BL Lacertae: Hard Optical Spectrum and GeV γ-ray Emission

作     者：Shan-JieQian Xi-ZhenZhang T.P.Krichbaum 

作者机构：NationalAstronomicalObservatoriesChineseAcademyofSciencesBeijing100012 Max-Planck-InstitutfürRadioastronomieAufdemHügel6953121BonnGermany 

出 版 物：《Chinese Journal of Astronomy and Astrophysics》 (中国天文和天体物理学报（英文版）)

年 卷 期：2004年第4卷第3期

页      面：231-246页

核心收录：

学科分类：07[理学] 070401[理学-天体物理] 0704[理学-天文学] 

主　　题：galaxies:compact gamma rays:observations quasars:individual:BL Lacertae 

摘      要：The spectral energy distribution (SED) of the γ-ray flare observed in July 1997 in BL Lacertae is re-considered. It is pointed out that the optical observa- tions made by Webb et al. showed the associated optical flare has a hard spectrum (the average spectral index αopt～ 0.48, F_v∝v^(-α)), and the ASCA observations made by Tanihata et al. showed very steep spectra in the soft X-ray band (0.7- 1.5 keV) (α_x～3-4). We find that the flux densities and spectral indices in both the optical and soft X-ray bands are closely consistent with a ‘canonical synchrotron spectrum emitted by relativistic electrons of a power-law energy distribution with a high energy cutoff, and thus the peak of the SED of the synchrotron radiation (in representation of vF_v) is located in the EUV -- soft X-ray bands. Therefore, the GeV γ-ray emission observed in the July 1997 outburst may be mainly due to the synchrotron self-Compton (SSC) process, contrasting with the current explanations in terms of external radiation Compton (ERC) process, in which the seed photons are mostly taken to be the UV emission from the clouds of the broad emission line region. We argue that the hard optical spectra observed during the γ-ray outburst may be an important signature for the acceleration of high energy electrons (γ_e～10~4) in the γ-ray emitting region.