Abstracts of Interest
Selected by:
Roger Clay
Abstract: 1803.07654
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Title: Fermi-LAT observations of the 2017 September 10$^{th}$ solar flare
(Submitted on 20 Mar 2018)
Abstract: The Fermi-Large Area Telescope (LAT) detection of the X8.2 GOES class solar flare of 2017 September 10 provides for the first time observations of a long duration high-energy gamma-ray flare associated with a Ground Level Enhancement (GLE). The >100 MeV emission from this flare lasted for more than 12 hours covering both the impulsive and extended phase. We present the localization of the gamma-ray emission and find that it is consistent with the active region (AR) from which the flare occurred over a period lasting more than 6 hours contrary to what was found for the 2012 March 7 flares. The temporal variation of the proton index inferred from the gamma-ray data seems to suggest two phases in acceleration of the proton population. Based on timing arguments we interpret the second phase to be tied to the acceleration mechanism powering the GLE, believed to be particle acceleration at a coronal shock driven by the CME.
Abstract: 1803.07803
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Title: A low threshold compact gamma-ray detector consisting of LaBr3 and SiPMs for GECAM
(Submitted on 21 Mar 2018 (v1), last revised 23 Mar 2018 (this version, v2))
Abstract: The Gravitational wave high-energy Electromagnetic Counterparts All-sky Monitor (GECAM) project is the planned Chinese space telescope for detecting the counterparts of gravitational wave events. It consists of two micro-satellites in low earth orbit with the advantage of an instantaneous full-sky coverage, and can be achieved within a short period and small budget. Due to the limitation of size, weight and power consumption in micro-satellites, Silicon Photomultipliers (SiPMs) are used to replace the Photomultiplier Tubes (PMTs) to assemble a novel gamma-ray detector. A prototype of a SiPM array with a LaBr3 crystal is tested and resulted in a good a high detection efficiency (72% at 5.9 keV) and an excellent uniformity. The low-energy X-ray of 5.9 keV can be detected by a simply readout circuit, and the energy resolution is 6.5% (FWHM) at 662 keV. The design and the detector performances are discussed in detail in this paper.
Abstract: 1803.08376
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Title: The Propagation of Cosmic Rays from the Galactic Wind Termination Shock: Back to the Galaxy?
(Submitted on 22 Mar 2018)
Abstract: Although several theories for the origin of cosmic rays in the region between the spectral `knee' and `ankle' exist, this problem is still unsolved. A variety of observations suggest that the transition from Galactic to extragalactic sources occurs in this energy range. In this work we examine whether a Galactic wind which eventually forms a termination shock far outside the Galactic plane can contribute as a possible source to the observed flux in the region of interest. Previous work by Bustard et al. (2017) estimated that particles can be accelerated up to energies above the `knee' up to $R_\mathrm{max} = 10^{16}$ eV for parameters drawn from a model of a Milky Way wind (Everett et al. 2017). A remaining question is whether the accelerated cosmic rays can propagate back into the Galaxy. To answer this crucial question, we simulate the propagation of the cosmic rays using the low energy extension of the CRPropa framework, based on the solution of the transport equation via stochastic differential equations. The setup includes all relevant processes, including three-dimensional anisotropic spatial diffusion, advection, and corresponding adiabatic cooling. We find that, assuming realistic parameters for the shock evolution, a possible Galactic termination shock can contribute significantly to the energy budget in the `knee' region and above. We estimate the resulting produced neutrino fluxes and find them to be below measurements from IceCube and limits by KM3NeT.
Abstract: 1803.08662
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Title: Muon content of extensive air showers: comparison of the energy spectra obtained by the Sydney University Giant Air-shower Recorder and by the Pierre Auger Observatory
(Submitted on 23 Mar 2018)
Abstract: The Sydney University Giant Air-shower Recorder (SUGAR) measured the energy spectrum of ultra-high-energy cosmic rays reconstructed from muon-detector readings, while the Pierre Auger Observatory, looking at the same Southern sky, uses the calorimetric fluorescence method for the same purpose. Comparison of their two spectra allows us to reconstruct the empirical dependence of the number of muons in the shower on the primary energy for energies between $10^{17}$ and $10^{18.5}$ eV. We compare this dependence with the predictions of hadronic interaction models \mbox{QGSJET-II-04} and \mbox{EPOS-LHC}. The empirically determined number of muons with energies above 0.75 GeV exceeds the simulated one by the factors $\sim$1.67 and $\sim$1.28 for $10^{17}$ eV proton and iron primaries, respectively. The muon excess grows moderately with the primary energy, increasing by an additional factor of $\sim 1.2$ for $10^{18.5}$ eV primaries.
