High Energy Astrophysical Phenomena (astro-ph.HE)

Abstract: Recent independent announcements by several collaborations have shown strong evidence of a Stochastic Gravitational-Wave Background (SGWB) detected through Pulsar Timing Arrays (PTAs). In this study, we investigate the implications of a first-order phase transition occurring within the early universe's dark quantum chromodynamics (dQCD) epoch, specifically within the framework of the mirror twin Higgs dark sector model. Our analysis indicates a distinguishable SGWB signal originating from this phase transition, which can explain the measurements obtained by PTAs. Remarkably, a significant portion of the parameter space within the mirror twin Higgs model that accounts for the SGWB signal also effectively resolves the existing tensions in both the $H_0$ and $S_8$ measurements in Cosmology. This intriguing correlation suggests a possible common origin for these three phenomena. Furthermore, the parameter region, $0.2 < \Delta N_{\rm eff} < 0.5$, where the mirror dark matter component constitutes less than $30\%$ of the total dark matter abundance, can accommodate all current cosmological observations and PTA measurements.

Abstract: The nearest GRB 170817A provided an opportunity to probe the angular structure of the jet of this short gamma-ray burst (SGRB), by using its off-axis observed afterglow emission. It is investigated that whether the afterglow-constrained jet structures can be consistent with the luminosity of the prompt emission of GRB 170817A. Furthermore, by assuming that all SGRBs including GRB 170817A have the same explosive mechanism and jet structure, we apply the different jet structures into the calculation of the flux and redshfit distributions of the SGRB population, in comparison with the observational distributions of the Swift and Fermi sources. As a result, it is found that the single-Gaussian structure can be basically ruled out, whereas the power-law and two-Gaussian models can in principle survive.

Abstract: Radio blazars have been linked both to individual high-energy neutrino events and to excesses in likelihood sky maps constructed from lower-energy neutrino data. However, the exact mechanism by which neutrinos are produced in these sources is still unknown. Here, we demonstrate that IceCube neutrinos with energies over 200 TeV, which were previously associated with bright radio blazars, are significantly more likely to be accompanied by flares of lower-energy events, compared to those lacking blazar counterparts. The parsec-scale core radio flux of blazars positioned within the error regions of energetic events is strongly correlated with the likelihood of a coincident day-scale lower-energy neutrino flare reported by IceCube. The probability of a chance correlation is 3.6*10^{-4}. This confirms the neutrino-blazar connection in a new and independent way, and provides valuable clues to understanding the origin of astrophysical neutrinos.

Abstract: The stability of astrophysical jets in the linear regime is investigated by presenting the methodology to find the growth rates of the various instabilities. We perturb a cylindrical axisymmetric steady jet, linearize the relativistic ideal magnetohydrodynamic (MHD) equations, and analyze the evolution of the eigenmodes of the perturbation by deriving the differential equations that need to be integrated subject to the appropriate boundary conditions, in order to find the dispersion relation. We also apply the WKBJ approximation and additionally give analytical solutions in some subcases corresponding to unperturbed jets with constant bulk velocity along the symmetry axis.

Abstract: Missing cascades from TeV blazar beams indicate that collective plasma effects may play a significant role in their energy loss. It is possible to mimic the evolution of such highly energetic pair beams in laboratory experiments using modern accelerators. The fate of the beam is governed by two different processes, energy loss through the unstable mode and energetic broadening of the pair beam through diffusion in momentum space. We chalk out this evolution using a Fokker-Planck approach in which the drift and the diffusion terms respectively describe these phenomena in a compact form. We present particle-in-cell simulations to trace the complete evolution of the unstable beam-plasma system for a generic narrow Gaussian pair beam for which the growth rate is reactive. We show that the instability leads to an energetic broadening of the pair beam, slowing down the instability growth in the linear phase, in line with the analytical and numerical solutions of the Fokker-Planck equation. Whereas in a laboratory experiment the change in the momentum distribution is an easily measured observable as a feedback of the instability, the consequence of diffusive broadening in an astrophysical scenario can be translated to an increase in the opening angle of the pair beam.

Abstract: The pattern of neutrino flavor oscillations could be altered by the influence of noisy perturbations such as those arising from a gravitational wave background (GWB). A stochastic process that is consistent with a GWB has been recently reported by the independent analyses of pulsar timing array (PTA) data sets collected over a decadal timescale by the North American Nanohertz Observatory for Gravitational Waves (NANOGrav), the European Pulsar Timing Array (EPTA), the Parkes Pulsar Timing Array (PPTA), and the Chinese Pulsar Timing Array (CPTA) collaborations. We investigate the modifications in the neutrino flavor oscillations under the influence of the GWB reported by the PTA collaborations and we discuss how such effects could be potentially revealed in near-future neutrino detectors, possibly helping the discrimination of different models for the GWB below the nHz frequency range.

