132 research outputs found
Dynamical Ne K Edge and Line Variations in the X-Ray Spectrum of the Ultra-compact Binary 4U 0614+091
We observed the ultra-compact binary candidate 4U 0614+091 for a total of 200
ksec with the high-energy transmission gratings onboard the \chandra X-ray
Observatory. The source is found at various intensity levels with spectral
variations present. X-ray luminosities vary between 2.0 \ergsec
and 3.5 \ergsec. Continuum variations are present at all times
and spectra can be well fit with a powerlaw component, a high kT blackbody
component, and a broad line component near oxygen. The spectra require
adjustments to the Ne K edge and in some occasions also to the Mg K edge. The
Ne K edge appears variable in terms of optical depths and morphology. The edge
reveals average blue- and red-shifted values implying Doppler velocities of the
order of 3500 \kms. The data show that Ne K exhibits excess column densities of
up to several 10 cm. The variability proves that the excess is
intrinsic to the source. The correponding disk velocities also imply an outer
disk radius of the order of cm consistent with an ultra-compact binary
nature. We also detect a prominent soft emission line complex near the \oviii
L position which appears extremely broad and relativistic effects from
near the innermost disk have to be included. Gravitationally broadened line
fits also provide nearly edge-on angles of inclination between 86 and
89. The emissions appear consistent with an ionized disk with
ionization parameters of the order of 10 at radii of a few 10 cm. The
line wavelengths with respect to \oviiia\ are found variably blue-shifted
indicating more complex inner disk dynamics.Comment: 24 pages, 8 figures, submitted to the Astrophyscial Main Journa
High-Resolution X-ray Spectroscopy of the Interstellar Medium: Structure at the Oxygen Absorption Edge
(Abbrev.) We present high-resolution spectroscopy of the oxygen K-shell
interstellar absorption edge in 7 X-ray binaries using the HETGS onboard
Chandra. Using the brightest sources as templates, we found a best-fit model of
2 absorption edges and 5 Gaussian absorption lines. All of these features can
be explained by the recent predictions of K-shell absorption from neutral and
ionized atomic oxygen. We identify the K alpha and K beta absorption lines from
neutral oxygen, as well as the S=3/2 absorption edge. The expected S=1/2 edge
is not detected in these data due to overlap with instrumental features. We
also identify the K alpha absorption lines from singly and doubly ionized
oxygen. The OI K alpha absorption line is used as a benchmark with which to
adjust the absolute wavelength scale for theoretical predictions of the
absorption cross-sections. We find that shifts of 30-50 mA are required,
consistent with differences previously noticed from comparisons of the theory
with laboratory measurements. Significant oxygen features from dust or
molecular components, as suggested in previous studies, are not required by our
HETGS spectra. With these spectra, we can begin to measure the large-scale
properties of the ISM. We place a limit on the velocity dispersion of the
neutral lines of <200 km s^{-1}, consistent with measurements at other
wavelengths. We also make the first measurement of the oxygen ionization
fractions in the ISM. We constrain the interstellar ratio of OII/OI to ~0.1 and
the ratio of OIII/OI to <0.1.Comment: 12 pages, 8 figures, accepted for publication in the Astrophysical
Journal (Vol. 612, September 1 issue
Low-energy absorption towards the ultra-compact binary 4U1850-087 located in the globular cluster NGC6712
We report the results of two XMM-Newton observations of the ultra-compact
low-mass X-ray binary 4U1850-087 located in the galactic globular cluster
NGC6712. A broad emission feature at 0.7keV was detected in an earlier ASCA
observation and explained as the result of an unusual Ne/O abundance ratio in
the absorbing material local to the source. We find no evidence for this
feature and derive Ne/O ratios in the range 0.14-0.21, consistent with that of
the interstellar medium. During the second observation, when the source was 10%
more luminous, there is some evidence for a slightly higher Ne/O ratio and
additional absorption. Changes in the Ne/O abundance ratio have been detected
from another ultra-compact binary, 4U1543-624. We propose that these changes
result from an X-ray induced wind which is evaporated from an O and Ne rich
degenerate donor. As the source X-ray intensity increases so does the amount of
evaporation and hence the column densities and abundance ratio of Ne and O.Comment: 9 pages, 6 figures, accepted for publication in Astronomy and
Astrophysic
On the ultra-compact nature of 4U1822-000
We report the discovery of a periodic modulation in the optical lightcurve of
the candidate ultra-compact X-ray binary 4U1822-000. Using time-resolved
optical photometry taken with the William Herschel Telescope we find evidence
for a sinusoidal modulation with a semi-amplitude of 8 percent and a period of
191 min, which is most likely close to the true orbital period of the binary.
