Abstracts of Interest
Selected by:
Justin Albury
Abstract: 1812.05861
Full Text: [ PostScript, PDF]
Full Text: [ PostScript, PDF]
Title:New developments in aerosol measurements using stellar photometry
(Submitted on 14 Dec 2018)
Abstract: The idea of using stellar photometry for atmospheric monitoring for optical experiments in high-energy astrophysics is seemingly straightforward, but reaching high precision of the order of 0.01 in the determination of the vertical aerosol optical depth (VAOD) has proven difficult. Wide-field photometry over a large span of altitudes allows a fast determination of VAOD independently of the absolute calibration of the system, while providing this calibration as a useful by-product. Using several years of data taken by the FRAM (F/(Ph)otometric Robotic Atmospheric Monitor) telescope at the Pierre Auger Observatory in Argentina and about a year of data taken by a similar instrument deployed at the planned future Southern site of the Cherenkov Telescope Array in Chile, we have developed methods to improve the precision of this measurement technique towards and possibly beyond the 0.01 mark. Detailed laboratory measurements of the response of the whole system to both the spectrum and intensity of incoming light have proven indispensable in this analysis as the usual assumption of linearity of the CCD detectors is not valid anymore for the conditions of the observations.
Abstract: 1812.05688
Full Text: [ PostScript, PDF]
Full Text: [ PostScript, PDF]
Title:Measurements of UHECR Mass Composition by Telescope Array
(Submitted on 13 Dec 2018)
Abstract: Telescope Array (TA) has recently published results of nearly nine years of $X_{\mathrm{max}}$ observations providing its highest statistics measurement of ultra high energy cosmic ray (UHECR) mass composition to date for energies exceeding $10^{18.2}$ eV. This analysis measured agreement of observed data with results expected for four different single elements. Instead of relying only on the first and second moments of $X_{\mathrm{max}}$ distributions, we employ a morphological test of agreement between data and Monte Carlo to allow for systematic uncertainties in data and in current UHECR hadronic models. Results of this latest analysis and implications of UHECR composition observed by TA are presented. TA can utilize different analysis methods to understand composition as both a crosscheck on results and as a tool to understand systematics affecting $X_{\mathrm{max}}$ measurements. The different analysis efforts underway at TA to understand composition are also discussed.
Abstract: 1812.04026
Full Text: [ PostScript, PDF]
Full Text: [ PostScript, PDF]
Title:A scenario for the Galactic cosmic rays between the knee and the second-knee
(Submitted on 10 Dec 2018)
Abstract: We perform a fit to measurements of the cosmic ray spectrum and of the depth of shower maximum in the energy range between $10^{15}$~eV and $10^{18}$~eV. We consider a Galactic component that is a mixture of five representative nuclear species (H, He, N, Si and Fe), for which we adopt rigidity dependent broken power-law spectra, and we allow for an extragalactic component which becomes strongly suppressed for decreasing energies. The relative abundances of the Galactic components at $10^{15}$~eV are taken to be comparable to those determined by direct measurements at $10^{13}$~eV. The main features of the spectrum and of the composition are reproduced in these scenarios. The spectral knee results from the break of the H spectrum at $E_{\rm k}\simeq 3\times 10^{15}$~eV, although it is broaden by the comparable contribution from He which has a break at about $6\times 10^{15}$~eV. The low-energy ankle at $E_{\rm la}\simeq 2\times 10^{16}$~eV is associated to the strong suppression of the H and He Galactic components and the increasing relative contribution of the heavier ones, but the observed hardening of the spectrum at this energy turns out to result from the growing contribution of the extragalactic component. The second-knee at $E_{\rm sk}\simeq 26 E_{\rm k}\simeq 8\times 10^{16}$~eV is associated with the steepening of the Galactic Fe component. The transition to the regime in which the total cosmic ray flux is dominated by the extragalactic component takes place at an energy of about $10^{17}$~eV. The parameters of the fit depend on the hadronic model that is used to interpret the $X_{\rm max}$ measurements as well as on the specific $X_{\rm max}$ dataset that is considered in the fit. The impact of the possible existence of a maximum rigidity cutoff in the Galactic components is also discussed.
