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LOFAR and the Magnetism Key Science Project Ger de Bruyn on behalf of the MKSP

LOFAR and the Magnetism Key Science Project Ger de Bruyn on behalf of the MKSP ASTRON & Kapteyn Institute. E-LOFAR meeting, Hamburg, 16-19 Sept 2008.

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LOFAR and the Magnetism Key Science Project Ger de Bruyn on behalf of the MKSP

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  1. LOFAR and the Magnetism Key Science Project Ger de Bruyn on behalf of the MKSP ASTRON & Kapteyn Institute E-LOFAR meeting, Hamburg, 16-19 Sept 2008

  2. Still young KSP (April 2006 kickoff) Project plan May 2007 (before rescope) Meetings in Bonn (3x), Leiden and Dwingeloo (June 08): about 20 - 25 people interestedMembership issue on tomorrows agendaManagement team: Rainer Beck, Wolfgang Reich, Ger de BruynWorking groups formed and Use Cases collected (James Anderson)Interested in contributing or want to know more? Come to our MKSP meeting tomorrow morning. Meeting agenda follows this afternoon (?) Organization and membership

  3. Outline • What is interesting about magnetic fields ? • Methods to study them? • Overview of current MKSP science themes • Magnetic fields and radio polarization • Low-frequency radio polarimetry : a difficult but exciting partnership • RM-synthesis: the why and how. Faraday thin please ! • Some examples (appetizers) of ‘low’ frequency polarimetry • Conclusions

  4. The Universe is one big plasma and magnetic fields are responsible for shaping the objects within (e.g. Sun, pulsars, Galaxies, radio sources,...). Understanding observed phenomena is hardly possible without understanding the magnetic field, indeed the magnetic energy density is often a dominant component. So what do we still want to know about them? How extended are galactic magnetic fields ? Are they strong enough to affect the dynamics in outer galaxies (halos, winds, interactions, general rotation) ? What can they tell us about the history of a galaxy ? Are they connected to intergalactic space ? What is their origin (primordial, dynamo, MRI) ? Open questions about magnetic fields

  5. Methods to study magnetic fields (strength, topology) • Nonthermal radio emission • - polarization studies • - equipartition arguments (synchrotron/IC losses, z-dependent) • - Galactic tomography (via absorption) • Faraday rotation: both a nuisance and a great diagnostic • - RM • - Faraday spectra (in mixed emitting/rotating media) • Pulsars (ratio RM/DM yields B) • UHECR (from ‘nearby’ AGN) and IGM propagation • Zeeman effect (HI, molecules) • Optical scattering (aligned dustgrains)

  6. Solar system (IPM) Stellar jets Our Galaxy (SNR, GMC, ……) Galactic foreground (…EoR nuisance) Disk and haloes of spiral Galaxies Dwarf galaxies AGN and giant radio galaxies (e.g. DDRG) Clusters , LSS and Cosmic Web anything that is polarized …. Overview of scientific themes in Magnetism KSP

  7. Overlap and synergy with other KSPs • EoR: Galactic foreground and ionospheric Faraday rotation • SRV: Nearby galaxies, clusters , AGN, RG’s • TRA: Transients often (circularly) polarized, e.g. ET’s TV • SSW: Interplanetary magnetic field • How to take the interests and contributions from all groups into account is going to take quite some interaction !

  8. Low frequency polarimetry aspects • Interstellar and ionospheric (time-variable) Faraday rotation  = o + RM.2 where RM   ne.B|| dl • Propagation and instrumentation lead to various depolarization effects:  beam need longer baselines (WSRT 3 km, LOFAR ~ 100 - 1000 km)  depth multiple layers along line of sight  bandwidth decoherence use multi-channel backends, but then S/Nissue ! Powerful new tool to combat at least the latter two effects is RM Synthesis

  9. RM synthesis Brentjens and de Bruyn (2005) 1) Linear polarization vector: P = Q + iU = p I e 2i where I, Q, U (V) are the Stokes parameters, p = % polarization, and  = 0.5 atan(U/Q) is the polarization angle 2) When observing the polarized power P at a range of 2 we can define: P(2) = W(2)  F() exp(2i2) d where F() is the complex polarized power per unitFaraday depth  first defined by Burn (1966), and W(2) is thewindow function of the instrument 3) This relation can be Fourier inverted to yield F() The derived quantity F(), also called the Faraday depth spectrum, is convolved with a response function, the RMSF, which gives the resolution in RM-space. In complex situations, deconvolution is still required. Note that the RMSF is the FT of the window function W(2) in 2 space. The output of RM synthesis is a cube of (Q,U) images in ‘Faraday depth’ space.

  10. Current WSRT RMSF’s and sidelobes 311 - 381 MHz (~ 1m) RMSF halfwidth ~ 11 rad/m2 138 - 157 MHz (~ 2m) RMSF halfwidth ~ 3 rad/m2

  11. Exquisite RMSF’s at LOFAR - frequencies ! 115 - 185 MHz 35 - 75 MHz halfwidth ~ 1.0 rad/m2 halfwidth ~ 0.05 rad/m2

  12. Fantastic RM-accuracy at LOFAR frequencies ! The plane of polarization rotates by:  = 2 RM. (/3) This angle can be measured to an accuracy:  ~ 1/2*SNR Where the Signal-to-Noise-Ratio equals: SNR~(S/SEFD) *(.t)0.5 Assume that we are dealing with steep spectra: S   -1±0.5 The accuracy in the measurement of RM then becomes: RM  2.5±0.5 . (/)-1.5 .SEFD Hence LOFAR ~ 100-1000 x better at 150 MHz than VLA/WSRT at 1500 MHz !! The relative bandwidth / and the SEFD (Jy) are similar for modern arrays: e.g. LOFAR: 30/150 MHz , 20 Jy VLA/WSRT: 300/1500 MHz , 15-25 Jy

