81 research outputs found
Search for a TeV gamma-ray halo of Mkn 501
For distant extragalactic sources of gamma-rays in the PeV energy range,
interactions of the gamma rays with intergalactic diffuse radiation fields will
initiate a pair cascade. Depending on the magnetic fields in the vicinity of
the source, the cascade can either result in an isotropic halo around an
initially beamed source, or remain more or less collimated. Data recorded by
the HEGRA system of imaging atmospheric Cherenkov telescopes are used to derive
limits on the halo flux from the AGN Mrk 501. This is achieved by comparing the
angular distribution of TeV gamma-rays during the 1997 burst phase -- where
direct photons should dominate -- with the distribution during the 1998/99
quiescent state, where a steady-state halo contribution should be most
pronounced. The results depend on the assumptions concerning the angular
distribution of the halo; limits on the halo flux within 0.5 to 1 degr. from
the source range between 0.1% and 1% of the peak burst flux.Comment: 7 Pages, 5 figures; A&A, in pres
TeV gamma rays from the blazar H 1426+428 and the diffuse extragalactic background radiation
The detection of TeV gamma-rays from the blazar H 1426+428 at an integral
flux level of (4 +- 2(stat) +- 1(syst)) 10^(-12) erg cm^(-2) s^(-1) above 1 TeV
with the HEGRA imaging atmospheric Cherenkov telescope system is reported. H
1426+428 is located at a redshift of z=0.129, which makes it the most distant
source detected in TeV gamma-rays so far. The TeV radiation is expected to be
strongly absorbed by the diffuse extragalactic background radiation (DEBRA).
The observed energy spectrum of TeV photons is in good agreement with an
intrinsic power law spectrum of the source ~E^(-1.9) corrected for DEBRA
absorption. Statistical errors as well as uncertainties about the intrinsic
source spectrum, however, do not permit strong statements about the density of
the DEBRA infrared photon field.Comment: 4 pages, 4 figures, accepted for publication in Astronomy &
Astrophysics as a lette
Energy Spectrum and Chemical Composition of Cosmic Rays between 0.3 and 10 PeV determined from the Cherenkov-Light and Charged-Particle distributions in Air Showers
Measurements of the lateral distribution of Cherenkov photons with the
wide-angle atmospheric Cherenkov light detector array AIROBICC and of the
charged particle lateral distribution with the scintillator matrix of the HEGRA
air-shower detector complex in air showers are reported. With the atmospheric
shower-front sampling technique these detectors measure the electromagnetic
component of an extensive air shower via the lateral density distribution of
the shower particles and of the Cherenkov photons. The data are compared with
events generated with the CORSIKA program package with the QGSJET
hadronic-event generator. Consistency checks performed with primary
energy-reconstruction methods based on different shower observables indicate
satisfactory agreement between these extensive air shower simulations and the
experimental data. The energy spectrum features a so called ``knee'' at an
energy of E_knee = 3.98 (+4.66) (-0.83) (stat) +- 0.53 (syst) PeV. Power law
fits to the differential energy spectrum yield indices of -2.72 (+0.02)(-0.03)
(stat) +- 0.07 (syst) below, and -3.22 (+0.47) (-0.59) (stat) +- 0.08 (syst)}
above the knee. The best-fit elongation rate for the whole energy range is
determined to 78.3 +- 1.0 (stat) +- 6.2 (syst) g/cm^2. At the highest energies
it seems to decrease slightly.The best-fit fraction of light nuclei decreases
from 37 (+28) (-21) % (combined statistical and systematic) to 8 (+32) (-8) %
(combined statistical and systematic) in the energy range discussed here. A
detailed study of the systematic errors reveals that a non-changing composition
cannot be excluded.Comment: 14 pages, 13 figures, accepted for publication in Astronomy and
Astrophysic
HEGRA search for TeV emission from BL Lac objects
The HEGRA system of four Imaging Atmospheric Cherenkov Telescopes (IACTs) has
been used to extensively observe extragalactic objects. In this paper we
describe the search for TeV emission from nine very promising potential TeV
sources, namely eight ``high frequency'' BL Lac objects (HBLs), and the object
``BL Lacertae'' itself. These objects were observed during 1997 and 1998
seasons, with total integration times ranging between one and fifteen hours.
No evidence for emission was found from any of these objects and the upper
limits on the integral energy flux above ~750 GeV are on the level of a few
times 10^-12 erg cm^-2 s^-1. For the two objects BL Lacertae and 1ES 2344+51.4,
we discuss the astrophysical implications of the TeV flux upper limit, using
also information from the X-ray and gamma-ray bands as measured with the All
Sky Monitor (ASM) of RXTE (1.3-12.0 keV) and with EGRET (30 MeV - 20 GeV).Comment: Accepted for publication by Astronomy and Astrophysics, 7 pages, 6
figure
Multi-wavelength observations of the TeV Blazars Mkn~421, 1ES1959+650, and H1426+428 with the HEGRA Cherenkov telescopes and the RXTE X-ray satellite
Recent results obtained with the HEGRA system of imaging Cherenkov telescopes
on the TeV emission of the Blazars Mkn~421 (), 1ES1959+650
(), and H1426+428 () are reported. For Mkn~421, a close
connection of the average flux level and spectral shape has been observed
during the periods of increased activity in the years 2000 and 2001.
