The Crab supernova remnant has been observed regularly with the stereoscopic system of five imaging air Cerenkov telescopes that was part of the High Energy Gamma Ray Astronomy (HEGRA) experiment. In ...total, close to 400 hr of useful data have been collected from 1997 to 2002. The differential energy spectrum of the combined data set can be approximated by a power law-type energy spectrum: d Phi /dE = Phi sub(0) (E/TeV) Gamma , Phi sub(0) = (2.83 plus or minus 0.04 sub(stat) plus or minus 0.6 sub(sys)) 10 super(-11) photons cm super(-2) s super(-1) TeV super(-1), and Gamma = -2.62 plus or minus 0.02 sub(stat) plus or minus 0.05 sub(sys). The spectrum extends up to energies of 80 TeV and is well matched by model calculations in the framework of inverse Compton scattering of various seed photons in the nebula, including for the first time a recently detected compact emission region at millimeter wavelengths. The observed indications for a gradual steepening of the energy spectrum in data is expected in the inverse Compton emission model. The average magnetic field in the emitting volume is determined to be 161.6 plus or minus 0.8 sub(stat) plus or minus 18 sub(sys) mu G. The presence of protons in the nebula is not required to explain the observed flux, and upper limits on the injected power of protons are calculated to be as low as 20% of the total spin-down luminosity for bulk Lorentz factors of the wind in the range of 10 super(4)-10 super(6). The position and size of the emission region have been studied over a wide range of energies. The position is shifted by 13" to the west of the pulsar, with a systematic uncertainty of 25". No significant shift in the position with energy is observed. The size of the emission region is constrained to be less than 2' at energies between 1 and 10 TeV. Above 30 TeV the size is constrained to be less than 3'. No indication of pulsed emission has been found, and upper limits in differential bins of energy have been calculated reaching typically 1%-3% of the unpulsed component.
Routine observations of the Crab Nebula for a total of about 250 h, performed with the HEGRA stereoscopic system of five imaging atmospheric Čerenkov telescopes in the standard operational mode, have ...proven the energy threshold of the system to be 500 GeV for small zenith angles (
θ⩽20°). A
topological trigger applied along with the
convergent observational mode allows to reduce noticeably the energy threshold of the system down to 350 GeV. Here we present the relevant Monte Carlo simulations as well as the analysis results of 15 h Crab Nebula data taken in such an observational mode. From the Crab Nebula data, the final energy threshold was found to be 350 GeV. The estimated γ-ray flux from the Crab Nebula above 350 GeV is (8.1±0.1
stat±0.2
syst)×10
−11 ph
cm
−2
s
−1, which is consistent with recent measurements reported by the STACEE, CELESTE, CAT, and Whipple groups.
Measurement of diffraction efficiencies relevant to crystal lens telescopes Kohnle, Antje; Smither, Robert; Graber, Timothy ...
Nuclear instruments & methods in physics research. Section A, Accelerators, spectrometers, detectors and associated equipment,
11/1998, Letnik:
416, Številka:
2
Journal Article
Recenzirano
The Toulouse/Argonne collaboration is working on a novel detector for gamma-ray astronomy based on a crystal lens that uses Bragg reflection of Ge crystals to concentrate photons onto a small ...detector in the image plane. The diffraction efficiency of the crystals is decisive for the energy bandpass, the field of view and the effective area of the crystal lens telescope. We have measured diffraction efficiencies of Ge crystals from 200 to 500
keV at the Advanced Photon Source synchrotron at Argonne National Laboratory in two experiments carried out at the sector 1 bending magnet beamline. The high brilliance and high particle energy of third-generation synchrotrons permit measurements at gamma-ray energies. The low angular divergence of synchrotron radiation is similar to that of an astronomical point source. The efficiencies ranged from 20% to 31% for diffraction of Ge
(1
1
1)
and
(2
2
0)
using crystals with a mosaic width of a few arc seconds. Neglecting absorption, the values ranged from 31% to 49%. The continuum flux diffracted in second-order diffraction
(4
4
0)
was a factor of 2.4–2.6 less than in first-order diffraction
(2
2
0)
. For third-order diffraction (the
(3
3
3)
and the
(1
1
1)
planes), the flux was reduced by a factor of 6.8.
The peak efficiency and the rocking curve integral for mosaic widths ranging from a few arc seconds to 0.4° were measured. The results agree with the expectation of the Darwin model for mosaic crystals. This gives us confidence in the calculations of the crystal lens telescope performance, and allows an optimization of a balloon-borne and a satellite-based telescope configuration.
Measurements were carried out with a wedged crystal, that was used to increase the diffraction efficiency of the lens crystals for a source at finite distance. The efficiency increase is due to both a bending and a straining of the crystal. We measured rocking curve widths of a crystal before and after etching to determine the effect of surface damage on the diffraction efficiency. The mean decrease in rocking curve width due to the etching was 1.7
arc
sec in the center of the crystal and 6.9
arc
sec at the crystal edge.
Photodetectors for HESS Kohnle, A; Mattes, J; Hermann, G ...
