A&A 459, 535-544 (2006)
DOI: 10.1051/0004-6361:20065061
G. Castelletti1, - G. Dubner1,
-
K. Golap2 - W. M. Goss2
1 - Instituto de Astronomía y Física del Espacio (IAFE),
CC 67, Suc. 28, 1428 Buenos Aires, Argentina
2 - National Radio Astronomy Observatory (NRAO), New Mexico, USA
Received 21 February 2006 / Accepted 29 June 2006
Abstract
Context. High-resolution, high-sensitivity multifrequency radio images of supernova remnants (SNRs) are essential in advancing the understanding of both the global SNR dynamics and particle acceleration mechanisms.
Aims. In this paper we report on a new study of the SNR Puppis A based on VLA observations at 1425 MHz; the improvement represents a factor of two in angular resolution and almost ten times in sensitivity compared to the best previous image of Puppis A. This new image is used to compare with re-processed 327 MHz data and ROSAT and Chandra images to investigate morphological and spectral characteristics.
Methods. The observations were carried out with the VLA in the DnC and CnB configurations in 2004. After combining with single-dish data from Parkes, an angular resolution of 34
and an rms noise of 0.5 mJy beam-1, were achieved. Archival VLA data at 327 MHz were also processed. The spectral index distribution was then determined by a direct comparison of the homogenized data at both 327 and 1425 MHz. In addition, to identify different spectral components, tomographic spectral analysis was performed.
Results. The new 1425 MHz radio image reveals a highly structured border encircling a diffuse, featureless interior. In particular, the northern half of Puppis A displays a complex structure along the periphery, consisting of short arcs resembling "wave-like'' features. These are oriented essentially perpendicular to the shock front on the NE side, but are tangential to the shock on the NW side. A remarkable correspondence between such "wave-like'' features and spectral changes is observed. On the other hand, the brightest radio features (located to the E of the SNR and also detected in X-rays) have no counterpart in the spectral index distribution. Based on a uniform compilation of integrated flux densities between 19 and 8400 MHz, a global spectral index
(
)
has been determined. The new 1425 MHz image of Puppis A was compared with the ROSAT X-ray image of the entire SNR and with the resolved arcsec Chandra image of the bright E region. There is good overall agreement between the radio and high resolution X-ray images. At the arcsec level, however, the agreement of the radio and X-ray images is less striking. A noticeable similarity is found between Puppis A, with its E and W extensions, and analogous morphological features observed in the SNR W50. This suggests that Puppis A could be another case of a SNR shaped by the action of energetic jets.
Key words: ISM: individual objects: Puppis A - ISM: supernova remnants - radio continuum: ISM
Dubner et al. (1991) have presented an earlier VLA
image of the SNR Puppis A at 1515 MHz based on observations from a
combination of 26 different
pointings observed with the VLA in the
DnC array. The angular resolution of
this image is 77
with a noise of
3 mJy beam-1. This image shows some agreement between the bright
radio features and the X-ray emission detected by Einstein
X-ray
Observatory.
However, the angular resolution and sensitivity of this previous VLA image is not high
enough to allow a detailed comparison with improved X-ray images,
e.g. that of Petre et al. (1996) obtained using ROSAT,
or the
detailed high resolution
Chandra study of the eastern region carried out by Hwang et al. (2005).
Because of advances in both instrumentation
and imaging algorithms for the production of sensitive
radio mosaics of large regions, a new set of 20 cm radio observations
have been carried out using the VLA.
In this paper, we present a new 1425 MHz image of Puppis A generated from VLA DnC and CnB configuration data sets obtained in 2003 and 2004. The improved sensitivity and resolution have led to the discovery of new morphological features near the outer shock front and at the E side of the SNR shell. From the combination of the radio observations at 1425 MHz with re-processed data at 327 MHz from the VLA archives, we have investigated the spatial spectral index distribution across the remnant. Based on the 1425 MHz image, we have also performed a detailed study of the correlation between radio and X-ray emission, focusing special attention on the complex region to the E.
The radio emission of Puppis A is of synchrotron origin and takes the form of a highly distorted clumpy shell, with the brightest section along the E border. Optical emission is present as a collection of compact condensations rather than delicate filaments, and shows little relation to the radio features (Milne et al. 1983).
