ATUC Science Meeting 24th Oct 2011

Radio emission from CU Virginis

Kitty Lo

Collaborators: Justin Bray, George Hobbs,

Tara Murphy, Bryan Gaensler, Don Melrose

ATUC Science Meeting 24th Oct 2011

Introducing CU Virginis

› CU Virginis is a magnetic chemically peculiar Ap star

› V magnitude of 5.01

› Rotation period of 0.52 days

› Polar magnetic field of ~3kG (determined optically)

› Distance of ~80pc

Source: DSS2 image from Aladin

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CU Vir emits two coherent radio pulses every rotation period

1.37GHz

1.67GHz

1.87GHz

2.17GHz

2.62GHz

Lo et. al, 2011 (submitted)

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The Pulsar lighthouse model

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Period of CU Vir

› Pyper (1998) found an abrupt increase in period of ~2 seconds around 1984

Spectrosopic variation versus phase (Pyper A&A 1998)

› More recently, Mikulášek (2011) shows CU Vir goes through intervals of rotational acceleration and braking

Period variation of CU Vir (Mikulasek A&A 2011)

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Radio observations

20cm 13cm

Lo et. al, 2011 (submitted)

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Electron Cyclotron Maser

› Electron cyclotron frequency, eB/2πm >> plasma frequency, e√(ne/m)/2π

- Usually in magnetized plasmas with low electron density and/or high magnetic field strength

› Population inversion in the electron distribution

- E.g. Loss cone distribution

Melrose & Dulk (1982)

Electrons with small pitch angle precipitate to the surface

Electrons reflected by magnetic mirroring

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Why ECM?

› High degree of circular polarization

› High brightness temperature (> 1010K)

› Highly beamed emission

› Very narrow band

› Emission frequency ~ cyclotron frequency or its second harmonic

- Magnetic field of Earth – 0.6G, ECM in kHz

- Magnetic field of CU Vir - ~3kG, ECM in GHz

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ECM is related to auroras

› ECM from Earth is known as the Auroral Kilometric Radiation (AKR)

› AKR is generated on auroral magnetic field lines above the auroral zone

› Typical frequency is 50 to 500kHz

› AKR closely correlated with occurrence of auroras (Gurnett 1974)

Mutel (2004)

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ECM has been observed on planets and brown dwarfs

Composite of optical and IR image from Cassini of Saturn

Radio light curve of TVLM-513 Hallinan et al, ApJ, 2007

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Model of CU Vir magnetosphere

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Simple ECM model does not produce the correct frequency dependence

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We can get the right frequency dependence if emission is along B field

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To get radio pulses to arrive at the right phase, emission need to be ducted

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Ducting of ECM

Ergun, ApJ, 2004

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Summary

› CU Vir is a unique source which emits pulses of coherent radio emission

› Electron cyclotron maser is believed to be the emission mechanism

› Simple models of ECM does not fit the frequency dependence of the pulse arrival time

› Propagation effect must play a role

› ASKAP, LOFAR, MWA could potentially find a lot more of these objects