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1992AJ....104..704Sramek+
A VLA search for young Galactic Supernova Remnants
(OCR+proof by H.Andernach 4+10/98)
R.A. SRAMEK
National Radio Astronomy Observatory, P.O. Box 0, Socorro, New Mexico 87801
J.J. COWAN
University of Oklahoma, Department of Physics & Astronomy, 440 W. Brooks,
Norman, Oklahoma 73019
D.A. ROBERTS AND W. M. Goss
National Radio Astronomy Observatory, P.O. Box 0, Socorro, New Mexico 87801
R.D. EKERS
There are tables 1-4 in CATS.
In an attempt to identify very young galactic supernova remnants (SNRs),
290 known compact (<2') galactic plane radio sources were observed at 20 cm
using the Very Large Array (VLA) in its 36 km configuration. The VLA
observations described could detect supernovae with diameters between 5"
and 20"; these would have an age of between 25 and 100 yr. Compact
structure was detected in 168 (58%) of the 290 different sources observed;
73 sources (25%) were unresolved or slightly resolved point sources, 21 (7%)
were single-well resolved, 56 (19%) were double, 18 (6%) were triple or
complex. The large scale structure was completely resolved out for 122
(42%) of the sources. Additional observations at 6 cm with the VLA in the
11 km configuration were made of 14 sources with apparent shell structures
that might have been characteristic of young SNRs. Low resolution
observations were made at 20 cm of 62 fields where the source was completely
resolved out in the high resolution images. Only one source, G25.5+0.2, is
a possible very young SNR. New H66alpha recombination line observations place
severe constraints on any thermal interpretation for this object.
1. INTRODUCTION
Despite a predicted supernova rate of one every 20 to 50 yr, there have
been no optical detections of supernovae in our Galaxy in nearly 400 yr.
The usual explanation for this discrepancy is that optical obscuration has
prevented detection of recent supernovae. However, such obscuration would
not prevent radio detections.
It is well established that radio emission can be detected from supernovae
within a few years of maximum light and from supernova remnants (SNRs) with
ages in excess of 300 yr (Weiler et al. 1986; Weiler & Sramek 1988). Now
there is a growing body of evidence that some extragalactic supernovae
remain radio emitters for at least several decades after optical maximum
and these objects would have a radio flux density of many tens of Janskys
even if located across the Galaxy (Cowan & Branch 1985; Cowan et al. 1988;
Cowan et al. 1991). These observations suggest that if recent galactic
supernovae were missed due to optical obscuration, a radio search might
identify the very young SNRs.
There have been several searches attempting to identify galactic SNRs.
Helfand et al. (1984) put an upper limit on the number of Crab-like SNRs in
the Galaxy. In an attempt to identify young SNRs with ages between 100 and
1000 yr, Green & Gull (1984) and Green (1985, 1989) observed a number of
compact galactic plane radio sources using a 4 arcsec beam. In addition to
these SNR searches, there have been several recent surveys of the galactic
plane that identified compact radio sources (see, e.g., Garwood et al.
1988; Jones et al. 1988; Zoonematkermani et al. 1990; Becker et al. 1990).
We report on here a search for young SNRs or old radio supernovae using the
Very Large Array (VLA). Supernovae at a distance of 20 kpc which have been
expanding at 10 000 km/s for 10 to 100 yrs will have diameters between 2
and 20 arcsec. The VLA, in its largest configuration at 20 cm wavelength
can resolve structures with diameter greater than 1" but a short
observation is not responsive to structure with angular extent greater than
about 20 arcsec. The VLA can therefore serve as a spatial filter to detect
young supernovae remnants in our Galaxy; this paper presents the results of
such a search. The higher angular resolution of this search will select
younger, less evolved objects than the Green and Gull searches mentioned
above.
Our criterion for a young SNR is a compact source in the galactic plane
with a shell-like structure and a nonthermal spectrum. The nonthermal
spectrum is seen in young radio supernovae and distinguishes the supernovae
from compact galactic H II regions which also have shell-like structures,
but with rising or flat spectra at cm wavelengths (Wood & Churchwell 1989).
