Äîêóìåíò âçÿò èç êýøà ïîèñêîâîé ìàøèíû. Àäðåñ îðèãèíàëüíîãî äîêóìåíòà : http://www.mrao.cam.ac.uk/yerac/vessey/vessey.ps Äàòà èçìåíåíèÿ: Thu Feb 23 02:07:31 1995 Äàòà èíäåêñèðîâàíèÿ: Tue Oct 2 01:47:27 2012 Êîäèðîâêà: Ïîèñêîâûå ñëîâà: galactic plane

A 151 MHz Galactic plane survey
By S i m o n Ve s s e y
e­mail: s.j.vessey@mrao.cam.ac.uk
Mullard Radio Astronomy Observatory, Cavendish Laboratory, Madingley Road, Cambridge
CB3 0HE, UNITED KINGDOM
The Cambridge Low Frequency Synthesis Telescope (CLFST) is currently being used to under­
take a systematic survey of the Galactic plane at a frequency of 151 MHz. Over the past four
years observations covering the Galactic plane with jbj Ÿ 5 ffi between the Crab Nebula (near
l = 180 ffi ) and Cyg A (near l = 70 ffi ) have been made, with a resolution of just over 1 0 . This
is a relatively high survey resolution for such low frequency observations (cf. 5 0 resolution for
the Effelsberg 2.7 GHz survey (Reich et al. (1990))). The CLFST is not merely limited to the
detection of compact sources and since non­thermal sources are relatively bright at low frequen­
cies the CLFST is an ideal instrument for imaging extended non­thermal structure on scales of
arcmin to a few degrees, allowing the systematic study of a number of well­known supernova
remnants (SNRs) which lie within the Galactic plane.
1. The CLFST
The CLFST is an East--West Earth­rotation synthesis telescope operating at 151 MHz
with a bandwidth of 800 kHz. A single observation takes 12 hours, sampling every
30 seconds, and has a useful field of view of about 8 ffi . The telescope consists of 60
steerable aerials, each composed of 4 Yagi antennae, spaced at irregular intervals along
a 4:6 km baseline. This maximum baseline translates to a maximum resolution of 71 00 \Theta
71 00 cosec(ffi). The CLFST has an extremely well­filled aperture, with 784 spacings at 3–
intervals, the lowest spacing being 6–.
2. The purpose of a 151 MHz Galactic plane survey
The object of any Galactic survey is to discover new Galactic objects and study their
statistics. Obviously, most Galactic objects will lie close to the Galactic plane, but even
directly in the plane, the vast majority of the observed radio sources are extragalactic,
and so a fairly large region of the plane has to be surveyed to have a chance of obtaining
a reasonable sample of Galactic objects.
Other surveys of the Galactic plane have been made, for example the Effelsberg 2.7­
GHz survey of Reich et al. (1990), and a number of surveys cover a substantial fraction of
the plane, e.g. the 87GB surveys of Becker et al. (1991), and Gregory & Condon (1991),
however these surveys tend to be at relatively high radio frequencies with comparatively
low resolution.
The usefulness of a 151 MHz survey is that at low frequencies synchrotron emitters
are much more prominent than thermal emitters, so a low frequency survey is ideal to
look for Galactic synchrotron emitters, e.g. pulsars and SNRs. No other survey has been
made at such a low frequency with such a high resolution.
3. Observations and data reduction
The fairly large field of view of the CLFST means that 25 survey fields were observed
over the four year period to cover the whole survey area, each field being observed for
1

2 Simon Vessey: A 151 MHz Galactic plane survey
Figure 1. A quadrant in the l = 83 ffi
:5 field, before the removal of Cyg A grating responses.
The greyscale is \Gamma0:1 to +0:3 Jy beam \Gamma1 but is uncalibrated, and should be scaled by a factor
of 2:50.
at least 2 days. The fields were chosen so that the survey has complete coverage of the
plane over the same latitude range as the Effelsberg survey (jbj Ÿ 5 ffi ). Maps made from
individual days' data were combined before a source fitting algorithm was applied to
obtain the highest signal to noise possible.
Data processing was performed in a standard way, using interference clipping, removal
of bright sources and correction for ionospheric effects. The processing software was
custom­written at MRAO for the CLFST system. One thing that is not done is beam
deconvolution --- this is difficult because the synthesised beam shape is dependent on
position, however the excellent u­v coverage of the telescope means that the synthesised
beam is very ``clean''.
3.1. Interference clipping
Interference can come from a variety of sources, and falls into two catagories, namely:
(a) ``One­off'' interference affecting a large number of spacings leading to parallel­line
artefacts with position angle related to the time at which the interference occurred. This
type of interference can easily be dealt with by applying a spacing­dependent clip to the
visibility data, eliminating all samples where the signal rises above the clip level.
(b) More persistent low­level interference with a constant phase relative to the tele­
scope baselines throughout the whole of (or a large part of) an observation leads to
grating responses due to an apparent source at the North Pole, as this is the only point
in the sky whose phase remains constant with respect to an Earth­rotation synthesis tele­
scope during the 12­hour observation period. If a simple interference clip is not sufficient
it is necessary to treat the false source at the North Pole as a ``real'' one and remove it
(see below).

