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KuPol:
A New Ku-Band Polarimeter for the OVRO 40-Meter Telescope
Kirit Karkare Caltech Radio Astronomy Laboratory
CASPER Workshop – August 17, 2010
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In Collaboration With
Tony Readhead
Timothy Pearson
Kieran Cleary
Glenn Jones
Oliver King
Rodrigo Reeves
Vasiliki Pavlidou
Martin Shepherd
Walter Max-Moerbeck
Joey Richards
Matthew Stevenson
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The OVRO 40-Meter Telescope
Located near Big Pine, CA, 4 hours north of Los Angeles
Built in 1966
Alt-azimuth, f = 0.4
Previously used for VLBI with Parkes and CMB experiments
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The OVRO 40-Meter Telescope
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Current Activity
Monitoring 1158 candidate gamma-ray blazars CGRaBS objects with δ > -20°
In collaboration with Fermi Gamma-ray Space Telescope
(Healey et al, 2008)
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Blazars
Active galactic nuclei driven by matter accreting into supermassive black holes at the centers of galaxies
Blazars have jets oriented down line of sight
No accepted model for jet acceleration, emission, composition
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Science Goals
Correlate radio and gamma-ray light curves
– Choose between different models of jet composition, distance from central engine
Delay between radio and gamma-ray peaks can tell us where they are created in the blazar
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First Results – Light Curves
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First Results
Radio/gamma-ray flux density correlation is significant
Radio flux density
Gam
ma-
ray
flux
den
sity
Radio lagsRadio lags Radio precedesRadio precedes
Radio/gamma-ray time lags need longer duration light curves
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Current System
• Dual-beam Dicke-switch radiometer– Single band from 13-16 GHz, 30 K system temp– Lose a factor of sqrt(2) in sensitivity from ideal
receiver
• What would we like?– Increased sensitivity– Wider bandwidth– Spectral capabilities (not so important for blazars)– Polarization – variability is related to magnetic field
structure in jet emission region
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Current System
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New Receiver Plans
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New Receiver Plans
• Analog front end:– Combined correlation polarimeter and balanced
dual-beam radiometer• Intensity difference between two beams,
polarization through correlation
– 12-18 GHz• 12 * 500 MHz bands
– 20 K system temperature– RF over Optical link down the feed legs to the back
end in the control room
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Front End Plans
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New Receiver Plans
• Digital back end:– One ROACH, two iADCs for each of the twelve 500
MHz bands• ROACH at 250 MHz, iADCs at 1 GHz
– MHz spectral resolution– Identical programming for each ROACH• Inputs: (A_LCP – B_LCP), (A_LCP + B_LCP),
(A_RCP – B_RCP), (A_RCP + B_RCP)• FFT, Demodulate → A_LCP, A_RCP, B_LCP, B_RCP• Stokes → For each horn we get LCP_pow, RCP_pow, real
and imaginary components of Q and U
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Flexibility
• Each of the 12 * 500 MHz bands is independent – can add identical modules to increase bandwidth
• Different instruments on same receiver– High resolution
spectrometer– RFI excision
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Status
• Horn design complete
• Entire front-end RF chain purchased or being fabricated
– OMTs, waveguide phase shifters in fab queue at NRAO
• ROACH design almost complete
• Commissioning in early 2011
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Acknowledgements
CASPER Group
CfA travel funding
Caltech Summer Undergraduate Research Fellowship (SURF program)
Rose Hills Foundation SURF Fellowship