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  1. Abstract

    Mounting evidence suggests that luminous fast blue optical transients (LFBOTs) are powered by a compact object, launching an asymmetric and fast outflow responsible for the radiation observed in the ultraviolet, optical, infrared, radio, and X-ray bands. Proposed scenarios aiming to explain the electromagnetic emission include an inflated cocoon, surrounding a jet choked in the extended stellar envelope. Alternatively, the observed radiation may arise from the disk formed by the delayed merger of a black hole with a Wolf–Rayet star. We explore the neutrino production in these scenarios, i.e., internal shocks in a choked jet and interaction between the outflow and the circumstellar medium (CSM). If observed on axis, the choked jet provides the dominant contribution to the neutrino fluence. Intriguingly, the IceCube upper limit on the neutrino emission inferred from the closest LFBOT, AT2018cow, excludes a region of the parameter space otherwise allowed by electromagnetic observations. After correcting for the Eddington bias on the observation of cosmic neutrinos, we conclude that the emission from an on-axis choked jet and CSM interaction is compatible with the detection of two track-like neutrino events observed by the IceCube Neutrino Observatory in coincidence with AT2018cow, and otherwise considered to be of atmospheric origin.more »While the neutrino emission from LFBOTs does not constitute the bulk of the diffuse background of neutrinos observed by IceCube, the detection prospects of nearby LFBOTs with IceCube and the upcoming IceCube-Gen2 are encouraging. Follow-up neutrino searches will be crucial for unraveling the mechanism powering this emergent transient class.

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  2. Abstract Initially classified as a Type Ib supernova (SN), ∼100 days after the explosion SN 2014C made a transition to a Type II SN, presenting a gradual increase in the H α emission. This has been interpreted as evidence of interaction between the SN shock wave and a massive shell previously ejected from the progenitor star. In this paper we present numerical simulations of the propagation of the SN shock through the progenitor star and its wind, as well as the interaction of the SN ejecta with the massive shell. To determine with high precision the structure and location of the shell, we couple a genetic algorithm to a hydrodynamic and a bremsstrahlung radiation transfer code. We iteratively modify the density stratification and location of the shell by minimizing the variance between X-ray observations and synthetic predictions computed from the numerical model, allowing the shell structure to be completely arbitrary. By assuming spherical symmetry, we found that our best-fit model has a shell mass of 2.6 M ⊙ ; extends from 1.6 × 10 16 cm to 1.87 × 10 17 cm, implying that it was ejected ∼ 60/( v w /100 km s −1 ) yr before the SNmore »explosion; and has a density stratification with an average behavior ∼ r −3 but presenting density fluctuations larger than one order of magnitude. Finally, we predict that if the density stratification follows the same power-law behavior, the SN will break out from the shell by mid-2022, i.e., 8.5 yr after explosion.« less
    Free, publicly-accessible full text available May 1, 2023
  3. Abstract

    We present the stellar population properties of 69 short gamma-ray burst (GRB) host galaxies, representing the largest uniformly modeled sample to date. Using theProspectorstellar population inference code, we jointly fit photometry and/or spectroscopy of each host galaxy. We find a population median redshift ofz=0.640.32+0.83(68% confidence), including nine photometric redshifts atz≳ 1. We further find a median mass-weighted age oftm=0.80.53+2.71Gyr, stellar mass of log(M*/M) =9.690.65+0.75, star formation rate of SFR =1.441.35+9.37Myr−1, stellar metallicity of log(Z*/Z) =0.380.42+0.44, and dust attenuation ofAV=0.430.36+0.85mag (68% confidence). Overall, the majority of short GRB hosts are star-forming (≈84%), with small fractions that are either transitioning (≈6%) or quiescent (≈10%); however, we observe a much larger fraction (≈40%) of quiescent and transitioning hosts atz≲ 0.25, commensurate with galaxy evolution. We find that short GRB hosts populate the star-forming main sequence of normal field galaxies, but do not include as many high-mass galaxies as the general galaxy population, implying that their binary neutron star (BNS) merger progenitors are dependent on a combination of host star formation and stellar mass. The distribution of ages and redshifts implies a broad delay-time distribution,more »with a fast-merging channel atz> 1 and a decreased neutron star binary formation efficiency from high to low redshifts. If short GRB hosts are representative of BNS merger hosts within the horizon of current gravitational wave detectors, these results can inform future searches for electromagnetic counterparts. All of the data and modeling products are available on the Broadband Repository for Investigating Gamma-ray burst Host Traits website.