Previous multiwavelength studies of the surroundings of Puppis A
suggested
the existence of dense interstellar clouds along the
E-NE border of the SNR (Arendt et al. 1990,1991; Teske & Petre 1987; Petre et al. 1982).
This was
later supported by atomic and molecular studies of the surrounding
interstellar medium; HI and
CO J:1-0 line
observations have revealed the existence of a chain of interstellar clouds
concentrated along the NE periphery of the remnant (Reynoso et al. 1995; Dubner & Arnal 1988).
From these studies, a kinematical distance of
kpc
has been derived
for Puppis A.
The X-ray remnant consists of non-uniform emission that includes both extended features and knots (Petre et al. 1982), the most conspicuous of which is the bright knot located at the E limb (called by those authors "the bright eastern knot'', or "BEK''). This feature coincides with an "indentation'' in the radio morphology, suggesting an encounter between the SN shock and an external inhomogeneity. Based on an OH 1667 MHz line study, Woermann et al. (2000) suggested that the shock front expanding to the E has completely engulfed a cloud giving rise to the X-ray knot, leaving no observable unshocked remainder of the cloud. Recently, Hwang et al. (2005) presented Chandra X-ray images and spectral analysis of the region around the BEK. These images reveal details of substructure in this part of Puppis A. The spectral analysis shows a temperature gradient rising behind the shock front, suggesting possible heating by reflected shocks due to the interaction with denser gas.
An unresolved X-ray source has been detected near the centre of Puppis A (RX J0822-4300, Pavlov et al. 1999, and references therein). This source may well be the compact stellar remnant formed in the SN event. However, no associated pulsar wind nebula has been detected (Gaensler et al. 2000).
327 MHz data acquired with the VLA in the CnB configuration in 1988 (Dubner et al. 1991) were re-processed. The data were flagged and calibrated using the AIPS software package. Further processing was done in AIPS++. The W-Projection imaging algorithm (Cornwell et al. 2003) was used to correct for the non-coplanarity of the VLA array at this frequency (Cornwell & Perley 1992). We made use of multiscale clean to deconvolve the image. The deconvolution was done in iterations, with the visibility data being self-calibrated in phase after each iteration.
To recover the very low spatial frequencies that the VLA observations miss, we made use of single-dish
data extracted from the 408 MHz All-Sky Continuum Survey
(Haslam et al. 1982).
We scaled the flux density of the single dish image with a spectral index
,
which we derived from
comparing the flux densities at 86 and 408 MHz (Milne et al. 1993).
The resultant single-dish
image was combined with our interferometric image. The
combination is based on a method referred to as "feathering''. This
method involves Fourier transformation of both the single dish and
interferometric data onto identical grids. The single-dish Fourier
data is multiplied by
the ratio of the Point Spread Function (PSF) volumes of the
interferometric image to that of the single dish image. This scaled
data is added to the Fourier interferometric data multiplied by a
feathering function. The feathering function used in AIPS++ is
1-FT(PB
), where FT(PB
)
denotes the Fourier transform of the primary beam
of the single-dish. This combined data is then Fourier
transformed back to the image domain.
Although the angular resolution of the final image at 327 MHz is 90
at a
position angle of 164
,
and
the rms noise level is 10 mJy beam-1, comparable to the earlier
results of Dubner et al. (1991), the newly processed 327 MHz image
gives a more reliable representation of Puppis A at all spatial
frequencies.
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Figure 1:
The SNR Puppis A at 1425 MHz. This image was obtained
from a combination of 39 different pointings observed with the VLA in the DnC and CnB configurations in 2004. Single dish observations from the Parkes Southern Galactic Plane Survey (McClure-Griffiths et al. 2001) have been added to the data. The angular resolution is
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We observed Puppis A at 1425 MHz using the VLA in the DnC and CnB configurations. The DnC data were obtained on 2003 January 21,
22, and 23, while the CnB observations were carried out on
2004 February 4, 5, 7, and 15. The flux density calibrators used were
3C 147
and 3C 286 for the DnC observations and
3C 286 for the CnB observations.
The phase and amplitude calibrator used for all the data observed was the
source J0828-375.
Due to the large size of this SNR (about 55
in diameter) we used a
mosaicking technique with 39 different pointings with a spacing of half of
the primary beam width.
Each sub-field was observed in two 50 MHz bands centered near 1465
and 1385 MHz in continuum mode.
