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Evolution of Xíray sources at the Galactic center after the
burst of star formation
V.M. Lipunov
Sternberg Astronomical Institute, Universitetskij pr. 13, 119899 Moscow
L.M. Ozernoy
Physics Dept. and Inst. for Comput. Sci. & Inform., George Mason
Univ.,
also Lab. for Astron. & Solar Phys., NASA/GSFC
S.B. Popov, K.A. Postnov & M.E. Prokhorov
Sternberg Astronomical Institute, Universitetskij pr. 13, 119899 Moscow
Abstract. Xíray binary population at the Galactic center is proposed
here to form as a result of a starburst, which is synthesized by using the
``Scenario Machine''-- the code of binary star evolution. For the currently
assumed starburst age of 6í8 millions years, our results are consistent
with the recent GRANAT Xíray observations of the Galactic center and
predict a substantial number of Xíray binaries with black holes.
We are exploring the evolution of massive binary star populations for a
starburst occurring in the conditions similar to the Milky Way galaxy, in its
central part (see Ozernoy 1994), on a time scale of 10 Myr. A computer code is
applied allowing for computing, by using Monte Carlo simulations the evolution
of a large ensemble of binary systems, with proper accounting for spin evolution
of magnetized neutron stars (NSs) (see the description of the code in Kornilov
& Lipunov 1983 and Lipunov et al 1995).
MonteíCarlo simulations of binary star evolution allow one to investigate
the evolution of a large ensemble of binaries and to estimate the number of
binaries at different evolutionary stages. We also take into account that the
collapse of a massive star into a NS can be asymmetrical, so that an additional
kick velocity, v kick
, presumably randomly oriented in space, should be imparted
to the newborn NS. In the present calculations, the kick velocity was taken to
be 75 km/s.
Our results include the number of Xíray transients (NS + Be star on highly
eccentric orbit), superíaccreting black holes (SS 433í like sources), and binaries
consisting of a BH and a supergiant (Cyg Xí1--like sources), all as functions of
time. In order to obtain statistically significant results, the evolution of 300,000
binary systems was computed. Then we normalized the figures so as to be
in agreement with the Tamblyn & Rieke (1993) calculations of the number of
massive OBístars (1,900 stars with M ? 10 M fi ) that survived ¦ 6 \Gamma 8 Myrs
after the starburst onset.
1

.01
.1
1
10
5.0 7.5 10.0
T, Myr
Number
SS433íílike sources
.01
.1
1
10
100
5.0 7.5 10.0
T, Myr
Number
CYG Xí1íílike sources
.01
.1
10
100
5.0 7.5 10.0
T, Myr
Ratio
Ratio BH/NS
.01
.1
1
10
100
5.0 7.5 10.0
T, Myr
Number
Xíray transients
Figure 1. The evolution of some selected types of Xíray binaries
during the starburst at the Galactic center.
We find that by 7 Myr after such a starburst one expects ¦1 Xíray source
with a BH (Cyg Xí1-- like), ¦1 SS 433ílike source (BH in the regime of superací
cretion), and ¦37 transient sources with a NS, all to be within the central 1 kpc
or so. An interesting result that can be considered as a specific starburst feature,
is that the ratio of the number of systems like SS 433 to the number of Xíray
transients is about 1:100, compared to 1:1000, characteristic to the average ratio
in the galactic field. The ratio of the total number of Xíray sources containing
a BH (of Cyg Xí1 + SS 433 types) to the number of Xíray transients with NSs
turns out to be a sensitive function of the age of the starburst, and its computed
value ¦0.04 is consistent with the observations of the GRANAT satellite (see
Pavlinsky et al 1994).
Acknowledgments. The authors acknowledge I. Panchenko and S. Nazin
for useful discussions. The work was partially supported by the COSMION
grant, INTAS grant No 93í3364 and Grant No JAP100 from the International
Science Foundation and Russian Government. PSB thanks ISF for the travel
grant (N5883).
References
Kornilov, V.G., Lipunov, V.M. 1983, AZh, 60, 574
Lipunov, V.M., Postnov, K.A. & Prokhorov, M.E. 1995, ApJ 441, 776
Ozernoy, L.M. 1994, in ``MultiíWavelength Continuum Emission of AGN'', eds.
T.J.íL. Courvoisier & A. Blecha, Kluwer, p. 351
Pavlinsky, M.N., Grebenev, S.A., and Sunyaev, R.A. 1994, ApJ 425, 110
Tamblyn, P. & Rieke, G.H. 1993, ApJ 414, 573