C-Band All Sky Survey (C-BASS)

BPOL workshop 27th October 2006

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


Galactic foregrounds

• 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!

WMAP polarized brightness: 23 GHz, 4° beam


C-BASS motivation

Lens

ing CLO

VER N

oise

B 90 GHzSynch.

• B-POL probably has primary frequencies at ≥ 90 GHz

• Satellite → nearly all sky survey: not just regions of minimum foreground

• Even at 90 GHz, extrapolation of 22 GHz WMAP polarization outside P06 mask (73% of sky) is larger than r=0.1 B-mode signal

– For r=0.002, signal is 7 times weaker

• We must correct for synchrotron emission to get even close to B-POL sensitivity requirements, even for > 90 GHz.

C-BASS


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


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:

– T(ν) = T0 (ν/ν0)β • Faraday rotation RM:

– χ(ν) = χ0 + RM λ2

• Depolarization:– Unresolved RM structure


Spectral Index 21:1.3 cm


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?


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)


• Dust polarization well measured by Planck

• Synchrotron dominates, at best, only in lowest Planck channels – need extra info to fix

spectrum.

• WMAP takes us down only to 23 GHz– weak lever arm for

extrapolation• Gap between 2.4 and

23 GHzGround-based surveys needed to fix synchrotron emission

ThermalDust

Faraday Rotation

AnomalousDust

Pinning down the Galactic synchrotron spectrum


• Dust polarization well measured by Planck

• Synchrotron dominates, at best, only in lowest Planck channels – need extra info to fix

spectrum.

• WMAP takes us down only to 23 GHz – weak lever arm for

extrapolation• Gap between 2.4 and

23 GHzC-BASS fills the gap!

ThermalDust

Faraday Rotation

AnomalousDust

Pinning down the Galactic synchrotron spectrum


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


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


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


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


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 backend, control, site

support• Rhodes/HartRAO:

– 7.6 m telescope, ground screen/baffles, site support

All partners contribute to observations, analysis & interpretation

FUNDEDFUNDED


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!


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).


UK Costings (PRD grant)

• Staff:– 0.8 FTE Academic– 3 FTE PDRA– 1.2 FTE Engineer– 2 FTE Technician– Direct costs £215k

• Equipment– £104k

• T & S: – £22k

• Estate & indirect– £158.6k

FEC Total: £500k (pre-FEC: £416k)


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


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


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…).

C-BASS Workpackage Breakdown

WP 1Project

ManagementTJP/JPL/MEJ/JLJ

WP 2Rx Design

Richard Davis

WP 3Optics Design

Mike Jones

WP 4Survey Design

Paddy Leahy

WP 7Rx Construction

Mike Jones

WP 8Rx Integration

Mike Jones

WP 9Rx Testing (UK)

Paddy Leahy

WP 5OVRO RFI

CharacterisationTim Pearson

WP 6Karoo RFI

CharacterisationJustin Jonas

WP 11Prepare 5 m

TelescopeTim Pearson

WP 10Software

Tim Pearson

WP 12Rx Shipping &

Installation/OVROMike Jones

WP 13OVRO

CommissioningTim Pearson

WP 14Write technical

PapersPDRA

WP 16Northern

Data AnalysisPDRA

WP 18Prepare 7.6 m

TelescopeJustin Jonas

WP 19Rx Shipping &

Installation/KarooTim Pearson

WP 20Karoo

CommissioningJustin Jonas

WP 21Southern Survey

OperationsJustin Jonas

WP 22Southern

Data AnalysisPDRA

WP 23CombineSurveysPDRA

WP 24Foreground

AnalysisClive Dickinson

WP 15Northern Survey

OperationsTim Pearson

WP 17PR & Outreach

Erik Leitch

WP 2Rx DesignR. J. Davis

WP 2.2Specify

JBO/Oxford I/F

WP 2.1Specify

Mechanical I/F

WP 2.4Design Rx Cryo

Components

WP 2.5Design Rx Backend

WP 2.6Design Cold

Load

WP 2.7Design

Cryostat

WP 2.8Design

Polarimeter

WP 2.9Adapt CCB

design

WP 3Optics DesignM. E. Jones

WP 3.25 m

Subreflector

WP 3.1OVRO

Ground Screen

WP 3.35 m

Feedhorn

WP 3.4Karoo

Ground Screen

WP 3.57.6 m

Subreflector

WP 3.67.6 m

Feedhorn

WP 2.3Specify

Oxford/CCB I/F

C-BASS WP Breakdown

C-BASS WP BreakdownWP 7

Rx ConstructionM. E. Jones

WP 7.2Backend amps

& filters

WP 7.1RF cryo

components

WP 7.4Cryostat

WP 7.5Phase switch

system

WP 7.6Detectors

WP 7.7Feedhorn

WP 9Rx TestingJ. P. Leahy

WP 9.4Noise diode

WP 9.5Polarization

purity

WP 9.6Phase stability

& zero point

WP 7.8CCB

WP 7.3Cold Load

WP 9.7Cold LoadStability

WP 9.1White Noise optimization

WP 9.2Bandpass

measurement

WP 9.8Feed radiation

pattern

WP 9.9Backendmodes

WP 9.31/f noise

optimisation

C-BASS WP BreakdownWP 10

SoftwareTim Pearson

WP 10.3CalibrationSoftware

WP 10.2Quick-Look

Software

WP 10.5Foreground

Analysis S/W

WP 15Northern OpsTim Pearson

WP 15.2PreventativeMaintenance

WP 15.1Night-timeScheduling

WP 15.3Far-sidelobe

Mapping

WP 15.4Main Beam

Mapping

WP 15.5Cryo

Maintenance

WP 10.4MappingSoftware

WP 10.1Data logging


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


C-BASS Motivation

• Holy Grail for CMB work: – ‘smoking gun’ of inflation: – B-mode polarization from

gravitational waves

• < 3% of small-scale E-modes that are already detected.

• Accurate E/B separation needs contiguous large solid angle.

• If B-modes too weak, masked by gravitational lensing converting E→ B Le

nsin

g

E

B

r = 0.1


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.


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!


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.


Data Analysis

• Npix ~ 5x105 (cf Planck ~ 5x107)

• Ndata ~ 109 (cf Clover ~ 1013)

• Long-solved problem (e.g. Haslam et al 1981)• Improved techniques for eliminating residual

striping, but all algorithms Ndata

– No higher powers of N


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

Documents

C-Band All Sky Survey (C-BASS)