2 Observations and results

The blazar optical monitoring program was started at the Torino Astronomical Observatory in November 1994 (e.g. Villata et al. [1997]; Raiteri et al. [1998b]). Data are obtained with the $1.05\rm\,m$ REOSC telescope equipped with a $1242 \times 1152$ pixels CCD camera. The scale of the CCD is $0.47\rm\,arcsec$ per pixel. In order to get spectral information, we generally observe in the Johnson's B, V, and Cousins' R bands. Flat-field frames are taken both at sunset and at dawn, when possible.

 

 

Table 1: List of the observed BL Lacs

Source	RA (2000)	Dec. (2000)	Field photometry
1ES 0145+138	$01\ 48\ 29.7$	$+14\ 02\ 18$	Torino, unpublished
1ES 0323+022	$03\ 26\ 14.0$	$+02\ 25\ 15$	Smith et al. [1991]
1ES 1028+511	$10\ 31\ 18.5$	$+50\ 53\ 36$	Villata et al. [1998]
1ES 1118+424	$11\ 20\ 48.1$	$+42\ 12\ 12$	Torino, unpublished
1ES 1959+650	$19\ 59\ 59.8$	$+65\ 08\ 55$	Villata et al. [1998]

Data are reduced with the Robin procedure locally developed (Lanteri [1999]). The bias level is estimated from the image overscan; no dark current correction is needed since the CCD is cooled with liquid nitrogen. The point spread function is fitted by means of a circular Gaussian, the background being subtracted by fitting it with an inclined plane. The blazar magnitude and corresponding uncertainty are derived by comparison with two reference stars in the same field of the source.

The coordinates at the 2000 equinox of the observed BL Lacs are given in Table 1, together with the reference for the photometric calibration of the comparison stars. In two cases (1ES 0145+138, 1ES 1118+424) we adopted unpublished standard magnitudes derived from observations performed at the REOSC telescope during photometric nights. The photometric calibration procedure is described in Villata et al. ([1998]).

The results of our observations are presented in Tables 2-6 and Figs. 1-5. The figures show light curves in the R band, which is the best sampled one. The BeppoSAX pointings are indicated. In the subsets to the figures we plot the deviation from the mean of the magnitude difference between the two reference stars chosen in the same field of the source. This gives information on the reliability of the reference stars for each frame. The magnitude error lies between 0.02 and $0.09\rm\,mag$ and is obtained as the maximum between the value in the subset and a lower limit that takes into account the magnitude difference between the source and the reference stars.

2.1 1ES 0145+138

1ES 0145+138 is an X-ray selected BL Lac, discovered by the Einstein satellite Slew Survey (Perlman et al. [1996]). For this object, only one magnitude estimate was found in the literature: V = 17.9, reported by Perlman et al. ([1996]).

We collected 11 frames in the R band and 8 frames in the V band, from October 22, 1996 to December 22, 1998. As can be seen from Fig. 1 and Table 2, we have two observations in the R band and one in the V band very close to the BeppoSAX pointing of December 30, 1997 ( $\rm JD-2449000=1813.1$). During our observing period no significant variation of the magnitude was seen: the brightest value for the R band is $17.17\rm\,mag$, and $\Delta R = 0.44$.

Figure 1: Light curve of 1ES 0145+138 in the R band

2.2 1ES 0323+022

This is an X-ray source discovered in the all-sky survey of the HEAO 1 satellite (Doxsey et al. [1983]); it was also detected by the Slew Survey (Perlman et al. [1996]). The first magnitude estimate of the optical counterpart was V = 16.5 (Doxsey et al. [1983]). Feigelson et al. ([1986]) obtained V = 16.55 on November 28, 1983; observations made between October 14, 1982 and October 22, 1982 revealed a magnitude variation (V band) from 15.56 to $16.43\rm\,mag$ and B-V = 0.41-0.96; in December 1982, V= 16.08, B-V = 0.59, V-R = 0.46, R-I = 0.52; on January 1, 1984, V = 16.5. Feigelson et al. ([1986]) made an estimate of the brightness of the nebulosity surrounding 0323+022: they found V=17.5. They could also calculate the redshift and absolute magnitude of the host galaxy: $z = 0.13 \pm 0.05$, $M_{V} = -21.6 \pm 0.7$.

Filippenko et al. ([1986]) reported large variations in the optical spectrum of 0323+022 over a period of only 2 days, in late February 1985. They measured V = 17.2 for the nucleus, 18.2 for the nebulosity, and M_V= -21-22 for the host galaxy.

