1976AJ 81. . 2 98M the ASTRONOMICAL JOURNAL VOLUME 81, NUMBER 5 MAY 1976 on the Intraday Variability of OJ 287 H. R. Miller, D
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98M 2 . 81. THE ASTRONOMICAL JOURNAL VOLUME 81, NUMBER 5 MAY 1976 On the intraday variability of OJ 287 1976AJ H. R. Miller, D. W. Wingert, B. Q. McGimsey, E. S. Anderson, and T. L. Mullikin Department of Physics, Georgia State University, Atlanta, Georgia 30303 G. H. Folsom Department of Physics, Agnes Scott College, Decatur, Georgia 30332 (Received 19 December 1975; revised 3 March 1976) Multiple exposures were taken of OJ 287 on 11 nights in an attempt to detect large-amplitude intraday optical variability. No conclusive evidence was found for statistically significant intraday variations. INTRODUCTION However, other investigations (Kiplinger 1974; Kip- linger 1975) have failed to detect these low-amplitude THE class of extragalactic sources known as BL periodic variations, suggesting that they may be Lac objects has been of considerable interest for transitory in nature and occur relatively infrequently. a number of years because of the rapid variations The purpose of this paper is to (1) report on the observed in their optical, infrared, and radio fluxes. A optical variability of OJ 287, observed on a time scale member of this class, OJ 287, has been the subject of a of days, from early 1973 to late 1975 and (2) report on number of investigations into the nature and time scale an attempt to observe large-amplitude intraday varia- of the flux variations at different wavelengths. Folsom tions of OJ 287. et al. (1971) and Brandie and Stull (1971) first reported observations of rapid variations at optical and radio I. OBSERVATIONS AND DATA ANALYSIS wavelengths. Similar observations were also reported by Kinman and Conklin (1971). The observations of OJ 287 reported here were made Recent observations (Kinman et al. 1974; Smith with the 76-cm telescope of the Bradley Observatory. et al. 1975; McGimsey et al. 1975; Veron and Veron All exposures were made using Kodak type 103a-O 1975) indicate that significant variations occur with plates. These plates were exposed at the//15 Cassegrain time scales on the order of a day. This suggests the focus with no filter for all observations made prior to 1 possibility of large-amplitude variations occurring on September 1975. The more recent observations were time scales of hours or less. Small-amplitude periodic made using a GG 385 filter which yields standard B optical variations with a period of less than an hour magnitudes. The plates were reduced on the Cuffey have been reported by Visvanathan and Elliot (1973). iris photometer at Georgia State University utilizing 1973 1974 1975 1976 YEAR Fig. 1. The light curve of OJ 287 from 1973 to 1976. 298 © American Astronomical Society • Provided by the NASA Astrophysics Data System 98M 2 . 81. VARIABILITY OF OJ 287 299 Table I. Variation of comparison stars. II. RESULTS Comparison star a The long-term light curve of OJ 287 is shown in Fig. 1976AJ 1. All magnitudes of OJ 287 shown are the average 1 0m046 2 0m068 of two or more plates taken on the same night with m the exception of the nights of 5 May 1973, 2 November 3 0m098 4 0 058 1973, 23 October 1974, 25 October 1974, 4 April 1975, 5 0m051 m and 31 October 1975. Table II tabulates the data 6 0m081 7 0m083 plotted in Fig. 1 along with the number of observations, 8 0 091 w, made on each night. A decline of nearly 2 mag is OJ 287 0m286 observed from early 1973 to late 1974, confirming the end of the long-term decline reported by McGimsey the photoelectric comparison sequence of Penston et al. (1975). In early 1975 the source is observed to and Wing (1973). An empirically determined correction flare by more than a magnitude and then to decline of the photographic to B magnitudes, determined by to its preflare level near 15.5 mag. This flare appears comparing filtered and unfiltered exposures of OJ 287 taken on the same night, was found to be 0.29 mag. Table II. Observational results for OJ 287. The uncertainties of these measurements were estimated in the following manner. Calibration curves Nightly average for each plate were formed using the iris reading and the Date B magnitudes of the comparison stars. The rms scatter 4 February 1973 11 13.36 of the comparison sequence about the calibration 5 February .1973 8 13.48 curve may then be used as an indication of the relia- 7 February 1973 18 13.46 