Abstracts of Interest
Selected by:
Ryan Burley
Abstract: 2305.06375
Full Text: [ PostScript, PDF]
Full Text: [ PostScript, PDF]
Title:A Leptonic Model for Neutrino Emission From Active Galactic Nuclei
Download PDFAbstract: It is often stated that the observation of high-energy neutrinos from an astrophysical source would constitute a smoking gun for the acceleration of hadronic cosmic rays. Here, we point out that there exists a purely leptonic mechanism to produce TeV-scale neutrinos in astrophysical environments. In particular, very high energy synchrotron photons can scatter with X-rays, exceeding the threshold for muon-antimuon pair production. When these muons decay, they produce neutrinos without any cosmic-ray protons or nuclei being involved. In order for this mechanism to be efficient, the source in question must feature both kG-scale magnetic fields and a high density of keV-scale photons. As an example, we consider the active galaxy NGC 1068, which IceCube has recently detected as a source of TeV-scale neutrinos. We find that the neutrino emission observed from this source could potentially be generated through muon pair production for reasonable choices of physical parameters.
Abstract: 2305.06948
Full Text: [ PostScript, PDF]
Full Text: [ PostScript, PDF]
Title:Galactic diffuse gamma-ray emission from GeV to PeV energies in light of up-to-date cosmic ray measurements
Download PDFAbstract: The diffuse gamma-ray emission between 10 and 1000 TeV from the Galactic plane was recently measured precisely by the Large High Altitude Air Shower Observatory (LHAASO), which is very useful in constraining the propagation and interaction of cosmic rays in the Milky Way. On the other hand, new measurements of CR spectra reach a very high precision up to 100 TeV energies, revealing multiple spectral structures of various species. In this work, we confront the model prediction of the diffuse gamma-ray emission, based on up-to-date measurements of the local cosmic ray spectra and simplified propagation setup, with the measurements of diffuse gamma-rays. To better constrain the low-energy part of the model, we analyze Fermi-LAT data to extract the diffuse emission between 1 and 500 GeV from the same sky regions of LHAASO. Compared with the prediction, we find that clear excesses between several GeV and ~60 TeV of the diffuse emission exist. Possible reasons to explain the excesses may include unresolved sources or more complicated propagation models. We illustrate that an exponential-cutoff-power-law component with an index of -2.40 and cutoff energy of ~30 TeV is able to account for such excesses.
Abstract: 2305.06729
Full Text: [ PostScript, PDF]
Full Text: [ PostScript, PDF]
Title:Searching for temporary gamma-ray dark blazars associated with IceCube neutrinos
Download PDFAbstract: Tensions between the diffuse gamma-ray sky observed by the Fermi Large Area Telescope (LAT) and the diffuse high-energy neutrino sky detected by the IceCube South Pole Neutrino Observatory question our knowledge about high-energy neutrino sources in the gamma-ray regime. While blazars are among the most energetic persistent particle accelerators in the Universe, studies suggest that they could account for up to for 10-30% of the neutrino flux measured by IceCube. Our recent results highlighted that the associated IceCube neutrinos arrived in a local gamma-ray minimum (dip) of three strong neutrino point-source candidates. We increase the sample of neutrino-source candidates to study their gamma-ray light curves. We generate the one-year Fermi-LAT light curve for 8 neutrino source candidate blazars (RBS 0958, GB6 J1040+0617, PKS 1313-333, TXS 0506+056, PKS 1454-354, NVSS J042025-374443, PKS 0426-380 and PKS 1502+106), centered on the detection time of the associated IceCube neutrinos. We apply the Bayesian block algorithm on the light curves to characterize their variability. Our results indicate that GB6 J1040+0617 was in the phase of high gamma-ray activity, while none of the other 7 neutrino source candidates were statistically bright during the detection of the corresponding neutrinos and that indeed even most of the times neutrinos arrived in a faint gamma-ray phase of the light curves. This suggests that the 8 source-candidate blazars (associated with 7 neutrino events) in our reduced sample are either not the sources of the corresponding IceCube neutrinos, or that an in-source effect (e.g. suppression of gamma rays due to high gamma-gamma opacity) complicates the multimessenger scenario of neutrino emission for these blazars.
