The Astrophysical Journal, 796:82 (19pp), 2014 December 1 doi:10.1088/0004-637X/796/2/82
C
2014. The American Astronomical Society. All rights reserved. Printed in the U.S.A.
SPECTRAL AND MORPHOLOGICAL ANALYSIS OF THE REMNANT OF
SUPERNOVA 1987A WITH ALMA AND ATCA
Giovanna Zanardo
1
, Lister Staveley-Smith
1,2
, Remy Indebetouw
3,4
, Roger A. Chevalier
3
,
Mikako Matsuura
5
, Bryan M. Gaensler
2,6
, Michael J. Barlow
5
, Claes Fransson
7
, Richard N. Manchester
8
,
Maarten Baes
9
, Julia R. Kamenetzky
10
,Ma
ˇ
sa Laki
´
cevi
´
c
11
, Peter Lundqvist
7
, Jon M. Marcaide
12,13
,
Ivan Mart
´
ı-Vidal
14
, Margaret Meixner
15,16
, C.-Y. Ng
17
, Sangwook Park
18
, George Sonneborn
19
,
Jason Spyromilio
20
, and Jacco Th. van Loon
11
1
International Centre for Radio Astronomy Research (ICRAR), M468, University of Western Australia, Crawley, WA 6009, Australia;
giovanna.zanardo@gmail.com
2
Australian Research Council, Centre of Excellence for All-sky Astrophysics (CAASTRO)
3
Department of Astronomy, University of Virginia, P.O. Box 400325, Charlottesville, VA 22904, USA
4
National Radio Astronomy Observatory (NRAO), 520 Edgemont Road, Charlottesville, VA 22903, USA
5
Department of Physics and Astronomy, University College London, Gower Street, London WC1E 6BT, UK
6
Sydney Institute for Astronomy, School of Physics, University of Sydney, NSW 2006, Australia
7
Department of Astronomy, Oskar Klein Center, Stockholm University, AlbaNova, SE-106 91 Stockholm, Sweden
8
CSIRO Astronomy and Space Science, Australia Telescope National Facility, P.O. Box 76, Epping, NSW 1710, Australia
9
Sterrenkundig Observatorium, Universiteit Gent, Krijgslaan 281 S9, B-9000 Gent, Belgium
10
Steward Observatory, University of Arizona, 933 North Cherry Avenue, Tucson, AZ 85721-0065, USA
11
Institute for the Environment, Physical Sciences and Applied Mathematics, Lennard-Jones Laboratories, Keele University, Staffordshire ST5 5BG, UK
12
Departamento de Astronom
´
ıa, Universidad de Valencia, C/Dr. Moliner 50, E-46100 Burjassot, Spain
13
Donostia International Physics Center, Paseo de Manuel de Lardizabal 4, E-20018 Donostia-San Sebastian, Spain
14
Department of Earth and Space Sciences, Chalmers University of Technology, Onsala Space Observatory, SE-439 92 Onsala, Sweden
15
Space Telescope Science Institute, 3700 San Martin Drive, Baltimore, MD 21218, USA
16
Department of Physics and Astronomy, Johns Hopkins University, 366 Bloomberg Center, 3400 North Charles Street, Baltimore, MD 21218, USA
17
Department of Physics, University of Hong Kong, Pokfulam Road, Hong Kong, China
18
Department of Physics, University of Texas at Arlington, 108 Science Hall, Box 19059, Arlington, TX 76019, USA
19
NASA Goddard Space Flight Center, 8800 Greenbelt Road, Greenbelt, MD 20771, USA
20
European Southern Observatory (ESO), Karl-Schwarzschild-Str. 2, D-85748 Garching b. M
¨
unchen, Germany
Received 2014 August 9; accepted 2014 September 26; published 2014 November 10
ABSTRACT
We present a comprehensive spectral and morphological analysis of the remnant of supernova (SN) 1987A with
the Australia Telescope Compact Array (ATCA) and the Atacama Large Millimeter/submillimeter Array (ALMA).
The non-thermal and thermal components of the radio emission are investigated in images from 94 to 672 GHz
(λ 3.2 mm to 450 μm), with the assistance of a high-resolution 44 GHz synchrotron template from the ATCA, and
a dust template from ALMA observations at 672 GHz. An analysis of the emission distribution over the equatorial
ring in images from 44 to 345 GHz highlights a gradual decrease of the east-to-west asymmetry ratio with frequency.
