Multi-wavelength observations of H 2356–309

Abstract

Aims. The properties of the broad-band emission from the high-frequency peaked BL Lac H 2356-309 (z = 0.165) are investigated. Methods. Very high energy (VHE; E > 100 GeV) observations of H 2356-309 were performed with the High Energy Stereoscopic System ( Results. A strong VHE signal, ∼13σ total, was detected by HESS after the four years HESS observations (116.8 h live time). The integral flux above 240 GeV is I(>240 GeV) = (3.06 ± 0.26 stat ± 0.61 syst ) × 10 −12 cm −2 s −1 , corresponding to ∼1.6% of the flux observed from the Crab Nebula. A time-averaged energy spectrum is measured from 200 GeV to 2 TeV and is characterized by a power law (photon index of Γ = 3.06 ± 0.15 stat ± 0.10 syst ). Significant small-amplitude variations in the VHE flux from H 2356-309 are seen on time scales of months and years, but not on shorter time scales. No evidence for any variations in the VHE spectral slope are found within these data. The XMM-Newton X-ray measurements show a historically low X-ray state, characterized by a hard, broken-power-law spectrum on both nights. Conclusions. The broad-band spectral energy distribution (SED) of the blazar can be adequately fit using a simple one-zone synchrotron selfCompton (SSC) model. In the SSC scenario, higher VHE fluxes could be expected in the future since the observed X-ray flux is at a historically low level.

The nature of the intrinsic spectra from the VHE emission of H 2356-309 and 1ES 1101-232

“…The aging of the HESS detector and the accumulation of dust on the optical elements of the telescopes affect the optical efficiency of the detector system and it can reduce the efficiency by about 26% for the entire data sample. So the HESS collaboration reanalyzed the previously published result of 2004 [15] and added results of new observations from 2005 to 2007 in another article [41]. As a consequence of the above correction the individual event energy is renormalized and correspondingly the flux changed.…”

“…Again, this new A γ corresponds to a 10% increase in the flux compared to the original fit. A power-law fit (red dotted curve) with I = I 0 E −Γ γ,T eV , where Γ = 2.97 and I 0 = 4.69 × 10 −13 erg cm −2 s −1[41], is shown for comparison. Although the EBL-F and EBL-I fits to the observe data are similar, for E γ < 200 GeV and E γ > 2 TeV we can see a difference in their behavior.…”

“…The following parameters are taken from the one-zone leptonic model of ref [41]. to fit the SED of H 2356-309 and from ref [42].…”