10,014 research outputs found
Soft-excess in ULX spectra: disc emission or wind absorption?
We assess the claim that Ultra-luminous X-ray sources (ULXs) host
intermediate-mass black holes (BH) by comparing the cool disc-blackbody model
with a range of other models, namelly a more complex physical model based on a
power-law component slightly modified at various energies by smeared
emission/absorption lines from highly-ionized gas. Our main conclusion is that
the presence of a soft excess, or a soft deficit, depends entirely on the
energy range to which we choose to fit the ``true'' power-law continuum; hence,
we argue that those components should not be taken as evidence for accretion
disc emission, nor used to infer BH masses. We speculate that bright ULXs could
be in a spectral state similar to (or an extension of) the steep-power-law
state of Galactic BH candidates, in which the disc is completely comptonized
and not directly detectable, and the power-law emission may be modified by the
surrounding, fast-moving, ionized gas.Comment: To appear in the proceedings of "The Multicoloured Landscape of
Compact Objects and their Explosive Progenitors: Theory vs. Observations",
Cefalu', Sicily, June 11-24, 2006 (AIP). Compilation needs specific AIP .clo,
.cls, .sty and .tex files (included along with the paper .tex file and
figures
On the weakness of disc models in bright ULXs
It is sometimes suggested that phenomenological power-law plus cool
disc-blackbody models represent the simplest, most robust interpretation of the
X-ray spectra of bright ultraluminous X-ray sources (ULXs); this has been taken
as evidence for the presence of intermediate-mass black holes (BHs) (M ~ 10^3
Msun) in those sources. Here, we assess this claim by comparing the cool
disc-blackbody model with a range of other models. For example, we show that
the same ULX spectra can be fitted equally well by subtracting a disc-blackbody
component from a dominant power-law component, thus turning a soft excess into
a soft deficit. Then, we propose a more complex physical model, based on a
power-law component slightly modified at various energies by smeared emission
and absorption lines from highly-ionized, fast-moving gas. We use the
XMM-Newton/EPIC spectra of two ULXs in Holmberg II and NGC 4559 as examples.
Our main conclusion is that the presence of a soft excess or a soft deficit
depends on the energy range over which we choose to fit the ``true'' power-law
continuum; those small deviations from the power-law spectrum are well modelled
by disc-blackbody components (either in emission or absorption) simply because
they are a versatile fitting tool for most kinds of smooth, broad bumps. Hence,
we argue that those components should not be taken as evidence for accretion
disc emission, nor used to infer BH masses. Finally, we speculate that bright
ULXs could be in a spectral state similar to (or an extension of) the
steep-power-law state of Galactic BH candidates, in which the disc is now
completely comptonized and not directly detectable, and the power-law emission
may be modified by the surrounding, fast-moving, ionized gas.Comment: 12 pages, accepted by MNRA
Black hole formation from massive scalar field collapse in the Einstein-de Sitter universe
We study the spherically symmetric collapse of a real, minimally coupled,
massive scalar field in an asymptotically Einstein-de Sitter spacetime
background. By means of an eikonal approximation for the field and metric
functions, we obtain a simple analytical criterion---involving the physical
size and mass scales (the field's inverse Compton wavelength and the spacetime
gravitational mass) of the initial matter configuration---for generic
(non-time-symmetric) initial data to collapse to a black hole. This analytical
condition can then be used to place constraints on the initial primordial black
hole spectrum, by considering spherical density perturbations that re-entered
the horizon during an early matter-dominated phase that immediately followed
inflation.Comment: 9 pages, RevTeX, 3 eps figures; to appear in Phys. Rev.
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