Abstract: 1812.04854
Full Text: [ PostScript, PDF]
Full Text: [ PostScript, PDF]
Title:Discovery of TeV $γ$-ray emission from the neighbourhood of the supernova remnant G24.7+0.6 by MAGIC
(Submitted on 12 Dec 2018)
Abstract: SNR G24.7+0.6 is a 9.5 kyrs radio and $γ$-ray supernova remnant evolving in a dense medium. In the GeV regime, SNR G24.7+0.6 (3FHL\,J1834.1--0706e/FGES\,J1834.1--0706) shows a hard spectral index ($Γ$$\sim$2) up to $200$\,GeV, which makes it a good candidate to be observed with Cherenkov telescopes such as MAGIC. We observed the field of view of \snr\ with the MAGIC telescopes for a total of 31 hours. We detect very high energy $γ$-ray emission from an extended source located 0.34\degr\ away from the center of the radio SNR. The new source, named \mgc\ is detected up to 5\,TeV, and its spectrum is well-represented by a power-law function with spectral index of $2.74 \pm 0.08$. The complexity of the region makes the identification of the origin of the very-high energy emission difficult, however the spectral agreement with the LAT source and overlapping position at less than 1.5$σ$ point to a common origin. We analysed 8 years of \fermi-LAT data to extend the spectrum of the source down to 60\,MeV. \fermi-LAT and MAGIC spectra overlap within errors and the global broad band spectrum is described by a power-law with exponential cutoff at $1.9\pm0.5$\,TeV. The detected $γ$-ray emission can be interpreted as the results of proton-proton interaction between the supernova and the CO-rich surrounding.
Abstract: 1812.05682
Full Text: [ PostScript, PDF]
Full Text: [ PostScript, PDF]
Title:All-Sky Measurement of the Anisotropy of Cosmic Rays at 10 TeV and Mapping of the Local Interstellar Magnetic Field
(Submitted on 13 Dec 2018)
Abstract: We present the first full-sky analysis of the cosmic ray arrival direction distribution with data collected by the HAWC and IceCube observatories in the Northern and Southern hemispheres at the same median primary particle energy of 10 TeV. The combined sky map and angular power spectrum largely eliminate biases that result from partial sky coverage and holds a key to probe into the propagation properties of TeV cosmic rays through our local interstellar medium and the interaction between the interstellar and heliospheric magnetic fields. From the map we determine the horizontal dipole components of the anisotropy $δ_{0h} = 9.16 \times 10^{-4}$ and $δ_{6h} = 7.25 \times 10^{-4}~(\pm0.04 \times 10^{-4})$. In addition, we infer the direction ($229.2\pm 3.5^\circ$ RA , $11.4\pm 3.0^\circ$ Dec.) of the interstellar magnetic field from the boundary between large scale excess and deficit regions from which we estimate the missing corresponding vertical dipole component of the large scale anisotropy to be $δ_N \sim -3.97 ^{+1.0}_{-2.0} \times 10^{-4}$.
Abstract: 1812.06025
Full Text: [ PostScript, PDF]
Full Text: [ PostScript, PDF]
Title:Relativistic Jets in Active Galactic Nuclei
(Submitted on 14 Dec 2018)
Abstract: The nuclei of most normal galaxies contain supermassive black holes, which can accrete gas through a disk and become active. These Active Galactic Nuclei, AGN, can form jets which are observed on scales from AU to Mpc and from meter wavelengths to TeV gamma ray energies. High resolution radio imaging and multi-wavelength/messenger campaigns are elucidating the conditions under which this happens. Evidence is presented that: AGN jets are formed when the black hole spins and the accretion disk is strongly magnetized, perhaps on account of gas accretingat high latitude beyond the black hole sphere of influence; AGN jets are collimated close to the black hole by magnetic stress associated with a disk wind; higher power jets can emerge from their galactic nuclei in a relativistic, supersonic and proton-dominated state and they terminate in strong, hot spot shocks; lower power jets are degraded to buoyant plumes and bubbles; jets may accelerate protons to EeV energies which contribute to the cosmic ray spectrum and which may initiate pair cascades that can efficiently radiate synchrotron gamma rays; jets were far more common when the universe was a few billion years old and black holes and massive galaxies were growing rapidly; jets can have a major influence on their environments, stimulating and limiting the growth of galaxies. The observational prospects for securing our understanding of AGN jets are bright.
Abstract: 1812.04020
Full Text: [ PostScript, PDF]
Full Text: [ PostScript, PDF]
Title:The enhanced X-ray Timing and Polarimetry mission - eXTP
(Submitted on 10 Dec 2018)
Abstract: In this paper we present the enhanced X-ray Timing and Polarimetry mission - eXTP. eXTP is a space science mission designed to study fundamental physics under extreme conditions of density, gravity and magnetism. The mission aims at determining the equation of state of matter at supra-nuclear density, measuring effects of QED, and understanding the dynamics of matter in strong-field gravity. In addition to investigating fundamental physics, eXTP will be a very powerful observatory for astrophysics that will provide observations of unprecedented quality on a variety of galactic and extragalactic objects. In particular, its wide field monitoring capabilities will be highly instrumental to detect the electro-magnetic counterparts of gravitational wave sources. The paper provides a detailed description of: (1) the technological and technical aspects, and the expected performance of the instruments of the scientific payload; (2) the elements and functions of the mission, from the spacecraft to the ground segment.
