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

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 agenda

Management team: Rainer Beck, Wolfgang Reich, Ger de Bruyn

Working 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

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

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

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)

Overview of scientific themes in Magnetism KSP

— 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 ….

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 !

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/N issue !

Powerful new tool to combat at least the latter two effects is

RM Synthesis

Low frequency polarimetry aspects

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(2i2) d

where F() is the complex polarized power per unit Faraday depth first defined by Burn (1966), and W(2) is the window 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.

Current WSRT RMSF’s and sidelobes

311 - 381 MHz (~ 1m)

RMSF halfwidth ~ 11 rad/m2

138 - 157 MHz (~ 2m)

RMSF halfwidth ~ 3 rad/m2

Exquisite RMSF’s at LOFAR - frequencies !

115 - 185 MHz 35 - 75 MHz

halfwidth ~ 1.0 rad/m2 halfwidth ~ 0.05 rad/m2

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

Fantastic RM-accuracy at LOFAR frequencies !

Polarized radiation transfer in a ‘Burn-slab’

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

thermal

nonthermal

Burn slabs & Faraday depth: ‘depth depolarization’A ‘back-to-front’ Faraday depth of

60 rad/m2 at 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/m2 will almost completely

quench the polarized signals !

Hence to see linear polarization

at < 200 MHz requires extremely

Faraday thin emission regions

Milky Way: distribution of cosmic-ray electronsUse tomographic techniques via absorption/emission

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)

WSRT 325 MHz polarisation in our Galaxy’s 2d quadrant

Schnitzeler et al (2007)

Haverkorn et al (2003)

Still very strong polarised signals !

WSRT LFFE observations of the FAN area

• 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++ Gianni Bernardi et a

l,

poster

Polarised intensity distributions in/around ‘the ring’

RM = - 5 rad/m2 RM = - 2 rad/m2

No good model as yet beyond: ‘ionised magnetized bubble …..’

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’

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

VLA/Effelsberg data VLA/Effelsberg data

POLINT 6cm POLINT 20cmPOLINT 6cm POLINT 20cm

Galaxies get bigger at low frequencies but start to Faraday depolarize Galaxies get bigger at low frequencies but start to Faraday depolarize at 20cm (Faraday depth ~ 60 rad/mat 20cm (Faraday depth ~ 60 rad/m22))

NGC6946 polarization studies Beck, 2007

NGC 6946 Faraday rotation gradient/structure

WSRT data 18+21cm Heald 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

Modification of background emission by Faraday rotation in a foreground galaxy (Fomalont et al. 1989)

Extragalactic Faraday screens

NGC 1310 NGC 1310 against against Fornax AFornax A

Intergalactic magnetic fields? Last frontier …

— 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

The radio galaxy B1834+62

Schoenmakers et al (2000)

z = 0.54, 1.2 Mpc size !

all 4 lobes polarized

RM = 56 - 60 rad/m2

(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 ?)

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