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 Gamma-ray flux density Radio lags Radio precedes Radio/gamma-ray time lags need longer duration light curves

Current System Dual-beam Dicke-switch radiometer – Single band from 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

Analog front end: – Combined correlation polarimeter and balanced dual-beam radiometer Intensity difference between two beams, polarization through correlation – 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