777 research outputs found
Discovery of a new branch of the Taurid meteoroid stream as a real source of potentially hazardous bodies
Taurid meteor shower produces prolonged but usually low activity every
October and November. In some years, however, the activity is significantly
enhanced. Previous studies based on long-term activity statistics concluded
that the enhancement is caused by a swarm of meteoroids locked in 7:2 resonance
with Jupiter. Here we present precise data on 144 Taurid fireballs observed by
new digital cameras of the European Fireball Network in the enhanced activity
year 2015. Orbits of 113 fireballs show common characteristics and form
together a well defined orbital structure, which we call new branch. We found
that this branch is characterized by longitudes of perihelia lying between
155.9-160o and latitudes of perihelia between 4.2-5.7o. Semimajor axes are
between 2.23-2.28 AU and indeed overlap with the 7:2 resonance. Eccentricities
are in wide range 0.80-0.90. The orbits form a concentric ring in the inner
solar system. The masses of the observed meteoroids were in a wide range from
0.1 g to more than 1000 kg. We found that all meteoroids larger than 300 g were
very fragile, while those smaller than 30 g were much more compact. Based on
orbital characteristics, we argue that asteroids 2015 TX24 and 2005 UR, both of
diameters 200-300 meters, are direct members of the new branch. It is therefore
very likely that the new branch contains also numerous still not discovered
objects of decameter or even larger size. Since asteroids of sizes of tens to
hundreds meters pose a treat to the ground even if they are intrinsically weak,
impact hazard increases significantly when the Earth encounters the Taurid new
branch every few years. Further studies leading to better description of this
real source of potentially hazardous objects, which can be large enough to
cause significant regional or even continental damage on the Earth, are
therefore extremely important.Comment: 24 pages, 22 figures, 5 tables. Accepted in Astronomy and
Astrophysic
Statistics of Cosmological Black Hole Jet Sources: Blazar Predictions for GLAST
A study of the statistics of cosmological black-hole jet sources is applied
to EGRET blazar data, and predictions are made for GLAST. Black-hole jet
sources are modeled as collimated relativistic plasma outflows with radiation
beamed along the jet axis due to strong Doppler boosting. The comoving rate
density of blazar flares is assumed to follow a blazar formation rate (BFR),
modeled by analytic functions based on astronomical observations and fits to
EGRET data. The redshift and size distributions of gamma-ray blazars observed
with EGRET, separated into BL Lac object (BL) and flat spectrum radio quasar
(FSRQ) distributions, are fit with monoparametric functions for the
distributions of the jet Lorentz factor \Gamma, comoving directional power
l'_e, and spectral slope. A BFR factor ~10 x greater at z ~ 1 than at present
is found to fit the FSRQ data. A smaller comoving rate density and greater
luminosity of BL flares at early times compared to the present epoch fits the
BL data. Based on the EGRET observations, ~1000 blazars consisting of ~800
FSRQs and FR2 radio galaxies and ~200 BL Lacs and FR1 radio galaxies will be
detected with GLAST during the first year of the mission. Additional AGN
classes, such as hard-spectrum BL Lacs that were mostly missed with EGRET,
could add more GLAST sources. The FSRQ and BL contributions to the EGRET
gamma-ray background at 1 GeV are estimated at the level of ~10 - 15% and ~2 -
4%, respectively. EGRET and GLAST sensitivities to blazar flares are considered
in the optimal case, and a GLAST analysis method for blazar detection is
outlined.Comment: 17 pages, 9 figures, ApJ, in press, v.660, May 1, 2007 (minor changes
from previous version
Point-like gamma ray sources as signatures of distant accelerators of ultra high energy cosmic rays
We discuss the possibility of observing distant accelerators of ultra high
energy cosmic rays in synchrotron gamma rays. Protons propagating away from
their acceleration sites produce extremely energetic electrons during
photo-pion interactions with cosmic microwave background photons. If the
accelerator is embedded in a magnetized region, these electrons will emit high
energy synchrotron radiation. The resulting synchrotron source is expected to
be point-like and detectable in the GeV-TeV energy range if the magnetic field
is at the nanoGauss level.Comment: 4 pages 2 figures. To be published in PR
Analyzing the Multiwavelength Spectrum and Variability of BL Lacertae During the July 1997 Outburst
The multiwavelength spectrum of BL Lacertae during its July 1997 outburst is
analyzed in terms of different variations of the homogeneous leptonic jet model
for the production of high-energy radiation from blazars. We find that a
two-component gamma-ray spectrum, consisting of a synchrotron self-Compton and
an external Compton component, is required in order to yield an acceptable fit
to the broadband spectrum. Our analysis indicates that in BL Lac, unlike other
BL Lac objects, the broad emission line region plays an important role for the
high-energy emission. Several alternative blazar jet models are briefly
discussed. In the appendix, we describe the formalism in which the process of
Comptonization of reprocessed accretion disk photons is treated in the
previously developed blazar jet simulation code which we use.Comment: Now accepted for publication in The Astronomical Journal.