The observations were made in cycles with 28 s in
each pointing of the mosaic. This observing cycle was repeated 9 times in the DnC array and 12 times in the CnB configuration.
The data were reduced using the AIPS++ software package. The data from each day were calibrated, imaged and self-calibrated separately to ensure that there were no day-to-day amplitude discrepancies. All the calibrated data sets were then concatenated into a single visibility dataset. We imaged and self-calibrated the combined visibility data in an iterative fashion. The imaging was done using the standard mosaicking technique that jointly deconvolves all the fields, taking into account the primary beam effect at each pointing. The multiscale clean method was applied to deconvolve the image. We used uniformly weighted visibility data for the high resolution image. For the image that is used to compare with the 327 MHz data, the visibility data were tapered in such a fashion that the resulting restoring beam is identical to that of the 327 MHz image.
To recover the large scale structure missed by the interferometer, we used an image from the Parkes Southern Galactic Plane Survey (McClure-Griffiths et al. 2001). The single-dish and the interferometric mosaic images were combined using the same feathering technique described above.
The uniform weighting image that was produced has a resolution of
34
at a
position angle of -174
,
and an rms noise of 0.5 mJy beam-1,
representing almost an order of magnitude improvement
in sensitivity over the image of Dubner et al. (1991).
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Figure 2: Enlargements of the new VLA 1425 MHz image. a) E region, containing the brightest features of the remnant. Feature A marks the position of a bright point-like emission region, while B indicates the location of the straight N-S filament. The grayscale ranges from 5 to 52 mJy beam-1. b) N part of Puppis A; the irregular outer border is prominent. Note the presence of "wave-like'' features almost perpendicular to the shock front on the NE and parallel to it on the NW. The grayscale ranges from 6 to 16 mJy beam-1. c) The SW region showing the presence of the only bright feature located at the outer boundary of Puppis A. The grayscale varies between 5 and 18 mJy beam-1. |
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Figure 1 shows the new VLA 1425 MHz image of Puppis A.
At this frequency the remnant has the appearance of an asymmetric shell
with an average diameter of 55
or 35 pc at a
distance of 2.2 kpc.
The intensity of the brightest features along the periphery is
spatially variable,
ranging from
8 mJy beam-1 in the N to
60 mJy beam-1 towards the E.
No sharp boundaries are observed,
although the presence of a
weak radio halo at a mean level of 3.5 mJy beam-1
is apparent in the original images extending about 5
ahead
of the bright rim.
With the addition of short spacing contributions,
all the flux density is represented in the image in Fig. 1. This
image shows
diffuse interior emission at an average level of 8 mJy beam-1.
These new sensitive radio observations
show no evidence for any radio counterpart
(
of 1.5 mJy beam-1)
to the X-ray point source
RX J0822-4300 nor for a pulsar wind nebula,
consistent with
previous negative results obtained by Gaensler et al. (2000).
Enlargements showing the detailed 1425 MHz total intensity morphology in
three selected regions of the Puppis A SNR are shown in
Fig. 2.
Due to the high dynamic range (1000:1) of the 1425 MHz image, several
grayscale ranges are used to display the different relevant regions.
Figure 2a shows the E portion of the SNR shell, Fig. 2b displays the N part
of Puppis A,
while Fig. 2c shows the locations of the
brightest
features in the SW edge of the remnant.
The new high resolution image reveals that the boundaries
of Puppis A
are quite structured, in contrast with the
smooth
interior. The outer border consists of a series of
arc-like features, resembling a "wave-like'' structure.
Such features are more evident in the
N half of Puppis A (Fig. 2b),
although some are detected to the SW.
We note that along the NW and SW borders, the "wave-like''
features are observed tangential to the
outer rim, but their orientation
changes to approximately radial (i.e perpendicular to
the local shock front) along the NE border.
Petre et al. (1982) reported the detection of bright X-ray emission near
,
(J2000),
the BEK
(see Fig. 2a). Intense radio emission is observed coincident
with the X-ray emitting area (see Fig. 7 below).
In addition, the new high resolution 1425 MHz observations
reveal the presence of a
compact (size
45
)
feature of enhanced synchrotron emission (feature A in
Fig. 2a), exactly at the site
where the shock front forms an angle, near
,
(observed in
Fig. 2a as a
bright region adjacent to the BEK).
