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Nature of the compact X­ray source in supernova remnant
RCW103 and related problems
Sergei B. Popov
Sternberg Astronomical Institute, Moscow, Russia
Abstract. I discuss the nature of the compact X­ray source in the center of the
supernova remnant RCW 103. Several models, based on the accretion onto a compact
object are analyzed. I show that it is more likely that the central X­ray source is an
accreting neutron star than an accreting black hole. I also argue that models of a
disrupted binary system consisting of an old accreting neutron star and a new one
observed as a 69­ms pulsar are most favored.
Keywords: neutron stars, supernova remnants
1. Introduction
Among all astrophysical objects neutron stars (NSs) and black holes
(BHs) attract most attention of physicists. Now we know more than
1000 NSs as radio pulsars and more than 100 NSs emitting X­ or/and
fl­rays. The Galactic population of these objects is much larger: about
10 8 -- 10 9 .
It is generally accepted that NSs and BHs are the products of su­
pernova (SN) explosions. In most cases a supernova remnant (SNR)
appears after a formidable explosion of a massive star (with M ?
10 \Gamma 35M fi ). Although sometimes a young NS is observed inside a
SNR as a radio pulsar (e.g., Crab, Vela, etc.) or as a X­ray source, in
most cases no compact object is found inside a SNR, or an accidental
coincidence of the radio pulsar and the SNR is very likely (e.g., (Kaspi,
1998); (Frail, 1997)).
It is possible, that about 50% of NSs are born with low magnetic
fields, so they never appear as radio pulsars and spend most of their
lives on the Ejector and Propeller stages. These NSs with low magnetic
fields can not spin­down significantly even during the Hubble time. For
high­velocity NSs the characteristic Ejector period is higher, and NSs
spend most of their lives as Ejectors.
Recently, Gotthelf et al. described a compact X­ray source in the
center of SNR RCW 103 with the X­ray luminosity L x ¸ 10 34 erg s \Gamma1
(for the distance 3:3 kpc) and the black­body temperature about 0:6 keV.
The source flux has varied since previous observations (Petre and Got­
thelf, 1999). The nature of the central compact source is unclear. No
radio or optical compact counterpart was observed. Also a 69­ms X­ray
c
fl 1999 Kluwer Academic Publishers. Printed in the Netherlands.
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2 Sergei Popov
and radio pulsar with a characteristic age about 8 kyr was discovered
7' from the center of the remnant (Kaspi, 1998), but the reality of the
association of the pulsar with the SNR is unclear (Dickel and Carter,
1999).
Here I discuss possible models of that compact central source and
its possible connections with the 69­ms pulsar (see some preliminary
results in (Popov, 1998a)).
2. What is inside the RCW 103?
Gotthelf et al. discussed why the source cannot be a cooling NS, a
plerion, or a binary with a normal companion. The reader is referred
to their paper for the details. In the present analysis I assume that
the X­ray luminosity of the source is produced due to accretion of the
surrounding material onto a compact object (a NS or a BH). I analyze
thus only models with compact objects, isolated or with a compact
companion (most probably the binary system was destroyed after the
second explosion, when the 69­ms pulsar was formed).
The main challenge for the models of accretion of the surrounding
material onto isolated compact object is to answer the question of where
a NS or a BH finds enough matter to accrete. I don't discuss it here,
assuming that the material is available in the surrounding medium (see,
for example, (Page et al., 1999)).
2.1. Accreting isolated young black hole or accreting
old black hole in pair with a young compact object
An isolated BH accreting the interstellar medium can be, in principle,
observed by X­ray satellites such as ROSAT , ASCA etc. (Heckler and
Kolb, 1996). To achieve high X­ray luminosity, a compact object must
move with a low velocity relative the ISM:
—
M = 2ú
`
(GM) 2 ae
(V 2
s + V 2 ) 3=2
'
; (1)
where V s is the speed of sound, V is the velocity of the compact object
with respect to the ambient medium, M -- the mass of the accreting star
and ae is the density of the accreting material. One can introduce the
effective velocity, V eff , and rewrite eq. (1) as follows:
—
M = 2ú((GM) 2 ae)=(V 3
eff ):
During the SN explosions a compact object can obtain an additional
kick velocity. At the present time the distribution of the kick velocity is
popov.tex; 12/11/1999; 12:51; p.2

Compact X­ray source in RCW103 3
not known well enough (e.g., (Lipunov et al., 1996)). Although observa­
tions of radio pulsars favor high kick velocities about 300 \Gamma 500 km s \Gamma1
(Lyne and Lorimer, 1994). We mark here, that if the 69­ms pulsar is
a new born NS, and the central source is the older object, it is not
surprising, that the 69­ms pulsar is farther from the center of the SNR.
Because the new born NS received a high kick velocity and the old one
only saved its orbital velocity, because the system survived in the first
explosion. X­ray radiation of the new born NS of course doesn't have
the accretion nature.
To explain the observed X­ray luminosity of the compact object
in the center of RCW 103 the accretion rate, —
M , should be about
10 14 g s \Gamma1 .
One can then estimate the size of the emmiting region, using ob­
served luminosity and temperature: L = 4ú \Delta R 2
emm oeT 4 . For observed
values of L x and T this equation gives Remm ¸ 1 km. For BHs such
a low value of Remm is very unlikely because the gravitational radius
is about RG ¸ 3 km (M=M fi ), and most of the present BH­candidates
have masses about 7 \Gamma 10M fi . This is probably the main argument
against isolated accreting BH as a model for the RCW 103. Also the
efficiency of spherically symmetric accretion onto a BH is very low
resulting in a significantly higher density required to achieve the same
luminosity. The same arguments can be used against models with a bi­
nary system (probably disrupted): BH+NS (NS was born in the recent
SN explosion -- a 69­ms pulsar).
