arXiv subjects

Kohta Murase

Publications and source records attributed to Kohta Murase.

At least 145 records · Page 8Linked to original sources

arXiv2020

Charm contribution to ultrahigh-energy neutrinos from newborn magnetars

Newborn, strongly magnetized neutron stars (so-called magnetars) surrounded by their stellar or merger ejecta are expected to be sources of ultrahigh-energy neutrinos via decay of mesons produced in hadronic interactions of protons which are accelerated to ultrahigh energies by magnetic dissipation of the spindown energy. We show that not only pions and kaons but also charm hadrons, which are typically neglected due to their small production cross sections, can represent dominant contributions to neutrino fluence at ultrahigh energies, because of their short lifetimes, while the ultrahigh-energy neutrino fluence from pion and kaon production is suppressed at early times due to their significant cooling before their decay. We show that the next-generation detectors such as Probe Of Extreme Multi-Messenger Astrophysics (POEMMA), Giant Radio Array for Neurtino Detection (GRAND) and IceCube-Gen2 have a good chance of observing neutrinos, primarily originating from charm hadrons, from nearby magnetars. We also show that neutrinos from nearby magnetar-driven merger novae could be observed in the time interval between $10^2$ s and $10^3$ s, where the charm hadron contribution is dominant for neutrino energies above $10^8$ GeV, of relevance to next generation detectors. We also comment on potential impacts of the charm hadron contribution to the diffuse neutrino flux.

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arXiv2020

High-energy neutrino emission subsequent to gravitational wave radiation from supermassive black hole mergers

Supermassive black hole (SMBH) coalescences are ubiquitous in the history of the Universe and often exhibit strong accretion activities and powerful jets. These SMBH mergers are also promising candidates for future gravitational wave detectors such as Laser Space Inteferometric Antenna (LISA). In this work, we consider neutrino counterpart emission originating from the jet-induced shocks. The physical picture is that relativistic jets launched after the merger will push forward inside the premerger disk wind material, and then they subsequently get collimated, leading to the formation of internal shocks, collimation shocks, forward shocks and reverse shocks. Cosmic rays can be accelerated in these sites and neutrinos are expected via the photomeson production process. We formulate the jet structures and relevant interactions therein, and then evaluate neutrino emission from each shock site. We find that month-to-year high-energy neutrino emission from the postmerger jet after the gravitational wave event is detectable by IceCube-Gen2 within approximately five to ten years of operation in optimistic cases where the cosmic-ray loading is sufficiently high and a mildly super-Eddington accretion is achieved. We also estimate the contribution of SMBH mergers to the diffuse neutrino intensity, and find that a significant fraction of the observed very high-energy ($E_ν\gtrsim1$ PeV) IceCube neutrinos could originate from them in the optimistic cases. In the future, such neutrino counterparts together with gravitational wave observations can be used in a multimessenger approach to elucidate in greater detail the evolution and the physical mechanism of SMBH mergers.

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arXiv2020

Constraining photohadronic scenarios for the unified origin of IceCube neutrinos and ultrahigh-energy cosmic rays

The diffuse neutrino flux measured in IceCube is comparable with the ultrahigh-energy cosmic ray (UHECR) flux, which has led to the concept of a unified origin of high-energy neutrino and UHECR backgrounds. We construct a generic unification model of sources to explain UHECR data at $\gtrsim 10^{19}$ eV, and high-energy neutrinos with energies that exceed $\sim100$ TeV in the framework of photo-meson production processes, and provide general constraints on the source properties. A source environment with moderately efficient in-situ production of $\gtrsim 100$ TeV neutrinos with an optical depth of $0.1 \lesssim τ_{pγ}\lesssim 0.6$ must be realized to accelerate cosmic rays to ultrahigh energies. The measured fluxes of cosmic rays and neutrinos set a bound on the source luminosity and its rate density. Although the results are rather general and applicable to unknown source population, among the proposed source candidates, low-luminosity gamma-ray bursts (GRBs) and tidal disruption events (TDEs) could satisfy the requirements if the Lorentz bulk factor of plasma outflow and the equipartition parameters for cosmic rays and magnetic field are appropriately selected.

