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