C-Band All Sky Survey (C-BASS)

J. P. Leahy (PI, Manchester), M. E. Jones (PI, Oxford) Clive Dickinson (JPL)

AIMS: • Definitive survey of Galactic synchrotron radiation and its polarization • Anchor for synchrotron emission in future CMB polarimetry experiments up to CMBPOL. • Prototype for possible ground-based surveys at frequencies up to CMB band: 10, 15, 30… GHz • New window on Galactic magnetic field and cosmic rays

BPOL workshop 27th October 2006 Galactic foregrounds

WMAP polarized brightness: 23 GHz, 4° beam

• Sky is full of polarized interstellar synchrotron emission – 91% of pixels detected at this resolution • All components have significant spectral variations We must have more measurements than parameters!

BPOL workshop 27th October 2006 Foreground brightness

• Smoothing removes emission that fluctuates on scales smaller than the beam – E.g. CMB ‘E-modes’ • Similar amplitudes at 2° and 4°: – Looking at signal, not noise! – Polarization has little small- scale structure. • (Foreground, high latitude) • At 4° resolution, polarization clearly detected in 91% of pixels • Median: 15 µK @ 22.5 GHz • Histogram of polarization values at 22.5 GHz, 4° beam, with various masks

BPOL workshop 27th October 2006 4° Pol. Maps in other bands

33 GHz 41 GHz

61 GHz 94 GHz

BPOL workshop 27th October 2006 C-BASS motivation

• CMB polarization splits T into orthogonal modes: • E-modes fix optical depth to re-ionization – dramatic reduction in parameter degeneracy • B-modes define energy E scale of inflation • Obscured by Galactic B foreground emission – minimum at ~60 GHz – synchrotron below – dust above.

BPOL workshop 27th October 2006 C-BASS motivation: 60 GHz

• E-modes fix optical depth to re-ionization C-BASS – dramatic reduction in E parameter degeneracy 60 GHz • B-modes define energy scale of B inflation • Obscured by Galactic r = 0.1 foreground emission – minimum at ~60 GHz g in – synchrotron below s n e – dust above. L

BPOL workshop 27th October 2006 C-BASS motivation

• B-POL probably has primary frequencies at ≥ 90 GHz C-BASS • Satellite → nearly all sky survey: not just regions of minimum foreground • Even at 90 GHz, extrapolation of 22 GHz WMAP polarization B 90 GHz outside P06 mask (73% of sky) is Synch. larger than r=0.1 B-mode signal e is – For r=0.002, signal is 7 times o N weaker R E V • We must correct for O L synchrotron emission to get g C in s even close to B-POL sensitivity n e requirements, even for L > 90 GHz.

BPOL workshop 27th October 2006 Galactic Science

• Faraday-free polarization → Reliable magnetic field directions at medium & high latitudes: local disk and halo field lines (more distant than starlight polarization) • 0.4-1.4-5-23 GHz spectra allows tracking of both low-frequency spectral index and high-frequency curvature of synchrotron spectrum across the sky – Initial spectral index characteristic of acceleration sites – Convex curvature is signature of radiative losses – Concave curvature (if present) suggests preferential diffusion of high-energy cosmic ray electrons • New diagnostics for life-cycle (acceleration, diffusion, energy loss) of cosmic rays in the Galaxy.

BPOL workshop 27th October 2006 Foreground Separation

• Template fitting: – Using external templates (Hα etc) – Blind template construction (FASTICA etc) • Spectral fitting: – Independent analysis at each resolution element – Requires high S/N in large majority of pixels, at least at one frequency per foreground

BPOL workshop 27th October 2006 Synchrotron spectral are smooth!

