Download The supernova of AD1181 – an update

Supernovae
The supernova of
AD1181 – an update
F Richard Stephenson and David A Green summarize observations of the supernova
of AD 1181 in East Asia and recent observations of its remnant, 3C 58.
O
+80°
+60°
Kui
Ziwe
i
5h
Chu
gai
ω
e
h
+70°
3C 58 ψ
tic eq
uator
ε
3C 10
δ
h
3
rig
ht
ce
as
ion
ns
ν
1h
following are translations of four key records,
the first of which – noted by Li Jinyi – provides
the most detailed information.
Key records
1. South China. “In the 8th year, sixth month,
day jisi [6 August AD 1181], a ‘guest star’
appeared in Kui lunar lodge. It was trespassing
against Chuanshe. According to divination, the
guest star was a star of ill omen ... (astrological
commentary) ... The guest star appeared at the
edge of Ziwei among the stars of Chuanshe ...
(further astrological commentary) ... On the day
jiaxu (11 August) the guest star guarded the 5th
star of Chuanshe. In the 9th year, 1st month, day
kuiyou [6 February AD 1182] the guest star disappeared. From the previous year, 6th month,
day jisi until the present was a total of 185
days.” (Wenxian Toungkao – “Comprehensive
History of Civilisation” by Ma Duanlin. Note: a
summary of this account is to be found in the
Songshu – “History of the Song Dynasty”.)
2. North China. “In the 21st year of the Ta-ting
reign period, sixth month, day jiawu [11 August
+60°
β
α
Kui
1 Chart showing
positions of stars,
Chinese star groups, or
“asterisms”, and the
radio sources 3C 10
and 3C 58 in
Cassiopeia. Asterisms
marked with solid lines
are following Shitong
(1988), with the broken
line indicating the less
certain Chuanshe. The
dotted green lines
indicate the limits of
the Chinese lunar
lodge Kui.
κ
γ
υ
η
θ
2h
+80°
Hua
ι
ansh
galac
4
April 1999 Vol 40
6h
E
ast Asian observations of the star which
appeared in AD 1181 were discussed by
Stephenson (1971) and later by Clark
and Stephenson (1977). These investigations
led to the identification of the remnant of the
star as 3C 58 (=G130.7+3.1). As pointed out
by Li Jinyi (1983, paper in Chinese), a further
Chinese record, not discussed in the above references, gives additional support to this identification. This record is also reported, briefly, in
English, by Wang Zhenru (1987). In this paper
we update the historical information and summarize recent observations of 3C 58.
The new star which appeared in the late summer of AD 1181 attracted considerable attention in South and North China (which were
then two independent empires) and in Japan.
However, there are no known European or Arabic records. Occurring in the Cassiopeia region,
the star was visible for about six months and
then faded from sight. From the descriptions in
Chinese and Japanese history, it is possible to
obtain a good fix on the position of the star, but
not to constrain its light curve in any detail. The
declination
+70°
bservations of the supernova of
1181 were recorded
independently at different locations
in North and South China and
Japan. These descriptions fix the
position of the star, coincident with
the radio source 3C 58, and indicate
that it was visible for up to 186
days, although they do not give any
detailed information useful for
constructing its light curve. We
discuss in detail a new historical
record of the supernova, and recent
observations of 3C 58 that reinforce
its identification as the remnant of
this supernova.
ζ
Wan
gl
iang
0h
1181] a guest star was seen at Huagai altogether for 156 days; then it was extinguished.” (Jinshu – “History of the Jin Dynasty”.)
3. Japan. “In the first year of the Yowa reign
period, sixth month, 25th day [7 August 1181],
a guest star appeared at the north near Wangliang and guarding Chuanshe.” (Meigetsuki –
“Diary of the Full Moon” by Fujiwara Sadaie.)
4. Japan. “In the first year of the Yowa reign
period, sixth month, 25th day [7 August 1181],
at the hour xu [19 to 21 h] a guest star was
seen at the north-east. (It was like) Saturn and
its colour was purple [blue-red] it had rays.
