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

Very high energy gamma-rays from Centaurus X-3: indications and implications

We present the results of a detailed timing analysis of observations of Cen X-3 taken by the University of Durham Mark 6 imaging atmospheric Cherenkov telescope in 1997-1999. The presence of a TeV gamma -ray signal at the overall >= 4.5sigma significance level in the ``fully cut'' image selected data, as reported earlier, is confirmed. A search for possible modulations of gamma -rays with the pulsar spin period P0~ 4.8 s was performed by the step-by-step application of image parameter cuts of gradually increasing hardness. The data of each of 23 days of observations have not revealed any statistically significant Rayleigh power peak, except for 1 day when a peak with a chance probability p=6.8x 10-7 was found in ``soft-cut'' data sets. This modulation, if real, is blue shifted by 6.6 ms (>103 km s-1) from the nominal second harmonic of the X-ray pulsar. Taking the large number of frequency trials into account, the estimated final probability of finding such a peak by chance still remains = 100 MeV) and very high energy (VHE, E>= 100 GeV) gamma -rays. We propose on the basis of the energetics required that all reasonable options for the gamma -ray production in Cen X-3 must be connected to jets emerging from the inner accretion disc around the neutron star. One of the principal options is a large-scale source, with R_s ~ 1013 - 1014 cm; this assumes effective acceleration of electrons up to ~ 10 TeV by shocks produced by interaction of these jets with the dense atmosphere of the binary. It is shown that such a quasi-stationary model could explain the bulk of the gamma -radiation features observed except for the gamma -ray modulations with the pulsar spin. These modulations, if genuine, would require an alternative source with R_s << 1011 cm. We consider two principal models, hadronic and leptonic, for the formation of such a compact source in the jet. Both models predict that the episodes of pulsed gamma -ray emission may be rather rare, with a typical duration not exceeding a few hours, and that generally the frequency of pulsations should be significantly shifted from the nominal frequency of the X-ray pulsar. The opportunities to distinguish between different models by means of future gamma -ray observations of this X-ray binary are also discussed

Active atmospheric calibration for HESS applied to PKS 2155-304

Using data derived from the H.E.S.S. telescope system and the LIDAR facility on site, a method of correcting for changing atmospheric quality based on cosmic-ray trigger-rate is presented and applied to data taken on the active galactic nuclei PKS 2155-304. These data were taken during August and September 2004, when the quality of the atmosphere at the site was highly variable. Corrected and uncorrected fluxes are shown, and the method is presented as a first step towards a more complete atmospheric calibration

Atmospheric sensing for the H.E.S.S. array

Several atmospheric monitoring instruments have been installed at the H.E.S.S. gamma‐ray observatory in Namibia. Firstly, Heitronics KT19 infrared radiometers, aligned paraxially with the H.E.S.S. telescopes, measure the infrared radiation of the water molecules. These allow us to detect clouds crossing the telescopes’ field of view and to estimate the humidity present in the atmosphere. For a general estimate of the atmosphere’s transmittance, i.e. the detection of any light‐attenuating aerosols, a ceilometer, which is a LIDAR with built‐in atmospheric data reduction code, is being used. It will be complemented soon by an instrument which will measure the transmissivity of the atmosphere at different wavelengths up to 500m above the ground. The overall status of the weather is monitored by a fully automated weatherstation. This paper describes the setup, the data analysis and how this will be used in order to improve the knowledge of the telescopes’ effective collection area