Synchrotron and other Low-Frequency Science

J. P. Leahy

Beyond COrE Workshop, Paris

Picture: GALFACTS Collaboration

Overview Synchrotron Physics (Galactic) Component separation for foregrounds What can COrE tell us that we don’t

know already from WMAP and Planck? Current ground-based work What about extragalactic low-

frequency sources (AGN)?

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Synchrotron Physics

Cosmic ray leptons (electrons, mostly)› Origin› Propagation› Distribution

Magnetic Fields (at source)› Structure

Coherent/Ordered/Random› Intensity

Propagation effects› Faraday Rotation› Free-free absorption

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Origin of CR Supernova

remnants prominent in Galactic synchrotron emission

Strong shocks Pulsar winds

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Shock acceleration First-order Fermi (e.g. Bell 1978):

› N(E) E−s, I −, s = 2+1› s = (r+2)/(r − 1)› r = 4 for strong adiabatic shock: s = 2, = 0.5› r < 4 weak shock: s > 2, > 0.5› r = 7 relativistic shock

s = 4/3, = 0.167 (Test particle) Complicated (CR-dominated)

› r >> 4 cooling shock but fast particles don’t see this compression ratio

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SN 1006

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VLA (Dyer et al 2009) Chandra (NASA/CXC)

SN 1006 Shock front Radio-X-ray:

› = 0.50 0.02 › Decourchelle et al (2011)

X-ray synchrotron = 1.5› Cutoff in spectrum just below X-ray

band › Highest-energy electrons ~ 100 TeV

Thin shock suggests r > 4 Barrel shape:

› Efficient acceleration at parallel shocks? Contrary to Bell model! SN1006 has radial B-field

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Complexities Significant variation in

SNR spectral indices Pulsar Wind nebulae

flatter, e.g. Crab  = 0.3

Balance between steepening spectrum of old material and injection of new material:› Young SNR (+radio SN)

have steeper spectra.

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Spectral Index: 13:7 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)

Kogut et al (2007) claim detection of flattening from βP ≈ −3.2 to −3.0 from WMAP data alone…› Use smoothing from 7° to

18°› No allowance for pol. bias at

23 GHz: artifact?

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(3-year WMAP data)

Spatial distribution of Synchrotron External galaxies

show SR most intense in spiral arms

(M51 extreme case) Highest fractional

polarization in interarms› Field more ordered

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3D Emission models Milky Way also has distinct

radio spiral arms Consistent with arms in

NE2001 model. Cosmic ray analysis suggests

CRs very smoothly distributed› e.g. Fermi: outer galaxy ≈

constant density Major variation in B-field Hammurabi code

› Waelkens, Jaffe et al.› Sun et al (2008)› Jaffe et al (2010)

Fit:› 408 MHz I› 23 GHz p

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NE2001 electrondensity contours

Propagation effects Ambient spectrum of CRs in

ISM is steady state between injection & loss› radiative, diffusive,

convective Direct measurement in good

agreement with inferred spectrum from synchrotron emission (B ~ 6 G)

Detailed modelling suggests injection spectrum with several breaks, very hard at low frequency (s ~ 1.6)

C.f. Orlando, Strong & Jaffe (2011)

Jaffe et al 2011: E3 scaling GALPROP prediction fitted

to synchrotron data vs local e− spectrum.

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Galactic Halo: not a plane slab! 408 MHz I + 23 GHz P Minimum intensity at mid

latitudes Synchrotron monopole:

› Cosec|b|fit to Haslam map: zero level = 9.8 K (N), 10.1 K (S)

› But already zeroed to 3 K› Would give negative

intensity at faintest points› cf ARCADE2

Isotropic component!› Local bubble or halo?

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The Synchrotron Sky

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On large scale mostly dominated by coherent structures› Loops (local)› Fan (c. 1 kpc)› SNRs

Not a lot of scope for meaningful statistical analysis› higher resolution

needed!

Loops

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Only Loop I and top half of Loop III clear

??

? ?

NGP Polarization Polarization well

organized across NGP Fractional

polarization low outside Spur: ~ 10%› Complex structure

along LOS Field in Spur follows

outside field› Bright rim effect?

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Component separation Needed to do

foreground science as well as for CMB!

Since Planck designed, Anomalous Microwave Emission discovered› Significant contributor

10-60 GHz› Quite variable

spectrum, peak 20-40 GHz

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Polarized Foregrounds rms Q,U @

1° E B,

r = 0.1 3.4%anomalous dust 10%

thermal dust

artefacts at 100 & 217 GHz from CO lines

QUIJOTE

WMAP/Fermi/Planck “Haze”

Planck, after subtraction of templates for all major components

NB: template fit pretty good! ESA press release Feb 2012

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Fermi , E > 10 GeV NASA press release

Jan 2012

Interpreting the haze Existence of “haze”

conclusively demonstrated Interpretation:

› Two components,  = 0.5 and  = 1 Implicit in template method

› Region with single unusual  = 0.7

› Very hard to distinguish without ultra-precise measurements

› Illustration assumes 2% errors for WMAP/Planck, except at > 50 GHz

› Actual errors dominated by residuals from other foregrounds

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Continuous continuum Synchrotron spectra

are very smooth› Monoenergetic

kernel› Nearly power-law

electron distribution

Similarly› free-free› Spinning dust› Thermal dust

Synchrotron spectrum of mono-energetic electrons

Black body

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Continuous Continuum

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

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Power-law electron distribution?

