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 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 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 • 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 C-BASS fills the gap! 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