1,214 research outputs found
A Possible Explanation for the "Parallel Tracks" Phenomenon in Low-Mass X-Ray Binaries
An explanation is proposed for the fact that in LMXBs the correlation between
most observable X-ray spectral and timing parameters (such as kHz QPO
frequency) on the one hand, and Lx on the other, while generally good in a
given source on a time scale of hours, is absent both on longer time scales and
between sources. This leads to parallel tracks in plots of such parameters vs.
Lx. Where previous explanations require at least two time-variable independent
parameters, e.g. Mdot through the disk and through a radial inflow, one is in
fact sufficient if the systemic response to time variations in this variable
has both a prompt and a time-averaged component. I explore a scenario in which
most observable spectral and timing parameters to first order depend on disk
accretion rate normalized by its own long-term average rather than on any
individual Mdot; Lx just depends on total Mdot. Thus, parameters can be
uncorrelated to Mdot, yet vary in response to Mdot variations. Numerical
simulations of the model describing the dependence of kHz QPO frequency on Lx,
which observationally is characterized by a striking pattern of parallel tracks
both in individual sources and between sources, reproduce the observations
remarkably well. A physical interpretation involving a radial inflow with a
rate that derives through a time averaging process from the disk accretion
rate, and an inner disk radius that depends on the balance between the
accretion through the disk and the total luminosity seems particularly
promising. The consequences of this idea for our understanding of states and
tracks in LMXBs are discussed, and the applicability of the idea to black-hole
candidates, where the observational situation is more complex, is briefly
addressed.Comment: 17 pages 3 figures - version accepted for publication in the ApJ;
tentatively scheduled for the v561 n2 p1 ApJ November 10, 2001 issue. Some
corrections and clarifications w/r to details of the argumen
Deep Extragalactic X-ray Surveys
Deep surveys of the cosmic X-ray background are reviewed in the context of
observational progress enabled by the Chandra X-ray Observatory and the X-ray
Multi-Mirror Mission-Newton. The sources found by deep surveys are described
along with their redshift and luminosity distributions, and the effectiveness
of such surveys at selecting active galactic nuclei (AGN) is assessed. Some key
results from deep surveys are highlighted including (1) measurements of AGN
evolution and the growth of supermassive black holes, (2) constraints on the
demography and physics of high-redshift AGN, (3) the X-ray AGN content of
infrared and submillimeter galaxies, and (4) X-ray emission from distant
starburst and normal galaxies. We also describe some outstanding problems and
future prospects for deep extragalactic X-ray surveys.Comment: 32 pages; Annu. Rev. Astron. Astrophys., Volume 43 (2005); updated to
match accepted versio
Kilohertz quasi-periodic oscillations in low mass X-ray binary sources and their relation with the neutron star magnetic field
Starting from the observation that kilohertz Quasi Period Oscillations (kHz
QPO) occur in a very narrow range of X-ray luminosities in neutron star low
mass X-ray binaries, we try to link the kHz QPO observability to variations of
the neutron star magnetospheric radius, in response to changing mass inflow
rate. At low luminosities, the drop off of kHz QPOs activity may be explained
by the onset of the centrifugal barrier, when the magnetospheric radius reaches
the corotation radius. At the opposite side, at higher luminosities, the
magnetospheric radius may reach the neutron star and the vanishing of the
magnetosphere may led to the stopping of the kHz QPOs activity. If we apply
these constraints, the magnetic fields of atoll (B ~ 0.3-1 10^8 G for Aql X-1)
and Z (B ~ 1-8 10^8 G for Cyg X-2) sources can be derived. These limits
naturally apply in the framework of beat frequency models but can also work in
the case of general relativistic models.Comment: 4 pages (emulateapj macro), 1 figure. Accepted for publication in ApJ
Letter
The new X-ray transient SAX J1711.6-3808: decoupling between its 3-20 keV luminosity and its state transitions
We present a study of the correlated spectral and timing behavior of the new
X-ray transient SAX J1711.6-3808 during its 2001 outburst using data obtained
with the RXTE. We also investigate the correlations between those source
properties and the 3-20 keV X-ray luminosity. The behavior of the source during
the observations can be divided into two distinct state types. During the hard
state, the energy spectra are relatively hard and can be described by only a
power-law component, and the characteristic frequencies (i.e., the frequency of
the 1-7 Hz QPOs observed for the first time in this source) in the power
spectra are low. However, during the ``soft'' state, the spectra are
considerably softer (in addition to the power-law component, a soft component
is necessary to fit the spectra) and the frequencies are the highest observed.
