Periodic Bursts of Coherent Radio Emission from an Ultracool Dwarf

Abstract

We report the detection of periodic (p = 1.96 hours) bursts of extremely bright, 100% circularly polarized, coherent radio emission from the M9 dwarf TVLM 513-46546. Simultaneous photometric monitoring observations have established this periodicity to be the rotation period of the dwarf. These bursts, which were not present in previous observations of this target, confirm that ultracool dwarfs can generate persistent levels of broadband, coherent radio emission, associated with the presence of kG magnetic fields in a large-scale, stable configuration. Compact sources located at the magnetic polar regions produce highly beamed emission generated by the electron cyclotron maser instability, the same mechanism known to generate planetary coherent radio emission in our solar system. The narrow beams of radiation pass our line of sight as the dwarf rotates, producing the associated periodic bursts. The resulting radio light curves are analogous to the periodic light curves associated with pulsar radio emission highlighting TVLM 513-46546 as the prototype of a new class of transient radio source.Comment: 12 pages, 3 figures, accepted for publication in ApJ Letter

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

“…From previous observations we know that the rotation period is ≈1.96 h (Hallinan et al 2006;Lane et al 2007). The main difference between the present data and the 2006 data reported by Hallinan et al (2007) is that the intervals of activity are longer, typically spread over 0.25 phase, comprising of several pulses. For the June 2007 data shown in Fig.…”

“…In addition, we have an inter-pulse at ≈3.8 h (best seen in Stokes I but also visible in Stokes V), i.e. 0.5 phase later than the pulse at ≈2.8 h. As shown by Hallinan et al (2007), due to the beaming of the ECM emission, additional activity can be observed ≈0.5 phase later. However, depending on the local density condition, this secondary activity 0.5 phase later is not always observed.…”

“…In January 2005, Hallinan et al (2006) observed the same target simultaneously at 8.4 GHz and 4.9 GHz for a total of 5 h. These authors reported a periodicity at both frequencies of ≈2 h (later confirmed as the rotation period of the dwarf, Lane et al 2007), with a ≈400 μJy flux at 8.4 GHz. The first report of pulsed emission from TVLM 513 was by Hallinan et al (2007) (2008) report a nonperiodicity in the pulses/flaring activity (see later). A summary of the radio observations of TVLM 513 is given in Table 1.…”

Atmospheric Electrification in Dusty, Reactive Gases in the Solar System and Beyond

“…By contrast, brown dwarfs are very bright radio emitters, but no correlation between X-ray and radio emission exists as is established for stars and solar events (Güdel-Benz relation, Benz and Güdel 1994). Since the discovery of the first radio emitting brown dwarf by Berger et al (2001), numerous surveys have detected radio emission at GHz frequencies in nearly 200 objects (Berger 2002(Berger , 2006Berger et al 2010;Hallinan et al 2006Hallinan et al , 2007Hallinan et al , 2008McLean et al 2012), including emission in the coolest brown dwarfs with spectral types as late as T6.5 (Route and Wolszczan 2012). In twelve of these objects, the radio emission is highly polarised, coherent and pulses on the rotation period of the dwarf.…”

“…In five of these systems, WD0137-349B (Maxted et al 2006), NLTT5306 (Steele et al 2013), SDSS141126þ200911 (Beuermann et al 2013), WD0837þ185 (Casewell et al 2012) and GD1400B (Farihi and Christopher 2004), the brown dwarf is known to have survived a phase of common envelope Fig. 13 Light curves of the total intensity (Stokes I) and the circularly polarised (Stokes V) radio emission detected at 8.44 GHz from TVLM 513-46546, an M9.5 dwarf, taken from Hallinan et al (2007). Right circular polarisation is represented by positive values, and left circular polarisation is represented by negative values in the Stokes V light curve.…”