The spectral energy distribution (SED) of the γ-ray flare observed inJuly 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...The spectral energy distribution (SED) of the γ-ray flare observed inJuly 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 observationsmade 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 theoptical and soft X-ray bands are closely consistent with a ‘canonical' synchrotronspectrum emitted by relativistic electrons of a power-law energy distribution witha high energy cutoff, and thus the peak of the SED of the synchrotron radiation (inrepresentation of vF_v) is located in the EUV -- soft X-ray bands. Therefore, theGeV γ-ray emission observed in the July 1997 outburst may be mainly due to thesynchrotron self-Compton (SSC) process, contrasting with the current explanationsin terms of external radiation Compton (ERC) process, in which the seed photonsare mostly taken to be the UV emission from the clouds of the broad emission lineregion. We argue that the hard optical spectra observed during the γ-ray outburstmay be an important signature for the acceleration of high energy electrons (γ_e～10~4)in the γ-ray emitting region.展开更多
文摘The spectral energy distribution (SED) of the γ-ray flare observed inJuly 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 observationsmade 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 theoptical and soft X-ray bands are closely consistent with a ‘canonical' synchrotronspectrum emitted by relativistic electrons of a power-law energy distribution witha high energy cutoff, and thus the peak of the SED of the synchrotron radiation (inrepresentation of vF_v) is located in the EUV -- soft X-ray bands. Therefore, theGeV γ-ray emission observed in the July 1997 outburst may be mainly due to thesynchrotron self-Compton (SSC) process, contrasting with the current explanationsin terms of external radiation Compton (ERC) process, in which the seed photonsare mostly taken to be the UV emission from the clouds of the broad emission lineregion. We argue that the hard optical spectra observed during the γ-ray outburstmay be an important signature for the acceleration of high energy electrons (γ_e～10~4)in the γ-ray emitting region.