Fermi shows electron spectrum really is smooth

NB: Emission frequency E

Stay tuned for AMS2 results

Departures from power law expected at high E where propagtion time from nearest source ~ radiative lifetime

~50 TeV? Optical band!

Assumes 6 G

Jaffe et al (2011)

COrE contribution More frequencies:

› Maybe we will have more frequency points than parameters to fit! Better-defined frequencies because bands are narrower

› Including significantly smaller colour correction› Hopefully more resources devoted to getting accurate bands in pre-

launch calibration High sensitivity: component discrimination depends on

differences between adjacent bands› Easy to run out of SNR› Especially if bands are close together

BUT fundamentally, with all components smooth, subdividing bands rapidly stops giving new information

Very likely CMB+AME+Synch + Free-Free + dust is fundamentally ambiguous

NB: monopole uncertainty doesn’t help.Beyond COrE Workshop, Paris

Meanwhile, back on the ground… Many surveys exist but

quality uneven› Strong et al (2011) for

an up-to-date list Complications:

› Below 300 MHz: free-free absorption, especially in the plane

› Above 1 GHz: free-free emission

› Below 5 GHz Faraday rotation

› Above 10 GHz: AME

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Guzmán et al 2011

GMIMS/STAPS: IQU at 21 cm Replacement for

DRAO/Villa Elisa 21 cm survey

Fully sampled, 30′ beam 1.3 -1.8 GHz 2048 channels

› for RM Synthesis I as well as Q U South from Parkes

› STAPS (PI Haverkorn) Also ‘low’ band: 300 -

900 MHz.

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Wolleben et al (2010, ApJL)

GALFACTS Continuum Transit

Survey with Arecibo L-Band Feed Array› 32% of sky

3 arcmin beam 300 MHz 2048 channels Final maps will use

GMIMS to recover large-scale structure

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GALFACTS fieldsoverlaid on Stockert Survey

S-PASS 2.3 GHz Southern-sky

polarization survey with Parkes

9′ beam Data collected,

processing under way Much less depolarized

than 21 cm. Figs from Carretti

(2011)› ATNF Newsletter

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C-BASS 5 GHz all-sky survey IQU, 48′ beam Planck-like pseudo-

correlation total power receiver combined with correlation polarimeter.

Telescope optimised to minimise sidelobes

Owens Valley (north) Karoo (south)

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QUIJOTE QUI JOint TEnerife experiment 11-30 GHz Telescope recently installed on Teide Aim to map ~ ¼ of sky at 15 GHz in full

polarization

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Point sources before WMAP Prior to WMAP, there were no large-area surveys from 6

cm to 100 m Previously, pioneering this much new territory has yield

unexpected phenomena› quasars, pulsars, X-ray binaries, Gamma-ray bursts, ULIRGs…

But WMAP and Planck LFI ERCSC show only expected blazar-type AGN

HFI point sources are also the expected SMGs Deeper surveys @ < 70 GHz with COrE-sized telescope

will rapidly hit the confusion limit› LFI close to confusion at 30 GHz

Ground-based surveys already far deeper› and SKA is coming…

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Extragalactic source (AGN) Old questions:

› What is the central engine?

› How does it make relativistic jets?

› How do jets propagate to Mpc scales?

› How does the AGN population evolve cosmologically?

New questions› What do AGN do to

their environment?› Regulate cluster

cooling?› Provide entropy

floor?› Use as Faraday

probes to study growth of cosmic magnetic fields

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Point sources

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FARADAY ROTATION IN RADIO GALAXIES

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Guidetta et al (2011)

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Faraday rotation in radio galaxies

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What we want to know about AGN

Radio/X-ray interactions to probe dynamics

VLBI movies to probe dynamics Variability Large samples for statistics/rare cases Polarization structure and wavelength-

dependence to probe local and line-of-sight Faraday rotation› Must be spatially resolved!

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The Competition

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ALMA

JVLA

GBT

Meerkat / SKA

LMT

ASKAP

The Competition (cont)

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Summary COrE features:

› many bands, high sensitivity, narrow(er) bandswill all help untangle low frequency foregrounds

› If frequency coverage extends 100 GHz› …and we avoid spectral lines!

BUT problem is fundamentally insoluable: not clear whether approximate solutions can be good enough to give interasting astrophysics.

COrE band is not of critical interest for synchrotron radiation

COrE measurements of AGN are not competitive with ground-based instruments.

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