A synchrotron self-Compton scenario for the very high energyγ-ray emission of the intermediate BL Lacertae object W Comae

W Comae has significant variability in multi-wavelengthes, from radio to gamma-ray bands. A bright outburst in optical and X-ray bands was observed in 1998, and most recently, a strong TeV flare was detected by VERITAS in 2008. It is the first TeV intermediate-frequency-peaked BL Lacertae source. I find that both the broadband spectral energy distributions (SEDs) which were quasi-simultaneously obtained during the TeV flare and during the optical/X-ray outburst are well fit by using a single-zone synchrotron + synchrotron-self-Compton model. The satisfactory fitting requires a large beaming factor, i.e., δ～25 and δ～20 for the TeV flare and the optical/X-ray outburst, respectively, suggesting that both the optical/X-ray outburst and the TeV flare are from a relativistic jet. The size of the emission region of the TeV flare is three times larger than that of the optical/X-ray outburst, and the strength of the magnetic field for the TeV flare is～14 times smaller than that of the X-ray/optical outburst, likely indicating that the region of the TeV flare is more distant from the core than that of the X-ray/optical outburst. The inverse Compton component of the TeV flare peaks around 1.3 GeV, but it is around 0 MeV for the X-ray/optical outburst, lower than that for the TeV flare by two orders of magnitude. The model predicts that the optical/X-ray outburst might be accompanied by a strong MeV/GeV emission, but the TeV flare may be not associated with the X-ray/optical outburst. The GeV emission is critical for characterizing the SEDs of the optical/X-ray outburst and the TeV flare. The predicted GeV flux is above the sensitivity of Fermi/LAT, and it could be verified with the observations by Fermi/LAT in the near future.     

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

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 observations made by Webb et al. showed the associated optical flare has a hard spectrum (the average spectral index αopt～0.48, Fν∝ν^-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) (αx～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 νFν) 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 (γe-10^4) in the γ-ray emitting region.     

Injected spectrum for TeV γ-ray emission from the galactic center

The detection of very high energy γ-ray emission from the Galactic center has been reported by four independent groups. One of these γ-ray sources, the 10 TeV -γ-ray radiation reported by HESS, has been suggested as having a hadronic origin when relativistic protons are injected into and interact with the dense ambient gas. Assuming that such relativistic protons required by the hadronic model come from the tidal disruption of a star by the massive black hole of Sgr A＊, we explore the spectrum of the relativis- tic protons. In the calculations, we investigate cases where different types of stars are tidally disrupted by the black hole of Sgr A＊, and we consider that different diffusion mechanisms are used for the propagation of protons. The initial energy distribution of the injected spectrum of protons is assumed to follow a power-law with an exponential cut-off, and we derive the different indices of the injected spectra for the tidal disruption of different types of stars. For the best fit to the spectrum of photons detected by HESS, the spectral index of the injected relativistic protons is about 2.05 when a red giant is tidally disrupted by the black hole of Sgr A＊ and the diffusion mechanism is the Effective Confinement of Protons.     

MACHETE: A transit imaging atmospheric Cherenkov telescope to survey half of the very high energy γ-ray sky

Current imaging atmospheric Cherenkov telescopes for very high energy γ-ray astrophysics are pointing instruments with a field of view up to a few tens of sq deg. We propose to build an array of two non-steerable (drift) telescopes. Each of the telescopes would have a camera with a FOV of 5 × 60 sq deg oriented along the meridian. About half of the sky drifts through this FOV in a year. We have performed a Monte Carlo simulation to estimate the performance of this instrument. We expect it to survey this half of the sky with an integral flux sensitivity of ∼0.77% of the steady flux of the Crab Nebula in 5 years, an analysis energy threshold of ∼150 GeV and an angular resolution of ∼0.1°. For astronomical objects that transit over the telescope for a specific night, we can achieve an integral sensitivity of 12% of the Crab Nebula flux in a night, making it a very powerful tool to trigger further observations of variable sources using steerable IACTs or instruments at other wavelengths.