IN the discovery paper of BL Lac forty years ago, Hoffmeister1 classified the object as a short-period variable with a range of mpg of 13 to 15. Subsequent unpublished observations were made by van Schewick2, and Semakin3 observed a range of mpg of 13.0 to 16.1 between 1950 and 1954. Schmitt4 identified BL Lac with the radio source VRO 42.22.01, whose radio spectrum has been investigated by MacLoed and Andrew5. Optical and near infrared spectrophotometry by Visvanathan6 and by Oke et al.7 showed a steepening decline in flux density from 1014 to 1015 Hz. The nonthermal nature of the radiation and the optical variability resemble the characteristics of compact galaxies and quasistellar sources. Recently Tschpe8 re-examined the Sonneberg plate material and found a range of mpg=12.4: to 16.1 during 1928 and early 1929. Modern photographic and photo-electric monitoring912 showed that the variability of BL Lac is extremely irregular over periods of a few hundred days of continual observation. In view of the interest in the past variability of BL Lac expressed at the recent meeting of the International Astronomical Union, we present here photometric data derived from the archive plate collection of Harvard College Observatory. Our conclusions, based on the available BL Lac light history, are (1) the brightness of the object is fainter than 14 magnitudes most of the time; (2) the object has reached or exceeded magnitude 13 during two periods in its history; (3) during the second outburst, the brightness increased secularly throughout practically its entire range, and subsequently decreased again, the whole episode lasting about 10 years; and (4) there is a similarity in the optical behaviour of BL Lac and the N-galaxy 3C371.
- 1 - general ; 2 - general science & technology ; 3 - general science & technology
- 1 - sciences appliquees, technologies et medecines ; 2 - sciences biologiques et medicales ; 3 - sciences medicales