8,097 research outputs found
Weighing the black holes in ultraluminous X-ray sources through timing
We describe a new method to estimate the mass of black holes in Ultraluminous
X-ray Sources (ULXs). The method is based on the recently discovered
``variability plane'', populated by Galactic stellar-mass black-hole candidates
(BHCs) and supermassive active galactic nuclei (AGNs), in the parameter space
defined by the black-hole mass, accretion rate and characteristic frequency. We
apply this method to the two ULXs from which low-frequency quasi-periodic
oscillations have been discovered, M82 X-1 and NGC 5408 X-1. For both sources
we obtain a black-hole mass in the range 100~1300 Msun, thus providing evidence
for these two sources to host an intermediate-mass black hole.Comment: 5 pages, 2 figures, Accepted by MNRA
How young are early-type cluster galaxies ? Quantifying the young stellar component in a rich cluster at z=0.41
We present a new method of quantifying the mass fraction of young stars in
galaxies by analyzing near-ultraviolet (NUV)-optical colors. We focus our
attention on early-type cluster galaxies, whose star formation history is at
present undetermined. Rest-frame NUV (F300W) and optical (F702W) images of
cluster Abell 851 (z=0.41) using HST/WFPC2 allow us to determine a NUV-optical
color-magnitude relation, whose slope is incompatible with a monolithic
scenario for star formation at high redshift. A degeneracy between a young
stellar component and its fractional mass contribution to the galaxy is found,
and a photometric analysis comparing the data with the predictions for a simple
two-stage star formation history is presented. The analysis shows that some of
the early-type galaxies may have fractions higher than 10% of the total mass
content in stars formed at z~0.5. An increased scatter is found in the
color-magnitude relation at the faint end, resulting in a significant fraction
of faint blue early-type systems. This would imply that less massive galaxies
undergo more recent episodes of star formation, and this can be explained in
terms of a positive correlation between star formation efficiency and
luminosity.Comment: Accepted for publication in ApJ Letters. Uses emulateapj.sty. 5 pages
with 3 embedded EPS figure
A Unified Description of the Timing Features of Accreting X-ray Binaries
We study an empirical model for a unified description of the power spectra of
accreting neutron stars and black holes. This description is based on a
superposition of multiple Lorentzians and offers the advantage that all QPO and
noise components are dealt with in the same way, without the need of deciding
in advance the nature of each component. This approach also allows us to
compare frequencies of features with high and low coherences in a consistent
manner and greatly facilitates comparison of power spectra across a wide range
of source types and states. We apply the model to six sources, the
low-luminosity X-ray bursters 1E 1724-3045, SLX 1735-269 and GS 1826-24, the
high-latitude transient XTE J1118+480, the bright system Cir X-1, and the Z
source GX 17+2. We find that it provides a good description of the observed
spectra, without the need for a scale-free (1/f) component. We update
previously reported correlations between characteristic frequencies of timing
features in the light of this new approach and discuss similarities between
different types of systems which may point towards similar underlying physics.Comment: 13 pages, to appear in The Astrophysical Journa
The ABC of low-frequency quasi-periodic oscillations in Black-Hole Candidates: analogies with Z-sources
Three main types of low-frequency quasi-periodic oscillations (LFQPOs) have
been observed in Black Hole Candidates. We re-analyzed RXTE data of the bright
systems XTE J1859+226, XTE J1550-564 and GX 339-4, which show all three of
them. We review the main properties of these LFQPOs and show that they follow a
well-defined correlation in a fractional rms vs. softness diagram. We show that
the frequency behavior through this correlation presents clear analogies with
that of Horizontal-, Normal- and Flaring-Branch Oscillations in Z sources, with
the inverse of the fractional rms being the equivalent of the curvilinear
coordinate Sz through the Z track.Comment: 5 pages, 4 figures, accepted by ApJ, uses emulateap
Studying the X-ray hysteresis in GX 339-4: the disc and iron line over one decade
We report on a comprehensive and consistent investigation into the X-ray
emission from GX 339-4. All public observations in the 11 year RXTE archive
were analysed. Three different types of model - single powerlaw, broken
powerlaw and a disc + powerlaw - were fitted to investigate the evolution of
the disc, along with a fixed gaussian component at 6.4 keV to investigate any
iron line in the spectrum. We show that the relative variation in flux and
X-ray colour between the two best sampled outbursts are very similar. The decay
of the disc temperature during the outburst is clearly seen in the soft state.
