Photometric History of BL Lacertae

IN the discovery paper of BL Lac forty years ago, Hoffmeister 1 classified the object as a short-period variable with a range of m pg of 13 to 15. Subsequent unpublished observations were made by van Schewick 2 , and Semakin 3 observed a range of m pg of 13.0 to 16.1 between 1950 and 1954. Schmitt 4 identified BL Lac with the radio source VRO 42.22.01, whose radio spectrum has been investigated by MacLoed and Andrew 5 . Optical and near infrared spectrophotometry by Visvanathan 6 and by Oke et al. 7 showed a steepening decline in flux density from 10 14 to 10 15 Hz. The nonthermal nature of the radiation and the optical variability resemble the characteristics of compact galaxies and quasistellar sources. Recently Tschäpe 8 re-examined the Sonneberg plate material and found a range of m pg =12.4: to 16.1 during 1928 and early 1929. Modern photographic and photo-electric monitoring 9–12 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.