Discovery of VHE γ-rays from the distant BL Lacertae 1ES 0347-121

Abstract

Aims. Our aim is to study the production mechanism for very-high-energy (VHE; >100 GeV) γ-rays in distant active galactic nuclei (AGN) and use the observed VHE spectrum to derive limits on the Extragalactic Background Light (EBL). We also want to determine physical quantities through the modeling of the object's broad-band spectral energy distribution (SED). Methods. VHE observations (∼25 h live time) of the BL Lac 1ES 0347-121 (redshift z = 0.188) were conducted with the High Energy Stereoscopic System (HESS) between August and December 2006. Contemporaneous X-ray and UV/optical observations from the SWIFT satellite are used to interpret the SED of the source in terms of a synchrotron self Compton (SSC) model.Results. An excess of 327 events, corresponding to a statistical significance of 10.1 standard deviations, is detected from 1ES 0347-121. Its photon spectrum, ranging from ∼250 GeV to ∼3 TeV, is well described by a power law with a photon index of Γ = 3.10 ± 0.23 stat ± 0.10 sys . The integral flux above 250 GeV corresponds to ∼2% of the flux of the Crab Nebula above the same threshold. No VHE flux variability is detected within the data set. Conclusions. Constraints on the EBL density at optical to near-infrared wavelengths derived from the photon spectrum of 1ES 0347-121 are close to the strongest limits derived previously. The strong EBL limits confirm earlier findings, that the EBL density in the near-infrared is close to the lower limits from source counts. This implies that the universe is more transparent to VHE γ-rays than previously believed. An SSC model provides a reasonable description of the contemporaneous SED.

The NuSTAR view on hard-TeV BL Lacs

“…This locates their Compton peak in the SED assuredly above 2-10 TeV, the highest Compton-peak energies ever seen in blazars (e.g. Aharonian et al 2006Aharonian et al , 2007bAcciari et al 2009Acciari et al , 2010b. This is different from all the other VHE-detected HBL, and in particular those bright in Fermi, which are characterized by steep intrinsic VHE spectra (ΓVHE > 2) and Compton peak energies around ∼ 100 − 200 GeV.…”

Extremely High energy peaked BL Lac nature of the TeV blazar Mrk 501

“…The H.E.S.S. telescopes first discovered a VHE γ-ray emission from this source in 2006 above an energy threshold of 250 GeV [71]. The observed differential photon spectrum was described by a power law with a spectral index of 3.10±0.23 and no evidence of short term variability on monthly timescales was observed.…”

The Diffuse Near-Infrared Background Spectrum From Akari

“…Such IHL model was suggested by Cooray et al (2012b) to explain the observed fluctuation of the EBL. The spatial fluctuations of the EBL were observed by CIBER (Zemcov et al, 2014), AKARI (Matsumoto et al, 2011), and Spitzer (Kashlinsky et al, 2005, 2007, 2012) from 1.1 to 4.5 µm in order to avoid uncertainty of the ZL model, since the ZL is known to be spatially smooth (Pyo et al, 2012). The observed fluctuations are consistent with each other and show significant large fluctuations at angular scales larger than 100 arcsec, which cannot be explained by known foreground emission.…”

“…Results from TeV-γ blazars are problems for the extragalactic origin of the EBL, since a high NIR EBL level makes intergalactic space opaque to TeV-γ photons (Dwek et al, 2005b;Aharonian et al, 2006Aharonian et al, , 2007Mazin & Raue, 2007;Raue et al, 2009). However, a recent discovery of a high redshift (z> 0.6) blazar (Furniss at al., 2013) contradicts the standard scenario above, and it requires a new physical process, such as the secondary TeV γ-ray model (Essey & Kusenko, 2010) or the Axion-like particles model (Sánchez-Conde et al, 2007).…”