9 research outputs found
The STACEE-32 Ground Based Gamma-ray Detector
We describe the design and performance of the Solar Tower Atmospheric
Cherenkov Effect Experiment detector in its initial configuration (STACEE-32).
STACEE is a new ground-based gamma ray detector using the atmospheric Cherenkov
technique. In STACEE, the heliostats of a solar energy research array are used
to collect and focus the Cherenkov photons produced in gamma-ray induced air
showers. The large Cherenkov photon collection area of STACEE results in a
gamma-ray energy threshold below that of previous detectors.Comment: 45 pages, 25 figures, Accepted for publication in Nuclear Instruments
and Methods
Discovery of the Binary Pulsar PSR B1259-63 in Very-High-Energy Gamma Rays around Periastron with H.E.S.S
We report the discovery of very-high-energy (VHE) gamma-ray emission of the
binary system PSR B1259-63/SS 2883 of a radio pulsar orbiting a massive,
luminous Be star in a highly eccentric orbit. The observations around the 2004
periastron passage of the pulsar were performed with the four 13 m Cherenkov
telescopes of the H.E.S.S. experiment, recently installed in Namibia and in
full operation since December 2003. Between February and June 2004, a gamma-ray
signal from the binary system was detected with a total significance above 13
sigma. The flux was found to vary significantly on timescales of days which
makes PSR B1259-63 the first variable galactic source of VHE gamma-rays
observed so far. Strong emission signals were observed in pre- and
post-periastron phases with a flux minimum around periastron, followed by a
gradual flux decrease in the months after. The measured time-averaged energy
spectrum above a mean threshold energy of 380 GeV can be fitted by a simple
power law F_0(E/1 TeV)^-Gamma with a photon index Gamma =
2.7+-0.2_stat+-0.2_sys and flux normalisation F_0 = (1.3+-0.1_stat+-0.3_sys)
10^-12 TeV^-1 cm^-2 s^-1. This detection of VHE gamma-rays provides unambiguous
evidence for particle acceleration to multi-TeV energies in the binary system.
In combination with coeval observations of the X-ray synchrotron emission by
the RXTE and INTEGRAL instruments, and assuming the VHE gamma-ray emission to
be produced by the inverse Compton mechanism, the magnetic field strength can
be directly estimated to be of the order of 1 G.Comment: 10 pages, 8 figures, accepted in Astronomy and Astrophysics on 2 June
2005, replace: document unchanged, replaced author field in astro-ph entry -
authors are all members of the H.E.S.S. collaboration and three additional
authors (99+3, see document
A low level of extragalactic background light as revealed by big gamma-rays from blazars
The diffuse extragalactic background light consists of the sum of the starlight emitted by galaxies through the history of the Universe, and it could also have an important contribution from the 'first stars', which may have formed before galaxy formation began. Direct measurements are difficult and not yet conclusive, owing to the large uncertainties caused by the bright foreground emission associated with zodiacal light1. An alternative approach2, 3, 4, 5 is to study the absorption features imprinted on the -ray spectra of distant extragalactic objects by interactions of those photons with the background light photons6. Here we report the discovery of -ray emission from the blazars7 H 2356 - 309 and 1ES 1101 - 232, at redshifts z = 0.165 and z = 0.186, respectively. Their unexpectedly hard spectra provide an upper limit on the background light at optical/near-infrared wavelengths that appears to be very close to the lower limit given by the integrated light of resolved galaxies8. The background flux at these wavelengths accordingly seems to be strongly dominated by the direct starlight from galaxies, thus excluding a large contribution from other sources—in particular from the first stars formed9. This result also indicates that intergalactic space is more transparent to -rays than previously thought
Discovery of extended VHE gamma-ray emission from the asymmetric pulsar wind nebula in MSH 15-52 with H.E.S.S
The Supernova Remnant MSH 15-52 has been observed in very high energy (VHE)
gamma-rays using the H.E.S.S. 4-telescope array located in Namibia. A gamma-ray
signal is detected at the 25 sigma level during an exposure of 22.1 hours live
time. The image reveals an elliptically shaped emission region around the
pulsar PSR B1509-58, with semi-major axis 6' in the NW-SE direction and
semi-minor axis 2' approximately. This morphology coincides with the diffuse
pulsar wind nebula as observed at X-ray energies by ROSAT. The overall energy
spectrum from 280 GeV up to 40 TeV can be fitted by a power law with photon
index Gamma = 2.27 +/- 0.03(stat.) +/- 0.20(syst.). The detected emission can
be plausibly explained by inverse Compton scattering of accelerated
relativistic electrons with soft photons.Comment: 5 pages, 3 figures, accepted by A&A letter
