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Astron. Astrophys. 339, 41-51 (1998)

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2. Observations

2.1. Optical

The observations were carried out with several telescopes (see Table 1).


Table 1. List of the instruments used

The 1.25m telescope at the Crimean Astrophysical Observatory was equipped with a Double Image Chopping UBVRI Photopolarimeter (Piirola, 1988). The instrument provides measurements of both the intensity and the polarization simultaneously in the UBVRI bands (0.36, 0.44, 0.54, 0.69, 0.83 µm respectively) by using four dichroic filters which split the light into the five spectral passbands. The passbands are close to the standard UBV(Johnson) and RI(Cousins) systems, allowing an accurate conversion of the photometric data from the instrumental to the standard system. All photometric data were calibrated using standard comparison stars (Landolt 1983a, 1983b, 1992).

The other telescopes were equipped with CCD cameras and broad band Johnson-Cousins filters. The integration times during the exposures varied from 3 to 6 min depending on the brightness of the objects and the filter used. Data reduction was performed with locally developed software packages, which operate all the standard corrections required (bias and dark subtraction, flat field correction) and the instrumental magnitudes computation.

Mkn 421 is in the centre of a bright host galaxy, in addition, a nearby galaxy in the N-E direction from the source is well visible. The distance between the two nuclei is of about 14 arcsec (Hickson et al., 1982), as a consequence the photometric reduction is related to the choice of the aperture radius. The data analysed in this paper were obtained using an aperture diameter of 10 arcsec centered in the nucleus of Mkn 421.

Another major obstacle to the CCD monitoring of this source is caused by the presence of two bright stars in the field of view (FOV), which are able to saturate the frame and, sometimes, to cover the BL Lac object. Because of these bright stars and the small FOV of the Tuorla and Perugia CCD cameras, we could not have Mkn 421 and comparison stars in the same image. Therefore, we observed Mkn 421 with these instruments only on photometric nights, jumping between the object and the Landolt comparison stars. The photometric images obtained at the Torino Astronomical Observatory were large enough to include the three reference stars calibrated by Villata et al. (1998).

The comparison among the data acquired with different telescopes in the same night is fully satisfying, and no significant colour effect was found.

In Fig. 1 we report the light curve in the UBVRI filters collecting the data obtained with the above mentioned instruments during the monitoring campaign, from November 1994 to June 1997 (JD 2449676-2450606). The more sampled light curves are those obtained with the V and R filters (255 and 236 observations respectively). Optical variations are synchronous in all the observed broad bands, but the amplitudes are generally enhanced toward higher frequencies.

[FIGURE] Fig. 1. UBVRI light curves of Mkn 421

During 1995 and the first half of 1996 (from JD 2449676 to 2450233), Mkn 421 showed small amplitude variations and a modulation with a time-scale of some months. In the winter 1996-1997 a large outburst occurred (see Fig. 2), with the observed maximum of about V[FORMULA]12.0 (JD 24500403, 14 November 1996 UT), one of the highest values of brightness reported in the historical light curve. It can be considered ended in March 1997, when the source faded to V[FORMULA]13.1 (JD 2450525), although this is not the fainter state observed in Mkn 421. From our data it is impossible to estimate the beginning of the outburst, nevertheless visual estimates reported by the Variable Star Network (VSNET; http://www.kusastro.kyoto-u.ac.jp/vsnet/) show that it probably started around the end of October 1996, only a few weeks before our first detection. Therefore the outburst time scale was [FORMULA] months.

[FIGURE] Fig. 2. V light curves of Mkn 421 during the large outburst observed in autumn-winter 1996-97. Observations are obtained at Tuorla (box), Torino (triangle), Crimea (diamond) and Perugia (star).

The light curve shows many flares superimposed to the overall trend. Very interesting is a flare that occurred around JD 2450158 (15 March 1996, see Fig. 3), with a temporal scale of a few days and a V amplitude of half a magnitude. Although the maximum is reported as only one photometric point, it is the average of eighteen observations and therefore it is reliable. Many other flares have similar time-scales but smaller amplitudes. In any case, we have never observed rapid flares with temporal scales less than a day (however, we have not obtained well sampled intranight observations).