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arXiv2020

High-Energy Neutrino and Gamma-Ray Emission from Tidal Disruption Events

Tidal disruption events (TDE) have been considered as cosmic-ray and neutrino sources for a decade. We suggest two classes of new scenarios for high-energy multi-messenger emission from TDEs that do not have to harbor powerful jets. First, we investigate high-energy neutrino and gamma-ray production in the core region of a supermassive black hole. In particular, we show that about 1-100 TeV neutrinos and MeV gamma-rays can efficiently be produced in hot coronae around an accretion disk. We also study the consequences of particle acceleration in radiatively inefficient accretion flows (RIAFs). Second, we consider possible cosmic-ray acceleration by sub-relativistic disk-driven winds or interactions between tidal streams, and show that subsequent hadronuclear and photohadronic interactions inside the TDE debris lead to GeV-PeV neutrinos and sub-GeV cascade gamma-rays. We demonstrate that these models should be accompanied by soft gamma-rays or hard X-rays as well as optical/UV emission, which can be used for future observational tests. Although this work aims to present models of non-jetted high-energy emission, we discuss the implications of the TDE AT2019dsg that might coincide with the high-energy neutrino IceCube-191001A, by considering the corona, RIAF, hidden sub-relativistic wind, and hidden jet models. It is not yet possible to be conclusive about their physical association and the expected number of neutrinos is typically much less than unity. We find that the most optimistic cases of the corona and hidden wind models could be consistent with the observation of IceCube-191001A, whereas jet models are unlikely to explain the multi-messenger observations.

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arXiv2020

Comprehensive Multimessenger Modeling of the Extreme Blazar 3HSP J095507.9+355101 and Predictions for IceCube

3HSP J095507.9+355101 is an extreme blazar which has been possibly associated with a high-energy neutrino (IceCube-200107A) detected one day before the blazar was found to undergo a hard X-ray flare. We perform a comprehensive study of the predicted multimessenger emission from 3HSP J095507.9+355101 during its recent X-ray flare, but also in the long term. We focus on one-zone leptohadronic models, but we also explore alternative scenarios: (i) a blazar-core model, which considers neutrino production in the inner jet, close to the supermassive black hole; (ii) a hidden external-photon model, which considers neutrino production in the jet through interactions with photons from a weak broad line region; (iii) a proton synchrotron model, where high-energy protons in the jet produce $γ$-rays via synchrotron; and (iv) an intergalactic cascade scenario, where neutrinos are produced in the intergalactic medium by interactions of a high-energy cosmic-ray beam escaping the jet. The Poisson probability to detect one muon neutrino in ten years from 3HSP J095507.9+355101 with the real-time IceCube alert analysis is $\sim 1\%$ ($3\%$) for the most optimistic one-zone leptohadronic model (the multi-zone blazar-core model). Meanwhile, detection of one neutrino during the 44-day-long high X-ray flux-state period following the neutrino detection is $0.06\%$, according to our most optimistic leptohadronic model. The most promising scenarios for neutrino production also predict strong intra-source $γ$-ray attenuation above $\sim100$ GeV. If the association is real, then IceCube-Gen2 and other future detectors should be able to provide additional evidence for neutrino production in 3HSP J095507.9+355101 and other extreme blazars.

