Chandra Observations of Supernova 1987A
Chandra Observations of Supernova 1987A
Sangwook Park∗, David N. Burrows∗, Gordon P. Garmire∗, Richard
McCray†, Judith L. Racusin∗ and Svetozar A. Zhekov∗∗
∗Department of Astronomy and Astrophysics, 525 Davey Lab., Pennsylvania State University,
University Park, PA 16802, USA
†JILA, University of Colorado, Box 440, Boulder, CO 80309, USA
∗∗Space Research Institute, Moskovska Strasse 6, Sofia 1000, Bulgaria
Abstract. We have been monitoring Supernova (SN) 1987A with Chandra X-Ray Observatory
since 1999. We present a review of previous results from our Chandra observations, and some pre-
liminary results from new Chandra data obtained in 2006 and 2007. High resolution imaging and
spectroscopic studies of SN 1987A with Chandra reveal that X-ray emission of SN 1987A origi-
nates from the hot gas heated by interaction of the blast wave with the ring-like dense circumstellar
medium (CSM) that was produced by the massive progenitor’s equatorial stellar winds before the
SN explosion. The blast wave is now sweeping through dense CSM all around the inner ring, and
thus SN 1987A is rapidly brightening in soft X-rays. At the age of 20 yr (as of 2007 January), X-ray
luminosity of SN 1987A is LX ∼ 2.4 × 10
36 ergs s−1 in the 0.5−10 keV band. X-ray emission is
described by two-component plane shock model with electron temperatures of kT ∼ 0.3 and 2 keV.
As the shock front interacts with dense CSM all around the inner ring, the X-ray remnant is now
expanding at a much slower rate of v ∼ 1400 km s−1 than it was until 2004 (v ∼ 6000 km s−1).
Keywords: supernova remnants; supernovae; SN 1987A; X-rays
PACS: 98.38.Mz
1. INTRODUCTION
Supernova (SN) 1987A, the nearest SN in four centuries, occurred in the Large Margel-
lanic Cloud (LMC). The identification of a Type II SN from a blue supergiant progenitor
and the detection of neutrino bursts associated with the SN indicate a core-collapse ex-
plosion of a massive star [1]. SN 1987A, providing these fundamental parameters and
being located at a near distance (d = 50 kpc), is a unique opportunity for the study of a
massive star’s death and the subsequent birth of a supernova remnant (SNR) in unprece-
dented detail.
About 10 yr after the SN explosion, the blast wave started to interact with the “inner
ring” of dense circumstellar medium (CSM) [5], which is believed to be produced by
the equatorial stellar winds of the massive progenitor star. This shock-CSM interaction
resulted in a dramatic brightening of SN 1987A in soft X-rays, which provides an
excellent laboratory for the X-ray study of the evolution of an optically thin thermal
plasma in nonequilibrium ionization (NEI) as the shock propagates through a complex
density gradient of the dense CSM. As the rapidly brightening X-rays begin to illuminate
the interior of the SN, metal-rich ejecta expelled from the massive star’s core will begin
to glow optically, allowing us to study SN nucleosynthesis yields. Then, a few decades
from now, when these newly-formed elements begin to cross the reverse shock surface,
we will be able to measure the distribution of these elements in more detail through their
http://arxiv.org/abs/0704.0209v1
TABLE 1. Chandra Observations of SNR 1987A
Observation ID
(Age)∗
Instrument
(Subarray)
(ks) Counts
124+1387† 1999-10-6 (4609) ACIS-S+HETG 116.1 690∗∗
122 2000-1-17 (4711) ACIS-S3 (None) 8.6 607
1967 2000-12-07 (5038) ACIS-S3 (None) 98.8 9030
1044 2001-4-25 (5176) ACIS-S3 (None) 17.8 1800
2831 2001-12-12 (5407) ACIS-S3 (None) 49.4 6226
2832 2002-5-15 (5561) ACIS-S3 (None) 44.3 6427
3829 2002-12-31 (5791) ACIS-S3 (None) 49.0 9277
3830 2003-7-8 (5980) ACIS-S3 (None) 45.3 9668
4614 2004-1-2 (6157) ACIS-S3 (None) 46.5 11856
4615 2004-7-22 (6359) ACIS-S3 (1/2) 48.8 17979
4640+4641+5362+5363+6099† 2004-8-26∼9-5 (∼6400) ACIS-S+LETG 289.0 16557∗∗
5579+6178† 2005-1-12 (6533) ACIS-S3 (1/8) 48.3 24939
5580+6345† 2005-7-14 (6716) ACIS-S3 (1/8) 44.1 27048
6668 2006-1-28 (6914) ACIS-S3 (1/8) 42.3 30940
6669 2006-7-28 (7095) ACIS-S3 (1/8) 36.4 30870
7636 2007-1-19 (7271) ACIS-S3 (1/8) 33.5 32798
∗ Days since SN
† These observations were splitted by multiple sequences which were combined for the analysis.
