Abstracts of Interest
Selected by:
Ryan Burley
Abstract: 1805.01902
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Title: Fornax A, Centaurus A and other radio galaxies as sources of ultra-high energy cosmic rays
(Submitted on 4 May 2018)
Abstract: The origin of ultra-high energy cosmic rays (UHECRs) is still unknown. It has recently been proposed that UHECR anisotropies can be attributed to starbust galaxies or active galactic nuclei. We suggest that the latter is more likely and that giant-lobed radio galaxies such as Centaurus A and Fornax A can explain the data.
Abstract: 1805.02496
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Title: IMAGINE: A comprehensive view of the interstellar medium, Galactic magnetic fields and cosmic rays
(Submitted on 7 May 2018)
Abstract: In this white paper we introduce the IMAGINE Consortium and its scientific background, goals and structure. Our purpose is to coordinate and facilitate the efforts of a diverse group of researchers in the broad areas of the interstellar medium, Galactic magnetic fields and cosmic rays, and our goal is to develop more comprehensive insights into the structures and roles of interstellar magnetic fields and their interactions with cosmic rays. To achieve a higher level of self-consistency, depth and rigour can only be achieved by the coordinated efforts of experts in diverse areas of astrophysics involved in observational, theoretical and numerical work. We present our view of the present status of this topic, identify its key unsolved problems and suggest a strategy that will underpin our work. The backbone of the consortium is the Interstellar MAGnetic field INference Engine, a publicly available Bayesian platform that employs robust statistical methods to explore the multi-dimensional likelihood space using any number of modular inputs. It provides an interpretation and modelling framework that has the power and flexibility to include a variety of observational, theoretical and numerical lines of evidence into a self-consistent and comprehensive picture of the thermal and non-thermal interstellar media. An important innovation is that a consistent understanding of the phenomena that are directly or indirectly influenced by the Galactic magnetic field, such as the deflection of ultra-high energy cosmic rays or extragalactic backgrounds, is made an integral part of the modelling. The IMAGINE Consortium, which is informal by nature and open to new participants, hereby presents a methodological framework for the modelling and understanding of Galactic magnetic fields that is available to all communities whose research relies on a state-of-the-art solution to this problem. (Abridged.)
Abstract: 1805.02361
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Title: The origins of the gamma-ray flux variations of NGC 1275 based on 8 years of Fermi-LAT observations
(Submitted on 7 May 2018)
Abstract: We present an analysis of 8 years of Fermi-LAT ( > 0.1 GeV) gamma-ray data obtained for the radio galaxy NGC 1275. The gamma-ray flux from NGC 1275 is highly variable on short (~ days to weeks) timescales, and has steadily increased over this 8-year timespan. By examining the changes in its flux and spectral shape in the LAT energy band over the entire dataset, we found that its spectral behavior changed around 2011 February (~ MJD 55600). The gamma-ray spectra at the early times evolve largely at high energies, while the photon indices were unchanged in the latter times despite rather large flux variations. To explain these observations, we suggest that the flux changes in the early times were caused by injection of high-energy electrons into the jet, while later, the gamma-ray flares were caused by a changing Doppler factor owing to variations in the jet Lorentz factor and/or changes in the angle to our line of sight. To demonstrate the viability of these scenarios, we fit the broad-band spectral energy distribution data with a one-zone synchrotron self-Compton (SSC) model for flaring and quiescent intervals before and after 2011 February. To explain the gamma-ray spectral behavior in the context of the SSC model, the maximum electron Lorentz factor would have changed in the early times, while a modest change in the Doppler factor adequately fits the quiescent and flaring state gamma-ray spectra in the later times.
