(July 17 - August 11, 2000)

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Last updated on August 4, 2000
(but the light curve is updated more or less daily)

(October 2000)


WEBT observers have organized an optical campaign on BL Lacertae contemporaneous with the high-energy campaign coordinated by Markus Boettcher involving X-ray and TeV observatories such as BeppoSAX, RXTE, STACEE, CAT, HEGRA.

Our purpose is to monitor BL Lac in the optical bands from one week before to one week after the high-energy campaign, that is from July 17 to August 11, with particularly dense coverage in the central period. We are collaborating with observatories located at different longitudes, in order to obtain as continuous as possible light curves during all 24 hours of each day. In particular, our goal is high-density and high-precision coverage, possibly in two optical bands (B and R). Observers interested in participating in this optical campaign are invited to contact us at, providing some information on their instrumentation (telescope, detector, filters) and possible observing period.

In the map below the location of participating observatories is shown.


Click here to have a look at the electronic correspondence relevant to the campaign.

A preliminary R-band light curve of BL Lac in the last days is available in postscript format (data from Torino, University of Victoria, Clark and Coyote, and Palomar Observatories).
The finest intranight variations have been observed on July 19, July 26-27, and August 2-3 (preliminary reduction).
Participants interested in showing their preliminary data in the above light curve may send them in a simple JD-mag-err format. The photometric sequence by Fiorucci & Tosti (1996, A&AS 116, 403) is used. (Errors must be calculated as errors on the variations, disregarding any calibration error, since we are interested in variations, rather than in a precise evaluation of the magnitude.)
You may also have a look at the longer-term behaviour (1996-2000 light curve) of BL Lac as observed from the Torino Observatory.


We suggest participating observers to perform optical observations alternately in two bands (Johnson's B and Cousins' R, if possible, other R filters are also accepted) in order to obtain a B,R,B,R,B,R,... series of frames during all the available time in each observing night. This allows to get two simultaneous light curves in the two bands for their comparison. Exposure times should be chosen in view of a good compromise between high precision (instrumental errors less than 0.03 mag for small telescopes and less than 0.01 for large ones) and high temporal density. When high precision implies gaps of 15-20 minutes in each light curve, we suggest to carry out observations in the R band only. As a matter of fact, the intensive B monitoring should be appropriate only with telescopes larger than 1 m. Moreover, at the beginning and end of the B-R (or R-only) sequence, a complete set of filters (U)BVRI (Johnson-Cousins when possible) would be very useful to follow the whole optical spectrum behaviour of the source.
A finding chart of the BL Lacertae field is available at the Heidelberg finding chart site.
All four reference stars should be included in each frame; for small-size frames we request to include at least Stars B and C. Star B is the brightest one (unless BL Lac is very bright), so that we must be careful that its counts are inside the linear response of the detector, specially under good seeing conditions.


Data will be collected as instrumental mags of the source and reference stars, in order to apply the same analysis and calibration procedures to all datasets. An example of instrumental mag format adopted by the WEBT is available, together with other useful notes on reduction procedure. We know that not all observers can easily provide all the information requested there; thus, simpler data formats are also accepted, provided that they contain the strictly necessary information, i.e. Gregorian or (geocentric) Julian Date and instrumental mags of the relevant objects for each frame. In any case, Gregorian Date must be provided with second precision at mid-exposure UT (e.g. 2000 07 17 00 00 01); the same for Julian Date (without heliocentric correction; you may omit the first 4-5 digits, e.g. 42.50001 would correspond to the date above); mags and errors should have three decimal digits.
Observers are thus requested to perform bias/dark correction and flat-fielding on their frames, and to obtain the instrumental mags with some procedure. Both aperture photometry (possibly using IRAF or CCDPHOT) or Gaussian fitting are allowed. In the case of aperture photometry, we suggest that all people use the same parameters, which could be 8, 10, and 16 arcsec for the radii of the aperture and of the edges of the background annulus; however, the aperture and annulus radii can be seeing-dependent in order to include "all" the object and to avoid the inclusion of other objects. The rather small background annulus is limited by the closeness of Star B and BL Lac (about 24 arcsec).
If any observer cannot perform the reduction, the frames can be reduced by us, provided that they have been de-biased and flat-fielded (pre-reduced frames).


During the campaign M. Aller and H. Aller plan to obtain daily observations of total flux and linear polarization at centimeter wavelengths (4.8, 8.0, or 14.5 GHz) with the University of Michigan 26-meter telescope. Each observation consist of a series of integrations over a 30-40 minute time period. Additionally, the source is observed 3 times per week with RXTE during the campaign by A. Marscher, S. Marchenko-Jorstad, and M. Aller as part of an ongoing study of BL Lac.


*The Torino Blazar Monitoring Group Home Page


This page is maintained by Massimo Villata