We attribute this to the shorter synchrotron lifetime at high frequencies. Across the transition from radio to far
infrared, both the synchrotron/dust-subtracted images and the spectral energy distribution (SED) suggest additional
emission beside the main synchrotron component (S
ν
∝ ν
−0.73
) and the thermal component originating from dust
grains at T ∼ 22 K. This excess could be due to free–free flux or emission from grains of colder dust. However,
a second flat-spectrum synchrotron component appears to better fit the SED, implying that the emission could be
attributed to a pulsar wind nebula (PWN). The residual emission is mainly localized west of the SN site, as the
spectral analysis yields −0.4 α −0.1 across the western regions, with α ∼ 0 around the central region. If there
is a PWN in the remnant interior, these data suggest that the pulsar may be offset westward from the SN position.
Key words: ISM: supernova remnants – radiation mechanisms: non-thermal – radiation mechanisms: thermal –
radio continuum: general – stars: neutron – supernovae: individual (SN 1987A)
Online-only material: color figures
1. INTRODUCTION
The evolution of the remnant of supernova (SN) 1987A in
the Large Magellanic Cloud has been closely monitored since
the collapse of its progenitor star, Sanduleak (Sk) −69
◦
202,
on 1987 February 23. Models of Sk −69
◦
202 indicated that it
had an initial mass of ∼20 M
(Hillebrandt et al. 1987). The
mass range of the progenitor is consistent with the formation
of a neutron star (NS; Thielemann & Arnett 1985), and thus
with the neutrino events reported by the Kamioka NDE (Hirata
et al. 1987) and IMB (Bionta et al. 1987; Haines et al. 1988)
detectors. Models by Crotts & Heathcote (2000) suggest a
transition from red supergiant into blue supergiant to explain
the hourglass nebula structure, which envelopes the SN with
three nearly stationary rings (Chevalier & Dwarkadas 1995;
Blondin & Lundqvist, 1993; Martin & Arnett 1995;Morris
& Podsiadlowski 2007). The two outer rings, imaged with
the Hubble Space Telescope (HST; Jakobsen et al. 1991; Plait
et al. 1995), are located on either side of the central ring in
the equatorial plane (equatorial ring, ER), and likely formed at
the same time as the ER (Crotts & Heathcote 2000; Tziamtzis
et al. 2011). The synchrotron emission, generated by the shock
propagating into the clumpy circumstellar medium (CSM) close
to the equatorial plane, was detected in the mid-1990s (Turtle
et al. 1990; Staveley-Smith et al. 1992), and has become brighter
over time (Manchester et al. 2002; Zanardo et al. 2010). Radio
1
The Astrophysical Journal, 796:82 (19pp), 2014 December 1 Zanardo et al.
observations have stretched from flux monitoring at 843 MHz
with the Molonglo Observatory Synthesis Telescope (Staveley-
Smith et al. 1993;Balletal.2001) to images of sub-arcsec
resolution with the Australia Telescope Compact Array (ATCA;
Gaensler et al. 1997; Manchester et al. 2005a;Ngetal.2008;
Potter et al. 2009; Zanardo et al. 2013). ATCA observations
at 94 GHz (Laki
´
cevi
´
cetal.2012b) have been followed by
observations from 100 GHz up to 680 GHz with the Atacama
Large Millimeter/submillimeter Array (ALMA; Kamenetzky
et al. 2013; Indebetouw et al. 2014).
The ongoing shock expansion has been monitored at 9 GHz
since 1992 (Gaensler et al. 1997, 2007). Shock velocities of
∼4000 km s
−1
have been measured between day 4000 and 7000
(Ng et al. 2008), while signs of a slower expansion have been
tentatively detected after day ∼7000, as the shock has likely
propagated past the high-density CSM in the ER (Ng et al.
2013). Similar evidence of slower shock expansion since day
∼6000 has been found in X-ray data (Park et al. 2005, 2006;
Racusin et al. 2009) as well as in infrared (IR) data (Bouchet
et al. 2006).
Since the early super-resolved images at 9 GHz (Gaensler
et al. 1997), a limb-brightened shell morphology has been
characteristic of the remnant. The radio emission, over the
years, has become more similar to an elliptical ring rather than
the original truncated-shell torus (Ng et al. 2013). The radio
remnant has shown a consistent east–west asymmetry peaking
on the eastern lobe, which has been associated with higher
expansion velocities of the eastbound shocks (Zanardo et al.