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arXiv2020

Multimessenger observations of counterparts to IceCube-190331A

High-energy neutrinos are a promising tool for identifying astrophysical sources of high and ultra-high energy cosmic rays (UHECR). Prospects of detecting neutrinos at high energies ($\gtrsim$TeV) from blazars have been boosted after the recent association of IceCube-170922A and TXS 0506+056. We investigate the high-energy neutrino, IceCube-190331A, a high-energy starting event (HESE) with a high likelihood of being astrophysical in origin. We initiated a Swift/XRT and UVOT tiling mosaic of the neutrino localisation, and followed up with ATCA radio observations, compiling a multiwavelength SED for the most likely source of origin. NuSTAR observations of the neutrino location and a nearby X-ray source were also performed. We find two promising counterpart in the 90% confidence localisation region and identify the brightest as the most likely counterpart. However, no Fermi/LAT $γ$-ray source and no prompt Swift/BAT source is consistent with the neutrino event. At this point it is unclear whether any of the counterparts produced IceCube-190331A. We note that the Helix Nebula is also consistent with the position of the neutrino event, and we calculate that associated particle acceleration processes cannot produce the required energies to generate a high-energy HESE neutrino.

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arXiv2020

Hidden Cores of Active Galactic Nuclei as the Origin of Medium-Energy Neutrinos: Critical Tests with the MeV Gamma-Ray Connection

Mysteries about the origin of high-energy cosmic neutrinos have deepened by the recent IceCube measurement of a large diffuse flux in the 10-100 TeV range. Based on the standard disk-corona picture of active galactic nuclei (AGN), we present a phenomenological model enabling us to systematically calculate the spectral sequence of multimessenger emission from the AGN coronae. We show that protons in the coronal plasma can be stochastically accelerated up to PeV energies by plasma turbulence, and find that the model explains the large diffuse flux of medium-energy neutrinos if the cosmic rays carry only a few percent of the thermal energy. We find that the Bethe-Heitler process plays a crucial role in connecting these neutrinos and cascaded MeV gamma rays, and point out that the gamma-ray flux can even be enhanced by the reacceleration of secondary pairs. Critical tests of the model are given by its prediction that a significant fraction of the MeV gamma-ray background correlates with about 10 TeV neutrinos, and nearby Seyfert galaxies including NGC 1068 are promising targets for IceCube, KM3Net, IceCube-Gen2, and future MeV gamma-ray telescopes.

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arXiv2020

Gamma-ray counterparts of 2WHSP high-synchrotron-peaked BL Lac objects as possible signatures of ultra-high-energy cosmic-ray emission

We present a search for high-energy $γ$-ray emission from 566 Active Galactic Nuclei at redshift $z > 0.2$, from the 2WHSP catalog of high-synchrotron peaked BL Lac objects with eight years of Fermi-LAT data. We focus on a redshift range where electromagnetic cascade emission induced by ultra-high-energy cosmic rays can be distinguished from leptonic emission based on the spectral properties of the sources. Our analysis leads to the detection of 160 sources above $\approx$ $5σ$ (TS $\geq 25$) in the 1 - 300 GeV energy range. By discriminating significant sources based on their $γ$-ray fluxes, variability properties, and photon index in the Fermi-LAT energy range, and modeling the expected hadronic signal in the TeV regime, we select a list of promising sources as potential candidate ultra-high-energy cosmic-ray emitters for follow-up observations by Imaging Atmospheric Cherenkov Telescopes.

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arXiv2020

Oscillation of high-energy neutrinos from choked jets in stellar and merger ejecta

We present a comprehensive study on oscillation of high-energy neutrinos from two different environments: blue supergiant progenitors that may harbor low-power gamma-ray burst (GRB) jets and neutron star merger ejecta that would be associated with short gamma-ray bursts. We incorporate the radiation constraint that gives a necessary condition for nonthermal neutrino production, and account for the time evolution of the jet, which allows us to treat neutrino oscillation in matter more accurately. For massive star progenitors, neutrino injection inside the star can lead to nonadiabatic oscillation patterns in the 1 TeV - 10 TeV and is also visible in the flavor ratio. For neutron star merger ejecta, we find a similar behavior in the 100 GeV - 10 TeV region and the oscillation may result in a $ν_e$ excess around 1 TeV. These features, which enable us to probe the progenitors of long and short GRBs, could be seen by future neutrino detectors with precise flavor ratio measurements. We also discuss potential contributions to the diffuse neutrino flux measured by IceCube, and find parameter sets allowing choked low-power GRB jets to account for the neutrino flux in the 10 TeV - 100 TeV range without violating the existing constraints.

