3 Discussion

3.1 Source S

The soft X-ray spectrum of the source S and the existence of the candidate optical counterpart strongly suggest that this source is an active star. Assuming that S is indeed a star and given the observed X-ray to optical flux ratio, one can exclude the possibility that this star is of the OB-type. Therefore it cannot be a member of a group of OB stars located in the direction of G 315.4-2.30 nearly at the same distance as the SNR (Westerlund 1966). Most likely the source S is a foreground active star of late spectral type. A follow-up study of this object could clarify its nature.

Note that the point X-ray source discovered by Vink et al. (2000) has much in common with the source S. The spectra of both sources are soft and could be fitted with the optically thin plasma model with depleted abundances (typical of late-type stars; e.g. Giampapa et al. 1996). Both sources have candidate optical counterparts with almost the same photographic magnitudes. In both cases the X-ray to optical flux ratios suggest that the optical objects are late-type stars. Evidence for long-term variability of the former source (Vink et al. 2000) also suggest that it is an active star.

3.2 Source N

The spectral characteristics of the source N coupled with the absence of an optical counterpart allow us to consider it as a candidate stellar remnant. We now discuss this possibility.

3.2.1 Power law model

The PL fit of the spectrum (with the photon index typical of pulsars) suggest that the source N can be an active (rotation-powered) neutron star. Assuming that N is a pulsar and using the empirical relationships between the non-thermal X-ray and spin-down luminosities of pulsars (e.g. Becker & Trümper 1995; Possenti et al. 2002), one can estimate the latter, $\dot{E}\sim 10^{35} ~ d_{2.8} ^2 ~ {\rm erg} ~ {\rm s}^{-1}$, where d_2.8 is the distance to the SNR in units of 2.8 kpc.

The inferred low spin-down luminosity can be considered as an argument against the possibility that the source N is a young fast-rotating pulsar with a standard magnetic field ( 10¹¹ - 10¹³ G). Therefore N can be either a young pulsar born with a low surface magnetic field (e.g. Blandford et al. 1983; Urpin et al. 1986) and/or a large rotation period (e.g. Spruit & Phinney 1998) or an aged pulsar with a decaying magnetic field. The first possibility would deserve detailed consideration if a candidate period of 12 ms found for the central X-ray source in Cas A (Murray et al. 2002a) is confirmed by an independent timing analysis. The second one implies that N is an old pulsar (of an arbitrary age) projected by chance on the SNR G 315.4-2.30.

Note, however, that the above-mentioned empirical relationships are quite uncertain and should be used with caution. For example, the 267 ms radio pulsar PSR B 1853+01 associated with the SNR W 44 (Wolszczan et al. 1991) has almost the same spin-down luminosity ($\simeq$ $4\times 10^{35}~{\rm erg} ~ {\rm s}^{-1})$ as that derived above for the source N, while the characteristic age of the pulsar ($\simeq$ $2\times 10^4$ yr; consistent with a range of ages derived for W 44 using the standard Sedov-Taylor model, see Smith et al. 1985) points to its relative youth. On the other hand, the pulsar PSR B 1853+01 could be much older if its high spin-down rate ( $\dot{P} \simeq 0.2\times 10^{-12} ~ {\rm s} ~ {\rm s}^{-1}$) is connected to the interaction between the pulsar's magnetosphere and the dense circumstellar matter (cf. Gvaramadze 2001). The age of W 44 also could be much greater if the origin of this mixed-morphology SNR is due to the cavity SN explosion (cf. Gvaramadze 2002).

The uncertainties inherent to the empirical relationships are more obvious in the case of the pulsar PSR J 0205+6449, recently discovered in the SNR 3C 58. The spin-down luminosity of this young (characteristic age $\simeq$5400 yr) pulsar is $2.6\times 10^{37} ~ {\rm erg} ~ {\rm s}^{-1}$ (Murray et al. 2002b), while its (non-thermal; see Slane et al. 2002) X-ray luminosity is $\simeq 2.1\times 10^{32} ~ d_{2.6} ^2 ~ {\rm erg} ~ {\rm s}^{-1}$, i.e. about two orders of magnitude less than that predicted by the empirical relationships. The disparity between the predicted and observed non-thermal luminosities is even higher in the case of another young pulsar - the Vela pulsar (Pavlov et al. 2001). Note that the photon indices and non-thermal luminosities of these two pulsars are almost the same as those predicted for the source N by the PL (or BB+PL) model, while their characteristic ages are of the same order of magnitude as the age of the SNR G 315.4-2.30 inferred in Sect. 4. Therefore one cannot exclude that the source N is a young pulsar with "ordinary" parameters. This should be tested observationally.

3.2.2 Two-temperature blackbody model

In the two-temperature blackbody model, the origin of the soft component can be attributed to the cooling surface of a neutron star, while the hard component to the polar caps heated by the backflow of relativistic particles (e.g. Wang et al. 1998).

The best-fit BB+BB model yields the temperature of the soft component of 0.11 keV and the effective radius of 2.3 km. The inferred temperature is somewhat larger than that predicted by standard cooling models (e.g. Page 1998), while the radius is significantly smaller than the radius of the neutron star. In principle, one can expect that the use of realistic neutron star atmosphere models (e.g. Pavlov et al. 1995) would adjust these parameters to acceptable values. But the interpretation of the hard component is more problematic. The effective polar cap area inferred from the best-fit BB+BB model ($\sim$ $10^8 ~ {\rm cm}^{-2}$) is too small and the observed temperature is too large to be consistent with models for heating of polar caps (e.g. Wang et al. 1998 and references therein). Therefore we consider this model as unplausible and suggest that at least at high energies the X-ray emission of the source N is non-thermal.

3.2.3 Blackbody plus power law model

The use of the BB+PL model assumes that the soft X-ray emission comes from the entire surface of a cooling neutron star or from some smaller hot areas, while the hard X-rays are due to the non-thermal magnetospheric emission.

The best fit BB+PL model shows that the spectrum of the source N is dominated by the non-thermal emission at energies >1 keV, and suggests the existence of a soft thermal component. However, the unknown interstellar absorption and uncertainties due to the time-dependent decrease in ACIS low-energy quantum efficiency make the estimates of the flux and bolometric luminosity of the BB component unconstrained. The best fit model with $N_{\rm H}$ as a free parameter (see Table 1) requires large interstellar absorption and consequent large bolometric luminosity. Although the large absorption, in principle, is consistent with values derived by Vink et al. (2002) for the SNR G 315.4-2.30, the large inferred bolometric luminosity implies an uncomfortably large effective radius of $\simeq$60 km. The normalization of the BB component, however, is plausible for lower values of $N_{\rm H}$. A more detailed consideration of this problem will be possible only with updated Chandra calibration at low energies. Note that dense clumps of circumstellar matter (the natural products of interaction between post-main-sequence winds of the SN progenitor star; e.g. Gvaramadze 2001 and references therein; see also Sect. 4) could add a significant contribution to the neutral hydrogen absorption towards the source N, as well as possibly causing the enhanced absorption towards the central compact source in Cas A (required by some model fits of its spectrum; e.g. Murray et al. 2002a).

Note also that at present we cannot definitively confirm or reject the existence of a soft BB component. Therefore we consider the PL model with a reasonable value of $N_{\rm H}$ as a good approximation for the spectrum of the source N. The follow-up multiwavelength observations of this source, including the search for pulsed emission, long term variability, and radio, optical or $\gamma$-ray counterparts, may provide crucial information for an understanding of its nature and therefore are highly desirable.