High Energy Astrophysical Phenomena (astro-ph.HE)

Abstract: The origin of high-energy cosmic rays, atomic nuclei that continuously impact Earth's atmosphere, has been a mystery for over a century. Due to deflection in interstellar magnetic fields, cosmic rays from the Milky Way arrive at Earth from random directions. However, near their sources and during propagation, cosmic rays interact with matter and produce high-energy neutrinos. We search for neutrino emission using machine learning techniques applied to ten years of data from the IceCube Neutrino Observatory. We identify neutrino emission from the Galactic plane at the 4.5$\sigma$ level of significance, by comparing diffuse emission models to a background-only hypothesis. The signal is consistent with modeled diffuse emission from the Galactic plane, but could also arise from a population of unresolved point sources.

Abstract: We study the recent outburst of the black hole candidate EXO 1846-031 which went into an outburst in 2019 after almost 34 years in quiescence. We use archival data from Swift/XRT, MAXI/GSC, NICER/XTI and NuSTAR/FPM satellites/instruments to study the evolution of the spectral and temporal properties of the source during the outburst. Low energy X-ray flux of the outburst shows multiple peaks making it a multipeak outburst. Evolving type-C quasi-periodic oscillations (QPOs) are observed in the NICER data in the hard, hard intermediate and soft intermediate states. We use the physical Two Component Advective Flow (TCAF) model to analyze the combined spectra of multiple satellite instruments. According to the TCAF model, the accreting matter is divided into Keplerian and sub-Keplerian parts, and the variation in the observed spectra in different spectral states arises out of the variable contributions of these two types of accreting matter in the total accretion rate. Studying the evolution of the accretion rates and other properties of the accretion flow obtained from the spectral analysis, we show how the multiple peaks in the outburst flux arises out of discontinuous supply and different radial velocities of two types of accreting matter from the pile-up radius. We detect an Fe emission line at $\sim6.6$ keV in the hard and the intermediate states in the NICER spectra. We determine the probable mass of the black hole to be $12.43^{+0.14}_{-0.03}~M_\odot$ from the spectral analysis with the TCAF model. We also estimate viscous time scale of the source in this outburst to be $\sim 8$ days from the peak difference of the Keplerian and sub-Keplerian mass accretion rates.

Abstract: A spinning black hole accreting from a disk of strongly magnetized plasma via a magnetically arrested disk is known to produce an efficient electromagnetic jet powered by the black hole's spin energy. We present general relativistic radiative magnetohydrodynamic simulations of magnetically arrested systems covering a range of sub- to super-Eddington accretion rates. Using the numerical results from these simulation, we develop formulae to describe the magnetization, jet efficiency, and spin evolution of an accreting black hole as a function of its spin and accretion rate. A black hole with near-Eddington accretion experiences a mild degree of spin-down because of angular momentum loss through the jet, leading to an equilibrium spin of 0.8 rather than 1.0 at the Eddington limit. As the accretion rate increases above Eddington, the spin-down effect becomes progressively stronger, ultimately converging on previous predictions based on non-radiative simulations. In particular, spin evolution drives highly super-Eddington systems toward a black hole spin near zero. The formulae developed in this letter may be applied to galaxy and cosmological scale simulations that include black holes. If magnetically arrested disk accretion is common among supermassive black holes, the present results have broad implications for active galactic nucleus feedback and cosmological spin evolution.

Abstract: In this work, we study the correlation between Quasi-periodic Oscillation (QPO) frequency and the spectral parameters during various X-ray states in the black hole binary GRS 1915+105 which matches well with the predicted relativistic dynamic frequency (i.e. the inverse of the sound crossing time) at the truncated radii. We have used broadband data of LAXPC and SXT instruments onboard AstroSat. Spectral fitting shows that the accretion rate varies from $\sim 0.1$ to $\sim 5.0 \times 10^{18}$ gm/s and the truncated radius changing from the last stable orbit of an almost maximally spinning black hole, $\sim$ 1.2 to $\sim$ 19 Gravitational radii. For this wide range, the frequencies of the C-type QPO (2 - 6 Hz) follow the trend predicted by the relativistic dynamical frequency model and interestingly, the high-frequency QPO at $\sim$ 70 Hz also follows the same trend, suggesting they originate from the innermost stable circular orbit with the same mechanism as the more commonly observed C-type QPO. While the qualitative trend is as predicted, there are quantitative deviations between the data and the theory, and the possible reasons for these deviations are discussed.

Abstract: Strong evidence for the Helling-Downs correlations have been reported by several pulsar timing array collaborations in middle 2023. In this work, we study the state-of-the-art graviton mass bounds by analyzing the observational data of overlap ruduction functions from NANOGrav 15-year data release and CPTA first data release. The data analysis places upper limits on the graviton mass at 95\% confidence level, namely, $m_{g}\lesssim0.43\times10^{-23}\mathrm{eV}$ for NANOGrav and $m_{g}\lesssim0.57\times10^{-23}\mathrm{eV}$ for CPTA. In addition, we discuss implications of these results for scenarios of ultralight tensor dark matter.

Abstract: Experiments seeking to detect radio emission stemming from neutrino interactions will soon reach sensitivities that bring a detection within reach. Since experiments like RNO-G or the future IceCube-Gen2 target more than an order of magnitude more effective volume than existing experiments, the renewed and detailed study of rare backgrounds is needed. In this paper, we study the potential background from energy losses of highly energetic atmospheric muons. Due to both limited experimental measurements and limited modeling in hadronic interaction models, the expected event rate is subject to large uncertainties. Here, we estimate rate predictions and their uncertainties for different models and instrumental parameters. We also study possible routes towards mitigation, such as parent air shower detection, and illustrate what is needed to make the first measurement of the prompt muon flux at energies above 10 PeV.

Abstract: We constrain a broad class of "hairy" black hole models capable of directly sourcing electromagnetic radiation during a binary black hole merger. This signal is generic and model-independent since it is characterized by the black hole mass ($M$) and the fraction of that mass released as radiation ($\epsilon$). For field energy densities surpassing the Schwinger limit, this mechanism triggers pair-production to produce a gamma-ray burst. By cross-referencing gravitational wave events with gamma-ray observations, we place upper bounds of $\epsilon<10^{-5}-10^{-4}$ for $10-50$ $M_\odot$ black holes depending on the black hole mass. We discuss the weak detection of a gamma-ray burst following GW150914 and show that this event is consistent with rapid electromagnetic emission directly from a "hairy" black hole with $\epsilon\sim10^{-7}-10^{-6}$. Below the Schwinger limit, ambient charged particles are rapidly accelerated to nearly the speed of light by the strong electromagnetic field. For 1-50 $M_\odot$ black holes and $\epsilon$ ranging from $10^{-20}$ to $10^{-7}$, the typical proton energies are $\sim20$ GeV-20 TeV and electron energies are $\sim0.01-10$ GeV. At these energies, cosmic ray protons and electrons quickly diffuse into the Milky Way's background magnetic field, making it difficult to identify a point source producing them. Overall, constraining $\epsilon$ in this less energetic regime becomes difficult and future constraints may need to consider specific models of "hairy" black holes.