Search results for " galactic"
showing 10 items of 140 documents
Constraints on ultra-high-energy cosmic ray sources from a search for neutrinos above 10 PeV with IceCube
2016
We report constraints on the sources of ultra-high-energy cosmic ray (UHECR) above $10^{9}$ GeV, based on an analysis of seven years of IceCube data. This analysis efficiently selects very high energy neutrino-induced events which have deposited energies from $\sim 10^6$ GeV to above $10^{11}$ GeV. Two neutrino-induced events with an estimated deposited energy of $(2.6 \pm 0.3) \times 10^6$ GeV, the highest neutrino energies observed so far, and $(7.7 \pm 2.0) \times 10^5$ GeV were detected. The atmospheric background-only hypothesis of detecting these events is rejected at 3.6$\sigma$. The hypothesis that the observed events are of cosmogenic origin is also rejected at $>$99% CL because of…
Sensitivity of the Cherenkov Telescope Array for probing cosmology and fundamental physics with gamma-ray propagation
2021
Full list of authors: Abdalla, H.; Abe, H.; Acero, F.; Acharyya, A.; Adam, R.; Agudo, I; Aguirre-Santaella, A.; Alfaro, R.; Alfaro, J.; Alispach, C.; Aloisio, R.; Batista, R. Alves; Amati, L.; Amato, E.; Ambrosi, G.; Anguner, E. O.; Araudo, A.; Armstrong, T.; Arqueros, F.; Arrabito, L.; Asano, K.; Ascasibar, Y.; Ashley, M.; Backes, M.; Balazs, C.; Balbo, M.; Balmaverde, B.; Baquero Larriva, A.; Martins, V. Barbosa; Barkov, M.; Baroncelli, L.; de Almeida, U. Barres; Barrio, J. A.; Batista, P-, I; Becerra Gonzalez, J.; Becherini, Y.; Beck, G.; Tjus, J. Becker; Belmont, R.; Benbow, W.; Bernardini, E.; Berti, A.; Berton, M.; Bertucci, B.; Beshley, V; Bi, B.; Biasuzzi, B.; Biland, A.; Bissaldi, …
A search for anisotropy in the arrival directions of ultra high energy cosmic rays recorded at the Pierre Auger Observatory
2012
Observations of cosmic ray arrival directions made with the Pierre Auger Observatory have previously provided evidence of anisotropy at the 99% CL using the correlation of ultra high energy cosmic rays (UHECRs) with objects drawn from the Véron-Cetty Véron catalog. In this paper we report on the use of three catalog independent methods to search for anisotropy. The 2pt–L, 2pt+ and 3pt methods, each giving a different measure of selfclustering in arrival directions, were tested on mock cosmic ray data sets to study the impacts of sample size and magnetic smearing on their results, accounting for both angular and energy resolutions. If the sources of UHECRs follow the same large scale structu…
Millimeter-VLBI observations of low-luminosity active galactic nuclei with source-frequency phase-referencing
2021
We report millimeter-VLBI results of low-luminosity active galactic nuclei (M 84 and M 87) up to 88 GHz with source-frequency phase-referencing observations. We detected the weak VLBI core and obtained the first image of M 84 at 88 GHz. The derived brightness temperature of M 84 core was about 7.2$\times$10$^9$ K, which could serve as a lower limit as the core down to 30 Schwarzschild radii was still un-resolved in our 88 GHz observations. We successfully determined the core-shifts of M 87 at 22-44 GHz and 44-88 GHz through source-frequency phase-referencing technique. The jet apex of M 87 could be deduced at about 46 $\mu$as upstream of the 43 GHz core from core-shift measurements. The est…
A Connection Between Apparent VLBA Jet Speeds and Initial Active Galactic Nucleus Detections Made by the Fermi Gamma-ray Observatory
2009
In its first three months of operations, the Fermi Gamma-Ray Observatory has detected approximately one quarter of the radio-flux-limited MOJAVE sample of bright flat-spectrum active galactic nuclei (AGNs) at energies above 100 MeV. We have investigated the apparent parsec-scale jet speeds of 26 MOJAVE AGNs measured by the Very Long Baseline Array (VLBA) that are in the LAT bright AGN sample (LBAS). We find that the gamma-ray bright quasars have faster jets on average than the non-LBAS quasars, with a median of 15 c, and values ranging up to 34 c. The LBAS AGNs in which the LAT has detected significant gamma-ray flux variability generally have faster jets than the nonvariable ones. These fi…
