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1. TeV Neutrinos and Hard X-Rays from Relativistic Reconnection in the Corona of NGC 1068

   https://doi.org/10.3847/2041-8213/ad192b  

   Fiorillo, Damiano F. G. ; Petropoulou, Maria ; Comisso, Luca ; Peretti, Enrico ; Sironi, Lorenzo ( January 2024 , The Astrophysical Journal Letters)

   The recent discovery of astrophysical neutrinos from the Seyfert galaxy NGC 1068 suggests the presence of nonthermal protons within a compact “coronal” region close to the central black hole. The acceleration mechanism of these nonthermal protons remains elusive. We show that a large-scale magnetic reconnection layer, of the order of a few gravitational radii, may provide such a mechanism. In such a scenario, rough energy equipartition between magnetic fields, X-ray photons, and nonthermal protons is established in the reconnection region. Motivated by recent 3D particle-in-cell simulations of relativistic reconnection, we assume that the spectrum of accelerated protons is a broken power law, with the break energy being constrained by energy conservation (i.e., the energy density of accelerated protons is at most comparable to the magnetic energy density). The proton spectrum is${\mathit{dn}}_p/{\mathit{dE}}_p\propto E_p^{-1}$below the break and${\mathit{dn}}_p/{\mathit{dE}}_p\propto E_p^{-s}$above the break, with IceCube neutrino observations suggestings≃ 3. Protons above the break lose most of their energy within the reconnection layer via photohadronic collisions with the coronal X-rays, producing a neutrino signal in good agreement with the recent observations. Gamma rays injected in photohadronic collisions are cascaded to lower energies, sustaining the population of electron–positron pairs that makes the corona moderately Compton thick.

    

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2. A Population of Short-duration Gamma-Ray Bursts with Dwarf Host Galaxies

   https://doi.org/10.3847/1538-4357/ad17c0  

   Nugent, Anya E. ; Fong, Wen-fai ; Castrejon, Cristian ; Leja, Joel ; Zevin, Michael ; Ji, Alexander P. ( February 2024 , The Astrophysical Journal)

   We present a population of 11 of the faintest (>25.5 AB mag) short gamma-ray burst (GRB) host galaxies. We model their sparse available observations using the stellar population inference codeProspector-βand develop a novel implementation to incorporate the galaxy mass–radius relation. Assuming these hosts are randomly drawn from the galaxy population and conditioning this draw on their observed flux and size in a few photometric bands, we determine that these hosts have dwarf galaxy stellar masses of$7.0\lesssim \log (M_{*}/M_{\odot })\lesssim 9.1$. This is striking as only 14% of short GRB hosts with previous inferred stellar masses hadM_*≲ 10⁹M_⊙. We further show these short GRBs have smaller physical and host-normalized offsets than the rest of the population, suggesting that the majority of their neutron star (NS) merger progenitors were retained within their hosts. The presumably shallow potentials of these hosts translate to small escape velocities of ∼5.5–80 km s⁻¹, indicative of either low postsupernova systemic velocities or short inspiral times. While short GRBs with identified dwarf host galaxies now comprise ≈14% of the total Swift-detected population, a number are likely missing in the current population, as larger systemic velocities (observed from the Galactic NS population) would result in highly offset short GRBs and less secure host associations. However, the revelation of a population of short GRBs retained in low-mass host galaxies offers a natural explanation for the observedr-process enrichment via NS mergers in Local Group dwarf galaxies, and has implications for gravitational-wave follow-up strategies.

