Very long baseline interferometric (VLBI) observations of the supernova remnant Cassiopeia A, at 74 MHz with a 12,000-wavelength baseline and at 111 MHz with a 18,500-wavelength baseline, are reported. The fringe amplitudes are strongly varying on a time scale of about 15 to 30 minutes. The location of the extra source must lie outside the supernova remnant shell possibly associated with a concentration of emission north of the shell, or lying outside the gap in the northeastern side of the shell. The flux and spectral index deduced for the compact source depend on the assumed size, with a range of 100 Jy to 500 Jy at 74 MHz. If the source is associated with the supernova explosion, it must have been traveling at least 5000 km s/2

Clark, T. A.

Erickson, W. C.

Hutton, L. K.

Knowles, S. H.

Resch, G. M.

Vandenberg, N. R.

Youmans, A. B.

English

NASA Technical Reports Server

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THIS PAPER HAS BEEN SUBMITTED FOR
PUBLICATION IN THE ASTRONOMICAL JOURNAL
I/
HIGH RESOLUTION OBSERVATIONS OF CASSIOPEIA A
AT METER WAVELENGTHS
L.K. Hutton, T.A. Clark, W.C. Erickson,
G.M. Resch, N.R. Vandenberg
Astronomy Program, University of Maryland,
College Park and Radio Astronomy Branch,
Goddard Space Flight Center
Greenbelt, Maryland
and
S.H. Knowles, A.B. Youmans
Naval Research Laboratory,
Washington, D.C.
Abstract: Very long baseline interferometric (VLBI) ob-
servations of the supernova remnant Cassiopeia A, at 74 MHz
with a 12,000-wavelength baselineoand at 111 MHz with a
18,500-wavelength baseline, are reported. The fringe
amplitudes are strongly varying on a time scale of about
15 to 30 minutes, which we feel can be explained by much
the same complex structure as that observed at higher
frequencies, plus one other compact source. Due to the
poor (u,v)-plane coverage, we cannot isolate the location
of the extra source unambiguously, but we can suggest
possibilities. The source must lie outside the supernova
remnant shell, possibly associated with a concentration of
emission north of the shell, or lying outside the gap in
the northeastern side of the shell. The flux and spectral
index deduced for the compact source depend on the assumed
size, with a range of 100 Jy to 500 Jy at 74 MHz. If the
source is associated with the supernova explosion, it must
-l
have been traveling at least 5000 km s .
2
I. INTRODUCTION
The supernova remnant Cassiopeia A (Cas A) shows a
very complicated brightness distributiop at high radio
frequencies. Various aperture synthesis maps have been0
made at centimeter wavelengths (Hogg et al. 1969; Rosenberg
1970a,b; Strom and Duin 1973) all showing the shell structure
typical of supernova remnants, as well as a number of small
"knots" of emission. It is of interest to have observa-
tions of comparable resolution at meter wavelengths for a
number of reasons. First, differences in the spectra of
various parts of the source might be distinguished over
the wide frequency range provided. And secondly, Cas A
could contain a compact source similar to the Crab Nebula
pulsar (Hewish and Okoye 1964; Andrew, Branson and Wills
1964), which by virtue of its steep spectrum is detectable
only at low frequencies. No pulsar searches have yet been
successful in Cas A (e g. Reifenstein, Brundage and
Staelin 1969; Davies and Large 1970), but these surveys
are generally not sensitive to very fast or very highly
dispersed pulsars. For a supernova remnant as young as
Cas A, the detection of a pulsar as a compact source rather
than as a pulsating source might be more likely.
3
II. OBSERVATIONS
To obtain resolution comparable to the aperture
synthesis maps at meter wavelengths, interferometers with
10 to 100-km baselines are necessary. Cas A had previously
shown fringes on a 265-km baseline at 26.3 MHz (Cronyn
et al. 1971; Hutton, Clark and Cronyn 1973), but only
at one point in the (u,v) plane since the phased arrays
used were essentially transit instruments. The source was
therefore included in an extensive meter wavelength VLBI
program utilizing the 300-ft. (94-m) telescope of the
National Radio Astronomy Observatory in Green Bank, West
Virginia and the 150-ft. (46-m) antenna of the Naval Research
Laboratory at Sugar Grove, West Virginia (Clark and Erickson
1973; Vandenberg et al. 1973). They form a 50-km baseline,