Abstract: 2305.06693
Full Text: [ PostScript, PDF]
Full Text: [ PostScript, PDF]
Title:Interpreting the GeV-TeV Gamma-Ray Spectra of Local Giant Molecular Clouds using GEANT4 Simulation
Download PDFAbstract: Recently, the Fermi-LAT gamma-ray satellite has detected six Giant Molecular Clouds (GMCs) located in the Gould Belt and the Aquila Rift regions. In half of these objects (Taurus, Orion A, Orion B), the observed gamma-ray spectrum can be explained using the Galactic diffused Cosmic Ray (CR) interactions with the gas environments. In the remaining three GMCs (Rho Oph, Aquila Rift, Cepheus), the origin of the gamma-ray spectrum is still not well established. We use the GEometry ANd Tracking (GEANT4) simulation framework in order to simulate gamma-ray emission due to CR/GMC interaction in these three objects, taking into account the gas density distribution inside the GMCs. We find that propagation of diffused Galactic CRs inside these GMCs can explain the Fermi-LAT detected gamma-ray spectra. Further, our estimated TeV-PeV fluxes are consistent with the HAWC upper limits, available for the Aquila Rift GMC. As last step, we compute the total neutrino flux estimated for these GMCs and compare it with the IceCube detection sensitivity.
Abstract: 2305.07590
Full Text: [ PostScript, PDF]
Full Text: [ PostScript, PDF]
Title:On the nature of M31 gamma-ray halo in its relation to dark matter annihilation
Download PDFAbstract: The present work analyzes various aspects of M31 gamma-ray halo emission in its relation to annihilating dark matter (DM). The main aspect is the predicted effect of asymmetry of the intensity of emission due to inverse Compton scattering (ICS) of a possible population of relativistic electrons and positrons ($e^\pm$) in the galactic halo on starlight photons. This asymmetry is expected to exist around the major galactic axis, and arises due to anisotropy of the interstellar radiation field and the inclination of M31. ICS emission and its asymmetry were modeled by GALPROP code for the trial case of $e^\pm$ generated by annihilating weakly interacting massive particles (WIMPs) with various properties. The asymmetry was obtained to appear at photon energies above $\sim$ 0.1 MeV. Morphological and spectral properties of the asymmetry were studied in detail. Potential observational detection of the asymmetry may allow to infer the leptonic fraction in the emission generation mechanism, thus providing valuable inferences for understanding the nature of M31 gamma-ray halo emission. Specific asymmetry predictions were made for the recently claimed DM interpretation of the outer halo emission. The paper also studied the role of secondary -- ICS and bremsstrahlung -- emissions due to DM annihilation for that interpretation. And, finally, the latter was shown to be in significant tension with the recently derived WIMP constraints by radio data on M31.
Abstract: 2305.07086
Full Text: [ PostScript, PDF]
Full Text: [ PostScript, PDF]
Title:IceCube: Neutrinos from Active Galaxies
Download PDFAbstract: The IceCube project transformed a cubic kilometer of transparent, natural Antarctic ice into a Cherenkov detector. It discovered neutrinos of TeV-PeV energy originating beyond our Galaxy with an energy flux that exceeds the one of high-energy gamma rays of extragalactic origin. Unlike at any other wavelength of light, extragalactic neutrinos outshine the nearby sources in our own Milky way. Updated measurements of the diffuse cosmic neutrino flux indicate that the high-energy gamma rays produced by the neutral pions that accompany cosmic neutrinos lose energy in the sources and are likely to be observed at MeV energy, or below. After the reanalysis of 10 years of archival data with an improved data selection and enhanced data analysis methods, the active galaxy NGC 1068 emerged as the hottest spot in the neutrino sky map. It is also the most significant source in a search at the positions of 110 preselected high-energy gamma-ray sources. Additionally, we find evidence for neutrino emission from the active galaxies PKS 1424+240 and TXS 0506+056. TXS 0506+056 had already been identified as a neutrino source in a multimessenger campaign triggered by a neutrino of 290 TeV energy and, by the independent observation of a neutrino burst in 2014 from this source in archival IceCube data. The observations point to active galaxies as the sources of cosmic neutrinos, and cosmic rays, with the gamma-ray-obscured dense cores near the supermassive black holes at their center as the sites where neutrinos originate, typically within $10\sim100$ Schwarzschild radii.
Abstract: 2305.07043
Full Text: [ PostScript, PDF]
Full Text: [ PostScript, PDF]
Title:Identifying Extended PeVatron Sources via Neutrino Shower Detection
Download PDFAbstract: Identifying the Milky Way's very high energy hadronic cosmic-ray accelerators -- the PeVatrons -- is a critical problem. While gamma-ray observations reveal promising candidate sources, neutrino detection is needed for certainty, and this has not yet been successful. Why not? There are several possibilities, as we delineated in a recent paper [T. Sudoh and J. F. Beacom, Phys. Rev. D 107, 043002 (2023)]. Here we further explore the possibility that the challenges arise because PeVatrons have a large angular extent, either due to cosmic-ray propagation effects or due to clusters of sources. We show that while extended neutrino sources could be missed in the commonly used muon-track channel, they could be discovered in the all-flavor shower channel, which has a lower atmospheric-neutrino background flux per solid angle. Intrinsically, showers are quite directional and would appear so in water-based detectors like the future KM3NeT, even though they are presently badly smeared by light scattering in ice-based detectors like IceCube. Our results motivate new shower-based searches as part of the comprehensive approach to identifying the Milky Way's hadronic PeVatrons.
