451 research outputs found
Using radio emission to detect isolated and quiescent accreting black holes
We discuss the implications of new relations between black holes' masses,
X-ray luminosities and radio luminosities, as well as the properties of the
next generation of radio telescopes, for the goal of finding isolated accreting
black holes. Because accreting black holes have radio-to-X-ray flux ratios that
increase with decreasing luminosity in Eddington units, and because deep
surveys over large fields of view should be possible with planned
instrumentation such as LOFAR, radio surveys should be significantly more
efficient than X-ray surveys for finding these objects.Comment: 5 pages, 1 table, accepted to MNRAS Letter
Limits on [OIII] 5007 emission from NGC4472's globular clusters: constraints on planetary nebulae and ultraluminous black hole X-ray binaries in globular clusters
We have searched for [OIII] 5007 emission in high resolution spectroscopic
data from Flames/Giraffe VLT observations of 174 massive globular clusters
(GCs) in NGC4472. No planetary nebulae (PNe) are observed in these clusters,
constraining the number of PNe per bolometric luminosity,
\alpha<0.8*10^{-7}PN/L_{\odot}. This is significantly lower than the rate
predicted from stellar evolution, if all stars produce PNe. Comparing our
results to populations of PNe in galaxies, we find most galaxies have a higher
\alpha than these GCs (more PNe per bolometric luminosity - though some massive
early-type galaxies do have similarly low \alpha). The low \alpha required in
these GCs suggests that the number of PNe per bolometric luminosity does not
increase strongly with decreasing mass or metallicity of the stellar
population. We find no evidence for correlations between the presence of known
GC PNe and either the presence of low mass X-ray binaries (LMXBs) or the
stellar interaction rates in the GCs. This, and the low \alpha observed,
suggests that the formation of PNe may not be enhanced in tight binary systems.
These data do identify one [OIII] emission feature, this is the (previously
published) broad [OIII] emission from the cluster RZ 2109. This emission is
thought to originate from the LMXB in this cluster, which is accreting at
super-Eddington rates. The absence of any similar [OIII] emission from the
other clusters favors the hypothesis that this source is a black hole LMXB,
rather than a neutron star LMXB with significant geometric beaming of its X-ray
emission.Comment: 10 pages, 3 figures, accepted for publication in Ap
Constraints on jet X-ray emission in low/hard state X-ray binaries
We show that the combination of the similarities between the X-ray properties
of low luminosity accreting black holes and accreting neutron stars, combined
with the differences in their radio properties argues that the X-rays from
these systems are unlikely to be formed in the relativistic jets. Specifically,
the spectra of extreme island state neutron stars and low/hard state black
holes are known to be indistinguishable, while the power spectra from these
systems are known to show only minor differences beyond what would be expected
from scaling the characteristic variability frequencies by the mass of the
compact object. The spectral and temporal similarities thus imply a common
emission mechanism that has only minor deviations from having all key
parameters scaling linearly with the mass of the compact object, while we show
that this is inconsistent with the observations that the radio powers of
neutron stars are typically about 30 times lower than those of black holes at
the same X-ray luminosity. We also show that an abrupt luminosity change would
be expected when a system makes a spectral state transition from a radiatively
inefficient jet dominated accretion flow to a thin disk dominated flow, but
that such a change is not seen.Comment: 5 pages, no figures, accepted to MNRAS Letter
Very fast X-ray spectral variability in Cygnus X-1: Origin of the hard and soft-state emission components
The way in which the X-ray photon index, {\Gamma}, varies as a function of
count rate is a strong diagnostic of the emission processes and emission
geometry around accreting compact objects. Here we present the results from a
study using a new, and simple, method designed to improve sensitivity to the
measurement of the variability of {\Gamma} on very short time-scales.
We have measured {\Gamma} in ~2 million spectra, extracted from observations
with a variety of different accretion rates and spectral states, on time-scales
as short as 16 ms for the high mass X-ray binary Cygnus X-1, and have
cross-correlated these measurements with the source count rate. In the
soft-state cross-correlation functions (CCFs) we find a positive peak at zero
lag, stronger and narrower in the softer observations. Assuming that the X-rays
are produced by Compton scattering of soft seed photons by high energy
electrons in a corona, these results are consistent with Compton cooling of the
corona by seed photons from the inner edge of the accretion disc, the
truncation radius of which increases with increasing hardness ratio.
The CCFs produced from the hard-state observations, however, show an
anti-correlation which is most easily explained by variation in the energy of
the electrons in the corona rather than in variation of the seed photon flux.
The hard-state CCFs can be decomposed into a narrow anti-correlation at zero
lag, which we tentatively associate with the effects of self-Comptonisation of
cyclo-synchrotron seed photons in either a hot, optically thin accretion flow
or the base of the jet, and a second, asymmetric component which we suggest is
produced as a consequence of a lag between the soft and hard X-ray emission.
The lag may be caused by a radial temperature/energy gradient in the
Comptonising electrons combined with the inward propagation of accretion rate
perturbations.Comment: 12 pages, 14 figures; accepted for publication in Monthly Notices of
the Royal Astronomical Society, 2013 June
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