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

APD photodetectors in the Geiger photon counter mode

International audienc

APD photodetectors in the Geiger photon counter mode.

International audienceGeiger APD technology, which has been used for a few years now, is evolving towards better performances, including integration in multifunctional Microsystems; one such achievement is today the so-called SiPM [ref 1]. The present work has been conducted by a consortium of researchers from CESR and LAAS/CNRS and the manufacturing of components was achieved in the clean room of LAAS/CNRS. We present here an original N/P technology of photodiode, designed so as to offer a very good homogeneity in the electrical operating characteristics. For this, we have chosen a design and technological process which defines the breakdown voltage from the substrate doping. We present the technological process which we developed, in which we took a special care to maintain, by low transit temperature processes, at the highest quality level the initial characteristics of the materials. We will also present the performances of the diodes produced, with sizes ranging from 10 to 100µm, as a function of many parameters (gain, dark current, etc). We also produced SiPM, and also 8X8 arrays of SiPM. Typical characteristics for a single diode are a Vbr between 43V and 44V, and a dark current below 1 pA at ambient temperature. But the most important feature seems to be the high homogeneity of these performances all over the wafer surface. This gives us a great confidence in the next step of our work, which is the manufacturing of very high sensitivity imaging devices

APD photodetectors in the Geiger photon counter mode.

International audienceThe best detector in Cerenkov experiments still remains the PM tube, thanks to its characteristics of sensitivity and speed. But its disadvantages are its low quantum efficiency and its cost. We are currently working on solid state silicon detectors, used in the Geiger photon counter mode. We have conducted a series of tests using standard APD, but with an electronic circuitry to raise the polarisation towards the Geiger mode. The photodiode is polarized over its own breakdown bias, one single photon passing through it may start an electron avalanche resulting in about 106 electrons collected. After that, the diode should recover as soon as possible to be available for the next photon. This process is under modelization: electrical diagrams (PSPICE), differential equations (VHDL-AMS) and components physics equations (SABER) are needed to reproduce closely the physical processes and to allow optimisation and improvement of the electronics both for triggering and for the readout of the detectors. Our most promising results will be presented

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

Evidence for VHE gamma-ray emission from the distant BL Lac PG 1553+113

The high-frequency peaked BL Lac PG 1553+113 was observed in 2005 with the H.E.S.S. stereoscopic array of imaging atmospheric-Cherenkov telescopes in Namibia. Using the H.E.S.S. standard analysis, an excess was measured at the 4.0 sigma level in these observations (7.6 hours live time). Three alternative, lower-threshold analyses yield >5 sigma excesses. The observed integral flux above 200 GeV is (4.8 +/- 1.3_{stat} +/- 1.0_{syst}) x10^{-12} cm^{-2} s^{-1}, and shows no evidence for variability. The measured energy spectrum is characterized by a very soft power law (photon index of Gamma=4.0 +/- 0.6). Although the redshift of PG 1553+113 is unknown, there are strong indications that it is greater than z=0.25 and possibly larger than z=0.78. The observed spectrum is interpreted in the context of VHE gamma-ray absorption by the Extragalactic Background Light, and is used to place an upper limit on the redshift of z<0.74