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

A rotation rate and X‐ray luminosity analysis is presented for rapidly rotating single and binary M‐dwarf systems. X‐ray luminosities for the majority of both single and binary M‐dwarf systems with periods below ≃5–6 d (equatorial velocities, V eq ≳6 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 phenomenon observed in some ultra‐fast rotating (≳100 km s −1 ) G‐ and K‐dwarfs in the IC 2391, IC 2602 and Alpha Persei clusters. The IC 2391 M star VXR60b is the least X‐ray active and most rapidly rotating of the short period ( P rot ≲2 d) stars considered 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‐dwarf 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 break‐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 difference in the X‐ray behaviour between the single and binary systems, because for the single stars, the mean log  L x L bol =−3.21±0.04 (0.2≲ P rot ≲10.1 d), whereas for the binary stars, the mean log  L x L bol =−3.19±0.10 (0.8≲ P rot ≲10.4 d). Furthermore, we show that extremely X‐ray active M‐dwarfs exhibit a blue excess of about 0.1 magnitudes in U–B compared with less active field M‐dwarfs. Such an excess level is comparable to that observed for extremely chromospherically active M‐dwarfs. Moreover, as is the case for M‐dwarf Ca  ii H and K activity levels, there is an exclusion zone of X‐ray activity between the extremely active M‐dwarfs and the less active ones.