  13. Polarized radiation transfer in a ‘Burn-slab’ thermal nonthermal A uniform mixed ‘slab’ of nonthermal emitting and thermal Faraday rotating plasma produces a sinc-like response in the observed P(2) response. There is a null in our polarized signal if: RM . 2 = n  radians (n=1,2,3 …) If RM. 2 >> 1 the medium is called ‘Faraday thick’. But note that a polarized background source can still be seen through it, i.e. Faraday depth  optical depth and, because B|| is sign-sensitive, Faraday depth  physical depth

  14. A ‘back-to-front’ Faraday depth of 60 rad/m2at 21cm(L-band) will significantly depolarize signals. This happens in some spiral galaxies (NGC6946, shown later) In the LOFAR HBA-band even a Faraday depth of only 2 rad/m2will almost completely quench the polarized signals ! Hence to see linear polarization at < 200 MHz requires extremely Faraday thin emission regions Burn slabs & Faraday depth: ‘depth depolarization’

  15. Milky Way: distribution of cosmic-ray electrons Use tomographic techniques via absorption/emission

  16. Full-sky ‘high-freq’ polarized images of our Galaxy 22.8 GHz WMAP image 1.4 GHz Reich et al (more sensitive, but serious depolarization effects already become visible)

  17. WSRT 325 MHz polarisation in our Galaxy’s 2d quadrant Still very strong polarised signals ! Schnitzeler et al (2007) Haverkorn et al (2003)

  18. Target location (J2000): 03h10m +65.5o 6x12h (9A=36,48,60,72,84,96m) in Nov/Dec 2007 (nighttime) Field of View ~ 6o HPBW, but all sky imaging needed 8 adjacent bands of 2.5 MHz from 138 -157 MHz each band has 512 spectral channels 10s integration time 400 GByte raw data Reduction/processing in AIPS++ WSRT LFFE observations of the FAN area Gianni Bernardi et al, poster

  19. Polarised intensity distributions in/around ‘the ring’ RM = - 5 rad/m2 RM = - 2 rad/m2 No good model as yet beyond: ‘ionised magnetized bubble …..’

  20. But there is more info  see e.g. Q and U images Note the very large polarization angle gradients at the edge of ‘the ring’

  21. Calibration challenges (but not only for the MKSP !) Faraday rotation by the ionosphere (rather mild now at 150 MHz) Instrumental polarized beams, time/position variable Automate polarization calibration ? Polarization typically <5% , rather than 10 - 50% at 6-21cm !  interactive processing needed  store uv-data for accurate calibration ! Analysis of 3-D RM-cubes and use in calibration major cycle ! Daytime versus nighttime ! LBA polarization (< 50 MHz) very tough ! and, Solar Max coming soon

  22. NGC6946 polarization studies Beck, 2007 VLA/Effelsberg data POLINT 6cm POLINT 20cm Galaxies get bigger at low frequencies but start to Faraday depolarize at 20cm (Faraday depth ~ 60 rad/m2)

  23. NGC 6946 Faraday rotation gradient/structure WSRT data 18+21cmHeald et al, in prep Interarm regoins have about 3 mJy/30” beam of polarized emission at 150 MHz IF not Faraday-thick and no beam-depolarization

  24. Modification of background emission by Faraday rotation in a foreground galaxy (Fomalont et al. 1989) Extragalactic Faraday screens NGC 1310 against Fornax A

  25. Use high redshift radio sources (Kronberg e tal, 2008) (relation to MgII absorbers at z=1-2 ?). Use giant radio sources ( > ~ 1 Mpc) Image cosmic web and LSS, e.g. around clusters Use DoA and (an-)isotropy of UHECR Intergalactic magnetic fields? Last frontier …

  26. Schoenmakers et al (2000) z = 0.54, 1.2 Mpc size ! The radio galaxy B1834+62 all 4 lobes polarized RM = 56 - 60 rad/m2

  27. (Lack of) depolarization in lobes of B1834+62 WSRT project on low freq polarization in giants with Tigran Arshakian et al (searching a.o. for LOFAR polarization calibrators) • Inner lobes depolarize at 350 MHz. • Outer lobes still at 13% polarization. • Data at 115-165 MHz in hand …. • very little internal gas in outer lobes but worries about beam depolarization  LOFAR • What is origin of RM difference of 3 rad/m2 between outer lobes ? • Either due to: • - our Galaxy (gradients ?) • - cocoons of lobes (rather high ne, B needed) • - variations in true IGM on scales of ~ 1 Mpc ? • RM ~3 rad/m2 ~ 10-6 x 0.1 G x 30 Mpc • (filaments of cosmic web ?)

  28. Conclusions Science applications: To study/use polarization we need Faraday thin regions  extreme conditions in plasma ! There is still Galactic polarization left at 2m wavelength In nearby galaxies: interarm regions and/or halo are best targets Extragalactic RM and IGM magnetic field : avoid beam depolar.! RM synthesis a powerful/essential tool Complicated distribution functions likely (both Faraday thin/thick) Many instrumental challenges ahead: some fundamental and some in processing Probably need to save uv-data (certainly initially)  reprocessing storage + cluster needed

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