To the S of this feature,
the presence of a
6
prominent, straight filament (labeled as B in
Fig. 2a) is noticed
near
,
running
N-S
from
to
.
To the E of this filament, a fainter semi-circular
protrusion (extending about 6
)
is observed.
The SW region of Puppis A (Fig. 2c) shows the only bright feature
located on the edge
of the SNR. Short arcs (size
4
), similar to those observed
to the NW, are
also observed in this region, although in this case they appear displaced
by
8
from the limb to the interior.
Figure 3 shows the re-processed image at 327 MHz of Puppis A. These data were used with the 1425 MHz image for the spectral analysis discussed in Sect. 4.
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Figure 3:
Radio continuum image of SNR Puppis A at 327 MHz obtained
with the VLA in the CnB configuration in 1988 and now re-processed.
The interferometric data were combined with single dish data
from MPIfR Bonn radiotelescope (at 408 MHz, Haslam et al. 1982).
The final beam size
is
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We have calculated integrated flux densities
for Puppis A based on the new images, obtaining:
Jy and
Jy. The errors reflect the
uncertainties both in the choice of background emission and integration
boundaries, in addition to the inherent rms noise of the images.
To construct accurate spectra of individual SNRs it is necessary that
all flux density values be brought on to the same absolute flux
density scale.
Table 1 contains the flux densities
between 19 and 8400 MHz. All the flux densities
for frequencies above 400 MHz are on the Baars et al. (1977) scale.
For those below that frequency no scaling was applied.
Figure 4 shows the resulting radio
spectrum of the Puppis A SNR; the new determinations are
plotted as filled circles.
The flux density values are adequately fitted by a single power law
of slope
,
in good agreement with
estimates presented by Milne & Hill (1969) (
0.50) and
Dubner et al. (1991) (
0.53).
It is important to discern whether the electron energy spectrum is a global property of this SNR, or if it is locally affected by the neutron star and/or the shock-ISM interaction. Hence, we performed a careful study of the variations in the spectrum as a function of position within the remnant.
Table 1: Integrated flux densities for the SNR Puppis A.
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Figure 4:
Radio spectrum of Puppis A
from the scaled flux density values listed in Table 1.
The data from this paper are shown by filled circles (327 and 1425 MHz). The
linear fit to the values of flux density indicates a global spectral
index of
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The previous study of the spectral index distribution was
carried out by Dubner et al. (1991) using VLA observations at 327 and 1515 MHz.
The distribution of the spectral indices was estimated by integrating flux
densities
within five rectangular boxes (of different sizes according to the region,
varying between approximately 16 and 190 arcmin2), and
comprising the brightest portions of Puppis A.
An index as steep as
was suggested for the E border,
while for the other four regions the spectral index was found to be
about -0.5.
Using more recent spectral analysis techniques and the new VLA 1425 MHz data,
we are able to produce a
detailed analysis of the spatial spectral transition from the bright
features to the smooth weaker interior of this SNR.
To accurately determine the spectral index distribution from the comparison of two interferometric images at different frequencies, it is necessary to ensure that the corresponding images are matched in uv coverage. As mentioned in Sect. 2.2, the data at 1425 MHz were processed by applying appropiate weights to the visibilities to obtain an image with the same beam size and beam shape as the 327 MHz image. Also, in order to avoid any positional offsets, the images were aligned and interpolated to identical projections (field center, pixel separation, etc.). In addition, we checked for possible flux density calibration bias. That is, an extrapolation back to zero flux density at one frequency should correspond to zero flux density at the other frequency, otherwise there is a zero-point offset that must be corrected. We have checked the 327 and 1425 MHz images, finding only an insignificant offset of 0.1 mJy beam-1 between frequency pairs. Therefore, no further correction was applied.
Based on these images, we carried out the spatial spectral analysis. This was done using two different methods in order to confirm that the observed spectral features are real, independent of any possible contamination from the databases used for the comparison.
In Fig. 5 we show the spectral index
distribution of Puppis A as obtained from the direct comparison of
the matched images at 327 and 1425 MHz
after clipping at the 5 level for both
frequencies.
We find that the spectral index varies across the SNR approximately between
0.8 and -0.2.
In this grayscale representation, darker means
flatter spectrum.
It is interesting to note that the feature with the flattest spectrum
(seen as a black band running from NE to SW in the interior of
Puppis A)
includes the location of the neutron star
RX J0822-4300 and
of the geometrical center of the explosion
as derived from optical observations by Winkler et al. (1988)
(shown in Fig. 5 as a triangle and a plus sign
respectively).
