1ES 0229+200: An extreme blazar with a very high minimum Lorentz factor

Abstract

The blazar 1ES 0229+200 is a high-frequency-peaked BL Lac object with a hard TeV spectrum extending to 10TeV. 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 (Γ ≈ 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 peak frequency and the narrowest electron distribution among the extreme blazars known to date. © 2011 ESO.