3 X-ray emission from an SN fragment interacting with a molecular cloud

Core-collapsed SNe from massive progenitors are expected to be correlated with massive molecular clouds. The most spectacular manifestations of the phenomena are the SNRs in starburst galaxies like NGC 253, Arp 220, M82 etc. (see e.g. Chevalier & Fransson 2001). There are also the star-forming molecular clouds Sgr A, Sgr B2 in the Galactic Center region with a strong CS emission from dense molecular gas (e.g. Blitz et al. 1993). To model such a case we considered an SN fragment of radius 3$\times$10¹⁶ cm and of oxygen mass of 10 $^{-3}~\mbox{$M_{\odot}$ }$, containing also $\sim$10 $^{-4}~\mbox{$M_{\odot}$ }$ of an impurity (Fe, Ar, Si). The fragment is propagating through a molecular clump of number density 10 $^3~\rm ~cm^{-3}$ with a velocity $\ga$1000 $\rm ~km~s^{-1}$. The magnetic field value in the cloud is about 100 $\rm\mu G$and $k_0 \approx 10^{18} \rm ~cm^2~s^{-1}$ at 1 keV.

 

 

Table 1: K-shell line luminosities of the fragment interacting with a molecular cloud.

Linea	$v_{\rm k}({\rm km s^{-1}})$			$\tau_{\rm max}$
	1080	1620	2700
O (0.54 keV)	40.4	104	1638	33 880
Si (1.7 keV)	1.3	10	48	592b
Ar (2.9 keV)	0.5	4	20	272b
Fe (6.4 keV)	0.4	3	15	78 b
T(2) [107 K]	0.3	0.5	1.0

^a The luminosities are in 10³⁸ ph s^-1.
^b The absorption depths can be applied only for the ionization states Si VI, Ar X, Fe XVIII and higher.

We calculated the local emissivities of K-shell lines through the fragment depth as well as the integrated line luminosities. Table 1 contains the luminosities of K $_{\rm\alpha}$ lines of O, Si, Ar, Fe. These luminosities are not corrected for the optical depth effect. We have also given the maximal depths $\tau_{\rm max}$ for resonant scattering calculated on the assumption that the column density of a given charge state of the ion A is equal to $N^{(A)}_{\rm max}$. To obtain a real estimation one should correct the depth for the actual column densities of the ions with incomplete L-shells. Note that in Tables 1 and 2 we presented only the integral luminosities of the K $_{\rm\alpha}$ complex. A prediction for the line shape depends on the details of the ion charge state profile inside the fragment. The exact modeling of the ionization profile is beyond the scope of the present paper. Nevertheless, one could see that in the case when the bow shock is radiative (see below) the dominant iron charge state in the fragment becomes lower than Fe XVIII, providing the low resonant absorption depth and the line centroid to be close to 6.4 keV. We assumed in Table 1 that the gas micro-turbulence velocity w₆ = 1, and the ion temperature $T \la10^4$ K in the fragment body. The ion temperatures T⁽²⁾ presented in Tables 1 and 2 were calculated for the position just after the viscous subshock. The observable electron temperatures in the postshock relaxation region are somewhat lower.

As seen in Table 1, an increase in the fragment velocity results in a strong enhancement of the line luminosities. For the lower knot velocities the line emission drops down drastically because the Coulomb losses are dominating in that case. Note however that if the value of k₀ was less than 10 $^{18} \rm ~cm^2~s^{-1}$ (at 1 keV) even the fragments of lower velocity ( $v_{\rm k} < 1000 \rm ~km~s^{-1}$) could provide a substantial line luminosity. A fast fragment interacting with CS-emitting gas of density $\ga$10 $^4 \rm ~cm^{-3}$ could reach even higher line luminosity if $k_0 \sim 10^{17} \rm ~cm^2~s^{-1}$ (at 1 keV). Since the larger fragments (for a given knot mass) are more transparent for the K-shell lines of oxygen (and other elements) a fast SN fragment of $R \ga3\times 10^{17}$ cm, propagating through the inter-clump medium of density $\ga$ $10 \rm ~cm^{-3}$, might have a prominent oxygen K-line.

A fast fragment of a larger scale $\ga$10¹⁷ cm, of the same mass 10 $^{-3}~\mbox{$M_{\odot}$ }$, entering the molecular clump will have $\rho_{\rm a}/\rho_{\rm k} \ga1$ and would drive a strong shock into the metal-rich fragment. Such a fragment should be a source of gamma-ray lines and also light (and other spallogenic origin) elements produced by accelerated ion interactions with the metal-rich knot. It would appear as a bright transient source.