71 research outputs found
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
TeV Gamma-ray Observations of the Crab and Mkn 501 during Moonshine and Twilight
TeV Gamma-ray signals from the Crab Nebula and Mkn 501 were detected with the
HEGRA CT1 imaging Cerenkov telescope during periods when the moon was shining
and during twilight. This was accomplished by lowering the high voltage supply
of the photomutipliers in fixed steps up to 13%. No other adjustments were made
and no filters were used. Laser runs could not establish any non-linearity in
the gain of the individual pixels, and the trigger rate was uniform over the
whole camera. The energy threshol was increased by up to a factor of two,
depending on the amount of HV reduction. In a series of observations lasting
11.7 hours, a signal with a 3.4 sigma significance was detected from the Crab.
During the 1997 multiple flare episode of Mkn 501 a 26 sigma combined excess
has been recorded during 134 hours of observations under various
moonshine/twilight conditions. The results show that this technique can easily
be adapted to increase the exposure of a source, which is important for sources
showing rapid time variability such as AGNs or GRBs. Observations can be made
up to ~20 deg. angular separation from the moon and until the moon is 85%
illuminated (ten to eleven days before and after new moon), as well as during
20 to 40 minutes during twilight, before the commencement of astronomical
darkness.Comment: 16 pages, 5 figures, submitted to Astroparticle Physic
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 very-high-energy gamma-ray emission from the vicinity of PSR J1913+1011 with HESS
The HESS experiment, an array of four Imaging Atmospheric Cherenkov Telescopes with high sensitivity and large field-of-view, has been used to search for emitters of very-high-energy (VHE, >100 GeV) -rays along the Galactic plane, covering the region 30° 60°, 280° 330°, and -3° 3°. In this continuation of the HESS Galactic Plane Scan, a new extended VHE -ray source was discovered at =191249, =+10°09´06´´(HESS J1912+101). Its integral flux between 1-10 TeV is ~10% of the Crab Nebula flux in the same energy range. The measured energy spectrum can be described by a power law d with a photon index \Gamma = 2.7 \pm 0.2_{\mbox{stat}}\pm 0.3_{\mbox{sys}}. HESS J1912+101 is plausibly associated with the high spin-down luminosity pulsar PSR J1913+1011. We also discuss associations with an as yet unconfirmed SNR candidate proposed from low frequency radio observation and/or with molecular clouds found in 13CO dat
HESS upper limits for Kepler's supernova remnant
Aims. Observations of Kepler's supernova remnant (G4.5+6.8) with the HESS telescope array in 2004 and 2005 with a total live time of 13 h are presented. Methods. Stereoscopic imaging of Cherenkov radiation from extensive air showers is used to reconstruct the energy and direction of the incident gamma rays. Results. No evidence for a very high energy (VHE: >100 GeV) gamma-ray signal from the direction of the remnant is found. An upper limit (99% confidence level) on the energy flux in the range of 8.6 is obtained. Conclusions. In the context of an existing theoretical model for the remnant, the lack of a detectable gamma-ray flux implies a distance of at least . A corresponding upper limit for the density of the ambient matter of is derived. With this distance limit, and assuming a spectral index , the total energy in accelerated protons is limited to . In the synchrotron/inverse Compton framework, extrapolating the power law measured by RXTE between 10 and down in energy, the predicted gamma-ray flux from inverse Compton scattering is below the measured upper limit for magnetic field values greater than $52~\mu {\rm G}
Very high energy gamma-ray observations of the galaxy clusters Abell 496 and Abell 85 with HESS
Aims. The nearby galaxy clusters Abell 496 and Abell 85 are studied in the very high-energy (VHE, GeV) band to investigate VHE cosmic rays (CRs) in this class of objects which are the largest gravitationally bound systems in the Universe.
Methods. HESS, an array of four imaging atmospheric cherenkov telescopes (IACT), was used to observe the targets in the range of VHE gamma rays.
Results. No significant gamma-ray signal is found at the respective position of the two clusters with several different source size assumptions for each target. In particular, emission regions corresponding to the high-density core, to the extension of the entire X-ray emission in these clusters, and to the very extended region where the accretion shock is expected are investigated. Upper limits are derived for the gamma-ray flux at energies GeV for Abell 496 and GeV for Abell 85.
Conclusions. From the non-detection in VHE gamma rays, upper limits on the total energy of hadronic CRs in the clusters are calculated. If the cosmic-ray energy density follows the large-scale gas density profile, the limit on the fraction of energy in these non-thermal particles with respect to the total thermal energy of the intra-cluster medium (ICM) is 51% for Abell 496 and only 8% for Abell 85 due to its higher mass and higher gas density. These upper limits are compared with theoretical estimates. They predict about ~10% of the thermal energy of the ICM in non-thermal particles. The observations presented here can constrain these predictions especially for the case of the Abell 85 cluste
Exploring a SNR/molecular cloud association within HESS J1745-303
Aims. HESS J1745−303 is an extended, unidentified VHE (very high energy) gamma-ray source discovered using HESS in the Galactic Plane
Survey. Since no obvious counterpart has previously been found in longer-wavelength data, the processes that power the VHE emission are not
well understood.
Methods. Combining the latest VHE data with recent XMM-Newton observations and a variety of source catalogs and lower-energy survey data,
we attempt to match (from an energetic and positional standpoint) the various parts of the emission of HESS J1745−303 with possible candidates.
Results. Though no single counterpart is found to fully explain the VHE emission, we postulate that at least a fraction of the VHE source may be
explained by a supernova-remnant/molecular-cloud association and/or a high-spin-down-flux pulsa
Simultaneous HESS and Chandra observations of Sagitarius A* during an X-ray flare
The rapidly varying (~10 min timescale) non-thermal X-ray emission observed from Sgr A implies that particle acceleration is occuring close to the event horizon of the supermassive black hole. The TeV γ-ray source HESS J1745-290 is coincident with Sgr A and may be closely related to its X-ray emission. Simultaneous X-ray and TeV observations are required to elucidate the relationship between these objects. We report on joint HESS/Chandra observations performed in July 2005, during which an X-ray flare was detected. Despite a factor of ≈9 increase in the X-ray flux of Sgr A, no evidence is found for an increase in the TeV γ-ray flux from this region. We find that an increase in the γ-ray flux of a factor of 2 or greater can be excluded at a confidence level of 99%. This finding disfavours scenarios in which the keV and TeV emission are associated with the same population of accelerated particles and in which the bulk of the γ-ray emission is produced within ~1014 cm (~100) of the supermassive black hol
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