Transitions in magnetic behavior at the substellar boundary

We aim to advance our understanding of magnetic activity and the underlying dynamo mechanism at the end of the main sequence. To this end, we have embarked on collecting simultaneous X-ray and radio observations for a sample of M7..L0 dwarfs in the solar neighborhood using XMM-Newton jointly with the Jansky Very Large Array (JVLA) and the Australia Telescope Compact Array (ATCA). We supplemented the data from these dedicated campaigns with X-ray data from the all-sky surveys of the ROentgen Survey with an Imaging Telescope Array (eROSITA) on board the Russian Spektrum-Roentgen-Gamma mission (SRG). Moreover, we complement this multiwavelength data set with rotation periods that we measured from light curves acquired with the Transiting Exoplanet Survey Satellite (TESS). We limited the sample to objects with rotation period of < 1 day, focusing on the study of a transition in magnetic behavior suggested by a drastic change in the radio detection rate at υ sin i ≈ 38 km s⁻¹, corresponding to P_rot ≈ 0.2 day for a typical ultracool dwarf (UCD) radius of R_★ = 0.15 R_⊙. Finally, to enlarge the target list, we have compiled archival X-ray and radio data for UCDs from the literature, and we have analyzed the abovementioned ancillary eROSITA and TESS observations for these objects' analogous to the targets from our dedicated X-ray/radio campaigns. We compiled the most up to date radio/X-ray luminosity (L_R,ν − L_x) relation for 26 UCDs with rotation periods (P_rot) lower than 1 day, finding that rapid rotators lie the furthest away from the "Güdel-Benz" relation previously studied for earlier-type stars. Radio bursts are mainly (although not exclusively) experienced by very fast UCDs (P_rot ≤ 0.2 day), while X-ray flares are seen by objects spanning the whole range of rotation. Finally, we examined the L_x/L_bol versus P_rot relation, where our sample of UCDs spans a large activity level range, that is log(L_x/L_bol) = −5.5 to log(L_x/L_bol) = −3. Although they are all fast rotating, X-ray activity evidently decouples from that of normal dynamos. In fact, we found no evident relation between the X-ray emission and rotation, reinforcing previous speculations on a bimodal dynamo across late-type dwarfs. One radio-detected object, 2MJ0838, has a rotation period consistent with the range of auroral bursting sources; while it displays moderately circularly polarized emission, there is no temporal variation in the polarized flux. A radio flare from this object is interpreted as gyrosynchrotron emission, and it displays X-ray and optical flares. Among the ten UCDs observed with the dedicated X-ray/radio campaigns, we found a slowly rotating apparent auroral emitter (2MJ0752) that is also one of the X-ray brightest radio-detected UCDs. We speculate that this UCD is experiencing a transition in its magnetic behavior since it produces signatures expected from higher-mass M dwarfs along with emerging evidence of auroral emission.