Very high energy $\gamma$-ray emission from two blazars of unknown redshift and upper limits on their distance

Abstract

We report on the detection of very-high-energy (VHE; $E>100$ GeV) $\gamma$-ray emission from the BL Lac objects KUV 00311-1938 and PKS 1440-389 with the High Energy Stereoscopic System (H.E.S.S.). H.E.S.S. observations were accompanied or preceded by multi-wavelength observations with Fermi/LAT, XRT and UVOT on board the Swift satellite, and ATOM. Based on an extrapolation of the Fermi/LAT spectrum towards the VHE $\gamma$-ray regime, we deduce a 95% confidence level upper limit on the unknown redshift of KUV 00311-1938 of z < 0.98, and of PKS 1440-389 of z < 0.53. When combined with previous spectroscopy results the redshift of KUV 00311-1938 is constrained to $0.51\le z<0.98$ and for PKS 1440-389 to $0.14⪅z<0.53$.

Accepted for publication in MNRAS. 13 pages, 4 figures

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