Äîêóìåíò âçÿò èç êýøà ïîèñêîâîé ìàøèíû. Àäðåñ îðèãèíàëüíîãî äîêóìåíòà : http://xray.sai.msu.ru/~polar/html/publications/rcw103/andrey.ps Äàòà èçìåíåíèÿ: Sat Dec 27 15:34:49 1997 Äàòà èíäåêñèðîâàíèÿ: Sat Dec 22 04:54:25 2007 Êîäèðîâêà: Ïîèñêîâûå ñëîâà: m 103

On the nature of the compact X­ray source
inside RCW 103
Sergei B.Popov
Sternberg Astronomical Institute, Universitetskii pr.13,
119899, Moscow, Russia
e­mail: polar@xray.sai.msu.su
Abstract
I discuss the nature of the compact X­ray source inside the supernova
remnant RCW 103. Several models, based on the accretion onto a compact
object such as a neutron star or a black hole (isolated or binary), are
analyzed. I show that it is more likely that the X­ray source is an accreting
neutron star than an accreting black hole. I also argue that models of a
binary system with an old accreting neutron star are most favored.
1 Introduction
It is generally accepted that most of neutron stars (NSs) and black holes (BHs)
are the products of supernova (SN) explosions (although there is also a possi­
bility of a ``quiet collapse''). In some cases a supernova remnant (SNR) appears
after a formidable explosion of a massive star. Although a young NS is often
observed inside a SNR as a radio pulsar (e.g., Crab, Vela, etc.), in most cases
no compact object is found inside a SNR, or an accidential coincidence of the
radio pulsar and the SNR is very likely (e.g., Kaspi 1996; Frail 1997).
Recently, Gotthelf et al. (1997) found a compact X­ray source inside the SNR
RCW 103 with the X­ray luminosity L x ¸ 10 34 erg=s (for the distance 3:3 kpc)
and the black­body temperature about 0:6 keV . The nature of the compact
source is unclear. No radio or optical compact counterpart was observed. Here,
I discuss possible models of that compact source.
2 What is inside the RCW 103?
Gotthelf et al. (1997) 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 analyse thus only models with compact objects,
isolated or with compact companions. Massive normal companions are excluded
by optical observations. If the companion is a low­mass star, it is difficult to
explain the X­ray luminosity as high as observed in the RCW 103 because in
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low­mass systems accretion usually occurs after the Roche lobe is overflowed
with higher luminosities.
The main challenge for the models of accretion of the surrounding material
is to answer the question of where the compact object finds enough matter to
accrete. I don't discuss it here, assuming that the material is available in the
surrounding medium.
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 satellities such as ROSAT , ASCA etc (Heckler & Kolb 1996). Using
the fact that neither radio pulsar nor X­ray pulsations are actually observed in
the case of RCW 103, one can argue that after the SN explosion a BH is formed
more likely than a NS. We then can explain why a compact X­ray source inside
a SNR without a radio pulsar are rare: the BHs are born by the most massive
stars, and so BHs are an order of magnitude less abundant than the NSs.
To achieve high X­ray luminosity, a compact object must move with a rela­
tively low velocity (the Bondi's formula):
—
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
!
:
The effective velocity cannot be much lower than 10 km=s, which corre­
sponds to the sound speed in the ISM with the temperature of ¸ 10 4 K. I will
therefore use this value of the velocity, 10 km=s, because the luminosity is high
for an isolated object, and with the lower velocity much higher density of the
surrounding medium is required.
During the SN explosions a compact object can obtain an additional kick
velocity. The value and the distribution of the kick velocity is not known well
enough (e.g., Lipunov et al. 1996). Although observations of radio pulsars
favour high kick velocities about 300 \Gamma 500 km=s (Lyne & Lorimer 1994), al­
ternative scenarios in which the velocity increases after the SN explodes are
possible (Kaspi 1996; Frail 1997). The most popular scenarios usually predict
the mean kick velocities to be much higher than 10 km=s.