    

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3. Interpretation of the Observed Neutrino Emission from Three Tidal Disruption Events 

   https://doi.org/10.3847/1538-4357/acbe9e  

   Winter, Walter ; Lunardini, Cecilia ( May 2023 , The Astrophysical Journal)

   Three Tidal Disruption Event candidates (AT2019dsg, AT2019fdr, and AT2019aalc) have been associated with high-energy astrophysical neutrinos in multimessenger follow-ups. In all cases, the neutrino observation occurred$\mathit{}(100)$days after the maximum of the optical-ultraviolet (OUV) luminosity. We discuss unified fully time-dependent interpretations of the neutrino signals where the neutrino delays are not a statistical effect, but rather the consequence of a physical scale of the post-disruption system. Noting that X-ray flares and infrared (IR) dust echoes have been observed in all cases, we consider three models in which quasi-isotropic neutrino emission is due to the interactions of accelerated protons of moderate, medium, and ultra-high energies with X-rays, OUV, and IR photons, respectively. We find that the neutrino time delays can be well described in the X-ray model assuming magnetic confinement of protons in a calorimetric approach if the unobscured X-ray luminosity is roughly constant over time, and in the IR model, where the delay is directly correlated with the time evolution of the echo luminosity (for which a model is developed here). The OUV model exhibits the highest neutrino production efficiency. In all three models, the highest neutrino fluence is predicted for AT2019aalc, due to its high estimated supermassive black hole mass and low redshift. All models result in diffuse neutrino fluxes that are consistent with observations.

    

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4. The First Short GRB Millimeter Afterglow: The Wide-angled Jet of the Extremely Energetic SGRB 211106A

   https://doi.org/10.3847/2041-8213/ac8421  

   Laskar, Tanmoy ; Escorial, Alicia Rouco ; Schroeder, Genevieve ; Fong, Wen-fai ; Berger, Edo ; Veres, Péter ; Bhandari, Shivani ; Rastinejad, Jillian ; Kilpatrick, Charles D. ; Tohuvavohu, Aaron ; et al ( August 2022 , The Astrophysical Journal Letters)

   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,A_V≳ 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 at$t_{\mathrm{jet}}={29.2}_{-4.0}^{+4.5}$days. For a presumed redshift ofz= 1, we infer an opening angle,θ_jet= (15.°5 ± 1.°4), and beaming-corrected kinetic energy of$\log (E_{\mathrm{K}}/\mathrm{erg})=51.8\pm 0.3$, 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 should 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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5. Inverse Compton signatures of gamma-ray burst afterglows

   https://doi.org/10.1093/mnras/staa1583  

   Zhang, H ; Christie, I M ; Petropoulou, M ; Rueda-Becerril, J M ; Giannios, D ( July 2020 , Monthly Notices of the Royal Astronomical Society)

   ABSTRACT The afterglow emission from gamma-ray bursts (GRBs) is believed to originate from a relativistic blast wave driven into the circumburst medium. Although the afterglow emission from radio up to X-ray frequencies is thought to originate from synchrotron radiation emitted by relativistic, non-thermal electrons accelerated by the blast wave, the origin of the emission at high energies (HE; ≳GeV) remains uncertain. The recent detection of sub-TeV emission from GRB 190114C by the Major Atmospheric Gamma Imaging Cherenkov Telescopes (MAGIC) raises further debate on what powers the very high energy (VHE; ≳300 GeV) emission. Here, we explore the inverse Compton scenario as a candidate for the HE and VHE emissions, considering two sources of seed photons for scattering: synchrotron photons from the blast wave (synchrotron self-Compton or SSC) and isotropic photon fields external to the blast wave (external Compton). For each case, we compute the multiwavelength afterglow spectra and light curves. We find that SSC will dominate particle cooling and the GeV emission, unless a dense ambient infrared photon field, typical of star-forming regions, is present. Additionally, considering the extragalactic background light attenuation, we discuss the detectability of VHE afterglows by existing and future gamma-ray instruments for a wide range of model parameters. Studying GRB 190114C, we find that its afterglow emission in the Fermi-Large Area Telescope (LAT) band is synchrotron dominated. The late-time Fermi-LAT measurement (i.e. t ∼ 104 s), and the MAGIC observation also set an upper limit on the energy density of a putative external infrared photon field (i.e. ${\lesssim} 3\times 10^{-9}\, {\rm erg\, cm^{-3}}$), making the inverse Compton dominant in the sub-TeV energies. 

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