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arXiv2020

New constraints on the origin of medium-energy neutrinos observed by IceCube

The recent IceCube publication claims the observation of cosmic neutrinos with energies down to $\sim 10$ TeV, reinforcing the growing evidence that the neutrino flux in the 10-100 TeV range is unexpectedly large. Any conceivable source of these neutrinos must also produce a $γ$-ray flux which degrades in energy en route to the Earth and contributes to the extragalactic $γ$-ray background measured by the Fermi satellite. In a quantitative multimessenger analysis, featuring minimalistic assumptions, we find a $\geq 3σ$ tension in the data, reaching $\sim 5σ$ for cosmic neutrinos extended down to $\sim 1$ TeV, interpreted as evidence for a population of hidden cosmic-ray accelerators.

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arXiv2020

Multi-Epoch Modeling of TXS 0506+056 and Implications for Long-Term High-Energy Neutrino Emission

The IceCube report of a $\sim 3.5σ$ excess of $13\pm5$ neutrino events in the direction of the blazar TXS 05056+056 in 2014-2015 and the 2017 detection of a high-energy neutrino, IceCube-170922A, during a gamma-ray flare from the same blazar, have revived the interest in scenarios for neutrino production in blazars. We perform comprehensive analyses on the long-term electromagnetic emission of TXS 05056+056 using optical, X-ray, and gamma-ray data from the All-Sky Automated Survey for Supernovae (ASAS-SN), the Neil Gehrels Swift Observatory (Swift), the Monitor of All-sky X-ray Image (MAXI), and the Fermi Large Area Telescope (Fermi-LAT). We also perform numerical modeling of the spectral energy distributions (SEDs) in four epochs prior to 2017 with contemporaneous gamma-ray and lower energy (optical and/or X-ray) data. We find that the multi-epoch SEDs are consistent with a hybrid leptonic scenario, where the gamma-rays are produced in the blazar zone via external inverse Compton scattering of accelerated electrons, and high-energy neutrinos are produced via the photomeson production process of co-accelerated protons. The multi-epoch SEDs can be satisfactorily explained with the same jet parameters and variable external photon density and electron luminosity. Using the maximal neutrino flux derived for each epoch, we put an upper limit of $\sim0.4-2$ on the muon neutrino number in ten years of IceCube observations. Our results are consistent with the IceCube-170922A detection, which can be explained as an upper fluctuation from the average neutrino rate expected from the source, but in strong tension with the 2014-2015 neutrino flare.

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arXiv2020

Linking extended and plateau emissions of short gamma-ray bursts

Some short gamma-ray bursts (SGRBs) show a longer lasting emission phase, called extended emission (EE) lasting $\sim 10^{2-3}\,\rm s$, as well as a plateau emission (PE) lasting $\sim10^{4-5}\,\rm s$. While a long-lasting activity of the central engines is a promising explanation for powering both emissions, their physical origin and their emission mechanisms are still uncertain. In this work, we study the properties of the EEs and their connection with the PEs. First, we constrain the minimal Lorentz factor $Γ$ of the outflows powering EEs, using compactness arguments and find that the outflows should be relativistic, $Γ\gtrsim10$. We propose a consistent scenario for the PEs, where the outflow eventually catches up with the jet responsible for the prompt emission, injecting energy into the forward shock formed by the prior jet, which naturally results in a PE. We also derive the radiation efficiency of EEs and the Lorentz factor of the outflow within our scenario for 10 well-observed SGRBs accompanied by both EE and PE. The efficiency has an average value of $\sim3\,\%$ but shows a broad distribution ranging from $\sim0.01$ to $\sim100\%$. The Lorentz factor is $\sim20-30$, consistent with the compactness arguments. These results suggest that EEs are produced by a slower outflow via more inefficient emission than the faster outflow which causes the prompt emission with a high radiation efficiency.