• Power law is just an approximation… • …but a good one • The best-measured synchrotron sources are well fit by a 2nd- order log-log polynomial over 2 decades of frequency

BPOL workshop 27th October 2006 The Penticton Survey

• Wollaben, Landecker, Reich & Wielebinski (2006) • survey of northern sky polarization at λ21 cm with Pentiction 25-m dish • Comparison with WMAP: • Spectral index β: β – T(ν) = T0 (ν/ν0) • Faraday rotation RM: 2 – χ(ν) = χ0 + RM λ • Depolarization: – Unresolved RM structure

BPOL workshop 27th October 2006 Spectral Index 21:1.3 cm

• Affected by depolarization @ λ21 cm, especially near Galactic plane – Tail of relatively flat apparent spectral indices • Relatively well-defined peak at βP = −3.2 – Seems unaffected by depol. • C.f. usual assumptions: – (− 2.7 ≥ β ≥ −3) • Polarized emission steeper than total? • Less contaminated by free- free, spinning dust?

BPOL workshop 27th October 2006 Spectral Index: 1.3:3 mm

• Low sensitivity in WMAP data at λ < 1.3 cm gives limited sky coverage • Note flat spectrum for Crab nebula

• Mean βP ≈ −3.0 – Slightly flatter than at lower frequencies. (−3.1 in same regions)

BPOL workshop 27th October 2006 Faraday Rotation

• PA differences between WMAP bands (22.5 – 33 GHz) suggest large Faraday rotation near Galactic Centre: – 3°-4° at 1.3 cm, – RM ≈−700 rad m-2 • A few pixels show up to 22° rotation between 22.5-33 GHz – Random errors (~ 5σ, but non- Gaussian) – Change of emission mechanism • Away from Galactic plane, RMS (dust polarization?) Faraday rotation between λ1.3 cm – Very large RM?? and λ21 cm is 33° – < 3° at 6 cm – < 0.2° at 1.3 cm • Significantly less than Faraday rotation of extragalactic sources – Diffuse synchrotron emission is mixed with ionized layer.

BPOL workshop 27th October 2006 Pinning down the Galactic synchrotron spectrum

• Dust polarization well measured by Planck Thermal • Synchrotron dominates, at Dust best, only in lowest Planck channels Anomalous – need extra info to fix Dust spectrum. • WMAP takes us down only to 23 GHz Faraday Rotation – weak lever arm for extrapolation • Gap between 2.4 and 23 GHz Ground-based surveys needed to fix synchrotron emission

BPOL workshop 27th October 2006 Pinning down the Galactic synchrotron spectrum

BPOL workshop 27th October 2006 5 GHz because…

• Halfway between quasi-reliable surveys at 1.4 GHz (Stockert, Reich & Reich) and 23 GHz (WMAP). • Expected high-latitude Faraday rotation a few degrees, c.f. ~30° at 2.3 GHz. – Residual correction at high latitude via 1.4 GHz polarization survey from Penticton/Villa Elisa (Wolleben/Testori et al.) • Below main emission from anomalous dust, so predominantly synchrotron. • Signal still strong enough (few mK) to map the sky in a reasonable time (< 1 year) with a single receiver.

BPOL workshop 27th October 2006 Impact of C-BASS

• C-BASS adds value to all future CMB polarization experiments (Planck, Clover, B-pol etc)

– Planck (& Clover) alone hardly constrain synchrotron emission in the CMB band (~ 60 GHz)

– with C-BASS, get 5-7 times better

• C-BASS will be the definitive 5 GHz survey: will be cited for decades

BPOL workshop 27th October 2006 The Survey

• Novel purpose-built single-feed polarization and total power receiver (Manchester/Oxford) • Northern survey from OVRO 5.5 m dish (California) – sub-reflector tripod designed for low spillover – high accuracy surface (mm-λ telescope) • Southern survey from 7.6 m at Karoo (KAT) site, South Africa – high quality communication antenna • Exquisite control of spillover – new, large sub-reflectors – ground screens & baffles – simulations & measurements