2013). The asymmetry degree appears to have changed with
the shock expansion, as images at 9 GHz exhibit a decreasing
trend in the east–west asymmetry since day ∼7000 (Ng et al.
2013). High-resolution observations at 1.4–1.6 GHz (Ng et al.
2011) via Very Long Baseline Interferometry (VLBI) with the
Australian Large Baseline Array (LBA) have highlighted the
presence of small-scale structures in the brightest regions in
both lobes.
The relation between the radio emission and the synchrotron
spectral indices, α (S
ν
∝ ν
α
), has been investigated via both flux
monitoring (Manchester et al. 2002; Zanardo et al. 2010) and
imaging observations (Potter et al. 2009; Laki
´
cevi
´
cetal.2012b;
Zanardo et al. 2013) with the ATCA. The progressive flattening
of the radio spectrum derived from 843 MHz to 8.6 GHz at
least since day 5000, coupled with the e-folding rate of the
radio emission, has pointed to an increasing production of non-
thermal electrons and cosmic rays (CR) by the shock front
(Zanardo et al. 2010). On the other hand, the association of
steeper spectral indices with the brightest eastern sites implies a
higher injection efficiency on the eastern side of the supernova
remnant (SNR; Zanardo et al. 2013). Flatter spectral indices
in the center of the remnant have been tentatively identified
from low-resolution two-frequency spectral maps (Potter et al.
2009; Laki
´
cevi
´
cetal.2012b), while at 18–44 GHz the central
and center–north regions have −0.5 α −0.3 (Zanardo
et al. 2013). With ALMA, the spectral energy distribution
(SED) of the remnant has been mapped where the non-thermal
and thermal components of the emission overlap, identifying
cold dust in the SNR interior (Indebetouw et al. 2014), which
accommodates 0.4–0.7 M
of the dust mass discovered with the
Herschel Space Observatory (Herschel; Matsuura et al. 2011)
in the ejecta.
This paper combines the results presented by Indebetouw
et al. (2014) with a comprehensive morphological and spectral
analysis of SNR 1987A based on both ATCA and ALMA data.
In Section 2, we present the ALMA Cycle 0 super-resolved
images before and after subtraction, in the Fourier plane, of the
synchrotron and dust components (Section 3). In Section 4,we
assess the remnant asymmetry from 44 to 345 GHz. In Section 5,
we update the SED derived by Indebetouw et al. (2014), while,
in Section 6, we investigate the spectral index variations in the
SNR across the transition from radio to far infrared (FIR). In
Section 7, we discuss possible particle flux injection by a pulsar
situated in the inner regions of the remnant.
2. OBSERVATIONS AND ANALYSIS
The ATCA and ALMA observations used in this study were
performed in 2011 and 2012. ATCA observations at 44 and
94 GHz are detailed in Zanardo et al. (2013) and Laki
´
cevi
´
cetal.
(2012b), respectively. ALMA observations were made in 2012
(Cycle 0) from April to November, over four frequency bands:
Band 3 (B3, 84–116 GHz, λ 3 mm), Band 6 (B6, 211–275 GHz,
λ 1.3 mm), Band 7 (B7, 275–373 GHz, λ 850 μm), and Band
9 (B9, 602–720 GHz, λ 450 μm). Each band was split over
dual 2 GHz wide sidebands, with minimum baselines of 17 m
(B9) to maximum baselines of 400 m (B3). All observations
used quasars J0538-440 and J0637-752 as bandpass and phase
calibrators, respectively. Callisto was observed as an absolute
flux calibrator in B3 and B6, while Ceres was used in B7 and B9
(see also Kamenetzky et al. 2013). It is noted that, while ALMA
is designed to yield data with flux density calibration uncertainty
as low as ∼1%, in Cycle 0 this uncertainty is estimated at ∼5%
at all frequencies. Relevant observational parameters are listed
in Table 1 (see also Table 1 in Indebetouw et al. 2014).
