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Humboldt-Universität zu Berlin - Mathematisch-Naturwissen­schaft­liche Fakultät - Experimentelle Elementarteilchenphysik

The H Collaboration (2006)

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

aap, 448:L19-L23.

The high-frequency peaked BL Lac PG 1553+113 was observed in 2005 with the HESS stereoscopic array of imaging atmospheric-Cherenkov telescopes in Namibia. Using the HESS standard analysis, an excess was measured at the 4.0sigma level in these observations (7.6 hours live time). Three alternative, lower-threshold analyses yield >5sigma excesses. The observed integral flux above 200 GeV is (4.8±1.3^stat±1.0^syst)×10^-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.

Provided by the SAO/NASA Astrophysics Data System