Issue |
A&A
Volume 514, May 2010
|
|
---|---|---|
Article Number | A70 | |
Number of page(s) | 8 | |
Section | Astronomical instrumentation | |
DOI | https://doi.org/10.1051/0004-6361/200913834 | |
Published online | 21 May 2010 |
Measurement of the Crab nebula polarization at 90 GHz as a calibrator for CMB experiments
J. Aumont1,8 - L. Conversi2 - C. Thum3 - H. Wiesemeyer4 - E. Falgarone5 - J. F. Macías-Pérez6 - F. Piacentini7 - E. Pointecouteau8 - N. Ponthieu1 - J. L. Puget1 - C. Rosset9 - J. A. Tauber10 - M. Tristram11
1 - Institut d'Astrophysique Spatiale, Centre
Universitaire d'Orsay, Bât. 121, 91405 Orsay Cedex, France
2 - European Space Astronomy Center, PO Box 78, 28691 Villanueva de la
Cañada, Madrid, Spain
3 - IRAM - Institut de Radioastronomie Millimétrique, 300 rue de la
Piscine, 38406 Saint-Martin d'Hères, France
4 - IRAM - Institut de Radioastronomie Millimétrique, Avenida Divina
Pastora 7, Núcleo Central, 18012 Granada, Spain
5 - LERMA/LRA, CNRS UMR 8112, École Normale Supérieure and Observatoire
de
Paris, 24 rue Lhomond, 75231 Paris Cedex 05, France
6 - Laboratoire de Physique Subatomique et de Cosmologie, Université
Joseph Fourier Grenoble 1, CNRS/IN2P3, Institut National Polytechnique
de Grenoble, 53 avenue des Martyrs, 38026
Grenoble, France
7 - Dipartimento di Fisica, Università di Roma ``La Sapienza'', Rome,
Italy
8 - Centre d'Étude Spatiale des Rayonnements, Université de Toulouse,
CNRS, 9 Av. du Colonel Roche, BP 44346, 31028 Toulouse Cedex 4, France
9 - APC, Université Denis Diderot-Paris 7, CNRS/IN2P3, CEA,
Observatoire
de Paris, 10 rue A. Domon & L. Duquet, Paris, France
10 - European
Space Agency, Astrophysics Division, Keplerlaan 1, 2201AZ Noordwijk,
The Netherlands
11 - LAL - Laboratoire de l'Accélérateur Linéaire, Université Paris-Sud
11,
CNRS/IN2P3, Bât. 200, 91898 Orsay Cedex, France
Received 9 December 2009 / Accepted 16 February 2010
Abstract
Context. CMB experiments aiming at a precise
measurement of the CMB polarization, such as the Planck satellite, need
a strong polarized absolute calibrator on the sky to accurately set the
detectors polarization angle and the cross-polarization leakage. As the
most intense polarized source in the microwave sky at angular scales of
few arcminutes, the Crab nebula will be used for this purpose.
Aims. Our goal was to measure the Crab nebula
polarization characteristics at 90 GHz with unprecedented
precision.
Methods. The observations were carried out with the
IRAM 30 m telescope employing the correlation polarimeter XPOL
and using two orthogonally polarized receivers.
Results. We processed the Stokes I,
Q, and U maps from our
observations in order to compute the polarization angle and linear
polarization fraction. The first is almost constant in the region of
maximum emission in polarization with a mean value of
in equatorial coordinates, and the second is found to reach a maximum
of
% for the
most polarized pixels. We find that a CMB experiment having a 5 arcmin
circular beam will see a mean polarization angle of
and a mean polarization fraction of
%.
Key words: ISM: supernova remnants - polarization - cosmology: cosmic background radiation
1 Introduction
The Crab nebula (Tau A, M1 or NGC 1952, at
coordinates
and
,
J2000) is a supernova remnant that
emits a highly polarized signal due to both the synchrotron emission of
the
central pulsar and its interaction with the surrounding gas (see e.g. Hester 2008). In this
paper, we present the measurements of intensity and polarization of
the Crab nebula at 90 GHz (3.3 mm) performed at the IRAM
30 m telescope using
the XPOL instrument (Thum
et al. 2008). The strength of
this instrument is the precision in the angle of polarization with
respect to the sky reference frame, which highly improves
previous measurements. This precision is needed in particular to
use the Crab nebula as an absolute polarization calibrator for cosmic
microwave background (CMB) experiments, such as the Planck ESA mission.
