Discovery of VHE γ-ray emission from the very distant BL Lac KUV00311-1938 by HESS

The observed spectra of very-high-energy (VHE) blazars may be deformed by gamma-gamma absorption onto the Extra-galactic Background Light (EBL) or, more speculatively, by photon-axion oscillations. As the resulting imprint on observed spectral characteristics may be redshift-dependent, it is important to enrich the catalogue of VHE high-redshift blazars. HESS-II [1], with its low-energy threshold and enhanced E > 100 GeV sensitivity, will be a major instrument in this field of study. While awaiting this new phase of the experiment, H. E. S. S. observed the very distant BL Lac KUV00311 - 1938 in a multi-year campaign, leading to the discovery of its VHE gamma-ray emission with a significance of 5.1 standard deviations. The detection of this source has been possible thanks to an enhanced low-energy sensitivity of recent-developed analysis techniques for IACTs (Paris-MVA) and a full refurbishment which markedly lowered the overall analysis threshold. The evaluation of the redshift of this source is difficult, due to the weakness of the absorption lines in the optical spectrum. A first attempt to evaluate this redshift was performed by [2], and [3], where a value of 0.61 was quoted with a question mark. More recently, [4] used the data from the X-Shooter spectrograph operating on the VLT to estimate a solid lower limit on the redshift of the source of z > 0.506, confirming that KUV00311 - 1938 is the farthest BL Lac ever detected at VHE.