X-ray emission from nearby M-dwarfs: the super-saturation phenomenon

A rotation rate and X-ray luminosity analysis is presented for rapidly rota ting single and binary M-dwarf systems. X-ray luminosities for the majority of both single and binary M-dwarf systems with periods below similar or eq ual to5-6 d (equatorial velocities, V(eq)greater than or similar to6 km s(- 1)) are consistent with the current rotation-activity paradigm, and appear to saturate at about 10(-3) of the stellar bolometric luminosity. The single M-dwarf data show tentative evidence for the super-saturation ph enomenon observed in some ultra-fast rotating (greater than or similar to 1 00 km s(-1)) G- and K-dwarfs in the IC 2391, IC 2602 and Alpha Persei clust ers. The IC 2391 M star VXR60b is the least X-ray active and most rapidly r otating of the short period (P(rot)less than or similar to2 d) stars consid ered herein, with a period of 0.212 d and an X-ray activity level of about 1.5 sigma below the mean X-ray emission level for most of the single M-dwar f sample. For this star, and possibly one other, we cautiously believe that we have identified the first evidence of super-saturation in M-dwarfs. If we are wrong, we demonstrate that only M-dwarfs rotating close to their bre ak-up velocities are likely to exhibit the super-saturation effect at X-ray wavelengths. The M-dwarf X-ray data also show that there is no evidence for any differen ce in the X-ray behaviour between the single and binary systems, because fo r the single stars, the mean log LxLbol=-3.21 +/-0.04 (0.2 less than or sim ilar toP(rot)less than or similar to 10.1 d), whereas for the binary stars, the mean log LxLbol=-3.19 +/-0.10 (0.8 less than or similar toP(rot)less t han or similar to 10.4 d). Furthermore, we show that extremely X-ray active M-dwarfs exhibit a blue ex cess of about 0.1 magnitudes in U-B compared with less active field M-dwarf s. Such an excess level is comparable to that observed for extremely chromo spherically active M-dwarfs. Moreover, as is the case for M-dwarf Ca II H a nd K activity levels, there is an exclusion zone of X-ray activity between the extremely active M-dwarfs and the less active ones.