KuPol:

A New Ku-Band Polarimeter for the OVRO 40-Meter Telescope

Kirit Karkare Caltech Radio Astronomy Laboratory

CASPER Workshop – August 17, 2010

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

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

The OVRO 40-Meter Telescope

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)

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

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

First Results – Light Curves

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

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

Current System

New Receiver Plans

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

Front End Plans

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

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

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

Acknowledgements

CASPER Group

CfA travel funding

Caltech Summer Undergraduate Research Fellowship (SURF program)

Rose Hills Foundation SURF Fellowship