From the top portion of Fig. 1 (click here) and Fig. 2 (click here), it is obvious that the present analysis is severely limited by dynamic-range problems in the vicinity of Cygnus A, and one might ask whether the very low flux density found for G76.9+1.0 (it is barely visible in Fig. 2 (click here)) is an error of observation. From the plots in Fig. 3 (click here)a, however, the 232-MHz flux densities appear reliable. Apart from G76.9+1.0, most of the other extended sources are compact HII regions. They do have very low 232-MHz flux densities but these can generally be attributed to internal self-absorption. The one source for which the 232-MHz flux density seems to be particularly low, however, is Source 13 (DR 13). The HII region spectra are discussed in more detail in Sect. 4.6.3, where it is shown that a low flux density at 232 MHz may be expected for DR 13 because of its environment. Therefore, we believe that the 232-MHz flux densities are generally reliable to the quoted level of error.
Figure 3: a) Top and bottom left: Radio spectra of the sixteen
radio sources. The dashed lines merely show trends; they connect points on
a best-fitting unweighted quadratic
curve through the observed data points. For sources with typical thermal
spectra that have a sharp transition from optically thin to optically
thick, the quadratic curve is a poor representation even of the data trend
(cf. Source 13). b) Bottom right: Radio spectrum of the supernova
remnant G76.9+1.0. The solid line is a best-fit to the weighted data
points above 1 GHz. The low-frequency (below 1 GHz) flux densities
fall well below this line and this is may be an intrinsic property of the
source. The dashed line gives the spectrum best fitting all the weighted
data points, ignoring the 327-MHz point
Table 4: Approximate positions and sizes of all sources
Table 5: Flux densities of the comparison sources
On the basis of the data presented in Fig. 3 (click here)b, one may therefore conclude that the radio spectrum of G76.9+1.0 flattens below 1 GHz, or that the 327-MHz flux values are unreliable and the SNR has a spectral index of about 0.5 over the whole frequency range. We consider the former to be more likely.
The Cygnus X environment is one in which ionized interstellar gas is
widespread. Can foreground distributed HII regions provide significant
absorption at 232 MHz? The background brightness temperature at 408 MHz in the
general vicinity of G76.9+1.0 is about 100 K (Wendker et al. 1991).
Assuming an electron temperature of 6000 K (based on the
recombination line measurements of Landecker 1984)) this implies
an optical depth (
) of 0.017, which translates to
= 0.054 at 232 MHz. Absorption will therefore account for
only 
5% of the apparent flux density deficiency at 232 MHz,
rather than the >50% observed. We conclude that the observed
low flux density cannot be attributed to foreground absorption.
Landecker et al. (1993) discussed the nature of G76.9+1.0, considering that the object could be (a) a filled-centre SNR, (b) a (relatively nearby) normal galaxy, or (c) a large radio galaxy. Interpretation (b) was ruled out because of the very low level of infrared emission, and interpretation (a) was favoured over (c) on morphological grounds, in particular the absence of any discernible edge to the emission, corresponding to the working surface where the jets of a radio galaxy interact with the surrounding medium. On the other hand, the steep radio spectrum above 400 MHz seemed to favour the radio-galaxy interpretation.
Most SNRs classified as the filled-centre type have flat spectra
(
). This is hardly surprising, since a spectral index
in this range is considered a strong indicator of this class of object, as
long as it is accompanied by evidence that the emission is non-thermal
(usually the detection of polarized emission). However, some filled-centre
SNRs show a spectral break (e.g. G74.9+1.2, Morsi & Reich 1987), with a
considerably steeper spectrum above a
certain frequency. This fact has become apparent only recently, with the
increasing sensitivity of radio telescopes in the millimetre range
(Salter et al. 1989). On the basis of the data in Fig. 3 (click here)b, it is possible
that G76.9+1.0 is a member of this class of object, but with a break
frequency lower than that of any other known SNR - below 1 GHz.
It should be noted, however, that, regardless of the precise interpretation
of the data in Fig. 3 (click here)b, G76.9+1.0 has a spectral index above 1 GHz in
the range of 0.5- 0.6. This is steeper than normally expected for a
filled-centre remnant.
Table 6: Flux densities of other sources
This source is a blend of the filled-centre remnant, G74.9+1.2, and a
neighbouring background point source.
The latter has a spectrum that peaks near 10 GHz and then decreases
towards lower frequencies (with 
-0.3). The spectrum shown in
Fig. 3 (click here)a indicates that the total flux density
is dropping at frequencies below 400 MHz, whereas the blended spectrum
that is expected by extrapolating higher-frequency data
(Morsi & Reich 1987) is still rising at lower frequencies. (Note that
the 4C spectral point is a lower limit because of the ``limited'' response of
that telescope to extended sources). The data
presented here suggest that G74.9+1.2 may have a second break frequency
in its spectrum. On the other hand, Kovalenko et al. (1994)
report
relatively high flux densities of 17 
4 Jy and 16 
4 Jy at
frequencies
of 83 and 111 MHz, respectively - consistent with the extrapolated
higher-frequency data. The 232-MHz flux density reported in this paper
must therefore be considered with caution.
The ON 2 region consists of several compact
HII regions, with two main components. Seven components in all have been
identified by Matthews & Spoelstra (1983). The OH maser source, which gives
its name to the complex, is near the southern main component. In Fig. 4 (click here), the
low-frequency radio spectrum of this region is presented, using the results
from Table 6 (click here), together with data at 610 MHz (Matthews & Spoelstra 1983)
and 10.7 GHz (Matthews et al. 1977). Other existing data from interferometric
observations have been omitted since short-spacing flux density is missing.
