Abstracts of Interest
Selected by:
Ryan Burley
Abstract: 1809.05321
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Title: Ultra-high energy cosmic rays and neutrinos from light nuclei composition
(Submitted on 14 Sep 2018)
Abstract: The baryonic mass composition of ultra-high energy ($\gtrsim 10^{18}$ eV) cosmic rays (UHECRs) at injection along with other factors directly governs the UHECR flux on the earth. High energy neutrinos produced from UHECR interactions on cosmological photon backgrounds can further serve as crucial astrophysical messengers of the most powerful particle accelerators in the Universe. The latest measurements at the Pierre Auger Observatory (PAO) suggest a mixed element composition of UHECRs with the sub-ankle spectrum being explained by a different class of sources than the super-ankle region ($> 10^{18.7}$ eV). In this work, we achieve a fit to the UHECR spectrum with a single population of sources over an energy range commencing at $\approx 10^{18}$ eV. We test the credibility of p+He composition by considering various abundance fractions at injection and a simple power-law evolution in redshift for source emissivity. We use CRPropa 3, a Monte Carlo simulation tool, for propagating primary and secondary ultra-high energy particles through extragalactic space and for calculating UHECR and cosmogenic neutrino fluxes on the earth. Many good fits are found corresponding to a range of parameter values, that well explains the UHECR spectrum. We place limits on the source spectral index and cut-off rigidity of the source population. Cosmogenic neutrino fluxes can further constrain the abundance fraction and maximum source redshift in this light nuclei injection model.
Abstract: 1809.05333
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Title: Energy spectra of abundant cosmic-ray nuclei in the NUCLEON experiment
(Submitted on 14 Sep 2018)
Abstract: The NUCLEON satellite experiment is designed to directly investigate the energy spectra of cosmic-ray nuclei and the chemical composition (Z=1-30) in the energy range of 2-500 TeV. The experimental results are presented, including the energy spectra of different abundant nuclei measured using the new Kinematic Lightweight Energy Meter (KLEM) technique. The primary energy is reconstructed by registration of spatial density of the secondary particles. The particles are generated by the first hadronic inelastic interaction in a carbon target. Then additional particles are produced in a thin tungsten converter, by electromagnetic and hadronic interactions.
Abstract: 1809.05104
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Title: Stochastic cosmic ray sources and the TeV break in the all-electron spectrum
(Submitted on 13 Sep 2018)
Abstract: Despite significant progress over more than 100 years, no accelerator has been unambiguously identified as the source of the locally measured flux of cosmic rays. High-energy electrons and positrons are of particular importance in the search for nearby sources as radiative energy losses constrain their propagation to distances of about 1 kpc around 1 TeV. At the highest energies, the spectrum is therefore dominated and shaped by only a few sources whose properties can be inferred from the fine structure of the spectrum at energies currently accessed by experiments like AMS-02, CALET, DAMPE, Fermi-LAT, H.E.S.S. and ISS-CREAM. We present a stochastic model of the Galactic all-electron flux and evaluate its compatibility with the measurement recently presented by the H.E.S.S. collaboration. To this end, we have MC generated a large sample of the all-electron flux from an ensemble of random distributions of sources. We confirm the non-Gaussian nature of the probability density of fluxes at individual energies previously reported in analytical computations. For the first time, we also consider the correlations between the fluxes at different energies, treating the binned spectrum as a random vector and parametrising its joint distribution with the help of a pair-copula construction. We show that the spectral break observed in the all-electron spectrum by H.E.S.S. and DAMPE is statistically compatible with a distribution of astrophysical sources like supernova remnants or pulsars, but requires a rate smaller than the canonical supernova rate. This important result provides an astrophysical interpretation of the spectrum at TeV energies and allows differentiating astrophysical source models from exotic explanations, like dark matter annihilation. We also critically assess the reliability of using catalogues of known sources to model the electron-positron flux.
