J/A+A/583/A137 LOFAR-HBA 3C196 field RM cubes (Jelic+, 2015)
Linear polarization structures in LOFAR observations of the interstellar medium
in the 3C196 field.
Jelic V., de Bruyn A.G., Pandey V.N., Mevius M., Haverkorn M.,
Brentjens M.A., Koopmans L.V.E., Zaroubi S., Abdalla F.B., Asad K.M.B.,
Bus S., Chapman E., Ciardi B., Fernandez E.R., Ghosh A., Harker G.,
Iliev I.T., Jensen H., Kazemi S., Mellema G., Offringa A.R., Patil A.H.,
Vedantham H.K., Yatawatta S.
<Astron. Astrophys. 583, A137 (2015)>
=2015A&A...583A.137J 2015A&A...583A.137J (SIMBAD/NED BibCode)
ADC_Keywords: Interstellar medium ; Polarization ; Magnetic fields
Keywords: ISM: general - ISM: magnetic fields - ISM: structure -
radio continuum: ISM - techniques: interferometric -
techniques: polarimetric
Abstract:
This study aims to characterize linear polarization structures in
LOFAR observations of the interstellar medium (ISM) in the 3C196
field, one of the primary fields of the LOFAR-Epoch of Reionization
key science project. We have used the high band antennas (HBA) of
LOFAR to image this region and Rotation Measure (RM) synthesis to
unravel the distribution of polarized structures in Faraday depth. The
brightness temperature of the detected Galactic emission is 5-15K
in polarized intensity and covers the range from -3 to +8 rad/m2 in
Faraday depth. The most interesting morphological feature is a
strikingly straight filament at a Faraday depth of +0.5 rad/m2
running from north to south, right through the centre of the field and
parallel to the Galactic plane. There is also an interesting system of
linear depolarization canals conspicuous in an image showing the peaks
of Faraday spectra. We used the Westerbork Synthesis Radio Telescope
(WSRT) at 350MHz to image the same region. For the first time, we see
some common morphology in the RM cubes made at 150 and 350MHz. There
is no indication of diffuse emission in total intensity in the
interferometric data, in line with results at higher frequencies and
previous LOFAR observations. Based on our results, we determined
physical parameters of the ISM and proposed a simple model that may
explain the observed distribution of the intervening magneto-ionic
medium. The mean line-of-sight magnetic field component is determined
to be 0.3-/+0.1uG and its spatial variation across the 3C196 field is
0.1uG. The filamentary structure is probably an ionized filament in
the ISM, located somewhere within the Local Bubble. This filamentary
structure shows an excess in thermal electron density
(ne*B||>6.2cm-3uG) compared to its surroundings.
Description:
Rotation Measure (RM) cubes of the 3C196 field in polarized intensity
(PI), Stokes Q and Stokes U, synthesized from the LOw Frequency ARray
(LOFAR) High Band Antennas (HBA) observation taken on 16 Dec 2012
(observational ID: L80508). The RM cubes cover a Faraday depth range
from -3rad/m2 to +8rad/m2 in 0.25rad/m2 steps. A resolution in
Faraday depth is 0.9rad/m2, while the largest Faraday structure that
can be resolved is 1.1rad/m2. Images are 8.3°x8.3° in size
with a PSF of 3.6-arcminx3.9-arcmin. Selected frames of these cubes
are presented in the paper in Figure 4.
A header of the FITS files provides more details about the RM cubes
using the standard FITS header keywords.
Objects:
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RA (2000) DE Designation(s)
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08 13 36.06 +48 13 02.6 3C 196 = 3C 196.0
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File Summary:
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FileName Lrecl Records Explanations
--------------------------------------------------------------------------------
ReadMe 80 . This file
list.dat 98 3 List of fits RMcubes
cubes/* . 3 Individual fits RM cubes
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Byte-by-byte Description of file: list.dat
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Bytes Format Units Label Explanations
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1- 11 F11.7 deg RAdeg Right ascension of image centre (J2000)
12- 22 F11.7 deg DEdeg Declination of image centre (J2000)
24- 26 F3.1 arcmin/pix Scale Pixel scale
28- 31 I4 --- Nx Number of pixels along X-axis
33- 36 I4 --- Ny Number of pixels along Y-axis
38- 39 I2 --- Nz Number of slices
41- 44 F4.1 rad/m2 bRM Lower value of Faraday depth interval
46- 49 F4.1 rad/m2 BRM Upper value of Faraday depth interval
51- 54 F4.2 rad/m2 dRM Step in Faraday depth
56 A1 --- Pol [PQU] Polarized intensity (P), Stokes Q or U
58- 60 I3 Mbyte FileSize Size of the file
62- 70 A9 --- FileName Name of the fits file in subdirectory cubes
72- 99 A28 --- Title Title of the file (1)
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Note (1): RM cubes are given in units of Jy/PSF/RMSF, where PSF is Point Spread
Function (3.6-arcmin x 3.9-arcmin) and RMSF is Rotation Measure Spread
Function (0.9rad/m2).
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Acknowledgements:
Vibor Jelic, vjelic(at)astro.rug.nl
(End) Vibor Jelic [RuG and ASTRON], Patricia Vannier [CDS] 09-Nov-2015