low frequency sky surveys with the murchison widefield array … · 2013-02-11 · low frequency...

Low frequency sky surveys with the

Murchison Widefield Array (MWA)

Gianni Bernardi

Harvard-Smithsonian Center for Astrophysics

SKA SA project/MeerKAT observatory

IRA-INAF, February 4th 2013

The MWA is an SKA low frequency precursor:

low frequency (80-300 MHz: 32 MHz

bandwidth), large-N (correlation rich) array,

3km maximum baselines;

Science cases:

• search for the 21cm emission from the Epoch

of Reionization;

• Solar and heliospheric science;

• radio transients;

• Galactic and extragalactic science (this talk

today);

Aperture array antenna elements, 4x4 arrays of

dual polarization dipole – “tiles”;

Initially 128 tiles, expandable to 256

350m

2008-2011: 32 element (32T) prototype

Survey concept and calibration

The drift scan maintains the tile primary beams

constant with time and all equal to each other,

because all the dipoles have zero delay (do not

underestimate this simplification!);

J0444-2809 (45 Jy @ 160 MHz, α = -0.81, a

few arcmin in size at 1.4 GHz) is used as flux,

phase and passband calibrator;

Each 5 min uv data set (“snapshot”) is

calibrated from the J0444-2809 solutions and

imaged. All the snapshots are mosaiced

together and deconvolved jointly;

Polarization leakage is less than 4% over 20º

in average less than 1.8%

(Calibration: Mitchell et al. 2008, Imaging: Ord et al. 2010, Deconvolution: Bernardi et al. 2011)

The primary beam model fitted to the data is

good at the 2% level over 30.72 MHz;

α

δ

0h

23h

22h

1h 2h 3h

4h

5h

-30

º -1

5º

6h

A 188.8 MHz drift scan survey with the 32T array (Bernardi et al. 2013, ApJ, submitted)

172-202 MHz, 15.6 arcmin resolution, 8 hour integration, 2400 square degrees

confusion limited at 200 mJy/beam (polarization noise ~ 15 mJy/beam)

α

α

α

δ

δ

δ

0h 23h

22h

1h 2h 3h 4h

5h

0h 23h

22h

1h 2h 3h 4h

5h

0h 23h

22h

1h 2h 3h 4h

5h

-30

º -1

5º

-30

º -1

5º

-30

º -1

5º

Stokes I

Stokes Q

Stokes U

The bright source sample (calibration accuracy)

A catalogue of 137 unresolved sources brighter

than 4 Jy (29 sources measured for the first

time below 200 MHz)

Comparison with 160 MHz measurement (Slee

1977) from the literature: 98% of sources

matched, 19% rms difference above 4 Jy.

The average spectral index between 188.8 and

160 MHz is α = -0.80 ± 0.17

Polarization: RM synthesis

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:

where F() is the complex polarized power per unit Faraday depth first

defined by Burn (1966), and W(2) is the window function

3) This relation can be Fourier inverted to yield F()

The quantity F() is convolved with a response function, called the RMSF,

which is the Fourier transform of the window function W(2) in 2 space.

The output of the RM synthesis is

a cube of images in Faraday depth space with 4.3 rad m2 resolution

22 2 2( ) ( ) ( ) iP W F e d

ϕ = 0 rad m2

ϕ = +2 rad m2

ϕ = +4 rad m2

ϕ = +6 rad m2

ϕ = +8 rad m2

ϕ = +34 rad m2

PMN J0351-2744

1.2% polarized

Where are polarized AGNs?

Radio sources at 1.4 GHz have an average

polarization fraction of ~ 7%, with peaks up to 20%

(Taylor et al. 2009);

A 4 Jy source, 7% polarized ~18σ detection…

ionospheric Faraday rotation 15% depolarization

why do we see only one polarized source?

20% polarized @ 1.4 GHz, 1.2% @ 188.8 MHz

J0351-2744 @1.4 GHz

(polarized intensity)

J0351-2744 @160 MHz

Stokes I

significant RM variations on scales

smaller than the sources size

(within the synthesized beam)

significant RM variations on scales smaller than the sources size (within the synthesized beam)

lead to beam depolarization:

p′ =p

p0= e2σRM

2λ4

J0351-2744: p’ ~ 18 σRM ~ 0.5 rad m2

very plausible also for smaller depolarization fractions

Where do RM variations occur?

“Small scale variation in the Galactic Faraday rotation”, Leahy ,1987, MNRAS, 226, 433

2 samples of 3C sources

variations due to faint HII

regions along the line of sight

(Galactic foreground)

What is the origin of the diffuse polarization?

