91 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
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
A Search for Very High Energy Gamma Rays from Active Galactic Nuclei Visible from the Southern Hemisphere
Observations have been made, using the University of Durham Mark 6 gamma ray
telescope, of the very high energy gamma ray emission from a number of active
galactic nuclei visible from the Southern hemisphere. Limits are presented to
the VHE gamma ray emission from 1ES 0323+022, PKS 0829+046, 1ES 1101--232, Cen
A, PKS 1514-24, RXJ 10578-275, and 1ES 2316-423, both for steady long-term
emission and for outbursts of emission on timescales of 1 day.Comment: 13 pages, 1 figure, accepted for publication in Ap.
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