Each data set was calibrated with the casa
21
package, then
exported in miriad
22
for imaging. After clean-ing (H
¨
ogbom
1974), both phase and amplitude self-calibration were applied in
B3 over a 2 minute solution interval, while only phase calibration
was applied in B6 and B7. No self-calibration was performed
in B9. As in Zanardo et al. (2013), we note that since the
self-calibration technique removes position information, each
image was compared with that prior to self-calibration and,
in case of positional changes, the self-calibrated images were
shifted. Further adjustments were made in the comparison with
the ATCA observations at 44 GHz, based on prominent features
on the eastern lobe and location of the remnant center. As from
Zanardo et al. (2013), the 44 GHz image was aligned with VLBI
observations of the SNR (G. Zanardo et al., in preparation).
Adding in quadrature these positional uncertainties and the
accuracy of the LBA VLBI frame, the errors in the final image
position are estimated at ∼60 mas.
Deconvolution was carried out via the maximum entropy
method (MEM; Gull & Daniell 1978) in B3, B6, and B7. A
weighting parameter of robust = 0.5 (Briggs 1995) was used in
all bands. The resultant diffraction-limited images, which have
central frequency at 102 GHz in B3, 213 GHz in B6, 345 GHz
in B7, and 672 GHz in B9, were then super-resolved with a
circular beam of 0.
8inB3,0.
6 in B6, and 0.
3inB7andB9.
The diffraction limited and super-resolved images are shown in
the first column of Figures 1 and 2, below the ATCA image
at 94 GHz (Laki
´
cevi
´
cetal.2012b). Integrated Stokes I flux
densities, dynamic range and related rms are given in Table 2.
To decouple the non-thermal emission from that originating
from dust, the synchrotron component, as resolved with ATCA
at 44 GHz (Zanardo et al. 2013), and the dust component, as
21
http://casa.nrao.edu/
22
http://www.atnf.csiro.au/computing/software/miriad/
2
The Astrophysical Journal, 796:82 (19pp), 2014 December 1 Zanardo et al.
Tab le 1
ALMA Observing Parameters
Parameter 102 GHz 213 GHz 345 GHz 672 GHz
(Band 3, B3) (Band 6, B6) (Band 7, B7) (Band 9, B9)
Date (2012) Apr 5, 6 Jul 15 and Aug 10 Jul 14 and Aug 24 Aug 25and Nov 5
Day since explosion 9174 9287 9294 9351
Frequency bands
a
(GHz) 100.093–101.949 213.506–213.597 336.979–340.917 661.992–665.992
102.051–103.907 349.010–352.963 678.008–682.008
Center frequency (GHz) 101.918 213.146 345.364 672.165
Channel width (MHz) 4.883 4.883 31.250 15.625
Max baselines (u, v)(kλ) 150, 120 180, 260 400, 400 700, 700
No. of antennas 14–18 14–23 28 19–25
Total observing time (hr) 0.83 1.03 0.62 3.40
Note.
a
In B3 and B6, the frequency range is selected to avoid CO and SiO emission (Kamenetzky et al. 2013).
Tab le 2
Image Parameters
Image S
ν
a
SR Beam
b
DL Beam
c
P.A. Rms Noise Dynamic Range
(GHz) (mJy) (
)(
)(
◦
) (mJy beam
−1
)
94
d
24.2 ± 3.90.7 ··· ··· 0.085 137
102
e
23.1 ± 3.10.81.74 × 1.25 5.70.033 285
213 19.7 ± 1.60.61.16 × 0.74 −68.50.034 75
345 16.7 ± 1.50.30.65 × 0.48 −40.20.023 121
672 52.8 ± 14.20.30.34 × 0.28 68.51.219 28
94−I
B9
f
23.2 ± 3.90.70.78 × 0.63 15.40.094 81
102−I
B9
19.4 ± 3.20.81.66 × 1.19 6.30.034 143
213−I
B9
16.9 ± 1.90.61.16 × 0.74 −68.50.029 64
345−I
B9
11.5 ± 1.90.30.65 × 0.48 −44.00.031 43
672−I
B9
−1.0 ± 14.30.30.34 × 0.28 68.51.193 2
94−I
44
g
0.9 ± 3.90.70.78 × 0.63 15.40.095 60
102−I
44
3.5 ± 3.10.81.48 × 0.96 10.10.027 81
213−I
44
2.9 ± 1.70.61.16 × 0.74 −68.50.024 26
345−I
44
5.8 ± 1.60.30.65 × 0.48 −44.00.014 90
672−I
44
47.4 ± 14.80.30.57 × 0.55 78.60.816 24
Notes.