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arXiv2020

Cosmic rays escaping from Galactic starburst-driven superbubbles

We calculate spectra of escaping cosmic rays (CRs) accelerated at shocks produced by expanding Galactic superbubbles powered by multiple supernovae producing a continuous energy outflow in star-forming galaxies. We solve the generalized Kompaneets equations adapted to expansion in various external density profiles, including exponential and power-law shapes, and take into account that escaping CRs are dominated by those around their maximum energies. We find that the escaping CR spectrum largely depends on the specific density profiles and power source properties, and the results are compared to and constrained by the observed CR spectrum. As a generic demonstration, we apply the scheme to a superbubble occurring in the centre of the Milky Way, and find that under specific parameter sets the CRs produced in our model can explain the observed CR flux and spectrum around the second knee at $10^{17}$ eV.

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arXiv2019

A Neutral Beam Model for High-Energy Neutrino Emission from the Blazar TXS 0506+056

The IceCube collaboration reported a $\sim 3.5σ$ excess of $13\pm5$ neutrino events in the direction of the blazar TXS 0506+56 during a $\sim$6 month period in 2014-2015, as well as the ($\sim3σ$) detection of a high-energy muon neutrino during an electromagnetic flare in 2017. We explore the possibility that the 2014-2015 neutrino excess and the 2017 multi-messenger flare are both explained in a common physical framework that relies on the emergence of a relativistic neutral beam in the blazar jet due to interactions of accelerated cosmic rays (CRs) with photons. We demonstrate that the neutral beam model provides an explanation for the 2014-2015 neutrino excess without violating X-ray and $γ$-ray constraints, and also yields results consistent with the detection of one high-energy neutrino during the 2017 flare. If both neutrino associations with TXS 05065+056 are real, our model requires that (i) the composition of accelerated CRs is light, with a ratio of helium nuclei to protons $\gtrsim5$, (ii) a luminous external photon field ($\sim 10^{46}$ erg s$^{-1}$) variable (on year-long timescales) is present, and (iii) the CR injection luminosity as well as the properties of the dissipation region (i.e., Lorentz factor, magnetic field, and size) vary on year-long timescales.

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arXiv2019

Complementarity of Stacking and Multiplet Constraints on the Blazar Contribution to the Cumulative High-Energy Neutrino Intensity

We investigate the blazar contribution to the cumulative neutrino intensity assuming a generic relationship between neutrino and gamma-ray luminosities, $L_ν\propto (L_{\rm ph})^{γ_{\rm lw}}$. Using the gamma-ray luminosity functions for blazars including flat spectrum radio quasars (FSRQs) and BL Lac objects, as well as the $Fermi$-LAT detection efficiency, we estimate contributions from {blazars resolved by $Fermi$-LAT as well as the unresolved counterpart.} Combining the existing upper limits from stacking analyses, the cumulative neutrino flux from all blazars ({including $Fermi$-LAT resolved and unresolved ones}) are constrained { in the range $0\lesssimγ_{\rm lw}\lesssim2.5$}. We also evaluate the effects of the redshift evolution and the effective local number densities for each class of FSRQs, BL Lacs, and all blazars, by which we place another type of constraints on the blazar contribution using the non-detection of high-energy neutrino multiplets. We demonstrate that these two upper limits are complementary, and that the joint consideration of the stacking and multiplet analyses { not only supports the argument that blazars are disfavored as the dominant sources of the 100-TeV neutrino background, but it extends this argument by including also $Fermi$-LAT-unresolved blazars as well, for a more generic luminosity correlation $L_ν\propto (L_{\rm ph})^{γ_{\rm lw}}$}.