A possible jet precession in the periodic quasar B0605-085
2010
The quasar B0605-085 (OH 010) shows a hint for probable periodical variability in the radio total flux-density light curves. We study the possible periodicity of B0605-085 in the total flux-density, spectra and opacity changes in order to compare it with jet kinematics on parsec scales. We have analyzed archival total flux-density variability at ten frequencies (408 MHz, 4.8 GHz, 6.7 GHz, 8 GHz, 10.7 GHz, 14.5 GHz, 22 GHz, 37 GHz, 90 GHz, and 230 GHz) together with the archival high-resolution very long baseline interferometry data at 15 GHz from the MOJAVE monitoring campaign. Using the Fourier transform and discrete autocorrelation methods we have searched for periods in the total flux-de…
A survey of active galaxies at TeV photon energies with the HAWC gamma-ray observatory
2020
The High Altitude Water Cherenkov Gamma-Ray Observatory (HAWC) continuously detects TeV photons and particles within its large field-of-view, accumulating every day a deeper exposure of two thirds of the sky. We analyzed 1523~days of HAWC live data acquired over four and a half years, in a follow-up analysis of {138} nearby ($z<0.3$) active galactic nuclei from the {\em Fermi} 3FHL catalog culminating within $40^\circ$ of the zenith at Sierra Negra, the HAWC site. This search for persistent TeV emission used a maximum-likelihood analysis assuming intrinsic power-law spectra attenuated by pair production of gamma-ray photons with the extragalactic background light. HAWC clearly detects persi…
The unusual multiwavelength properties of the gamma-ray source PMN J1603-4904
2013
We investigate the nature and classification of PMNJ1603-4904, a bright radio source close to the Galactic plane, which is associated with one of the brightest hard-spectrum gamma-ray sources detected by Fermi/LAT. It has previously been classified as a low-peaked BL Lac object based on its broadband emission and the absence of optical emission lines. Optical measurements, however, suffer strongly from extinction and the absence of pronounced short-time gamma-ray variability over years of monitoring is unusual for a blazar. We are combining new and archival multiwavelength data in order to reconsider the classification and nature of this unusual gamma-ray source. For the first time, we stud…
TANAMI monitoring of Centaurus A: The complex dynamics in the inner parsec of an extragalactic jet
2014
Centaurus A is the closest radio-loud active galaxy. Very Long Baseline Interferometry (VLBI) enables us to study the jet-counterjet system on milliarcsecond (mas) scales, providing essential information for jet emission and propagation models. We study the evolution of the central parsec jet structure of Cen A over 3.5 years. The proper motion analysis of individual jet components allows us to constrain jet formation and propagation and to test the proposed correlation of increased high energy flux with jet ejection events. Cen A is an exceptional laboratory for such detailed study as its proximity translates to unrivaled linear resolution, where 1 mas corresponds to 0.018 pc. The first 7 …
TANAMI: tracking active galactic nuclei with austral milliarcsecond interferometry
2010
We introduce the TANAMI program (Tracking Active Galactic Nuclei with Austral Milliarcsecond Interferometry) which is monitoring an initial sample of 43 extragalactic jets located south of -30 degrees declination at 8.4 GHz and 22 GHz since 2007. All aspects of the program are discussed. First epoch results at 8.4 GHz are presented along with physical parameters derived therefrom. We present first epoch images for 43 sources, some observed for the first time at milliarcsecond resolution. Parameters of these images as well as physical parameters derived from them are also presented and discussed. These and subsequent images from the TANAMI survey are available at http://pulsar.sternwarte.uni…