High-energy photons from the Crab Nebula: The Crab Nebula contains a pulsar that excites the surrounding gas to emit high-energy radiation. The combination of the pulsar's youth and nearby location makes the nebula the brightest gamma-ray source in the sky. The LHAASO Collaboration report observations of this source at energies of tera– to peta–electron volts, extending the spectrum of this prototypical object. They combine these data with observations at lower energies to model the physics of the emission process. The multiwave-length data can be explained by a combination of synchrotron radiation and inverse Compton scattering. Science , abg5137, this issue p. 425

The increasingly deep limit on the neutrino emission from gamma-ray bursts (GRBs) with IceCube observations has reached a level that could place useful constraints on the fireball properties. We first present a revised analytic calculation of the neutrino flux that predicts a flux of one order of magnitude lower than that obtained by the IceCube Collaboration. For the benchmark model parameters (e.g., the bulk Lorentz factor is Γ = 10^2.5, the observed variability time for the long GRBs is t ^ob _v = 0.01 s, and the ratio between the energy in the accelerated protons and in the radiation is η _p = 10 for every burst) in the standard internal shock scenario, the predicted neutrino flux from 215 bursts during the period of the 40- and 59-string configurations is a factor of ~3 below the IceCube sensitivity. However, if we accept the recently found inherent relation between the bulk Lorentz factor and the burst energy, then the expected neutrino flux significantly increases and the spectral peak shifts to a lower energy. In this case, the nondetection implies that the baryon-loading ratio should be η _p 10 if the variability time of the long GRBs is fixed to t ^ob _v = 0.01 s. Instead, if we relax the standard internal-shock scenario but still assume η _p = 10, then the nondetection constrains the dissipation radius, R 4 × 10¹² cm, assuming the same dissipation radius for every burst and benchmark parameters for the fireballs. We also calculate the diffuse neutrino flux from the GRBs for different luminosity functions from the literature. The expected flux exceeds the current IceCube limit for some of the luminosity functions, and, thus, the nondetection constrains η _p 10 when the variability time of the long GRBs is fixed at t ^ob _v = 0.01 s.

Recently, very high-energy photons above 100 GeV were reported to be detected from GRB 190114C and GRB 180720B at, respectively, 100–1000 s and 10 hr after the burst. We model the available broadband data of both GRBs with the synchrotron plus synchrotron self-Compton (SSC) emission of the afterglow shocks. We find that the sub-TeV emission of GRB 180720B can be interpreted as the SSC emission from afterglow shocks expanding in a constant-density circumburst medium. The SSC emission of GRB 190114C dominates over the synchrotron component from GeV energies at ∼100 s, which can explain the possible hard spectrum of the GeV emission at this time. The extrapolated flux of this SSC component to sub-TeV energies can explain the high-significance detection of GRB 190114C by the MAGIC telescope. The parameter values (such as the circumburst density and shock microphysical parameters) in the modeling are not unusual for both gamma-ray bursts, implying that the detection of sub-TeV photons from these two bursts should be attributed to their large burst energies and low redshifts.

A high-energy muon neutrino event, IceCube-170922A, was recently discovered in both spatial and temporal coincidence with a gamma-ray flare of the blazar TXS 0506+056. It has been shown with standard one-zone models that neutrinos can be produced in the blazar jet via hadronic interactions, but with a flux that is mostly limited by the X-ray data. In this work, we explore the neutrino production from TXS 0506+056 by invoking two physically distinct emission zones in the jet, with an inner blob inside of or close to the broad-line region (BLR) and an outer one well beyond the BLR. Using the Doppler-boosted radiation of the BLR as the target photon field, the inner zone accounts for the neutrino and gamma-ray emission via pγ interactions and inverse Compton scattering, respectively, while the outer zone produces the optical and X-ray emission via synchrotron and synchrotron self-Compton processes. The different conditions of the two zones allow us to suppress the X-ray emission from the electromagnetic cascade, and set a much higher upper limit on the muon neutrino flux (i.e., ∼10⁻¹¹ erg cm⁻² s⁻¹) than in one-zone models. We compare our scenario in detail with one-zone models discussed in the literature, and argue that differentiating between such scenarios will become possible with next-generation neutrino telescopes, such as IceCube-Gen2.

Although many high-energy neutrinos detected by the IceCube telescope are believed to have an extraterrestrial origin, their astrophysical sources remain a mystery. Recently, an unprecedented discovery of a high-energy muon neutrino event coincident with a multiwavelength flare from a blazar, TXS $0506+056$, shed some light on the origin of the neutrinos. It is usually believed that a blazar is produced by a relativistic jet launched from an accreting supermassive black hole (SMBH). Here, we show that the high-energy neutrino event can be interpreted by the inelastic hadronuclear interactions between the accelerated cosmic-ray protons in the relativistic jet and the dense gas clouds in the vicinity of the SMBH. Such a scenario only requires a moderate proton power in the jet, which could be much smaller than that required in the conventional hadronic model which instead calls upon the photomeson process. Meanwhile, the flux of the multiwavelength flare from the optical to gamma-ray band can be well explained by invoking a second radiation zone in the jet at a larger distance to the SMBH. In our model, the neutrino emission lasts a shorter time than the multiwavelength flare, so the neutrino event is not necessarily correlated with the flare, but it is probably accompanied by a spectrum hardening above a few giga-electron-volt (GeV).