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Register a new userThe hard TeV spectrum of 1ES 0229+200: new clues from Swift
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Added by
Gabriele Ghisellini
Publication date
2009
fields
Physics
and research's language is
English
Authors
F. Tavecchio
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The BL Lac object 1ES 0229+200 (z=0.14) has been detected by HESS during observations taking place in 2005-2006. The TeV spectrum, when corrected for the absorption of gamma-ray photons through the interaction with the extragalactic background light, is extremely hard, even if the most conservative level for the background is considered. The case of 1ES 0229+200 is very similar to that of 1ES 1101-232, for which a possible explanation, in the framework of the standard one-zone synchrotron-self Compton model, is that the high-energy emission is synchrotron-self Compton radiation of electrons distributed as a power law with a large value of the minimum energy. In this scenario the hard TeV spectrum is accompanied by a very hard synchrotron continuum below the soft X-ray band. We will show that recent Swift observations of 1ES 0229+200 in the critical UV-X-ray band strongly support this model, showing the presence of the expected spectral break and hard continuum between the UV and the X-ray bands.
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The blazar 1ES 0229+200 is a high frequency peaked BL Lac object with a hard TeV spectrum extending to 10 TeV. Its unusual spectral characteristics make it a frequently used probe for intergalactic radiation and magnetic fields. With new, simultaneous observations in the optical, ultraviolet (UV) and X-rays, the synchrotron emission is probed in great detail. The X-ray emission varies by a factor of ~2 in 2009, while being rather stable in 2010. The X-ray spectrum is very hard (Gamma ~ 1.8) and it shows an indication of excess absorption above the Galactic value. The X-ray emission is detected up to ~100 keV without any significant cut-off, thus 1ES 0229+200 belongs to the class of extreme blazars. The simultaneous measured, host galaxy- and extinction-corrected optical and UV fluxes illustrate that the cut-off of the low energy part of the synchrotron emission is located in the UV regime. The minimum energy of the electron distribution has to be rather high to account for this cut-off. This implies that there is a narrow-band energy distribution function of radiating electrons, which is responsible for the unusually hard TeV spectrum. Other extreme blazars have similar synchrotron peak frequencies but much softer TeV spectra, hence 1ES 0229+200 has one of the highest inverse Compton (IC) peak frequency and the narrowest electron distribution among the extreme blazars known to date.
The high-frequency-peaked BL Lacertae object 1ES 0229+200 is a relatively distant (z = 0.1396), hard-spectrum (Gamma ~ 2.5), very-high-energy-emitting (E > 100 GeV) gamma-ray blazar. Very-high-energy measurements of this active galactic nucleus have been used to place constraints on the intensity of the extragalactic background light and the intergalactic magnetic field. A multi-wavelength study of this object centered around very-high-energy observations by VERITAS is presented. This study obtained, over a period of three years, an 11.7 standard deviation detection and an average integral flux F(E>300 GeV) = (23.3 +- 2.8_stat +- 5.8_sys) x 10^-9 photons m^-2 s^-1, or 1.7% of the Crab Nebulas flux (assuming the Crab Nebula spectrum measured by H.E.S.S). Supporting observations from Swift and RXTE are analyzed. The Swift observations are combined with previously published Fermi observations and the very-high-energy measurements to produce an overall spectral energy distribution which is then modeled assuming one-zone synchrotron-self-Compton emission. The chi^2 probability of the TeV flux being constant is 1.6%. This, when considered in combination with measured variability in the X-ray band, and the demonstrated variability of many TeV blazars, suggests that the use of blazars such as 1ES 0229+200 for intergalactic magnetic field studies may not be straightforward and challenges models that attribute hard TeV spectra to secondary gamma-ray production along the line of sight.
We have observed 1ES 1426+428 with INTEGRAL detecting it up to $sim$150 keV. The spectrum is hard, confirming that this source is an extreme BL Lac object, with a synchrotron component peaking, in a $ u F_ u$ plot, at or above 100 keV, resembling the hard states of Mkn 501 and 1ES 2344+514. All these three sources are TeV emitters, with 1ES 1426+428 lying at a larger redshift (z=0.129): for this source the absorption of high energy photons by the IR cosmic background is particularly relevant. The observed hard synchrotron tail helps the modeling of its spectral energy distribution, giving information on the expected intrinsic shape and flux in the TeV band. This in turn constrains the amount of the poorly known IR background.
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-high energy (VHE, E>100 GeV) gamma-rays due to its high X-ray and radio flux. Observations in the VHE gamma-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 gamma-ray spectrum might provide further limits on the level of the Extragalactic Background Light (EBL). 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. 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 7.8 sigma. The photon spectrum of the source is well described by a power law, with photon index of 3.45 pm 0.25stat pm 0.20syst. The integral flux above 200 GeV is (1.88 pm 0.20stat pm 0.38syst) times10-12 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 pm 0.2stat) times 10-9 erg cm-2 s-1, and a spectrum well described by a power-law function with a photon index 1.85 pm 0.18. Swift/XRT observations show an X-ray flux between 2 and 10 keV of (0.8 - 1) times 10-11 erg cm-2 s-1, and a steep spectrum (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. ...
Atmospheric neutrinos are produced during cascades initiated by the interaction of primary cosmic rays with air nuclei. In this paper, a measurement of the atmospheric u_mu + bar{ u}_mu energy spectrum in the energy range 0.1 - 200 TeV is presented, using data collected by the ANTARES underwater neutrino telescope from 2008 to 2011. Overall, the measured flux is ~25% higher than predicted by the conventional neutrino flux, and compatible with the measurements reported in ice. The flux is compatible with a single power-law dependence with spectral index gamma_{meas}=3.58pm 0.12. With the present statistics the contribution of prompt neutrinos cannot be established.
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