which amounts to 12,000 and 18,500 wavelengths at our
observing frequencies of 74 MHz and 111 MHz, respectively.
The data were taken in left circular polarization and
recorded on computer tape using the Mark I VLBI system
(Bare et al. 1967) with a bandwidth of 350 kHz. The observa-
tions were made between 1972 December 4 and 10, and between
1973 February 23 and March 1.
4
Although the 300-ft. telescope is normally a transit
instrument, at these frequencies the use of the "traveling
feed" allows a certain amount of hour angle range. Cas A,
at declination +58 , can be tracked for nearly two and
one half hours, so a number of points in the (u,v) plane
were sampled. The Sugar Grove telescope is fully steerable
and does not limit the mutual visibility. The resultant
(u,v)-plane coverage is shown in Figure 1.
The data tapes were correlated using the standard GSFC-
MIT VLBI reduction programs(Hinteregger 1973; Whitney 1974)
on the IBM 360/91 and 360/75 computers at Goddard Space
Flight Center. Fringe amplitudes were coherently averaged
over three minutes, and expressed as a fraction of the
geometric mean of the total system temperatures at the
two stations. Since the antenna temperature due to the
source alone accounted for over 75% of the total system
temperature, the observed correlation coefficients were
to first order true visibilities. Additional corrections
were made on the basis of the total power data from both
stations, but calibration errors were of little consequence
due to the strength of the source. The resulting visibility
curves at the two frequencies are presented in Figure 2.
5
III. ANALYSIS
The first step in the data analysis was to make as
direct a comparison as possible with the existing higher
frequency data. To accomplish this we used the map made
at 2695 MHz with the NRAO interferometer (Hogg et al. 1969).
"Observing" this map by computer on our baselines produced
visibility curves of the shapes shown by the dotted curves
in Figure 2. The reliability of this procedure was checked
by also "observing" the map on the 2700-m baseline of the
NRAO interferometer and comparing the results with the raw
NRAO data. The normalization provided by this comparison
revealed that the map accounts for about 40% of the total
flux of Cas A at 2695 MHz, with the remaining flux being
distributed in a broad plateau slightly greater than the
shell source in size. (This plateau is not observable with
the NRAO interferometer due to the lack of spacings less
than 100 m, but is clearly seen on other maps, e.g. Rosenberg
(1970a)). If the map is strictly extrapolated to the lower
frequencies, assuming that it accounts for the same per-
centage of the total flux, then the normalization is that
of the dotted curves in Figure 2. We see that, although
6
the normalization appears to be reasonable, the correspondence
between the models and the data is not good at either
frequency.
Since the 74-MHz data appears to present a more clear
cut case of disigreemen than the 111-MHz data, and is no
doubt simpler due to the-lower angular resolution, let us
initially concentrate on the lower frequency. There is clearly
"something extra" in tht form of a quasi-sinusoidal varia-
tion of about 30-minute period in the data which does not
appear in the model curves. Since a sinusoidal visibility
function is what one expects from a pair of point sources,
we see that we may have:detected at least one such pair which
is not present at higher frequencies. Because of the (u,v)-
plane coverage, the north-south separation for the fastest
variation is fairly well determined (about 4'), but the
east-west separation is virtually unknown. The small knots
of emission on the high frequency maps are immediately
suspect; if one of these were enhanced at the lower
frequencies, the beat pattern from it and the other knots
could qualitatively produce.the observed effect. The
approximate north-south separation required is present
7
between a knot on the north side of the remnant, outside
the shell, and any one of the knots on the southern edge.
The same effect could also be produced by an extra compact
source, unrelated to those observed at high frequencies,
beating with the knots in the shell.
To make the above analysis more quantitative, we used
an iterative least squares routine (Bevington 1969) to fit
the fluxes and relative positions of the NRAO map and one