OVRO 5.5 m BPOL workshop 27th October 2006 Receiver: combining technologies

• Novel architecture: analogue correlation radiometer + polarimeter • Unique ultra-stable cold load (collaboration with RAL) • Draws on current technology (e-MERLIN, Clover, Planck) – e-MERLIN amplifiers: broad-band, low-noise – correlation receiver prototyped under Oxford Experimental Cosmology grant BPOL workshop 27th October 2006 Survey Parameters

• FWHM resolution 52 arcmin – Same as 408 MHz survey – Smooth to 1º for high-latitude analysis, to reduce pixel noise

• Sensitivity: < 0.1 mK / beam rms. – Extrapolated map at 60 GHz has SNR > 2 for 90% of pixels even at high latitudes (outside WMAP polarization mask ‘P06’)

• Timescale: Complete by end 2010 – Northern survey released 2009

7.6 m Telescope

BPOL workshop 27th October 2006 Receiver Architecture

• Balanced radiometer + polarimeter, All-RF system, 20% bandwidth

• E-MERLIN 4-8 GHz LNA, Tsys < 20 K, BW ≅ 1 GHz • Analogue correlation polarimeter • Current technology (e.g. MERLIN, Planck) except for cold load

BPOL workshop 27th October 2006 Survey Strategy

• Based on Effelsberg experience • Long, fast sweeps – small dish can be scanned rapidly! • Full coverage of one quadrant of the sky after ~ 1 week. • Many observations per pixel – spread over many months – several different parallactic angles • Gives redundancy and robustness of polarization solution • Bonus: transients! Example 1-night coverage High sensitivity allows identification & control of systematics

BPOL workshop 27th October 2006 Project Partners

• Manchester: – front end systems and backend amps & filters – low-level and calibration software • Oxford: – cryostat, cold load, polarimeter and detectors, sub-reflector, optical design – mapping software • Caltech: – 5.5 m telescope, ground screen/baffles, digital backenFUNDEDd, control, site support • Rhodes/HartRAO: – 7.6 m telescope, ground screen/baffles, site support FUNDED

All partners contribute to observations, analysis & interpretation

BPOL workshop 27th October 2006 An Experienced Team

• Mike Jones: • Richard Davis: – Planck Radiometer Working – CAT, VSA, AMI, Clover, Group (Chair), MERLIN SKA telescope at Cambridge, VSA • Paddy Leahy: • Peter Wilkinson – Polarimetry with Planck, – SKA, VLBI etc Effelsberg, VLA etc • Ghassan Yassin – Clover, CAT, VSA optics • Tim Pearson: • Rod Davies – CBI, CCB, PGPLOT – Tenerife experiments, VSA, Planck • Justin Jonas: • Angela Taylor – Rhodes/HartRAO – VSA, AMI, CBI, Clover 2.3 GHz survey • + experienced US & SA teams.

BPOL workshop 27th October 2006 PDRA Tasks (18 month PRD period)

Manchester Oxford – OMT & polarizer design and – Radiometer/polarimeter simulations design and simulations – Other RF design, – Contribution to contribution to construction Radiometer/polarimeter & integration development, & integration – RF Testing – Optics simulations of – Calibration & foreground telescopes & groundscreen modelling software – mapping simulations & – Scan strategy simulations software – Commissioning at OVRO – Testing polarimeter and – Paper preparation cold load – Commissioning at OVRO – Paper preparation Vital experience for effective contribution to operations phase – continuity essential! BPOL workshop 27th October 2006 Impact of C-BASS

• Planck alone → Planck + C-BASS • Typical high-latitude pixel (2° beam): – Spectral index bias • Stokes I: −0.14 → 0.015 • Stokes Q,U: −0.16 → 0.03 – 70 GHz synchrotron amplitude error (assuming straight spectrum) • Stokes I σ: 0.9 µK → 0.3 µK (SNR: 3.5 → 12) • Stokes Q,U σ: 0.3 µK → 0.045 µK (SNR: 1 → 7) – 70 GHz synch. Amp. Bias • Stokes I: 0.9 µK → 0.15 µK • Stokes Q,U: 0.015 µK → 0.003 µK

