aristeidis noutsos the galactic magnetic field from pulsar rms and the low-frequency arrays...

Aristeidis NoutsosAristeidis Noutsos

The Galactic Magnetic The Galactic Magnetic Field from Pulsar RMsField from Pulsar RMs

and the Low-Frequency and the Low-Frequency ArraysArrays

Aristeidis NoutsosJodrell Bank Centre for Astrophysics,

Manchester, UK

Faraday Rotation of Pulsar Faraday Rotation of Pulsar EmissionEmission

Pulsars are amongst the most polarised radio sources.Some, e.g. Vela, are ¼100% linearly polarised!

LinearCircular

TotalVela

The plane of linearly polarised emission rotates (ΔPA) as highly-polarised pulsar emission propagates through the magnetised ISM.

The amount of rotation across the observation band is expressed by the RM:

PA

Telescope

ISMPulsa

r

B

d1d2

LOS

Potentially, field reversals can be revealed along the LOS

B

The Interstellar Magnetic Field The Interstellar Magnetic Field from Pulsar Rotation from Pulsar Rotation

MeasuresMeasuresOne can obtain the radial function of <B||> using pairs of nearly aligned pulsars, e.g. at d1 and d2

<B||

>

The average strength and direction of the B-field along the LOS to the pulsar is often estimated as

B||

B

d

<B||

><B||

>

<B||

>

Advantages:Pulsars …


MeasuresMeasures

… are highly polarised radio sources… are scattered throughout the entire Galactic volume

… lie at approximately known distances (derived from DM + ne models)

l=180ol=180o

60o240o

Galactic Hammer–Aitoff projectionNorth-pole projection of Galactic

plane

All known PSRsNE2001 ne

model

Bsy

nchr

otro

n

where’s my pulsar?!

(Actual pulsar position.)

Galactic Hammer–Aitoff projection

Problems, Problems …


MeasuresMeasures

We have RMs for only ~ 1/3 of the known pulsars:

• Some are simply too weak or their pulses are too scattered to measure their polarisation properties• Not all pulsars have measurable degree of polarisation Pulsar-distance estimates on the GP based on NE2001 can be up to ~ 20% in error. For high-b pulsars this error can be much higher (~ 50% in some cases!) (see Gaensler et al. 2008; PASA submitted)

… and more problems!The Galactic magnetic field can be seen as

a regular, large-scale field mixed with a turbulent, small-scale component.


MeasuresMeasures

<B||> ∝ RM∕DM only if ne(l) and B(l) are independent! For the turbulent field δ ne and δ B are correlated under/over estimation of <B||> (see Beck et al., 2003).

The existence of small-scale magnetised regions affect the mapping of the ISM field

from RMs.

+~ kpc~ 10 – 100 pc

HII region

 ~ 4 µG ~ 5 µG

New Pulsar Rotation New Pulsar Rotation Measures and the Galactic Measures and the Galactic

Magnetic FieldMagnetic Field

New Pulsar Rotation New Pulsar Rotation Measures and the Galactic Measures and the Galactic

Magnetic FieldMagnetic Field

l=180o l=180o60o240o

We performed independent measurements of 150 pulsar RMs at 20cm with the 64m Parkes telescope (Noutsos et al. 2008):

positive negative

l=180o

l=180o

60o240o

‣ 46 new RMs‣ 12 new RMs in Q1: a quadrant that benefited from the new sample

‣~20 RMs were revised from their previously published values

We plotted a map of the projected values of <B||> on the GP using all LOS with available pulsar data and looked for field reversals in the best sampled directions.

B|| towards observer

B|| away from observer

Large-Scale Magnetic Field Large-Scale Magnetic Field ReversalsReversals

CCW

CW

CCW



distance

l = 305–310o

A field reversal is seen between

Carina and Crux in Q4.

CCW

distance

l = 305–310o

Between 6–8 kpc, in the Crux arm, the field appears to reverse from

CCW to CW.

CW

CCW

CW

CW

distance

l = 280–285o

The field reverses from CW to CCW in the Carina arm region, where anomalous RM values have been reported (“Carina anomaly”; see e.g Han et al. 2006).

Local fieldQuadrant 1

Local field reverses from CW to CCW at ~ 1 kpc(confirms Lyne & Smith 1989).

Quadrant 4A reversal within r☉ ~ 2

kpc towards ℓ = 285–290º is consistent with Frick et al. (2001) within the distance uncertainties.

Local Neighbourhood Field Local Neighbourhood Field ReversalsReversals

Q1Q4

We selected 4 large-scale magnetic-field models to compare to the data

3 bisymmetric spirals +1 dipolar–toroidal model

face

-on

ed

ge-o

n

TT HMR

PS Dipol.–Toroidal

PS Dipol.–Toroidal

Bspiral + Bhalo

Bspiral + Bhalo

Bspiral + Bhalo +

Btoroidal + Bdipolar

Bdipole + Btoroidal

TT

HMR

PS

D.–T.

Testing the Large-Scale Field Testing the Large-Scale Field ModelsModels

Comparison with the data revealed that the large-scale component alone cannot explain the B fluctuations.

data

PS

TT HMR

Dipol.–Toroid.

Also, HII regions have a significant impact on the

RMs and cause an ‘anomalous’ variation of <B||> on top of a smooth large-scale component

(see e.g. Mitra et al 2003).