The
results for circular polarization are irrelevant for current CMB
polarimetry experiments and will be published elsewhere
(Wiesemeyer et al., in prep.).
The last Thomson scattering of cosmological photons by free
electrons at the time of decoupling between matter and radiation at
(Dunkley et al.
2009) have led to a
linearly polarized CMB radiation at a level of
a few percent. The linear polarization pattern on the sky can be
decomposed into a scalar and a pseudo-scalar field, respectively called
E- and
B-modes, which have the advantage of being
independent of
the reference frame
(Zaldarriaga
2001; Zaldarriaga & Seljak 1997).
This decomposition is
particularly useful because primordial density fluctuations (scalar
perturbations) can only produce E polarization,
while B can only be
produced by lensing effects, exotic phenomena such as cosmological
birefringence (Carroll
et al. 1990) and by the stochastic background of
gravity
waves arising from the inflation epoch (tensor perturbations). The
detection of the latter would improve the
constraints on the inflationary model parameters in an invaluable
manner and constitutes one
of the most ambitious goals of observational cosmology.
The first measurement of the CMB E
polarization was
made by the 30 GHz radio interferometer, DASI, in 2002 (Kovac et al. 2002).
Since then, in addition to a further measurement by
the DASI experiment (Leitch
et al. 2005), E modes
measurements have
been made with the CBI (Readhead
et al. 2004), CAPMAP (Bischoff et al. 2008),
BOOMERanG (Montroy
et al. 2006), Wmap (Page
et al. 2007), MAXIPol
(Wu et al. 2007),
QUaD (Ade et al.
2008)
and Bicep (Chiang
et al. 2010). With these measurements,
the
angular power spectrum has now been detected over a
wide range of angular scales (
).
Additionally, the Wmap satellite has given a
very precise measurement of the temperature-E
polarization
correlation, the
angular cross power spectrum, at large
scales up to multipoles
(Nolta et al. 2009).
To date, no detection of non-zero
angular power spectrum
has been reported and only upper limits on the effect of gravity waves
from other spectra
can be inferred. The best indirect limit to date on the tensor to
scalar ratio r, given by Wmap data and other
cosmological observables together is r<0.2
(95% C.L.) (Komatsu
et al. 2009).
Upcoming experiments target the detection of lower values of the tensor-to-scalar ratio. The low signal of the B-modes compared to the temperature and E-modes signals will require a high control of systematics and good knowledge of the instrument. In particular, the knowledge of its polarization characteristics will require a highly and well known polarized source.
The Crab nebula represents the most suitable candidate as absolute calibrator for the polarization angle and the linear polarization degree, given that it is the most intense polarized source in the microwave sky at angular scales of few arcminutes (Flett & Henderson 1983,1981,1979).
The paper is organized as follows: in Sect. 2 we describe the required calibration accuracy for a CMB experiment aiming at the detection of the B-modes; in Sect. 3 we briefly describe the XPOL instrument; Sect. 4 reports the observations, data reduction and systematic error analysis; in Sect. 5 we give our results; we discuss the extension of our measurements to other frequencies in Sect. 6 and summarize our results in Sect. 7.
2 Required calibration accuracy
The expected primordial B-mode level at large
angular scale, for r=0.01, is
typically 1% of the E-mode level. More precisely,
for ,
the
E-mode is on the order of
(from the WMAP best-fit model, see Nolta
et al. 2009), while the B-mode
is expected
at
.
For comparison, the temperature
anisotropies are at the level
at large scale.
The Planck High Frequency Instrument, for example, has the potential
to detect or constrain the tensor-to-scalar ratio down to
(Efstathiou & Gratton
2009). However, such a low level will be
reachable only if the systematics are well controlled, otherwise,
given the hierarchy of signal levels,
,
we may
expect leakage from temperature to polarization and a leakage of
E-mode to B-mode.
