Observations were made at meter wave-lengths using very long base-line interferometry techniques. At 196.5 MHz no resolution of the pulsar are observed; all the pulse shapes observed with the interferometers are similar to single dish profiles, and all the power pulsates. At 111.5 MHz besides the pulsing power there is always a steady component, presumably due to interstellar scattering. The pulsar is slightly resolved at 111.5 MHz with an apparent angular diameter of 0.07 sec ? 0.01 sec. A 50 percent linear polarization of the time-averaged power is noted at 196.5 MHz; at 111.5 MHz, 20 percent of the total time-averaged power is polarized, 35 percent of the pulsing power is polarized, and the steady component is unpolarized

Broderick, J. J.

Clark, T. A.

Erickson, W. C.

Knowles, S. H.

Payne, R. G.

Resch, G. M.

Vandenberg, N. R.

Youmans, A. B.

English

NASA Technical Reports Server

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VLBI OBSERVATIONS OF THE
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https://ntrs.nasa.gov/search.jsp?R=19730002125 2020-03-23T09:20:41+00:00Z
VLBI OBSERVATIONS OF THE CRAB NEBULA PULSAR
N.R. Vandenberg, T~A. Clark, W.C. Erickson, and G.M.
"
Astronomy Program
University of Maryland
College Park, Maryland
and
Resch
Radio Astronomy Branch
Goddard Space Fl~ght Center
Greenbelt, ~aryland
J.J. Broderick and R.R. Payne
National Astronomy and Ionospheric Center
Arecibo Obsp-rvatory
Arecibo, Puerto Rico
S.H. Knowles and A.B. Youmans
Naval Research,Laboratory
Washington, D.C.
October, 1972
Received
-------
ABSTRACT
Observations of the Crab Nebula pulsar at meter wave-
lengths using VLBI techniques have been made. At 196.5 MHz
we observe no resolution of the pulsar, all the pulse shapes
observed with the interferometers are similar to single
dish profiles, and all the power pulsates. At 111.5 MHz
besides the pulsing power there is always a steady component,
presumably due to interstellar scattering. The pulsar is
slightly resolved at 111.5 MHz with an apparent angular
diameter of 0~07 + 0~01. We" observe 50% linear polariza-
tion of the time-averaged power at 196.5 MHz; at 111.5 MHz,
20% of the total time-averaged power is polarized, 35% of
the pulsing power is polarized and the steady component is
unpolarized.
I. ~NTRODUCTION
From 1971 November through 1972 March we observed
the Crab Nebula pulsar (PSR 0531+21) at monthly intervals
using very long baseline interferometry (VLBI) techniques
at meter wavelengths. The results presented in this paper
are based on the November and December observations. The
purpose of the experiment was to monitor the apparent
angular size of the pulsar and to compare variations in,the
pulse shape, as reported by Rankin and Couns~lman (~972),
to angular size changes. The relation between spatial and
temporal broadening can indicate the general location of the
scattering screen.
The telescopes used were the 305-m (1000 ft.) Arecibo
dish, the 92-m (300 ft.) NRAO dish, and the 46-m (150 ft.)
dish at Sugar Grove, W.Va. These telescopes form two long
baselines of about 2000 km each and one short baseline about
. " .,..
50 km long. Table 1 shows the interferometer parameters:
baseline in wavelengths,. minimum detectable flux densi~y,
and fringe spacing for the two observing frequenci,es , Ill.!?
and 196.5 MHz. The standard ~AO Mark-I recording terminal~
were used; and the normal Arecibo Crab pulsar timing
2
observations, as described by Rankin et ale (1970), were
made simultaneously.
The VLBI data were analyzed into the Fourier components
of the pulse shape using the techniques described by
Erickson et ale (1972). This technique coherently de-disperses
the pulsar and also removes the differential Faraday rotation
across the passband. Reconstruction of the pulse shapes is
done by adding the components in a Fourier sum. At 196.5 MHz
wegemeraliy obtained five Fourier harmonics in addition to
the fundamental; at 111.5 MHz three or four components were
detectable.
II. RESULTS
a) Positional agreement of pulsar and compact source
The 26 pulse profiles derived from the 111.5 MHz data
indicate that the average deviation of the phase of the
Fourier components (associated with the pulsar) from the
fundamental phase (associated with the compact source) is
aborit 1.2 degrees. This corresponds to a positional agree-
'ment of ~bbut 0~01, and supports the previous conclusion
that the compact source in the Crab Nebula must be identified
with the pulsar.
3
b) Timing observations
In determining the epoch of the reconstructed interfero-
meter pulse profiles, we find that it agrees with the time of
arrival determined at Arecibo to within about 1% of the pulse
period at 196.5 MHz and 2-3% at 111.5 MHz.
c) Profiles
At both frequencies the low resolution interferometer
pulse profiles agree in overall shape with those observed
with the single dish at Arecibo. This is illustrated in
Figure 1 which shows typical 196.5 MHz pulse profiles ob-
served in 1971 December. At this frequency, the pUI~e profiles
