernovae with broad lines (SNe Ic-bl) almost always accompany the nearby (z < 0.3) jetted relativistic explosions known as long duration gamma-ray bursts (GRBs). However, the majority of SNe Ic-bl have no detected GRB counterpart. At least some of these SNe should harbor off-axis jets, whose afterglow may become detectable at late times, particularly at radio wavelengths. Here, we present Karl G. Jansky Very Large Array (VLA) radio observations (rest-frame times of ∼3–4 × 10³days post SN discovery) of a sample of 14 SNe Ic-bl discovered by the Palomar Transient Factory (PTF) that have been demonstrated to originate from the same host environments as the SNe Ic-bl associated with nearby GRBs. Of the 14 SNe, we identify three that are radio detected, one of which (PTF10tqv,z= 0.0795) is consistent with an off-axis jet with energy similar to classical GRBs (∼10⁵¹–10^51.7erg). Using recently developed synchrotron radiation code, we find that for our 11 nondetections, which are among the deepest limits obtained for Ic-bl, we rule out an off-axis jet with an energy of ≳10⁵¹erg in circumburst densities of ≳10⁻¹cm⁻³. We predict that well-spaced monitoring of newly discovered SNe Ic-bl from ∼10 days to ∼10 yr (rest frame) to luminosities of ∼10²⁷erg s⁻¹Hz⁻¹will constrain the existence of highly off-axis jets (≳60°) with classical GRB energies. The VLA Sky Survey will probe jets that are ≲60° off-axis, whereas the Deep Synpotic Array 2000 will probe jets out to ∼90° off-axis, demonstrating the importance of utilizing radio surveys to supplement targeted observations. 

Abstract For over 25 yr, the origin of long-duration gamma-ray bursts (lGRBs) has been linked to the collapse of rotating massive stars. However, we have yet to pinpoint the stellar progenitor powering these transients. Moreover, the dominant engine powering the explosions remains open to debate. Observations of both lGRBs, supernovae associated with these GRBs, such as broad-line (BL) stripped-envelope (type Ic) supernovae (hereafter, Ic-BL), supernovae (SNe), and perhaps superluminous SNe, fast blue optical transients, and fast x-ray transients, may provide clues to both engines and progenitors. In this paper, we conduct a detailed study of the tight-binary formation scenario for lGRBs, comparing this scenario to other leading progenitor models. Combining this progenitor scenario with different lGRB engines, we can compare to existing data and make predictions for future observational tests. We find that the combination of the tight-binary progenitor scenario with the black hole accretion disk engine can explain lGRBs, low-luminosity GRBs, ultra-long GRBs, and Ic-BL. We discuss the various progenitor properties required for these different subclasses and note such systems would be future gravitational-wave merger sources. We show that the current literature on other progenitor-engine scenarios cannot explain all of these transient classes with a single origin, motivating additional work. We find that the tight-binary progenitor with a magnetar engine is excluded by existing observations. The observations can be used to constrain the properties of stellar evolution, the nature of the GRB, and the associated SN engines in lGRBs and Ic-BL. We discuss the future observations needed to constrain our understanding of these rare, but powerful, explosions. 

Abstract The observational link between long gamma-ray bursts (GRBs) and broad-lined stripped-envelope core-collapse supernovae (SNe Ic-BL) is well established. Significant progress has been made in constraining what fraction of SNe Ic-BL may power high- or low-luminosity GRBs when viewed at small off-axis angles. However, the GRB–SN connection still lacks a complete understanding in the broader context of massive-star evolution and explosion physics. Models predict a continuum of outcomes for the fastest ejecta, from choked to ultrarelativistic jets, and observations from radio to X-rays are key to probing these scenarios across a range of viewing angles and velocities. Here, we present results from a coordinated radio-to-X-ray campaign targeting nearby (z ≲ 0.1) SNe Ic-BL designed to explore this diversity. With eight new radio-monitored events and updated data for one previously observed SN, we further tighten constraints on the fraction of SNe Ic-BL as relativistic as SN 1998bw (GRB 980425). We identify SN 2024rjw as a new radio-loud event likely powered by strong interaction with circumstellar material, and add evidence supporting a similar interpretation for SN 2020jqm. We also establish new limits on the properties of radio-emitting ejecta with velocities consistent with cocoons from choked jets, highlighting SN 2022xxf as a promising cocoon-dominated candidate. These results refine our understanding of the continuum linking ordinary SNe Ic-BL, engine-driven explosions, and GRBs, and contribute to building a sample that will inform future multimessenger searches for electromagnetic counterparts to high-energy neutrinos. 

ABSTRACT Follow-up observations of neutrino events have been a promising method for identifying sources of very-high-energy cosmic rays. Neutrinos are unambiguous tracers of hadronic interactions and cosmic rays. On 2020 June 15, IceCube detected a neutrino event with an 82.8 per cent probability of being astrophysical in origin. To identify the astrophysical source of the neutrino, we used X-ray tiling observations to identify potential counterpart sources. We performed additional multiwavelength follow-up with NuSTAR and the VLA in order to construct a broadband spectral energy distribution (SED) of the most likely counterpart. From the SED, we calculate an estimate for the neutrinos we expect to detect from the source. While the source does not have a high predicted neutrino flux, it is still a plausible neutrino emitter. It is important to note that the other bright X-ray candidate sources consistent with the neutrino event are also radio-quiet active galactic nuclei. A statistical analysis shows that 1RXS J093117.6+033146 is the most likely counterpart (87.5 per cent) if the neutrino is cosmic in origin and if it is among X-ray detectable sources. This result adds to previous results suggesting a connection between radio-quiet AGN and IceCube neutrino events. 

