987 research outputs found
Evaluating Spectral Models and the X-ray States of Neutron-Star X-ray Transients
We propose a hybrid model to fit the X-ray spectra of atoll-type X-ray
transients in the soft and hard states. This model uniquely produces luminosity
tracks that are proportional to T^4 for both the accretion disk and boundary
layer. The model also indicates low Comptonization levels for the soft state,
gaining a similarity to black holes in the relationship between Comptonization
level and the strength of integrated rms variability in the power density
spectrum. The boundary layer appears small, with a surface area that is roughly
constant across soft and hard states. This result may suggestion that the NS
radius is smaller than its inner-most stable circular orbit.Comment: 15 pages, 15 figures, accepted for publication in the Ap
XMM-Newton observations of the spiral galaxy M74 (NGC 628)
The face-on spiral galaxy M74 (NGC 628) was observed by XMM on 2002 February
2. In total, 21 sources are found in the inner 5' from the nucleus (after
rejection of a few sources associated to foreground stars). Hardness ratios
suggest that about half of them belong to the galaxy. The higher-luminosity end
of the luminosity function is fitted by a power-law of slope -0.8. This can be
interpreted as evidence of ongoing star formation, in analogy with the
distributions found in disks of other late-type galaxies. A comparison with
previous Chandra observations reveals a new ultraluminous X-ray transient (L_x
\~ 1.5 x 10^39 erg/s in the 0.3--8 keV band) about 4' North of the nucleus. We
find another transient black-hole candidate (L_x ~ 5 x 10^38 erg/s) about 5'
North-West of the nucleus. The UV and X-ray counterparts of SN 2002ap are also
found in this XMM observation.Comment: submitted to ApJL. Based on publicly available data, see
http://xmm.vilspa.esa.es/external/xmm_news/items/sn_2002_ap/index.shtm
Variable-Frequency QPOs from the Galactic Microquasar GRS 1915+105
We show that the galactic microquasar GRS 1915+105 exhibits quasi-periodic
oscillations (QPOs) whose frequency varies continuously from 1-15 Hz, during
spectrally hard dips when the source is in a flaring state. We report here
analyses of simultaneous energy spectra and power density spectra at 4 s
intervals. The energy spectrum is well fit at each time step by an optically
thick accretion disk plus power law model, while the power density spectrum
consists of a varying red noise component plus the variable frequency QPO. The
features of both spectra are strongly correlated with one another. The 1-15 Hz
QPOs appear when the power law component becomes hard and intense, and
themselves have an energy spectrum consistent with the power law component
(with root mean square amplitudes as high as 10%). The frequency of the
oscillations, however, is most strikingly correlated with the parameters of the
thermal disk component. The tightest correlation is between QPO frequency and
the disk X-ray flux. This fact indicates that the properties of the QPO are not
determined by solely a disk or solely a corona.Comment: Accepted to ApJ Letters, 12 pages, 3 figures, AASTEX forma
Kilohertz QPO Peak Separation Is Not Constant in Scorpius X-1
We report on a series of twenty ~10^5 c/s, 0.125 msec time-resolution RXTE
observations of the Z source and low-mass X-ray binary Scorpius X-1. Twin
kilohertz quasi-periodic oscillation (QPO) peaks are obvious in nearly all
observations. We find that the peak separation is not constant, as expected in
some beat-frequency models, but instead varies from ~310 to ~230 Hz when the
centroid frequency of the higher-frequency peak varies from ~875 to ~1085 Hz.
We detect none of the additional QPO peaks at higher frequencies predicted in
the photon bubble model (PBM), with best-case upper limits on the peaks' power
ratio of 0.025. We do detect, simultaneously with the kHz QPO, additional QPO
peaks near 45 and 90 Hz whose frequency increases with mass accretion rate. We
interpret these as first and second harmonics of the so-called
horizontal-branch oscillations well known from other Z sources and usually
interpreted in terms of the magnetospheric beat-frequency model (BFM). We
conclude that the magnetospheric BFM and the PBM are now unlikely to explain
the kHz QPO in Sco X-1. In order to succeed in doing so, any BFM involving the
neutron star spin (unseen in Sco X-1) will have to postulate at least one
additional unseen frequency, beating with the spin to produce one of the kHz
peaks.Comment: 6 pages including 3 figure
Non-LTE Spectra of Accretion Disks Around Intermediate-Mass Black Holes
We have calculated the structures and the emergent spectra of stationary,
geometrically thin accretion disks around 100 and 1000 M_sun black holes in
both the Schwarzschild and extreme Kerr metrics. Equations of radiative
transfer, hydrostatic equilibrium, energy balance, ionization equilibrium, and
statistical equilibrium are solved simultaneously and consistently. The six
most astrophysically abundant elements (H, He, C, N, O, and Fe) are included,
as well as energy transfer by Comptonization. The observed spectrum as a
function of viewing angle is computed incorporating all general relativistic
effects. We find that, in contrast with the predictions of the commonly-used
multi-color disk (MCD) model, opacity associated with photoionization of heavy
elements can significantly alter the spectrum near its peak. These ionization
edges can create spectral breaks visible in the spectra of slowly-spinning
black holes viewed from almost all angles and in the spectra of
rapidly-spinning black holes seen approximately pole-on. For fixed mass and
accretion rate relative to Eddington, both the black hole spin and the viewing
angle can significantly shift the observed peak energy of the spectrum,
particularly for rapid spin viewed obliquely or edge-on. We present a detailed
test of the approximations made in various forms of the MCD model. Linear
limb-darkening is confirmed to be a reasonable approximation for the integrated
flux, but not for many specific frequencies of interest.Comment: 30 pages, 11 eps figures, accepted for publication in Ap
Hysteresis of spectral evolution in the soft state of black-hole binary LMC X-3
We report the discovery of hysteresis between the x-ray spectrum and
luminosity of black-hole binary LMC X-3. Our observations, with the
Proportional Counter Array on the Rossi X-ray Timing Explorer, took place
entirely within the soft spectral state, dominated by a spectral component that
was fitted well with a multicolor disk blackbody. A power-law component was
seen only during times when the luminosity of the disk blackbody was declining.
The x-ray luminosity at these times was comparable to that seen in transient
systems (x-ray novae) when they return to the hard state at the end of an
outburst. Our observations may represent partial transitions to the hard state;
complete transitions have been seen in this system by Wilms et al. (2001). If
they are related to the soft-to-hard transition in transients, then they
demonstrate that hysteresis effects can appear without a full state transition.
We discuss these observations in the context of earlier observations of
hysteresis within the hard state of binaries 1E 1740.7-2942 and GRS 1758-258
and in relation to published explanations of hysteresis in transients.Comment: 14 pages, 6 figures, accepted by The Astrophysical Journa
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