The expected decay is S_Disc \propto T^4; we measure T^4.75\pm0.23. This
implies that the inner disc radius is approximately constant in the soft state.
We also show a significant anti-correlation between the iron line significant
width and the X-ray flux in the soft state while in the hard state the EW is
independent of the flux. This results in hysteresis in the relation between
X-ray flux and both line flux and EW. To compare the X-ray binary outburst to
the behaviour seen in AGN, we construct a Disc Fraction Luminosity Diagram for
GX 339-4, the first for an X-ray binary. The shape qualitatively matches that
produced for AGN. Linking this with the radio emission from GX 339-4 the change
in radio spectrum between the disc and power-law dominated states is clearly
visible.Comment: Accepted for publication in MNRAS, 20 pages, 17 figures. For high-res
version see http://www.astro.soton.ac.uk/~r.j.dunn/publications.htm
Hard X-ray states and radio emission in GRS 1915+105
We have compared simultaneous Ryle Telescope radio and Rossi X-Ray Timing
Explorer X-ray observations of the galactic microquasar GRS 1915+105, using the
classification of the X-ray behaviour in terms of three states as previously
established. We find a strong (one-to-one) relation between radio oscillation
events and series of spectrally hard states in the X-ray light curves, if the
hard states are longer than ~100s and are "well separated" from each other. In
all other cases the source either shows a low-level or a high level radio
emission, but no radio oscillation events. During intervals when the source
stays in the hard spectral state for periods of days to months, the radio
behaviour is quite different; during some of these intervals a quasi-continuous
jet is formed with an almost flat synchrotron spectrum extending to at least
the near-infrared. Based on the similarities between the oscillation profiles
at different wavelengths, we suggest a scenario which can explain most of the
complex X-ray : radio behaviour of GRS 1915+105. We compare this behaviour with
that of other black hole sources and challenge previous reports of a relation
between spectrally soft X-ray states and the radio emission.Comment: 21 pages, 7 figures. Accepted for publication in MNRAS. Full
resolution version of figure 4 available at
http://zon.wins.uva.nl/~klein/figure4.eps.g
Variability of X-ray binaries from an oscillating hot corona
The spectral and timing properties of an oscillating hot thermal corona are
investigated. This oscillation is assumed to be due to a magneto-acoustic wave
propagating within the corona and triggered by an external, non specified,
excitation. A cylindrical geometry is adopted and, neglecting the rotation, the
wave equation is solved in for different boundary conditions. The resulting
X-ray luminosity, through thermal comptonization of embedded soft photons, is
then computed, first analytically, assuming linear dependence between the local
pressure disturbance and the radiative modulation. These calculations are also
compared to Monte-Carlo simulations. The main results of this study are: (1)
the corona plays the role of a low band-pass medium, its response to a white
noise excitation being a at top noise Power Spectral Density (PSD) at low
frequencies and a red noise at high frequency, (2) resonant peaks are present
in the PSD. Their powers depend on the boundary conditions chosen and, more
specifically, on the impedance adaptation with the external medium at the
corona inner boundary. (3) The flat top noise level and break as well as the
resonant peak frequencies are inversely proportional to the external radius rj.
(4) Computed rms and f-spectra exhibit an overall increase of the variability
with energy. Comparison with observed variability features, especially in the
hard intermediate states of X-ray binaries are discussed.Comment: 12 pages, 7 figures, accepted for publication in MNRA
Connection between accretion disk and superluminal radio jets and the role of radio plateau state in GRS 1915+105
We investigate the association between the accretion disk during radio
plateau state and the following superluminal relativistic radio jets with peak
intensity varies from 200 mJy to 1000 mJy observed over a period of five years
and present the evidences of direct accretion disc-jet connection in
microquasar GRS 1915+105. We have analysed RXTE PCA/HEXTE X-ray data and have
found that the accretion rate, , as inferred from the X-ray
flux, is very high during the radio plateaux. We suggest that the accretion
disk during the radio plateaux always associated with radiation-driven wind
which is manifested in the form of enhanced absorption column density for X-ray
and the depleted IR emission. We find that the wind density increases with the
accretion disk luminosity during the radio plateaux. The wind density is
similar to the density of the warm absorber proposed in extragalactic AGNs and
Quasars. We suggest a simple model for the origin of superluminal relativistic
jets. Finally, We discuss the implications of this work for galactic
microquasars and the extragalactic AGNs and Quasars.Comment: 9 pages, 6 Figures, Accepted for publication in Ap
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