Compact objects across the mass spectrum–from neutron stars to supermassive black holes–are progenitors and/or central engines for some of the most cataclysmic phenomena in the Universe. As such, they are associated with radio emission on a variety of timescales and represent key targets for multi-messenger astronomy. Observations of transients in the radio band can unveil the physics behind their central engines, ejecta, and the properties of their surroundings, crucially complementing information on their progenitors gathered from observations of other messengers (such as gravitational waves and neutrinos). In this contribution, we summarize observational opportunities and challenges ahead in the multi-messenger study of neutron stars and black holes using radio observations. We highlight the specific contribution of current U.S. national radio facilities and discuss expectations for the field focusing on the science that could be enabled by facilities recommended by the 2020 Decadal survey such as the next generation Very Large Array (ngVLA). 

The landmark detection of both gravitational waves (GWs) and electromagnetic (EM) radiation from the binary neutron star merger GW170817 has spurred efforts to streamline the follow-up of GW alerts in current and future observing runs of ground-based GW detectors. Within this context, the radio band of the EM spectrum presents unique challenges. Sensitive radio facilities capable of detecting the faint radio afterglow seen in GW170817, and with sufficient angular resolution, have small fields of view compared to typical GW localization areas. Additionally, theoretical models predict that the radio emission from binary neutron star mergers can evolve over weeks to years, necessitating long-term monitoring to probe the physics of the various postmerger ejecta components. These constraints, combined with limited radio observing resources, make the development of more coordinated follow-up strategies essential—especially as the next generation of GW detectors promises a dramatic increase in detection rates. Here, we presentRADAR, a framework designed to address these challenges by promoting community-driven information sharing, federated data analysis, and system resilience, while integrating AI methods for both GW signal identification and radio data aggregation. We show that it is possible to preserve data rights while sharing models that can help design and/or update follow-up strategies. We demonstrate our approach through a case study of GW170817, and discuss future directions for refinement and broader application. 

Abstract In 2026 March, the Einstein Probe (EP) discovered its nearest (z= 0.0343) Fast X-ray Transient (FXT), EP 260321a, the first EP FXT to provide a strong match to expectations for X-ray “shock breakout” (SBO) emission. Here, we present our multiwavelength follow-up campaign of EP 260321a and its broad-line Type Ic (Ic-BL) supernova (SN) counterpart, SN 2026gzf. We show that our radio follow-up extending over 5.8–54.5 days post-FXT rules out an on-axis jet counterpart of isotropic-equivalent kinetic energyE_K ≳ 10⁴⁹ erg for circumburst densitiesn > 10⁻²cm⁻³and assuming microphysical parametersϵ_e = ϵ_B = 0.1. Our radio data also constrain a median mass-loss rate of $\dot{M}\lesssim 1.2\times 10^{-5}{M}_{\odot }{\mathrm{yr}}^{-1}$ for a Wolf–Rayet progenitor. In addition, we derive SN 2026gzf’s properties, including⁵⁶Ni mass, diffusion timescale, and expansion velocities, from our ∼nightly cadence optical data and compare them with those of optically discovered Type Ic-BL SNe, finding that SN 2026gzf is well within the 90% confidence interval across all properties. We further fit SN 2026gzf’s light curve and determine that combined emission from both interaction with CSM and⁵⁶Ni radioactive decay provides the best fit with plausible model parameters. Finally, using the rate of Ic-BL SNe from the Zwicky Transient Facility Bright Transient Survey and assuming all Type Ic-BL SNe produce EP 260321a-like FXTs, we infer an expected rate of EP-detected SBOs of 4.4–16 yr⁻¹. This is inconsistent at the 90% confidence level with current EP detection rates, potentially indicating that most Type Ic-BL SNe produce less luminous X-ray SBO signals compared to EP 260321a. 

Abstract The multimessenger detection of GW170817 showed that binary neutron star (BNS) mergers are progenitors of (at least some) short gamma-ray bursts (GRBs), and that short GRB jets (and their afterglows) can have structures (and observational properties) more complex than predicted by the standard top-hat jet scenario. Indeed, the emission from the structured jet launched in GW170817 peaked in the radio band (centimeter wavelengths) at ≈100 days since merger—a timescale much longer than the typical time span of radio follow-up observations of short GRBs. Moreover, radio searches for a potential late-time radio flare from the fast tail of the neutron-rich debris that powered the kilonova associated with GW170817 (AT 2017gfo) have extended to even longer timescales (years after the merger). In light of this, here we present the results of an observational campaign targeting a sample of seven, years-old GRBs in the Swift/BAT sample with no redshift measurements and no promptly identified X-ray counterpart. Our goal is to assess whether this sample of short GRBs could harbor nearby BNS mergers, searching for the late-time radio emission expected from their ejecta. We found one radio candidate counterpart for one of the GRBs in our sample, GRB 111126A, though an origin related to emission from star formation or from an active galactic nucleus in its host galaxy cannot be excluded without further observations.