The quasar PKS 1510 $ - $ 089 ( z = 0.361 ) was observed with the H.E.S.S .
array of imaging atmospheric Cherenkov telescopes during high states in the optical and GeV bands , to search for very high energy ( VHE , defined as E \geq 0.1 TeV ) emission .
VHE \gamma -rays were detected with a statistical significance of 9.2 standard deviations in 15.8 hours of H.E.S.S .
data taken during March and April 2009 .
A VHE integral flux of I ( 0.15 TeV < E < 1.0 TeV ) = ( 1.0 \pm 0.2 _ { \mathrm { stat } } \pm 0.2 _ { \mathrm { sys } } ) \times 10 ^ { -11 } cm ^ { -2 } s ^ { -1 } is measured .
The best-fit power law to the VHE data has a photon index of \Gamma = 5.4 \pm 0.7 _ { \mathrm { stat } } \pm 0.3 _ { \mathrm { sys } } .
The GeV and optical light curves show pronounced variability during the period of H.E.S.S .
observations .
However , there is insufficient evidence to claim statistically significant variability in the VHE data .
Because of its relatively high redshift , the VHE flux from PKS 1510 - 089 should suffer considerable attenuation in the intergalactic space due to the extragalactic background light ( EBL ) .
Hence , the measured \gamma -ray spectrum is used to derive upper limits on the opacity due to EBL , which are found to be comparable with the previously derived limits from relatively-nearby BL Lac objects .
Unlike typical VHE-detected blazars where the broadband spectrum is dominated by non-thermal radiation at all wavelengths , the quasar PKS 1510 - 089 has a bright thermal component in the optical to UV frequency band .
Among all VHE detected blazars , PKS 1510 - 089 has the most luminous broad line region ( BLR ) .
The detection of VHE emission from this quasar indicates a low level of \gamma - \gamma absorption on the internal optical to UV photon field .