These positions are known with an accuracy of tenths of an arcsec for
RX J0822-4300 and
1 arcmin
for the proposed center of the explosion.
We can confidently assume that this spectral feature is real since
both images have recovered all flux density.
The present spectral map
shows that along most of the periphery the spectrum is
clearly steep (steeper than 0.7).
To the W, the ear-like feature shows a combination of different
structures with variable spectral indices.
Curiously the spectral pattern observed near the bright eastern radio
features (approximately near
,
from
to
)
does not correlate with the morphology observed in total power
(see Fig. 1).
Although it is somewhat flatter than the rest of the shell, it is apparent
that the spectrum varies following a pattern
of parallel fringes that alternate steeper and flatter spectral indices.
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Figure 5:
Grayscale image of the spectral index distribution across the
SNR Puppis A as derived between 327 and 1425 MHz. The wedge displays
the spectral index value (
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In order to test the spectral index distribution determined above, we
also
constructed a gallery of tomographic maps.
Spatial tomography between two images is a technique developed
by Katz-Stone & Rudnick (1997a), based on the use of a test spectral index
.
A tomographic map is calculated by scaling the
brightness
in the higher frequency image (
,
in our case) by the
test spectral index, and subtracting this scaled image from the lower
frequency image (
). Then, a tomographic gallery is
obtained by calculating
,
stepping
through a range of spectral indices
.
Features which have a spectral index
identical to the test value will vanish in the tomographic map. Spatial
components that have different spectral indices will appear as positive
or negative features depending upon whether the spectrum is steeper or
flatter than the assumed test value.
This method has been succesfully applied to analyze spatial
spectral variations in the SNRs of Tycho (Katz-Stone et al. 2000),
Kepler (DeLaney et al. 2002),
G292.0+1.8 (Gaensler & Wallace 2003) and in the study of the radio galaxies
3C 67, 3C 190
and 3C 449 (Katz-Stone & Rudnick 1997b,a). The technique is
particularly useful to disentagle components with different spectra
that can overlap along the line of sight. Also, when the goal is to locate
small-scale spectral variations, this method can provide a more
accurate
picture than the simple comparison between images at different frequencies.
In Fig. 6 we show the resulting tomographic
images for four test values:
0.2 (Fig. 6a), -0.4 (Fig. 6b),
-0.6 (Fig. 6c), and -0.8 (Fig. 6d).
In these maps the spatial components appear as
positive (light) or negative (dark)
features for a spectrum steeper or flatter than
the assumed test value respectively.
The point sources overlapping the remnant are always seen
as positive residuals in the tomographic images (white spots) as they
have spectral indices steeper than -0.8, thus supporting their
extragalactic nature,
as already noted by Milne et al. (1983) and Dubner et al. (1991).
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Figure 6:
Series of tomographic images for the SNR Puppis A
constructed from data at 327 and 1425 MHz,
which were matched in uv coverage.
a)
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The most interesting tomographic images are those obtained from the
comparison
with
0.4 (Fig. 6b)
and with
0.6 (Fig. 6c), where,
as noticed before, a
notable striation is evident along the periphery.
It is worth noting that this spectral pattern formed by short
fringes with
alternatively steeper and flatter than the
background, mimics the "wave-like'' morphology noticed in the total
power image along the NE, NW and S borders. That is, the spectral components
appear almost
parallel to the outer boundary to the NW and SW, and transverse
to it along the E side.
The newly identified point-like feature to the E of Puppis A (feature A in Fig. 2a), has a spectrum steeper than -0.8 (i.e. it appears as a positive residual in all tomographic images). We thus conclude that, in all probability, it is an extragalactic object.
The central band seen in Fig. 5 disappears against the
local background in the tomographic image traced with
0.2 (Fig. 6a), thus confirming its very flat
spectrum.
The grayscale corresponding to the
0.8 image (Fig. 6d)
reveals that the spectral distribution in Puppis A has practically
no components steeper than this value.
The X-ray properties of Puppis A have been investigated using the Einstein, ASCA and ROSAT telescopes (Pavlov et al. 1999, and references therein). The highest resolution X-ray image of the entire source was obtained with ROSAT (HRI) in the energy band from 0.1 to 2 keV (Petre et al. 1996).