Various instrument parameters, if not precisely known, will
induce
systematic effects. In the case of the Planck HFI polarization
sensitive bolometers, the detectors are sensitive to the power carried
by the electric field aligned with the detector's grid. The signal smeasured
by a detector can be written (Rosset
et al. 2010)
s | = | ![]() |
|
![]() |
(1) |
where I, Q and U are the Stokes parameters of the sky signal (intensity and linear polarization in the sky reference frame), g is the detector gain, A is the instrument beam,






In the case of a unique detector scanning the sky, it has been
shown
(Rosset
et al. 2010; O'Dea et al. 2007)
that the error on the B-mode
power spectrum due to an error
in the polarization
orientation and an error
on the polarization efficiency is given by:
![]() |
(2) |
For example, an experiment targeting a tensor-to-scalar ratio measurement of r=0.01, using the large angular scale B-mode (at

![]() |
(3) |
in order to have a leakage from E to B-modes lower than one tenth of the expected

These tolerances set the precision needed for the calibrator, and define our goal for the Crab nebula observations presented in this paper.
3 Instrumental setup
3.1 General
For the observation of the Crab nebula, we used the IRAM 30 m telescope
at 89.189 GHz, the frequency of the
HCO+(1-0) transition, where the angular
resolution (the full width at half
power of the near-Gaussian beam) is .
We used the following IRAM receivers: A100 cryostat
(vertical linear polarization with respect to Nasmyth reference frame)
and B100 cryostat (horizontal linear polarization). The mean system
temperatures
were 87 K (A100) and 113 K (B100) while mean receiver
temperatures
(single-sideband) were 67 K (A100) and 59 K (B100).
The effective cold
loads used for temperature calibration had a mean temperature of
87 K
(A100) and 98 K (B100). We used the VESPA
backends in XPOL mode (see Sect. 3.2),
with a 500 MHz bandwith (200 channels spaced by
2.5 MHz). Finally, the conversion factor from antenna
temperature outside atmosphere to flux
density was 1 K = 6.0 Jy and the mean zenith opacity
was
at our observing frequency.
3.2 XPOL
The signals from the two orthogonal
linearly polarized heterodyne receivers were detected in auto- and
cross-correlation, from which the four Stokes parameters were derived
as
described in detail by Thum
et al. (2008). This procedure, designated
XPOL, employs a precise calibration of the phase between the two
receivers,
resulting in a phase error
per channel of 2.5 MHz, across the 500 MHz bandwidth.
Particular care was taken in the absolute calibration of the
polarization angle .
This angle was re-measured, for the purpose
of this experiment in the Nasmyth cabin, with respect to its horizontal
axis, where the receivers are stationary, as
where Q and U refer to the quantities measured in the Nasmyth reference frame. The precision with which the receivers, labelled H and V, actually measure horizontally and vertically polarized power is determined by the orientation of the grid (G3 in Fig. 1 of Thum et al. 2008) which splits the incoming beam into a transmitted fraction which is horizontally polarized, and a reflected fraction which is vertically polarized. The horizontality of the incoming beam, and thus also that of the H-fraction rests on the correct alignment of the telescope Nasmyth mirror M3, the subreflector, and the validity of the implemented homology corrections and the pointing model. Their combined error is less than 1 arcmin. The verticality of the V-fraction additionally depends on the orientation of G3 whose normal should make an angle of 45



Equation (4)
shows that the precision of the measurement further depends on the correct relative
calibration of
Stokes U and Q. Since
XPOL measures Q as the power difference between
the H and V receivers, but derives Stokes U from
their correlation,
we need to make a precise measurement of the inevitable correlation
losses. This is done as a by-product of the frequent phase
calibration measurements where a wire grid of precisely known
orientation is observed against hot and cold loads. Due
to the high signal-to-noise of these calibrations, the correlation
losses
can be determined with an rms error of
%, resulting in an rms error
of
of
.