∗∗ Photon statistics are from the zeroth-order data.
X-ray emission. Neutrino bursts were a strong support for a core-collapse explosion, and
thus for the creation of a neutron star which should become bright in X-rays.
High resolution imaging and spectroscopic studies of SN 1987A with Chandra X-
Ray Observatory are an ideal tool for the X-ray study of SN 1987A. We have thus
been observing SN 1987A with Chandra since its launch in 1999, roughly twice a
year, in order to monitor the earliest stages of the evolution of the X-ray remnant of
SN 1987A. We here review previous results from our Chandra observations of SNR
1987A [2, 6, 7, 8, 9, 10, 12, 13], and present some preliminary results from the latest
Chandra observations.
2. OBSERVATIONS
Our Chandra observations of SNR 1987A are listed in Table 1. As of 2007 January, we
have performed a total of sixteen Chandra observations of SNR 1987A, including two
deep gratings observations. Data reduction and analysis process have been described in
the literatures [2, 6, 8, 12].
3. X-RAY IMAGES
Broadband Chandra ACIS images of SNR 1987A are presented in Fig. 1. We applied
a subpixel resolution method [11], deconvolved images with the detector point spread
function (PSF), and then smoothed. The ring-like overall morphology of the X-ray rem-
2007−1−19 (7271)2006−7−28 (7095)
2006−1−28 (6914)2005−7−14 (6716)2005−1−12 (6533)2004−7−22 (6359)
2004−1−2 (6157)2003−7−8 (5980)2002−5−15 (5561) 2002−12−31 (5791)
2001−12−12 (5407)2001−4−25 (5176)2000−12−7 (5038)2000−1−17 (4711)
1 arcsecond
FIGURE 1. Chandra ACIS (in the 0.3−8 keV band) false-color images of SNR 1987A. In each panel,
the observation date and age (days since the SN, in parentheses) are presented.
nant is evident. SNR 1987A has been brightening and expanding for the last 7 yr. Ini-
tially, the eastern side was brighter, but then the western side began brightening in early
2004 (day ∼6200). SNR 1987A is now bright all around the ring. Early images showed
that the soft X-ray band images (E < 1.2 keV) were correlated with the optical images
while the hard band (E > 1.2 keV) image matched the radio images [7]. These differ-
ential X-ray morphologies supported our interpretation that soft X-rays are produced by
the decelerated shock entering dense protrusions of the inner ring and that hard X-rays
originate from the fast shock propagating through less dense regions between protru-
sions. Recent data show that the X-ray morphology is now nearly identical between the
hard and soft bands, which is perhaps expected as an increasing fraction of the blast
FIGURE 2. Radial expansion of SNR 1987A (taken from Racusin et al. in preparation). Data taken
with the gratings are excluded. Day 4711 has also been excluded because of the low photon statistics.
wave shock front is reaching dense CSM all around the inner ring [8]. The 0.3−8 keV
band count rate is now ∼0.98 c s−1, which is ∼14 times brighter than it was in 2000.
Assuming the apparent X-ray morphology of SNR 1987A (i.e., an elliptical torus
superposed with 3−4 bright lobes), we model X-ray images to derive the best-fit radius
at each epoch. The details of our image modeling are presented in the literature (Racusin
et al. in preparation). Measured radii indicate that the X-ray remnant is expanding
with an overall expansion rate of v ∼ 3900 km s−1 (Fig. 2), which is consistent with
our previous estimates [8]. It is, however, intriguing to note that the expansion rate is
significantly reduced to v ∼ 1400 km s−1 since day ∼ 6200 (Fig. 2). Deceleration of the
expansion rate is in fact in good agreement with our interpretation of the shock reaching
dense CSM all around the inner ring on days ∼6000−6200 [9].
The putative neutron star has not yet become visible [2, 7, 8]. If the extinction for the
SNR’s center were similar to that for the entire SNR, an upper limit of LX(2−10 keV)
∼ 1.5 × 1034 ergs s−1 has been estimated for an embedded point source [8].