Pica et al. ([1988]) reported seven measured magnitudes from September 1986 to January 1987, which indicate a maximum brightness of this object in that period of B = 17.45, with variation $\Delta B = 0.13$. Significant brightness variations were also reported by Jannuzi et al. ([1993]). In 101 nights of polarimetry and/or photometry observations during September 1987 - March 1990, the following extreme values were recorded: B = 16.90-17.34, V = 16.13-17.03, R = 15.91-16.19, I = 15.26-15.60. Fiorucci & Tosti ([1996]) do not report any dramatic event in the optical monitoring program of this BL Lac. From February 1993 to January 1995 a mean value $\overline{V} = 16.70$ was found, with $\Delta V = 0.30$, and $\overline{I} = 15.68$ with $\Delta I = 0.32$; from October 1994 to January 1995, $\overline{R}= 16.28$, with $\Delta R = 0.40$ ( V-R = 0.36, R - I = 0.57).

Xie et al. ([1996]) reported BVRI data taken in the period December 7, 1993 - December 31, 1994. No significant variations were detected: B = 17.44-17.52, V = 16.78-16.92, R = 16.15-16.41, I = 15.76-15.84.

Our dataset includes 26 frames in the R band, 13 in the V band, 9 in the I band, and 3 in the B band between October 23, 1996 and January 23, 1999 (see Table 3). The BeppoSAX pointing was scheduled for January 20, 1998 ( $\rm JD-2449000=1833.8$), a date around which we obtained a good sampling. In particular, we have VRI data taken exactly in the same day of the BeppoSAX observation.

The light curve in the R band, as can be seen in Fig. 2, shows an overall variation $\Delta R = 0.67$, with a maximum brightness R = 16.16.

Figure 2: Light curve of 1ES 0323+022 in the R band

2.3 1ES 1028+511

This is a RASS and Slew Survey source for which very few photometric information exist. Only one value of the V magnitude was found in the literature: V = 16.6 (Perlman et al. [1996]), and three values for the B magnitude: B = 17.0, derived from the spectrum based on an observation on May 29, 1992 (Schachter et al. [1993]), B=16.8, from a spectrum taken between February and November 1992 (Bade et al. [1994]), and B=16.8, as derived from HQS plates (Nass et al. [1996]).

Our dataset regarding this object consists of 18 frames in the R band, 2 in the V band, and only one in the B band, and covers the period December 3, 1996 - May 8, 1997 (see Table 4). BVR data were taken on May 1, 1997, simultaneously with the BeppoSAX pointing (JD-2449000 = 1570.4).

As seen in Fig. 3, a very thin variation of the magnitude was detected: $\Delta R = 0.18$, and the maximum brightness was found at R = 16.53.

Figure 3: Light curve of 1ES 1028+511 in the R band

2.4 1ES 1118+424

This is an X-ray selected BL Lacertae object as was discovered by EXOSAT (and seen also with the Slew Survey). Giommi et al. ([1991]) reported a V magnitude of 17.3, and Perlman et al. ([1996]) V=17.0. The average brightness in the R band in March 1993 was R = 18.0 (Heidt & Wagner [1998]).

As can be seen in Fig. 4 and Table 5, we collected 11 data in the R band between January 17 and May 21, 1997, and one of them is contemporaneous with the BeppoSAX observation of May 1, 1997 (JD-2449000 = 1570.1). The minimum value of the R magnitude is 16.94, and its variation is $\Delta R = 0.32$.

Figure 4: Light curve of 1ES 1118+424 in the R band

2.5 1ES 1959+650

It was identified in the $200\rm\,mJy$ radio-selected sample and it was seen also in the Slew Survey. A detailed study of its host galaxy (M_R=-23.0) was recently performed by Heidt et al. ([1999]), showing a complex structure possibly composed by an elliptical galaxy plus a disc and an absorption dust lane. Three photometric optical values were found in the literature, showing a great variation of the source brightness in the optical band. It was reported as V=16 by Marcha ([1994]); V = 12.8, as derived from digitalized plates, using B-V = 0.5 (Schachter et al. [1993]); and an approximate POSS O magnitude of 13.67 obtained from the Cambridge Automatic Plate Measurement (Perlman et al. [1996]). More recently, Kurtanidze et al. ([1999]) report data taken in 1997-1999, with variations in the range R=14.50-14.92.

We observed this source from February 29, 1996 to May 30, 1997, and collected 16 frames in the R band, 7 in the V band, and 5 in the B band (see Table 6). The BeppoSAX observation occurred on May 4, 1997 (JD-2449000 = 1573.4), near the end of our observing period. The light curve in the R band is characterized by rapid flickering (see Fig. 5); in particular, a decrease of $0.28\rm\,mag$ in 4 days was recorded on JD-2449000 = 1242.55-1246.55.

Figure 5: Light curve of 1ES 1959+650 in the R band

This is the only object in the present sample showing rapid variability. Indeed, this BL Lac is also known as a radio source, so that it is not surprising that it presents the typical optical variability of radio-selected BL Lacs. Moreover, it has also been detected at TeV energies by the Utah Seven Telescope Array.