We discovered the >100 GeV γ-ray source, HESS J1713-381, apparently associated with the shell-type supernova remnant (SNR) CTB 37B, using HESS in 2006. In 2007 we performed X-ray follow-up ...observations with Chandra with the aim of identifying a synchrotron counterpart to the TeV source and/or thermal emission from the SNR shell. These new Chandra data, together with additional TeV data, allow us to investigate the nature of this object in much greater detail than was previously possible. The new X-ray data reveal thermal emission from a ~4' region in close proximity to the radio shell of CTB 37B. The temperature of this emission implies an age for the remnant of ~5000 years and an ambient gas density of ~0.5 cm-3. Both these estimates are considerably uncertain due to the asymmetry of the SNR and possible modifications of the kinematics due to efficient cosmic ray (CR) acceleration. A bright (≈ 7 $\times$ 10-13 erg cm-2 s-1) and unresolved (<1″) source (CXOU J171405.7-381031), with a soft (Γ≈3.3) non-thermal spectrum is also detected in coincidence with the radio shell. Absorption indicates a column density consistent with the thermal emission from the shell, suggesting a genuine association rather than a chance alignment. The observed TeV morphology is consistent with an origin in the complete shell of CTB 37B. The lack of diffuse non-thermal X-ray emission suggests an origin of the γ-ray emission via the decay of neutral pions produced in interactions of protons and nuclei, rather than inverse Compton (IC) emission from relativistic electrons.
The extreme synchrotron BL Lac object H 2356-309, located at a redshift of z = 0.165, was observed from June to December 2004 with a total exposure of approx40 h live-time with the HESS (High Energy ...Stereoscopic System) array of atmospheric-Cherenkov telescopes (ACTs). Analysis of this data set yields, for the first time, a strong excess of 453 gamma-rays (10 standard deviations above background) from H 2356-309, corresponding to an observed integral flux above 200 GeV of I (>200 GeV) = (4.1 pm 0.5) times 10 super(-12) cm super(-2) s super(-1) (statistical error only). The differential energy spectrum of the source between 200 GeV and 1.3 TeV is well-described by a power law with a normalisation (at 1 TeV) of N sub(0) = (4.1 pm 0.5) times 10 super(-13) cm super(-2) s super(-1) TeV super(-1) and a photon index of Gamma = gamma-rays so far. Results from simultaneous observations from ROTSE-III (optical), RXTE (X-rays) and NRT (radio) are also included and used together with the HESS data to constrain a single-zone homogeneous synchrotron self- Compton (SSC) model. This model provides an adequate fit to the HESS data when using a reasonable set of model parameters.
The γ-ray emission above $250 \mathrm{GeV}$ from the BL Lac object Markarian 421 was observed by the CAT Cherenkov imaging telescope between December, 1996, and June, 2000. In 1998, the source ...produced a series of small flares, making it the second extragalactic source detected by CAT. The time-averaged differential spectrum has been measured from 0.3 to $5 \mathrm{TeV}$, which is well fitted with a power law: $\frac{\mathrm{d}\phi}{\mathrm{d}E}\varpropto E_{\mathrm{TeV}}^{-2.88\pm0.12^\mathrm{stat}\pm0.06^\mathrm{syst}}$. In 2000, the source showed an unprecedented activity, with variability time-scales as short as one hour, as for instance observed during the night between 4 and 5 February. The 2000 time-averaged spectrum measured is compatible with that of 1998, but some indication of a spectral curvature is found between 0.3 and $5 \mathrm{TeV}$. The possibility of $\mathrm{TeV}$ spectral hardening during flares is also discussed, and the results are compared to those obtained on the other $\mathrm{TeV}$ BL Lac, Markarian 501.
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 107m2 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.
The serendipitous discovery of an unidentified extended TeV γ-ray source close to the galactic plane named HESS J1303-631 at a significance of 21 standard deviations is reported. The observations ...were performed between February and June 2004 with the HESS stereoscopic system of Cherenkov telescopes in Namibia. HESS J1303-631 was discovered roughly $0.6\degr$ north of the binary system PSR B1259-63/SS 2883, the target object of the initial observation campaign which was also detected at TeV energies in the same field of view. HESS J1303-631 is extended with a width of an assumed intrinsic Gaussian emission profile of $\sigma = (0.16 \pm 0.02)\degr$ and the integral flux above $380\,\mathrm{GeV}$ is compatible with constant emission over the entire observational period of $(17 \pm 3)$% of the Crab Nebula flux. The measured energy spectrum can be described by a power-law $\mathrm{d}N/\mathrm{d}E \sim E^{-\Gamma}$ with a photon index of $\Gamma = 2.44 \pm 0.05_{\mathrm{stat}} \pm 0.2_{\mathrm{syst}}$. Up to now, no counterpart at other wavelengths is identified. Various possible TeV production scenarios are discussed.
