Discovery of very-high-energy emission from RGB J2243+203 and derivation of its redshift upper limit

Abstract

Very-high-energy (VHE; $>$ 100 GeV) gamma-ray emission from the blazar RGB J2243+203 was discovered with the VERITAS Cherenkov telescope array, during the period between 21 and 24 December 2014. The VERITAS energy spectrum from this source can be fit by a power law with a photon index of $4.6\pm 0.5$, and a flux normalization at 0.15 TeV of $(6.3\pm 1.1)\times 10^{-10}{\text{cm}}^{-2}{\text{s}}^{-1}{\text{TeV}}^{-1}$. The integrated \textit{Fermi}-LAT flux from 1 GeV to 100 GeV during the VERITAS detection is $(4.1\pm 0.8)\times 10^{\text{-8}}{\text{cm}}^{\text{-2}}{\text{s}}^{\text{-1}}$, which is an order of magnitude larger than the four-year-averaged flux in the same energy range reported in the 3FGL catalog, ($4.0\pm 0.1\times 10^{\text{-9}}{\text{cm}}^{\text{-2}}{\text{s}}^{\text{-1}}$). The detection with VERITAS triggered observations in the X-ray band with the \textit{Swift}-XRT. However, due to scheduling constraints \textit{Swift}-XRT observations were performed 67 hours after the VERITAS detection, not simultaneous with the VERITAS observations. The observed X-ray energy spectrum between 2 keV and 10 keV can be fitted with a power-law with a spectral index of $2.7\pm 0.2$, and the integrated photon flux in the same energy band is $(3.6\pm 0.6)\times 10^{-13}{\text{cm}}^{-2}{\text{s}}^{-1}$. EBL model-dependent upper limits of the blazar redshift have been derived. Depending on the EBL model used, the upper limit varies in the range from z $<0.9$ to z $<1.1$.