74 research outputs found
Observations of H1426+428 with HEGRA -- Observations in 2002 and reanalysis of 1999&2000 data
The HEGRA system of imaging air Cherenkov telescopes has been used to observe
the BL Lac object H1426+428 () for 217.5 hours in 2002. In this data
set alone, the source is detected at a confidence level of ,
confirming this object as a TeV source. The overall flux level during the
observations in 2002 is found to be a factor of lower than during
the previous observations by HEGRA in 1999&2000. A new spectral analysis has
been carried out, improving the signal-to-noise ratio at the expense of a
slightly increased systematic uncertainty and reducing the relative energy
resolution to over a wide range of energies. The new
method has also been applied to the previously published data set taken in 1999
and 2000, confirming the earlier claim of a flattening of the energy spectrum
between 1 and 5 TeV. The data set taken in 2002 shows again a signal at
energies above 1 TeV. We combine the energy spectra as determined by the CAT
and VERITAS groups with our reanalyzed result of the 1999&2000 data set and
apply a correction to account for effects of absorption of high energy photons
on extragalactic background light in the optical to mid infrared band. The
shape of the inferred source spectrum is mostly sensitive to the
characteristics of the extragalactic background light between wavelengths of 1
and 15~mComment: 12 pages, 4 Figures, submitted to A&
Rejection of the hypothesis that Markarian 501 TeV photons are pure Bose-Einstein condensates
The energy spectrum of the Blazar type galaxy Markarian 501 (Mrk 501) as
measured by the High-Energy-Gamma-Ray Astronomy (HEGRA) air Cerenkov telescopes
extends beyond 16 TeV and constitutes the most energetic photons observed from
an extragalactic object. A fraction of the emitted spectrum is possibly
absorbed in interactions with low energy photons of the diffuse extragalactic
infrared radiation, which in turn offers the unique possibility to measure the
diffuse infrared radiation density by TeV spectroscopy. The upper limit on the
density of the extragalactic infrared radiation derived from the TeV
observations imposes constraints on models of galaxy formation and stellar
evolution. One of the recently published ideas to overcome severe absorption of
TeV photons is based upon the assumption that sources like Mrk 501 could
produce Bose-Einstein condensates of coherent photons. The condensates would
have a higher survival probability during the transport in the diffuse
radiation field and could mimic TeV air shower events. The powerful
stereoscopic technique of the HEGRA air Cerenkov telescopes allows to test this
hypothesis by reconstructing the penetration depths of TeV air shower events:
Air showers initiated by Bose-Einstein condensates are expected to reach the
maximum of the shower development in the atmosphere earlier than single photon
events. By comparing the energy-dependent penetration depths of TeV photons
from Mrk 501 with those from the TeV standard-candle Crab Nebula and simulated
air shower events, we can reject the hypothesis that TeV photons from Mrk 501
are pure Bose-Einstein condensates.Comment: 9 pages, 2 figures, published by ApJ Letters, revised version
(simulation results added
Is the giant radio galaxy M 87 a TeV gamma-ray emitter?
For the first time an excess of photons above an energy threshold of 730 GeV from the giant radio galaxy M 87 has been measured at a significance level above 4 σ. The data have been taken during the years 1998 and 1999 with the HEGRA stereoscopic system of 5 imaging atmospheric Cherenkov telescopes. The excess of 107.4 ± 26.8 events above 730 GeV corresponds to an integral flux of 3.3% of the Crab flux or Nγ (E > 730 GeV) = (0.96 ± 0.23) × 10-12 phot cm-2 s-1. M 87 is located at the center of the Virgo cluster of galaxies at a relatively small redshift of z = 0.00436 and is a promising candidate among the class of giant radio galaxies for the emission of TeV γ-radiation. The detection of TeV γ-rays from M 87 - if confirmed - would establish a new class of extragalactic source in this energy regime since all other AGN detected to date at TeV energies are BL Lac type objects.F. A. Aharonian ...G. P. Rowell...et al
A study of Tycho's SNR at TeV energies with the HEGRA CT-System
Tycho's supernova remnant (SNR) was observed during 1997 and 1998 with the
HEGRA Cherenkov Telescope System in a search for gamma-ray emission at
energies above
~1 TeV. An analysis of these data, ~65 hours in total, resulted in no
evidence for TeV gamma-ray emission. The 3sigma upper limit to the gamma-ray
flux (>1 TeV) from Tycho is estimated at 5.78x10^{-13} photons cm^{-2} s^{-1},
or 33 milli-Crab. We interpret our upper limit within the framework of the
following scenarios:
(1) that the observed hard X-ray tail is due to synchrotron emission. A lower
limit on the magnetic field within Tycho may be estimated B>=22 microG,
assuming that the RXTE-detected
X-rays were due to synchrotron emission. However, using results from a
detailed model of the ASCA emission, a more conservative lower limit B>=6
microG is derived.
(2) the hadronic model of Drury, Aharonian & Voelk, and (3) the more recent
time-dependent kinetic theory of Berezhko & Voelk.