In Fig. 7 we present a false color image showing the spatial
comparison
between the radio image at 1425 MHz (in red) and the soft X-ray
emission
(in green) for the entire
remnant. For the first time a comparison with a spatial
resolution <30
is possible.
Features where both spectral bands overlap are shown in yellow.
The X-ray emission associated with Puppis A has been
interpreted as resulting from several thermal components (Berthiaume et al. 1994).
The N half of Puppis A is clearly the brighter and more
complex in X-rays. A network of small
scale structures that include filaments, short arcs and knots are, in
general, poorly correlated with radio features. The brightest features
are preferentially located to the E although the N-NW rim is also well
delineated by bright X-ray filaments.
Figure 7 demonstrates that the most striking radio/X-ray
correspondence
occurs predominantly along the
E limb of the remnant.
Three features are well matched:
the BEK, the curved filament near
,
(see below) and the indentation around
,
.
In contrast, little radio/X-ray agreement is observed in the
SE sector of Puppis A;
the radio emission extends over 5
beyond the
X-ray limb. Moreover, the protrusion ahead of the bright,
straight
N-S radio filament (feature B in Fig. 2a)
lacks a counterpart in X-rays.
A similar radio/X-ray comparison cannot be carried out to the SW of
Puppis A
due to a missing ROSAT pointing in this direction
(see the ROSAT archive); thus the reconstruction of the image
is likely incomplete in this region.
Towards the NW border of Puppis A
the X-ray emission accompanies the "wave-like'' morphology
revealed in the radio,
with the same orientation parallel to the shock front. The
dense X-ray knot located
immediately behind the NW shock front
(the "bright northern knot'', BNK,
,
)
is prominent in both spectral
domains.
Petre et al. (1982) have suggested that this knot originated as a result
of the interaction with a dense neutral
cloudlet, similiar to the BEK.
In the radio domain, the spectrum of this knot between 327 and 1425 MHz
has a spectral index
(see Sect. 4.2), similar to
the global spectral index of this SNR.
A careful spectral study of this clump in X-rays would
help towards understanding its origin.
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Figure 7:
A high resolution X-ray/radio comparison of the
Puppis A SNR.
The green image corresponds to soft X-ray emission from
ROSAT in the 0.1-2 keV range (Petre et al. 1996), while in red the
1425 MHz radio emission is shown. The yellow regions are areas where emission
in both spectral ranges overlap. The X-ray image is smoothed to
the resolution of the radio image,
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Hwang et al. (2005) have used the Chandra X-ray telescope to
resolve the fine structure
around the BEK over a field of view of
.
These authors describe the inner structure in this part of the
remnant as consisting of: (1) a bright compact knot,
(2) a curved vertical feature (named the "bar'')
about 1
W behind the shock,
and (3) a smaller bright cloud (the "cap'') separated from the
"bar'' by faint
diffuse emission. Nomenclature for these X-ray features is
contained in Fig. 8.
To study the relation between the X-rays and
the radio components in the Puppis A SNR,
in Fig. 8 we show a direct comparison of the new 1425 MHz VLA image with the Chandra X-ray image obtained from the
combination
of the soft, medium, and hard X-rays (note that the Chandra
image has an angular resolution about 5 times finer than the radio
image).
Figure 8 reveals that the X-ray emitting plasma does not
coincide with radio features at a detailed level.
However, on a larger scale reasonable agreement is observed.
The radio emission is compressed in the
indented
eastern X-ray boundary, confirming that the shock wave is wrapping
around an obstacle in the neutral ISM as was earlier proposed by
Hwang et al. (2005).
A local radio maximum matches the BEK, as shown in Fig. 8.
In this portion of the SNR, this feature
is not the brightest
feature in radio as is the case for X-rays.
The most intense radio
emission is found to the E of the X-ray "bar''
(around
,
), where it
correlates with a secondary X-ray peak (see Fig. 8).
No conspicuous radio feature can be associated with
the "cap''.
The region of faint diffuse X-ray emission between
the "bar'' and the "cap'' is a region of low surface brightness
radio emission.
In summary, based on the present comparison only moderate agreement between radio and X-ray emission has been observed at a detailed level. This conclusion is not surprising since the X-rays can be associated with thermal radiation from a hot plasma, while the radio continuum arises via synchrotron emission from relativistic electrons.