To explain the observed X­ray luminosity of the compact object inside RCW
103 the accretion rate, —
M , should be about 10 14 g=s (assuming that one gramm
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of accreted material produces 10 20 erg). For all models that consider accretion
onto an isolated compact object, the density required to obtain L x ¸ 10 34 erg=s
is as high as 10 \Gamma22 g=cm 3 .
One can then estimate the size of the emmiting region, using observed lu­
minosity and temperature:
L = 4ú \Delta R 2
emm oeT 4
For observed values of L x and T this equation gives Remm ¸ 0:9 km. For
BHs such a low value of Remm is very unlikely because the gravitational radius
is about RG ¸ 3 km (M=M fi ). 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. This is probably the main argument against
isolated accreting black hole as a model for the RCW 103.
The same argument can be used against models with a binary system:
BH+BH (BH is born in the recent SN explosion) and BH+NS (NS is born
in the recent SN explosion and the pulsar is not observed due, for example,
to unfortunate orientation), or against models in which no compact remnant
survives after the recent SN explosion of the massive star in a binary system
with a BH as a companion.
In the next subsections I present more viable models with accreting NS.
2.2 Accreting isolated young neutron star
In the past few years isolated accreting NSs have become a subject of great
interest due to the new observations by the ROSAT satellite (Treves & Colpi,
1991; Walter et al. 1996; Haberl et al. 1996). In this subsection I will present
an argument that the compact X­ray source in RCW 103 can be an isolated
accreting NS and will estimate some properties of that NS.
There are four main possible stages for a NS in a low­density plasma: 1):
Ejector (or a radio pulsar); 2): Propeller; 3): Accretor; and 4): Georotator
(Lipunov & Popov 1995; Konenkov & 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 in the Accretor stage, then its period is longer than the accretor
period, PA :
PA = 2 5=14 ú (GM ) \Gamma5=7 (¯ 2 = —
M ) 3=7 sec; (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, M = 1:4 M fi ,
RNS = 10 6 cm which give:
B ¸ 10 10 \Delta p 7=6 G: (3)
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This critical line for this period (eq. 2), PA , is shown in the Figure 1. The
region below the line is allowed for accreting NS.
If material is accreted from the turbulent interstellar medium, a new equi­
librium period can occur (Konenkov & 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
sec; (4)
where V t is the turbulent velocity (all velocities are in units of 10 km=s); 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)
The corresponding critical line for the equilibrium period (eq. 4) is also
shown in the Figure 1.
It is obvious from the Figure 1 that to explain the luminosity of the RCW
103 by an isolated accreting NS, one must assume that the NS was born with
extremely 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.
2.3 Accreting old neutron star in pair with a young neu­
tron star or a young black hole
Binary compact objects are 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
neutron star requires unusual initial parameters. However, 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
(or there was no remnant at all).
In that case, the parameters determined by eqs.(3), (5) are not unusual: old
NS can have low magnetic fields and long periods. Due to the fact that Gotthelf
et al. (1997) 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 sec), which is contrary to what is expected (Lipunov &
Popov 1995).
The evolutionary scenario for such a system is clear enough (Lipunov et al.,
1996). One can easily calculate it using the ``Scenario Machine'' WWW­facility
(http://xray.sai.msu.su/sciwork/scenario.html; Nazin et al. 1996). For
example, two stars with masses 15 M fi and 14 M fi on the main sequence with
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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) 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 & 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 ).
Young NS can be unobsereved as a radiopulsar due to several reasons. The
simplest one is the effect of orientation.
3 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 (or
no remnant survived after the explosion). Such systems a rare, but natural
products of the binary evolution. Scenarios with single compact objects or with
accreting BH are less probable.
Acknowledgements
I thank M.E. Prokhorov for helpful discussions, A.V. Kravtsov for his com­
ments on the text and the ``Scenario Machine'' group for the WWW­version of
the program. The work was supported by the RFFI (95­02­6053), the INTAS
(93­3364) and the ISSEP (a96­1896) grants.
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