Multiwavelength Observing Campaign
on 3C 66A

September 2003 - January 2004

Current News and Updates


A multiwavelength campaign on the BL Lac object 3C 66A was proposed to the WEBT collaboration by Markus Böttcher. Following the approval of the source observing proposal for RXTE, coordinated broadband monitoring observations during the period Sept. 20 - Dec. 15, 2003 have been planned. The campaign includes radio (UMRAO, Metsähovi, VLBA), optical (WEBT; see WEBT News 26 and 28), X-ray (RXTE), and VHE (STACEE, CELESTE, VERITAS) observations.

Purpose of the Campaign

The purpose of our broadband monitoring campaign is to probe broadband spectra and spectral variability of the blazar 3C66A with as dense a temporal sampling as possible. In particular, we aim for a probe of simultaneous intraday spectral variability measurements at optical and X-ray frequencies.

Core Campaign Periods

Both RXTE and the ground-based VHE observations will focus on the dark-moon periods, which are

Throughout those periods, we have a total of 10 pointings of 20 ksec each with RXTE. Optical observers should try to contribute as many observations as possible during these core campaign periods.

Optical Observing Strategy

We suggest that optical observers perform 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 ... series of frames during all the available time in each observing night. This allows us to get two simultaneous light curves in the two bands for color variability studies. 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, say, 60 cm. At the beginning and end of the B-R (or R-only) sequence, a complete set of filters B-V-R-I (Johnson-Cousins when possible) would be very useful to follow the optical spectrum behaviour of the source. We are providing a finding chart of the field of 3C 66A, including the B, V, R, and I magnitudes of four comparison stars. All four reference stars should be included in each frame. Star C1 is the brightest one, so that we must be careful that its counts are inside the linear response of the detector, especially under good seeing conditions.

Data Reduction

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. The suggested data format for instrumental magnitudes is the same of BL Lac 2000 and BL Lac 2001:

JD-2452000 3C 66A
18.131 0.004 17.492 0.003 18.389 0.004 16.798 0.002 17.742 0.003

where JD (or equivalent date + UT) must be geocentric at mid exposure (if different, specify it and provide exposure times). Times are requested with second precision (e.g. 2003 10 01 00 00 01, or the preferred corresponding JD - 2452000 = 913.50001 with five decimal digits); mags and errors should have three decimal digits.

Observers are 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 observers use the same parameters, i.e. 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 the "entire" flux from the object and to avoid the inclusion of other objects. The rather small background annulus is due to the closeness of a faint star to Star C1.

Infrared Observations

Observers having access to infrared telescope facilities, are invited to contribute infrared observations. Our finding chart + magnitude table also contains the J, H, and K magnitudes of 3 comparison stars (from González-Pérez, Kidger & Martín-Luis, 2001) for calibration.


Observers who have access to an optical spectrometer are invited to take medium resolution spectra of this source. The aim is to try to detect the possible Mg II emission feature at 4044 A, which may be not an easy task, as it is witnessed by the literature. The chance to detect the line may increase when the source is fainter.
In any case, spectral observations repeated in time can provide important information on the continuum variations.


For more information and suggestions, please contact Markus Böttcher

All observers interested in participating in the campaign are invited to send a message to the Campaign Manager, Markus Böttcher, and the WEBT President, Massimo Villata , providing information on their instrumentation (telescope, detector, filters) and possible observing periods.

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