Two hundred and ninety previously cataloged radio sources were observed at
20cm wavelength. Radio spectra, radio recombination line, hydrogen emission
line, and low resolution observations were obtained for a subset of these SNR
candidates. In the end only one object, G25.5+0.2, is considered a possible
young sUpernova remnant (see Cowan et al. 1989). Here we present the results
on the other 289 sources, many of which show interesting structure and may
warrant further study.
2. OBSERVATIONS
The search list was compiled from three single-dish radio surveys of the
galactic plane. Unresolved sources brighter than 0.6 Jy were selected from
the 408 MHz survey of Clark & Crawford (1974) (beamwidth of 2.9 arcmin),
brighter than 0.26 Jy from the 5 GHz survey of Haynes et al. (1979)
(beamwidth of ~4.2 arcmin) and from the 4.9 GHz survey of Altenhoff et al.
(1978; beamwidth of 2.6'). The H110alpha radio recombination line survey
of Downes et al. (1980) was used to remove HII regions from the search
list. A list of 500 SNR candidates was produced in this way. Owing to
limited observing time, only the brightest 291 of these sources were
observed during several VLA sessions in late 1984. Using either a measured
spectral index or assuming a spectral index of 0.5, the limiting total 20 cm
flux density of the sources observed was about 0.5 Jy for 340<1<60 and 0.2 Jy
for 190d< l < 260d. Due to confusion in the single-dish observations,
G21.05+1.9 and G21.06+1.9, listed as separate sources in different
catalogs, turned out to be the same source. Thus, a total of 290 unique
sources were studied.
The search for young SNRs progressed in five steps.
(a) A high resolution survey was done at 20 cm of the 290 objects to
select sources with shell structure and size <20".
(b) Follow-up observations were made at 6 cm to obtain radio spectra of 14
candidate shell sources. Based on their spectra and morphology, nine
sources were determined to be compact HII regions or background
extragalactic radio sources and were eliminated from further study.
(c1) Spectral line studies were made of the remaining five objects.
Hydrogen recombination lines observations were made of three sources with
shell structure and flat spectra. Sources with recombination line emission
were likely optically thin, compact HII regions and were eliminated from
our list. Neutral hydrogen 21 cm observations were made of two sources with
shell structure and steep spectra; the presence of absorption features due
to galactic hydrogen was used to establish the extragalactic nature of
these sources.
(c2) Additional continuum observations were made of the five objects at
6 and 20 cm to study their radio structure in detail. After these
observations there was one possible galactic SNR, G25.5+0.2.
(d) 20 cm low resolution observations were made of 62 sources which were
not detected in the original VLA survev. The high resolution survey
would have rejected SNR which were larger than 20"; these later observations
would have detected structures as large as 210".
(a) VLA 20 cm high resolution observations. Data were obtained in November
and December 1984 in the largest VLA configuration (the 36 km, A
configuration) at 20 cm wavelength. A two minute observation was made of
each field and a 512 x 512 image with a cell size of 0.4" was produced.
The synthesized beam was typically 1.7 X 1.3 arcsec with an rms noise level
(mostly due to confusion) of about 1 mJy. Following normal procedures, 3C
286 and 3C 48 were used as the primary flux density calibrators for the
galactic center and galactic anticenter regions, respectively.
Only very basic information was extracted from these images. The source
structure and peak flux density were noted, and if the source position of
the compact component differed significantly from the catalogued position
then a position correction was recorded. A source was regarded as detected
if there was a feature with a peak flux density greater than five times the
rms noise level within a 3.0 arcmin box centered on the catalogued
position. Otherwise the field was designated as "empty". The quoted
accuracy of positions in the catalogues ranged from 0.15' to 0.5'. The
fact that we call a field "empty" usually means that the source detected in
the single-dish survey was resolved out by the interferometer array.
With only 2 min of data in a complicated region like the galactic plane,
the images often have very high background noise levels owing to partially
sampled large scale structure in the image or from sidelobes of sources
somewhere in the primary beam. The detection limits for these observations
varies greatly. The results of the initial survey are presented in Table 1.
Keep in mind that these results refer only to the compact structure in the
sources.
The columns in Table 1 are as follows:
Column 1: Galactic longitude derived from the original survey catalog.
Column 2: Galactic latitude derived from the original survey catalog.
Column 3: Right ascension (1950.0) as listed in the original survey catalog.
Column 4: Declination (1950.0) as listed in the original
survey catalog.
