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Is The RXJ1713.7-3946 Nebula The SNeR Of The AD393 Guest Star?

29/12/2011

Since the middle of the 19th century, astronomers have known about pre-Tychonic bright new or ‘temporary’ stars recorded in ancient Asian, Arabic, and European texts chiefly through the works of Biot (1846) and Chambers (1867). After the acceptance of Zwicky & Baade’s (1934) seminal work “On Super-nova” and later by Pacini (1968) a concerted effort was made to link all known historical (pre-1900s) supernovae (SN) to their accompanying remnants (Hsi 1955; Clark & Stephenson 1976, 1977).

Apart from the well-known supernovae of AD 1054 (the Crab Nebula), 1572 (Tycho’s SN) and 1604 (Kepler’s SN) only four other historical guest stars seemed to merit observational consideration as possible candidate galactic supernovae: AD 185, 393, 1006 and 1181 . Of the four possible candidates the remnants, RCW86, G.327.6+14.6 (PKS 1459-41) and G.130.7+3.1 (3C58) have now been conclusively established to belong to AD185, AD 1006 and AD 1181 respectively. The one remaining plausible supernova candidate, AD 393, still has not had its remnant irrefutably located, having not satisfied all the criteria for a successful SNeR location: e.g. SNeR found within the region of the guest star, the distance, and hence magnitude estimation, is correct to the historical records and the age of the remnant is in concordance with the guest star date.

Fig. 1: Somewhere within the Wěi asterism, which appears in the lower half of the Scorpius constellation, lies the elusive SNR 393. Its “elusivity” may be due to the guest star of AD393 in fact being a nova. However, the recurrent theory of novae would limit this theory. Yet, the The supernova remnant RX J1713.7-3946 is consistent with a type-II or type-Ib SNe. (Credit: Berezhko & Völk 2010.)

Recent work by Stephenson & Green (2002) has given a greater understanding of the locations of many historical Galactic supernovae events, as well as providing details to the coupling of the pulsar wind to the rotation kinematics of the remnant to allow for a better technique to couple known supernovae compact remnants and nebulae. Clark & Stephenson focused on three Chinese guest stars* each observed within a 25 year margin of each other in the 4th century A.D.: namely, 369, 386, and 393 AD. Of these three guest stars the most likely SN event is the one seen in 393 A.D.

Fig. 2: A γ-ray contonour image of the RXJ1713.7-3946 supernova remnant showing where γ-ray intensity is highest, as measured by the High Energy Stereoscopic System (HESS) in Namibia. This supernova remnant is the gaseous remnant of a massive star that exploded and is approximate 1600 years old. (Credit: JAXA/ Takaaki Tanaka/HESS).

N.B. Historical guest stars are now considered to be due to either a novae, a dwarf novae or a supernovae event.

393 A.D. is somewhat difficult to classify. The translated texts state that during the second month of the 18th year (27 Feb – 28 Mar of 393 AD) a guest star appeared within the tail of Scorpius and lasted until nine months. However, work to classify it as either a nova, a supernova or a recurrent nova has been problematic. For example, Pskovskii (1972) argued that the 393 star was likely a recurrent nova since Chinese records also reported a star within the tail of Scorpius in 1600 AD. A nova interpretation was also suggested by van den Bergh (1978) due to the lack of a supernova remnant (SNeR) in that region. Hence, identification, or lack of, of an SNeR that coincides with the AD393 guest star would allow a definitive classification to either be made, or ruled out as the case may be.

Identifying the remnant of a historic SN is often difficult and the guest star of 393 is no exception, with nearly a dozen Galactic supernova remnants located within the tail of Scorpius (Green 2009). The remnants of G348.5+0.1 (CTB 37A) and G348.7+0.3 (CTB 37B) were initially seen as possible SNR candidates to the 393 guest star due to their small angular sizes of 15′ and 17′, respectively (Clark & Stephenson 1977; Stephenson & Green 2002). However, these remnants lie ~10 kpc away (Aharonian et al. 2008; Nakamura et al. 2009) and such large distances near the Galactic center likely imply considerable interstellar extinction decreasing the chance of an associated visually bright guest star. The same is true for the apparently very young SNR G350.1-3.0 whose distance is only ∼3.4 kpc but lies behind an estimated 20 magnitudes of visual extinction (Gaensler et al. 2008). Hence, Chinese astronomers would have been extremely unlikely to record an such supernovae at these remnant locations.

In 1996 Pfeffermann & Aschenbach (1996) announced the ROSAT discovery of the Galactic remnant RX J1713.7-3946 (G347.3-0.5) in the Scorpius constellation. The remnant’s location near the SN 393 reported position along with Pfeffermann & Aschenbach’s estimated remnant distance of 1.1 kpc and 2100 year age led Wang et al. (1997) to suggest it as the likely remnant of SN393. However, Fesen et al. (2012) have recently shown that the search for the remnant of the 393 guest star is far from certain and may not be as conclusive as Wang et al. (1997) would perhaps like.

At a distance less than 2kpc for the remnant RX J1713.7-3946 raises problems with the expected supernova maximum apparent brightness and durations which appear in conflict with the Chinese records. Although, the remnant RX J1713.7-3946 exhibits several properties suggesting a relatively young age, probably less than a few thousand years and thus potentially consistent with a SN event around 393 AD estimated absolute visual magnitude, , values between −12 and −14 imply a very sub-luminous core-collapse SN event. Neither the Chinese nor the Roman descriptions are easily reconciled with an expected RX J1713.7-3946 supernova brightness and duration. However, at the remnant’s current estimated 1-1.3 kpc distance and AV sime 3, its supernova (SN) should have been a visually bright object at maximum light (–3.5 to –5.0 mag) if MV = – 17 to –18 and would have remained visible for over a year. The peak brightness sime0 mag adopted by Wang et al. and others would require the RX J1713.7-3946 supernova to have been a very sub-luminous event similar to or fainter than SN 2005cs in M51.

Fesen et al. (2012) also noted that if RX J1713.7-3946 were the SN 393 remnant, it would then rank as having been the nearest of all the known historic Galactic supernovae during the last 2000 years. It’s relatively small distance of around 1kpc plus a moderate amount of optical extinction also means its supernova would have likely have been a visually brilliant object, certainly as bright as Jupiter and maybe as bright as Venus. This was not quite what was seemed to have been observed in historical records. Hence, the link between RX J1713.7-3946 and the guest star of AD393 first proposed by Wang et al. (1997) seems to have been killed. The hunt for a new remnant which suitably concurs with all the main criteria for establishing a link now need to be made.

Journal References:

  • Wang, Z.R. et al. (1997) Is RX J1713.7-3946 The Remnant Of The AD393 Guest Star? Astronomy & Astrophysics, 318 pp.L59-L61.
  • Berezhko, E. G.; Völk, H.J. (2010). Non-Thermal & Thermal Emission From The Supernova Remnant RX J1713.7-3946. Astronomy & Astrophysics, 511 Article #34.
  • Fesen, R.A. et al. (2012) The SN 393-SNR RX J1713.7-3946 (G347.3-0.5) ConnectionThe Astronomical Journal, 143 (2): Article I.D.: 27.
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