Most of the diffuse polarization at low frequencies has no counterpart in total intensity, originated

by small scale structure in the ISM which rotates a fairly smooth polarized synchrotron

background

ne, B

uniform Stokes Q background

ne, B ISM clouds

emerging Stokes Q with

structure on the cloud size

Stokes Q structures detected

against a uniform background

which remains resolved out

What is the origin of the diffuse polarization?

Most of the diffuse polarization at low frequencies has no counterpart in total intensity,

originating by small scale structure in the ISM

ne, B

uniform Stokes Q background

ne, B ISM clouds

emerging Stokes Q with

structure on the cloud size

Stokes Q structures detected

against a uniform background

which remains resolved out

WSRT observations at 350 MHz, 5 arcmin resolution (Haverkorn, Katgert & de Bruyn, 2003)

Total intensity Polarized intensity

What is the origin of the diffuse polarization?

WSRT observations at 150 MHz, 4 arcmin resolution (Bernardi et al. 2009)

Total intensity Polarized intensity

What is the origin of diffuse polarization?

Polarized intensity @ 188 MHz Polarized intensity @ 1.4 GHz (Gaensler et al. 2011)

Low observed RM values ( < 15 rad m2) indicate that the emission should be more local than 120-

150 pc. Confirmed by the comparison with pulsars of known RM. LOCAL ISM

Magnetized, subsonic turbulence in the local, diffuse ionized gas is able to generate a complex

filamentary web of discontinuities in gas density and magnetic field (Gaensler et al. 2011)

The only resolved source: Fornax A

Lanz et al. 2009

two X-ray cavities

Either the 1.4 GHz radio image does not account for

the full radio emission or the central SMBH generated

at least two outbursts.

This question can be answered by high brightness

sensitivity observations, especially at low frequencies

An MWA 32T image of Fornax A

integrated flux @ 188.8 MHz:

~ 519 ± 26 Jy

Fornax A @ 1.4 GHz

with the VLA

FornaxA @ 1.4 GHz (VLA)

Fornax A is four beams across not

enough resolution to claim the

existence of a bridge at low

frequencies, need to wait for the 128T

Polarization in Fornax A?

Fornax A @ 1.4 GHz

with the VLA

Fornax A @ 1.4 GHz (VLA)

Polarization fraction at 1.5 GHz, 22” resolution

(Fomalont et al. 1989): dark is 40-65%, average

is 20%

Depolarization: region 5 is due to a foreground

elliptical galaxy which belongs to the cluster;

unknown the rest

No polarized emission detected at 188.8 MHz

Polarization fraction must be less than 1% (set

by the polarization calibration)

Easily explained by beam depolarization

MWA and all sky survey program development

• Deployment of the full array started in August 2012;

• New receivers, new hybrid CPU-GPU correlator;

• Commissioning of the full array started in August 2012;

• For practical reasons, the array was divided in four sub-arrays of 32T each (separate

deployment and commissioning);

• Deployment completed in December 2012;

• Full array operations (all the 128 tiles simultaneously) expected to start in February 2013;

• First call for proposal (including open sky) later in the year (see

http://mwatelescope.org/info/docs/MWA-AO-01.pdf);

beta array all-sky survey (114 MHz, 40 kHz bandwidth, max baselines ~380m). Courtesy N. Hurley-Walker

gamma array drift-scan survey (114 MHz, 40 kHz bandwidth, max baselines ~2.9 km). Courtesy N. Hurley-Walker

Conclusions

We have conducted a 2400 survey with the MWA 32 element array with 16 arcmin resolution at

189 MHz:

• Total intensity images are limited by confusion at ~200 mJy/beam. A source catalogue only

marginally improves over previous measurements

• Drift scans have been demonstrated to be a very effective observational strategy: beam

stability, gain (fairly) stability, relatively easy to calibrate and to obtain full polarization

images to be employed for the 128 all-sky survey;

• We detected polarization from only one catalogue source: a comparison with their cm-

wavelength polarization fraction indicate that they are likely to be beam depolarized

• A wealth of diffuse polarization across almost the whole area at low RM values with peaks up

to ~20 K/RMSF tracing turbulence in the local (< 150 pc) magnetized, diffuse, ionized gas

• The all sky survey program has continued during commissioning and it will receive dedicated

observing time in 2013 with the full 128T array (~200 h, full 90-200 MHz coverage, a survey

team and theme, lead by Dr. R. Wayth)