Column 5: Peak 20 cm flux density on VLA image in a 3.0 x 3.0 arcmin box
centered on the catalogued position. For double and complex sources the
peak flux density refers only to the brightest component. No attempt was
made to measure integrated flux densities. Approximate upper limits (5 sigma)
are given for fields in which the source is resolved out.
Column 6: The radio structure of the dominant high brightness compact
source. The symbols are P= point source or slightly resolved, S=single
extended source, D= double, C= complex structure with several knots and/or
filaments, and E= empty field, the catalog source is probably resolved out.
In addition to these descriptions of the compact structure, some sources
may have weak extended background features (see notes). Other sources in
the vicinity which are outside the 3 x 3 arcmin "detection" box are also
referred to in the notes.
Column 7: Epoch of the observation. The "B" observations were of objects
toward the galactic center, while the "A" observations were toward the
galactic anticenter. The dates of the observations were:
A1= 29 November 1984;
A2= 30 November 1984;
B1= 7 December 1984;
B2= 24 December 1984; and B3, 28 December 1984.
Column 8: Reference to the original survey catalog used for source positions.
The catalogs were:
1= Clark & Crawford 1974
2= Haynes et al. 1979
3= Altenhoff et al. 1978.
Column 9: New right ascension; if the peak of the compact structure is more
than 0.5 arcmin from the catalogued position, an improved R.A. is given
(seconds of time, error +-0.8s/cos(DEC)).
Column 10: New declination: if the peak of the compact structure is more
than 0.5' from the cataloged position, an improved Dec. is given
(arcmin, error +-0.2') .
Column 11: See notes for more information.
(b) The 6 cm observations. The initial 20 cm observations indicated that 14
sources had some type of shell structure. To determine the spectral index
of these apparent shell sources, further observations lasting 8 min on each
object were made at 6 cm with the VLA in the 11 km, B configuration, which
matches the resolution of the 20 cm observations. The results of these
observations are listed in Table 2. The sources toward the galactic center
(340 < l < 60) were observed on April 17, 1985. The sources in the region
190 < l < 260 were observed on April 19, 1985. The beamwidth for these
images was 1.7"x 1.2", the same as for the 20 cm observations.
Four shell sources, G2.6+0.1, G5.4-0.2, G10.1+0.7, and G31.4+0.3, had
rising spectra (alpha>+0.3) which characterizes them as optically thick
HII regions [see Wood & Churchwell (1989) for follow-up observations].
Another four sources, G213.9-0.7, G226.3-0.1, G248.4+0.6, and G251.2-0.6,
had steep spectra and the 6 cm images did not confirm their shell structure.
Based on their morphology and steep spectrum these objects are thought to be
extragalactic background sources. Another source, G356.9+0.0, had a
flat spectrum but no shell structure. It is possibly a compact H II region.
No further work was done in this program on these nine sources. This left
five sources, G10.3+0.8, G21.8+0.0, G25.5+0.2, G34.1+0.4, and G213.6-0.6,
which had confirmed shell structure and spectral indices of < 0.3.
These five were considered candidate SNRs for future observations.
(c1) The final five, radio recombination, and HI line observations. H76alpha
radio recombination line observations were made of the three flat spectrum
shell sources G21.8+0.0, G25.5+0.2, and G34.1+0.4. These observations were
made on February 1 and 2, 1986 with the VLA at 2 cm in the D configuration.
The line to continuum ratio for G21.8+0.0 was 0.18 and for G34.1+0.4 it
was 0.14. These observations establish that both objects are H II regions.
The H76alpha observations of G25.5+0.2 showed no recombination lines suggesting
that this source is not an HII region (see Cowan et al. 1989).
Neutral hydrogen line observations made at the VLA of the steep spectrum
sources G10.3+0.8 and G213.6-0.6 showed galactic absorption features that
suggests that these objects are extragalactic.
(c2) The final five, extended observations. Extended observation of the
final five objects were made at 6 and 20 cm to provide better u,v coverage
and more complete images. These sources were observed for 2 hr each with
the VLA in the 36 km configuration on May 11, 1986 and June 2, 1986. These
longer observations had a beamwidth of 0.4" at 6 cm and 1.2" at
20 cm. This is slightly smaller than the beamwidth for the survey and,
therefore, the brightness of these extended sources (Jy/beam area) appears
slightly smaller than the values given in Tables 1 and 2).