5-7 times reduction in systematic synchrotron residuals in the CMB Band! BPOL workshop 27th October 2006 C-BASS: Summary

• C-BASS provides anchor for polarized synchrotron spectrum – c.f. also Parkes 2.3 GHz survey (Caretti et al.) • Requires at least one more frequency close to primary CMB frequencies to fix synchrotron spectral index (70-90 GHz) • We probably need 1 or 2 more intermediate frequencies, e.g. 10-15 GHz; 30-40 GHz – Fix spectral curvature – Check for polarized emission from anomalous dust, free-free – Can be obtained from ground/ VLDF balloon (especially if we can calibrate very large scales from space).

BPOL workshop 27th October 2006

UK Costings (PRD grant)

• Staff: • Equipment – 0.8 FTE Academic – £104k – 3 FTE PDRA • T & S: – 1.2 FTE Engineer – £22k – 2 FTE Technician • Estate & indirect – Direct costs £215k – £158.6k

FEC Total: £500k (pre-FEC: £416k)

BPOL workshop 27th October 2006 UK Phasing

• As suggested by PPARC secretariat: • C-BASS PRD Bid: – Receiver design & construction – Commissioning • C-BASS Exploitation Grant – submitted June 2007 – Observation, analysis, publication • Future Project bid – Submission 2009 if justified by C-BASS, CLOVER et al. – 10 GHz survey exploiting C-BASS technology

BPOL workshop 27th October 2006 C-BASS as a PRD scheme

• Exploitation of PPARC technology infrastructure? – World-class Expertise and equipment at Jodrell Bank and Oxford • High-Priority Science? – Internationally identified as such (e.g. Dark Energy Task Force report) • Novel technology? – New receiver architecture; stabilised cold load • Paves the way for UK intellectual leadership in international projects? – Provides leadership of international C-BASS project, and likely successor at 10 GHz • Paves the way for UK industrial return? – A 10 GHz multi-feed system would involve industrial contracts for receiver components (~ £1M) and possibly for custom telescopes (~£1M) • Pre-construction phase? – Exploratory research for a major instrument at 10 GHz, as well as versatile working 5 GHz instrument

BPOL workshop 27th October 2006 Timeliness

• Planck proprietary period ends Q1 2011 • We must start now to complete C-BASS (North & South) in time to incorporate in official Planck analysis. • Similar time-line for ground-based and balloon B-mode experiments (Clover, BICEP, QUIET, EBEX, SPIDER…).

BPOL workshop 27th October 2006 C-BASS Workpackage Breakdown WP 1 WP 2 WP 3 WP 4 WP 5 WP 6 Project Rx Design Optics Design Survey Design OVRO RFI Karoo RFI Management Characterisation Characterisation TJP/JPL/MEJ/JLJ Richard Davis Mike Jones Paddy Leahy Tim Pearson Justin Jonas

WP 7 WP 8 WP 9 WP 10 WP 11 WP 12 Rx Construction Rx Integration Rx Testing (UK) Software Prepare 5 m Rx Shipping & Telescope Installation/OVRO Mike Jones Mike Jones Paddy Leahy Tim Pearson Tim Pearson Mike Jones

WP 13 WP 14 WP 15 WP 16 WP 17 WP 18 OVRO Write technical Northern Survey Northern PR & Outreach Prepare 7.6 m Commissioning Papers Operations Data Analysis Telescope Tim Pearson PDRA Tim Pearson PDRA Erik Leitch Justin Jonas

WP 19 WP 20 WP 21 WP 22 WP 23 WP 24 Rx Shipping & Karoo Southern Survey Southern Combine Foreground Installation/Karoo Commissioning Operations Data Analysis Surveys Analysis Tim Pearson Justin Jonas Justin Jonas PDRA PDRA Clive Dickinson C-BASS WP Breakdown

