Papers - TAJIMA, Hiroyasu

TARGET: A multi-channel digitizer chip for very-high-energy gamma-ray telescopes Reviewed International coauthorship

Bechtol, K.; Funk, S.; Okumura, A.; Ruckman, L. L.; Simons, A.; Tajima, H.; Vandenbroucke, J.; Varner, G. S.

Astroparticle Physics   Vol. 36   page： 156-165   2012.8

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Language：English   Publishing type：Research paper (scientific journal)  

The next-generation very-high-energy (VHE) gamma-ray observatory, the Cherenkov Telescope Array, will feature dozens of imaging atmospheric Cherenkov telescopes (IACTs), each with thousands of pixels of photo-sensors. To be affordable and reliable, reading out such an array of up to a million channels requires event recording technology that is highly integrated and modular, with a low cost per channel. We present the design and performance of a chip targeted to this application: the TeV Array Readout with GSa/s sampling and Event Trigger (TARGET). This application-specific integrated circuit (ASIC) has 16 parallel input channels, a 4096-sample buffer for each channel, adjustable input termination, self-trigger functionality, and tight window-selected readout. We report the performance of the first-generation version of this chip (TARGET 1) in terms of sampling frequency, power consumption, dynamic range, current-mode gain, analog bandwidth, and cross talk. The large number of channels per chip allows a low cost per channel (10 to 20 including front-end and back-end electronics but not including photosensors) to be achieved with a TARGET-based IACT readout system. In addition to basic performance parameters of the TARGET 1 chip, we present a camera module prototype as well as a second-generation chip (TARGET 2), both of which have now been produced.

DOI： 10.1016/j.astropartphys.2012.05.016

Fermi Large Area Telescope Discovery of GeV Gamma-Ray Emission from the Vicinity of SNR W44 Reviewed International coauthorship

Uchiyama, Yasunobu; Funk, Stefan; Katagiri, Hideaki; Katsuta, Junichiro; Lemoine-Goumard, Marianne; Tajima, Hiroyasu; Tanaka, Takaaki; Torres, Diego F.

The Astrophysical Journal Letters   Vol. 749   page： L35   2012.4

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We report the detection of GeV gamma-ray emission from the molecular cloud complex that surrounds the supernova remnant (SNR) W44 using the Large Area Telescope on board Fermi. While the previously reported gamma-ray emission from SNR W44 is likely to arise from the dense radio-emitting filaments within the remnant, the gamma-ray emission that appears to come from the surrounding molecular cloud complex can be ascribed to the cosmic rays (CRs) that have escaped from W44. The non-detection of synchrotron radio emission associated with the molecular cloud complex suggests the decay of pi0 mesons produced in hadronic collisions as the gamma-ray emission mechanism. The total kinetic energy channeled into the escaping CRs is estimated to be Wesc ~ (0.3-3) x 10^50 erg, in broad agreement with the conjecture that SNRs are the main sources of Galactic CRs.

DOI： 10.1088/2041-8205/749/2/L35

Gamma-Ray Observations of the Supernova Remnant RX J0852.0-4622 with the Fermi Large Area Telescope Reviewed International coauthorship

Tanaka, T.; Allafort, A.; Ballet, J.; Funk, S.; Giordano, F.; Hewitt, J.; Lemoine-Goumard, M.; Tajima, H.; Tibolla, O.; Uchiyama, Y.

The Astrophysical Journal Letters   Vol. 740   page： L51   2011.10

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Language：English   Publishing type：Research paper (scientific journal)  

We report on gamma-ray observations of the supernova remnant (SNR) RX J0852.0-4622 with the Large Area Telescope (LAT) on board the Fermi Gamma-ray Space Telescope. In the Fermi-LAT data, we find a spatially extended source at the location of the SNR. The extension is consistent with the SNR size seen in other wavelengths such as X-rays and TeV gamma rays, leading to the identification of the gamma-ray source with the SNR. The spectrum is well described as a power law with a photon index of 1.85±0.06 (stat)+0.18-0.19 (sys), which smoothly connects to the H.E.S.S. spectrum in the TeV energy band. We discuss the gamma-ray emission mechanism based on multiwavelength data. The broadband data can be fit well by a model in which the gamma rays are of hadronic origin. We also consider a scenario with inverse Compton scattering of electrons as the emission mechanism of the gamma rays. Although the leptonic model predicts a harder spectrum in the Fermi-LAT energy range, the model can fit the data considering the statistical and systematic errors.

DOI： 10.1088/2041-8205/740/2/L51