Relationship between X-ray and ultraviolet emission of flares from dMe stars observed by XMM-Newton

¹ Mullard Space Science Laboratory, University College London, Holmbury St. Mary, Dorking, Surrey RH5 6NT, UK e-mail: umk@mssl.ucl.ac.uk
² Paul Scherrer Institut, Würenlingen & Villigen, 5232 Villigen PSI, Switzerland
³ Columbia Astrophysics Laboratory, Columbia University, 550 West 120th Street, New York, NY 10027, USA
⁴ SRON National Institute for Space Research, Sorbonnelaan 2, 3584 CA Utrecht, The Netherlands
⁵ Astronomical Institute “Anton Pannekoek”, Kruislaan 403, 1098 SJ Amsterdam, The Netherlands
⁶ Observatoire de Paris, LESIA, 92195 Meudon, France
⁷ Konkoly Observatory, 1525 Budapest, Hungary

Received: 30 April 2004
Accepted: 30 September 2004

Abstract

We present simultaneous ultraviolet and X-ray observations of the dMe-type flaring stars AT Mic, AU Mic, EV Lac, UV Cet and YZ CMi obtained with the XMM-Newton observatory. During 40 h of simultaneous observation we identify 13 flares which occurred in both wave bands. For the first time, a correlation between X-ray and ultraviolet flux for stellar flares has been observed. We find power-law relationships between these two wavelength bands for the flare luminosity increase, as well as for flare energies, with power-law exponents between 1 and 2. We also observe a correlation between the ultraviolet flare energy and the X-ray luminosity increase, which is in agreement with the Neupert effect and demonstrates that chromospheric evaporation is taking place.