VHE $\gamma$-ray discovery and multiwavelength study of the blazar 1ES 2322−409

A hotspot at a position compatible with the BL Lac object 1ES 2322−409 was serendipitously detected with H.E.S.S. during observations performed in 2004 and 2006 on the blazar PKS 2316−423. Additional data on 1ES 2322−409 were taken in 2011 and 2012, leading to a total live-time of 22.3 h. Point-like very-high-energy (VHE; |$E\gt 100\, \mathrm{GeV}$|⁠) γ-ray emission is detected from a source centred on the 1ES 2322−409 position, with an excess of 116.7 events at a significance of 6.0σ. The average VHE γ-ray spectrum is well described with a power law with a photon index Γ = 3.40 ± 0.66_stat ± 0.20_sys and an integral flux |$\Phi (E\gt 200\, \mathrm{GeV}) = (3.11\pm 0.71_{\rm stat}\pm 0.62_{\rm sys})\times 10^{-12} \, \mathrm{cm}^{-2} \, \mathrm{s}^{-1}$|⁠, which corresponds to 1.1 |${{\ \rm per\ cent}}$| of the Crab nebula flux above |$200\, \mathrm{GeV}$|⁠. Multiwavelength data obtained with Fermi LAT, Swift XRT and UVOT, RXTE PCA, ATOM, and additional data from WISE, GROND, and Catalina are also used to characterize the broad-band non-thermal emission of 1ES 2322−409. The multiwavelength behaviour indicates day-scale variability. Swift UVOT and XRT data show strong variability at longer scales. A spectral energy distribution (SED) is built from contemporaneous observations obtained around a high state identified in Swift data. A modelling of the SED is performed with a stationary homogeneous one-zone synchrotron-self-Compton leptonic model. The redshift of the source being unknown, two plausible values were tested for the modelling. A systematic scan of the model parameters space is performed, resulting in a well-constrained combination of values providing a good description of the broad-band behaviour of 1ES 2322−409.