2 Point X-ray sources

2.1 X-ray images

Figure 1 shows the Chandra (0.7-2 keV) image of the highly structured southwest corner of the SNR G 315.4-2.30. Two point X-ray sources are clearly visible at the northwest of the image at $\alpha _{2000} = 14^{\rm h} 40^{\rm m} 31{\hbox{$.\!\!^{\rm s}$ }}33, \delta _{2000} = -62^{\circ} 38' 22{\hbox{$.\!\!^{\prime\prime}$ }}8$ and $\alpha _{2000} = 14^{\rm h} 40^{\rm m} 31{\hbox{$.\!\!^{\rm s}$ }}05, \delta _{2000} = -62^{\circ} 38' 16{\hbox{$.\!\!^{\prime\prime}$ }}9$. For both sources we measured a FWHM of $1{\hbox{$.\!\!^{\prime\prime}$ }}1$, which is consistent with the Chandra point spread function at the off-axis angle  $2\hbox{$.\mkern-4mu^\prime$ }5$. Figure 2 shows a close-up of the region around the sources, labelled S (southern source) and N (northern source).

Figure 1: Chandra image of the southwest region of the SNR G 315.4-2.30. Both X-ray sources are surrounded by a circle. North is up and east is to the left.

Figure 2: The enlarged view of the region around the point X-ray sources, labelled S (southern) and N (northern).

2.2 Optical images

Figure 3 shows the image of the hemispherical optical nebula to the southwest of the SNR G 315.4-2.30 from the Digital Sky Survey (DSS-2, red plates). Figure 4 shows an enlarged view of the region around the point X-ray sources. The source S is in good positional agreement with a point-like object immersed in a diffuse optical filament. This object is indicated in the HST Guide Star Catalog as non-stellar, with a photographic magnitude of $13.38\pm0.40$. We believe, however, that it was misclassified due to the effect of the background diffuse emission and in fact it is a star.

Figure 3: The DSS-2 image of the optical nebula southwest of the SNR G 315.4-2.30. The position of both point X-ray sources is surrounded by a circle.

The source N has no optical counterpart to a limiting magnitude $\sim$21.

Figure 4: The enlarged DSS-2 image of the region around the point X-ray sources. The circles are centered at the position of X-ray sources.

2.3 Spectral analysis

The spectra of the point sources were extracted from circular regions with radii of 1.5 arcsec. The background spectrum was taken from a circle with a radius of 4.5 arcsec at 6.5 arcsec northeast of the point sources. The PSF model shows that the fraction of the source flux in the background region is negligible. The spectral modeling was performed in the 0.5-10 keV energy range using the XSPEC spectra fitting package. The estimates of the interstellar absorption, $N_{\rm H}$, towards G 315.4-2.30 are quite uncertain (cf. e.g. Vink et al. 1997 with Vink et al. 2002), so the spectra were fitted with $N_{\rm H}$ as a free parameter. In one case, however, we used the fixed value of $N_{\rm H}$ (see below).

The spectrum of the source S is shown in Fig. 5;

Figure 5: The background-subtracted Chandra ACIS spectrum from the source S. The solid line represents the best fit optically thin plasma model.

the solid line represents the best fit optically thin plasma model, with a temperature $=0.69\pm 0.05$ keV, abundance $=0.07\pm 0.02$, $N_{\rm H} =(1.2\pm 0.3)\times 10^{21} ~ {\rm cm}^{-2}$, luminosity (in the 0.5-10 keV energy range) $\simeq 2.8\times 10^{31} ~ {\rm erg} ~ {\rm s}^{-1}$, and $\chi ^2$/d.o.f. = 21.6/25. The blackbody or power law models give unacceptable fits.

The spectrum of the source N (shown in Fig. 6) can be fitted almost equally well with a power law (PL), two-temperature blackbody (BB+BB), or blackbody plus power law (BB+PL) models. A simple BB model gives unacceptable fits. The best fit spectrum predicted by the BB+PL model is shown in Fig. 6 by the solid line.

We note that the best fit PL model requires $N_{\rm H} =0$ (we consider this fact as an indication of the galactic origin of the source N), although models with $N_{\rm H}$ up to $1.5\times 10^{21} ~ {\rm cm}^{-2}$ (presented in Table 1) also give acceptable fits.

Figure 6: The background-subtracted Chandra ACIS spectrum from the source N. The solid line corresponds to the best fit blackbody plus power law model.

The results of a spectral analysis for the source N are summarized in Table 1.

   

Table 1: Spectral fits to the point X-ray source N.

Model	Photon index	Temperature,	$N_{\rm H},$	Luminosity,	$\chi ^2$ /d.o.f. $^{{\rm a}}$
		keV	$10^{21} ~ {\rm cm}^{-2}$	$10^{32} ~ {\rm erg} ~ {\rm s}^{-1}$
PL	$1.87\pm 0.09$		1.5 (fixed)	0.44 ( $0.5{-}10~{\rm keV}$)	51.5/27
B+B		$0.11 \pm 0.03$	$4.3\pm 1.8$	1.32 (bolometric)	47.3/25
		$0.71\pm 0.05$		0.35 (bolometric)
BB+PL		$0.070\pm 0.003$	$9.2\pm 3.4$	112 (bolometric)	38.9/25
	$2.31\pm 0.30$			0.67 ( $0.5{-}10~{\rm keV}$)

$^{{\rm a}}$ Note that a single point near the Chandra mirror edge contributes $\simeq 10$ to the $\chi ^2$ in all cases.