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;
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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)
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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)