other point source (a total of four parameters). The pro-
gram was provided with initial positions for the point
source relative to the phase center of the NRAO map spaced
15" apart throughout the area within the dotted line in
Figure 3. Convergent solutions which considerably improve
the fit were found for point sources in the areas marked
A,B and C, several solutions occurring in each. As would
be expected, the individual solutions always occur in a
series, spaced about one fringe apart in the east-west
direction.
These regions were then grid searched by the same
procedure at 111 MHz, yielding acceptable solutions in all
three. The solutions are again arranged in a series, spaced
8
about one fringe apart. None of the 111-MHz solutions
correspond exactly in position to any of the 74-MHz ones,
nor are the resulting fits exact at either frequency.
However, such problems can probably be attributed to
changes in the structure of the shell source with frequency.
Figure 4 shows visibility curves typical of each of the three
areas, along with the curves from the NRAO map alone for
comparison. The solutions chosen are the ones with the
closest coincidence in position between the two frequencies.
Table 1 summarizes the parameters for each curve shown.
The spectrum of the point source is very steep (a " -2.5
where S(v) a v-a) in all three cases, perhaps explaining
why it does not appear in higher frequency maps. The
spectral index of the fraction of the total flux contained
in the map (referred to as "map flux" in Table 1), between
our frequencies and 2695 MHz, is generally consistent with
the known spectral index of Cas A (a = -0.75), even though
the agreement between the 74-MHz and 111-MHz fluxes is poor
in some cases.
All the above analysis assumes that the added source
is a point source. On such a limited range of resolution
9
we have virtually no information about the size of the
extra source; larger sources simply require more flux.
However, due to the difference in resolution, they require
relatively more flux at 111 MHz than at 74 MHz and the
spectral index changes. Least squares solutionssimilar
to the ones just described, but using Gaussian sources of
various fixed sizes, were performed for region C. The
analysis showed that Gaussian sources with half power
diameters greater than about 6" have spectra flatter than
the total flux of Cas A and therefore should have appeared
on high frequency maps, provided that they have power law
spectra. Such sources fit at about 500 Jy at 74 MHz, so
we may tentatively consider this figure to be an upper
limit. Table 2 summarizes the fluxes for sources of
various assumed sizes. In any event, the compact source
contains less than 10% of the total flux.
IV. SUMMARY
To summarize, we have succeeded in fitting visibility
curves at 74 MHz and 111 MHz with a 2695-MHz map extrapolated
directly to these frequencies, plus one other compact source.
The compact source may be anywhere from 100 Jy to 500 Jy
10
at 74 MHz depending on its size and its spectrum. The fact
that we have had any success at all suggests that the SNR
shell itself changes remarkably little with frequency, the
changes being concentrated in one compact source.
From our data alone,,the position of the compact
source is ambiguous; we can at best suggest a few possible
locations. The nature pf the source is therefore only
hinted at. The existence of solutions near the northernmost
0
knot (region C) suggest that it may be responsible. No
fits at either frequency are centered on the knot as it
appears in the map, but again changes in the complicated
structure of the shell with frequency may account for such
problems. The consistency of the map fluxes for the region
C solutions, as well as the more nearly coincident positions
at the two frequencies," certainly speaks in favor of such an
explanation. On the other hand, region B and perhaps 
even
region A lie generally outside the gap in the shell. 
There
are known optical filaments outside this gap (van den Bergh
and Dodd 1970; van den Bergh 1971), although not as far as
either of these regions, and as long ago as 1959 Jennison
(1959) reported the existence of "something extra" 
on the
11
eastern side of Cas A at meter wavelengths.
It is significant that no improvements to the fit
were obtained by adding a point source inside the supernova
