We present an 8.5-hour simultaneous radio, X-ray, UV, and optical observation of the L dwarf binary 2MASSW J0746425+200032. We detect strong radio emission, dominated by short-duration periodic pulses at 4.86 GHz with P = 124.32 ± 0.11 min. The stability of the pulse profiles and arrival times demonstrates that they are due to the rotational modulation of a B ≈ 1.7 kG magnetic field. A quiescent non-variable component is also detected, likely due to emission from a uniform large-scale field. The Hα emission exhibits identical periodicity, but unlike the radio pulses it varies sinusoidally and is offset by exactly 1/4 of a phase. The sinusoidal variations require chromospheric emission from a large-scale field structure, with the radio pulses likely emanating from the magnetic poles. While both light curves can be explained by a rotating mis-aligned magnetic field, the 1/4 phase lag rules out a symmetric dipole topology since it would result in a phase lag of 1/2 (poloidal field) or zero (toroidal field). We therefore conclude that either (i) the field is dominated by a quadrupole configuration, which can naturally explain the 1/4 phase lag; or (ii) the Hα and/or radio emission regions are not trivially aligned with the field. Regardless of the field topology, we use the measured period along with the known rotation velocity (vsini ≈ 27 km s −1 ), and the binary orbital inclination (i ≈ 142 • ), to derive a radius for the primary star of 0.078 ± 0.010 R ⊙ . This is the first measurement of the radius of an L dwarf, and along with a mass of 0.085 ± 0.010 M ⊙ it provides a constraint on the mass-radius relation below 0.1 M ⊙ . We find that the radius is about 30% smaller than expected from theoretical models, even for an age of a few Gyr. The origin of this discrepancy is either a breakdown of the models at the bottom of the main sequence, or a significant mis-alignment between the rotational and orbital axes.

As part of our on-going investigation into the magnetic field properties of ultracool dwarfs, we present simultaneous radio, X-ray, and Hα observations of three M9.5-L2.5 dwarfs (BRI 0021-0214, LSR 060230.4+391059, and 2MASS J052338.2−140302). We do not detect X-ray or radio emission from any of the three sources, despite previous detections of radio emission from BRI 0021 and 2M0523−14. Steady and variable Hα emission are detected from 2M0523−14 and BRI 0021, respectively, while no Hα emission is detected from LSR 0602+39. Overall, our survey of nine M8-L5 dwarfs doubles the number of ultracool dwarfs observed in X-rays, and triples the number of L dwarfs, providing in addition the deepest limits to date, log(L X /L bol ) −5.With this larger sample we find the first clear evidence for a substantial reduction in X-ray activity, by about two orders of magnitude, from mid-M to mid-L dwarfs. We find that the decline in both X-rays and Hα roughly follows L X ,Hα /L bol ∝ 10 −0.4×(SP−M6) for SP M6. In the radio band, however, the luminosity remains relatively unchanged from M0 to L4, leading to a substantial increase in L rad /L bol . Our survey also provides the first comprehensive set of simultaneous radio/X-ray/Hα observations of ultracool dwarfs, and reveals a clear breakdown of the radio/X-ray correlation beyond spectral type M7, evolving smoothly from L ν,rad /L X ≈ 10 −15.5 to ∼ 10 −11.5 Hz −1 over the narrow spectral type range M7-M9. This breakdown reflects the substantial reduction in X-ray activity beyond M7, but its physical origin remains unclear since, as evidenced by the uniform radio emission, there is no drop in the field dissipation and particle acceleration efficiency. Based on the results of our survey, we conclude that a further investigation of magnetic activity in ultracool dwarfs will benefit from a two-pronged approach: multi-rotation observations of nearby known active sources, and a snapshot survey of a large sample within ∼ 50 pc to uncover rare flaring objects. 10 The FeH Zeeman broadening technique leads to fields of 3.9 kG for EV Lac, 2.9 kG for AD Leo, and > 3.9 kG for YZ CMi (Reiners & Basri 2007), while the ZDI technique leads to much weaker fields of 0.5 − 0.6 kG, 0.2 kG, and 0.55 kG for the three objects, respectively (Morin et al. 2008).

As part of our ongoing investigation of magnetic activity in ultracool dwarfs we present simultaneous radio, X-ray, UV, and optical observations of LSR1835+32 (M8.5), and simultaneous X-ray and UVobservations of VB10 (M8), both with a duration of about 9 hr. LSR1835+32 exhibits persistent radio emission, and H variability on timescales of 0.5-2 hr. The detected UV flux is consistent with photospheric emission, and no X-ray emission is detected to a deep limit of L X /L bol P10 À5:7 . The H and radio emission are temporally uncorrelated, and the ratio of radio to X-ray luminosity exceeds the correlation seen in F-M6 stars by >2 ; 10 4 . The lack of radio variability during four rotations of LSR1835+32 requires a uniform stellar-scale field of~10 G, and indicates that the H variability is dominated by much smaller scales, <10% of the chromospheric volume. VB10, on the other hand, shows correlated flaring and quiescent X-ray and UV emission, similar to the behavior of early M dwarfs. Delayed and densely sampled optical spectra exhibit a similar range of variability amplitudes and timescales. Along with our previous observations of the M8.5 dwarf TVLM513À46546 we conclude that late M dwarfs exhibit a mix of activity patterns, which points to a transition in the structure and heating of the outer atmosphere by large-scale magnetic fields. We find that rotation may play a role in generating the fields as evidenced by a tentative correlation between radio activity and rotation velocity. The X-ray emission, however, shows evidence for supersaturation at v sin i > 25 km s À1 , which could be the result of secondary effects such as inefficient heating or centrifugal stripping of extended coronal loops. These effects may underlie the severe violation of the radio/ X-ray correlation in ultracool dwarfs.

We present the first simultaneous radio, X-ray, ultraviolet, and optical spectroscopic observations of the M8.5 dwarf TVLM 513-46546, with a duration of 9 hours. These observations are part of a program to study the origin of magnetic activity in ultracool dwarfs, and its impact on chromospheric and coronal emission. Here we detect steady quiescent radio emission superposed with multiple short-duration, highly polarized flares; there is no evidence for periodic bursts previously reported for this object, indicating their transient nature. We also detect soft X-ray emission, with L X /L bol ≈ 10 −4.9 , the faintest to date for any object later than M5, and a possible weak X-ray flare. TVLM 513-46546 continues the trend of severe violation of the radio/X-ray correlation in ultracool dwarfs, by nearly 4 orders of magnitude. From the optical spectroscopy we find that the Balmer line luminosity exceeds the X-ray luminosity by a factor of a few, suggesting that, unlike in early M dwarfs, chromospheric heating may not be due to coronal X-ray emission. More importantly, we detect sinusoidal Hα and Hβ equivalent width light curves with a period of 2 hr, matching the rotation period of TVLM 513-46546. This is the first known example of such Balmer line behavior, which points to a co-rotating chromospheric hot spot or an extended magnetic structure, with a covering fraction of about 50%. This feature may be transitory based on the apparent decline in light curve peak during the four observed maxima. From the radio data we infer a large scale and steady magnetic field of ∼ 10 2 G, in good agreement with the value required for confinement of the X-ray emitting plasma. A large scale field is also required by the sinusoidal Balmer line emission. The radio flares, on the other hand, are produced in a component of the field with a strength of ∼ 3 kG and a likely multi-polar configuration. The overall lack of correlation between the various activity indicators suggests that the short duration radio flares do not have a strong influence on the chromosphere and corona, and that the chromospheric emission may not be the result of coronal heating.