Abstract: 1809.05064
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Title: PSR J2234+0611: A new laboratory for stellar evolution
(Submitted on 13 Sep 2018)
Abstract: We report timing results for PSR J2234+0611, a 3.6-ms pulsar in a 32-day, eccentric (e = 0.13) orbit with a helium white dwarf companion discovered as part of the Arecibo Observatory 327 MHz drift scan survey. The precise timing and the eccentric nature of the orbit allow precise measurements of an unusual number of parameters: a) a precise proper motion of 27.10(3) mas/yr and a parallax of 1.05(4) mas resulting in a pulsar distance of 0.95(4) kpc; this allows a precise estimate of the transverse velocity, 123(5) km/s. Together with previously published spectroscopic measurements of the systemic radial velocity, this allows a full 3-D determination of the system's velocity; b) precise measurements of the rate of advance of periastron, which after subtraction of the contribution of the proper motion yields a total system mass of $1.6518^{+0.0033}_{-0.0035}$ solar masses; c) a Shapiro delay measurement, h_3 = $82 \pm 14$ ns despite the orbital inclination not being near 90 deg; combined with the measurement of the total mass, this yields a pulsar mass of $1.353^{+0.014}_{-0.017}$ solar masses and a companion mass of $0.298^{+0.015}_{-0.012}$ solar masses; d) we measure precisely the secular variation of the projected semi-major axis and detect significant annual orbital parallax; together these allow a determination of the full 3-D orbital geometry, including an unambiguous orbital inclination (i = $138.7^{+2.5}_{-2.2}$ deg) and a position angle for the line of nodes (Omega = $44^{+5}_{-4}$ deg). We discuss the component masses to investigate hypotheses previously advanced to explain the origin of eccentric MSPs. The unprecedented determination of the full 3-D position, motion and orbital orientation of the system, plus the precisely measured pulsar and WD mass and the latter's optical detection make this system an unique test of our understanding of white dwarfs and their atmospheres.
Abstract: 1809.05031
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Title: Exoplanet Terra Incognita
(Submitted on 4 Sep 2018)
Abstract: Exoplanet surface imaging, cartography and the search for exolife are the next frontiers of planetology and astrophysics. Here we present an over-view of ideas and techniques to resolve albedo features on exoplanetary surfaces. Albedo maps obtained in various spectral bands (similar to true-colour images) may reveal exoplanet terrains, geological history, life colonies, and even artificial structures of advanced civilizations.
Abstract: 1809.04984
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Full Text: [ PostScript, PDF]
Title: Locations of optical and $γ$-ray emitting regions in the jet of PMN J2345-1555
(Submitted on 13 Sep 2018)
Abstract: We collect long term $\gamma$-ray, optical and radio $15$ GHz light curves of the flat-spectrum radio quasar (FSRQ) object PMN J2345-1555. The correlation analyses between them are performed via the local cross-correlation function (LCCF). We found that all the optical $V$, $R$ band and the infrared $J$ band are correlated with the radio 15 GHz with significance larger than $3\sigma$, and the lag times are $-221.81^{+6.26}_{-6.72}$, $-201.38^{+6.42}_{-6.02}$ and $-192.27^{+8.26}_{-7.37}$ days, respectively. However, the $\gamma$-ray is correlated with the radio at only $1.5 \sigma$ significance level, and the lag time is $-103.51^{+9.57}_{-8.69}$ days. The lag times enable us to derive that the optical $V$ band and the $\gamma$-ray emitting regions are located at $5.47\pm0.31$ and $11.16\pm0.46$ parsec from the base of the jet, respectively. We present that time lags between different frequencies can be used as an alternative parameter to derive the core-shift measurement, and the magnetic field and particle density of jets can be derived by this method in a direct way. The variation of $R-J$ color index shows a bluer when brighter trend. We also found that a $3\sigma$ correlation between $\delta V-\delta R$ and the radio light curve, which indicates that opacity plays an important role in the variation. As hinted from radio images, we proposed that the upstream and downstream emission component contributes the bluer and redder spectral index, respectively. Such spatial dependent spectral index model is complementary for the shock in jet model, and provides a new mechanism to explain various spectral index behaviors of blazars in a unified way.