A new population of very high energy gamma-ray sources in the Milky Way
Very high energy {gamma}-rays probe the long-standing mystery of the origin of cosmic rays. Produced in the interactions of accelerated particles in astrophysical objects, they can be used to image cosmic particle accelerators. A first sensitive survey of the inner part of the Milky Way with the High Energy Stereoscopic System (HESS) reveals a population of eight previously unknown firmly detected sources of very high energy {gamma}-rays. At least two have no known radio or x-ray counterpart and may be representative of a new class of "dark" nucleonic cosmic ray sources
Calibration of cameras of the H.E.S.S. detector
H.E.S.S.—the high energy stereoscopic system—is a new system of large atmospheric Cherenkov telescopes for GeV/TeV astronomy. Each of the four telescopes of 107 m2 mirror area is equipped with a 960-pixel photomultiplier-tube camera. This paper describes the methods used to convert the photomultiplier signals into the quantities needed for Cherenkov image analysis. Two independent calibration techniques have been applied in parallel to provide an estimation of uncertainties. Results on the long-term stability of the H.E.S.S. cameras are also presented
Energy dependent -ray morphology in the pulsar wind nebula HESS J1825-137
Aims. We present results from deep -ray observations of the Galactic pulsar wind nebula HESS J1825-137 performed with the HESS array. Methods. Detailed morphological and spatially resolved spectral studies reveal the very high-energy (VHE) -ray aspects of this object with unprecedented precision. Results. We confirm previous results obtained in a survey of the Galactic Plane in 2004. The -ray emission extends asymmetrically to the south and south-west of the energetic pulsar PSR J1826-1334 , that is thought to power the pulsar wind nebula. The differential -ray spectrum of the whole emission region is measured over more than two orders of magnitude, from 270 GeV to 35 TeV, and shows indications for a deviation from a pure power law. Spectra have also been determined for spatially separated regions of HESS J1825-137. The photon indices from a power-law fit in the different regions show a softening of the spectrum with increasing distance from the pulsar and therefore an energy dependent morphology. Conclusions. This is the first time that an energy dependent morphology has been detected in the VHE -ray regime. The VHE -ray emission of HESS J1825-137 is phenomenologically discussed in the scenario where the -rays are produced by VHE electrons via Inverse Compton scattering. The high -ray luminosity of the source cannot be explained on the basis of constant spin-down power of the pulsar and requires higher injection power in past
Energy dependent gamma-ray morphology in the pulsar wind nebula HESSJ1825-137
Aims. We present results from deep γ-ray observations of the Galactic pulsar wind nebula HESS J1825-137 performed with the HESS array. Methods. Detailed morphological and spatially resolved spectral studies reveal the very high-energy (VHE) γ-ray aspects of this object with unprecedented precision. Results. We confirm previous results obtained in a survey of the Galactic Plane in 2004. The γ-ray emission extends asymmetrically to the south and south-west of the energetic pulsar PSR J1826-1334, that is thought to power the pulsar wind nebula. The differential γ-ray spectrum of the whole emission region is measured over more than two orders of magnitude, from 270 GeV to 35 TeV, and shows indications for a deviation from a pure power law. Spectra have also been determined for spatially separated regions of HESS J1825-137. The photon indices from a power-law fit in the different regions show a softening of the spectrum with increasing distance from the pulsar and therefore an energy dependent morphology. Conclusions. This is the first time that an energy dependent morphology has been detected in the VHE γ-ray regime. The VHE γ-ray emission of HESS J1825-137 is phenomenologically discussed in the scenario where the γ-rays are produced by VHE electrons via Inverse Compton scattering. The high γ-ray luminosity of the source cannot be explained on the basis of constant spin-down power of the pulsar and requires higher injection power in past. © ESO 2006.F. Aharonian ... G. Rowell ... et al
Very high energy gamma rays from the direction of Sagittarius A*
We report the detection of a point-like source of very high energy (VHE) gamma-rays coincident within 1' of Sgr A*, obtained with the H.E.S.S. array of Cherenkov telescopes. The gamma-rays exhibit a power-law energy spectrum with a spectral index of -2.2 +/- 0.09 +/- 0.15 and a flux above the 165 GeV threshold of (1.82 +/- 0.22) \times 10^{-7} m^{-2} s^{-1}. The measured flux and spectrum differ substantially from recent results reported in particular by the CANGAROO collaboration