BVR photometry of comparison stars in selected blazar fields - II. Photometric sequences for 9 quasars

Astronomy and Astrophysics Supplement Series, Jun 1998

In the course of the blazar monitoring activity at the Torino Astronomical Observatory, we obtained standard BVR magnitudes for 36 stars and standard VR magnitudes for 10 stars in the fields of 9 quasars. Observations were done during 14 photometric nights with the Cassegrain REOSC telescope equipped with a pixel CCD camera. All stars are brighter than and are useful comparison objects for the evaluation of the quasar magnitudes.

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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)


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C. M. Raiteri, M. Villata, L. Lanteri, M. Cavallone, G. Sobrito. BVR photometry of comparison stars in selected blazar fields - II. Photometric sequences for 9 quasars, Astronomy and Astrophysics Supplement Series, 1998, pp. 495-500, Volume 130, Issue 3, DOI: 10.1051/aas:1998420