BL Lacertae

3 Observations during the WEBT campaign

Originally, the WEBT campaign was planned for the period July 17-August 11, 2000, that is from one week before to one week after the planned high-energy campaign (Böttcher et al. 2002). Simultaneous observations were also performed in the radio band from the University of Michigan Radio Astronomy Observatory (UMRAO) and from the Metsähovi Radio Observatory, in Finland.

At the campaign start, BL Lacertae was in a relatively faint phase. A subsequent, considerable brightening of the source in late September 2000 caused a campaign extension up to January 2001.

Data were finally collected from May 2000 to January 2001.

Table 1 shows the list of observatories which participated in the WEBT campaign. The name and country of the observatory (Col. 1) is followed by the telescope diameter (Col. 2), by the total number of observations done ( $N_{\rm obs}$, Col. 3), and by the number of data points in UBVRI derived, sometimes after binning (Cols. 4-8).

3.1 Observing strategy

The participating observers were suggested to perform optical observations alternately in two bands (Johnson's B and Cousins' R, but other Rfilters were also accepted) in order to obtain a BR sequence of frames throughout each observing night. Exposure times were chosen in view of a good compromise between high precision and high temporal density. In those cases where high precision would have required gaps of 15- $20\rm ~min$ in each light curve, it was suggested that observations be obtained in the R band only. As a matter of fact, intensive B monitoring was considered to be appropriate only for telescopes larger than $1\rm ~m$. Moreover, at the beginning and end of the BR (or R-only) sequence, a complete set of filters (U)BVRI (Johnson-Cousins when possible) was suggested in order to follow the whole optical spectrum behaviour of the source.

 

 

Table 1: Offsets given to the datasets in order to remove systematic differences; when a range is given, it means that different values were used in different nights; the accuracy of the offsets is similar to that of the corresponding datasets, ranging from less than 0.01 to about $0.05\rm~mag$; in few cases no comparison was possible and no offset was given.

Observatory	U	B	V	R	I
Kyoto	-	-	-	0	-
Osaka Kyoiku	-	-	-0.05	-0.04-0	0
Mt. Maidanak (AZT-22; SITe)	0	0	0	0	0
Mt. Maidanak (AZT-22; ST-7)	-	-0.27--0.25	-0.10	-0.21--0.11	-0.10
Mt. Maidanak (T60-K)	-0.15	+0.10	+0.10	-	-
Abastumani	-	-	-	-0.04-+0.06	-
Crimean	-	-0.20	-0.05	-0.07--0.04	-0.05
Nyrölä	-	-	-	+0.01	-
Skinakas	-	0	-	+0.02	-
Catania	0	+0.05-+0.15	+0.05	-	-
Vallinfreda	-	+0.10	-0.05	0	-0.05
Monte Porzio	-	+0.10	-0.05	-0.07	-0.05
Perugia	-	-	0	0	0
Torino	-	-0.13--0.04	-0.10	-0.13--0.01	-0.10
Guadarrama	-	-	-	0	-
Roque de los Muchachos (KVA)	-	0	0	+0.02	-
Roque de los Muchachos (NOT)	-	+0.05	-	-0.01	-0.05
Bell Farm	-	-	-	0	-
St. Louis	-	0	0	+0.01-+0.02	0
Sommers-Bauch	-	-	0	+0.04	-
Lowell	-	+0.06	0	0	0
San Pedro Martir	-	-0.10	-0.10	-0.10	-0.10
Palomar	-	-0.05	-	-0.125	-
Clarke and Coyote	-	-	-	-0.04	-
University of Victoria	-	-	-	+0.04-+0.11	-

3.2 Data reduction and assembling

Data were collected as instrumental magnitudes of the source and reference stars, in order to apply the same analysis and calibration procedures to all datasets. Frame pre-reduction, i.e. bias/dark correction and flat-fielding, was performed by the observers. Instrumental magnitudes were obtained by either standard aperture photometry procedures or Gaussian fitting, in most cases by the observers themselves.

A minority of data came from photometer observations: these data were provided directly as standard magnitudes

A careful data assembling was required, since the whole dataset consisted of more than 15 000 observations coming from 22 different observatories, 24 different telescopes, and 25 different detectors.

The analysis was performed through the following steps:

• Obtaining standard magnitudes of BL Lac from instrumental ones. Stars B C H K originally identified by Bertaud et al. (1969; their b c h k) were used as reference stars for the magnitude calibration of the source. The photometry adopted was that published by Bertaud et al. (1969) for the U and B bands, and the one from Fiorucci & Tosti (1996) for the VRIbands.
• Discarding bad and unreliable points.
• Binning the data when needed.
• Check of the final light curves for each observatory (see Table 1 for the relevant data numbers).
• Putting together different datasets, night by night.
• Estimating the offsets existing among different datasets (see Table 2) from overlapping data, and correcting them, night by night.
• Discarding less precise data in favour of higher-precision simultaneous data.

Table 3 shows the number of data points remained in the various bands at the end of the data assembling procedure (to be compared with the totals of Table 1, where the last step of the assembling procedure is not considered), and the minimum and maximum magnitudes reached.

 

 

Table 3: Number of points plotted in the UBVRI light curves; minimum and maximum magnitudes reached.

	U	B	V	R	I
$N_{\rm points}$	290	1094	611	4015	396
Min	14.49	14.76	13.75	13.01	12.35
Max	16.08	16.26	15.21	14.50	13.74

Figure 1: UBVRI light curves of BL Lacertae from May 2000 to January 2001; the numbers on the right are the numbers of data points in each light curve; the grey (yellow in the electronic version) strip corresponds to the period of the core WEBT campaign.

BL Lacertae