Abstract: 1803.08017
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Title: The KM3NeT Neutrino Telescope and the potential of a neutrino beam from Russia to the Mediterranean Sea
(Submitted on 21 Mar 2018 (v1), last revised 22 Mar 2018 (this version, v2))
Abstract: KM3NeT is a new generation neutrino telescope currently under construction at two sites in the Mediterranean Sea. At the Capo Passero site, 100 km off-shore Sicily, Italy, a volume of more than one cubic kilometre of water will be instrumented with optical sensors. This instrument, called ARCA, is optimized for observing cosmic sources of TeV and PeV neutrinos. The other site, 40 km off-shore Toulon, France, will host a much denser array of optical sensors, ORCA. With an energy threshold of a few GeV, ORCA will be capable to determine the neutrino mass hierarchy through precision measurements of atmospheric neutrino oscillations. In this contribution, we review the scientific goals of KM3NeT and the status of its construction. We also discuss the scientific potential of a neutrino beam from Protvino, Russia to ORCA. We show that such an experiment would allow for a measurement of the CP-violating phase in the neutrino mixing matrix. To achieve a sensitivity competitive with that of the other planned long-baseline neutrino experiments such as DUNE and T2HK, an upgrade of the Protvino accelerator complex will be necessary.
Abstract: 1803.07478
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Title: Neutrinos from Choked Jets Accompanied by Type-II Supernovae
(Submitted on 20 Mar 2018)
Abstract: The origin of the IceCube neutrinos is still an open question. Upper limits from diffuse gamma-ray observations suggest that the neutrino sources are either distant or hidden from gamma-ray observations. It is possible that the neutrinos are produced in jets that are formed in the core-collapsing massive stars and fail to break out, the so-called choked jets. We study neutrinos from the jets choked in the hydrogen envelopes of red supergiant stars. Fast photo-meson cooling softens the neutrino spectrum, making it difficult to explain the PeV neutrinos observed by IceCube in a one-component scenario, but a two-component model can explain the spectrum. Furthermore, we predict that a newly born jet-driven type-II supernova may be observed to be associated with a neutrino burst detected by IceCube.
Abstract: 1803.07873
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Title: Gamma Ray Bursts. Progenitors, accretion in the central engine, jet acceleration mechanisms
(Submitted on 21 Mar 2018)
Abstract: The collapsar model was proposed to explain the long-duration gamma-ray bursts (GRBs), while the short GRBs are associated with the mergers of compact objects. In the first case, mainly the energetics of the events is consistent with the proposed progenitor models, while the duration, time variability, as well as the afterglow emission may shed some light on the detailed properties of the collapsing massive stars. In the latter case, the recent discovery of the binary neutron star (NS-NS) merger in the gravitational wave observation made by LIGO (GW170817), and the detection of associated electromagnetic counterparts, for the first time gave a direct proof of the NS-NS merger being a progenitor of a short GRB.
In general, all GRBs are believed to be powered by accretion through a rotationally supported torus, or by fast rotation of a compact object. For long ones, the rotation of the progenitor star is a key property in order to support accretion over relatively long activity periods, and also to sustain the rotation of the black hole itself. The latter is responsible for ejection of the relativistic jets, which are powered due to the extraction of the BH rotational energy, mitigated by the accretion torus and magnetic fields. The jets must break through the stellar envelope though, which poses a question on the efficiency of this process. Similar mechanisms of powering the jet ejection may act in short GRBs, which in this case may freely propagate through the interstellar medium. The power of the jets launched from the rotating black hole is at first associated mostly with the magnetic Poynting flux, and then at large distances it is transferred to the kinetic and finally radiative energy of the expanding shells.