As noticed before, the general shell-like morphology of Puppis A
appears
distorted to the E by the protrusion near
,
and to the W by the "ear'' around
,
(Figs. 1, 2a and 7). If we
trace two intersecting lines oriented about a
position angle of
100
(from N through E),
with a semi-opening angle of
15
,
crossing at the position
of the neutron star candidate RX J0822-4300, the appearance of
Puppis A
strikingly resembles that of the SNR W50. The shell of
W50 is distorted by the precessing
jets that originate in the central compact source SS433, and
Fig. 9 illustrates the similarity between the two
SNRs. In
Fig. 9 (left) we have plotted a sketch
of the hypothetical "cones'' overlapping the new radio image of
Puppis A, while
on the right side an image of the
SNR W50 at 1.4 GHz (from Dubner et al. 1998) is shown
with the precession cones of
20
semi-opening angle
delimited by the intersecting white lines. In the case of W50,
the elongated shape has been
interpreted as being due to the action of the two oppositely directed
relativistic jets injected by the X-ray binary SS433.
The present comparison shows that not only the general
appearance with symmetrical lateral extension is
similar for both SNRs, but that the analogy also extends to
particular features. For example, the bright straight N-S filament in
Puppis A (named B in Fig. 2a) looks very similar to the
straight radio filament that stands out near the E extremity of W50, and which is
thought to be the radio "hot-spot''
produced by the interaction of the relativistic jet from
SS433 with
the ISM (Safi-Harb & Ögelman 1997). Beyond these bright, straight filaments,
diffuse radio emission is observed in both cases.
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Figure 8:
( Left) ROSAT HRI mosaic of the Puppis A SNR.
The box indicates
the region of the
E limb observed with Chandra (Hwang et al. 2005).
The plus (+) symbol shows the position of the compact source
RX J0822-4300.
( Right) The grayscale representation combines soft
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Figure 9: Comparison of the SNR Puppis A ( left) with the SNR W50 that contains the central X-ray binary SS433 ( right, from Dubner et al. 1998). In Puppis A, the white cross shows the location of the X-ray point source RX J0822-4300, a radio-quiet neutron star proposed to be the compact remnant of Puppis A explosion. The white intersecting lines delimit the "cones'' proposed to be associated with Puppis A in analogy with the case of W50. In the image of the SNR W50, the white lines trace the volume defined by the oppositely directed, precessing, relativistic jets. |
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In the X-ray domain, no conspicuous feature is apparent
in the immediate vicinity of RX J0822-4300
(see Figs. 7 and 8 left).
The first
bright X-ray features within the hypothetical cones are the thin
filament to the E near
,
and to the west near
,
(see Fig. 8 left for a
representation of the X-ray emission
in equatorial coordinates). At the assumed distance
of 2.2 kpc, these features
occur at about
32 pc E and
22 pc W of the central point
X-ray source. In the case of W50, the X-ray lobes are
symetrically
displaced E and W of SS433 starting at about 15
from
the center (approximately 17 pc at the distance of W50) and have
a knotty structure with enhanced emission at about 35
(i.e. about 40 pc) from SS433 (Safi-Harb & Ögelman 1997). The displacement
of the X-ray lobes in W50 has been interpreted by Safi-Harb & Ögelman (1997)
as due to the expansion of beams in a low density ISM.
A careful study of the X-ray features in the interior of Puppis A is highly needed to understand the interaction of the central neutron star with the surrounding SNR and to answer the question as to whether Puppis A is another example of a Galactic SNR distorted by the action of symmetric energetic jets.
In the current work we have presented a new image of the SNR Puppis A at 1425 MHz. This image has revealed with superb detail the presence of short filaments along most of the distorted periphery of Puppis A. In particular, in the northern half of Puppis A the radio emission near the limb resembles a "wave-like'' structure, oriented parallel to the shock front along the NW but almost perpendicular to it towards the brighter NE limb. Such a pattern is replicated in the spatial spectral distribution, where fringes with analogous orientation alternate between flat and steep spectra. We propose that the peripheral filaments are a manifestation of Rayleigh-Taylor (R-T) instabilities that are distorting the interface between the ejecta and the ambient material. As R-T instabilities develop, they stretch and compress the magnetic field (Jun & Norman 1996). This produces magnetic field amplification in the ambient medium close to the interface, which in turn enhances the synchrotron emission. In our model, the R-T fingers observed to the NE are radial, while to the NW they are tangential suggesting that along this border the shock is probably in a more advanced evolutionary stage. This could be related to the possibility that to the NE the shock front is expanding in a denser ISM. To confirm this hypothesis, it is important to investigate the magnetic field orientation in the region. A detailed study of polarization vectors at high angular resolution is planned for the near future.