Abstract: 1809.04905
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Title: Revisit cosmic ray propagation by using $^{1}$H, $^{2}$H, $^{3}$He and $^{4}$He
(Submitted on 13 Sep 2018)
Abstract: The secondary-to-primary ratios are unique tools to investigate cosmic ray propagation mechanisms. In this work, we use the latest data of deuteron-to-helium~4 ratio and helium~3-to-helium~4 ratio measured by PAMELA combined with other Z$\leq$2 primary fluxes measured by PAMELA and Voyager-1, to constrain the cosmic ray acceleration and propagation models. The analysis is performed by interfacing statistical tools with the GALPROP propagation package. To better fit both the modulated and unmodulated low energy cosmic ray data, we find that a time-, charge- and rigidity-dependent solar modulation model is better than the force-field approximation. Among all the studied cosmic ray propagation models, the diffusion-reacceleration-convection model is strongly supported by the derived Bayesian evidence. The robustness of the estimated diffusion slope $\delta$ is cross-checked by another low-mass secondary-to-primary ratio, i.e. the antiproton-to-proton ratio. It is shown that the diffusion-reacceleration-convection model can reconcile well with the high energy antiproton-to-proton ratio. This indicates that the estimated value of $\delta$ is reliable. The well constraint $\delta$ from the `best' model is found to be close to 1/3, inferring a Kolmogorov-type interstellar magnetic turbulence.
Abstract: 1809.04866
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Title: Cosmogenic Neutrinos Through the GRAND Lens Unveil the Nature of Cosmic Accelerators
(Submitted on 13 Sep 2018)
Abstract: The sources of cosmic rays with energies above 55 EeV are still mysterious. A guaranteed associated flux of ultra high energy neutrinos known as the cosmogenic neutrino flux will be measured by next generation radio facilities, such as the Giant Radio Array for Neutrino Detection (GRAND). By using the orthogonal information provided by the cosmogenic neutrino flux, we here determine the ability of GRAND to constrain the source redshift evolution and the chemical composition of the cosmic ray sources. If the redshift evolution is known, independently on GRAND's energy resolution, GRAND will constrain the proton/iron fraction to the $\sim5-10\%$ level after one year of data taking; on the other hand, if hints on the average source composition are given, GRAND will measure the redshift evolution of the sources to a $\sim 10\%$ uncertainty. However, GRAND alone will not be able to break the degeneracy between redshift evolution of the sources and their composition. Our findings underline the discriminating potential of next generation radio array detectors and motivate further efforts in this direction.
Abstract: 1809.04617
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Title: Europa's Optical Aurora
(Submitted on 12 Sep 2018)
Abstract: Auroral emissions provide opportunities to study the tenuous atmospheres of Solar System satellites, revealing the presence and abundance of molecular and atomic species as well as their spatial and temporal variability. Far-UV aurorae have been used for decades to study the atmospheres of the galilean satellites. Here we present the first detection of Europa's visible-wavelength atomic oxygen aurora at 6300/6364 \AA{} arising from the metastable O$(^1$D) state, observed with the Keck I and Hubble Space Telescopes while Europa was in eclipse by Jupiter on six occasions in February-April 2018. The disk-integrated O($^1$D) brightness varies from $<$500 R up to more than 2 kR between dates, a factor of 15 higher than the OI 1356 \AA{} brightness on average. The ratio of emission at 6300/5577 \AA{} is diagnostic of parent molecule; the 5577 \AA{} emission was not detected in our dataset, which favors O$_2$ as the dominant atmospheric constituent and rules out an O/O$_2$ mixing ratio above 0.35. For an O$_2$ atmosphere and typical plasma conditions at Europa's orbit, the measured surface brightness range corresponds to column densities of 1-9$\times$10$^{14}$ cm$^{-2}$.