Abstract: 1803.07871
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Title: Cosmic Ray Propagation in Turbulent Spiral Magnetic Fields associated with Young Stellar Objects
(Submitted on 21 Mar 2018)
Abstract: External cosmic rays impinging upon circumstellar disks associated with young stellar objects provide an important source of ionization, and as such, play an important role in disk evolution and planet formation. However, these incoming cosmic rays are affected by a variety of physical processes internal to stellar/disk systems, including modulation by turbulent magnetic fields. Globally, these fields naturally provide both a funneling effect, where cosmic rays from larger volumes are focused into the disk region, and a magnetic mirroring effect, where cosmic rays are repelled due to the increasing field strength. This paper considers cosmic ray propagation in the presence of a turbulent spiral magnetic field, analogous to that produced by the Solar wind. The interaction of this wind with the interstellar medium defines a transition radius, analogous to the Heliopause, which provides the outer boundary to this problem. We construct a new coordinate system where one coordinate follows the spiral magnetic field lines and consider magnetic perturbations to the field in the perpendicular directions. The presence of magnetic turbulence replaces the mirroring points with a distribution of values and moves the mean location outward. Our results thus help quantify the degree to which cosmic ray fluxes are reduced in circumstellar disks by the presence of magnetic field structures that are shaped by stellar winds. The new coordinate system constructed herein should also be useful in other astronomical applications.
Abstract: 1803.08046
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Title: Millisecond Pulsars, TeV Halos, and Implications For The Galactic Center Gamma-Ray Excess
(Submitted on 21 Mar 2018)
Abstract: Observations by HAWC indicate that many young pulsars (including Geminga and Monogem) are surrounded by spatially extended, multi-TeV emitting regions. It is not currently known, however, whether TeV emission is also produced by recycled, millisecond pulsars (MSPs). In this study, we perform a stacked analysis of 24 MSPs within HAWC's field-of-view, finding between 2.6-3.2 sigma evidence that these sources are, in fact, surrounded by TeV halos. The efficiency with which these MSPs produce TeV halos is similar to that exhibited by young pulsars. This result suggests that several dozen MSPs will ultimately be detectable by HAWC, including many "invisible" pulsars without radio beams oriented in our direction. The TeV halos of unresolved MSPs could also dominate the TeV-scale diffuse emission observed at high galactic latitudes. We also discuss the possibility that TeV and radio observations could be used to constrain the population of MSPs that is present in the inner Milky Way, thereby providing us with a new way to test the hypothesis that MSPs are responsible for the Galactic Center GeV excess.
Abstract: 1803.07595
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Title: From $γ$ to Radio - The Electromagnetic Counterpart of GW 170817
(Submitted on 20 Mar 2018)
Abstract: The gravitational waves from the first binary neutron star merger, GW170817, were accompanied by a multi-wavelength electromagnetic counterpart, from $\gamma$-rays to radio. The accompanying gamma-rays, seems at first to confirm the association of mergers with short gamma-ray bursts (sGRBs). The common interpretation was that we see an emission from an sGRB jet seen off-axis. However, a closer examination of the sub-luminous $\gamma$-rays and the peculiar radio afterglow were inconsistent with this simple interpretation. Here we present results of 3D and 2D numerical simulations that follow the hydrodynamics and emission of the outflow from a neutron star merger form its ejection and up to its deceleration by the circum-merger medium. Our results show that the entire set of $\gamma$-rays, X-rays and radio observations can be explained by the emission from a mildly relativistic cocoon material (Lorentz factor $\sim$2-5) that was formed while a jet propagated through the material ejected during the merger. The $\gamma$-rays are generated when the cocoon breaks out from the engulfing ejecta while the afterglow is produced by interaction of the cocoon matter with the interstellar medium. The strong early uv/optical signal may be a Lorentz boosted macronova/kilonova. The fate of the jet itself is currently unknown, but our full-EM models define a path to resolving between successful and choked jet scenarios, outputting coupled predictions for the image size, morphology, observed time-dependent polarization and light curve behavior from radio to X-ray. The predictive power of these models will prove key in interpreting the on-going multi-faceted observations of this unprecedented event.