Matthews and Spoelstra gave model parameters for the seven components, and
the expected spectrum from those components is given by the dotted curve
in Fig. 4 (click here) - a poor fit to the data. The fit can be improved, but only by
reducing the electron temperature to unreasonably low values. It seems that
a model of dense ``knots'' of emission fits the observed data better.
The simplest way of modelling
such a clumpy medium is to assume that each of the radio source components
consists of a number of small ``knots'', unresolved in existing observations.
The solid curve in the figure shows the spectrum of one such model: 260
spherical clumps, 6'' in
diameter, with an electron density of 3000 cm
and an electron
temperature of 7500 K, at a distance of 5.5 kpc. If a nearer distance
is assumed, say 900 pc (cf. Dent et al. 1988), the electron density
would have to be even higher, 
7400 cm
. Of course, the combination
of number of clumps and clump size is very arbitrary, and other choices
would be equally compatible with existing observations.
These HII
regions all have radio spectra showing self-absorption turn-overs.
Higher-resolution images of the last three objects exist in the literature:
DR 6 (Odenwald et al. 1986), DR 9 and 13 (Wendker et al. 1991).
These show that the sources have complex radio structures. If one
attempts to fit simple (uniform-density) thermal models to the observed
data in Table 6 (click here), one finds, as for the ON 2 complex, that a ``clumpy''
electron-density distribution is required. If an arbitrary distance of
1.5 kpc is assumed for all of these sources, a model in which the electron
density is restricted to spherical clumps 15'' in diameter of uniform
electron density, with an electron temperature of 7500 K, gives the solid
curves shown in Fig. 5 (click here). For the two stronger sources, DR 6 and DR 13, the
electron density of
the clumps is taken to be 1500 cm
, while for the other two,
2000 cm
was used. The numbers of such clumps required to give the
observed flux density were 40 (G74.76+0.62),
160 (DR 6), 60 (DR 9) and 390 (DR 13). If different distances, D, should
be more appropriate, these electron densities scale as 
.
Figure 4: The low-frequency spectrum of the ON 2 cluster of
radio sources. The dotted curve gives the predicted spectrum from the
model (of seven components) given by Matthews & Spoelstra (1983), while
the solid curve results from a model consisting of a large number of small,
high-density ``knots''
As noted earlier in the paper, the 232-MHz flux density for DR 13 seems
extremely low. If a linear spectrum is fitted to the data points in Fig. 5 (click here)
below 1 GHz, a spectral index of -2.7 
0.4 is found, steeper than
the -2.0 that internal self-absorption allows. The lowest 232-MHz flux
density that a thermal spectrum would allow would be close to 10 Jy. Indeed,
the fitted solid curve in the figure would predict a value close to 15 Jy.
On the other hand, of these four HII regions, DR 13 is the one which is
situated in the strongest area of extended surrounding radio emission.
Since flux densities are derived from excess brightness above a surrounding
background, the opacity properties of the background must be considered.
If, for example, the clumpy model of DR 13 used above were immersed in an
extended HII background of emission measure, E 
5 10
cm
pc
(about one fifth that of the clumps themselves),
the clumps will fade at lower frequencies and the dotted spectrum shown
in Fig. 5 (click here) would result. (This means that the apparent spectrum can drop much
faster than the normal black-body curve). Although the 232-MHz point
still lies below this modified spectrum, it does indicate that the detailed
structure of DR 13 and its environment may cause the source to disappear
rapidly as one observes at lower frequencies.
Figure 5: The low-frequency spectra of the optically-thick HII
regions in the area of this study. The solid curves give representative
spectra for models consisting of a large number of high-density ``knots''.
For DR 13, the dotted curve shows the effect of immersing these knots or
clumps in an extended medium of emission measure of about one fifth of
that of the clumps themselves
Of the sixteen sources
examined in this paper, two exhibit very steep radio spectra, No. 7 and No. 11
(4C 36.40). The spectral indices found for these sources, using the data
in Table 6 (click here) (without weighting), are 
and
, respectively. Both appear to be point sources
for the resolutions used in this study.
From Fig. 3 (click here)a, one sees that this source has a non-thermal spectrum above 1 GHz, but which appears to flatten at lower frequencies. The source appears to be slightly resolved at the resolutions used in this paper and hence would seem to be a candidate for further investigation - possibly a small-diameter SNR. However, Green (1985) has observed this source (G75.43-1.30) at high resolution and has found it to be unresolved. It is therefore likely to be extragalactic.
This object is a well-known bipolar nebula
(cf. Staude & Elsässer 1993) and has been the subject of many
radio studies (e.g. Israel & Felli 1978; Bally et al. 1983;
Felli et al. 1984). This object consists of two intense radio ``lobes''
bracketting a weak radio source with a mass-outflow spectrum. The lobes
have a complex radio structure, with much of the radio emission apparently
coming from ionization fronts on the surface of molecular lobes. The data
obtained in this paper define the low-frequency spectrum of
these radio-continuum lobes. In Fig. 6 (click here), a model spectrum
is shown along with the observed data points. The model, assumed to
be at a distance of 500 pc, consists of two
conical lobes, with their axes at right angles to the line of sight. The
cones have opening angles of 
, a minimum radius (from the central
outflow source) of 5'', a maximum radius of 1.2', an electron temperature
of 10
K, and an electron density varying as 
with a value of
cm
at a radius of 1'. This is, of course, a crude
approximation to a very complex object, but gives order-of-magnitude
estimates of the physical parameters.
Figure 6: The low-frequency spectrum of the S 106 HII region.
The representative spectrum indicated by the curve is that expected from
a bi-conical source with radially-decreasing electron density - a crude
approximation to the observed lobe structure