The detection of four blazars at TeV energies (Mrk421, Mrk501, 1ES 2344+514, PKS 2155-304) proves that high energy gamma-rays are indeed emitted from blazars. The favourite emission mechanisms suggested so far to explain the Spectral Energy Distribution (SED) of blazars, characterized by two broad peaks at IR-optical-UV and MeV-GeV energies respectively, are synchrotron radiation from the electrons of the relativistic jet for the low frequency peak and inverse Compton for the high frequency peak. Analysis of the SED of gamma-ray blazars observed so far has shown that BL Lac objects present higher energy peaks with respect to FSRQ. In particular, following the classification made in Giommi & Padovani (1994) of radio selected BL Lac (RBL) and X-ray selected BL Lac (XBL), these latter seem to be the blazars with the SED shifted to the highest energies: in effect the four objects observed at TeV energies are all classified as XBL.
In searching for the best candidate high energy gamma-ray blazars for ARGO-YBJ observations, we will then look mainly at the XBL type.
 

Interaction with low energy photons

The other limitation to gamma-ray observations of blazars in the Very High Energy (VHE) range is the possible interaction of these photons with low energy photons of intergalactic origin. The cross section of this interaction is maximized  in a way that TeV photons interact mainly with IR photons, while gamma-rays of some tens of GeV energies interact mainly with UV photons. Therefore, the intergalactic radiation fields which interact with photons detectable by ARGO-YBJ are the Intergalactic Infrared Radiation Field (IIRF), produced by dust reradiation of star formation light of all galaxies during their evolution, and the field made of extragalactic starlight photons from the IR through the UV range, which we will call ``IR-to-UV background''.

The effect of the photon-photon interaction modifies the observed VHE spectra of blazars, which intrinsically follow a power law. The photon-photon absorption produces an exponential cutoff  exp(-tau(E)), where tau(E) is the optical depth.

The expression of the optical depth depends on the photon density of the interacting low energy photon field. Unfortunately, the knowledge of both the IIRF and the IR-to-UV background is difficult to gain through direct measurements, only empirical models based on indirect observations having been recently proposed. The two low energy photon fields which interact with gamma-rays in the ARGO-YBJ energy range are of different origin, and therefore are treated in different works.

Adopting the IIRF modeled in Malkan & Stecker (1998) and the IR-to-UV background modeled in Salamon & Stecker (1998) we can evaluate the flux absorption amount at different energies and redshifts, and study the effect on observed blazar spectra.

In Figure 1 we report the percentage of VHE photons reaching the observer after absorption by the IIRF and the IR-to-UV background at different redshift values (top), and effect of this absorption on the simple power law fit to Whipple observational data for Mrk 501 (z = 0.034, Samuelson et al. 1998) (bottom).
The dashed curve represents the ARGO-YBJ 5 sigma sensitivity.
 
 

  ARGO-YBJ observation times

Blazars are characterized by a rapid flux variability, down to the hour time scale. Therefore to study their variability a detector with sufficiently low exposure times is necessary. The ARGO-YBJ 5 sigma sensitivity reported in the previous Figure is valid for a one year observation time.
This would seem to be too long for the detection of flaring objects or variability studies.

However it has to be remarked that the sensitivity scales as (t_obs )^0.5.

In order to evaluate the ARGO-YBJ capabilities for variability studies, we considered the 1997 outburst of Mrk 501. Taking the daily data collected by the stereoscopic HEGRA air Cerenkov Telescope during the period March to October 1997 (Aharonian et al. 1999), we calculated the expected observation times for a 5 sigma detection by ARGO-YBJ.

These are shown in the Figure 2,   where we report the daily integral fluxes for the 1997 Mkn 501 outburst, normalized to Crab flux (top), and relative expected observation times for achieving a 5 sigma detection with ARGO-YBJ (bottom). The dashed line refers to the Crab observation time.

As can be seen, in the favourable cases of sources flaring up to a few Crab fluxes, a few hours of exposure are sufficient for a 5 sigma detection.

For a 3 sigma signal, the observation time is a factor (5/3)² less.
 
 

Blazar candidates for ARGO-YBJ

Considering all the above results, in order to select a list of candidate blazars for ARGO-YBJ observations, we first took as a standard candle the Crab Nebula spectrum determined with the Whipple Observatory (Hillas et al. 1998):

F(E) = 3.2 10^-11 (E/TeV)^-2.49           photons cm^-2 s^-1 TeV^-1
 

Varying then the normalization factor and the photon spectral index, we moved this candle at various redshifts, finding the upper z value at which it should be possible to detect gamma-ray blazars with ARGO-YBJ: this limit should be ~0.2.

Finally, with this other limitation and taking into account the ARGO-YBJ site latitude, we have selected the blazar candidates for ARGO-YBJ observations,  i.e.,  mainly XBL with z < 0.2 and -10^o < delta  < 70^o . Their list is presented in the Table 1.