The accretion of matter onto a massive black hole is believed to feed the relativistic plasma jets found in many active galactic nuclei (AGN). Although some AGN accelerate particles to energies ...exceeding 1012 electron volts and are bright sources of very-high-energy (VHE) γ-ray emission, it is not yet known where the VHE emission originates. Here we report on radio and VHE observations of the radio galaxy Messier 87, revealing a period of extremely strong VHE γ-ray flares accompanied by a strong increase of the radio flux from its nucleus. These results imply that charged particles are accelerated to very high energies in the immediate vicinity of the black hole. PUBLICATION ABSTRACT
Mkn 421 was observed during a high flux state for nine nights in April and May 2004 with the fully operational High Energy Stereoscopic System (HESS) in Namibia. The observations were carried out at ...zenith angles of 60°–65°, which result in an average energy threshold of 1.5 TeV and a collection area reaching 2 km2 at 10 TeV. Roughly 7000 photons from Mkn 421 were accumulated with an average gamma-ray rate of 8 photons/min. The overall significance of the detection exceeds 100 standard deviations. The light-curve of integrated fluxes above 2 TeV shows changes of the diurnal flux up to a factor of 4.3. For nights of high flux, intra-night variability is detected with a decay time of less than 1 h. The time averaged energy spectrum is curved and is well described by a power-law with a photon index $\Gamma=2.1\pm0.1_{\rm stat}\pm0.3_{\rm sys}$ and an exponential cutoff at $E_{\rm c}=3.1(+0.5\,-0.4)_{\rm stat}\pm0.9_{\rm sys}$ TeV and an average integral flux above 2 TeV of 3 Crab flux units. Significant variations of the spectral shape are detected with a spectral hardening as the flux increases. Contemporaneous multi-wavelength observations at lower energies (X-rays and gamma-rays above ≈300 GeV) indicate smaller relative variability amplitudes than seen above 2 TeV during high flux state observed in April 2004.
APD photodetectors in the Geiger photon counter mode Pellion, D.; Borrel, V.; Esteve, D. ...
Nuclear instruments & methods in physics research. Section A, Accelerators, spectrometers, detectors and associated equipment,
11/2006, Letnik:
567, Številka:
1
Journal Article
Recenzirano
Odprti dostop
The best detector in Cherenkov 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 polarised over its own breakdown bias, one single photon passing through it may start an electron avalanche resulting in about 10
6 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.
Discovery of VHE gamma rays from PKS 2005–489 Aharonian, F.; Akhperjanian, A. G.; Aye, K.-M. ...
Astronomy and astrophysics (Berlin),
06/2005, Letnik:
436, Številka:
2
Journal Article
Recenzirano
Odprti dostop
The high-frequency peaked BL Lac PKS 2005-489 ($z=0.071$) was observed in 2003 and 2004 with the HESS stereoscopic array of imaging atmospheric-Cherenkov telescopes in Namibia. A signal was detected ...at the 6.7σ level in the 2004 observations (24.2 h live time), but not in the 2003 data set (27.3 h live time). PKS 2005-489 is the first blazar independently discovered by HESS to be an emitter of VHE photons, and only the second such blazar in the Southern Hemisphere. The integral flux above 200 GeV observed in 2004 is ($6.9\pm1.0_{\rm stat}\pm1.4_{\rm syst}) \times 10^{-12}$ cm-2 s-1, corresponding to ~2.5% of the flux observed from the Crab Nebula. The 99% upper limit on the flux in 2003, I(>200 GeV$) < 5.2 \times 10^{-12}$ cm-2 s-1, is smaller than the flux measured in 2004, suggesting an increased level of activity in 2004. However, the data show no evidence for significant variability on any time scale less than a year. An energy spectrum is measured and is characterized by a very soft power law (photon index of $\Gamma=4.0\pm0.4$).
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