4. X-RAY SPECTRUM
The X-ray spectrum of SNR 1987A is line-dominated, indicating a thermal origin
(Fig. 3). As the shock interacts with increasing amount of dense CSM, multiple com-
ponents of hot optically thin plasma are required to adequately fit the observed X-ray
5 10 15 20 25
Wavelength (Anstrom)
10.5 2 51
Energy (keV)
(a) (b)
FIGURE 3. (a) The ACIS spectrum of SNR 1987A as of 2007-1-19. The best-fit two-component plane
shock model is overlaid. (b) The LETG spectrum of SNR 1987A as of 2004-8 (taken from [12]).
TABLE 2. Best-Fit Parameters from the Two-Shock Model Fit of SNR 1987A
(days)
kT(soft)
(keV)
kT(hard)
(keV)
net(hard)
(1011 cm−3 s)
EM(soft)
(1058 cm−3)
EM(hard)
(1058 cm−3) χ2/ν
6914 0.31+0.04
−0.02 2.21
+0.16
−0.07 2.24
+0.48
−0.40 29.28
+5.86
−6.75 3.54
+0.27
−0.21 178.3/142
7095 0.29+0.01
−0.01 2.03
+0.13
−0.12 2.63
+0.62
−0.44 37.89
+1.53
−0.87 4.65
+0.30
−0.30 240.5/141
7271 0.31+0.07
−0.01 1.96
+0.09
−0.07 3.63
+1.05
−0.78 40.80
+3.00
−13.80 5.61
+0.44
−0.33 183.6/142
∗ Days since SN
spectrum [8, 10, 13]. In fact, a two-temperature NEI plane shock model fits the observed
ACIS spectrum of SNR 1987A (Fig. 3a). The soft and hard components characteristi-
cally represent the decelerated shock (by dense protrusions of the inner ring) and the fast
shock propagating into less-dense medium, respectively. Results from two-component
plane shock model fits of the ACIS spectrum for the latest three epochs, which have not
been published, are presented in Table 2. The foreground column is fixed at NH = 2.35
× 1021 cm−2 [10]. Metal abundances are fixed at values measured by Zhekov et al. [13],
which are generally consistent with the LMC abundances. Ionization timescales for the
soft component (kT ∼ 0.3 keV) are high (net > 10
12 cm−3 s), indicating the hot gas is
in collisional ionization equilibrium due to the shock interaction with dense CSM.
The high resolution dispersed spectrum obtained by the deep LETG observation re-
vealed detailed X-ray emission lines from various elemental species (Fig. 3b, [12]). The
high-quality LETG spectrum showed that the continuous distribution of the shock tem-
perature is represented by two dominant components (kT ∼ 0.5 and 2.5 keV) [13]. The
LETG spectrum indicated LMC-like metal abundances with a moderate enhancement
in N [13]. X-ray line broadening measurements using the deep LETG observation in-
dicated shock velocities of v ∼ 300−1700 km s−1 [12] which are significantly lower
than that deduced from the HETG observation performed ∼5 yr earlier (v ∼ 3400 km
s−1, [6]). These results are consistent with the ACIS spectral analysis, supporting the
interpretation of the blast wave recently interacting with the entire inner ring.
TABLE 3. Chandra Flux and Luminosity of SNR 1987A
Age∗ fX(0.5−2 keV)† fX(3−10 keV)† LX(0.5−10 keV)∗∗
4711 1.61±0.66 0.84±0.57 1.54
5038 2.40±0.22 0.92±0.21 2.22
5176 2.71±0.54 1.22±0.41 2.59
5407 3.55±0.43 1.20±0.44 3.24
5561 4.19±0.46 1.49±0.64 3.79
5791 5.62±0.45 1.82±0.46 5.05
5980 6.44±0.52 1.95±0.62 5.71
6157 7.73±0.62 2.38±0.57 6.82
6359 11.48±0.69 2.40±0.60 9.54
6533 16.29±0.65 2.80±0.73 13.58
6716 19.41±0.97 3.26±0.68 16.06
6914 21.96±1.10 3.45±0.69 17.99
7095 25.56±1.28 3.84±0.77 20.58
7271 29.62±1.48 4.41±0.88 23.54
∗ Days since SN
† Observed flux in units of 10−13 ergs cm−2 s−1
∗∗ In units of 1035 ergs s−1, after corrected for NH = 2.35 × 10
21 cm−2.
