Synchrotron and other Low-Frequency Science
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Transcript of Synchrotron and other Low-Frequency Science
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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