Similar short, curved filaments are also observed in the young SNRs of Tycho and Cas A, both in the radio and in X-rays, although in these cases a weaker plateau and a thin brighter outer rim is observed ahead of the filaments (Reynoso et al. 1997; Hwang et al. 2004). In Puppis A, a faint plateau can be clearly seen around almost all the periphery, but no sharp outer border, presumably associated with the foward shock, is detected in this intermediate-age SNR.
The most conspicuous radio features are situated at the E border, at the site where the shock front has engulfed a dense ISM cloud. Interestingly enough, the radio counterpart of the BEK and the narrow straight filament located just south of it do not appear as spectrally distinct features. If these synchrotron maxima are the result of the interaction with external clouds, then we are led to the conclusion that this condition is not reflected in the radio spectrum. The spectral map of Puppis A does not show diferences between the E and the W sides in spite of the gradient in density in the ISM (Petre et al. 1982). We can therefore conclude that in this case the shocks are probably still non-radiative and the compression is not very high. This contention is supported by the lack of optical filaments in the boundaries and the generally poor radio/optical correlation (Dubner et al. 1991).
Another spectral peculiarity in Puppis A is the
fact that the spectrum steepens in general from the interior to the periphery.
In the first-order Fermi, non-relativistic, test particle
limit, stronger shocks yield flatter indices up to the
limit.
Therefore, based on this acceleration model, higher emissivity
regions should
have flatter spectrum, while steep spectra are expected in the more diffuse
emission regions (see for example Anderson & Rudnick 1993), exactly
the opposite of
what we find in Puppis A. This result is confirmed through the
tomographic analysis of the spectrum.
It is interesting
to note that the region of Puppis A with the flattest
spectrum is a narrow
band towards the
interior, like a channel that connects the position of the neutron star
RX J0822-4300 with the presumed geometrical center of
explosion.
The continuous injection of relativistic particles by the neutron star could
provide an explanation for this spectral characteristic.
Based on a compilation of integrated flux densities over a wide spectral
range, we have recalculated the global
non-thermal radio spectrum of the SNR Puppis A.
The fit produces a
spectral index of
for the whole SNR.
We have also used the new radio image to perform the first accurate radio/X-ray comparison for Puppis A. In X-rays, Puppis A is bright on the E half, and the emission seen by ROSAT between 0.1 and 2 keV is very structured. The new radio image, although much more sensitive than previous ones, does not show detailed correlation with the thermal plasma imaged in X-rays, except for the bright E features. Even there, the matching is not accurate when the radio emission is compared with the high resolution Chandra image. The only remarkable coincidence revealed by the new results occur along the N-NW boundaries, where the "wave-like'' radio features near the shock front have their exact counterpart in the X-rays. A detailed X-ray study along the NW side, including the BNK, would be valuable. To the S-SW of Puppis A, we were unable to carry out a similar comparison because a ROSAT pointing is missing there.
On the other hand, based on the new observations, we have confirmed
that
down to 1.5 mJy beam-1 neither a radio counterpart
to the neutron star
RX J0822-4300, nor a surrounding pulsar wind
nebula, is observed.
Finally, based on the radio images and their comparison with the X-ray emission, a striking similarity between Puppis A and the SNR W50, which harbors the Galactic microquasar SS433 in its interior, is seen. This suggests that Puppis A is a candidate for being another SNR shaped by internal jets, probably at an earlier evolutionary stage. Detailed central X-ray images and spectral studies would be of great help to test this suggestion and to understand the nature of the central X-ray compact object.
Acknowledgements
We would like to thank Naomi McClure-Griffiths for providing us with the 1.4 GHz data from the Parkes Southern Galactic Plane Survey. We are grateful to the referee for all his/her suggestions which certainly has helped to make this a better paper. This work has been supported by Argentina grants ANPCYT-PICT 04-14018, UBACYT A055/04, ANPCYT-PICT 03-11235 and PIP-CONICET 6433.