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2. Observations and results

The observations presented here were obtained during the OJ-94 Project (Takalo 1996). We have been monitoring AO 0235+164 in the optical bands (mostly in the V, R and I bands) since fall 1993. The optical telescopes used for these observations are listed in Table 1; all of them were equipped with CCD cameras. Details on the observing and data reduction procedures can be found in Katajainen et al. (1997), Fiorucci & Tosti (1996), Takalo et al. (1996) and Villata et al. (1997). The results of the optical monitoring are shown in Table 2. Magnitudes have been calculated using the calibration sequence given by Smith et al. (1985) and Nilsson et al. (1996). The radio observations have been done at the Metsähovi radio telescope in connection with their ongoing monitoring program (e.g. Teräsranta et al. 1992 and references therein).


Telescope Size Filters Code
JKT 1.0 m VR JKT
Torino 1.05 m R TO
Tuorla 1.03 m V TU
Boltwood 17 cm VRI BO
Perugia 40 cm VRI PE
Table 1: The used optical telescopes. The code refers to the observers used in Table 2 to identify the telescopes  

Figure 1 (click here) shows the historical B-band light curve with our observations included. The minimum brightness level previously observed for AO 0235+164 has been around B=20.0. By adopting an approximate B-V colour index of 1.2, our observation of V=19.80 corresponds to B=21.0, which is the faintest value ever recorded for this object in this band. We have to mention, however, that the colour of the object is also variable (e.g. Webb & Smith 1989). In Figs. 2 (click here) and 3 (click here) show our V and R light curves, while Fig. 4 (click here) displays the radio observations. In Fig. 5 (click here) we show our sampling dates at the different frequences. As can be seen there are a lot of simultaneous observations (data taken during the same day). Although both the optical and radio light curves have extended gaps in the data, some trends can still be recognized. The optical light curves show the object in a faint state, with some large variability. The quick brightening at JD 9750 and toward the end of the observations is very well documented. During spring 1996 (after JD 49900) AO 0235+164 brightened by almost 3 magnitudes in three months. During this phase we observed an intense activity: the most noticeable event was a one magnitude flare, lasting at most a couple days (Fig. 3). When AO 0235+164 became observable again, after conjunction, it was again very faint (Table 2, Fig. 2 (click here)). This kind of flaring behaviour is typical for AO 0235+164 (e.g. Webb & Smith 1989; Schramm et al. 1994).

Figure 1: The historical B-band light curve of AO 0235+164. This light curve is based on the data from Pica et al. (1976); Webb & Smith (1989); Takalo (1990); Takalo et al. (1992); Barbieri et al. (1982); Webb et al. (1988); Pollock et al. (1979); Smith et al. (1987); Xie et al. (1992); Sillanpää et al. (1988)

Figure 2: The observed V-band light curve of AO 0235+164

Figure 3: The R-band light curve obtained during spring 1996. Note the sharp flare at JD 50040

Figure 4: The observed radio light curves. The dots mark the 22 GHz and the black squares the 37 GHz data

Figure 5: A scatter plot, showing our sampling at the different observed frequences

For the nights where data of different bands are available, colour indexes have been derived: the result is B-V=0.98 and 1.11 from the two nights with B observations. These values are slightly larger than those observed during outbursts in this object (Webb & Smith 1989). The V-R values we measured range between 0.44 and 0.93. These are 0.2 to 0.5 magnitudes smaller than what has been observed when the object was in a bright state (e.g. Webb & Smith 1989). Our data reveal a clear tendency for V-R to be larger when the object is brighter. By fitting a power-law to the observed fluxes (tex2html_wrap_inline1108), we obtained a spectral index of -2.7 between the B and I bands. This is very similar to the value measured by Nilsson et al. (1996) from a spectrum of this object taken during January 1994. Thus the spectrum is flatter than seen during the outbursts (Webb & Smith 1989). Incidentally, now that AO 0235+164 is really faint, the intervening galaxies along the line of sight are expected to contribute more to the overall brightness, especially in the R and I-bands.

A less violent behaviour has characterized the emission in the radio bands (Fig. 4 (click here)). A very sharp increase in the flux is evident at JD 9450, when unfortunately optical data are lacking. There was a weak detection of AO 0235+164 by the EGRET instrument onboard CGRO in tex2html_wrap_inline1050-rays and by ASCA in X-rays at this time (Madejski et al. 1996). From the light curve shown in Fig. 4 (click here), it seems that a huge outburst was actually starting with this flux increase, which would end with the minimum registered on JD 10020. This minimum radio flux is the lowest level ever measured at these frequencies for AO 0235+164. The extent of this outburst cannot be clearly defined, since there is a large gap in the data. There are some indications that the radio flux is slowly increasing towards the end of the observations.

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