The transfer of
to the polarization angle
as measured from north to east in the equatorial system is made through
where the elevation



4 Observations and data reduction
4.1 Mapping strategy
The Stokes maps of the Crab nebula were taken in the on-the-fly mode (hereafter OTF), allowing us to map a large area on the sky within a reasonable time interval. The telescope was scanning the source while XPOL dumped twice per second the total power from the orthogonally polarized receivers, and the complex cross-correlation. The Stokes maps were subsequently derived from these quantities. Throughout the paper, Q and U will refer to the quantities measured in the Nasmyth system and then rotated in equatorial coordinates according to Eq. (5). The Crab nebula was scanned along the projections of latitude and longitude onto the local tangential plane (hereafter respectively






4.2 Raw data reduction
The conversion from backend count rates to antenna temperatures follows the standard method applied at the IRAM 30 m observatory, involving total power measurements of the sky, a hot load, and a cold load. These calibration measurements are combined with the phase calibration measurements, namely an on-off between a fully linearly polarized signal at cold load temperature, and the unpolarized hot load signal at ambient temperature (Sect. 3). The calibrations were derived and applied to the data with the MIRA software (see Wiesemeyer 2009).Since the continuum emission from the Supernova remnant has a well-defined outer boundary, there is no need to observe a separate emission-free reference position. The atmosphere emission was rather removed by subtracting, from each OTF subscan observed along a given direction, a linear baseline defined by the Crab nebula emission-free map boundary. Subtracting only a zero order spatial baseline would leave ``stripes'' in the maps along the scanning direction, which could be removed in Fourier space with the PLAIT algorithm (Emerson & Graeve 1988). Both methods were shown to yield equivalent results. In fact, they are both limited by the uncertainty of the absolute reference flux if the source boundary is not precisely known. In the image plane approach, the source and sky emission cannot be clearly separated from each other, in the Fourier plane technique case, the PLAIT algorithm does not work close to zero spatial frequency (whose Fourier component is the total flux density contained in the map).
![]() |
Figure 1:
Standard deviation maps for the Crab nebula intensity |
Open with DEXTER |
Since for XPOL the signal is downconverted into eight adjacent spectral basebands of 62.5 MHz width each, total power variations between the sky/hot/cold scan and the OTF record result in calibration mismatches among the basebands that introduce platforms in the autocorrelation spectra. In an attempt to correct for this undesired effect, one may naively think of using the data from a filterbank, connected in parallel and with a monolithic spectral band, such that they are not affected by platforming. However, in the subsequent data processing step (rotation from the Nasmyth to equatorial reference frame, for getting a stationary astronomical polarization), the spectra will be linearly combined with the real part of XPOL's cross correlation (which is, after phase calibration, Stokes U in the telescope's Nasmyth reference frame). The small calibration differences observed between XPOL and the filterbank are believed to be caused by non-linearities when the backends operate close to the saturation limit. Mixing these data would thus lead to an inconsistent calibration and hence an error in the polarization angle map of the Crab nebula. Therefore, another strategy was used:
- for each autocorrelation spectrum (i.e. each record of an OTF map), the median across the whole band, with concatenated basebands, has been calculated;
- for each of the eight basebands of a given spectrum, the mean continuum flux and noise has been determined;
- when the absolute difference between the mean baseband signal and the median signal is greater than twice the baseband noise, the offset of the baseband signal has been corrected by the difference between the median and the baseband mean.
It should be noted here that the cross-correlation spectra (Stokes U in the Nasmyth reference frame, and Stokes V) do not suffer from platforming either. This derives from the fact that the clipping voltage of a sampler is adjusted with respect to the signal's noise level, which may change when a given interval of the time series is measured. The zero time lag channel measures the total power (Parseval's theorem), which is positive definite. After the FFT, this shows as an offset to the spectral baseline. A cross-correlator analyses the signals from two independent samplers. Variations in the noise power of the signals coming from the samplers are largely uncorrelated, and therefore cancel out in the spectral cross-power (which is not a positive definite quantity anymore).
The data were gridded at Nyquist sampling. Because of the slightly different values of the start or end scanning coordinate between different subscans, the sampling is irregular. Therefore the data had to be resampled to a regular grid. This has been done by applying a Gaussian convolution kernel with a FWHM of one-third the telescope's half-power beam width. The kernel is truncated at three FWHM.
4.3 Description of the data sets
We performed two observation campaigns of the Crab Nebula, respectively from September 5 to 10, 2007, and from January 9 to 12, 2009.