Abstract: 2305.08019
Full Text: [ PostScript, PDF]
Full Text: [ PostScript, PDF]
Title:TeV halos and the role of pulsar wind nebulae as sources of cosmic ray positrons
Download PDFAbstract: The recent detection of the Geminga PWN by HAWC in the multi-TeV band allows us to infer precious information about the transport of pairs in the immediate surroundings of the pulsar and on the spectrum of pairs contributed by a Geminga-like pulsar to the spectrum of pairs in the cosmic radiation. Moreover, this detection allows us to address the issue of how typical are the so-called TeV halos associated to PWNe. Our calculations confirm the need to have suppressed diffusion in a region of at least $20-50\,$pc around the pulsar, and are used here to infer precious constraints on the spectrum of pairs accelerated at the termination shock: more specifically, we discuss the conditions under which such a spectrum is consistent with that typically expected in a PWN and how it gets modified once it escapes the halo. Finally, we discuss the implications of the existence of a TeV halo around Geminga in terms of acceleration of protons in the pulsar environment, a topic of profound relevance for the whole field of particle acceleration and physics of pulsars.
Abstract: 2305.07697
Full Text: [ PostScript, PDF]
Full Text: [ PostScript, PDF]
Title:The Maximum Energy of Shock-Accelerated Cosmic Rays
Download PDFAbstract: Identifying the accelerators of Galactic cosmic ray protons (CRs) with energies up to a few PeV ($10^{15}$ eV) remains a theoretical and observational challenge. Supernova remnants (SNRs) represent strong candidates, as they provide sufficient energetics to reproduce the CR flux observed at Earth. However, it remains unclear whether they can accelerate particles to PeV energies, particularly after the very early stages of their evolution. This uncertainty has prompted searches for other source classes and necessitates comprehensive theoretical modeling of the maximum proton energy, $E_{\rm max}$, accelerated by an arbitrary shock. While analytic estimates of $E_{\rm max}$ have been put forward in the literature, they do not fully account for the complex interplay between particle acceleration, magnetic field amplification, and shock evolution. This paper uses a multi-zone, semi-analytic model of particle acceleration based on kinetic simulations to place constraints on $E_{\rm max}$ for a wide range of astrophysical shocks. In particular, we develop relationships between $E_{\rm max}$, shock velocity, size, and ambient medium. We find that SNRs can only accelerate PeV particles under a select set of circumstances, namely, if the shock velocity exceeds $\sim 10^4$ km s$^{-1}$ and escaping particles drive magnetic field amplification. However, older, slower SNRs may still produce observational signatures of PeV particles due to populations accelerated when the shock was younger. Our results serve as a reference for modelers seeking to quickly produce a self-consistent estimate of the maximum energy accelerated by an arbitrary astrophysical shock.
Abstract: 2305.09607
Full Text: [ PostScript, PDF]
Full Text: [ PostScript, PDF]
Title:Constraining the cosmic-ray pressure in the inner Virgo Cluster using H.E.S.S. observations of M 87
Download PDFAbstract: The origin of the gamma-ray emission from M87 is currently a matter of debate. This work aims to localize the VHE (100 GeV-100 TeV) gamma-ray emission from M87 and probe a potential extended hadronic emission component in the inner Virgo Cluster. The search for a steady and extended gamma-ray signal around M87 can constrain the cosmic-ray energy density and the pressure exerted by the cosmic rays onto the intra-cluster medium, and allow us to investigate the role of the cosmic rays in the active galactic nucleus feedback as a heating mechanism in the Virgo Cluster. H.E.S.S. telescopes are sensitive to VHE gamma rays and have been utilized to observe M87 since 2004. We utilized a Bayesian block analysis to identify M87 emission states with H.E.S.S. observations from 2004 until 2021, dividing them into low, intermediate, and high states. Because of the causality argument, an extended ($\gtrsim$kpc) signal is allowed only in steady emission states. Hence, we fitted the morphology of the 120h low state data and found no significant gamma-ray extension. Therefore, we derived for the low state an upper limit of 58"(corresponding to $\approx$4.6kpc) in the extension of a single-component morphological model described by a rotationally symmetric 2D Gaussian model at 99.7% confidence level. Our results exclude the radio lobes ($\approx$30 kpc) as the principal component of the VHE gamma-ray emission from the low state of M87. The gamma-ray emission is compatible with a single emission region at the radio core of M87. These results, with the help of two multiple-component models, constrain the maximum cosmic-ray to thermal pressure ratio $X_{CR,max.}$$\lesssim$$0.32$ and the total energy in cosmic-ray protons (CRp) to $U_{CR}$$\lesssim$5$\times10^{58}$ erg in the inner 20kpc of the Virgo Cluster for an assumed CRp power-law distribution in momentum with spectral index $\alpha_{p}$=2.1.