Abstract: Here we consider the results of direct measurements of muons in extensive air showers with zenith angles $\theta \le 45^{\circ}$ and energy above $10^{17}$ eV, obtained at the Pierre Auger Observatory and Yakutsk array. In both experiments muons were registered with underground scintillation detectors with $\approx 1.0 \times \sec\theta$ GeV energy threshold. Measured density values were compared to theoretical predictions calculated within the framework of the QGSJet-II.04 hadron interaction model. They differ by factor $1.53 \pm 0.13$(stat). We demonstrate that this difference is due to overestimation of muon densities by 1.22 times and underestimation of primary energy by 1.25 times in the Auger experiment.

Abstract: Axions coupled to nucleons might be copiously emitted from core-collapse supernovae (SNe). If the axion-nucleon coupling is strong enough, axions would be emitted from the SN as a burst and, reaching Earth, may excite the oxygen nuclei in water Cherenkov detectors (${}^{16}{\rm O} + a \to {}^{16}{\rm O}^{*}$). This process will be followed by radiative decay(s) of the excited state (${}^{16}{\rm O}^* \rightarrow {}^{16}{\rm O}+\gamma $) providing a strategy for a direct detection of axions from a Galactic SN in large underground neutrino Cherenkov detectors. Motivated by this possibility, we present an updated calculation of axion-oxygen cross section obtained by using self-consistent continuum Random Phase Approximation. We calculate the branching ratio of the oxygen nucleus de-excitation into gamma-rays, neutrons, protons and $\alpha$-particles. These results are used to revisit the detectability of axions from SN 1987A in Kamiokande-II.

Abstract: Swift J0549.7-6812 is an Be/X-ray binary system (BeXRB) in the Large Magellanic Cloud (LMC) exhibiting a 6s pulse period. Like many such systems the variable X-ray emission is believed to be driven by the underlying behaviour of the mass donor Be star. In this paper we report on X-ray observations of the brightest known outburst from this system which reached a luminosity of 8 x 10^37 erg/s. These observations are supported by contemporaneous optical photometric observations, the first reported optical spectrum, as well as several years of historical data from OGLE and GAIA. The latter strongly suggest a binary period of 46.1d. All the observational data indicate that Swift J0549.7-6812 is a system that spends the vast majority of its time in X-ray quiescence, or even switched off completely. This suggests that occasional observations may easily miss it, and many similar systems, and thereby underestimate the massive star evolution numbers for the LMC.

Abstract: The origin of interstellar and intergalactic magnetic fields is largely unknown, and the primordial magnetic fields (PMFs) produced by, e.g., phase transitions of the early Universe are expected to provide seeds for those magnetic fields. The PMFs affect the evolution of the Universe at an early time, resulting in a series of phenomena. In this work, we show that the PMF-induced turbulence can give rise to nanohertz (nHz) gravitational waves reported by several pulsar timing arrays, including NANOGrav, PPTA, EPTA, and CPTA. Using the nHz gravitational wave data, we obtain the constraints on the characteristic magnetic field strength ($B_{\rm ch}^* \sim \mathcal{O}(1)~\rm{\mu G}$) and coherent length scale ($\ell_{\rm ch}^* \sim \mathcal{O}(1)~\rm{pc}$) of PMFs, assuming a generation temperature of approximately the QCD temperature ($\sim 100$ MeV). In addition, the PMFs which evolve to the recombination era can induce baryon density inhomogeneities, and then alter the ionization process. This naturally results in an alleviation of the tension of the Hubble parameter $H_0$ and the matter clumpiness parameter $S_8$ between early and late-time measurements. Assuming an evolution form of $B_{\rm ch}\sim \ell_{\rm ch}^{-\alpha}$ from the epoch of the production of PMFs to the epoch of recombination, we find $0.91<\alpha<1.08$ (95\% credible region).

Abstract: Recently, the NANOGrav, PPTA, EPTA and CPTA collaborations reported compelling evidence of the existence of the Stochastic Gravitational-Wave Background (SGWB). The amplitude and spectrum of this inferred gravitational-wave background align closely with the astrophysical predictions for a signal originating from the population of supermassive black-hole binaries. In light of these findings, we explore the possibility to detect dark matter spikes surrounding massive black holes, which could potentially impact the gravitational-wave waveform and modulate the SGWB. We demonstrate that the SMBH binary evolution induced by the combined effects of GW radiation and the dynamical friction of the dark matter spike exhibits detectable manifestations within the nHz frequency range of the SGWB.

Abstract: Very recently, the Pulsar Timing Array (PTA) experiments reported strong evidence for the presence of the nanohertz stochastic gravitational wave background (SGWB). In this work we show that the cosmic string loops can account for the nanohertz SGWB data with a $G\mu \sim 2\times 10^{-12}$ and the loops number density $N \sim 10^{4}$. Though the presence of cosmic string loops can also effectively enhance the number density of massive galaxies at high redshifts, we do not find a reasonable parameter space to self-consistently interpret both the SGWB data and the JWST observations. This implies either an extension of the model adopted in this work or the different physical origins of these two phenomena.