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

Abstract

Context. Radio data obtained for the ultracool dwarf TVLM 513-46546 has indicated a rotation period of ≈1.96 h via regular radio pulses, but how stable is this period. This has major implications regarding the stability of the magnetic field structures responsible for the radio emission from the ultracool dwarf. Aims. The aim of the present work is to investigate the stability of this rotation period using two datasets taken ≈40 days apart, some 12 months after the first report of periodical pulses in the radio data. Methods. Here we use a Bayesian analysis method which is a statistical procedure that endeavours to estimate the parameters of an underlying model probability distribution based on the observed data. Results. Periodical pulses are detected in datasets taken in April and June 2007, with the pulses being confined to a narrow range in the rotation period. This is in contradiction to a previous report of only aperiodic activity in the April 2007 dataset, while in fact both datasets have a periodic signal with a false alarm probability 10 −12 . These two datasets are then used to derive a more accurate period (previously determined to be 1.96 h) of 1.96733 ± 0.00002 h. Conclusions. The similarly in the burst structure in datasets taken several weeks apart point towards the stability of an electric field structure which is somehow generated and sustained within the magnetosphere of the ultracool dwarf. The derived period of 1.96733 h is consistent with the period derived via radio and optical data taken some 12 months prior to the present observations and implies the near phase constancy of the pulsed emission. This suggest the presence of stable large-scale magnetic fields on timescales of more than 1 year. The characteristics of the pulses suggest that they are produced by the electron cyclotron maser (ECM) instability.

Modelling the environment around five ultracool dwarfs via the radio domain

“…Wolszczan & Route (2014) reported stable optical and radio emission for 7 and 5 years respectively, that produce the same period with precision up to ∼20ms. Berger et al (2008), Doyle et al (2010) and Wolszczan & Route (2014) report a correlation between the Hα peaks and radio flares with approximately 0.4 phase offset, suggesting that the optical and radio variability originates from a large-scale dipolar magnetic field that is stable for at least a decade. Wolszczan & Route (2014) also reported that the period between bursts may be gradually shortening, possibly due to migration of spots towards the equator and differential rotation.…”

“…Highly polarised radio bursts were reported by Hallinan et al (2007); Berger et al (2008) and Doyle et al (2010). Doyle et al (2010) showed the data contained a series of consecutive periodic bursts with stable shape of the light curve, but changing shape/structure of the bursts. Wolszczan & Route (2014) reported stable optical and radio emission for 7 and 5 years respectively, that produce the same period with precision up to ∼20ms.…”

325 MHz VERY LARGE ARRAY OBSERVATIONS OF ULTRACOOL DWARFS TVLM 513-46546 AND 2MASS J0036+1821104

“…Detected bursts were highly circularly polarized and lasted multiple minutes. Both the persistent and burst emission features were later confirmed by Osten et al (2006), Hallinan et al (2006), Hallinan et al (2007), Berger et al (2008), Forbrich &Doyle et al (2010) through observations at 8.5 GHz and 4.9 GHz. Further, Hallinan et al (2007) and Lane et al (2007, optical) detected a pulse periodicity of ∼2 hr, consistent with the stellar rotation rate.…”

Modelling the radio pulses of an ultracool dwarf

“…Figure 6 shows the comparison of our simulation with observations (see Hallinan et al 2010, for further details). The observation is taken from 2008 May 19 (UT) at 4725 MHz for the M8.5V dwarf TVLM 513.…”