The detection of multi-TeV gamma rays from the afterglow phase of GRB 190829A by the High Energy Stereoscopic System telescope is an addition to the already existing list of two GRBs observed in very high-energy (VHE) gamma rays in recent years. Jets of blazars and GRBs have many similarities and the photohadronic model is very successful in explaining the VHE gamma-ray spectra from the high-energy blazars. Recently, the photohadronic model has been successfully applied to study the sub-TeV gamma rays from the afterglow phases of GRB 180720B and GRB 190114C. We employed this model again to explain the VHE spectra observed for the two consecutive nights from GRB 190829A. We show that the spectra of GRB 190829A can be due to the interactions of high-energy protons with the synchrotron self-Compton photons in the forward shock region of the GRB jet, similar to the low emission state of the VHE flaring events of high-energy blazars. We speculate that, if in the future, it is possible to observe the VHE gamma-ray spectra from nearby GRBs in their afterglow phases, then some of them could only be explained by employing two different spectral indices. If confirmed, such VHE spectra could be interpreted as a result of the interactions of the high-energy protons with the photons, both from the synchrotron background and the synchrotron self-Compton background in the forward shock region.

A left-right-symmetric extension of the standard electroweak interaction model is presented in which baryon- ($B$-) and lepton- ($\mathcal{L}$-) number conservation are considered as spontaneously broken gauge symmetries. The gauge, global, and the mixed gauge-gravitational anomalies in the model are canceled by invoking new fermion matter that carries baryon as well as lepton numbers. We find a class of models in which the exotic fermions carry the same electric charges as the $N$ families of conventional fermions but different $B$ and $\mathcal{L}$ charges. The Higgs structure in this case allows a light massive neutral boson with a lower bound of 120 GeV.

On 2022 October 9, an extremely powerful gamma-ray burst, GRB 221009A, was detected by several instruments. Despite being obstructed by the Milky Way galaxy, its afterglow outburst outshone all other GRBs seen before. LHAASO detected several thousand very high energy photons extending up to 18 TeV. Detection of such energetic photons is unexpected due to the large opacity of the universe. It is possible that in the afterglow epoch, the intrinsic very high energy photon flux from the source might have increased manifolds, which could compensate for the attenuation by pair production with the extragalactic background light. We propose such a scenario and show that very high energy photons can be observed on the Earth from the interaction of very high energy protons with the seed synchrotron photons in the external forward shock region of the GRB jet.

Major outbursts have been observed from the well known high-energy-peaked blazar Markarian 501 since its discovery in 1996. Two episodes of very-high-energy gamma-ray flaring events during 2005 May–July and 2012 June are of special significance, as the source exhibited extreme HBL-like behavior. The successful standard photohadronic model does not adequately explain these extraneous behaviors. We propose a two-zone photohadronic scenario to overcome this problem. In this picture, the low-energy regime (zone-1) of the spectrum follows the standard photohadronic interpretation, while the high-energy regime (zone-2) of the spectrum is new, with a spectral index δ ₂ ≥ 3.1, which is solely due to the extreme nature of the flaring event. We also estimate the bulk Lorentz factor corresponding to these extreme flaring events. By analyzing many flaring events before and after these extreme events we argue that the extreme HBL-like events are transient and may repeat in the future.

The short-baseline neutrino oscillation experiments, the excess of radiation from the measurement of the cosmic microwave background radiation, the necessity of the nonbaryonic dark matter candidate, and the depletion of the neutrino flux in IceCube all seem to hint at new physics beyond the standard model. An economical way to address these issues is to invoke the existence of sterile neutrinos. We present simple extensions of the standard model with additionally three sterile neutrinos and discuss the corresponding PMNS like neutrino flavor mixing matrix. The noteworthy features of the sterile neutrino scenario advocated here are that the lightest one is almost degenerate with one of the active neutrinos, the second sterile has mass of order eV, and the heaviest one is in the keV range. In the present scenario, the short-baseline anomaly is explained through \(\Delta m^2\sim 1\, \mathrm{eV^2}\) , the depletion of the muon neutrino flux in IceCube is explained through \(\Delta m^2\sim 4.0\times 10^{-16}\, \mathrm{eV^2}\) , and the dark matter problem is addressed through \(\Delta m^2\sim 1\, \mathrm{keV^2}\) . Our proposed mixing matrix is also compatible with the observed neutrino oscillation data. We show that the high-energy muon and the tau neutrino fluxes from Gamma Ray Bursts can be depleted in IceCube by as much as 38 and 43 %, respectively. These substantial depletions in both muon and tau neutrino fluxes are due to their small but sizable mixing with the sterile neutrinos.