An estimate of the age t and distance d of the supernova remnant RX J0852.0-4622 / GRO J0852-4642 can be obtained by combining the ROSAT X-ray and COMPTEL -ray data. Assuming a 44Ti yield of the supenova of 5 10-5 and an expansion velocity of 5000 km s-1 t = 680 yrs and d = 200 pc are obtained. Actually, the expansion velocity is constrained by the X-ray data to lie in the range of 2000 km s-1 v 10 000 km s-1 yielding an uncertainty of the age of 100 years for a fixed 44Ti yield. For the highest 44Ti yield given by current supernova models, a firm upper limit of the distance is 500 pc and 1100 years for the age.
The determination of the age depends to some extent on the ionization state of 44Ti because 44Ti decays by electron capture. The values quoted above have been obtained under the assumption that the K-shell is fully populated. If the K-shell contains only one electron, the 44Ti mean lifetime is estimated to increase by a factor of two. But the age of the SNR will not increase by the same factor because of the angular diameter, and therefore distance constraint. Adopting the same mean expansion velocity t and d can change by about 35% at most and for a very strong ionization t and d may be even lower than the "nominal" estimate. Future X-ray spectroscopy measurements are needed to search for Ti X-ray emission lines to determine the ionization state of 44Ti and further constrain t and d.
The X-ray surface brightness of RX J0852.0-4622 is rather low and implies a rather low matter density of the shock wave heated plasma if the radiation is thermal. A formal analysis of the X-ray data in terms of a Sedov-type evolution of the SNR using the standard conversion of X-ray temperature in shock velocity turns out a rather low value of a few times 1049 erg for the explosion energy , which can be raised only significantly if the mean expansion velocity v would exceed 10000 km s-1. But if v is closer to 5000 km s-1 as the X-ray data indicate then the bulk of resides still in kinetic energy of the ejecta, not radiating in X-rays, which means that any reverse shock has not yet penetrated deep into the ejecta, and that the titanium is not highly ionized. In this case a lower limit for the mass of the progenitor star of 25 M is estimated from the energy balance.
There is evidence for 26Al 1.809 MeV line emission from RX J0852.0-4622, which has been measured by COMPTEL towards the Vela region. Admittedly this has still to be confirmed. But if a non-neglible part of this 1.809 MeV line flux is coming from RX J0852.0-4622 a similar age of 600-750 yrs for the SNR is obtained for similar yields of 26Al and 44Ti. Since t depends only on the logarithm of the yields and the fluxes t will not change significantly even for large changes of the 26Al flux. It is more a matter of whether or not there is 26Al emission. If the 26Al line flux is about what is indicated by the COMPTEL data the existing type Ia supernova models can be ruled out for the progenitor explosion because they predict a ratio of the 26Al and 44Ti yield which is by far too large. They could be reconsidered only if less than 1% of the 26Al line flux from Vela in total is associated with RX J0852.0-4622. Explosion models of core-collapse supernovae (Woosley & Weaver, 1995) are in general in agreement with the observations, i.e. the measurements of v, 44Ti line flux and 26Al line flux, judging from their prediction of the yields of 44Ti and 26Al. Their models with 10-8 M and 10-4 M can definitely be excluded because the yield of 26Al predicted is inconsistent with even the upper limit of the 26Al line flux measured for the Vela region in total. In summary, both the energy balance and the yield predictions of the currently available explosion models point towards a core-collapse event.
Within the vicinity of the explosion center two point-like X-ray emission regions have been found, either of which could be the manifestation of a neutron star. Spectra are not available, but if the radiation is assumed to be black-body emisson, the X-ray flux indicates a surface temperature of 3105 K, which is surprisingly low for a 700 years old neutron star. If neither one of these two sources is a neutron star and the neutron star hides somewhere else with an even lower X-ray count rate or if only a fraction of the radiation observed from the point-like objects is due to thermal radiation the surface temperature of the neutron star could be even lower.
In principle, the supernova could have been seen from the far-east astronomers of China, Corea or Japan or from geographical latitudes further south. The supernova could have been very bright and then there are records expected to exist, which should be searched for. If the supernova would have been of the sub-luminous class with a brightness as low as that of SN 1181 and a short peak-plateau duration it could have been missed. There is some chance that the progenitor star was sufficiently bright and could have been seen by the naked eye. Then the stellar pattern of Vela was different in ancient times and the astronomers who monitored the sky might not have noted the supernova as a "guest star" or a "new" star, because the star was existing and just brightening and eventually fading away.
© European Southern Observatory (ESO) 1999
Online publication: October 14, 1999