There had been no other example since that
appearing in the third year of the Kanko reign
period [i.e. AD 1006].” (Azuma Kagami –
“History of the Kamakura Shogunate”.)
Note that “asterism” has become a standard
term to describe the numerous small Chinese
constellations or star groups, to distinguish
these from Western constellations. In points 3
and 4 above, Chinese terminology has been
used for asterisms, and for the hour xu.
In each instance the new star is described as
2.27
Supernovae
a “guest star” (kexing); this is the usual oriental term for a star-like object. Neither of the
Japanese records gives any indication of the
period of visibility of the star. However, the
two independent records from South and
North China affirm a lengthy duration. As the
supernova was circumpolar, it would never be
close to the Sun, so that its duration of visibility would be determined by its faintness, and
by the local weather conditions in South and
North China. The Azuma Kagami seems to
imply that the star was about as bright as the
planet Saturn – i.e. among the brightest stars in
the sky, but by no means a really brilliant
object. The fact that the star “had rays” may
merely indicate an optical effect caused by its
brightness, being significantly brighter than the
surroundings stars in Cassiopeia. Its lack of
mention in Korea also suggests that it was not
outstandingly bright. (Reference to the Koryosa shows that Korean astronomical records
around this time are fairly detailed, but they all
relate to meteors or lunar and planetary movements; there is nothing which could be understood to relate to a strange star of long duration). The supernova of AD 1006, with which
the guest star was compared in Japan, was a
brilliant object; however, this occurred nearly
two centuries earlier so that no direct comparison could be made.
Locating the observations
The first of the records for which we have
given translations indicates the approximate
range of right-ascension (in Kui lunar lodge)
within which the star appeared. This is a zone
about 15° in width, extending north and south
from Andromeda and Pisces. The asterism
Ziwei (“Purple Palace”) was near the edge of
the circle of perpetual visibility. Other asterisms near which the guest star appeared are
Chuanshe, Wangliang and Huagai. These are
neighbouring asterisms in Cassiopeia and lie
near the galactic plane. Comparison between
early oriental star maps and detailed modern
charts shows that Wangliang, a group of five
stars named after a famed charioteer, includes
some of the brightest stars of Cassiopeia.
Although much fainter, the seven stars of Huagai (“Gilded Canopy”) form a well-defined
cluster shaped like a parasol and are fairly
readily identified. Chuanshe (“Inns”, “Guest
Houses”) is an extended asterism consisting of
nine dim stars barely visible to the unaided eye
and extending more or less along an east–west
direction. It lies roughly midway between
Wangliang and Huagai.
Clearly the most detailed record is that from
South China which asserts that the guest star
“guarded the 5th star of Chuanshe”. It is interesting that the account in the Meigetsuki also
notes that the guest star “guarded” this same
asterism; the term implies a stationary
2.28
position. According to the extensive historical
researches of the Beijing astronomer Yi Shitong
(1988), the fifth constituent of Chuanshe is a
faint star quite close to ε Cas.
A long period of visibility and proximity to
the galactic equator are both characteristic of
supernovae. Of the known supernova remnants (SNRs) which have been identified, only
two which are relatively young lie in this part
of the sky. One of these remnants is 3C 10
(=G120.1+1.4), but this is well established as
the remnant of the supernova which appeared
in AD 1572. The careful measurements of
Tycho Brahe and other European observers in
2: Radio emission from 3C 58 at 2.7 GHz observed
with the Cambridge 5 km telescope. This is the
remnant of the supernova of AD 1181, and is
~9 × 5 arcmin square.
that year leave no doubt as to this identification (see the discussion in Clark and Stephenson 1977). The other remnant is 3C 58
(=G130.7+3.1). Allowing for precession, a star
with this location would lie close to the eastern
edge of the lunar lodge Kui and roughly
between Wangliang and Huagai (see figure 1).
