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BVR photometry of comparison stars in selected blazar fields - II. Photometric sequences for 9 quasars
Astron. Astrophys. Suppl. Ser.
BV R photometry of comparison stars in selected blazar elds
II. Photometric sequences for 1
quasars 1
C.M. Raiteri 1 2
M. Villata 1 2
L. Lanteri 1 2
M. Cavallone 0 1
G. Sobrito 0 1
0 Istituto di Fisica Generale dell'Universita di Torino , Via P. Giuria 1, I-10125 Torino , Italy
1 Send o print requests to: C.M. Raiteri
2 Osservatorio Astronomico di Torino , Strada Osservatorio 20, I-10025 Pino Torinese , Italy
In the course of the blazar monitoring activity at the Torino Astronomical Observatory, we obtained standard BV R magnitudes for 36 stars and standard V R magnitudes for 10 stars in the elds of 9 quasars. Observations were done during 14 photometric nights with the 1:05 m Cassegrain REOSC telescope equipped with a 1242 1152 pixel CCD camera. All stars are brighter than V = 16:6 and are useful comparison objects for the evaluation of the quasar magnitudes.
galaxies; active | quasars; general
1. Introduction
According to the current classi cation scheme for the
active galactic nuclei, the class of blazars comprises BL
Lacertae objects and a subset of quasars, all objects that
are believed to emit their energy by a relativistic plasma
moving towards us at a small angle to the line of sight.
They are characterized by rapid variability, high and
variable polarization, high brightness temperature, and
superluminal motion of radio-emitting components
(e.g. Urry &
Padovani 1995)
.
Although several blazars have been monitored for
many years, calibrated stars in the source eld are still
lacking for a number of them. This implies more work
in order to obtain the blazar magnitude; moreover,
using di erent reference stars may lead to di erent values
of the blazar magnitude, thus making the comparison of
data from di erent observers less meaningful than when
a common photometric sequence is available. With this in
mind, we calibrated eld reference stars for a number of
blazars that we are monitoring at the Torino Astronomical
Observatory.
In this paper we present calibration of 46 stars in the
elds of 9 quasars of our monitoring object list. Standard
magnitudes for 56 stars in the elds of 10 BL Lacertae
objects are published in
Villata et al. (1998
a; Paper I).
2. Observations and data reduction
All data were taken with the 1:05 m Cassegrain
reflector of the Torino Astronomical Observatory. The
instrumentation comprises a 1242 1152 pixel CCD camera
(EEV) and standard Johnson's BV and Cousins' R
lters. The observations were done during 14 photometric
nights, from February 1995 to May 1997. Flat eld and
bias frames were taken each night for the image correction;
no dark current correction is needed. Several Landolt's
elds
(Landolt 1992)
and other standard stars were
observed during each night.
Frame reduction was performed with the Robin
procedure locally developed, including bias subtraction, flat
elding, and circular gaussian t after background
subtraction.
Transformation of the instrumental magnitudes into
standard ones was obtained with the Calib procedure (see
Paper I for a description of the transformation equations).
3. Results
Table 1 lists the 9 quasars for which photometric sequences
were obtained; coordinates at the 2000 equinox are given.
BV R magnitudes of the reference stars are reported in
Table 2, together with their uncertainties ( ) and the
number of observations (N ) for each band. For 10 stars
V R magnitudes only are given. The magnitude and
uncertainty evaluation has been performed as in Paper I.
Finding charts for the stars identi cation are shown in
Figs. 1{9. They are 10 arcmin wide; north is up and east
is on the left.
The blazar PKS 0528+134 has recently been pointed
for long periods of time by the Compton Gamma Ray
Observatory (CGRO); consequently, international
collaborations have been started in order to intensify the study
of this object at di erent wavelengths.
BV R magnitudes of Stars 1 and 5 are published in Villata
We are not aware of any photometric sequence in its et al. (1997; their Stars D and A); they are in agreement
eld, so that our work can o er a way to calibrate the large with the present values with the only exception of the V
amount of optical data expected from this huge monitor- magnitude of Star 5.
ing campaign. The object \S" in Fig. 6 is characterized by
large
We have to notice that Star 2 of our photometric se- amplitude and rapid variability. Its coordinates are
quence (see Fig. 2) is not a point-like source and, in good- RA(2000) = 12h 56m 8s:47, Dec:(2000) = −5 440 340:05. It
seeing nights, it appears as a double object. has been named \Simona" in
Villata et al. (1998
b).
Blazar
BV R magnitudes of two among the six comparison stars
calibrated in the present work are reported in
Villata et al.
(1997)
: Stars A and B in that paper are our Stars 4 and 6.
The respective magnitudes are in accordance within the
errors, apart from the B magnitude of Star 6.
3.6. 3C 454.3
A U BV photometric sequence for 3C 454. (...truncated)