Abstract: 1809.04168
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Title: Production of atomic hydrogen by cosmic rays in dark clouds
(Submitted on 11 Sep 2018)
Abstract: The presence of small amounts of atomic hydrogen, detected as absorption dips in the 21 cm line spectrum, is a well-known characteristic of dark clouds. The abundance of hydrogen atoms measured in the densest regions of molecular clouds can be only explained by the dissociation of H$_2$ due to cosmic rays. We want to assess the role of Galactic cosmic rays in the formation of atomic hydrogen, by using recent developments in the characterisation of the low-energy spectra of cosmic rays and advances in the modelling of their propagation in molecular clouds. We model the attenuation of the interstellar cosmic rays entering a cloud and compute the dissociation rate of molecular hydrogen due to collisions with cosmic-ray protons and electrons as well as fast hydrogen atoms. We compare our results with the available observations. The cosmic-ray dissociation rate is entirely determined by secondary electrons produced in primary ionisation collisions. These secondary particles constitute the only source of atomic hydrogen at column densities above $\sim10^{21}$ cm$^{-2}$. We also find that the dissociation rate decreases with column density, while the ratio between the dissociation and ionisation rates varies between about 0.6 and 0.7. From comparison with observations we conclude that a relatively flat spectrum of interstellar cosmic-ray protons, as the one suggested by the most recent Voyager 1 data, can only provide a lower bound for the observed atomic hydrogen fraction. An enhanced spectrum of low-energy protons is needed to explain most of the observations. Our findings show that a careful description of molecular hydrogen dissociation by cosmic rays can explain the abundance of atomic hydrogen in dark clouds. An accurate characterisation of this process at high densities is crucial for understanding the chemical evolution of star-forming regions.
Abstract: 1809.04150
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Title: High-energy neutrino interaction physics with IceCube
(Submitted on 11 Sep 2018)
Abstract: Although they are best known for studying astrophysical neutrinos, neutrino telescopes like IceCube can study neutrino interactions, at energies far above those that are accessible at accelerators. In this writeup, I present two IceCube analyses of neutrino interactions at energies far above 1 TeV. The first measures neutrino absorption in the Earth, and, from that determines the neutrino-nucleon cross-section at energies between 6.3 and 980 TeV. We find that the cross-sections is 1.30 $^{+0.21}_{-0.19}$ (stat.) $^{+0.39}_{-0.43}$ (syst.) times the Standard Model cross-section. We also present a measurement of neutrino inelasticity, using $\nu_\mu$ charged-current interactions that occur within IceCube. We have measured the average inelasticity at energies from 1 TeV to above 100 TeV, and found that it is in agreement with the Standard Model expectations. We have also performed a series of fits to this track sample and a matching cascade sample, to probe aspects of the astrophysical neutrino flux, particularly the flavor ratio.
Abstract: 1809.03426
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Title: Prospects for gamma-ray observations of narrow-line Seyfert 1 galaxies with the Cherenkov Telescope Array
(Submitted on 10 Sep 2018)
Abstract: Gamma-ray emitting narrow-line Seyfert 1 ($\gamma$-NLSy1) galaxies are thought to harbour relatively low-mass black holes (10$^6$-10$^8$ M$_{\odot}$) accreting close to the Eddington limit. They show characteristics similar to those of blazars, such as flux and spectral variability in the gamma-ray energy band and radio properties which point toward the presence of a relativistic jet. These characteristics make them an intriguing class of sources to be investigated with the Cherenkov Telescope Array (CTA), the next-generation ground-based gamma-ray observatory. We present our extensive set of simulations of all currently known $\gamma$-ray emitters identified as NLS1s (20 sources),investigating their detections and spectral properties, taking into account the effect of both the extra-galactic background light in the propagation of gamma-rays and intrinsic absorption components.We find that the prospects for observations of $\gamma$-NLSy1 with CTA are promising. In particular, the brightest sources of our sample, SBS 0846+513, PMN J0948+0022, and PKS 1502+036 can be detected during high/flaring states, the former two even in the case in which the emission occurs within the highly opaque central regions, which prevent $\gamma$ rays above few tens of GeV to escape. In this case the low-energy threshold of CTA will play a key role. If, on the other hand, high-energy emission occurs outside the broad line region, we can detect the sources up to several hundreds of GeV-depending on the intrinsic shape of the emitted spectrum. Therefore, CTA observations will provide valuable information on the physical conditions and emission properties of their jets.
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