3 Radio and optical fluxes: Visual correlation analysis

Radio light curves of AO 0235+16 are shown in Fig. 8: data at 22 and $37\rm ~GHz$ are from the Metsähovi Radio Observatory, those at 4.8, 8.0, and $14.5\rm ~GHz$ are from the University of Michigan Radio Astronomy Observatory (UMRAO). The observations at the Metsähovi Radio Observatory were made with the $13.7\rm ~m$antenna using standard ON/ON techniques with DR 21 used as a calibration source. Details on the observing procedure and data reduction can be found in Teräsranta et al. (1992, 1998). The data from UMRAO were taken with the $26\rm ~m$ paraboloid of the University of Michigan. A description of the observing and data reduction procedures is included in Aller et al. (1985, 1999).

As in the optical band, also in the radio AO 0235+16 presents intense activity at all wavelengths, with pronounced outbursts lasting from several months to a few years. The overall flux variations (maximum value over minimum one) detected in the various bands are: 18, 22, 32, 28, and 31 at 37, 22, 14.5, 8.0, and $4.8\rm ~GHz$, respectively. In particular, in the last four years ( ${\rm JD} > 2450300$) variations up to a factor 18 were observed.

In Fig. 8 radio fluxes (in Jy) are compared to the optical ones (in mJy), obtained in the following way: all magnitudes in the B band before ${\rm JD}=2449000$ have been transformed into R ones adopting the mean colour index $<B-R>\ =1.65 \pm 0.16$ (which was derived by considering all the B-R pairs from a same observatory separated by no more than half an hour); these data plus the real R magnitudes after ${\rm JD}=2449000$ have been converted into fluxes by adopting Rieke & Lebovski (1985) and the law by Cardelli et al. (1989), and using a Galactic extinction $A_B=0.341 ~ \rm mag$ (from NED).

The existence of radio-optical correlations for AO 0235+16 was investigated in a number of previous works; evidence for a simultaneous radio and optical variability was found in correspondence to the optical flares of 1975 and 1979 (MacLeod et al. 1976; Ledden et al. 1976; Rieke et al. 1976; Balonek & Dent 1980) and, more recently, to that which occurred in 1997 (Webb et al. 2000).

Clements et al. (1995) analyzed optical data taken in the period 1977-1991 and radio data at $8.0\rm ~GHz$ from UMRAO with the Discrete Correlation Function (DCF), and found that: "Overall, radio events lag optical events with lag times varying from 0 to 2 months". Takalo et al. (1998) visually compared optical data from 1980 to 1996 with combined 22 and $37\rm ~GHz$ data from the Metsähovi Radio Observatory and noticed that some of the optical spikes appear to be coincident with radio flares, while others have no counterparts. Moreover, the general trend looked very similar in both frequency regimes, suggesting some kind of correlation.

Figure 8: Optical (mJy) and radio (Jy) light curves of AO 0235+16; radio data at 22 and $37\rm ~GHz$ are from the Metsähovi Radio Observatory, those at 14.5, 8.0, and $4.8\rm ~GHz$ are from UMRAO.

A visual inspection of Fig. 8 shows that the big optical outburst of 1975 has a big radio counterpart at 14.5 and $8.0\rm ~GHz$, while in 1979 a noticeable optical peak corresponds to a modest radio peak. Prominent radio outbursts at 22, 14.5, 8.0, and $4.8\rm ~GHz$ were observed in 1982, towards the end of the optical season, so that a possible optical peak might have been missed. A strong brightness increase was detected in 1987 in both optical and radio bands, but in this case the double-peaked optical flare seems to preceed the radio ones. The behaviour of the long radio outburst of 1990-1991 appears more complex; during the radio outburst a sharper optical flare was detected but not followed in details. The radio outburst in 1992-1993 was double-peaked; just before the first radio peak, an optical flare was detected; no other optical data were taken at the time when the radio fluxes reached their maxima. One interesting feature, however, is that if one looks at the better-sampled UMRAO data, the first radio peak seems delayed when proceeding from the higher to the lower radio frequencies. Indeed, the maximum value was reached on October 4, 1992 at $14.5\rm ~GHz$, on October 13 at $8.0\rm ~GHz$, and on November 3 at $4.8\rm ~GHz$. A radio flux increase in 1994 was practically not followed in the optical band.

From these considerations it is clear that the main difficulty in performing a meaningful study on possible radio-optical correlations is the paucity of optical data.

The situation has been noticeably improved in the last years, because of the intense observational effort of the monitoring groups involved in the present work. Indeed, the big outbursts occurred at the end of 1997 and in 1998 were accurately followed in all the radio bands and in the optical one (Fig. 9).

Figure 9: Optical (mJy) and radio (Jy) light curves of AO 0235+16 in the last years; radio data at 22 and $37\rm ~GHz$ are from the Metsähovi Radio Observatory, those at 14.5, 8.0, and $4.8\rm ~GHz$ are from UMRAO.

A preliminary discussion on the radio-optical correlations in this period was presented by Villata et al. (1999). On December 28, 1997 ( ${\rm JD}=2450811.33$), the R-band flux reached $9.19 \pm 0.68 ~ \rm mJy$ (R=14.03); four days before it was $3.90 \pm 0.11 ~ \rm mJy$ and 24 hours after the peak the flux had dropped to $2.85 \pm 0.13 ~ \rm mJy$. This very sharp peak was observed by two different groups (Perugia and Roma) and also in different bands (see Figs. 1 and 2). At the time of this optical flare, the radio fluxes were in a rising phase: they reached their maximum values later, likely first the shorter (observed by Metsähovi) and then the longer (UMRAO) wavelengths. The peaks at 37 and $22\rm ~GHz$ were detected on March 3 and 2, 1998, respectively, although a gap in the radio datasets does not allow to establish whether the peak of the outburst had already occurred. The maximum value of the $14.5\rm ~GHz$flux was seen on April 14, 1998 ( ${\rm JD}=2450918$), and that at $8.0\rm ~GHz$ on April 5, 1998, but in this case the peak probably occurred later. This peak-delay effect has already been quoted above for the 1992-1993 outburst. The radio outburst at 22 and $14.5\rm ~GHz$ is clearly double-peaked; this feature is recognizable also in the $8.0\rm ~GHz$ data. It is noticeable that the second radio peak could reasonably be contemporaneous at 22, 14.5, and $8.0\rm ~GHz$, the best sampled bands, and corresponds to a second optical flare detected at the beginning of the observing season, in July-August 1998. The solar conjunction period prevented to follow the rising phase of the optical outburst, so that the possibility that the optical peak also in this case preceded the radio ones remains open. A second brighter optical peak detected on August 7, 1998 ( ${\rm JD}=2451032.63$, R flux of $8.94 \pm 0.16 ~ \rm mJy$) occurred when the radio flux was in a decreasing stage. Another important, sharp optical flare was finally detected on September 4, 1999 ( ${\rm JD}=2451425.66$), which again has a radio counterpart, whose peak shows several weeks of delay.

The above discussion demonstrates that, notwithstanding the great observational effort of the last years, we are still far from having the sufficient sampling to derive firm conclusions on the radio-optical correlations. We can only notice that, in general, when the observational coverage is sufficiently good, a long time scale radio flux increase corresponds to a short time scale optical brightness increase, whose peak may precede the radio one. Moreover, there are at least two cases (the 1992-1993 and 1998 double-peaked outbursts) where a progressive time delay in reaching the maximum value is observed when passing from the higher to the lower frequency radio fluxes.