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Synchrotron emission is seen in a vast array of astrophysical transients, such as gamma-ray bursts, radio supernovae, neutron star mergers, tidal disruption events (TDEs), and fast blue optical transients (FBOTs). Despite the ubiquity of synchrotron-emitting sources, modeling of the emergent flux from these events often relies on simplified analytic approximations for the radiative transfer. These approximations are inaccurate for high-velocity shocks, where special-relativistic effects are important. Properly incorporating these effects considerably complicates calculations, and generally requires a numerical treatment. In this work, we present a novel numerical model which solves the full radiative-transfer problem in synchrotron-emitting shocks, accounting for all relativistic effects. This “full-volume” model is capable of calculating synchrotron emission from a shock of arbitrary velocity, and is designed to be flexible and applicable to a wide range of astrophysical sources. Using this new code, we evaluate the accuracy of more commonly used approximate models. We find that the full-volume treatment is generally necessary once the shock proper velocity exceeds Γβ≳0.1, and that approximate models can be inaccurate by ≳ an order of magnitude in transrelativistic shocks. This implies that there may be a bias in the inferred physical properties of some FBOTs, jetted TDEs, and other relativistic explosions, where approximate analytic models are typically employed. The code associated with our model is made publicly available, and can be used to study the growing population of relativistic synchrotron-emitting transients. 

Abstract We presentFrankenBlast, a customized and improved version of theBlastweb application.FrankenBlastassociates transients to their host galaxies, performs host photometry, and runs a innovative spectral energy distribution fitting code to constrain host stellar population properties—all within minutes per object. We testFrankenBlaston 14,432 supernovae (SNe), ≈half of which are spectroscopically classified, and are able to constrain host properties for 9262 events. When contrasting the host stellar masses (M_*), specific star formation rates (sSFR), and host dust extinction (A_V) between spectroscopically and photometrically classified SNe Ia, Ib/c, II, and IIn, we determine that deviations in these distributions are primarily due to misclassified events contaminating the photometrically classified sample. We further show that the higher redshifts of the photometrically classified sample also force theirM_*and sSFR distributions to deviate from those of the spectroscopically classified sample, as these properties are redshift-dependent. We compare host properties between spectroscopically classified SN populations and determine if they primarily traceM_*or SFR. We find that all SN populations seem to both depend onM_*and SFR, with SNe II and IIn somewhat more SFR-dependent than SNe Ia and Ib/c, and SNe Ia moreM_*-dependent than all other classes. We find the difference in the SNe Ib/c and II hosts to be the most intriguing and speculate that SNe Ib/c must be more dependent on higherM_*and more evolved environments for the right conditions for progenitor formation. All data products andFrankenBlastare publicly available, along with a developingFrankenBlastversion intended for Rubin Observatory science products. 

Abstract Both the core collapse of rotating massive stars, and the coalescence of neutron star (NS) binaries result in the formation of a hot, differentially rotating NS remnant. The timescales over which differential rotation is removed by internal angular-momentum transport processes ( viscosity ) have key implications for the remnant’s long-term stability and the NS equation of state (EOS). Guided by a nonrotating model of a cooling proto-NS, we estimate the dominant sources of viscosity using an externally imposed angular-velocity profile Ω( r ). Although the magneto-rotational instability provides the dominant source of effective viscosity at large radii, convection and/or the Tayler–Spruit dynamo dominate in the core of merger remnants where d Ω/ dr ≥ 0. Furthermore, the viscous timescale in the remnant core is sufficiently short that solid-body rotation will be enforced faster than matter is accreted from rotationally supported outer layers. Guided by these results, we develop a toy model for how the merger remnant core grows in mass and angular momentum due to accretion. We find that merger remnants with sufficiently massive and slowly rotating initial cores may collapse to black holes via envelope accretion, even when the total remnant mass is less than the usually considered threshold ≈1.2 M TOV for forming a stable solid-body rotating NS remnant (where M TOV is the maximum nonrotating NS mass supported by the EOS). This qualitatively new picture of the post-merger remnant evolution and stability criterion has important implications for the expected electromagnetic counterparts from binary NS mergers and for multimessenger constraints on the NS EOS. 

Abstract We present high-resolution 1.5–6 GHz Karl G. Jansky Very Large Array and Hubble Space Telescope (HST) optical and infrared observations of the extremely active repeating fast radio burst (FRB) FRB 20201124A and its barred spiral host galaxy. We constrain the location and morphology of star formation in the host and search for a persistent radio source (PRS) coincident with FRB 20201124A. We resolve the morphology of the radio emission across all frequency bands and measure a star formation rate (SFR) ≈ 8.9M_⊙yr⁻¹, approximately ≈2.5–6 times larger than optically inferred SFRs, demonstrating dust-obscured star formation throughout the host. Compared to a sample of all known FRB hosts with radio emission, the host of FRB 20201124A has the most significantly obscured star formation. While HST observations show the FRB to be offset from the bar or spiral arms, the radio emission extends to the FRB location. We propose that the FRB progenitor could have formed in situ (e.g., a magnetar born from a massive star explosion). It is still plausible, although less likely, that the progenitor of FRB 20201124A migrated from the central bar of the host. We further place a limit on the luminosity of a putative PRS at the FRB position ofL_6.0GHz≲ 1.8 ×10²⁷erg s⁻¹Hz⁻¹, among the deepest PRS luminosity limits to date. However, this limit is still broadly consistent with both magnetar nebulae and hypernebulae models assuming a constant energy injection rate of the magnetar and an age of ≳10⁵yr in each model, respectively.