Abstract: 1805.02646
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Title: Search for a Non-Relativistic Component in the Spectrum of Cosmic Rays at Earth
(Submitted on 7 May 2018 (v1), last revised 9 May 2018 (this version, v2))
Abstract: Dark matter particles gravitationally bound to our galaxy should exhibit a characteristic speed distribution limited by their escape velocity at the position of the Earth ($v_{esc}\simeq$ 550 km/s). An ongoing search for anomalous cosmic rays at Earth, kinematically similar to cold dark matter, is described. The technique can discriminate between these and known slow-moving particles such as neutrons, would be sensitive to telltale signatures from presently unexplored candidates, and offers the possibility of identifying the mediating type of interaction (nuclear vs. electron recoils). Studies of background identification and abatement in a shallow underground site are presented. The expected reach of the method is discussed, and illustrated by obtaining the first limits for dark matter particles lighter than 100 MeV/c$^{2}$ interacting via nuclear recoils.
Abstract: 1805.02750
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Title: Limits on the flux of tau neutrinos from 1 PeV to 3 EeV with the MAGIC telescopes
(Submitted on 7 May 2018)
Abstract: A search for tau neutrino induced showers with the MAGIC telescopes is presented. The MAGIC telescopes located at an altitude of 2200 m a.s.l. in the Canary Island of La Palma, can point towards the horizon or a few degrees below across an azimuthal range of about 80 degrees. This provides a possibility to search for air showers induced by tau leptons arising from interactions of tau neutrinos in the Earth crust or the surrounding ocean. In this paper we show how such air showers can be discriminated from the background of very inclined hadronic showers by using Monte Carlo simulations. Taking into account the orography of the site, the point source acceptance and the event rates expected have been calculated for a sample of generic neutrino fluxes from photo-hadronic interactions in AGNs. The analysis of about 30 hours of data taken towards the sealeads to a 90\% C.L. point source limit for tau neutrinos in the energy range from $1.0 \times 10^{15}$ eV to $3.0 \times 10^{18}$ eV of about $E_{\nu_{\tau}}^{2}\times \phi (E_{\nu_{\tau}}) < 2.0 \times 10^{-4}$ GeV cm$^{-2}$ s$^{-1}$ for an assumed power-law neutrino spectrum with spectral index $\gamma$=-2. However, with 300 hours and in case of an optimistic neutrino flare model, limits of the level down to $E_{\nu_{\tau}}^{2}\times \phi (E_{\nu_{\tau}}) < 8.4 \times 10^{-6}$ GeV cm$^{-2}$ s$^{-1}$ can be expected.
Abstract: 1805.03216
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Title: Constraining the coherence scale of the interstellar magnetic field using TeV gamma-ray observations of supernova remnants
(Submitted on 8 May 2018)
Abstract: Galactic cosmic rays are believed to be accelerated at supernova remnant (SNR) shocks. In the hadronic scenario the TeV gamma-ray emission from SNRs originates from decaying pions that are produced in collisions of the interstellar gas and cosmic rays. Using cosmic ray-magnetohydrodynamic simulations, we show that magnetic obliquity-dependent shock acceleration is able to reproduce the observed TeV gamma-ray morphology of SNRs solely by varying the magnetic morphology. This allows us to constrain the magnetic coherence scale around SN1006 and Vela Jr. to $>200_{-62}^{+10}$ pc and $8.3_{-5.8}^{+15.1}$ pc, respectively.
Abstract: 1805.03406
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Title: Gamma-ray Production in the Extended Halo of the Galaxy and Possible Implications for the Origin of Galactic Cosmic Rays
(Submitted on 9 May 2018)
Abstract: Various studies have implied the existence of a gaseous halo around the Galaxy extending out to ~100 kpc. Galactic cosmic rays that propagate to the halo, either by diffusion or by convection with the possibly existing large-scale Galactic wind, can interact with the gas there and produce gamma-rays via proton-proton collision. We calculate the gamma-ray flux based on the recent studies on gas distribution in the halo, and find that the current measurement of isotropic gamma-ray background at $\lesssim$TeV with Fermi Large Area Telescope already approaches a level that may provide interesting constraints on the properties of Galactic cosmic ray. The resulting Galactic cosmic ray luminosity in some cases is smaller than that estimated with a cosmic-ray free-escape boundary in our Galaxy. Given that the isotropic gamma-ray background is likely to be dominated by unresolved extragalactic sources, future telescopes may extract more individual sources from the IGRB, and hence put even more stringent restriction on the Galactic cosmic ray luminosity in the presence of a turbulent halo that we consider. We also discuss the possibilities of the Fermi bubble and IceCube neutrinos originating from the proton-proton collision between cosmic rays and gas in the halo, as well as the implication of our results for the baryon budget of the hot circumgalactic medium of our Galaxy.