observed'with high resolution are also similar to the Arecibo
profiles; all the profiles at 196.5 MHz have the same shape
and height, and all the power pulsates. We conclude there
is no resolution of the pulsar at 196.5 MHz and place an
upper limit on its apparent size of O~04. The VLBI observa-
tion at 196.5 MHz are essentially identical to those of a
single dish.
From the low resolution 111.5 MHz observations we re-
construct a broad pulse shape which always has a steady non-
pulsing component. The detection of the steady component
4
illustrates one major advantage of interferometer observa-
tions, we are able to measure both the steady and the pulsing
flux. Typical pulse profiles observed in 1971 November are
shown in Figure 2. The high resolution observations also
show a broad pulse shape, implying that the temporal and
angular scattering are not simply related and probably do not
occur at the same place along the intervening path.
The average fringe visibility at 111.5 MHz is 0.8 + 0.1,
corresponding to an angular diameter of 0~07 + 0~01, assuming
a gaussian brightness distribution. The apparent angular
size predicted on the basis of interstellar scattering by
Harris et ale (1970) is 0~09 at 111.5 MHz, assuming a distance
of 2000 pc to the Crab Nebula. This size is consistent with
our observations.
We observe that the average value of the ratio of steady
to total flux is 0.45 + 0.1 for both months' observations.
The scattering function proposed by Rankin et ale (1970) predicts
a frac~ion, 0.5 + 0.2, of the total flux will be non-pulsing,
which is in good agreement with our observations.
d) Total flux
The total flux of the small diameter source was constant
5
during the observing sessions and was the same value in
both November and December. Preliminary analysis of the
1972 observations also gives the same total flux. Each of
these five epochs yields a time-averaged total flux of
35 + 5 f.u. at 111.5 MHz and 6 + 1 f.u. at 196.5 MHz. The
variations in pulsar strength observed with a single dish at 11.5
MHz should be attributed to different distributions of the power
between the pulsing and steady components, not to intrinsic
changes in the total flux. We have observed one example of
this redistribution in the average profiles observed between
06:00 and 07:00 D.T. on 1971 November 24, as shown in Figure
2. Although the general shape of all the low resolution and
Arecibo profiles is the same, the steady component is twice
as strong in the later profile. In one hour the power was
redistributed, the steady flux increased by a factor of two
while the pulsating flux decreased to conserve the total flux.
e) Polarization
In order to show the polarization effects more clearly
in Figure 3 we have superposed the 111.5 MHz pulse profiles
from Figure 2. Observations made with feeds of opposite
circular polarization at the ends of the baselines make the
6
interferometer sensitive to only linearly polarized radia-
tion of any position angle. The instrumental polarization
was calibrated by observing several quasars. When crossed
circular feeds were used, the steady component of the pulse
disappears. This occurs for both the high and low resolution
observations and is a polarization, not a resolution, effect.
Assuming there is no circular polarization of the pulsar
(Campbell et al., 1970), the disappearance of the steady
component means that it is unpolarized; only the pulsing
component retains partial linear polarization. The polariza-
tion of the steady component is evidently destroyed by the
multipath scattering effects which cause this component.
We observe 50% linear polarization of the average power
at 196.5 MHz. At 111.5 MHz, 20% of the total time-averaged
power is linearly polarized. Since the interferometer ob-
serves all the power from the small diameter source, some of
which is not detected by single dish observers, this is a
true measure of the polarization. In order to compare our
results to those of other observers, we consider only the
pulsing power at 111.5 MHz, and calculate 35% polarization,
which agrees with the observations of Manchester et al. (1972).
7
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REFERENCES
Campbell, D.B., Heiles, C., and Rankin, J.M. 1970, Nature
225, 527.
"-'''-'''-'
Counselman, C.C., and Rankin, J.M. 1971, Ap. J. !~' 513.
Cronyn, W.M.1970, Science 1~, 1453.
Erickson, W.C., Kuiper, T.B.H., Clark, T.A., Knowles, S.H.,
and Broderick, J.J. 1972, Ap. J. 111.
Harris, D.E., Zeissig, G.A. and Lovelace, R.V. 1970, Astron.
and Astrophys. ~' 98.
Manchester, R.N., Huguenin, G.R., and Taylor, J.H. 1972,
Ap. J. (Letters) !Z~' L19.
Rankin, J.M., Comella, J.M., Craft, H.D., Richards, D.W.,
Campbell, D.B., and Counselman, C.C. 1970, Ap. J. l~~, 707.
Rankin, J.M., and Counselman, C.C. 1972, Ap. J., submitted.
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VLBI observations of the Crab nebula pulsar

VLBI observations of the Crab nebula pulsar

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