2,594 research outputs found
Risk markers associated with challenging behaviours in people with developmental disabilities: a meta-analytic study.
X-ray reflected spectra from accretion disk models. III. A complete grid of ionized reflection calculations
We present a new and complete library of synthetic spectra for modeling the
component of emission that is reflected from an illuminated accretion disk. The
spectra were computed using an updated version of our code XILLVER that
incorporates new routines and a richer atomic data base. We offer in the form
of a table model an extensive grid of reflection models that cover a wide range
of parameters. Each individual model is characterized by the photon index
\Gamma of the illuminating radiation, the ionization parameter \xi at the
surface of the disk (i.e., the ratio of the X-ray flux to the gas density), and
the iron abundance A_{Fe} relative to the solar value. The ranges of the
parameters covered are: 1.2 \leq \Gamma \leq 3.4, 1 \leq \xi \leq 10^4, and 0.5
\leq A_{Fe} \leq 10. These ranges capture the physical conditions typically
inferred from observations of active galactic nuclei, and also stellar-mass
black holes in the hard state. This library is intended for use when the
thermal disk flux is faint compared to the incident power-law flux. The models
are expected to provide an accurate description of the Fe K emission line,
which is the crucial spectral feature used to measure black hole spin. A total
of 720 reflection spectra are provided in a single FITS
file{\url{http://hea-www.cfa.harvard.edu/~javier/xillver/}} suitable for the
analysis of X-ray observations via the atable model in XSPEC. Detailed
comparisons with previous reflection models illustrate the improvements
incorporated in this version of XILLVER.Comment: 70 pages, 21 figures, submitted to Ap
The X-ray Outburst of H1743-322: High-Frequency QPOs with a 3:2 Frequency Ratio
We observed the 2003 X-ray outburst of H1743-322 in a series of 130 pointed
observation with RXTE. We searched individual observations for high-frequency
QPOs (HFQPOs) and found only weak or marginal detections near 240 and 160 Hz.
We next grouped the observations in several different ways and computed the
average power-density spectra (PDS) in a search for further evidence of HFQPOs.
This effort yielded two significant results for those observations defined by
the presence of low-frequency QPOs (0.1-20 Hz) and an absence of
``band-limited'' power continua: (1) The 9 time intervals with the highest 7-35
keV count rates yielded an average PDS with a QPO at Hz. (; 3--35 keV); and (2) a second group with lower 7-35 keV count rates (26
intervals) produced an average PDS with a QPO at Hz (;
7--35 keV). The ratio of these two frequencies is . This finding
is consistent with results obtained for three other black hole systems that
exhibit commensurate HFQPOs in a 3:2 ratio. Furthermore, the occurrence of
H1743-322's slower HFQPO at times of higher X-ray luminosity closely resembles
the behavior of XTE J1550-564 and GRO J1655-40. We discuss our results in terms
of a resonance model that invokes frequencies set by general relativity for
orbital motions near a black-hole event horizon.Comment: 12 pages, 3 figures, submitted to Ap
HST/STIS UV Spectroscopy of Two Quiescent X-ray Novae: A0620-00 and Centaurus X-4
In 1998 we made UV spectroscopic observations with HST/STIS of A0620-00 and
Cen X-4, which are two X-ray novae (aka soft X-ray transients). These binary
systems are similar in all respects except that the former contains a black
hole and the latter contains a neutron star. A UV spectrum (1700-3100A) is
presented for the quiescent state of each system in the context of previously
published UV/optical and X-ray data. The non-stellar, continuum spectrum of
black hole A0620-00 has a prominent UV/optical peak centered at about 3500A. In
contrast the spectrum of neutron-star Cen X-4 lacks a peak and rises steadily
with frequency over the entire UV/optical band. In the optical, the two systems
are comparably luminous. However, black hole A0620-00 is about 6 times less
luminous at 1700A, and about 40 times less luminous in the X-ray band. The
broadband spectrum of A0620-00 is discussed in terms of the advection-dominated
accretion flow model.Comment: 18 pages including 4 figures; tentatively scheduled for the March 10,
2000 issue of ApJ; minor revision
Anti-correlated Soft Lags in the Intermediate State of Black Hole Source GX 339-4
We report the few hundred second anti-correlated soft lags between soft and
hard energy bands in the source GX 339-4 using RXTE observations. In one
observation, anti-correlated soft lags were observed using the ISGRI/INTEGRAL
hard energy band and the PCA/RXTE soft energy band light curves. The lags were
observed when the source was in hard and soft intermediate states, i.e., in a
steep power-law state.We found that the temporal and spectral properties were
changed during the lag timescale. The anti-correlated soft lags are associated
with spectral variability during which the geometry of the accretion disk is
changed. The observed temporal and spectral variations are explained using the
framework of truncated disk geometry. We found that during the lag timescale,
the centroid frequency of quasi-periodic oscillation is decreased, the soft
flux is decreased along with an increase in the hard flux, and the power-law
index steepens together with a decrease in the disk normalization parameter. We
argue that these changes could be explained if we assume that the hot corona
condenses and forms a disk in the inner region of the accretion disk. The
overall spectral and temporal changes support the truncated geometry of the
accretion disk in the steep power-law state or in the intermediate state.Comment: published in ApJ, 9 pages, 8 figure
A Chandra Observation of the Diffuse Emission Centered on the Low Mass X-ray Binary 4U 1755-33
We present an analysis of a Chandra observation of the field surrounding the
low-mass X-ray binary 4U 1755-33, which has been in quiescence since 1996. In
2003, Angelini and White reported the appearance of a narrow 7 arcmin long
jetlike feature centered on the position of 4U 1755-33 using the XMM-Newton
telescope. Though the source and jet are not visibly apparent in our
Chandra/ACIS-S image, there is a significant excess (4-6 sigma) of counts in a
region that encloses the jet when compared to adjacent regions. We examined a
knot of emission in the jet that was detected by XMM-Newton but not by Chandra
and calculated that if the knot flux observed by XMM-Newton was concentrated in
a point source, Chandra would have easily detected it; we therefore conclude
that this knot of emission is diffuse. In summary, we suggest that the jetlike
feature found previously in the XMM-Newton data is quite diffuse and likely a
true jet, and is not due to a chance alignment of discrete point sources or
point-like regions of emission associated with internal shocks.Comment: 5 pages, color figures, uses emulateapj.cls and apjfonts.sty,
submitted to ApJ Letter
Targeting Health Disparities: A Model Linking Upstream Determinants To Downstream Interventions
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