2007/08/10 by E. Berger, J. E. Gizis, John E. Gizis +16 · 4 citations
Physics and Astronomy · #Astronomy #Astrophysical Phenomena and Observations #Astrophysics #Balmer series #Emission spectrum #Flare #Galaxy #Gamma-ray bursts and supernovae #Light curve #Line (geometry) #Luminosity #Magnetic field #Physics #Rotation period #Spectral line #Stars #Stellar, planetary, and galactic studies #astro-ph
paper · pdf · doi:10.1086/524769
13 pages, 4 figures
arxiv created 2007/08/10 · openalex publication_date 2008/02/01 · arxiv updated 2009/12/01 · openalex created_date 2020/11/23 · openalex updated_date 2026/08/05
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 hr. 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 −5.1 , the faintest to date for any object later than M5, and a possible 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, ruling out chromospheric heating by 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 behavior points to a corotating 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. 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 multipolar 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 is not the result of coronal heating.