Extensive monitoring of the HBLs PKS 0548-322 and PKS 2005-489 over the lifetime of the University of Durham Mark 6 telescope has failed to provide any evidence for VHE gamma ray emission. However, the predicted VHE fluxes using the models of Stecker et al. (1996) and Fossati et al. (1998) are not in conflict with the present flux limits. These objects should be readily detectable in the VHE band when the next generation of Southern hemisphere ACTs become operational.
The broad-band spectral energy distributions of these two objects are not as well sampled as those of Mrk 501 and Mrk 421. In spite of this, we observe that the broad-band spectral energy distributions for PKS 0548-322 and PKS 2005-489 compiled here bear a resemblance to those compiled for Mrk 501 and Mrk 421, respectively (e.g. Kubo et al. 1998). PKS 0548-322 shows a broader, higher energy synchrotron peak than PKS 2005-489, which is similar to the comparison between Mrk 501 and Mrk 421.
Mrk 501 and Mrk 421 are both at the same distance () but show different VHE spectral shapes (Krennrich et al. 1999). Thus, the different VHE spectral shapes must be due to variation in the intrinsic source spectrum and so the effects of gamma ray absorption on the IR background cannot be determined. Studies of the VHE spectra from PKS 0548-322 and PKS 2005-489 (which are both at ) with the new generation of Southern hemisphere ACT arrays (HESS and CANGAROO III) hold out the prospect of separating the effects of source spectrum variation and IR absorption. We suggest that these objects should be a high-priority targets when these new VHE facilties become operational in 2001 - 2002.
© European Southern Observatory (ESO) 2000
Online publication: January 29, 2001