Abstract: 1812.05654
Full Text: [ PostScript, PDF]
Full Text: [ PostScript, PDF]
Title:Cascading Constraints from Neutrino Emitting Blazars: The case of TXS 0506+056
(Submitted on 13 Dec 2018)
Abstract: We present a procedure to generally constrain the environments of neutrino-producing sites in photomeson production models of jetted Active Galactic Nuclei (AGN) where any origin of the dominant target photon field can be accommodated. For this purpose we reconstruct the minimum target photon spectrum required to produce the (observed) neutrino spectrum, and derive all corresponding secondary particles. These initiate electromagnetic cascades with an efficiency that is linked to the neutrino production rate. The derived photon spectra represent the minimum radiation emerging from the source that is strictly associated to the photo-hadronically produced neutrinos.
Using the 2014-15 neutrino spectrum observed by IceCube from TXS 0506+056, we conduct a comprehensive study of these cascade spectra and compare them to the simultaneous multi-wavelength emission. For this set of observations, photopion production from a co-spatially produced (co-moving) photon target can be ruled out as well as a setup where synchrotron or Compton-synchrotron supported cascades on a stationary (AGN rest frame) target photon field operate in this source. However, a scenario where Compton-driven cascades develop in the stationary soft-X-ray photon target which photo-hadronically produced the observed neutrinos appears feasible with required proton kinetic jet powers near the Eddington limit. The source is then found to produce neutrinos inefficiently, and emits GeV photons significantly below the observed Fermi-LAT-flux. Hence, the neutrinos and the bulk of the gamma rays observed in 2014/2015 from TXS 0506+056 cannot have been initiated by the same process.
Abstract: 1812.04685
Full Text: [ PostScript, PDF]
Full Text: [ PostScript, PDF]
Title:IceCube Neutrinos from Hadronically Powered Gamma-Ray Galaxies
(Submitted on 11 Dec 2018)
Abstract: In this work we use a multi-messenger approach to determine if the high energy diffuse neutrino flux observed by the IceCube Observatory can originate from $γ$-ray sources powered by Cosmic Rays interactions with gas. Typical representatives of such sources are Starburst and Ultra-Luminous Infrared Galaxies. Using the three most recent calculations of the non-blazar contribution to the extragalactic $γ$-ray background measured by the Fermi-LAT collaboration, we find that a hard power-law spectrum with spectral index $α\leq 2.12$ is compatible with all the estimations for the allowed contribution from non-blazar sources, within 1$σ$. Using such a spectrum we are able to interpret the IceCube results, showing that various classes of hadronically powered $γ$-ray galaxies can provide the dominant contribution to the astrophysical signal. With the addition of neutrinos from the Galactic plane, it is possible to saturate the IceCube signal. Our result reverses previous findings in which evidence was claimed against hadronic sources being the dominant source of IceCube neutrinos.
Abstract: 1812.05764
Full Text: [ PostScript, PDF]
Full Text: [ PostScript, PDF]
Title:Simultaneous X-ray and Infrared Observations of Sagittarius A*'s Variability
(Submitted on 14 Dec 2018)
Abstract: Emission from Sgr A* is highly variable at both X-ray and infrared (IR) wavelengths. Observations over the last ~20 years have revealed X-ray flares that rise above a quiescent thermal background about once per day, while faint X-ray flares from Sgr A* are undetectable below the constant thermal emission. In contrast, the IR emission of Sgr A* is observed to be continuously variable. Recently, simultaneous observations have indicated a rise in IR flux density around the same time as every distinct X-ray flare, while the opposite is not always true (peaks in the IR emission may not be coincident with an X-ray flare). Characterizing the behaviour of these simultaneous X-ray/IR events and measuring any time lag between them can constrain models of Sgr A*'s accretion flow and the flare emission mechanism. Using 100+ hours of data from a coordinated campaign between the Spitzer Space Telescope and the Chandra X-ray Observatory, we present results of the longest simultaneous IR and X-ray observations of Sgr A* taken to date. The cross-correlation between the IR and X-ray light curves in this unprecedented dataset, which includes four modest X-ray/IR flares, indicates that flaring in the X-ray may lead the IR by approximately 10-20 minutes with 68% confidence. However, the 99.7% confidence interval on the time-lag also includes zero, i.e., the flaring remains statistically consistent with simultaneity. Long duration and simultaneous multiwavelength observations of additional bright flares will improve our ability to constrain the flare timing characteristics and emission mechanisms, and must be a priority for Galactic Center observing campaigns.
This page created: Tue Dec 18 10:49:53 ACDT 2018 by Justin Albury
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