2.2. Young isolated accreting neutron star
In the past few years isolated accreting NSs have become a subject
of great interest especially due to the observations with the ROSAT
satellite.
There are four main possible stages for a NS in a low­density plasma:
1): Ejector (a radio pulsar is an example of Ejector); 2): Propeller; 3):
Accretor; and 4): Georotator ((Lipunov and Popov, 1995); (Konenkov
and Popov, 1997)). The stage is determined by the accretion rate, —
M ,
the magnetic field of the NS, B, and by the spin period of the NS, p.
If the NS is on the Accretor stage, then its period is longer than the
so­called Accretor period, PA :
PA = 2 5=14 ú (GM) \Gamma5=7 (¯ 2 = —
M) 3=7 s; (2)
where ¯ = B \Delta R 3
NS is magnetic moment of the NS.
For the RCW 103 I use the following values: —
M = 10 14 g s \Gamma1 , M =
1:4 M fi , RNS = 10 6 cm which give:
popov.tex; 12/11/1999; 12:51; p.3

4 Sergei Popov
B ¸ 10 10 \Delta p 7=6 G: (3)
If material is accreted from the turbulent interstellar medium, a new
equilibrium period can occur (Konenkov and Popov, 1997):
P eq ¸ 30 B 2=3
12 I 1=3
45
—
M \Gamma2=3
14 R 2
NS 6
V 7=3
eff 6
V \Gamma2=3
t 6
M \Gamma4=3
1:4 s; (4)
where V t is the turbulent velocity (all velocities are in units of 10 km s \Gamma1 );
M 1:4 is the mass of the NS in units of 1:4 M fi , B 12 is the magnetic field
of the NS in unites 10 12 G and RNS is the radius of the NS in units of
10 6 cm.
We then obtain:
B ¸ 8 \Delta 10 9 \Delta p 3=2 G: (5)
It is obvious that to explain the luminosity of the RCW 103 by an
isolated accreting NS, one must assume that the NS was born with ex­
tremely 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 the magnetic field could not decay significantly (Konenkov
and Popov, 1997). The flux of the source is not constant (Petre and
Gotthelf, 1999), so the idea of cooling NS can be rejected. Thus, the
model with isolated young accreting NS is not a likely explanation for
the data.
2.3. Accreting old neutron star in pair with a young
neutron star (or in the disrupted system)
Binary compact objects are quite natural products of binary evolution
(Lipunov et al., 1996). One can, therefore, discuss these scenarios as a
viable alternative.
In the previous subsection I showed that accretion onto a young
isolated NS requires unusual initial parameters. However, there is a
chance that we observe a binary system (or a disrupted binary), where
one component is an old NS and the other component was formed in
a recent SN explosion and appears (for disrupted system) as a 69­ms
pulsar.
In that case, the parameters determined by eqs.(3), (5) are not un­
usual: old NS can have low magnetic fields and long periods (Lipunov
and Popov, 1995). Due to the fact that Gotthelf et al. did not find any
periodic change of the luminosity, one can argue that the field is too
low to produce the observable modulation (the accreting material is
not channeled to the polar caps: B ! 10 6 G) or that the period is very
long (p ? 10 4 s), which is possible for old NSs with ``normal'' magnetic
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Compact X­ray source in RCW103 5
fields (Lipunov and Popov, 1995): P ú 500 s: The last opportunity is,
probably, better, as the emmiting area is not large ú 1 km 2 .
The evolutionary scenario for such a system is clear enough (Lipunov
et al., 1996). One can easily calculate it using the ``Scenario Machine''
WWW­facility (Nazin et al., 1998). For example, two stars with masses
15 M fi and 14 M fi on the main sequence with the initial separation
200 R fi , R fi -- the solar radius, after 14 Myr (with two SN explosions
with low kick velocities: about 60 km s \Gamma1 ) 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 spin­down (Konenkov and Popov, 1997). The
binary NS+NS is relatively wide: 20 R fi with an orbital period 5:8 d , so
the orbital velocity is not high (the orbital velocity of the accreting NS
should be added to V eff ).
The 69­ms X­ray pulsing source and it's radio pulsar counterpart
that were discovered near RCW 103 (Kaspi, 1998) can be a new­born
radio pulsar. So, it means that the binary system was disrupted after
the second explosion. It means that in the first explosion the kick
velocity was small (about 50 km s \Gamma1 in the opposite case the system
could be disrupted after the first explosion and the older NS could leave
the SNR before the second explosion, but if the orbit was significantly
eccentric, the kick velocity in the first explosion could be high too) and
in the second explosion it was as high as 750 \Gamma 800 km s \Gamma1 for the same
initial parameters as in the previous example.
Of course other variants of the initial parameters are possible, and
I showed this one just as a simple example.
3. Conclusions
To conclude, I argued that the most likely model for the central compact
X­ray source of RCW 103 is that of an accreting old NS in a disrupted
binary system with a young compact object (the 69­ms pulsar) born in
the recent SN explosion that produced the observed supernova remnant
(some ideas about a disrupted binary in RCW 103 were also discussed
in the article (Torii et al., 1998)). Such systems are rare, but natu­
ral products of the binary evolution. Scenarios with a single compact
objects or with accreting BH are less probable.
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6 Sergei Popov
Acknowledgements
I thank M.E. Prokhorov and V.M.Lipunov for helpful discussions, E.V.
Gotthelf and K. Torii for the information about RCW 103 and A.V.
Kravtsov for his comments on the text. The work was supported by
the INTAS (96­0315) and NTP ''Astronomy'' (1.4.4.1) grants.
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