- First campaign: Observations were made
during 6 intervals,
under varying astmospheric opacity and stability conditions. Most of
the maps were affected by a substantial atmospheric contamination,
showing as strong linear stripes in the scanning direction. We obtained
66 I, Q, U
and V maps resulting in
maps after coadding
-scanned and
-scanned maps. These maps were centered on the reference point we called crabxpol having equatorial coordinates
and
(J2000). The centering and the size of these maps made the destriping delicate because the extension of the source emission was larger than the observed area and thus, strong mix-up with atmospheric contamination arose on maps' edges leading to uncontrolled systematics.
- Second campaign: Observations were made
during 4 nights during which we experienced stable atmospheric
conditions. As a consequence of the first observation campaign, we
increased the coaddition of the
-scanned and
-scanned maps to a size of
in order to be able to characterize the atmospheric stripes outside the source. 11 maps of I, Q, U and V were observed centered on the same reference point as in the first campaign, namely crabxpol, and then we changed to a new reference point for center, called crabcenter and having equatorial coordinates
and
(J2000), on which the next 28 maps were observed. This choice was made to have a better centering of the source extent in the north-south direction, allowing better destriping. The 11 first maps centered on crabxpol have a significantly higher noise level than the 28 centered on crabcenter due to worse weather condition at the beginning of the campaign.
Thus, using only the 28 maps from
2CC provides the best results and any addition
of 2CX maps, only
2CX maps scanned in
direction
(direction in which the borders of the maps are off-source) and
1CX lead to worse results in terms of noise
homogeneity and of signal-to-noise ratio, both in intensity and
polarization. Consequently, our analysis will use only the 28
maps of the second campaign centered on the crabcenter
reference point.
4.4 Systematic effects analysis
In order to check for residual systematic effects in the maps, we
performed jack-knife tests dividing the 28 maps in several subsets
using three
criteria, i.e.: randomly, between maps scanned in
and
direction
and between the three observation nights during which these
28 maps were taken. We also divided the data in two with respect
to the position angle of the Nasmyth reference frame projected onto
the sky,
,
in order to check for sidelobe polarization
effects (Forbrich
et al. 2008), which may show whether the difference
between the
of
subsets is close to 90
.
Table 1:
Polarization angle
and polarized fraction
computed for different divisions of the data set.
![]() |
Figure 2: Maps of the Crab nebula at 89.189 GHz in antenna temperature (K) for intensity I (top-left), Q polarization (top-right), U polarization (bottom-left) and polarized intensity P (bottom-right). The position of the Crab nebula pulsar is indicated by the black cross. |
Open with DEXTER |
The mean and dispersion of the 28 observations allow us to derive a
signal-to-noise map for I, Q, U
and
.
For the residual systematic effects
analysis, we have chosen a region corresponding to S/N(P)>10
that
includes 19 pixels. For these pixels, we compute the mean polarization
angle
and the mean
polarization fraction
for each
map j. Then,
and
are averaged for each
subset to give the mean
and
and the error on
the mean is given by the standard deviation over the subset of
maps, preventing correlations between Q and U,
or among pixels.
Results are displayed in Table 1. Values
are
stable against the selection of different data
subsets. For the polarization angle, for each subset selection
criteria, the values are compatible at less than two ,
showing no clue to residual
systematic effects in the data. We observe a similar behavior for the
polarization fraction, where only the subset corresponding to
and
scans shows a departure of slightly more than two
.
This systematic behavior is due to a
percent-level difference in the intensity maps corresponding to each
type of scan, caused by residual errors in the destriping which is less
precise for
-scanned
maps due to the limited number of
off-source pixels in this case and leading to an overestimation of the
intensity.
![]() |
Figure 3:
(Left) Map of the polarization angle |
Open with DEXTER |
5 Results
After the raw data reduction, including atmospheric decontamination by
linear baselines subtraction, we obtained a set of 28
systematic-reduced individual maps with which we compute average and
standard
deviation maps for I, Q, U,
,
and
.
The Crab nebula intensity I map is
displayed on top-left panel of Fig. 2, in
antenna temperature, showing a maximum 0.91 K emission half an arcmin
south from the pulsar position. The
flux density integrated over the source and its error bar derived from
the pixel-to-pixel standard deviation are
measured as
Jy. This value is compatible within
3
with the
value given by the WMAP satellite at 92.9 GHz of
Jy
(Page et al. 2007).