Simon Vessey: A 151 MHz Galactic plane survey 3
Figure 2. The same quadrant as before, with the same greyscale, but with Cyg A removed.
The ``shell'' SNR G84:2 \Gamma 0:8 is clearly visible towards the centre­left of the map, along with
some extended emission from the Cyg X region towards the south­west of the map.
3.2. Removal of bright sources
There are a number of bright radio sources within or close to the Galactic plane which
produce grating rings on maps (Figure 1), even though the sources in question may not
be within the 8 ffi field of view of the telescope. Cyg A is nearby the field shown, near RA
19 h 58 m , DEC 40 ffi 36 0 , and its grating responses are so strong that they dominate the map,
obscuring the real sources. These rings can be removed using a standard algorithm (see
Figure 2), but problems may arise due to ionospheric effects (see below) which prevent the
rings being completely eliminated. This is especially a problem with the three brightest
radio sources in the sky, the Crab Nebula, Cas A and Cyg A, all of which lie very close
to the survey region and have 151 MHz fluxes of thousands of janskies.
3.3. Correction for ionospheric effects
Metre­wavelength radio waves are effected by ionospheric scintillation, and the most
obvious problem with maps made from raw data is the ionosphere. The Sun can create
waves in the ionosphere which phase­shift incoming radio waves and effectively cause
point sources to ``twinkle'', thus smearing them out on a map. So long as the waves are
of fairly long wavelength with relatively long coherence lengths and times, it is possible
to correct for this by applying a phase correction to each individual 30 second sample.
The required set of phase corrections can be calculated by choosing a ``phase calibration
source'' which is known or assumed to be unresolved, and then determining the phase
corrections which must be applied to in order to actually make that source look like a
point source on the map. This assumes a simple linear phase gradient (i.e. position
shift) across the telescope for any given sample. The gradients are then simply removed
from the data. If the ionospheric waves are short wavelength or have short coherence

4 Simon Vessey: A 151 MHz Galactic plane survey
lengths or times, a correction that is appropriate for one part of the map will not work
for another part and in very bad cases the whole observation must be discarded.
3.4. Source fitting
The CLFST is about 3 ffi from East--West and is not quite straight. This means that
the visibility data should be laid down in a (u; v; w) space rather than a (u; v) plane.
Inverting the visibilities using 3­D Fourier transforms is impractical and so the data
is extrapolated into a (u; v) plane and standard 2­D transforms are used. This means
that the shape of the synthesised beam depends on the distance from the observation
phase centre. The beam becomes ``C''­shaped rather than being a simple Gaussian, the
distortion becoming much more noticeable towards the edge of a map. Apart from making
beam deconvolution very difficult, this position­dependent distortion also complicates the
source fitting procedure.
Since the cause of the beam distortion is known, and the shape of the synthesised beam
can be accurately calculated at any point in the sky, an appropriate synthesised beam
should be fitted to each source in order to derive its flux and position. Unfortunately,
this would take a prohibitive amount of computer time, and a compromise solution is
used, whereby a grid of synthesised beams is calculated accurately (a ``beam set'') and
the synthesised beam at any point of the map is then interpolated in a linear way from
the shapes of the nearest beams to that point (Waldram & Riley (1993)).
3.5. Final source list
Eventually, after a great deal of processing, map making, source fitting to theoretical
beams and calibration, the result is a list of approximately 6900 small­diameter sources,
some of which are Galactic objects. The sensitivity of the survey is not uniform, but
rather depends on two not entirely independent quantities, namely the noise level in each
survey field and the mean large­scale sky background temperature seen by the primary
beam of the CLFST.
The raw noise level, which depends on the quality of the individual observations, is
significantly reduced by the data processing described earlier and by combining more
than one day's data together.
The CLFST has an automatic gain correction (AGC) system which reduces the sensit­
ivity of the telescope (and thus the ability of the telescope to detect faint sources) when
the mean background temperature is large. The mean background changes considerably
over the Galactic plane, and so the sensitivity varies across the survey. The flux density
scale must be scaled accordingly, in some cases by a factor more than 2.5. The flux scale
used is that of Roger et al. (1973).
The noise level ranges from 24 mJy to an extreme of 314 mJy near Cyg A, with a
median noise level of 39 mJy. Figure 3 shows the absolute noise level as a function of
Galactic longitude for all the fields in the survey.
4. Galactic objects in the survey
4.1. Pulsars
In the radio continuum, pulsars generally have very steep spectra, with a spectral index
ff (defined by S / š \Gammaff ) of perhaps 2 or more. This makes them very distinctive, but
requires a high frequency survey that is sensitive enough to detect very faint compact
sources if ff is to be calculated accurately.
Current pulsar catalogues are heavily influenced by selection effects. The observed
concentration of known pulsars within the first and fourth Galactic quadrants does not

