Exciting results of observations of BL Lac after 2005 can be found on our publicity page for a paper we published in Nature in the 24 April 2008 issue
From the beginning of 1999 to March 2001 we monitored BL Lac at X-ray energies with RXTE. After an initial observation at the start of 1999, we observed once per week after mid-February of that year. Our observations that year ended in October, after which there was a 0.45-year gap before we resumed with twice-weekly monitoring. Unfortunately, we missed a major optical/radio outburst, although we did see an apparently correlated X-ray/optical flare between 1999.5 and 1999.6. The second half of our program included a period of strong X-ray and optical activity during which the X-ray spectrum tended to be steep, suggestive of synchrotron radiation (high-frequency tail).
A formal correlation analysis indicates that the long-term X-ray and optical light curves are strongly correlated. However, Bottcher et al. (2003) found that on short timescales (hours and days) the correlation is intermittent and usually weak or non-existent.
Go to BL Lac publicity page for data after March 2005