The Q and U maps of the Crab nebula are displayed on top-right and bottom-left panel of Fig. 2, in antenna temperature (K). Q map is showing a roughly positive value, up to 0.16 K while U is showing a negative one down to -0.20 K.
The polarized intensity P map is displayed
on bottom-right panel of Fig. 2, in
antenna temperature, showing a maximum of
0.25 K half an arcmin south from the pulsar position. We can see that
the Crab nebula is less extended in polarized intensity than in Stokes
I. The polarized flux density is
measured as
Jy, leading to a mean polarization
fraction for the whole Crab nebula source of
%. This
value is compatible with the value measured by WMAP of
% (Page et al. 2007).
The
map is displayed on the left panel of Fig. 3. It is
worth noting that
is
almost constant
around 150
in the region of maximum polarized intensity
(P>0.05 K). This region corresponds to the
region where each pixel's standard
deviation
is the lowest (
), going
down to
for the most intensely
polarized pixels. The value of the angle we measured at the flux peak,
,
is in very good agreement with previous measurements at 1350 and
1100
m
(Flett
& Murray 1991; Greaves et al. 2003).
On the map edges,
pixel-to-pixel deviation is high, except for the
northern region where the average angle is around 230
.
When
looking at the
map, we can see that pixels
on the edges have an undetermined value (
),
whereas in the northern
region they have
,
indicating a polarization angle there that is significantly
different from that near the center of the nebula.
![]() |
Figure 4: Map of the Crab nebula polarized intensity P at 89.189 GHz in antenna temperature on which polarization vectors have been overpolted. Intensity contours at 0.15, 0.30 and 0.45 K are also displayed. |
Open with DEXTER |
The polarization fraction
map is displayed on the right panel of Fig. 3. We have
set to 0 the pixels for which intensity
is lower than 0.02 K in order to avoid the divergence of
.
Maximum
polarization fraction is found to be spatially correlated with maximum
polarized intensity region, with a polarization fraction reaching 30%
for few
pixels. A 1.5 arcmin circular region around the maximum polarization
fraction has a significant
%.
To summarize all these results, we display in Fig. 4 the
polarized flux density P and the orientation
of the polarization vectors associated with each map pixel. The
polarization vectors pattern is compatible with 9 mm (Flett & Henderson 1979)
and 850 m
(Greaves et al.
2003) observations.
We finally
computed several values of the polarization angle
and
of the polarization fraction
for different regions, displayed in
Table 2.
We selected the pulsar position and high signal-to-noise ratio regions
for
the polarized intensity (
S/N(P)>3,
corresponding roughly to the
source's polarized emission extent and S/N(P)>10).
Furthermore,
we convolved
each individual map by a circular Gaussian beams of 5 arcmin and 10
arcmin of FWHM before computing the mean
polarization angle and fraction. These
beams mimic the Planck satellite's beams (10' for the 100 GHz
and 5'for the 217 and 353 GHz channels) and give an estimation
on how a
generic CMB experiment having Planck-like characteristic would see the
Crab nebula's polarization. For each of these cases, the mean value
and its associated error were computed similarly to Sect. 4.4.
6 Extension to other frequencies
The total intensity emission of the Crab nebula from 1 to 106 GHz
is dominated by the
well known synchrotron radiation observed at radio wavelengths with
only one extra dust component in the far infrared (Macías-Pérez et al. 2010).
This synchrotron emission shows a decrease of flux with increasing
frequency which can be represented by a power law of spectral index (Baars
et al. 1977; Macías-Pérez et al. 2010)
from the radio to the submillimeter domains. The flux
is also decreasing with time at a rate of
% yr-1
(Aller & Reynolds 1985).
Moreover, from the visible to the X-rays the
synchrotron emission evolves towards a much harder spectrum
represented by a power law of spectral index
(Macías-Pérez et al. 2010).
These statements allow us to postulate that where the synchrotron dominates, the emission at different wavelengths is produced by particles accelerated by the same magnetic field. The direction of polarization is thus expected to be constant while the polarization fraction may vary. Nevertheless, we expect values for both the polarization angle and the polarization fraction, at low-resolution, to be similar in the millimeter and submillimeter where the emission is produced by the same electron population.
