3. Rotational modulation of the radio emission
A very interesting result appears when we plot versus phase the data folded with a period equal to the orbital one, of 6.44 days, as shown in Fig. 3 (note: due to the synchronism this is the rotational period as well). Together with a general scatter of the data, indicating that the stellar activity is not related to the orbital period, this plot shows an evident hole of activity around phase 0.4. During the whole period of observations it was in fact noticed that the flux underwent sudden decreases when approaching the orbital phase 0.4. This decrease was of course much more evident if the star was flaring at that moment. During the phase interval between about 0.35-0.45 the observed flux values never exceeded 50-100 mJy, irrespective of the value observed before entering in this phase interval. This effect cannot be attributed to absorption by material between the two stars as already pointed out by Trigilio et al (1996) who observed the same phenomenon at =6 cm.
It is known (Klein and Chiuderi Drago, 1987; Franciosini and Chiuderi Drago, 1996) that, due to the magnetic field gradient in a magnetic loop, the radio emission at a given frequency is generated at a height increasing with decreasing frequency. Radio emission at high frequencies is therefore coming from the loop legs while the low frequency flux is coming from the loop top. Since the hole appearing in Fig. 3 is particularly evident for the high values of the flux, which, as already mentioned, are observed only at high frequencies, the only explanation for such a sudden flux decrease can be the disappearence of the corresponding source (i.e. the loop legs) behind the star. In other words such a minimum can be interpreted (Massi et al. 1996) by taking into account the 6.44 days rotation of the active star together with a two-component model for the emitting region: a component, compact enough to be oscured by the body of the star during each rotation and responsible for the high (up to 750 mJy) level of emission, and an extended component, always visible, responsible for the 50-100 mJy at phase 0.4. We note, in passing, that such a two-component model has been indeed observed by VLBI (Mutel et al. 1985). The extended component could be associated with the highest part of the loop, or alternately to a larger magnetized volume within the binary system filled by fast electrons escaped from the energy release site (Beasley and Bastian, 1997).
A very important result connected with the presence of this hole of activity at a particular phase is the stability in longitude of the stellar activity for 965 days. We recall here that at phase 0.5 the other star (a G5 V star), of this non eclipsing binary system, is between us and the active K0 star. As previously shown, around this phase the active region is located in the non visible side of the K0 star, consequently the preferred longitude on the K0 star for developing flares is in the hemisphere opposite to that facing the G5 V star. That the magnetic fields of the two stars might interact in the intervening medium has often been assumed by many authors after Uchida & Sakurai (1983) first advanced such an hypothesis. Our finding that no flare develops on the side facing the G5 V component could be the observational proof for a modified topology of the magnetic field between the two stars preventing any activity.
© European Southern Observatory (ESO) 1998
Online publication: March 10, 1998