J. Kushida

Design study of CANGAROO-III, stereoscopic imaging atmospheric Cherenkov telescopes for sub-TeV γ-ray detection

CANGAROO-III is an Imaging Atmospheric Cherenkov Telescope (IACT) array of four 10 m telescopes for very high energy (sub-TeV) gamma-ray astronomy. A design study of

Detection of diffuse TeV gamma-ray emission from the neaby starburst galaxy NGC 253

Department of Physics, Osaka City University, Osaka, Osaka 558-8585, JapanA&A 396, L1-L4(2002). A&A402, 443-455(2003)ABSTRACTContext.Aims. The CANGAROO-II telescope observed sub-TeV gamma-ray emission from the nearby starburst galaxy NGC 253. The emission regionwas extended with a radial size of 0.3-0.6 degree. On the contrary, H.E.S.S could not confirm this emission and gave upper l imits at the levelof the CANGAROO-II flux. In order to resolve this discrepancy, we analyzed new observational results for NGC 253 by CANGAROO-III andalso assessed the results by CANGAROO-II.Methods. Observation was made with three telescopes of the CANGAROO-III in October 2004. We analyzed three-fold coincidence data bythe robust Fisher Discriminant method to discriminate gamma ray events from hadron events.Results. The result by the CANGAROO-III was negative. The upper limit of gamma ray flux was 5.8% Crab at 0.58 TeV for po int-sourceassumption. In addition, the significance of the excess flux o f gamma-rays by the CANGAROO-II was lowered to less than 4 sigma afterassessing treatment of malfunction of photomultiplier tubes.Key words. gamma rays: observation – galaxies: starburst – galaxies: i ndividual: NGC 253 – galaxies: halos: cosmic rays

Erratum: Evidence of TeV gamma-ray emission from the nearby starburst galaxy NGC 253

The CANGAROO-II telescope observed sub-TeV gamma-ray emission from the nearby starburst galaxy NGC 253. The emission region was extended with a radial size of 0.3-0.6 degree. On the contrary, H.E.S.S could not confirm this emission and gave upper limits at the level of the CANGAROO-II flux. In order to resolve this discrepancy, we analyzed new observational results for NGC 253 by CANGAROO-III and also assessed the results by CANGAROO-II. Observation was made with three telescopes of the CANGAROO-III in October 2004. We analyzed three-fold coincidence data by the robust Fisher Discriminant method to discriminate gamma ray events from hadron events. The result by the CANGAROO-III was negative. The upper limit of gamma ray flux was 5.8% Crab at 0.58 TeV for point-source assumption. In addition, the significance of the excess flux of gamma-rays by the CANGAROO-II was lowered to less than 4 sigma after assessing treatment of malfunction of photomultiplier tubes.Aims. The CANGAROO-II telescope observed sub-TeV gamma-ray emission from the nearby starburst galaxy NGC 253. The emission region was extended with a radial size of 0.3–0.6 degree. On the contrary, HESS could not confirm this emission and gave upper limits at the level of the CANGAROO-II flux. In order to resolve this discrepancy, we analyzed new observational results for NGC 253 by CANGAROO-III and also assessed the results by CANGAROO-II. Methods. Observation was made with three telescopes of the CANGAROO-III in October 2004. We analyzed three-fold coincidence data by the robust Fisher Discriminant method to discriminate gamma ray events from hadron events. Results. The result by the CANGAROO-III was negative. The upper limit of gamma ray flux was 5.8% Crab at 0.58 TeV for pointsource assumption. In addition, the significance of the excess flux of gamma-rays by the CANGAROO-II was lowered to less than 4 sigma after assessing treatment of malfunction of photomultiplier tubes.

A technique for estimating the absolute gain of a photomultiplier tube

Abstract Detection of low-intensity light relies on the conversion of photons to photoelectrons, which are then multiplied and detected as an electrical signal. To measure the actual intensity of the light, one must know the factor by which the photoelectrons have been multiplied. To obtain this amplification factor, we have developed a procedure for estimating precisely the signal caused by a single photoelectron. The method utilizes the fact that the photoelectrons conform to a Poisson distribution. The average signal produced by a single photoelectron can then be estimated from the number of noise events, without requiring analysis of the distribution of the signal produced by a single photoelectron. The signal produced by one or more photoelectrons can be estimated experimentally without any assumptions. This technique, and an example of the analysis of a signal from a photomultiplier tube, are described in this study.

Simulating Cherenkov Telescope Array observation of RX J1713.7–3946

We perform simulations of Cherenkov Telescope Array (CTA) observations of a young supernova remnant RX J1713.7-3946. This target is not only one of the brightest sources ever discovered in very high-energy gamma rays but also well observed in other wavebands. In X-rays, the emission is dominated by synchrotron radiation, which links directly to the existence of high-energy electrons. Radio observations of CO and HI gas have revealed a highly inhomogeneous medium surrounding the SNR, such as clumpy molecular clouds. Therefore gamma rays from hadronic interactions are naturally expected. However, the spectrum in GeV energy range measured by Fermi/LAT indicates more typical of leptonic emission from accelerated electrons. Despite lots of multi-wavelength information, the competing interpretations have led to much uncertainty in the quest of unraveling the true origin of the gamma-ray emission from RX~J1713.7--3946. CTA will achieve highest performance ever in sensitivity, angular resolution, and energy resolution. We estimate CTA capability to examine the emission mechanisms of the gamma rays through simulated spatial distribution, spectra, and their time variation.