Testing the Large-Scale Field Testing the Large-Scale Field ModelsModels

LOFAR

MW

A LWA

RM Measurements withRM Measurements withLow Frequency ArraysLow Frequency Arrays

Pulsar All-Sky SurveysPulsar All-Sky Surveys‣Large effective area: ~ 105 m2 (i.e. full-size original LOFAR design)‣Wide field-of-view: multi-beaming capabilities provide wide instantaneous sky coverage‣Optimal sensitivity at frequencies where most pulsar spectra peak: ~ 100–200 MHz (high-frequency band)Huge potential for discovering new

pulsarsHuge potential for discovering new

pulsars

But the distance on the Galactic plane to which pulsars will be discovered by low-frequency arrays will be

limited by pulse scattering and sky background (high at low frequencies) Low frequency arrays will discover mostly low-DM

pulsars: Nearby pulsars on the Galactic plane and high-latitude

pulsars

All-Sky SurveysAll-Sky Surveys

X (kpc)

Y (

kpc)

–10 –5 0 5 10

05

10

Sun

Known pulsar-RM sky

van Leeuwen & Stappers (2008)

LOFAR 60-day Survey Simulation

X (kpc)

Y (

kpc)

–10 –5 0 5 10

05

10

Sun

1000+ pulsars detected1000+ pulsars detected~ 600 RMs measured~ 600 RMs measuredGC

Q1

Q2

Q1Q1

Q2Q2

New RMs in New Directions

will help map the large-scale field

A denser sample of RMs will

increase our knowledge of the

small-scale field

~ 100 pc

Polarimetry Polarimetry AdvantagesAdvantages

‣Large bandwidth (32 MHz) at low frequencies (20 ~ 300 MHz)

RM = 0 ?

Low-frequency arrays will have high sensitivity to small RMs, especially at their low-frequency bands: e.g. ~ 20–80 MHz.Hence, the small RMs (~ 1 rad m–2) of nearby, high-latitude pulsars will be accurately determined.

RM = – 0.3 rad m–2

Multi-channel spectro-polarimetry: RM synthesis* (Brentjens & de Bruyn 2005)

Decomposing RM Space Decomposing RM Space ‣High frequency resolution (up to 30,000 channels)

0 RM

A B

Pulsar

Regions of Faraday rotation and polarised

emission

RM

RM

TF ~ 0.1 rad m–2

BandwidthLow Band High Band

Selected frequency

rangesBy appropriately selecting the

frequency coverage, individual

RMs can be resolved down to ~

0.1 rad m–2 level

f

Side-lobes due to incomplete

frequency coverage

* RM Synthesis: decomposition of RM into discrete components (Fourier spectrum)

Ionospheric Faraday Rotation can contribute as much as ~ 5 rad m–2 (e.g. Junor et al. 2000).

By using e.g. bright, polarised pulsars as calibrators, low-frequency arrays can improve the ionospheric electron-density models and help correct for the systematic effect caused by Faraday rotation through the ionospheric plasma:

Ionospheric CalibrationIonospheric Calibration

MWA Science Goals (website)

Ionospheric Noon Sol. Max

Ionospheric Noon Sol. Min





This can reduce the systematic errors due to the ionosphere to as low as σiono ~ 0.01 rad m–2, thus improving pulsar-RM measurements.

The High-latitude SkyThe High-latitude SkyBy measuring the RMs of high-latitude pulsars, low-frequency arrays can shed light on the high-latitude Galactic B-field:

LOFAR 60-day Pulsar Survey Simulation

van Leeuwen & Stappers (2008)

GCz = 1.8 kpc

ne scale-height

B field @ y = 8.5 kpc

z < 0.5 kpc0.5 < z < 1.0 kpc1.0 < z < 1.5 kpc

z > 1.5 kpc

Latitudinal distribution of known pulsar RMs

Studies of high-latitude pulsars will also help improve

the electron-density models for high latitudes.

Studies of high-latitude pulsars will also help improve

the electron-density models for high latitudes.

SummarySummary

‣ Current efforts to map the Galactic Magnetic field using pulsar RMs show promise and have unveiled a number of features in the field’s structure (field direction, reversals, etc.)

‣ However, the sample of measured pulsar RMs is sparsely distributed across the sky, which makes the study of the large-scale Galactic magnetic field in certain directions difficult.‣ Low frequency arrays (LOFAR, MWA, LWA, etc.) have the potential to discover many more nearby and high-latitude pulsars, many of which will provide RM measurements in follow-up polarisation observations. The sensitivity of low-frequency arrays to the small RMs expected from this objects will provide accurate measurements.‣ The high frequency resolutions and high bandwidths of the low-frequency arrays open a new window into measuring the RM contributions of discrete sources of Faraday rotation along the LOS, as well as removing the systematic effects of the ionosphere.

‣ Current efforts to map the Galactic Magnetic field using pulsar RMs show promise and have unveiled a number of features in the field’s structure (field direction, reversals, etc.)

‣ However, the sample of measured pulsar RMs is sparsely distributed across the sky, which makes the study of the large-scale Galactic magnetic field in certain directions difficult.‣ Low frequency arrays (LOFAR, MWA, LWA, etc.) have the potential to discover many more nearby and high-latitude pulsars, many of which will provide RM measurements in follow-up polarisation observations. The sensitivity of low-frequency arrays to the small RMs expected from this objects will provide accurate measurements.

aristeidis noutsos the galactic magnetic field from pulsar rms and the low-frequency arrays...

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