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Circularly polarized emission from the transient bursting radio source GCRT J1745-3009: Emission from magnetized dw arf ?
Subhashis Roy NCRA-TIFR India
Collaborators: Scott Hyman, Sabyasachi Pal, Joseph Lazio, Paul S. Ray, Namir Kassim


The GCRT: Introduction


Bursting transient radio source J1745-3009 discovered by Hyman et al. (2005) at 330 MHz.



On 10 minutes, in each period of 77 minutes during discovery observation.
Brightness temp >1015 K if near the GC.





Likely to be coherent emission. 77 min too high for a typical pulsar.




GC R T


Radio detections


GMRT observations in 2003. Serendipitous detection from 2004 SNR data.
Light curve of GCRT emissions at different epochs. These have been folded with a periodicity of 77 minutes.




Results...


Results ...


Very steep spectrum (-13 ±3) (Hyman et al. 2007). Reanalysis of 2003 outburst.




Circularly polarised emission

Stokes V difference image made from data integrated between 35 to 103 and 103 to 155 seconds respectively from the scan start and bridging the polarisation reversal. Rms noise 19.2 mJy.Beam-1 .


Discussions


Stokes V reversal within 17sec. Size < 8.R. Brightness temperature >1010 K for distance >1 pc, and circular poln. rules out thermal and incoherent synchrotron emission. Integration time > known pulsar periods. Pulse averaged Pulsar Cir. Poln. frac. <<100%. Pulsar based scenario ruled out.






Discussions ...


Cyclotron or plasma emission produces high circular polarisation. Bandwidth 0.1. (~30 MHz). Cyclotron -- magnetic field 120 Gauss, typical in stellar corona. Plasma emission ­ 10 ions.cm , less likely in brown dwarf corona.
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Discussions ...
Distance and Classification:


TB of cyclotron emission could reach up to ~1020 K (Melnik 1994, Slee 1969). Distance upper limit 100 kpc (1 kpc, or a cool dwarf L5 star at ~200 pc (Kaplan et al. 2008).



Limits from IR observation:





Distance limits ...


Flare stars (typically dwarf stars of class G to M) in the Galaxy could emit strong high Cir. Pol. radio emission.

History: More than 50 obs. from 1958 (Gershberg 2005) in ~tens of MHz to ~GHz frequency.


Mostly M dwarfs (e.g., V 371 Ori, UV Cet, YZ CMi, EV Lac and AD Leo). Highest known luminosity: 230 Jy at 136 MHz from Orion nebula (400 pc away) (Slee & Higgins 1969). Comparable GCRT luminosity (2002 outbursts) places it ~4 kpc away (similar to IR limit).






Comparison with flare star Cir. Poln.


Cir. Poln. from flare stars 0 to 100% (AbadaSimon et al. 1994). 40-60% of Cir. Poln. in 8 cases (Nelson et al. 1979). Lang et al. (1983) -- 15% Cir. Poln. from AD Leo.





Reversal in Cir. Poln. seen from AD Leo (Jackson et al. 1989). Varying mode coupling change Poln. fraction and poln. sense reversal (Dulk 1985, Melrose 1980).




Periodicity of 2002 outbursts


Stellar flares not known to occur periodically. Rotation period 77 min. of the spot of emission ? Measured rotation period decreases toward lower mass (0.1.M) stars to ~0.1 day [e.g., in Pleaides, Irwin et al. (2008)]. Few of these ultracool dwarfs produce flares at radio frequencies (Burgasser & Putman 2005). Within one rotation there could be two pulses. One detected with radio flare varying with a time period of 2 hours (Hallinan et al. 2007).








Candidate progenitor of the GCRT


Kaplan et al. (2008) IR observation identified one object (C) as a possible counterpart. IR flux densities are not well fitted by emission from an ultracool dwarf (L4.5V). Goodness of the fit remains similar if their K7V star is replaced by a ~0.1.M star ~0.1 Gyr old (young mid to late M type of star) with Av ~3.5 (distance ~4 kpc).






Conclusions


Detection of time varying high (up to 100%) circular poln. from the GCRT J1745-3009. TB >10 K for distance >1 pc shows coherent emission. Properties inconsistent with Pulsar, but could result from cyclotron or plasma emission. Radius <8.R. 77 min periodicity could correspond to half of rotation period. GCRT could be outbursts from a highly subsolar flare star with a distance 4 kpc.
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Time series of radio emission from M9 dwarf TVLM 513-46546 (Hallinan et al.)