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  4. We describe the first observations of the same celestial object with gravitational waves and light. ▪  GW170817 was the first detection of a neutron star merger with gravitational waves. ▪  The detection of a spatially coincident weak burst of gamma-rays (GRB 170817A) 1.7 s after the merger constituted the first electromagnetic detection of a gravitational wave source and established a connection between at least some cosmic short gamma-ray bursts (SGRBs) and binary neutron star mergers. ▪  A fast-evolving optical and near-infrared transient (AT 2017gfo) associated with the event can be interpreted as resulting from the ejection of ∼0.05 M ⊙ of material enriched in r-process elements, finally establishing binary neutron star mergers as at least one source of r-process nucleosynthesis. ▪  Radio and X-ray observations revealed a long-rising source that peaked ∼160,d after the merger. Combined with the apparent superluminal motion of the associated very long baseline interferometry source, these observations show that the merger produced a relativistic structured jet whose core was oriented ≈20 deg from the line of sight and with properties similar to SGRBs. The jet structure likely results from interaction between the jet and the merger ejecta. ▪  The electromagnetic and gravitational wave information can bemore »combined to produce constraints on the expansion rate of the Universe and the equation of state of dense nuclear matter. These multimessenger endeavors will be a major emphasis of future work.« less
  5. Abstract

    We present the discovery of the first millimeter afterglow of a short-durationγ-ray burst (SGRB) and the first confirmed afterglow of an SGRB localized by the GUANO system on Swift. Our Atacama Large Millimeter/Sub-millimeter Array (ALMA) detection of SGRB 211106A establishes an origin in a faint host galaxy detected in Hubble Space Telescope imaging at 0.7 ≲z≲ 1.4. From the lack of a detectable optical afterglow, coupled with the bright millimeter counterpart, we infer a high extinction,AV≳ 2.6 mag along the line of sight, making this one of the most highly dust-extincted SGRBs known to date. The millimeter-band light curve captures the passage of the synchrotron peak from the afterglow forward shock and reveals a jet break attjet=29.24.0+4.5days. For a presumed redshift ofz= 1, we infer an opening angle,θjet= (15.°5 ± 1.°4), and beaming-corrected kinetic energy oflog(EK/erg)=51.8±0.3, making this one of the widest and most energetic SGRB jets known to date. Combining all published millimeter-band upper limits in conjunction with the energetics for a large sample of SGRBs, we find that energetic outflows in high-density environments are more likely to have detectable millimeter counterparts. Concerted afterglow searches with ALMA shouldmore »yield detection fractions of 24%–40% on timescales of ≳2 days at rates of ≈0.8–1.6 per year, outpacing the historical discovery rate of SGRB centimeter-band afterglows.

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  6. Abstract

    We present a comprehensive optical and near-infrared census of the fields of 90 short gamma-ray bursts (GRBs) discovered in 2005–2021, constituting all short GRBs for which host galaxy associations are feasible (≈60% of the total Swift short GRB population). We contribute 274 new multi-band imaging observations across 58 distinct GRBs and 26 spectra of their host galaxies. Supplemented by literature and archival survey data, the catalog contains 542 photometric and 42 spectroscopic data sets. The photometric catalog reaches 3σdepths of ≳24–27 mag and ≳23–26 mag for the optical and near-infrared bands, respectively. We identify host galaxies for 84 bursts, in which the most robust associations make up 56% (50/90) of events, while only a small fraction, 6.7%, have inconclusive host associations. Based on new spectroscopy, we determine 18 host spectroscopic redshifts with a range ofz≈ 0.15–1.5 and find that ≈23%–41% of Swift short GRBs originate fromz> 1. We also present the galactocentric offset catalog for 84 short GRBs. Taking into account the large range of individual measurement uncertainties, we find a median of projected offset of ≈7.7 kpc, for which the bursts with the most robust associations have a smaller median of ≈4.8 kpc. Our catalog captures more high-redshiftmore »and low-luminosity hosts, and more highly offset bursts than previously found, thereby diversifying the population of known short GRB hosts and properties. In terms of locations and host luminosities, the populations of short GRBs with and without detectable extended emission are statistically indistinguishable. This suggests that they arise from the same progenitors, or from multiple progenitors, which form and evolve in similar environments. All of the data products are available on the Broadband Repository for Investigating Gamma-Ray Burst Host Traits website.

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  7. Abstract We present the complete set of Hubble Space Telescope imaging of the binary neutron star merger GW170817 and its optical counterpart AT 2017gfo. Including deep template imaging in F814W, F110W, F140W, and F160W at 3.4 yr post-merger, we reanalyze the full light curve of AT 2017gfo across 12 bands from 5 to 1273 rest-frame days after merger. We obtain four new detections of the short γ -ray burst 170817A afterglow from 109 to 170 rest-frame days post-merger. These detections are consistent with the previously observed β = −0.6 spectral index in the afterglow light curve with no evidence for spectral evolution. We also analyze our limits in the context of kilonova afterglow or IR dust echo emission but find that our limits are not constraining for these models. We use the new data to construct deep optical and IR stacks, reaching limits of M = −6.3 to −4.6 mag, to analyze the local environment around AT 2017gfo and low surface brightness features in its host galaxy NGC 4993. We rule out the presence of any globular cluster at the position of AT 2017gfo to 2.3 × 10 4 L ⊙ , including those with the reddest V − Hmore »colors. Finally, we analyze the substructure of NGC 4993 in deep residual imaging and find shell features that extend up to 71.″8 (14.2 kpc) from NGC 4993. The shells have a cumulative stellar mass of 6.3 × 10 8 M ⊙ , roughly 2% of NGC 4993, and mass-weighted ages of >3 Gyr. We conclude that it was unlikely that the GW170817 progenitor system formed in the galaxy merger.« less
    Free, publicly-accessible full text available February 1, 2023
  8. Abstract