5 May 1973 1 14.18 bility of the magnitude determined for OJ 287 on that 20 May 1973 4 14.24 plate. A second method of evaluating the uncertainties 2 November 1973 1 14.60 19 December 1973 5 14.72 of these measurements is to observe the variation of 13 January 1974 14 14.67 each comparison star when it is treated as an unknown 21 January 1974 13 14.92 and its magnitude determined by the remaining stars in 22 January 1974 40 15.07 31 January 1974 12 15.08 the comparison sequence. The results of such an analysis 13 February 1974 12 15.14 for all stars in the comparison sequence over all plates 21 February 1974 15 15.51 taken of OJ 287 are shown in Table I. Column 1 is the 26 February 1974 30 15.39 27 February 1974 4 15.64 comparison-star designation following the scheme of 28 February 1974 27 15.40 Penston and Wing (1973) and column 2 indicates the 3 March 1974 25 15.33 15 March 1974 4 15.30 standard deviation of the variation of that comparison 18 April 1974 16 15.37 star. In addition, the standard deviation of the varia- 27 April 1974 8 15.28 tion of OJ 287 is also shown. The largest variations 17 October 1974 2 15.26 23 October 1974 1 15.54 are seen to be associated with star 3 which has a 25 October 1974 1 15.43 standard deviation of 0T098. Therefore, we will 21 November 1974 8 15.15 consider only variations of OJ 287 which exceed a 3-cr 17 December 1974 3 14.96 I 1 18 December 1974 4 14.97 level (Aw>0 i 294) significant. In addition, if variations 5 January 1975 12 14.79 which exceed this level are found to occur for plates 7 January 1975 2 14.77 14 January 1975 4 14.76 where the rms scatter of the comparison sequence is 15 January 1975 2 14.72 greater than 0V098, they will be considered significant 16 January 1975 18 14.74 only if the variation is greater than three times the 17 January 1975 2 14.82 21 January 1975 2 14.73 rms scatter of the comparison stars on that plate. We 1 February 1975 2 14.66 have imposed the following two additional conditions 8 February 1975 3 14.87 for the inclusion of data in our study of the variability 10 February 1975 6 14.78 15 February 1975 4 14.81 of OJ 287 : (1) The plate must have a minimum of five 3 March 1975 2 15.31 measurable comparison stars and (2) OJ 287 must be 4 March 1975 2 15.30 7 March 1975 3 15.40 bracketed in brightness by the stars of the comparison 9 March 1975 6 15.14 sequence. It is interesting to note that if these last 4 April 1975 1 15.09 two conditions are not imposed, one finds variations 5 April 1975 4 15.08 12 April 1975 3 15.14 of ^0V5 occurring on a time scale of less than an hour. 17 April 1975 2 15.34 We feel that these last two conditions are necessary 6 May 1975 2 15.21 in order to accurately determine the brightness of the 11 October 1975 2 15.10 12 October 1975 2 14.99 source and to have a meaningful assessment of 13 October 1975 2 14.92 the uncertainties associated with that particular 31 October 1975 1 15.02 observation. © American Astronomical Society • Provided by the NASA Astrophysics Data System 98M 2 . 81. 300 MILLER ET AL. to be eight months in duration with the rate of increase and decline in the object’s brightness approximately the 1976AJ same. There are day-to-day fluctuations of up to 0.5 mag superimposed on the overall change in brightness observed during this eight-month period. Multiple exposures of OJ 287 were taken on a number of nights from December 1973 to March 1975 which yield a time resolution as short as 7 min. In Fig. 2 are five nights on which five or more exposures were taken of OJ 287. With the exception of the night of 22 January 1974, the variations observed within a given evening do not exceed a 3-cr level and therefore cannot be considered significant. On the night of 22 January 1974, three of the four observations made near 0700 UT, indicating a possible 0.4-mag flare, have uncertainties of 0.15 mag. These uncertainties were determined from the rms scatter of the comparison sequence about the calibration curve for each of the plates. Therefore, this event should not be considered statistically significant. U.T. (HRS) Fig. 3. The intraday light curves of OJ 287 for the nights of 26 February 1974, 28 February 1974, 21 November 1974, 5 January 1975, 10 February 1975, and 9 March 1975. Six additional nights where multiple exposures of OJ 287 were made are shown in Tig. 3. No variations observed on any of these nights exceeds the 3-<r level with the exception of one plate taken on 26 February 1974 at 0550 UT.