Abstract: 1805.03372
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Title: Leaked GeV CRs from a broken shell: Explaining 9 years Fermi-LAT data of SNR W28
(Submitted on 9 May 2018)
Abstract: Supernova remnant (SNR)\,W28 is well known for its classic hadronic scenario, in which the TeV cosmic rays (CRs) released at the early stage of this mid-aged SNR are illuminating nearby molecular clouds (MCs). Overwhelming evidences have shown that the northeast of the SNR (W28-North) has already encountered with the MC clumps. Through this broken shell -- W28-North, we believe the CRs with energy down to $<$1\,GeV to be able to be injected into nearby MCs. To further testify this hadronic scenario, we first analyse the 9 years Fermi-LAT data in/around W28 with energy down to 0.3\,GeV. Our Fermi-LAT analysis display a 10-200 GeV skymap which spatially matches well with the known TeV sources -- HESS\,J1801-233 (W28-North), HESS\,J1800-240\,A,\,B\,\&\,C (240\,A\,B\,\&\,C). At low energy band, we has discovered a 0.5-1\,GeV blob located to the south of 240\,B\,\&\,C, and a low flux of 0.3-1\,GeV at 240\,A. A hadronic model is build to explain our analysis results and previous multi-wavelength observations of W28. Our model consists of three CR sources: The run-away CRs escaped from a strong shock; The leaked GeV CRs from the broken shell -- W28-North; And the local CR sea. Through modelling the SNR evolution, CR acceleration\,\&\,releasing, we have explained the GeV-TeV emission in/around SNR\,W28 (except for 240\,A) in one model. Both the damping of the magnetic waves by the neutrals and the decreased acceleration efficiency are taken into account in our model due to the mid-age of SNR W28.
Abstract: 1805.03537
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Title: A Luminous and Highly-variable Gamma-ray Flare Following the 2017 Periastron of PSR B1259-63/LS 2883
(Submitted on 9 May 2018)
Abstract: Three periastron passages of the PSR B1259$-$63/LS 2883 binary system, consisting of a 48 ms rotation-powered pulsar and a $\sim30$ M$_{\odot}$ Be star, have been observed by the Large Area Telescope (LAT) on board the Fermi Gamma-ray Space Telescope, in 2010, 2014, and 2017. During the most-recent periastron passage, sustained low-level gamma-ray emission was observed over a $\sim3$-week long interval immediately after periastron, which was followed by an interval of no emission. Sporadic flares were detected starting 40 days post-periastron and lasted approximately 50 days, during which the emission displayed significant spectral curvature, variability on timescales as short as 1.5 minutes, and peak flux levels well in excess of the pulsar spin-down power. By contrast, during the 2010 and 2014 periastron passages, significant gamma-ray emission was not observed with the LAT until 30 and 32 days post-periastron, respectively. The previous flares did not exhibit spectral curvature, showed no short term variability, and did not exceed the pulsar spin-down power. The high flux and short timescales observed in 2017 suggest significant beaming of the emission is required and constrain the size of the emission region. The flares occur long enough after periastron that the neutron star should already have passed through the extended disk-like outflow, thus constraining options for target material and seed photon sources for inverse Compton models.
