409 research outputs found
Roche-lobe overflow systems powered by black holes in young star clusters: the importance of dynamical exchanges
We have run 600 N-body simulations of intermediate-mass (~3500 Msun) young
star clusters (SCs) with three different metallicities (Z=0.01, 0.1 and 1
Zsun). The simulations include the dependence of stellar properties and stellar
winds on metallicity. Massive stellar black holes (MSBHs) with mass >25 Msun
are allowed to form through direct collapse of very massive metal-poor stars
(Z<0.3 Zsun). We focus on the demographics of black hole (BH) binaries that
undergo mass transfer via Roche lobe overflow (RLO). We find that 44 per cent
of all binaries that undergo an RLO phase (RLO binaries) formed through
dynamical exchange. RLO binaries that formed via exchange (RLO-EBs) are powered
by more massive BHs than RLO primordial binaries (RLO-PBs). Furthermore, the
RLO-EBs tend to start the RLO phase later than the RLO-PBs. In metal-poor SCs
(0.01-0.1 Zsun), >20 per cent of all RLO binaries are powered by MSBHs. The
vast majority of RLO binaries powered by MSBHs are RLO-EBs. We have produced
optical color-magnitude diagrams of the simulated RLO binaries, accounting for
the emission of both the donor star and the irradiated accretion disk. We find
that RLO-PBs are generally associated with bluer counterparts than RLO-EBs. We
compare the simulated counterparts with the observed counterparts of nine
ultraluminous X-ray sources. We discuss the possibility that IC 342 X-1, Ho IX
X-1, NGC 1313 X-2 and NGC 5204 X-1 are powered by a MSBH.Comment: 17 pages, 10 figures, 9 tables, ApJ, accepte
Modelling optical emission of Ultra-luminous X-ray Sources accreting above the Eddington limit
We study the evolution of binary systems of Ultra-luminous X-ray sources and
compute their optical emission assuming accretion onto a black hole via a non
standard, advection-dominated slim disc with an outflow. We consider systems
with black holes of and , and donor masses between
and . Super-critical accretion has considerable
effects on the optical emission. The irradiating flux in presence of an outflow
remains considerably stronger than that produced by a standard disc. However,
at very high accretion rates the contribution of X-ray irradiation becomes
progressively less important in comparison with the intrinsic flux emitted from
the disc. After Main Sequence the evolutionary tracks of the optical
counterpart on the colour-magnitude diagram are markely different from those
computed for Eddington-limited accretion. Systems with stellar-mass black holes
and donors accreting supercritically are characterized by
blue colors (F450W -- F555W ) and high luminosity (). Systems with more massive black holes accreting
supercritically from evolved donors of similar mass have comparable colours but
can reach . We apply our model to NGC 1313 X-2 and NGC 4559
X-7. Both sources are well represented by a system accreting above Eddington
from a massive evolved donor. For NGC 1313 X-2 the agreement is for a black hole, while NGC4559 X-7 requires a significantly more
massive black hole.Comment: 13 pages, 15 figures, Accepted for publication in MNRAS;
Acknowledgments adde
Will a Black Hole Soon Emerge from SN 1997D ?
Observations combined with theoretical modeling of the light curve of the
recently discovered supernova 1997D in NGC 1536 suggest that it might host a
black hole formed in the aftermath of the explosion. We consider some
observable consequences of a black hole in SN 1997D and estimate the late--time
accretion luminosity of the material which falls back onto the hole. We find
that this luminosity, decaying with a characteristic power--law dependence on
time, may emerge above the emission of the envelope in just a few years. Its
detection would thus provide unmistakable evidence for the presence of a black
hole.Comment: 8 pages, LaTeX, aaspp4.sty style file (AASTEX package), 2 postscript
figures (included). To appear in The Astrophysical Journal Letters (scheduled
for August 1, 1998 issue, Vol. 502
X--Ray Spectra from Neutron Stars Accreting at Low Rates
The spectral properties of X--ray radiation produced in a static atmosphere
around a neutron star accreting at very low rates are investigated. Previous
results by Alme \& Wilson (1973) are extended to the range to include the typical luminosities, , expected from isolated neutron stars
accreting the interstellar medium. The emergent spectra show an overall
hardening with respect to the blackbody at the neutron star effective
temperature in addition to a significant excess over the Wien tail. The
relevance of present results in connection with the observability of
low--luminosity X--ray sources is briefly discussed.Comment: 14 pages (3 postscript figures available on request), PlainTex,
submitted to Ap
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