Our upper limit lies within the range of predicted values of both hadronic
models, according to uncertainties in physical parameters of Tycho, and shock
acceleration details. In the latter case, the model was scaled to suit the
parameters of Tycho and re-normalised to account for a simplification of the
original model.
We find that we cannot rule out Tycho as a potential contributor at an
average level to the Galactic cosmic-ray flux.Comment: 9 pages, 6 figures. Accepted for publication in Astronomy and
Astrophysic
Evidence for TeV gamma ray emission from Cassiopeia A
232 hours of data were accumulated from 1997 to 1999, using the HEGRA
Stereoscopic Cherenkov Telescope System to observe the supernova remnant
Cassiopeia A. TeV gamma ray emission was detected at the 5 sigma level, and a
flux of (5.8 +- 1.2(stat) +- 1.2(syst)) 10^(-9) ph m^(-2) s^(-1) above 1 TeV
was derived. The spectral distribution is consistent with a power law with a
differential spectral index of -2.5 +- 0.4(stat) +- 0.1(syst) between 1 and 10
TeV. As this is the first report of the detection of a TeV gamma ray source on
the "centi-Crab" scale, we present the analysis in some detail. Implications
for the acceleration of cosmic rays depend on the details of the source
modeling. We discuss some important aspects in this paper.Comment: 9 pages, 6 figures, accepted for publication in Astronomy &
Astrophysic
VERITAS Upper Limit on the VHE Emission from the Radio Galaxy NGC 1275
The recent detection by the Fermi gamma-ray space telescope of high-energy
gamma-rays from the radio galaxy NGC 1275 makes the observation of the very
high energy (VHE: E > 100 GeV) part of its broadband spectrum particularly
interesting, especially for the understanding of active galactic nuclei (AGN)
with misaligned multi-structured jets. The radio galaxy NGC 1275 was recently
observed by VERITAS at energies above 100 GeV for about 8 hours. No VHE
gamma-ray emission was detected by VERITAS from NGC 1275. A 99% confidence
level upper limit of 2.1% of the Crab Nebula flux level is obtained at the
decorrelation energy of approximately 340 GeV, corresponding to 19% of the
power-law extrapolation of the Fermi Large Area Telescope (LAT) result.Comment: Accepted for publication in ApJ Letter
A search for TeV gamma-ray emission from SNRs, pulsars and unidentified GeV sources in the Galactic plane in the longitude range between -2 deg and 85 deg
Using the HEGRA system of imaging atmospheric Cherenkov telescopes, one
quarter of the Galactic plane (-2 deg < l < 85 deg) was surveyed for TeV
gamma-ray emission from point sources and moderately extended sources (diameter
<= 0.8 deg). The region covered includes 86 known pulsars (PSR), 63 known
supernova remnants (SNR) and nine GeV sources, representing a significant
fraction of the known populations. No evidence for emission of TeV gamma
radiation was detected, and upper limits range from 0.15 Crab units up to
several Crab units, depending on the observation time and zenith angles
covered. The ensemble sums over selected SNR and pulsar subsamples and over the
GeV-sources yield no indication for emission from these potential sources. The
upper limit for the SNR population is at the level of 6.7% of the Crab flux and
for the pulsar ensemble at the level of 3.6% of the Crab flux.Comment: 10 pages, 5 figures, 4 tables, accepted for publication in A&
Reanalysis of the high energy cutoff of the 1997 Mkn 501 TeV energy spectrum
Data taken with the HEGRA system of imaging atmospheric Cherenkov telescopes
during the 1997 flares of Mkn 501 are reanalyzed using an algorithm providing
improved energy resolution. A resolution of 10% to 12% is obtained by
accounting for the variation of the Cherenkov light yield with the height of
the shower maximum in the atmosphere. The improved energy resolution is
particularly relevant for the study of the high-energy cutoff in the spectrum,
which might be caused by interactions with the intergalactic infrared
background radiation. The reanalysis presented here confirms the results
obtained in the previous analysis, but hints a steeper slope of the spectrum in
the region around 20 TeV.Comment: 6 Pages, 4 figures; A&A, in pres
Observations of 54 Active Galactic Nuclei with the HEGRA System of Cherenkov Telescopes
A sample of 54 selected Active Galactic Nuclei (AGN) has been observed with
the HEGRA stereoscopic system of Cherenkov Telescopes between 1996 and 2002 in
the TeV energy regime. The observations were motivated by the positive results
obtained for Mkn 421 and Mkn 501. The distances of the selected objects vary
over a large range of redshifts between z = 0.004 and z = 0.7. Among the
observed AGN are the meanwhile established TeV-emitting BL Lac type objects
H1426+428 and 1ES1959+650. Furthermore the BL Lac object 1ES2344+514 and the
radio galaxy M87 show evidence for a signal on a 4 sigma level. The observation
of 1ES2344+514 together with the Whipple results firmly establishes this AGN as
a TeV source. Several objects (PKS2155-304, BL Lacertae, 3C066A) that have been
claimed as TeV gamma-ray emitters by other groups are included in this data
sample but could not be confirmed using data analysed here. The upper limits of
several AGN included in this analysis are compared with predictions in the
frame-work of SSC models.Comment: 9 pages, 2 figures, 7 tables, submitted to A&
- …