VHE γ-ray discovery and multiwavelength study of the blazar 1ES 2322-409

Abstract

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>100GeV⁠) 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 ± 0.66stat ± 0.20sys and an integral flux Φ(E>200GeV)=(3.11±0.71stat±0.62sys)×10⁻¹²cm⁻²s⁻¹⁠, which corresponds to 1.1 per cent of the Crab nebula flux above 200GeV⁠. 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.