The high-frequency peaked BL Lac PKS 2155-304 at redshift z=0.117 has been detected with high significance (∼45σ) at energies greater than 160 GeV, using the H.E.S.S. stereoscopic array of imaging air-Cherenkov telescopes in Namibia. A strong signal is found in each of the data sets corresponding to the dark periods of July and October, 2002, and June-September, 2003. The observed flux of VHE gamma rays shows variability on time scales of months, days, and hours. The monthly-averaged integral flux above 300 GeV varies between 10% and 60% of the flux observed from the Crab Nebula. Energy spectra are measured for these individual periods of data taking and are characterized by a steep power law with a time-averaged photon index of Γ=3.32±0.06. An improved χ2 per degree of freedom is found when either a power law with an exponential cutoff energy or a broken power law are fit to the time-averaged energy spectrum. However, the significance of the improvement is marginal (∼2σ). The suggested presence of features in the energy spectrum may be intrinsic to the emission from the blazar, or an indication of absorption of TeV gamma rays by the extragalactic infrared background light.
H.E.S.S. observations of PKS 2155-304
- F. Aharonian et al. (H.E.S.S. Collaboration)
- Astronomy & Astrophysics 430, 865-875 (2005)
- 2005
Abstract
Auxiliary informations
H.E.S.S. data points for Figure 5 (Time-Average Spectrum)
Energy interval Mean energy Flux Flux Error [TeV] [TeV] [/TeV cm^2 s] 0.200 0.267 0.228189 2.57312e-10 1.91652e-11 0.267 0.356 0.304475 8.80581e-11 6.3164e-12 0.356 0.474 0.405731 4.27828e-11 2.88159e-12 0.474 0.633 0.540882 1.56588e-11 1.23406e-12 0.633 0.844 0.721846 6.2626e-12 5.73691e-13 0.844 1.125 0.962343 2.18063e-12 2.74668e-13 1.125 1.500 1.2829 6.07232e-13 1.46382e-13 1.500 2.000 1.71053 2.77766e-13 7.59495e-14 2.000 2.667 2.28083 8.65328e-14 4.56978e-14
Remarks: Statistical errors only. Systematic error on energy scale estimated to 20%. See paper for details.
H.E.S.S. data points for Figure 4a (July 2002 Spectrum)
Energy interval Mean energy Flux Flux Error [TeV] [TeV] [/TeV cm^2 s] 0.356 0.474 0.407063 2.21116e-10 4.05839e-11 0.474 0.633 0.542694 1.03221e-10 1.777e-11 0.633 0.844 0.724238 2.70964e-11 7.70216e-12 0.844 1.125 0.965526 1.12623e-11 4.39106e-12 1.125 1.500 1.28715 9.54899e-12 3.49308e-12 1.500 2.000 1.7162 4.80433e-12 1.49672e-12 2.000 2.667 2.2884 2.3641e-12 1.32311e-12 2.667 3.557 3.05178 8.48101e-13 5.20042e-13 3.557 4.744 4.07019 3.55706e-13 3.69679e-13
Remarks: Statistical errors only. Systematic error on energy scale estimated to 20%. See paper for details.
H.E.S.S. data points for Figure 4b (October 2002 Spectrum)
Energy interval Mean energy Flux Flux Error [TeV] [TeV] [/TeV cm^2 s] 0.356 0.474 0.406348 1.0226e-10 3.0508e-11 0.474 0.633 0.541722 4.27171e-11 1.23132e-11 0.633 0.844 0.722954 1.60494e-11 5.92188e-12 0.844 1.125 0.963817 5.91376e-12 3.30237e-12 1.125 1.500 1.28487 4.0376e-12 2.05608e-12 1.500 2.000 1.71316 1.02904e-12 1.23482e-12
Remarks: Statistical errors only. Systematic error on energy scale estimated to 20%. See paper for details.
H.E.S.S. data points for Figure 4c (June 2003 Spectrum)
Energy interval Mean energy Flux Flux Error [TeV] [TeV] [/TeV cm^2 s] 0.356 0.474 0.405082 5.40824e-11 6.3948e-12 0.474 0.633 0.539999 2.42304e-11 2.85232e-12 0.633 0.844 0.72068 6.75204e-12 1.32567e-12 0.844 1.125 0.960792 3.36134e-12 6.24381e-13 1.125 1.500 1.28083 8.34634e-13 3.14273e-13 1.500 2.000 1.70777 2.48435e-13 1.63906e-13 2.000 2.667 2.27714 8.79799e-14 9.04906e-14
Remarks: Statistical errors only. Systematic error on energy scale estimated to 20%. See paper for details.