Nuclear instruments & methods in physics research. Section A, Accelerators, spectrometers, detectors and associated equipment,
03/2000, Letnik:
442, Številka:
1-3
Journal Article
Recenzirano
HESS is a next-generation array of imaging atmospheric Cherenkov telescopes (IACTs) for the detection of cosmic gamma rays above 40 GeV, to be built in the Southern Hemisphere in the Khomas Highland ...of Namibia. In its final stage, the HESS experiment will consist of up to 16 telescopes each with a 80 m2 reflector and 15 m focal length. The schedule foresees to start with scientific operation of the first 4-telescope subsystem in 2002 (Phase 1). In the focal plane, each of the cameras contains ∼700 photomultiplier pixels each viewing 0.16° of the sky, giving a total field of view of 4.3°. A large part of the trigger electronics, analog signal storage, and control and monitoring electronics are integrated in the camera.
The unidentified TeV source in Cygnus is now confirmed by follow-up observations from 2002 with the HEGRA stereoscopic system of Cherenkov Telescopes. Using all data (1999 to 2002) we confirm this ...new source as steady in flux over the four years of data taking, extended with radius 6.2′ (±$1.2^\prime_{\rm stat}$ ± $0.9^\prime_{\rm sys}$) and exhibiting a hard spectrum with photon index -1.9. It is located in the direction of the dense OB stellar association, Cygnus OB2. Its integral flux above energies $E>1$ TeV amounts to ~5% of the Crab assuming a Gaussian profile for the intrinsic source morphology. There is no obvious counterpart at radio, optical nor X-ray energies, leaving TeV J2032+4130 presently unidentified. Observational parameters of this source are updated here and some astrophysical discussion is provided. Also included are upper limits for a number of other interesting sources in the FoV, including the famous microquasar Cygnus X-3.
Deep observation (~113 hrs) of the Cygnus region at TeV energies using the HEGRA stereoscopic system of air Čerenkov telescopes has serendipitously revealed a signal positionally inside the core of ...the OB association Cygnus OB2, at the edge of the 95% error circle of the EGRET source 3EG J2033+4118, and ~$0.5^\circ$ north of Cyg X-3. The source centre of gravity is RA $\alpha_{\rm J2000}$: $20^{\rm hr} 32^{\rm m} 07^{\rm s}\pm 9.2^{\rm s}_{\rm stat} \pm2.2^{\rm s}_{\rm sys}$, Dec $\delta_{\rm J2000}$: $+41^\circ 30^\prime 30^{\prime\prime}\pm 2.0^\prime_{\rm stat} \pm 0.4^\prime_{\rm sys}$. The source is steady, has a post-trial significance of +4.6σ, indication for extension with radius $5.6^\prime$ at the ~$3\sigma$ level, and has a differential power-law flux with hard photon index of $-1.9 \pm0.3_{\rm stat}\pm0.3_{\rm sys}$. The integral flux above 1 TeV amounts ~3% that of the Crab. No counterpart for the TeV source at other wavelengths is presently identified, and its extension would disfavour an exclusive pulsar or AGN origin. If associated with Cygnus OB2, this dense concentration of young, massive stars provides an environment conducive to particle acceleration and likely subsequent interaction with a nearby gas cloud. Alternatively, one could envisage γ-ray production via a jet-driven termination shock.
232 hours of data were accumulated from 1997 to 1999, using the HEGRA Stereoscopic Cherenkov Telescope System to observe the supernova remnant Cassiopeia A. TeV γ-ray emission was detected at the $5 ...\sigma$ level, and a flux of $(5.8 \pm 1.2_{\mathrm{stat}} \pm 1.2_{\mathrm{syst}}) 10^{-9}\ {ph} {m}^{-2} {s}^{-1}$ above 1 TeV was derived. The spectral distribution is consistent with a power law with a differential spectral index of $-2.5 \pm 0.4_{\mathrm{stat}} \pm 0.1_{\mathrm{syst}}$ between 1 and 10 TeV. As this is the first report of the detection of a TeV γ-ray source on the "centi-Crab"scale, we present the analysis in some detail. Implications for the acceleration of cosmic rays depend on the details of the source modeling. We discuss some important aspects in this paper.
Using the HEGRA system of imaging atmospheric Cherenkov telescopes, a region of the Galactic plane ($-10^\circ < b < 5^\circ$, $38^\circ < l < 43^\circ$) was surveyed for TeV gamma-ray emission, both ...from point sources and of diffuse nature. The region covered includes 15 known pulsars, 6 known supernova remnants (SNR) and one unidentified EGRET source. No evidence for emission from point sources was detected; upper limits are typically below 0.1 Crab units for the flux above 1 TeV. For the diffuse gamma-ray flux from the Galactic plane, an upper limit of $6.1\times10^{-15}$ ph cm-2 s-1 sr-1 MeV-1 was derived under the assumption that the spatial distribution measured by the EGRET instrument extends to the TeV regime. This upper flux limit is a factor of about 1.5 larger than the flux expected from the ensemble of gamma-ray unresolved Galactic cosmic ray sources.
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