Gamma-ray bursts (GRBs) are considered as promising sources of ultra-high-energy cosmic rays (UHECRs) due to their large power output. Observing a neutrino flux from GRBs would offer evidence that GRBs are hadronic accelerators of UHECRs. Previous IceCube analyses, which primarily focused on neutrinos arriving in temporal coincidence with the prompt gamma-rays, found no significant neutrino excess. The four analyses presented in this paper extend the region of interest to 14 days before and after the prompt phase, including generic extended time windows and targeted precursor searches. GRBs were selected between 2011 May and 2018 October to align with the data set of candidate muon-neutrino events observed by IceCube. No evidence of correlation between neutrino events and GRBs was found in these analyses. Limits are set to constrain the contribution of the cosmic GRB population to the diffuse astrophysical neutrino flux observed by IceCube. Prompt neutrino emission from GRBs is limited to ≲1% of the observed diffuse neutrino flux, and emission on timescales up to 10⁴ s is constrained to 24% of the total diffuse flux.

Very long baseline interferometry (VLBI) allows for high-resolution and high-sensitivity observations of relativistic jets, that can reveal periodicities of several years in their structure. We perform an analysis of long-term VLBI data of the quasar S5 1928+738 in terms of a geometric model of a helical structure projected onto the plane of the sky. We monitor the direction of the jet axis through its inclination and position angles. We decompose the variation of the inclination of the inner 2 mas of the jet of S5 1928+738 into a periodic term with amplitude of ∼0$ $_{.}^{\circ}$$89 and a linear decreasing trend with rate of ∼0$ $_{.}^{\circ}$$05 yr⁻¹. We also decompose the variation of the position angle into a periodic term with amplitude of ∼3$ $_{.}^{\circ}$$39 and a linear increasing trend with rate of ∼0$ $_{.}^{\circ}$$24 yr⁻¹. We interpret the periodic components as arising from the orbital motion of a binary black hole inspiralling at the jet base and derive corrected values of the mass ratio and separation from the accumulated 18 yr of VLBI data. Then, we identify the linear trends in the variations as due to the slow reorientation of the spin of the jet emitter black hole induced by the spin–orbit precession and we determine the precession period T_SO = 4852 ± 646 yr of the more massive black hole, acting as the jet emitter. Our study provides indications, for the first time from VLBI jet kinematics, for the spinning nature of the jet-emitting black hole.

The combined study of the flaring of active galactic nuclei (AGNs) at radio wavelengths and parsec-scale jet kinematics with Very Long Baseline Interferometry has led to the view that (i) the observed flares are associated with ejections of synchrotron blobs from the core, and (ii) most of the flaring follows a one-to-one correlation with the ejection of the component. Recent results have added to the mounting evidence showing that the quasi-regular component injections into the relativistic jet may not be the only cause of the flux variability. We propose that AGN flux variability and changes in jet morphology can both be of deterministic nature, i.e., having a geometric/kinetic origin linked to the time-variable Doppler beaming of the jet emission as its direction changes due to precession (and nutation). The physics of the underlying jet leads to shocks, instabilities, or ejections of plasmoids. The appearance (morphology, flux, etc.) of the jet can, however, be strongly affected and modulated by precession. We demonstrate this modulating power of precession for OJ 287. For the first time, we show that the spectral state of the spectral energy distribution (SED) can be directly related to the jet’s precession phase. We model the SED evolution and reproduce the precession parameters. Further, we apply our precession model to 11 prominent AGNs. We show that for OJ 287 precession seems to dominate the long-term variability (≳1 yr) of the AGN flux, SED spectral state, and jet morphology, while stochastic processes affect the variability on short timescales (≲0.2 yr).

Despite the uncovered association of a high-energy neutrino with the apparent flaring state of blazar TXS 0506+056 in 2017, the mechanisms leading to astrophysical particle acceleration and neutrino production are still uncertain. Recent studies found that when transparent to γ-rays, γ-flaring blazars do not have the opacity for protons to produce neutrinos. Here we present observational evidence for an alternative explanation, in which γ-ray emission is suppressed during efficient neutrino production. A large proton and target photon density helps produce neutrinos while temporarily suppressing the observable γ-emission due to a large γ γ opacity. We show that the Fermi-LAT γ-flux of blazar PKS 1502+106 was at a local minimum when IceCube recorded the coincident high-energy neutrino IC-190730A. Using data from the OVRO 40 m Telescope, we find that radio emission from PKS 1502+106 at the time period of the coincident neutrino IC-190730A was in a high state, in contrast to earlier time periods when radio and γ fluxes are correlated for both low and high states. This points to an active outflow that is γ-suppressed at the time of neutrino production. We find similar local γ-suppression in other blazars, including in MAGIC’s TeV flux of TXS 0506+056 and Fermi-LAT’s flux of blazar PKS B1424-418 at the time of coincident IceCube neutrino detections. Using temporary γ-suppression, neutrino–blazar coincidence searches could be substantially more sensitive than previously assumed, enabling the identification of the origin of IceCube’s diffuse neutrino flux possibly with already existing data.

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