X-ray detection of GJ 581 and simultaneous UV observations

Context. The M3 dwarf GJ 581 hosts a rich system of exo-planets, some of which are potentially within or at the edge of the habitable zone. Nevertheless, the system habitability might be reduced by large and sterilizing high energy emission flares, if these are frequent. <BR /> Aims The GJ 581 radiation environment was studied with simultaneous X-ray and UV observations, which were performed with the XRT and UVOT instruments, respectively, on board of the Swift satellite. <BR /> Methods X-ray and UV data were analysed with the distributed standard tools. <BR /> Results The dwarf GJ 581 was detected for the first time in the 0.2-10 keV range with an intensity of (8 2) 10<SUP>-4</SUP> cts/s and a signal-to-noise ratio of 3.6. If black-body or APEC spectra are assumed, then the source X-ray flux is found to be between 1.8 and 3.3 10<SUP>-14</SUP> erg cm<SUP>-2</SUP> s<SUP>-1</SUP> and log<SUB>10</SUB>(L<SUB>X</SUB>) between 25.914 and 26.176. Despite hints of X-ray variability, better statistics are needed to establish robust evidence for this property. The UV measurements, obtained during 13 pointings, are also reported. A combination of these Swift X-ray and Hubble Space Telescope UV measurements (with Lyman-alpha) indicate a low X-ray to UV luminosity ratio of ~4%. <BR /> Conclusions Simultaneous X-ray and UV observations of GJ 581 are reported. These constitute an experimental view of the system radiation environment, which will be a useful input for the habitability studies of the GJ 581 planetary system.