Simultaneous Multiwavelength Observations of Magnetic Activity in Ultracool Dwarfs. I. The Complex Behavior of the M8.5 Dwarf TVLM 513−46546

Abstract

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.

Phase connecting multi-epoch radio data for the ultracool dwarf TVLM 513-46546

“…Looking at the Stokes I data, we can identify by eye four intervals of activity separated by ≈2 h; e.g. at ≈5.5 h, ≈7.4 h, ≈9.4 h and ≈11.3 h. These events were reported by Berger et al (2008) as random flaring events. We show in Fig.…”

“…2, data was also obtained in April 2007 for TVLM 513 (see Berger et al 2008). However, these authors did not report a periodicity, thus this data was obtained from the archive and reduced in a similar manner as the June 2007 data.…”

“…In fact, many authors, e.g. Berger et al (2008), initially interpreted both the non-pulsed emission and the pluses as gyrosynchrotron; however, if the ECM emission can become depolarized as it propagates through the ultracool dwarf magnetosphere, then a two-source model may not be required (Hallinan et al , 2009). Further work is however needed to investigate this aspect.…”

“…Many publications suggest gyrosynchrotron (e.g. Berger et al 2001) as the main source, in-fact, Berger et al (2008) argue towards the gyrosynchrotron interpretation as the A&A 524, A15 (2010) radio emission mechanism in UCDs for both the pulses and "quiescent" emission. A two emission model involving both incoherent gyrosynchrotron and the coherent electron cyclotron maser emission was suggested as a possibility by Hallinan et al (2006) for TVLM 513-46546 and by Berger et al (2009) for the observations of an L dwarf binary.…”

“…Here, we report on observations of TVLM 513-46546 (hereafter TVLM 513) taken in June 2007, these being followup observations to the earlier detection of pulses in data acquired in 2006. In addition, we re-analyze archival data, in particular, data taken in late April 2007 (Berger et al 2008), i.e. ≈40 days earlier than our data.…”