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Astron. Astrophys. 364, 732-740 (2000)

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5. Discussion

5.1. RX J0852.0-4622: young or not so young?

Several observations of RX J0852.0-4622 suggest that it is a young SNR. Most notably, both the X-ray temperatures derived by Aschenbach (1998) and the [FORMULA]-ray work of Iyudin et al. (1998) imply an age of [FORMULA] yr. Further, the more recent discussion of the [FORMULA]-ray observations by Aschenbach et al. (1999) suggest the remnant is [FORMULA] years old. This is in agreement with the approximately circular radio appearance of the remnant; a characteristic exhibited by other young, shell-type SNRs. Even the cautious upper distance limit of approximately 1 kpc provided by Aschenbach (1998), which leads to a linear diameter of [FORMULA] pc, implies the age of this remnant cannot be more than a few thousand years.

However, there are some radio properties of this remnant are difficult to reconcile with those of other young SNRs.

  • The radio shell is far from complete. The radio image of RX J0852.0-4622 is composed primarily of two, opposing regions of emission. This is in contrast to other young, shell-type SNRs, which exhibit essentially complete radio shells. We note a possible resemblance to the structure of SN1006, however, which also shows opposing arcs of emission.

  • The radio emission from this new SNR is of relatively low surface brightness. The mean radio surface brightness of this new remnant at 1 GHz (see Table 2) is a factor of 5 lower than that of SN1006. This is significant, because the SN1006 remnant has the lowest surface brightness of all the young, shell-type SNRs in current catalogues (e.g. Green 1998) - see the further discussion in Sect. 5.3.

  • The radio spectral index of [FORMULA] determined herein for RX J0852.0-4622 is flatter than those of other young shells, which have indices of [FORMULA] (e.g. Green 1988).

It is possible that some of the unusual radio properties of RX J0852.0-4622 may result from the SNR expanding into a hot, low-density region of the interstellar medium; Aschenbach (1998) determines an upper limit to this density of approximately 0.06 cm-3. If so, this would emphasise the role played by environmental effects on the detectability of young radio remnants. Alternatively, this SNR may be just beginning to "turn on" at radio wavelengths, although this scenario may be difficult to reconcile with the very incomplete radio shell.

Further insights into the unusual radio properties of this SNR must await more detailed investigations of the characteristics of both the remnant and the environment into which it is expanding.

5.2. The distance to RX J0852.0-4622

Unfortunately, the distance to RX J0852.0-4622 is highly uncertain. The X-ray data of Aschenbach (1998) provide an upper limit of approximately 1 kpc, based on the lack of absorption, but suggest that the remnant's distance could be as small as 200 pc. This lower limit is based upon a comparison of the new remnant's surface brightness with that of SN1006. However, SN1006 is atypically faint for known, young, Galactic SNRs (this is further discussed in Sect. 5.3).

The [FORMULA]-ray data discussed by Iyudin et al. (1998) and Aschenbach et al. (1999) suggest an age of approximately 700 yr, from a comparison of the observed 44Ti line flux with that expected from SN models, which corresponds to a distance of approximately 200 pc. However, the interpretation of the [FORMULA]-ray detection requires the assumption of both the supernova shock velocity and the 44Ti yield. Iyudin et al. (1998) note that increases in either of these quantities will lead to an increase in the derived distance of the remnant.

We have examined 21-cm HI observations in the region of RX J0852.0-4622, from Kerr et al. (1986), in an attempt to find any correlating features. However, since the remnant lies in a complex region in Vela, no associated features in HI could be found.

The ice core data of Burgess & Zuber (2000) may be able to provide an accurate age for RX J0852.0-4622, but these data are not able to constrain the distance to the SNR without further assumptions. Furthermore, as noted above, it is not possible to definitively associate the additional nitrate spike with this SNR. If we assume the age of [FORMULA] yr determined by Burgess & Zuber (2000) to be accurate, then assuming an upper limit to the mean shock velocity of [FORMULA] km s-1 places the remnant at a distance of [FORMULA] pc, with a linear diameter of [FORMULA] pc.

A value of [FORMULA] km s-1 was nominated as an upper limit to the shock velocity by Aschenbach et al. (1999), based on their analysis of the X-ray data. Should the mean shock velocity exceed this value, the remnant would lie at a distance of [FORMULA] pc, with a correspondingly larger linear diameter.

In summary, the distance is very poorly constrained by current observational data. Unfortunately, since the remnant is faint, is not detected optically, and is in a complex region, many direct techniques used to determine the distance to SNRs are not applicable in this case (e.g. HI absorption, association with other HI features or molecular clouds, or optical studies). Nevertheless, the radio properties of this remnant, even with the present uncertainty in its age and size, have some interesting implications for statistical studies of Galactic SNRs, which are discussed in the next section.

5.3. Statistical implications

Fig. 6 shows a surface-brightness versus diameter plot for Galactic SNRs for which there are reasonable distances available (Green 1984, 1991, 1998). RX J0852.0-4622 is plotted on this figure with a range of diameters corresponding to distances from 200 pc to 1 kpc. The surface brightness of RX J0852.0-4622, of [FORMULA] W m-2 Hz- 1 sr-1, is faint for a known Galactic SNR - among the faintest 20% of catalogued remnants. This is less than the nominal completeness limit of many radio surveys (e.g. Green 1991). Note that although most faint remnants are thought to be old, that the remnant of the SN of AD1006 is also faint, with [FORMULA] W m-2 Hz- 1 sr-1. We also note that, whilst RX J0852.0-4622 is one of the fainter remnants to appear on the [FORMULA] plot, the only other remnant detected in 44Ti [FORMULA]-ray emission is Cas A, which has the highest surface brightness.

[FIGURE] Fig. 6. A surface-brightness vs. diameter ([FORMULA]) plot for Galactic SNRs with relatively well known distance. RX J0852.0-4622 is plotted for a range of distances from 200 pc to 1 kpc, and known young SNRs are labelled.

From Fig. 6, it is clear that the properties of RX J0852.0-4622 are very unusual if it lies at the smaller distances suggested by the [FORMULA]-ray data. If the SNR is at a distance [FORMULA] pc, its diameter is less than 10 pc, but its surface brightness is two or more orders of magnitude less than other known young SNRs with similar diameters (e.g. Kepler's SN, Tycho's SN, and 3C58). This would have important consequences for statistical studies of Galactic SNRs (see Green 1991), as the range of [FORMULA] - or, equivalently, luminosity - for a given D may be even wider than was previously thought. This in turn would imply that the observational selection effect of faint SNRs being difficult to detect is important, not only for old SNRs, but also for young SNRs. The low radio surface brightness of RX J0852.0-4622 indicates that a fraction of young SNRs may be faint at radio wavelengths. The available sample of young SNRs (i.e. historical events) is small, however, so it is not possible to meaningfully estimate this proportion.

On the other hand, if the remnant is as distant as 1 kpc, then although it is faint for it's diameter of [FORMULA] pc, it is not strikingly unusual.

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Online publication: January 29, 2001