Abstract: 1803.07320
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Title: The $H_0$ Tension in Non-flat QCDM Cosmology
(Submitted on 20 Mar 2018)
Abstract: The recent local measurement of Hubble constant leads to a more than $3\sigma$ tension with Planck + LCDM (Riess et al, 2018). In this article we study the $H_0$ tension in non-flat QCDM cosmology, where Q stands for a minimally coupled and slowly rolling quintessence field $\phi$ with a smooth potential $V(\phi)$. By generalizing the QCDM parametrization in Huang et al, 2011 to non-flat universe and using the latest cosmological data, we find that the $H_0$ tension remains at $3.2\sigma$ level for this class of model.
Abstract: 1803.08592
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Full Text: [ PostScript, PDF]
Title: The cosmic transparency measured with Type Ia supernovae: implications for intergalactic dust
(Submitted on 22 Mar 2018)
Abstract: Observations of high-redshift Type Ia supernovae (SNe~Ia) are used to study the cosmic transparency at optical wavelengths. Assuming a flat $\Lambda$CDM cosmological model based on BAO and CMB results, redshift dependent deviations of SN~Ia distances are used to constrain mechanisms that would dim light. The analysis is based on the most recent Pantheon SN compilation, for which there is a $0.03\pm0.01 {\textrm \,(\rm stat)}$ mag discrepancy in the distant supernova distance moduli relative to the $\Lambda$CDM model anchored by supernovae at $z<0.05$. While there are known systematic uncertainties that combined could explain the observed offset, here we entertain the possibility that the discrepancy may instead be explained by scattering of supernova light in the intergalactic medium (IGM). We focus on two effects: Compton scattering by free electrons and extinction by dust in the IGM. We find that if the discrepancy is due entirely to dimming by dust, the measurements can be modeled with a cosmic dust density $\Omega_{\rm IGM}^{\rm dust} = 8 \cdot 10^{-5} (1+z)^{-1}$, corresponding to an average attenuation of $2\cdot 10^{-5}$ mag Mpc$^{-1}$ in V-band. Forthcoming SN~Ia studies may provide a definitive measurement of the IGM dust properties, while still providing an unbiased estimate of cosmological parameters by introducing additional parameters in the global fits to the observations.
Abstract: 1803.07298
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Title: Robust Limits on Photon Mass from Statistical Samples of Extragalactic Radio Pulsars
(Submitted on 20 Mar 2018)
Abstract: The photon zero-mass hypothesis has been investigated for a long time using the frequency-dependent time delays of radio emissions from astrophysical sources. However, the search for a rest mass of the photon has been hindered by the similarity between the frequency-dependent dispersions due to the plasma and nonzero photon mass effects. Here we propose a new method through which the degeneracy of these two effects can be alleviated, by fitting a combination of the dispersion measures of radio sources. This allows us to simultaneously establish a robust limit on the photon mass. Using the observed dispersion measures from two statistical samples of extragalactic pulsars, we show that at the 68\% confidence level, the constraints on the photon mass can be as low as $m_{\gamma}\leq1.5\times10^{-48}~\rm kg\simeq8.4\times10^{-13}~{\rm eV}/c^{2}$ for the sample of 22 radio pulsars in the Large Magellanic Cloud and $m_{\gamma}\leq1.6\times10^{-48}~\rm kg\simeq8.8\times10^{-13} {\rm eV}/c^{2}$ for the other sample of 5 radio pulsars in the Small Magellanic Cloud, which are a factor of $\sim1$ better than that obtained by a single extragalactic pulsar. Furthermore, the method presented here can also be used when more fast radio bursts with known redshifts are detected in the future.
Abstract: 1803.07570
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Title: Optimal Target Stars in the Search for Life
(Submitted on 20 Mar 2018)
Abstract: The selection of optimal targets in the search for life represents a highly important strategic issue. In this paper, we evaluate the relative benefits of searching for life around a potentially habitable planet orbiting a star of arbitrary mass relative to a Sun-like star. If recent physical arguments implying that the habitability of planets orbiting low-mass stars is selectively suppressed are correct, we find that planets around solar-type stars may represent the optimal targets.
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