Recurrent mini-outbursts and a magnetic white dwarf in the symbiotic system FN Sgr

Abstract

Aims. We investigated the optical variability of the symbiotic binary FN Sgr with photometric monitoring over a period of ≃55 years and with a high-cadence Kepler light curve lasting 81 days. Methods. The data obtained in the V and I bands were reduced with standard photometric methods. The Kepler data were divided into subsamples and were analysed with the Lomb-Scargle algorithm. Results. The V and I band light curves show a phenomenon never before observed with such recurrence in any symbiotic system, namely short outbursts starting between orbital phases 0.3 and 0.5 and lasting about 1 month, with a fast rise, a slower decline, and amplitudes of 0.5–1 mag. In the Kepler light curve, we discovered three frequencies with sidebands. We attribute a stable frequency of 127.5 d−1 (corresponding to a period of 11.3 min) to the white dwarf rotation. We suggest that this detection probably implies that the white dwarf accretes through a magnetic stream, as in intermediate polars. The small outbursts may be ascribed to the stream–disc interaction. Another possibility is that they are due to localised thermonuclear burning, perhaps confined by the magnetic field, such as those recently inferred in intermediate polars, albeit on different timescales. We also measured a second frequency around 116.9 d−1 (corresponding to about 137 min), which is much less stable and has a drift. This latter may be due to rocky detritus around the white dwarf, but is more likely caused by an inhomogeneity in the accretion disc. Finally, there is a third frequency close to the first one that appears to correspond to the beating between the rotation and the second frequency.