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Register a new userPulsars in a Bubble? Following Electron Diffusion in the Galaxy with TeV Gamma Rays
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Added by
Henrike Fleischhack Dr.
Publication date
2019
fields
Physics
and research's language is
English
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TeV Halos, extended regions of TeV gamma-ray emission around middle-aged pulsars, have recently been established as a new source class in gamma-ray astronomy. These halos have been attributed to relativistic electrons and positrons that have left the acceleration region close to the pulsar and are diffusing in the surrounding medium. Measuring the morphology of TeV Halos enables for the first time a direct measurement of the electron diffusion on scales of tens of parsecs. There are hints that the presence of relativistic particles affects the diffusion rate in the pulsars surroundings. Understanding electron diffusion is necessary to constrain the origins of the apparent `excess of cosmic-ray positrons at tens of GeV. TeV Halos can also be used to find mis-aligned pulsars, as well as study certain properties of the Galaxys pulsar population. Future VHE gamma-ray instruments will detect more of those TeV Halos and determine how much pulsars contribute to the observed cosmic-ray electron and positron fluxes, and how they affect diffusion in their environments.
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Its generally believed that young and rapidly rotating pulsars are important sites of particles acceleration, in which protons can be accelerated to relativistic energy above the polar cap region if the magnetic moment is antiparallel to the spin axis($vec{mu}cdotvec{Omega}<0$). To obtain the galactic diffusive neutrinos and gamma-rays for TeV, firstly,we use Monte Carlo(MC) method to generate a sample of young pulsars with ages less than $10^6$ yrs in our galaxy ; secondly, the neutrinos and high-energy gamma-rays can be produced through photomeson process with the interaction of energetic protons and soft X-ray photons ($p+gammarightarrow Delta^+rightarrow n+pi^+/p+pi^0$) for single pulsar, and these X-ray photons come from the neutron star surface. The results suggest that the diffusive TeV flux of neutrinos are lower than background flux, which indicated it is difficult to be detected by the current neutrino telescopes.
The Fermi Large Area Telescope (LAT) is a powerful pulsar detector, as demonstrated by the over one hundred objects in its second catalog of pulsars. Pass 8 is a new reconstruction and event selection strategy developed by the Fermi-LAT collaboration. Due to the increased acceptance at low energy, Pass 8 improves the pulsation detection sensitivity. Ten new pulsars rise above the 5 sigma threshold and are presented in this work, as well as one previously seen with the former Pass 7 reconstruction. More than 60$%$ of the known pulsars with spin-down power ($dot{E}$) greater than $10^{36}$ erg/s show pulsations in gamma-rays, as seen with the Fermi Large Area Telescope. Many non-detections of these energetic pulsars are thought to be a consequence of a high background level, or a large distance leading to a flux below the sensitivity limit of the instrument. The gamma-ray beams of the others probably miss the Earth. The new Pass 8 data now allows the detection of gamma ray pulsations from three of these high spin-down pulsars, PSRs J1828$-$1101, J1831$-$0952 and J1837$-$0604, as well as three others with $dot{E}$ $ge 10^{35}$ erg/s. We report on their properties and we discuss the reasons for their detection with Pass 8.
The Galactic Center (GC) has been long known to host gamma-ray emission detected to >10 TeV. HESS data now points to two plausible origins: the supermassive black hole (perhaps with >PeV cosmic rays and neutrinos) or high-energy electrons from the putative X-ray pulsar wind nebula G359.95-0.04 observed by Chandra and NuSTAR. We show that if the magnetic field experienced by PWN electrons is near the several mG ambient field strength suggested by radio observations of the nearby GC magnetar SGR J1745-29, synchrotron losses constrain the TeV gamma-ray output to be far below the data. Accounting for the peculiar geometry of GC infrared emission, we also find that the requisite TeV flux could be reached if the PWN is ~1 pc from Sgr A* and the magnetic field is two orders of magnitude weaker, a scenario that we discuss in relation to recent data and theoretical developments. Otherwise, Sgr A* is left, which would then be a PeV link to other AGN.
Recent studies suggest that pulsars could be strong sources of TeV muon neutrinos provided positive ions are accelerated by pulsar polar caps to PeV energies. In such a situation muon neutrinos are produced through the delta resonance in interactions of pulsar accelerated ions with its thermal radiation field. High energy gamma rays also should be produced simultaneously in pulsar environment as both charged and neutral pions are generated in the interactions of energetic hadrons with the ambient photon fields. Here we estimate TeV gamma ray flux at Earth from few nearby young pulsars. When compared with the observations we find that proper consideration of the effect of polar cap geometry in flux calculation is important. Incorporating such an effect we obtain the (revised) event rates at Earth due to few potential nearby pulsars. The results suggest that pulsars are unlikely to be detected by the upcoming neutrino telescopes. We also estimate TeV gamma ray and neutrino fluxes from pulsar nebulae for the adopted model of particle acceleration.
The detection of high-energy astrophysical neutrinos and ultra-high-energy cosmic rays (UHECRs) provides a new way to explore sources of cosmic rays. One of the highest energy neutrino events detected by IceCube, tagged as IC35, is close to the UHECR anisotropy region detected by Pierre Auger Observatory. The nearby starburst galaxy (SBG), NGC 4945, is close to this anisotropic region and inside the mean angular error of the IC35 event. Considering the hypernovae contribution located in the SB region of NGC 4945, which can accelerate protons up to $sim 10^{17} , {rm eV}$ and inject them into the interstellar medium, we investigate the origin of this event around this starburst galaxy. We show that the interaction of these protons with the SB regions gas density could explain Fermi-LAT gamma-ray and radio observations if the magnetic fields strength in the SB region is the order of $sim rm mG$. Our estimated PeV neutrino events, in ten years, for this source is approximately 0.01 ($4times10^{-4}$) if a proton spectral index of 2.4 (2.7) is considered, which would demonstrate that IC35 is not produced in the central region of this SBG. Additionally, we consider the superwind region of NGC 4945 and show that protons can hardly be accelerated in it up to UHEs.
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