Table 2:
Values of the polarization angle
and of the polarization fraction
.
Values of the polarization angle of the Crab nebula source
have been reported over this wide
range of wavelengths from the radio (e.g. at 9 mm, ,
Flett & Henderson
1979) to the millimeter (at 3.3 mm,
,
this paper) and to the X-rays (at 240 pm,
,
Weisskopf et al.
1978). We can see that the value
of the polarization angle is strikingly constant over nearly ten
decades of wavelength. Furthermore, the value measured in the X-ray,
similar to the one we measured at 90 GHz, indicates that we
are
probably dominated by synchrotron emission for the same regions in
both cases and that an extrapolation up to 353 GHz, where
present and future CMB experiments such as the Planck satellite are
observing the sky, is rather safe.
The polarization fraction can be compared to other
measurements too,
keeping in mind that it is a quantity which changes inside the source
rapidly and that shall be compared only for
experiments observing with a comparable beam size. The
comparison of our value of % to radio
observations
at 9 mm giving
% (Flett & Henderson 1979)
and the value we obtain when our maps were convolved by a
10' beam of
%
to the WMAP experiment (
%,
Page et al. 2007)
indicates that those measurements are
coherent with an emission coming from the same electron population
wich leads to a constant polarization fraction over the CMB frequency
range.
7 Summary
We mapped the polarized emission of the Crab nebula using the IRAM 30 m telescope at 89.189 GHz with an angular resolution of 27'', with two orthogonally linearly polarized heterodyne receivers. The Stokes parameters were derived from the auto- and cross-correlations using the XPOL procedure (Thum et al. 2008).
Observations have been undertaken during two campains, leading
to a
set of 105 individual I, Q and U
maps having changing weather
conditions, sizes and centering. Particular care has been taken in
choosing the set of individual maps leading to the lower level of
systematics in the final products. Additionnaly, jack-knife tests have
been carried out in order to demonstrate the robustness of the
data. As a result, we constructed 10
I, Q and U
coaddition maps of these
observations centered on the Crab nebula.
From these maps we have computed the polarized intensity
,
the polarization angle
and
the polarization
fraction
.
We derived our results on the
polarization characteristics of the Crab nebula with these maps:
- The measured flux density is
Jy and the polarized flux density is
Jy.
- The polarization angle is almost constant in the region of
maximum emission in polarization with a mean value of
. A region north to the pulsar is seen with an average angle of
230
, but it does not correspond to a strongly polarized emission region. When seen by a 5 arcmin beam, the mean polarization angle of the whole source as a value of
, in good agreement with the other measurements at radio, millimeter and X-rays wavelengths.
- The polarization fraction shows a maximum in a region south
from
the pulsar position, reaching 30%. When averaged by a 5 arcmin
beam,
the measured mean value of the polarization fraction is
%. This value is in very good agreement with the WMAP measurements at 94 GHz.
We gratefully acknowledge the support of IRAM to this program including the grant of discretion time by its director P. Cox. J. Aumont has been partly supported in this work by a post-doctoral position from CNES. E. Pointecouteau was supported by grant ANR-06-JCJC-01142.
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All Tables
Table 1:
Polarization angle
and polarized fraction
computed for different divisions of the data set.
Table 2:
Values of the polarization angle
and of the polarization fraction
.
All Figures
![]() |
Figure 1:
Standard deviation maps for the Crab nebula intensity |
Open with DEXTER | |
In the text |
![]() |
Figure 2: Maps of the Crab nebula at 89.189 GHz in antenna temperature (K) for intensity I (top-left), Q polarization (top-right), U polarization (bottom-left) and polarized intensity P (bottom-right). The position of the Crab nebula pulsar is indicated by the black cross. |
Open with DEXTER | |
In the text |
![]() |
Figure 3:
(Left) Map of the polarization angle |
Open with DEXTER | |
In the text |
![]() |
Figure 4: Map of the Crab nebula polarized intensity P at 89.189 GHz in antenna temperature on which polarization vectors have been overpolted. Intensity contours at 0.15, 0.30 and 0.45 K are also displayed. |
Open with DEXTER | |
In the text |
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