Up: Multi-colour optical monitoring of 0716+71
Subsections
The multi-star CCD photometric monitoring was carried out for 21 nights during
February/March 1994 using two 1-metre class telescopes located at Naini Tal and
Kavalur in India, at a separation of
2500 km. Due to the high declination
of the blazar the airmass was generally larger than 1.5, but the object could be
monitored typically for about 5 hours in a night. The positional offsets, the
calibrated BV Johnson and RI Cousins magnitudes and instrumental colours
of the three comparison
stars used for differential photometry of the blazar are listed in Table
1. The photometric measurements for stars 1 and/or 2 have been
carried out by Takalo et al. (1994);
Ghisellini et al. (1997) and
Villata et al.
(1998) in the different passbands. Within the errors, the magnitudes derived by
them for a star agree fairly well. For stars 1 and 2, the BVR data are taken
from Villata et al. (1998) as they are the most accurate measurements;
while the I magnitudes are adopted from
Ghisellini et al. (1997). For star 3,
these values are determined from stars 1 and 2, using the differential
instrumental magnitudes measured in our campaign. A log of observations for
the entire campaign is given in Table 2. Further details of the
observations are summarized below.
Table 1:
Positional offsets in arcmin and the differential (
) instrumental
(b - v) and (v - i) colours in mag of the comparison stars are relative to the
blazar S5 0716+71. Stars observed by Takalo et al. (1994),
Ghisellini et al.
(1997) and Villata et al. (1998) have been identified with prefix T-, G-
and V- respectively. BVRI are standard magnitudes
|
Table 2:
Log of the CCD observations
|
The BV Johnson and RI Cousins photometric observations were carried
out using the Photometrics CCD system at the f/13 Cassegrain focus of the
104-cm Sampurnanand reflector of the Uttar Pradesh State Observatory
(UPSO), Naini Tal on 19 nights in February/March 1994 (Table 2).
Typical seeing was around 1.5
. Details of the CCD system are
reported in Mohan et al. (1991). Typical exposure times were 20-60 s
in R and I, 1-2 min in V and 3-5 min in B.
The observed comparisons are stars 1 and 2 (Table 1).
CCD photometric observations in R and I Cousins passbands were made using
the Thomson-CSF TH7882 CCD chip mounted at the f/13 Cassegrain
focus of the 102-cm telescope of the Vainu Bappu Observatory (VBO) at
Kavalur. Typical seeing was around 2
. Details of the
instrumental set up can be found in Sagar & Pati (1989).
Typical exposure times were 5-10 min in both R and I
passbands (owing to large zenith distance of the object and
frequent cloudy conditions). The observed comparisons are stars
1 and 3 (Table 1).
At both sites, bias and dark frames were taken intermittently
throughout the observations. The pixel size of 23
23
(
m)2 of both CCD systems corresponds to 0.36
(arcsec)2 on the sky. The chip size of 384
576
pixels covers an area of 2.3
3.4 (arcmin)2. In order
to improve the signal/noise ratio the data were acquired in the
binning mode of 2
2 (pixels)2. In order to detect
small intensity fluctuations, we performed differential
photometry, generally ensuring that the locations of the blazar
and the comparison stars did not change by more than a few pixels
from exposure to exposure during a night. This largely obviated
the need to flat-field the frames (which introduces additional
noise). The background was subtracted using the measured counts
within the annuli (between radii of 25 and 35 pixels) around the
circular aperture of 5 pixel radius, centred at the desired
object.
![\begin{figure}
\includegraphics []{8009f1.eps}\end{figure}](/articles/aas/full/1999/03/ds8009/Timg15.gif) |
Figure 1:
DLCs in the B, V, R, I passbands deduced from the UPSO
observations on 19 nights (excepting for February 28 on which I-band data
could not be obtained, whereas for the last 3 nights B-band observations could
not be made). The symbols crosses, squares, filled circles and
triangles refer to B, V, R and I passbands respectively. In
order to plot the DLCs with clarity in the B, V, R, I sequence
from bottom to top, we have applied fixed offsets of 0.25 mag
in B, 0.0 mag in V and -0.1 mag in R and I for the DLCs involving
the blazar and 0.1 mag in B, 0.0 mag in V and -0.05 mag in R and
-0.1 mag in I for the DLCs involving the two comparison stars 1 and 2 |
![\begin{figure}
\includegraphics []{8009f2.eps}\end{figure}](/articles/aas/full/1999/03/ds8009/Timg16.gif) |
Figure 2:
DLCs in the R and I passbands obtained at VBO for
5 consecutive nights. The temporal overlap with the UPSO data on
the first 3 nights (Fig. 1) shows good consistency between the
two datasets. Note that the comparison star 1 is common to the two datasets.
For clarity in plot, the offsets applied are -0.1 mag in both R and
I for the DLCs involving the blazar, and -0.1 mag in I and 0.0 mag
in R for the DLCs involving the two comparison stars 1 and 3 |
The data were reduced at the VBO VAX 11/780 system using DAOPHOT
software package, as described in Gopal-Krishna et al. (1993;
1995). In order to maximise the signal/noise ratio, we performed
the aperture-growth analysis described by Howell (1989). Accordingly,
a circular aperture of 10 pixels (=7
) diameter was adopted for
the photometry of the blazar as well as the comparison stars. The DAOPHOT
algorithm also provided estimates of the statistical errors of the
computed relative magnitudes, which were usually < 1% for all
the passbands. Using the computed instrumental magnitudes, differential light
curves (DLCs) were generated. These represent the relative instrumental
magnitudes of the blazar relative to the two comparison stars, and the same for
the pair of comparison stars (Figs. 1 and 2). Note that the
comparisons used are stars 1 and 2 for the UPSO DLCs and stars 1 and 3 for
the VBO DLCs. It is seen that the average instrumental colours of the blazar
are not much different from those of the three comparison stars
(Table 1).
Up: Multi-colour optical monitoring of 0716+71
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