a
All flux densities are derived from the diffraction-limited images. The errors are derived from the flux calibration
uncertainty combined with the uncertainties in the image subtraction.
b
Circular beam used for super-resolution (SR).
c
Beam associated with the diffraction-limited (DL) image.
d
The flux density is scaled to day 9280 via exponential fitting parameters derived for ATCA flux densities from day
8000, as measured at 8.6 and 9 GHz (G. Zanardo et al., in preparation). All other image parameters are as from Laki
´
cevi
´
c
et al. (2012b).
e
Images are shown in Figures 1 and 2.
f
Images obtained by subtracting the model flux density at 672 GHz (I
B9
) scaled to fit the central emission. See central
column of Figures 1 and 2.
g
Images obtained by subtracting the model flux density at 44 GHz (I
44
) scaled to fit the toroidal emission. See left column
of Figures 1 and 2.
imaged with ALMA at 672 GHz (B9; Indebetouw et al. 2014),
were separately subtracted from the data sets at 94, 102, 213,
345, and 672 GHz. All subtractions were performed in the
Fourier plane, via miriad task uvmodel, where the model flux
density at 44 GHz was scaled to fit the SNR emission over
the ER (I
44
), while the B9 model flux density was scaled to
fit the emission localized in the central region of the remnant
(I
B9
). Scaling of the 44 GHz model was tuned by minimizing
the flux density difference on the brighter eastern lobe, without
over-subtracting in other regions of the remnant. To separate
the emission in the SNR center, the central flux was firstly
estimated by fitting a Gaussian model via miriad task uvfit.
The image model at 672 GHz was then scaled to match the flux
of the Gaussian model. The scaling factor was further tuned to
minimize over-subtraction.
Super-Nyquist sampling was applied in all images, using a
pixel size of 8 mas to avoid artifacts when sources are not at
pixel centers. Deconvolution via MEM was carried out on the
residual images obtained from the subtraction of I
B9
, while
standard CLEANing was applied to the residuals obtained from
the I
44
subtraction. All diffraction-limited-subtracted images
are shown in the central and right columns of Figure 1, while
Figure 2 shows the residuals after super-resolution with the
circular beam used for the original images. All image parameters
are given in Table 2.
The flux densities were determined by integrating within
polygons enclosing the SNR emission. Uncertainties in the flux
densities include uncertainties in the image fitting/scaling pro-
cess combined with the uncertainty in the flux density cali-
bration. We note that the residual images from the subtraction
3
The Astrophysical Journal, 796:82 (19pp), 2014 December 1 Zanardo et al.
Figure 1. (Top to bottom) Left column: Stokes I continuum images of SNR 1987A at 94 (Laki
´
cevi
´
c et al. 2012b), 102, 213, 345, and 672 GHz. Images from 102 to
672 GHz are made from ALMA observations (Cycle 0) performed from 2012 April 5 to November 5 (see Table 1). Center column: images obtained by subtracting
the model flux density at 672 GHz (Band 9) scaled to fit the central emission. Right column: images obtained by subtracting a scaled model flux density at 44 GHz
(Zanardo et al. 2013), with 3σ flux density contours highlighted (white). The angular resolution is shown in the bottom left corner. The green cross indicates the VLBI
position of SN 1987A as determined by Reynolds et al. (1995) [R.A. 05
h
35
m
27.
s
968, decl. −69
◦
16
11.
09 (J2000)].
(A color version of this figure is available in the online journal.)
4
The Astrophysical Journal, 796:82 (19pp), 2014 December 1 Zanardo et al.
Figure 2. Panel layout is identical to that in Figure 1, but all Stokes I continuum-subtracted images are super-resolved. The circular beam used for the super-resolved
ALMA images is 0.
8 at 102 GHz (Band 3), 0.
6 at 213 GHz (Band 6), 0.
3 at 345 (Band 7) and 672 GHz (Band 9), and is plotted in the lower left corner. The 94 GHz
images are restored with a 0.
7 circular beam. For images in the right column, 3σ flux density contours are highlighted (white). The green cross indicates the VLBI
position of SN 1987A as determined by Reynolds et al. (1995).
(A color version of this figure is available in the online journal.)
5