The steep spectrum objects G10.3+0.8 and G213.6-0.6 are shown in
Figs. 1a and 1b. They have double structure which is consistent with their
being extragalactic. The compact HII regions with ringlike structure,
G21.8+0.0 and G34.1+0.4, are shown in Figs. 1c and 1d.
The remaining object, G25.5+0.2, has a relatively flat spectrum (alpha=-0.05),
but unlike G21.8+0.0 and G34.1 +0.4, it does not show the H76alpha
recombination line (Cowan et al. 1989). An additional search for the H92alpha
recombination line was also negative with a 3 sigma upper limit on the
line-to-continuum ratio, L/C, of 0.03. Cowan et al. conclude that the most
likely explanation for G25.5+0.2, is that it is a very young SNR.
White & Becker (1990) argue that the observations could better be explained if
G25.5+0.2 is an extremely massive planetary nebula. Green (1990) notes
that the apparent association of the IRAS source 18344-0632 with G25.5+0.2
suggests a thermal nature for this object. More recently, Zijlstra (1991)
argues against the planetary nebula interpretation and suggests instead
that the observations could be consistent with young outflow objects.
On 12 March 1991 further VLA observations at 1.3 cm were made of G25.5+0.2.
These observations were an attempt to detect the H66alpha line and spanned
11 hr. The total bandwidth observed was 25 MHz with 16 channels and the
channel separation was approximately 20 km/s with a total velocity range of
330 km/s. The beam size was about 3" and the rms noise in the continuum was
0.3 mJy; the noise on the spectral line was 1.4 mJy. The results were again
negative. No recombination iine was detected in the spectrum integrated over
the source, with a 3 sigma upper limit to the L/C ratio of 0.02.
These observations were designed to search for a broad, weak line.
An HII region with recombination lines Doppler broadened by 50 km/s and an
electron temperature of T_e= 10^4 K would typically show an L/C ratio of
approximately 0.10 for the H66alpha line. Our upper limit, which is much below
this, indicates than an HII region or a planetary nebula with these
velocity widths would have to have a temperature of about 40000 K to be
below our detection limit. For even larger velocities of 100 km/s our
observations set a lower limit on T_e of 22 000 for thermal sources.
This third and most stringent upper limit on hydrogen recombination line
emission from G25.5+0.2 puts severe constraints on any thermal interpretation.
We should also note that the flux density of 280 mJy at a minimum distance of
8.5 kpc is much larger than typical outflow objects (Rodriguez et al. 1990;
Rodriguez 1991). Thus, we conclude that G25.5+0.2 is either a very young SNR
or an extremely unusual galactic object.
(d) Low resolution 20 cm observations. The large number of blank fields
in the high resolution survey suggests that there may be many young SNRs
with somewhat older ages that were resolved out. These may be SNRs with
ages of 25 to 100 years at distances of 5-20 kpc.
In search of these objects, we observed 62 of the 122 sources that were
listed as empty fields in Table 1 in the VLA B/C configuration. These
sources were observed at 20 cm on 3 July and 20 July 1989. Images 43'
on a side were made with 5" pixels and a beam size of 20"x15".
The results are listed in Table 3.
[... see sramek92.tb3]
The low resolution observations have not yet produced any confirmed
identifications of young SNRs. However, six sources which are marked in the
notes have a shell morphology and deserve follow-up observations. These
objects could be SNRs, compact HII regions, or planetary nebulae. Three of
these sources are shown in Fig. 2.
Eighteen of our original 290 sources were also observed by Green & Gull
(1984) using the 3.4 km VLA C array at 6 cm. They were also searching for
young remnants by observing known galactic plane sources. Sixteen of the
eighteen sources common to both studies were detected in our high
resolution survey. The two sources not seen in our high resolution images
are listed by Green and Gull as ~1' in extent and were thus resolved out
in our observations. One of these sources is G227.1+1.0 which was originally
thought to be a young supernova remnant but was later classified as
extragalactic (Channan et al. 1986; Green & Gull 1986).
