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{\Large {\bf On the nature of the compact X-ray source inside RCW 103\\}}

{\large {\bf Sergei B.Popov \\}}


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Recently, Gotthelf et al. (1997) found a compact X-ray source
inside the SNR RCW 103 with the
X-ray luminosity $L_x\sim 10^{34}\, erg/s$ (for the distance $3.3\, kpc$)
and the black-body temperature about $0.6\, keV$.
No radio or optical compact
counterpart was observed. \\

{\Large {\bf Accreting isolated young BH or accreting
old BH in pair with a young compact object\\}}

To explain the observed X-ray luminosity of the compact object inside RCW 103
the accretion rate, $\dot M$, should be about $10^{14}\, g/s$.


\begin{equation}
\dot M=2 \pi \left(
\frac{(GM)^2 \rho}{V_{eff}^3}\right).
\end{equation}

One can then estimate the size of the emmiting region:

$$
L=4\pi \cdot R_{emm}^2 \sigma T^4
$$

For the observed values
of $ L_x$ and $ T$
this equation gives $R_{emm} \sim 0.9 \,km$.
For BHs such a low value of $R_{emm}$ is very unlikely.\\

{\Large {\bf Accreting isolated young NS\\}}

If the NS is in the Accretor stage, then its period is longer than the
accretor period, $P_A$:

\begin{equation}
P_A=2^{5/14}\pi \, (GM)^{-5/7} (\mu ^2/\dot M)^{3/7}\, sec,
\end{equation}
where $\mu = B\cdot R_{NS}^3$ is magnetic moment of the NS.

For the RCW 103 I use the following values: $\dot M= 10^{14}\, g/s$,
$M=1.4\, M_{\odot}$, $R_{NS}=10^6\, cm$ which give:

\begin{equation}
B\sim 10^{10}\cdot p^{7/6}\, G.
\end{equation}

%\begin{figure}
%\epsfxsize=0.9\hsize
%\centerline{\rotate[r]{\epsfbox{bp.eps}}}
%\caption{Possible values of the magnetic field, B, and period, p, for
%the accreting NS. The dotted line (1) corresponds to
%the equilibrium period, $P_{eq}$ (eq. 5), while
%solid line (2) corresponds to the accretor period, $P_A$ (eq.3).}
%\end{figure}

If material is accreted from the turbulent interstellar medium, a new
equilibrium period can occur (Konenkov \& Popov 1997):

\begin{equation}
P_{eq}\sim 30\, B_{12}^{2/3}I_{45}^{1/3}\dot M_{14}^{-2/3}
R_{{NS}_6}^2 V_{{eff}_6}^{7/3}V_{{t}_6}^{-2/3}M_{1.4}^{-4/3}\, s.
\end{equation}


\begin{equation}
B\sim 8\cdot 10^{9}\cdot p^{3/2}\, G.
\end{equation}


To explain the luminosity
of the RCW 103 by an isolated accreting NS,
one must assume that the NS was born with very low magnetic
field or with
unusually long spin period. The age of the SNR RCW 103 is about 1000 years
(Gotthelf et al., 1997), which
means that magnetic field could not decay significantly
(Konenkov \& Popov 1997). Thus, the model with
isolated young accreting NS is not a likely explanation for the data.\\

{\Large {\bf Accreting old NS in pair
with a young NS or a young BH\\}}

There is a chance that we observe a binary system, where one component is an
old neutron star and the other component (a NS or a BH)
was formed in a recent SN explosion.
In that case, the parameters
determined by eqs.(3), (5) are not unusual:
old NS can have low magnetic fields and long periods.
Two stars with masses $15 \, M_{\odot}$ and $14 \, M_{\odot}$ on the
main sequence with the initial separation $200 \, R_{\odot}$, $R_{\odot}$ --
the solar radius, after 14 Myr
end their evolution as a binary system NS+NS.
The second NS is 1 Myr younger. During 1 Myr the magnetic field can decrease
up to 1/100 of the initial value with a significant spin-down (Konenkov \&
Popov, 1997). The binary NS+NS is relatively wide: $ 20\, R_{\odot}$ with an
orbital period $5.8^d$, so the orbital velocity is not high.\\

{\Large {\bf Conclusions\\}}

To conclude, I argued that the most likely model for the RCW 103 is
that of an accreting old NS in a binary system with a young
compact object born in the recent SN explosion that produced
the observed supernova remnant.
Such systems a rare, but natural products
of the binary evolution. Scenarios with single compact objects or with
accreting BH are less probable.


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