Further, it would be very near to ε Cas – in
accord with Yi Shitong’s identification. Hence
there is remarkable accord between the recorded position of the star and that of the supernova remnant 3C 58.
The identification of 3C 58 as the remnant of
the supernova of AD 1181 has not always been
accepted, mainly because of discrepancies
between the distance estimates available for the
source of ~3 and ~8 kpc. At the larger distance
it would be difficult to reconcile the physical
size of 3C 58 with an age of only around a
thousand years. However, the smaller distance
– which was first proposed by Green and Gull
(1982) from neutral hydrogen (21 cm) line
observations – has been confirmed by more
recent 21 cm observations by Roberts et al.
(1993). These authors also confirmed that confusion with structure in hydrogen emission was
responsible for apparent absorption features
that had led to the erroneous larger distance.
Like the Crab nebula, 3C 58 shows a centrally brightened morphology at radio (see figure
2) and X-ray wavelengths, and this is generally taken to be an indication of the presence of
a central, compact energy source in these
objects. However, unlike the Crab nebula,
3C 58 does not have a pulsar definitely identified within it as its central power source,
although there is evidence for a central compact source from X-ray observations (Helfand,
Becker and White 1995).
The Crab nebula and 3C 58 are both members of the “filled-centre” class of SNRs, which
contains about 10 known objects in the
Galaxy, all of which show such centrally
brightened morphologies. (These SNRs are
also called “Crab-like” remnants, although it
is not clear whether or not the Crab is typical.)
These “filled-centre” SNRs are also characterized by their flat radio spectra, with spectral
indices α of ≈0.0 to 0.3 – where flux density S
varies with frequency υ as S ∝ υ–α – in contrast
to typical spectral indices of 0.4 to 0.6 seen on
the majority of SNRs, which are of “shell”
type. In the shell remnants the relativistic particle spectrum responsible for the observed
radio is thought to be due to shock acceleration, whereas the particle spectrum from the
central sources in “filled-centre” SNRs clearly
must be much harder.
Recently it has become clear that not only is
3C 58 much less luminous than the Crab nebula, despite being about the same age, but also
that (see Green and Scheuer 1993 or Woltjer et
al. 1997) the form of its synchrotron spectrum
is quite different from that of the Crab. The
flat radio spectra of the Crab nebula extends
from the radio through to the optical, with a
spectral break at ~104 GHz. On the other
hand, the upper limits on the infrared emission
from 3C 58 that are available from IRAS imply
that 3C 58’s spectrum turns over sharply somewhere near 50 GHz. This implies that either
the intrinsic spectrum produced by the central
power source in 3C 58 is quite different from
that produced by the pulsar in the Crab nebula, or that the break is produced by synchrotron losses, and the central source in 3C 58 has
not been active for some considerable time.
Crucial to this interpretation of 3C 58’s present-day synchrotron is the fact that the age of
3C 58 is known from the historical observations of its parent supernova. ●
F R Stephenson, Department of Physics, University
of Durham and D A Green, Cavendish Laboratory,
University of Cambridge.
References
Clark D H and Stephenson F R 1977 The Historical Supernovae
Pergamon, New York.
Green D A and Gull S F 1982 Nature 299 606.
Green D A and Scheuer P A G 1992 MNRAS 258 833.
Helfand D H, Becker R H and White R L 1995 ApJ 453 741.
Li Jinyi 1981 Studies in the History of Natural Sciences 2 45 (in
Chinese).
Roberts D A et al. 1993 A&A 274 427.
Stephenson F R 1971 QJRAS 12 10.
Wang Zhenru 1987 in The Origin and Evolution of Neutron Stars (IAU
Symposium No.125), eds Helfand D J and Huang Jiehao, p305
(Reidel, Dordrecht).
Woltjer L et al. 1997 A&A 325 295.
Yi Shitong 1988 Atlas comparing Chinese and Western Starmaps
and Catalogues (Science Press, Beijing) (in Chinese).
April 1999 Vol 40