3. DISCUSSION
Compact structure was detected in 168 (58%) of the 290 different sources
listed in Table 1; 73 (25%) were slightly resolved or unresolved point
sources, 21 (7%) were single well resolved, 56 (19%) were double, 18 (6%)
were triple or complex. The large scale structure was completely resolved
out for 122 (42%) of the sources. The results so far indicate that only one
source, G25.5+0.2, is possibly a remnant of a galactic supernova that
occurred in the last hundred years.
If the supernova rate in our Galaxy is one every twenty to fifty years,
where are the other remnants? There are several possibilities. If we assume
that supernovae occur uniformly in a disk 12 kpc in radius with the Sun at
8.5 kpc from the center, the range of galactic longitude in our sample only
covered 57% of the area of this disk. Within this area we examined
catalogued point sources brighter than 0.5 Jy at 20 cm wavelength.
Since one of our criteria for a detected SNR was an observed shell
structure, we would have missed SNRs smaller than 5". And since the array
was not sensitive to sources with angular extant larger than 20", at a
distance of 20 kpc, we could detect SNRs with ages between 25 and 100 yr
assuming an expansion velocity of 10 000 km/s. At a distance 5 kpc this age
range falls to 6-25 yr. If we integrate the area of observed galactic disk
with the SNR age range to which we are sensitive, we could have detected 48%
of the supernovae that went off in the observed sector during the past 100 yr,
or only 27% of the supernovae that exploded in the galaxy during that period.
We must also consider the supernovae that were undetected because of our
flux density limits. It is the flux density limit used in our selection
from the original single dish catalogues (about 0.5 Jy at 20 cm), not the
VLA detection limit (about 7 mJy), which determines the level to which our
search for SNRs is complete. First, it is unlikely we will detect any type I
supernovae. There have not yet been any detections of type 1a supernovae
in external galaxies and although the Type Ib supernovae that have been
detected are quite powerful, they decay very quickly. The best upper limit
we have for a middle aged supernova is for SN 1885A, a type Ia supernova in
M31, where a limit of 0.015 mJy at 3.4 cm wavelength (Crane et al. 1992)
translates to a flux density upper limit of 60 mJy at 20 kpc.
Type II supernovae like SN 1979C and SN 1980K remain strong radio emitters
for many years. The recent radio detections of SN 1950B, SN1957D, SN 1961V,
and SN 1970G (see Cowan et al. 1991) give examples of the radio luminosity
of some supernovae a few decades after their explosion. These objects are
at the high end of the luminosity distribution of radio supernovae and at a
distance of 20 kpc they would have a flux density of between 25 and 100 Jy.
If such radio supernovae currently exist in the Galaxy they would have been
discovered long ago. There are luminosity upper limits available for other
supernovae of this age, but they are at about the same level as the four
detections mentioned above (Weiler et al. 1989; Branch & Cowan 1985). In
their early years, the flux density of type II supernovae decays as S=t^-0.7.
The type II supernovae listed above would still be well above our detection
limit at age 100 yr. However, there is evidence that after a few decades,
the radio power law decay of these supernovae steepens rapidly (Cowan et al.
1991). Even if we assume that all type II supernovae remain above our
detection limit after 100 yr, since only a third of the supernovae in a
late type spiral galaxy are type II objects (Muller et al. 1992), our
overall detection rate for galactic supernovae then drops from 27% to 9%.
Finally, the single dish catalogues used to produce our observing lists may
well be incomplete in the 0.3 to 3 Jy range. This would further reduce our
anticipated detection rate of young SNRs. Thus with a supernovae rate for
the Galaxy of 3.3 per 100 yr (Muller et al. 1992), our probability of
finding a supernova is at most 0.3, which is consistent with our single
possible detection, G25.2+0.2.
Note Added in Proof: H105alpha and H106alpha recombination lines at 6 cm have
been detected from G25.5+0.2 by Ravi Subrahmanyan using the Australia Telescope
National Facility Compact Array at Narrabri. The linewidth is about 110 km/s
FWHM with a L/C ratio of 1.5%. This has been confirmed by us with the VLA in
the 3.4 km configuration using the H92alpha line at 3.6 cm. The linewidth is
150 to 200 km/s with a maximum L/C ratio of 1%. These lines are too broad to
have been seen in the previous searches. These linewidths are far larger than
would be expected from a planetary nebula or HII region.
We thank K. Venkatakrishna and D. Branch for their help in the initial phases
of this project. This research was supported in part by NSF Grant No. AST-8521705.
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