458 research outputs found
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
Soft-excess in ULX spectra: the chilled-disk scenario
Soft X-ray spectra of ULXs show small deviations from a power-law model, that
can be attributed to reprocessing in a fast, ionized outflow, or to thermal
emission from a cool disk. If it is thermal emission, the cool peak temperature
can be explained by an inner disk that radiates only a small fraction of the
gravitational power, transferring the rest to an upscattering medium which is
then responsible for the dominant power-law component. This scenario does not
require intermediate-mass black holes: we use a phenomenological model to show
that the observed X-ray luminosities and spectra of ULXs are consistent with
typical masses ~ 50-100 Msun.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
Recipes for ULX formation: necessary ingredients and garnishments
I summarize the main observational features that seem to recur more
frequently in the ULX population. I speculate that two of the most important
physical requirements for ULX formation are low metal abundance, and clustered
star formation triggered by external processes such as molecular cloud
collisions. In this scenario, most ULX are formed from recent stellar
processes, have BH masses < 100 Msun and do not require merger processes in
super star clusters.Comment: 4 pages, to appear in the proceedings of "Black Holes: from Stars to
Galaxies", IAU Symp. No. 238, V. Karas & G. Matt eds., Cambridge University
Pres
The optical emission lines as a probe of state transitions in black-hole candidates
Optical spectroscopic studies of emission lines in black-hole candidates can
help us investigate state transitions in those systems. Changes in the optical
line profiles reflect changes in the geometry of the accretion flow, usually
associated with X-ray state transitions in the inner region. We identify at
least four optical states in the black-hole candidate GRO J1655-40 in outburst,
and two optical states in GX339-4.Comment: Talk presented at the IX Marcel Grossmann Meeting, Rome 2000--to
appear in the Conference proceeding
Black hole masses and accretion states in ULXs
We summarize indirect empirical arguments used for estimating black hole (BH)
masses in ultraluminous X-ray sources (ULXs). The interpretation of the X-ray
data is still too model-dependent to provide tight constraints, but masses <~
100 Msun seem the most likely. It is getting clearer that ULXs do not show the
same evolutionary sequence between canonical spectral states as stellar-mass
BHs, nor the same timescale for state transitions. Most ULX spectra are
consistent either with a power-law-dominated state (apparently identical to the
canonical low/hard state), or with a very high state (or slim-disk state).
Despite often showing luminosity variability, there is little evidence of ULXs
settling into a canonical high/soft state, dominated by a standard disk
(disk-blackbody spectrum). It is possible that the mass accretion rate (but not
necessarily the luminosity) is always higher than Eddington; but there may be
additional physical differences between stellar-mass BHs and ULXs, which
disfavour transitions to the standard-disk, radio-quiet state in the latter
class. We speculate that the hard state in ULXs is associated with jet or
magnetic processes rather than an ADAF, can persist up to accretion rates ~
Eddington, and can lead directly to the very high state.Comment: 8 pages; to appear in the proceedings of the conference
"Observational Evidence of Black Holes", Kolkata, February 200
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