1000 2000 3000 4000 5000 6000 7000 8000
Square: ROSAT (0.5-2 keV)
Triangle: Chandra ACIS 3-10 keV
Circle: Chandra ACIS 0.5-2 keV
Dot: ATCA 3, 6, 13, 20 cm
Days since SN .
FIGURE 4. X-ray and radio light curves of SNR 1987A. Radio fluxes are arbitrarily scaled. The solid
line is the best-fit model by [9]
5. X-RAY LIGHT CURVES
We present the soft (0.5−2 keV) and hard (3−10 keV) band X-ray light curves in Table 3
and Fig. 4. We also present the ROSAT [4] and radio1 light curves (Fig. 4). The soft X-
ray light curve has been increasing nearly exponentially for the last several yr, with
apparent “upturns” on days ∼3500−4000 and days ∼6000−6200. These features were
interpreted as the time when the blast wave first made contact with the dense protrusions,
and the time when the shock reached the main body of the inner ring [9]. The latest data
points (days > 6700) suggest that the soft X-ray flux is still rapidly increasing, but
probably less steeply than it was for the previous ∼2 yr (Fig. 4). This latest behavior of
the soft X-ray light curve might have implications for the details of the density structure
of the inner ring. Periodic monitoring of the soft X-ray flux is important to study the
details of the density and abundance structures of the inner ring.
The hard X-ray light curve is increasing at a lower rate than the soft X-ray light
curve (Fig. 4). This slow increase rate appears to be roughly consistent with the radio
light curve (Fig. 4). Hard X-rays in SNR 1987A might thus originate from the same
synchrotron radiation as radio emission does. However, the morphology of hard X-ray
images is no longer distinguishable from that of soft X-ray images [9]. The origin of
hard X-ray emission from SNR 1987A is thus uncertain. Periodic monitoring of hard
X-ray and radio light curves and searching for X-ray lines in the hard band (e.g., Fe K
lines) will be important to reveal the origin of hard X-ray emission.
6. THE ACIS PHOTON PILE-UP
Based on their XMM-Newton data analysis, Haberl et al. [3] argued that our Chandra soft
X-ray light curve [9] was significantly contaminated by the ACIS photon pileup. They
re-estimated the 0.5−2 keV band ACIS fluxes of SNR 1987A using archival Chandra
data, and calculated pileup correction factors for the measured ACIS fluxes. Their 0.5−2
keV band flux correction factors were up to ∼25%, especially for recent epochs of days
∼6533 and 6716. Thus, they claimed that the reported upturn of the soft X-ray light
curve on days ∼6000−6200 [9] was an artifact caused by the photon pileup.
Haberl et al., however, misunderstood our Chandra instrument setup for three epochs:
we used the HETG on day 4609 and a 1/8 subarray of the ACIS on days 6533 and 6716,
while they assumed the bare ACIS on day 4609 and a 1/2 subarray of the ACIS on days
6533 and 6716. Their ACIS flux corrections for these epochs were thus incorrect. We
note that the ACIS photon pileup is not the sole contamination, and Haberl et al. did
not consider other issues such as the charge transfer inefficiency and the time-dependent
quantum efficiency degradation of the ACIS data. A moderate discrepancy (
<10%) is
also known between XMM-Newton and Chandra due to the imperfect cross-calibration
between them. Furthermore, SNR 1987A is an extended source as observed with the
Chandra ACIS, whereas a pointlike source was apparently assumed by Haberl et al.
1 Radio data obtained with Australian Telescope Compact Array (ATCA) have been provided by L.
Staveley-Smith.
Considering these technical issues, we have re-analyzed the possible effects of
ACIS photon pile-up on our Chandra observations using three independent methods:
PIMMS/XSPEC simulations, ACIS event grade distribution analysis, and the (modified)
standard ACIS photon pileup model. Our results from these three analyses agree that
flux correction factors due to the ACIS photon pileup are roughly several % or less, with
the exception of day 6157 where ∼15% of the soft X-ray flux appeared to be lost due to
photon pileup (see Park et al. [in preparation] for the detailed results). Based on these
results, we confirm that the scientific conclusions by Park et al. [9] were not affected by
the ACIS photon pileup.
ACKNOWLEDGMENTS
This work was supported in part by Smithsonian Astrophysical Observatory under
Chandra grant GO6-7047X.
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	Introduction
	Observations
	X-Ray Images
	X-Ray Spectrum
	X-Ray Light curves
	The ACIS Photon Pile-Up