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arXiv2019

Multi-Messenger Connections among High-Energy Cosmic Particles

The origin of high-energy cosmic neutrinos is one of the biggest mysteries in astroparticle physics. The fact that diffuse intensities of high-energy neutrinos, ultrahigh-energy cosmic rays, and GeV-TeV gamma rays are all comparable suggests that these messengers are physically connected. The IceCube data above 100 TeV energies can be naturally explained by cosmic-ray reservoir models. In particular, starburst galaxies and galaxy clusters/groups serve as natural storage rooms of cosmic rays, and it has been theoretically predicted that these sources are promising sites of high-energy neutrinos and gamma rays that are produced via inelastic pp interactions. Indeed, the predictions made before the discovery of IceCube neutrinos are consistent with the current high-energy neutrino data measured in IceCube, and that they could give a grand-unified view of sub-PeV neutrinos, sub-TeV gamma rays, and ultrahigh-energy cosmic rays. These unified models have strong prediction powers, which can be tested by next-generation neutrino detectors such as IceCube-Gen2 as well as gamma-ray telescopes such as CTA. The recent observations have also shown that the 10-100 TeV diffuse neutrino flux is higher than that at PeV energies, which suggests that they come from a different class of neutrino sources. The detailed comparison with the diffuse isotropic gamma-ray background measured by Fermi has revealed that these medium-energy neutrinos are likely to come from hidden cosmic-ray accelerators, from which neutrinos can escape while GeV-TeV gamma rays are attenuated. The candidate source classes are choked gamma-ray burst jets and active galactic nuclei (AGN) cores. In particular, the AGN corona model predicts a unique connection between 10-100 TeV neutrinos and MeV gamma rays, which can be robustly tested with future MeV gamma-ray missions such as AMEGO.

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arXiv2019

Neutrino Echoes from Multimessenger Transient Sources

The detection of the high-energy neutrino event, IceCube-170922A, demonstrated that multimessenger particle astrophysics triggered by neutrino alerts is feasible. We consider time delay signatures caused by secret neutrino interactions with the cosmic neutrino background and dark matter and suggest that these can be used as a novel probe of neutrino interactions beyond the Standard Model (BSM). The tests with BSM-induced neutrino echoes are distinct from existing constraints from the spectral modification and will be enabled by multimessenger observations of bright neutrino transients with future experiments such as IceCube-Gen2, KM3Net, and Hyper-Kamiokande. The constraints are complementary to those from accelerator and laboratory experiments and powerful for testing various particle models that explain tensions prevailing in the cosmological data.

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arXiv2019

Acceleration and Escape Processes of High-energy Particles in Turbulence inside Hot Accretion Flows

We investigate acceleration and propagation processes of high-energy particles inside hot accretion flows. The magnetorotational instability (MRI) creates turbulence inside accretion flows, which triggers magnetic reconnection and may produce non-thermal particles. They can be further accelerated stochastically by the turbulence. To probe the properties of such relativistic particles, we perform magnetohydrodynamic simulations to obtain the turbulent fields generated by the MRI, and calculate orbits of the high-energy particles using snapshot data of the MRI turbulence. We find that the particle acceleration is described by a diffusion phenomenon in energy space with a diffusion coefficient of the hard-sphere type: $D_ε\propto ε^2$, where $ε$ is the particle energy. Eddies in the largest scale of the turbulence play a dominant role in the acceleration process. On the other hand, the stochastic behaviour in configuration space is not usual diffusion but superdiffusion: the radial displacement increases with time faster than that in the normal diffusion. Also, the magnetic field configuration in the hot accretion flow creates outward bulk motion of high-energy particles. This bulk motion is more effective than the diffusive motion for higher energy particles. Our results imply that typical active galactic nuclei that host hot accretion flows can accelerate CRs up to $ε\sim 0.1-10$ PeV.

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