Remarkably, this distinction into two separate states cannot be extrapolated to
also include the 3-20 keV X-ray luminosity. Except for one observation, this
luminosity steadily decreased but the hard state was observed both at the
highest and lowest observed luminosities. In contrast, the soft state occurred
only at intermediate luminosities. This clearly demonstrates that the state
behavior of SAX J1711.6-3808 is decoupled from its X-ray luminosity and that if
the X-ray luminosity traces the accretion rate in SAX J1711.6-3808, then the
state transitions are not good accretion rate indicators, or vice versa. The
data of SAX J1711.6-3808 does not allow us to conclusively determine its exact
nature. The source resembles both neutron star and black hole systems when they
have low luminosities. We discuss our results with respect to the correlated
timing and spectral behavior observed in other LMXBs and the implications of
our results on the modeling of the outburst light curves of X-ray transients.Comment: Accepted for publication in ApJ Main Journal, 13 September 200
Properties of the integrated spectrum of serendipitous 2XMM catalogue sources
Our analysis is aimed at characterizing the properties of the integrated
spectrum of active galactic nuclei (AGNs) such as the ubiquity of the Fe
K{\alpha} emission in AGNs and the dependence of the spectral parameters on the
X-ray luminosity and redshift. We selected 2646 point sources from the 2XMM
catalogue at high galactic latitude (|BII| > 25 degrees) and with the sum of
EPIC-PN and EPIC-MOS 0.2-12 keV counts greater than 1000. Redshifts were
obtained for 916 sources from the NED. The final sample consists of 507 AGN.
Individual source spectra have been summed in the observed frame to compute the
integrated spectra in different redshift and luminosity bins over the range
0<z<5. Detailed analysis of these spectra has been performed. We find that the
narrow Fe K{\alpha} line at 6.4 keV is significantly detected up to z=1. The
line equivalent width decreases with increasing X-ray luminosity in the 2-10
keV band (''IT effect''). The anti-correlation is characterized by the relation
log(EWFe) = (1.66 +/- 0.09) + (-0.43 +/- 0.07) log(LX,44), where EWFe is the
rest frame equivalent width of the neutral iron K{\alpha} line in eV and LX,44
is the 2-10 keV X-ray luminosity in units of 10^{44} erg s^{-1}. The equivalent
width is nearly independent of redshift up to z ~ 0.8 with an average value of
101+/-40 (rms dispersion) eV in the luminosity range 43.5<= logLX <= 44.5. Our
analysis also confirmed the hardening of the spectral indices at low
luminosities implying a dependence of obscuration on luminosity. We confirm
that the neutral narrow Fe K{\alpha} line is an almost ubiquitous feature of
AGNs. We find compelling evidence for the ''IT effect'' over a redshift
interval larger than probed in any previous study. We detect no evolution of
the average rest frame equivalent width of the Fe K{\alpha} line with redshift.Comment: 10 pages, 7 figures, 2 Tables. A&A in pres
The Interpretations For the Low and High Frequency QPO Correlations of X-ray Sources Among White Dwarfs, Neutron Stars and Black Holes
It is found that there exists an empirical linear relation between the high
frequency \nhigh and low frequency \nlow of quasi-periodic oscillations
(QPOs) for black hole candidate (BHC), neutron star (NS) and white dwarf (WD)
in the binary systems, which spans five orders of magnitude in frequency.
For the NS Z (Atoll) sources,
and are identified as the lower kHz QPO frequency
and horizontal branch oscillations (HBOs) \nh (broad noise components); for
the black hole candidates and low-luminosity neutron stars, they are the QPOs
and broad noise components at frequencies between 1 and 10 Hz; for WDs, they
are the ``dwarf nova oscillations'' (DNOs) and QPOs of cataclysmic variables
(CVs). To interpret this relation, our model ascribes to the
Alfv\'en wave oscillation frequency at a preferred radius and to
the same mechanism at another radius. Then, we can obtain \nlow = 0.08
\nhigh and the relation between the upper kHz QPO frequency \nt and HBO to
be \nh \simeq 56 ({\rm Hz}) (\nt/{\rm kHz})^{2}, which are in accordance with
the observed empirical relations. Furthermore, some implications of model are
discussed, including why QPO frequencies of white dwarfs and neutron stars span
five orders of magnitude in frequency. \\Comment: 11 pages, 1 figure, accepted by PAS
Probing Elemental Abundances in SNR 1987A Using XMM-Newton
We report on the latest (2007 Jan) observations of supernova remnant (SNR)
1987A from the XMM-Newton mission. Since the 2003 May observations of Haberl et
al. (2006), 11 emission lines have experienced increases in flux by factors ~ 3
to 10, with the 775 eV line of O VIII showing the greatest increase; we have
observed 6 lines of Fe XVII and Fe XVIII previously unreported by XMM-Newton. A
two-shock model representing plasmas in non-equilibrium ionization is fitted to
the EPIC-pn spectra, yielding temperatures of ~ 0.4 and ~ 3 keV, as well as
elemental abundances for N, O, Ne, Mg, Si, S and Fe. We demonstrate that the
abundance ratio of N and O can be constrained to less than ~20% accuracy.
Within the same confidence interval, the same analysis suggests that the C+N+O
abundance varies from ~ 1.1 to 1.4 X 10^-4. Normalizing our obtained abundances
by the Large Magellanic Cloud (LMC) values of Hughes, Hayashi & Koyama (1998),
we find that O, Ne, Mg and Fe are under-abundant, while Si and S are
over-abundant, consistent with the findings of Aschenbach (2007). Such a result
has implications for both the single-star and binary accretion/merger models
for the progenitor of SNR 1987A. In the context of the binary merger scenario
proposed by Morris & Podsiadlowski (2006, 2007), material forming the inner,
equatorial ring was expelled after the merger, implying that either our derived
Fe abundance is inconsistent with typical LMC values or that iron is
under-abundant at the site of the progenitor star of SNR 1987A.Comment: 14 pages, 10 diagrams (2 omitted). Accepted by Ap
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