Significantly extended discussion w.r.t. original version. 3 Figures included
using epsf.sty, rotate.st
EGRET Observations of the Extragalactic Gamma Ray Emission
The all-sky survey in high-energy gamma rays (E30 MeV) carried out by the
Energetic Gamma Ray Experiment Telescope (EGRET) aboard the Compton Gamma-Ray
Observatory provides a unique opportunity to examine in detail the diffuse
gamma-ray emission. The observed diffuse emission has a Galactic component
arising from cosmic-ray interactions with the local interstellar gas and
radiation as well an almost uniformly distributed component that is generally
believed to originate outside the Galaxy. Through a careful study and removal
of the Galactic diffuse emission, the flux, spectrum and uniformity of the
extragalactic emission is deduced. The analysis indicates that the
extragalactic emission is well described by a power law photon spectrum with an
index of -(2.10+-0.03) in the 30 MeV to 100 GeV energy range. No large scale
spatial anisotropy or changes in the energy spectrum are observed in the
deduced extragalactic emission. The most likely explanation for the origin of
this extragalactic high-energy gamma-ray emission is that it arises primarily
from unresolved gamma-ray-emitting blazars.Comment: 19 pages latex, 10 figures, accepted for publication in Ap
Data on 824 fireballs observed by the digital cameras of the European Fireball Network in 2017-2018. I. Description of the network, data reduction procedures, and the catalog
A catalog of 824 fireballs (bright meteors), observed by a dedicated network
of all-sky digital photographic cameras in central Europe in the years
2017-2018 is presented. The status of the European Fireball Network,
established in 1963, is described. The cameras collect digital images of
meteors brighter than an absolute magnitude of about -2 and radiometric light
curves with a high temporal resolution of those brighter than a magnitude ~ -4.
All meteoroids larger than 5 grams, corresponding to sizes of about 2 cm, are
detected regardless of their entry velocity. High-velocity meteoroids are
detected down to masses of about 0.1 gram. The largest observed meteoroid in
the reported period 2017-2018 had a mass of about 100 kg and a size of about 40
cm. The methods of data analysis are explained and all catalog entries are
described in detail. The provided data include the fireball date and time,
atmospheric trajectory and velocity, the radiant in various coordinate systems,
heliocentric orbital elements, maximum brightness, radiated energy, initial and
terminal masses, maximum encountered dynamic pressure, physical classification,
and possible shower membership. Basic information on the fireball spectrum is
available for some bright fireballs (apparent magnitude < -7). A simple
statistical evaluation of the whole sample is provided. The scientific analysis
is presented in an accompanying paper.Comment: accepted in Astronomy and Astrophysic
EGRET Spectral Index and the Low-Energy Peak Position in the Spectral Energy Distribution of EGRET-Detected Blazars
In current theoretical models of the blazar subclass of active galaxies, the
broadband emission consists of two components: a low-frequency synchrotron
component with a peak in the IR to X-ray band, and a high-frequency inverse
Compton component with a peak in the gamma-ray band. In such models, the
gamma-ray spectral index should be correlated with the location of the
low-energy peak, with flatter gamma-ray spectra expected for blazars with
synchrotron peaks at higher photon energies and vice versa. Using the
EGRET-detected blazars as a sample, we examine this correlation and possible
uncertainties in its construction.Comment: 17 pages including 1 figure, accepted for publication in The
Astrophysical Journa
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