Discovery of hard-spectrumγ-ray emission from the BL Lacertae object 1ES 0414+009

Abstract

Context. 1ES 0414+009 (z = 0.287) is a distant high-frequency-peaked BL Lac object, and has long been considered a likely emitter of very-highenergy (VHE, E > 100 GeV) γ-rays due to its high X-ray and radio flux. Aims. Observations in the VHE γ-ray band and across the electromagnetic spectrum can provide insights into the origin of highly energetic particles present in the source and the radiation processes at work. Because of the distance of the source, the γ-ray spectrum might provide further limits on the level of the extragalactic background light (EBL). Methods. We report observations made between October 2005 and December 2009 with H.E.S.S., an array of four imaging atmospheric Cherenkov telescopes. Observations at high energies (HE, 100 MeV-100 GeV) with the Fermi-LAT instrument in the first 20 months of its operation are also reported. To complete the multi-wavelength picture, archival UV and X-ray observations with the Swift satellite and optical observations with the ATOM telescope are also used. Results. Based on the observations with H.E.S.S., 1ES 0414+009 is detected for the first time in the VHE band. An excess of 224 events is measured, corresponding to a significance of 7.8σ. The photon spectrum of the source is well described by a power law, with photon index of Γ VHE = 3.45 ± 0.25 stat ± 0.20 syst . The integral flux above 200 GeV is (1.88 ± 0.20 stat ± 0.38 syst ) ×10 −12 cm −2 s −1 . Observations with the Fermi-LAT in the first 20 months of operation show a flux between 200 MeV and 100 GeV of (2.3 ± 0.2 stat ) × 10 −9 erg cm −2 s −1 , and a spectrum well described by a power-law function with a photon index Γ HE = 1.85 ± 0.18. Swift/XRT observations show an X-ray flux between 2 and 10 keV of (0.8−1) × 10 −11 erg cm −2 s −1 , and a steep spectrum Γ X = (2.2−2.3). Combining X-ray with optical-UV data, a fit with a log-parabolic function locates the synchrotron peak around 0.1 keV. Conclusions. Although the GeV-TeV observations do not provide better constraints on the EBL than previously obtained, they confirm a low density of the EBL, close to the lower limits from galaxy counts. The absorption-corrected HE and VHE γ-ray spectra are both hard and have similar spectral indices (≈1.86), indicating no significant change of slope between the HE and VHE γ-ray bands, and locating the γ-ray peak in the SED above 1-2 TeV. As for other TeV BL Lac objects with the γ-ray peak at such high energies and a large separation between the two SED humps, this average broad-band SED represents a challenge for simple one-zone synchrotron self-Compton models, requiring a high Doppler factor and very low B-field.

The NuSTAR view on hard-TeV BL Lacs

“…However, the LAT spectrum extracted at the beginning of the Fermi mission and partly overlapping with the H.E.S.S. observations (years 2008-2010, see Abramowski et al 2012) is characterized by a slightly harder photon index and higher flux, more in line with the intrinsic VHE spectrum (Abramowski et al 2012). We confirm this using Pass 8 data (ΓLAT = 1.77 ± 0.14).…”

“…The spectral fits and residuals of the best fit models are shown in Figure 1. -4]: energy (in keV) and νFν flux (in erg cm −2 s −1 ) of the synchrotron peak in the SED, as resulting from the log-parabolic X-ray fit ( Aharonian et al (2006Aharonian et al ( , 2007aAharonian et al ( , 2007bAharonian et al ( , 2007c, Abramowski et al 2012and Acciari et al (2009. Col. [6][7]: approximate lower limits for the energy (in TeV) and νFν flux (in erg cm −2 s −1 ) of the Compton peak in the SED, assuming a power-law VHE spectrum.…”

Hardening of TeV gamma spectrum of active galactic nuclei in galaxy clusters by conversions of photons into axionlike particles

“…5 (color online). The spectra of 1ES 0414 þ 009 [43] (upper plot) and Mkn 501 [42] (lower plot). We are using the FRV EBL model [78,79].…”

“…As a specific example we discuss in Sec. V the effect on the energy spectra of two AGNs located in galaxy clusters, namely Mkn 501 at z ¼ 0:034 [42] and 1ES 0414 þ 009 at redshift z ¼ 0:287 [43]. Finally, in Sec.…”

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

“…[15], it was inferred that the Universe is more transparent to gamma rays than previously anticipated. Later on, harder spectra have also been observed from many HBLs [22][23][24]. Thereafter, many scenarios are suggested to achieve very hard VHE spectra which are discussed in ref.…”