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

We present simultaneous ultraviolet and X-ray observations of the dMe-type flaring stars <ASTROBJ>AT Mic</ASTROBJ>, <ASTROBJ>AU Mic</ASTROBJ>, <ASTROBJ>EV Lac</ASTROBJ>, <ASTROBJ>UV Cet</ASTROBJ> and <ASTROBJ>YZ CMi</ASTROBJ> 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.