H.E.S.S. data points for Figure 4d (July 2003 Spectrum)
Energy interval Mean energy Flux Flux Error [TeV] [TeV] [/TeV cm^2 s] 0.200 0.267 0.22828 2.30019e-10 3.13195e-11 0.267 0.356 0.304595 7.33201e-11 8.94987e-12 0.356 0.474 0.405889 3.48987e-11 4.00428e-12 0.474 0.633 0.541098 1.21015e-11 1.73843e-12 0.633 0.844 0.722131 5.68651e-12 8.34539e-13 0.844 1.125 0.962722 1.7727e-12 4.2794e-13 1.125 1.500 1.28341 6.98939e-13 2.37664e-13 1.500 2.000 1.71121 3.73034e-13 1.22536e-13 2.000 2.667 2.28173 6.88855e-14 5.86386e-14
Remarks: Statistical errors only. Systematic error on energy scale estimated to 20%. See paper for details.
H.E.S.S. data points for Figure 4e (August 2003 Spectrum)
Energy interval Mean energy Flux Flux Error [TeV] [TeV] [/TeV cm^2 s] 0.200 0.267 0.228123 2.52886e-10 2.38711e-11 0.267 0.356 0.304388 9.20111e-11 8.32373e-12 0.356 0.474 0.405616 4.11884e-11 3.83435e-12 0.474 0.633 0.540726 1.41557e-11 1.76842e-12 0.633 0.844 0.721639 6.49463e-12 9.0103e-13 0.844 1.125 0.962068 1.98742e-12 4.23007e-13 1.125 1.500 1.28253 3.76204e-13 2.3187e-13 1.500 2.000 1.71004 1.64224e-13 1.43269e-13 2.000 2.667 2.28018 1.59893e-13 7.0881e-14
Remarks: Statistical errors only. Systematic error on energy scale estimated to 20%. See paper for details.
H.E.S.S. data points for Figure 4f (September 2003 Spectrum)
Energy interval Mean energy Flux Flux Error [TeV] [TeV] [/TeV cm^2 s] 0.200 0.267 0.228036 3.45205e-10 4.50203e-11 0.267 0.356 0.304272 1.34474e-10 1.71069e-11 0.356 0.474 0.405463 6.44281e-11 9.29784e-12 0.474 0.633 0.540518 1.81436e-11 3.79184e-12 0.633 0.844 0.721365 7.03789e-12 1.69245e-12 0.844 1.125 0.961703 2.15205e-12 9.68276e-13 1.125 1.500 1.28205 6.21078e-13 7.37604e-13 1.500 2.000 1.7094 4.21595e-13 3.5292e-13
Remarks: Statistical errors only. Systematic error on energy scale estimated to 20%. See paper for details.
H.E.S.S. data points for Figures 7a-f (Nightly Light Curve)
MJD Flux(>300 GeV) Flux Error
[/cm^2 s]
52472 9.41117e-11 1.22807e-11
52473 6.36864e-11 1.16496e-11
52474 8.59335e-11 1.24806e-11
52475 5.92879e-11 1.04508e-11
52545 -3.15587e-12 2.02338e-11
52546 5.01365e-11 1.90679e-11
52547 5.23366e-11 1.45916e-11
52548 2.98907e-11 1.85178e-11
52549 7.13138e-11 3.20232e-11
52550 5.26789e-11 1.82371e-11
52551 1.08845e-11 9.10677e-12
52553 6.70484e-11 1.43938e-11
52555 1.56956e-11 1.20891e-11
52817 2.02999e-11 2.95349e-12
52818 1.57011e-11 2.20695e-12
52819 1.4089e-11 2.00946e-12
52820 1.5821e-11 1.86269e-12
52821 1.40957e-11 1.98143e-12
52822 1.49259e-11 2.0543e-12
52823 1.61846e-11 2.2431e-12
52826 1.86388e-11 2.17188e-12
52845 7.57236e-12 1.60136e-12
52846 9.25655e-12 1.38739e-12
52847 8.97007e-12 1.21619e-12
52848 7.27217e-12 1.43153e-12
52849 1.34032e-11 1.94997e-12
52850 6.93393e-12 1.06943e-12
52851 1.22392e-11 1.23845e-12
52852 9.3142e-12 1.71323e-12
52853 6.86159e-12 1.23443e-12
52854 9.16342e-12 1.45143e-12
52855 3.67771e-12 1.88297e-12
52856 9.65834e-12 1.33727e-12
52857 7.06173e-12 3.83837e-12
52858 7.23412e-12 1.62158e-12
52859 6.38725e-12 2.49889e-12
52872 6.51623e-12 1.4353e-12
52873 1.62583e-11 1.99124e-12
52874 1.09723e-11 1.13674e-12
52875 7.98983e-12 1.09902e-12
52876 1.09677e-11 1.2702e-12
52877 6.43512e-12 9.92843e-13
52878 1.39862e-11 1.30494e-12
52879 9.7967e-12 1.25112e-12
52880 1.0602e-11 1.21129e-12
52881 4.59318e-12 1.12367e-12
52883 6.1881e-12 9.42792e-13
52884 9.96856e-12 1.39136e-12
52885 1.20898e-11 1.50465e-12
52901 1.1852e-11 1.77809e-12
52903 1.37708e-11 1.46034e-12
52904 1.39656e-11 1.23763e-12
Remarks: Statistical errors only. Systematic error on flux estimated to 20%. See paper for details.