Simon Vessey: A 151 MHz Galactic plane survey 5
Figure 3. 1oe noise levels for the 151 MHz Galactic survey.
reflect the true pulsar distribution. There should be undiscovered pulsars within the
region covered by this 151 MHz survey that are detectable in the radio continuum.
4.2. Supernova remnants
Around 180 Galactic SNRs are known (Green (1991)), but some authors estimate that
almost this number remain undiscovered. SNR catalogues are also heavily affected by
selection effects (Green (1991)). The number of SNRs in the Galaxy at any one time and
the rate of supernova explosions are quite important numbers to know, since supernova
explosions are a primary mechanism for energizing the interstellar medium and re­cycling
material into it.
The CLFST is ideal for looking at extended SNRs because it is sensitive to structures
on scales of up to a few degrees. SNRs have synchrotron spectra with spectral indices in
the range 0:3 Ÿ ff Ÿ 0:7, and so even though many SNRs lie close to complex thermally
emitting regions, they are clearly visible using the CLFST because at low frequencies
non­thermal emission is enhanced and thermal emission depressed relative to that seen
at high frequencies (e.g. in the Cyg X region, where a number of non­thermal shells
are clearly visible at 151 MHz). Previous searches for SNRs at higher frequencies have
discovered new extended SNRs in the very recent past, but even if no new extended
SNRs are found I will be able to place an upper limit on the surface brightness of any
that do exist over a large portion of the Galactic plane.
A number of well­known extended SNRs are within the survey region, for example
IC443 (Figure 4), HB3 and HB9, and by comparing the 151 MHz data with images made
at other frequencies fairly detailed spectral index maps of the SNRs can be made. The
spectral index depends on the shape of the relativistic electron energy distribution, so
these comparisons can lead to knowledge of the actual physical conditions within the
SNR.
Another important question is whether or not SNRs have pulsars associated with them.
A type II supernova explosion is supposed to produce a neutron star, however very few
SNRs have ever had pulsars associated with them. Any new suspected pulsar that lies
within the boundary of a SNR would be extremely interesting.

6 Simon Vessey: A 151 MHz Galactic plane survey
Figure 4. 71 00 \Theta 185 00 resolution map of IC443.
In addition to these large SNRs, I am also looking for more compact objects, on the
scale of a few arcmin. Such objects are indistinguishable from extragalactic sources
using spectral data alone. Assuming an expansion velocity of around 10 4 km s \Gamma1 any
SNR within our own Galaxy that is around a thousand years or more old should have an
angular size of a few arcmin, and will therefore be resolved at the maximum resolution
of the CLFST. Thus searching for slightly resolved sources with synchrotron spectral
indices may be more productive.
Another way of proceeding is to use radio­infra­red comparisons. Galactic SNRs should
have a detectable infra­red (IR) flux due to dust heated by the supernova shock­wave,
whereas the IR from radio galaxies is negligible in comparison. The IRAS point source
catalogue is an ideal place to look for infra­red associations, because it has a very similar
resolution to the 151 MHz Galactic survey. Chance matches due to associations with
stars can be eliminated using the IRAS colours, and since SNRs have a distinctive ratio
of 60 ¯m to radio flux compared to that of thermal emitters (e.g. Haslam & Osbourne
(1987), F¨urst et al. (1987)) they can be distinguished from Hii regions and PNe by this
ratio or by using the IRAS colours once more. Work on this topic is currently in progress.
REFERENCES
Becker R.H., White R.L. & Edwards A.L., 1991, ApJS, 75, 1.
F¨urst E., Reich W. & Sofue Y., 1987, A&AS, 71, 63.
Green D.A., 1991, PASP, 103, 209.
Gregory P.C. & Condon J.J., 1991, ApJS, 75, 1011.
Haslam C.G.T. & Osbourne J.L., 1987, Nature, 327, 211.
Reich W., F¨urst E., Reich P. & Reif K., 1990, A&AS, 85, 633.
Roger R.S., Bridle A.H. & Costain C.H., 1973, AJ, 78, 1030.
Waldram E.M. & Riley J.M., 1993, MNRAS, 265, 853.