    Calcium-rich (Ca-rich) transients are a new class of supernovae (SNe) that are known for their comparatively rapid evolution, modest peak luminosities, and strong nebular calcium emission lines. Currently, the progenitor systems of Ca-rich transients remain unknown. Although they exhibit spectroscopic properties not unlike core-collapse Type Ib/c SNe, nearly half are found in the outskirts of their host galaxies, which are predominantly elliptical, suggesting a closer connection to the older stellar populations of SNe Ia. In this paper, we present a compilation of publicly available multiwavelength observations of all known and/or suspected host galaxies of Ca-rich transients ranging from far-UV to IR, and use these data to characterize their stellar populations withprospector. We estimate several galaxy parameters including integrated star formation rate, stellar mass, metallicity, and age. For nine host galaxies, the observations are sensitive enough to obtain nonparametric star formation histories, from which we recover SN rates and estimate probabilities that the Ca-rich transients in each of these host galaxies originated from a core-collapse versus Type Ia-like explosion. Our work supports the notion that the population of Ca-rich transients do not come exclusively from core-collapse explosions, and must either be only from white dwarf stars or a mixed populationmore »of white dwarf stars with other channels, potentially including massive star explosions. Additional photometry and explosion site spectroscopy of larger samples of Ca-rich host galaxies will improve these estimates and better constrain the ratio of white dwarf versus massive star progenitors of Ca-rich transients.

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  9. ABSTRACT

    Evidence is mounting that recent multiwavelength detections of fast blue optical transients (FBOTs) in star-forming galaxies comprise a new class of transients, whose origin is yet to be understood. We show that hydrogen-rich collapsing stars that launch relativistic jets near the central engine can naturally explain the entire set of FBOT observables. The jet–star interaction forms a mildly relativistic shocked jet (inner cocoon) component, which powers cooling emission that dominates the high velocity optical signal during the first few weeks, with a typical energy of ∼1050–1051 erg. During this time, the cocoon radial energy distribution implies that the optical light curve exhibits a fast decay of $L \,\, \buildrel\propto \over \sim \,\,t^{-2.4}$. After a few weeks, when the velocity of the emitting shell is ∼0.01 c, the cocoon becomes transparent, and the cooling envelope governs the emission. The interaction between the cocoon and the dense circumstellar winds generates synchrotron self-absorbed emission in the radio bands, featuring a steady rise on a month time-scale. After a few months the relativistic outflow decelerates, enters the observer’s line of sight, and powers the peak of the radio light curve, which rapidly decays thereafter. The jet (and the inner cocoon) becomes optically thinmore »to X-rays ∼day after the collapse, allowing X-ray photons to diffuse from the central engine that launched the jet to the observer. Cocoon cooling emission is expected at higher volumetric rates than gamma-ray bursts (GRBs) by a factor of a few, similar to FBOTs. We rule out uncollimated outflows, however, both GRB jets and failed collimated jets are compatible with all observables.

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  10. Abstract We present a population of 19 radio-luminous supernovae (SNe) with emission reaching L ν ∼ 10 26 –10 29 erg s −1 Hz −1 in the first epoch of the Very Large Array Sky Survey (VLASS) at 2–4 GHz. Our sample includes one long gamma-ray burst, SN 2017iuk/GRB 171205A, and 18 core-collapse SNe detected at ≈1–60 yr after explosion. No thermonuclear explosion shows evidence for bright radio emission, and hydrogen-poor progenitors dominate the subsample of core-collapse events with spectroscopic classification at the time of explosion (79%). We interpret these findings in the context of the expected radio emission from the forward shock interaction with the circumstellar medium (CSM). We conclude that these observations require a departure from the single wind–like density profile (i.e., ρ CSM ∝ r −2 ) that is expected around massive stars and/or from a spherical Newtonian shock. Viable alternatives include the shock interaction with a detached, dense shell of CSM formed by a large effective progenitor mass-loss rate, M ̇ ∼ 10 − 4 – 10 − 1 M ⊙ yr −1 (for an assumed wind velocity of 1000 km s −1 ); emission from an off-axis relativistic jet entering our line of sight; ormore »the emergence of emission from a newly born pulsar-wind nebula. The relativistic SN 2012ap that is detected 5.7 and 8.5 yr after explosion with L ν ∼ 10 28 erg s −1 Hz −1 might constitute the first detections of an off-axis jet+cocoon system in a massive star. However, none of the VLASS SNe with archival data points are consistent with our model off-axis jet light curves. Future multiwavelength observations will distinguish among these scenarios. Our VLASS source catalogs, which were used to perform the VLASS cross-matching, are publicly available at https://doi.org/10.5281/zenodo.4895112 .« less
    Free, publicly-accessible full text available December 1, 2022