BL Lac objects are in many ways the most extreme class of active galactic nuclei (AGNs), that have no or only weak emission lines in their spectra and exhibit strong and rapid variability and polarization from radio through X-ray regime. Based on their discovering techniques, they are divided into two subclasses, the radio-selected BL Lac objects discovered by optical spectroscopic identification of radio source, and the X-ray-selected BL Lac objects found by optical spectroscopic identification of candidates from X-ray surveys. Long term monitoring in the optical band shows that XBLs are less variable and polarized than RBLs, the variability amplitude of long term light curve of XBLs is much smaller than that of RBLs and the percent polarization is lower than that for RBLs (Jannuzi et al. 1994). It is widely believed that BL Lac objects are identical to low-luminosity radio galaxies (FR I, Fanaroff & Riley 1974) with relativistic jets oriented close to the line of sight. XBLs have the jets orienting at much larger angles to the line of sight than RBLs do (e.g. Stocke et al. 1985; Maraschi et al. 1986; Ghisellini & Maraschi 1989).
Intraday variability, as defined by Wagner & Witzel (1995), is used to describe the microvariations on time scales of one day or less. There have been many reports of optical microvariations in BL Lac objects. Wagner and Witzel have given a review for microvariability (Wagner & Witzel 1995, and reference therein). Heidt & Wagner (1996) find optical intraday variability in 80% of the sources in the Stickel's complete sample of radio-selected BL Lac objects (Stickel et al. 1991). If the difference between the XBLs and RBLs is truly caused by the orientation of relativistic jet, the intraday variability in RBLs and XBLs will show some differences. Miller & Noble (1997) have investigated much smaller microvariability in RBLs and XBLs, and found that RBLs systematically exhibit larger amplitude, more rapid microvariability than XBLs. This is consistent with the RBLs having the jets oriented closer to the line of sight than XBLs.
In this paper, we present the recent observational results from CCD photometric monitoring of seven BL Lac objects. As a preliminary discussion of these data, we have compared the difference in intraday variability between radio-selected and X-ray-selected BL Lac objects.
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