Simultaneously taken data with the RXTE X-ray satellite reveal a complex light
curve at X-ray and TeV energies. After a deep exposure of 94 hrs, the object
1ES1959+650 was detected at the significance level of 5.4 with a soft
energy spectrum following a power-law with a photon-index of .
During recent observations in May 2002, the source has shown increased activity
with indications for a flattening of the energy spectrum.
The high energy peaked Blazar H1426+428 has recently been identified as a
source of TeV photons. Since the source is fairly distant (z=0.129), absorption
of TeV photons due to pair-production on the optical and near infrared
extragalactic light becomes important and should leave a signature in the
observed TeV energy spectrum. Notably, the TeV energy spectrum determined with
the HEGRA system of Cherenkov telescopes agrees with the expectation of a
strongly absorbed source spectrum.Comment: 6 pages, 4 figures, will appear in the Proceedings for "High Energy
Blazar astronomy", Turku, Finland 200
Measurement of the flux, spectrum, and variability of TeV gamma-rays from Mkn 501 during a state of high activity
Between March 16, 1997 and April 14, 1997, a high flux level of TeV
gamma-rays was observed from Mkn 501, using the HEGRA stereoscopic system of
four imaging Cherenkov telescopes. The flux level varied during this period
from about one half up to six times the flux observed from the Crab Nebula.
Changes of the detection rate by a factor of up to 4 within 1 day have been
observed. The measured differential energy spectrum of the radiation follows a
power law from 1 TeV to 10 TeV. The differential spectral index of
2.47{+-}0.07{+-}0.25 is close to that of the Crab Nebula of
2.66{+-}0.12{+-}0.25.Comment: 4 Pages, 4 figures, Latex, uses l-aa.sty; revised version with minor
corrections; Accepted for publication in A&A (Letters
First Results on the Performance of the HEGRA IACT Array
First results concerning the performance characteristics of the HEGRA IACT
array are given, based on stereoscopic observations of the Crab Nebula with
four telescopes. The system provides a gamma-ray energy threshold around 0.5
TeV. The Crab signal demonstrates an angular resolution of about 0.1 deg. Shape
cuts allow to suppress cosmic ray background by almost a factor 100, while
maintaining 40% efficiency for gamma-rays. The Crab signal is essentially
background free. For longer observation times of order 100 h, the system in its
present form provides sensitivity to point sources at a level of 3% of the Crab
flux. Performance is expected to improve further with the inclusion of the
fifth telescope and the implementation of advanced algorithms for shower
reconstruction.Comment: 19 Pages, 11 figures, Latex, uses epsf, elsart.st
Correlations between parameters of extended air showers and their proper use in analyses
In air shower experiments information about the initial cosmic ray particle
or about the shower development is obtained by exploiting the correlations
between the quantities of interest and the directly measurable quantities. It
is shown how these correlations are properly treated in order to obtain
unbiased results. As an example, the measurement of the average penetration
depth as a function of the shower energy is presented.Comment: 4 Pages, 4 Figures; to appear in the proceedings of the 26th ICRC,
Salt Lake City, August 199
A search for gamma-ray emission from the Galactic plane in the longitude range between 37 deg and 43 deg
Using the HEGRA system of imaging atmospheric Cherenkov telescopes, a region
of the Galactic plane (-10 deg < b < 5 deg, 38 deg < l < 43 deg) was surveyed
for TeV gamma-ray emission, both from point sources and of diffuse nature. The
region covered includes 15 known pulsars, 6 known supernova remnants (SNR) and
one unidentified EGRET source. No evidence for emission from point sources was
detected; upper limits are typically below 0.1 Crab units for the flux above 1
TeV. For the diffuse gamma-ray flux from the Galactic plane, an upper limit of
6.1*10E-15 ph/(cm2 s sr MeV) was derived under the assumption that the spatial
distribution measured by the EGRET instrument extends to the TeV regime. This
upper flux limit is a factor of about 1.5 larger than the flux expected from
the ensemble of gamma-ray unresolved Galactic cosmic ray sources.Comment: 10 pages, 8 figures, accepted for publication by A&
The time structure of Cherenkov images generated by TeV gamma-rays and by cosmic rays
The time profiles of Cherenkov images of cosmic-ray showers and of gamma-ray
showers are investigated, using data gathered with the HEGRA system of imaging
atmospheric Cherenkov telescopes during the 1997 outbursts of Mrk 501. Photon
arrival times are shown to vary across the shower images. The dominant feature
is a time gradient along the major axis of the images. The gradient varies with
the distance between the telescope and the shower core, and is maximal for
large distances. The time profiles of cosmic-ray showers and of gamma-ray
showers differ in a characteristic fashion. The main features of the time
profiles can be understood in terms of simple geometrical models. Use of the
timing information towards improved shower reconstruction and cosmic-ray
suppression is discussed.Comment: 25 Pages, 16 figures, Late
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