[FIGURE] Fig. 3. V light curves of Mkn 421 during early 1996

In Fig. 4 we report the colour index (U-I) as a function of the V magnitude: we observe a reddening of the spectral index when the object is fainter. The reddening is well visible with the other colour indices, too. This is a typical feature discovered in other BL Lacs that have an evident host galaxy, therefore this phenomenon is generally interpreted as due to the thermal emission of the galaxy (Hagen-Thorn et al., 1983; Sillanpää et al., 1988): when the synchrotron continuum of the AGN becomes less luminous then the spectrum of the galaxy is well observable and the colour index is redder.

[FIGURE] Fig. 4. Behaviour of the (U-I) colour index vs V magnitude

2.2. Polarimetry

The polarimetric study of Mkn 421 has a long history. First polarimetry of this source was made in 1973 by Ulrich et al. (1975) and Hagen-Thorn & Semenova (1974). Successively, many works were made by various groups of observers from the ultraviolet to near infrared regions (see, e.g., Mead et al. 1990, Takalo 1991, Takalo et al. 1992, Takalo & Sillanpää 1993). The object showed moderate polarization varying from about 1% to 7% (Takalo, 1991), with the position angle of polarization varying in the range [FORMULA]-[FORMULA]. Frequency dependent polarization (FDP) was found increasing toward the short wavelengths, while the wavelength dependence of position angle (FDPA) was not so definite. The data on circular polarization are very rare and controversial: no circular polarization was found in May 1976 (Maza et al., 1978), but small circular polarization was detected in one night in January 1992 (Takalo & Sillanpää, 1993).

Our photopolarimetric observations of Mkn 421 were carried out at the Crimean Astrophysical Observatory. All polarimetric data were corrected for instrumental polarization, calculated from the measurements of standard stars with low and high polarization.

The general behaviour was very similar in all wavebands. The results in the V band, since 1995, are shown in Fig. 5, where we can note that most of the time the degree of polarization varied between 0% and 5%. During the 1997 outburst the degree of polarization rose up to [FORMULA] 14% and there was clear correlation between the variations of polarization degree and the brightness of Mkn 421. No such high polarization was detected before ([FORMULA] on JD 2450466.6).

[FIGURE] Fig. 5. V linear polarization of Mkn 421

An historical analysis of the polarization angle (PA) is reported by Efimov et al. (1998). Mkn 421 showed a rotation of the position angle of about [FORMULA] from May to October 1995. However, such rapid jumps of position angle of polarization were observed also in 1974-1980 (Hagen-Thorn et al.,1983), in April 1985 and in January 1990 (Valtaoja et al., 1991).

The comparison of the V light curve with the observed polarization shows that the enhancement of the linear polarization is strictly related to the great outburst, but the polarization decays faster than the source brightness. The polarization angle do not seem to be affected by the optical outburst.

There are also evidences for changes in the polarization with frequency (FDP) in this object, while no definite FDPA was seen. Some FDP, selected from our observations, are shown in Fig. 6.

[FIGURE] Fig. 6. FDP examples

2.3. Radio observations

Our radio observations were carried out with the Metsähovi 13.7 meter radiotelescope using 22 and 37 GHz receivers. These observations are part of an ongoing quasar monitoring program running at Metsähovi since 1981.

The 22 GHz and 37 GHz light curves of Mkn 421 are reported in Figs. 7-8, where we can see an enhancement of the radio emission peaking around January-February 1997. We have to remember that this is a very rare phenomenon in Mk421 which is normally a very stable and weak radio source. Very interesting is the fact that the radio outburst has a delay of only a few months with respect to the optical outburst, and they have the same temporal scale. Therefore, they are probably correlated.

[FIGURE] Fig. 7. 22 GHz flux curve (in Jy) of Mkn 421

[FIGURE] Fig. 8. 37 GHz flux curve (in Jy) of Mkn 421

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© European Southern Observatory (ESO) 1998

Online publication: September 30, 1998