The Neupert Effect in Active Stellar Coronae: Chromospheric Evaporation and Coronal Heating in the dMe Flare Star Binary UV Ceti

Abstract

Evidence for coronal heating by chromospheric evaporation in flares ofactive dMe stars is presented through observations of the Neuperteffect in high-frequency microwaves and soft X-rays. The Neuperteffect, as originally found in solar flares, manifests itself in aclose similarity between the soft X-ray light curve and the timeintegral of the simultaneous microwave light curve. It is interpretedas the signature of the accumulation of hot plasma due to heating byaccelerated electrons in the chromosphere.We used the ROSAT and ASCA soft X-ray observatories and the Very LargeArray (VLA) radio telescope (at 6 cm and 3.6 cm wavelengths) tomonitor simultaneously the nearby dMe flare star binary Gliese 65 A +B = UV Ceti during 9 hours on each of two consecutive days. We findseveral weakly polarized radio events that start contemporaneously(within a few minutes) with X-ray flares and then peak and decay asthe X-ray flares develop gradually. A striking similarity to thetemporal evolution of solar gradual events is found. We argue thatthe Neupert effect is best observed in relatively hard bands of thesoft X-ray emission, but that its presence can be inferred from themuch softer bands commonly used for stellar observations by use of thesolar analogy. Together with spectral hardness observations of softX-rays, the data suggest the operation of chromospheric evaporation onUV Cet. The observations thus indicate a causal relation between thenonthermal and thermal energies of the underlying electronpopulations.We find that stellar flares are, relative to solar flares, X-ray-weak.The ratio between the total energy radiated into the radio and thesoft X-ray bands closely matches the corresponding ratio between thequiescent luminosities of active stars, perhaps implying similarmechanisms and similar efficiencies for the quiescent emission and forlarger, single flares. Estimating the total kinetic energy in theelectrons from the radio flux, we find that only a part is observed insoft X-rays, a discrepancy well known from solar flares.