Abstract
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 4.0σ level in these observations (7.6 hours live time). Three alternative, lower-threshold analyses yield >5σ excesses. The observed integral flux above 200 GeV is (4.8 ± 1.3stat ± 1.0syst) × 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 Γ = 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 γ-ray absorption by the Extragalactic Background Light, and is used to place an upper limit on the redshift of z < 0.74.
Original language | English (US) |
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Pages (from-to) | L19-L23 |
Journal | Astronomy and Astrophysics |
Volume | 448 |
Issue number | 2 |
DOIs | |
State | Published - Mar 2006 |
Keywords
- Galaxies: active
- Gamma rays: observations