shell (no pair can then produce the required 4' north-south
spacing). Since the shell itself appears to be expanding
-I
at 5000 km s (van den Bergh 1971), any object that
originated in the supernova explosion and now lies outside
the shell must have been expelled at a velocity greater
than this. Although the point source spectrum is suggestive
of a Crab-like pulsar, the velocities implied are certainly
far greater than any "runaway" velocity yet reported.
Of course, the compact source may be a background
source, not associated with the supernova at all. In that
case, it might well have been one of the prominent sources
in the 3C Catalogue, had it not had the misfortune of being
too close to Cas A.
The authors would like to thank C. Grant, H. Huffman,
G.L. Mader, G.E. Marandino and R.A. Perley for assistance
in taking the data; Dr. T.A. Matthews for his suggestions
and encouragement; and especially Dr. D.E. Hogg for his
suggestions and encouragement and his digital map. We also
0 12
thank the staff of the GSFC Computer Facility, the National
Radio Astronomy Observatory and the Naval Research Laboratory's
Sugar Grove Facility for their apparently infinite patience.
The National Radio Astronomy Observatory is operated by
Associated Universities, Inc., under contract with the
National Science Foundation.
13
T able 1: Parameters for Point Source Solutions
Region Freq Position Map Point Map 
Point
W N Flux Flux Spectral Spectral
(MHz) (arc sec) (Jy) (Jy) Index Index
A 74 -431 76 14300 122
111 -430 57 3100 44 0.8 2.5
B 74 -200 69 13300 91
111 -197 85 2600 45 0.8 1.7
C 74 -19 99 6600 110
111 -21 93 6200 32 0.7 3.0
Table 2: Parameters For Gaussian Source Solutions
Point 74 MHz 111 MHz Point
Size Flux Flux Spectral
(,,) (Jy) (Jy) Index
0.0 110 32 3.0
4.7 140 56 2.2
9.4 330 410 -0.54
14.1 1400 11000 -5.0
- /S--
Figure Captions:
Figure 1. (u,v)-plane coverage for the meter wavelength
observations. u and v are given in wavelengths
and times are marked on the tracks in Greenwich
Sidereal Time. Note that the longest baseline
on the NRAO interferometer at 2695 MHz is
24,300 wavelengths.
Figure 2. Visibility curves observed on the meter wavelength
experiment. The error bar is a 3-a error bar
estimated from the noise in fringe amplitude.
The dotted curve is the visibility curve predicted
from the 2695-MHz map alone.
Figure 3. Map of the Cas A region showing possible locations
of the extra point source. The dotted line
represents the area searched for solutions at
74 MHz. Distances are in seconds of arc.
Figure 4. Visibility curves obtained by the addition of
the extra point source at the locations marked
in Figure 3. The curves for the NRAO map alone
are also shown for comparison.
-I(0-
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Androw, B., Branson, H., Wills, D. (1964), Nature 203, 171.
Bare, C., Clark, B.G., Kellermann, K.I., Cohen, M.H.,
Jauncey, D.L. (1967) Science 157, 189.
Bergh, S. van den, Dodd, W.W. (1970) Ap. J. 162, 485.
Bergh. S. van den (1971) Ap. J. 165, 457.
Bevington, P.R. (1969) Data Reduction and Error Analysis for
the Physical Sciences (McGraw-Hill, New York), p. 238.
Clark, T.A., Erickson, W.C. (1973) Proc. I.E.E.E. -61,1230.
Cronyn, W.M., Klemperer, W.K., Rufenach, C.L. Shawhan, S.D.,
Basart, J., Clark, T.A., Erickson, W.C. (1971) B.A.A.S.
3, 438.
Davies, J.G., Large, M.I. (1970) M.N.R.A.S. 149, 301.
Hewish, A., Okoye, S. (1964) Nature 203,171
Hinteregger, H.F. (1973) Ph.D. Dissertation, M.I.T.
Hogg, D.E., Macdonald, G.H., Conway, R.G., Wade, C.M.
(1969) A.J. 74, 1206.
Hutton, L.K., Clark, T.A., Cronyn, W.M. (1973) B.A.A.S. 5 ,35.
Jennison, R.C. (1959) I.A.U. SymposiumS, 309.
Reifenstein, E.C., Brundage, W.D., Staelin, D.H. (1969)
Ap. J. Lett. 156, L125
-/7-
Rosenberg, I. (1970a) M.N.R.A.S. 147,215.
Rosenberg, I. (1970b) M.N.R.A.S. 151, 109.
Strom, R.G., Duin,R.M. (1973) Astr. & Ap. 25, 351.
Vandenberg, N.R., Clark, T.A., Erickson, W.C., Resch, G.M.,
Broderick, J.J., Payne, R.R., Knowles, S.H., Youmans,
A.B. (1973) Ap. J. Lett. 180, L27.
Whitney, A.R. (1974) Ph.D. Dissertation, M.I.T.
V
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NASA-GSFC


High resolution observations of Cassiopeia A at meter wavelengths

High resolution observations of Cassiopeia A at meter wavelengths

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