Abstract: 1805.04500
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Title: Heavy decaying dark matter and IceCube high energy neutrinos
(Submitted on 11 May 2018)
Abstract: We examine the hypothesis of decaying heavy dark matter (HDM) in the context of the IceCube highest energy neutrino events and recent limits on the diffuse flux of high-energy photons. We consider DM particles $X$ of mass $10^{6}~\leq~M_X~\leq~10^{16}$ GeV decaying on tree-level into $X~\rightarrow~\nu\bar{\nu}$, $X~\rightarrow~e^+e^-$ and $X~\rightarrow~q\bar{q}$. The full simulation of hadronic and electroweak decay cascades and the subsequent propagation of the decay products through the interstellar medium allows us to determine the permitted values of $M_X$. We show that for leptonic decay channels it is possible to explain the IceCube highest energy neutrino signal without overproducing high-energy photons for $M_X~\lesssim~5.5 \cdot 10^{7}$ GeV and $1.5 \cdot 10^{8}~\lesssim~M_X~\lesssim~1.5 \cdot 10^{9}$ GeV, while hadronic decays contradict the gamma-ray limits for almost the whole range of $M_X$ values considered. The leptonic hypothesis can be probed by operating and planned gamma-ray observatories: for instance, the currently upgrading Carpet experiment will be capable to test a significant part of the remaining parameter window within one year of observation.
Abstract: 1805.04163
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Title: Hyper-Kamiokande Design Report
(Submitted on 9 May 2018)
Abstract: On the strength of a double Nobel prize winning experiment (Super)Kamiokande and an extremely successful long baseline neutrino programme, the third generation Water Cherenkov detector, Hyper-Kamiokande, is being developed by an international collaboration as a leading worldwide experiment based in Japan. The Hyper-Kamiokande detector will be hosted in the Tochibora mine, about 295 km away from the J-PARC proton accelerator research complex in Tokai, Japan. The currently existing accelerator will be steadily upgraded to reach a MW beam by the start of the experiment. A suite of near detectors will be vital to constrain the beam for neutrino oscillation measurements. A new cavern will be excavated at the Tochibora mine to host the detector. The experiment will be the largest underground water Cherenkov detector in the world and will be instrumented with new technology photosensors, faster and with higher quantum efficiency than the ones in Super-Kamiokande. The science that will be developed will be able to shape the future theoretical framework and generations of experiments. Hyper-Kamiokande will be able to measure with the highest precision the leptonic CP violation that could explain the baryon asymmetry in the Universe. The experiment also has a demonstrated excellent capability to search for proton decay, providing a significant improvement in discovery sensitivity over current searches for the proton lifetime. The atmospheric neutrinos will allow to determine the neutrino mass ordering and, together with the beam, able to precisely test the three-flavour neutrino oscillation paradigm and search for new phenomena. A strong astrophysical programme will be carried out at the experiment that will also allow to measure precisely solar neutrino oscillation.
Abstract: 1805.03959
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Title: Gamma-Ray Burst Optical Afterglows with Two-Component Jets: Polarization Evolution Revisited
(Submitted on 7 May 2018)
Abstract: Gamma-ray bursts (GRBs) have been widely argued to originate from binary compact object mergers or core collapses of massive stars. Jets from these systems may have two components, an inner, narrow sub-jet and an outer, wider sub-jet. Such a jet subsequently interacts with its ambient gas, leading to a reverse shock (RS) and a forward shock (FS). The magnetic field in the narrow sub-jet is very likely to be mixed by an ordered component and a random component during the afterglow phase. In this paper, we calculate light curves and polarization evolution of optical afterglows with this mixed magnetic field in the RS region of the narrow sub-jet in a two-component jet model. The resultant light curve has two peaks: an early peak arises from the narrow sub-jet and a late-time rebrightening is due to the wider sub-jet. We find the polarization degree (PD) evolution under such a mixed magnetic field confined in the shock plane is very similar to that under the purely ordered magnetic field condition. The two-dimensional `mixed' magnetic fields confined in the shock plane are essentially the ordered magnetic fields only with different configurations. The position angle (PA) of the two-component jet can change gradually or abruptly by $90^\circ$. In particular, an abrupt $90^\circ$ change of the PA occurs when the PD changes from its decline phase to rise phase.
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