WP 2 Rx Design R. J. Davis

WP 2.1 WP 2.2 WP 2.3 WP 2.4 WP 2.5 Specify Specify Specify Design Rx Cryo Design Rx Mechanical I/F JBO/Oxford I/F Oxford/CCB I/F Components Backend

WP 2.6 WP 2.7 WP 2.8 WP 2.9 Design Cold Design Design Adapt CCB Load Cryostat Polarimeter design

WP 3 Optics Design M. E. Jones

WP 3.1 WP 3.2 WP 3.3 WP 3.4 WP 3.5 WP 3.6 OVRO 5 m 5 m Karoo 7.6 m 7.6 m Ground Screen Subreflector Feedhorn Ground Screen Subreflector Feedhorn C-BASS WP Breakdown

WP 7 Rx Construction M. E. Jones

WP 7.1 WP 7.2 WP 7.5 WP 7.3 WP 7.4 RF cryo Backend amps Phase switch Cold Load Cryostat components & filters system

WP 7.6 WP 7.7 WP 7.8 Detectors Feedhorn CCB

WP 9 Rx Testing J. P. Leahy

WP 9.1 WP 9.2 WP 9.3 WP 9.5 WP 9.4 White Noise Bandpass 1/f noise Polarization Noise diode optimization measurement optimisation purity

WP 9.6 WP 9.7 WP 9.8 WP 9.9 Phase stability Cold Load Feed radiation Backend & zero point Stability pattern modes C-BASS WP Breakdown

WP 10 Software Tim Pearson

WP 10.2 WP 10.3 WP 10.4 WP 10.5 WP 10.1 Quick-Look Calibration Mapping Foreground Data logging Software Software Software Analysis S/W

WP 15 Northern Ops Tim Pearson

WP 15.1 WP 15.2 WP 15.3 WP 15.4 WP 15.5 Night-time Preventative Far-sidelobe Main Beam Cryo Scheduling Maintenance Mapping Mapping Maintenance Technology in place:

• E-Merlin C-band LNA: • 1/f knee, with differencing, ~ 1 mHz • Allows full rotation scan at ~ 1°/sec – Several times faster in practice

BPOL workshop 27th October 2006 C-BASS Motivation

• Holy Grail for CMB work: – ‘smoking gun’ of inflation: – B-mode polarization from E gravitational waves • < 3% of small-scale E- modes that are already B detected. • Accurate E/B separation r = 0.1 needs contiguous large solid angle.

• If B-modes too weak, g in masked by gravitational s n e lensing converting E→ B L

BPOL workshop 27th October 2006 5 GHz because…

BPOL workshop 27th October 2006 Impact of C-BASS

5-7 times reduction in systematic synchrotron residuals in the CMB Band! BPOL workshop 27th October 2006 A Proof of Concept

• The SPLASH survey (Abidin et al 2004) used the Effelsberg dish at 1.4 GHz to measure faint synchrotron polarization at high Galactic Latitude. • Absolute polarization levels recorded to within ± 8 mK, ~10% of mean signal. – Limited by relatively infrequent (90 min cycle) calibration to counter baseline drifts.

BPOL workshop 27th October 2006 Data Analysis

5 7 • Npix ~ 5x10 (cf Planck ~ 5x10 ) 9 13 • Ndata ~ 10 (cf Clover ~ 10 ) • Long-solved problem (e.g. Haslam et al 1981) • Improved techniques for eliminating residual striping, but all algorithms ∝ Ndata – No higher powers of N

BPOL workshop 27th October 2006 Competition?

• “Galactic Emission Mapping” • Recently began preparation for 5 GHz polarization survey • Operational at various frequencies since 1991 • No results to date • Originally intended to complement COBE • Sensitivity too low to achieve goals of C-BASS – 10 x noisier GEM Brazil

BPOL workshop 27th October 2006