Abstract: 2305.09428
Full Text: [ PostScript, PDF]
Full Text: [ PostScript, PDF]
Title:Hunting for gamma-ray emission from Fast Radio Bursts
Download PDFAbstract: Fast radio bursts (FRBs) are a recently discovered class of GHz-band, ms-duration, Jy-level-flux astrophysical transients, which origin is still a mystery. Exploring their gamma-ray counterpart is crucial for constraining their origin and emission mechanism. Thanks to more than 13 years of gamma-ray data collected by the Fermi-Large Area Telescope, and to more than 1000 FRB events, one of the largest sample created as of today, we perform the largest and deepest search for gamma-ray emission from FRB sources to date. In addition to the study of individual FRB events on different time-scales (from few seconds up to several years), we performed, for the first time, a stacking analysis on the full sample of FRB events as well as a search for triplet photons in coincidence with the radio event. We do not detect significant emission, reporting the most stringent constraints, on short time scales, for the FRB-like emission from SGR 1935+2154 with $E<10^{41}$ erg, corresponding to a factor $<10^7$ with respect to the emitted radio energy. For the stacked signal of steady emission from all repeaters, the obtained upper limit (UL) on the FRBs luminosity ($L<1.6\times10^{43}$ erg s$^{-1}$) is more than two orders of magnitudes lower than those derived from the individual sources. Finally, no individual or triplet photons have been significantly associated with FRB events. We derived the LAT ms energy sensitivity to be $E<10^{47}$ (D$_L$/150 Mpc)$^2$ erg, ruling out a gamma-ray-to-radio energy ratio greater than $10^9$ on ms timescales. The results reported here represent the most stringent UL reported so far on the high-energy emission from FRBs on short and long time scales, as well as on cumulative emission and individual photon searches. While the origin of FRBs is still unclear, our work provides important constraints for FRB modeling, which might shed light on their emission mechanism.
Abstract: 2305.10251
Full Text: [ PostScript, PDF]
Full Text: [ PostScript, PDF]
Title:Model of Cosmic Ray Propagation in the Milky Way at the Knee
Download PDFAbstract: We present a new model of anisotropic cosmic ray propagation in the Milky Way, where cosmic rays are injected at discrete transient sources in the disc and propagated in the Galactic magnetic field. In the framework of our model, we show that the cosmic ray spectrum is time-dependent and space-dependent around the energy of the knee. It has a major contribution of one or a few nearby recent sources at any given location in the Galaxy, in particular at the position of the Solar system. We find that the distribution of $\sim$ PeV cosmic rays in our Galaxy is significantly clumpy and inhomogeneous, and therefore substantially different from the smoother distribution of GeV cosmic rays. Our findings have important implications for the calculation and future interpretation of the diffuse Galactic gamma-ray and neutrino fluxes at very high energies.
Abstract: 2305.09879
Full Text: [ PostScript, PDF]
Full Text: [ PostScript, PDF]
Title:Jump-starting relativistic flows, and the M87 jet
Download PDFAbstract: We point out the dominant importance of plasma injection effects for relativistic winds from pulsars and black holes. We demonstrate that outside the light cylinder the magnetically dominated outflows while sliding along the helical magnetic field move in fact nearly radially with very large Lorentz factors $\gamma_0 \gg 1 $, imprinted into the flow during pair production within the gaps. Only at larger distances, $r \geq \gamma_0 (c/\Omega)$, the MHD acceleration $\Gamma \propto r$ takes over. As a result, Blandford-Znajek (BZ) driven outflows would produce spine-brightened images, contrary to observations of the edge-brightened jet in M87. We conclude that M87 jet is not BZ-driven. Other implications include: (i) variability time scale of emission even from non-blazar AGNe like M87 and Cen A can be much shorter than the BH spin period; (ii) the model explains blazar-like phenomena in clearly non-aligned AGNe; (iii) in PIC simulations of pulsars' and black holes' magnetospheres with pair production, the pairs should be injected with large Lorentz factors - this would drastically change the resulting magnetospheric and inner-wind structure, if compared with pair injection at rest; (iv) the model resolves the problem of cyclotron absorption in the Crab pulsar wind.
This page created: Thu May 18 10:35:50 ACST 2023 by rburley
For a printable title listing click here
For details on generating this page see the instructions. If there are problems with this page (and I expect there will be from time to time) contact Jose.
For previous lists of abstracts of interest click Previous abstracts of interest