Using the van Paradijs & McClintock relation for the absolute magnitude and the
distance modulus allowing for interstellar reddening, we estimate the distance
to 4U1822-000 to be 6.3 kpc. The long orbital period and casts severe doubts on
the ultra-compact nature of 4U1822-000.Comment: 3 pages, 1 figure, accepted by MNRA
XMM-Newton observations of the low-mass X-ray binary XB1832-330 in the galactic globular cluster NGC 6652
We report on two XMM-Newton observations performed in 2006 of the luminous
low-mass X-ray binary XB1832-330 which is located in the galactic globular
cluster NGC 6652 and is probably an ultracompact binary (orbital period less
than 1 hour). The aim of these observations is to investigate the low-energy
absorption towards NGC6652 and in particular to search for Ne-rich material
local to the binary, which has been suggested as a possible spectral signature
of Neon rich degenerate companions. XMM-Newton observed the source twice, in
2006 September and October. High resolution spectroscopy with the RGS was used
to estimate the ratio of the neutral neon to oxygen column densities to search
for an anomalous Ne abundance in this X-ray binary. We find no evidence for
anomalous Ne/O ratios, finding Ne/O=0.18+/-0.06 and Ne/O=0.17+/-0.03 (1 sigma
uncertainties), in the two observations, respectively. These values are
consistent with that in the interstellar medium. Timing analysis of EPIC data
suggests possible periodicities at 9170+/-235 s and 18616 +/- 531 s, which need
confirmation. A by-product of these observations consists of the spatial
analysis of the source field, which resulted in the detection of 46 faint
sources within the EPIC field of view, which are not present in the Second
XMM-Newton Serendipitous Source Catalogue. All these faint sources are likely
foreground objects. We performed the first high spectral resolution
observations of XB1832-33, a probable ultracompact binary, without finding any
evidence for an anomalous Ne abundance.Comment: Accepted for publication in A&
The neutron star soft X-ray transient 1H1905+000 in quiescence
In this Paper we report on our analysis of a ~25 ksec. Chandra X-ray
observation of the neutron star soft X-ray transient (SXT) 1H1905+000 in
quiescence. Furthermore, we discuss our findings of the analysis of optical
photometric observations which we obtained using the Magellan telescope and
photometric and spectroscopic observations which we obtained using the Very
Large Telescope at Paranal. The X-ray counterpart of 1H1905+000 was not
detected in our Chandra data, with a 95 per cent confidence limit to the source
count rate of 1.2x10^-4 counts s^-1. For different spectral models this yields
an upper limit on the luminosity of 1.8x10^31 erg s^-1 (for an upper limit on
the distance of 10 kpc.) This luminosity limit makes 1H1905+000 the faintest
neutron star SXT in quiescence observed to date. The neutron star luminosity is
so low that it is similar to the lowest luminosities derived for black hole
SXTs in quiescence. This low luminosity for a neutron star SXT challanges the
hypothesis presented in the literature that black hole SXTs in quiescence have
lower luminosities than neutron star SXTs as a result of the presence of a
black hole event horizon. Furthermore, the limit on the neutron star luminosity
obtained less than 20 years after the outburst has ceased, constrains the
thermal conductivity of the neutron star crust. Finally, the neutron star core
must be so cold that unless the time averaged mass accretion rate is lower than
2x10^-12 M_sun yr^-1, core cooling has to proceed via enhanced neutrino
emission processes. We derive a limit on the absolute I-band magnitude of the
quiescent counterpart of M_I>7.8 assuming the source is at 10 kpc. This is in
line with 1H1905+000 being an ultra-compact X-ray binary, as has been proposed
based on the low outburst V-band absolute magnitude.Comment: 10 pages, 5 figures, accepted for publication in MNRA
K-Shell Photoabsorption Studies of the Carbon Isonuclear Sequence
K-shell photoabsorption cross sections for the isonuclear C I - C IV ions
have been computed using the R-matrix method. Above the K-shell threshold, the
present results are in good agreement with the independent-particle results of
Reilman & Manson (1979). Below threshold, we also compute the strong 1s -> np
absorption resonances with the inclusion of important spectator Auger
broadening effects. For the lowest 1s -> 2p, 3p resonances, comparisons to
available C II, C III, and C IV experimental results show good agreement in
general for the resonance strengths and positions, but unexplained
discrepancies exist. Our results also provide detailed information on the C I
K-shell photoabsorption cross section including the strong resonance features,
since very limited laboratory experimental data exist. The resultant R-matrix
cross sections are then used to model the Chandra X-ray absorption spectrum of
the blazar Mkn 421
The ultra-compact binary 4U1850-087 observed with INTEGRAL: hard X-ray emission from an X-ray burster
The X-ray burster 4U1850-087, located in the Galactic globular cluster
NGC6712, is an ultracompact binary (orbital period~21 min),likely harbouring a
degenerate companion.The source has been observed at soft gamma-rays several
times with the INTEGRAL satellite,during the monitoring of the Galactic plane,
with an unprecedented exposure time.We analysed all available INTEGRAL
observations, with the main aim of studying the long-term behaviour of this
Galactic bulge X-ray burster.The spectral results are based on the systematic
analysis of all INTEGRAL observations covering the source position performed
between March 2003 and November 2005.The source X-ray emission is hard and is
observed, for the first time, up to 100keV.A broad-band spectrum obtained
combining the INTEGRAL spectrum together with a quasi-simultaneous XMM-Newton
observation performed in September 2003 is well modeled with a disk-blackbody
emission (with kTin=0.8keV) together with a power-law (photon index=2.1).The
2-100keV luminosity is 1.5E36 erg/s (assuming a distance of 6.8kpc).Comment: Accepted for publication in Astronomy & Astrophysic
Disk Dominated States of 4U 1957+11: Chandra, XMM, and RXTE Observations of Ostensibly the Most Rapidly Spinning Galactic Black Hole
We present simultaneous Chandra-HETG and RXTE observations of a moderate flux
`soft state' of the black hole candidate 4U1957+11. These spectra, having a
minimally discernible hard X-ray excess, are an excellent test of modern disk
atmosphere models that include the effects of black hole spin. The HETG data
show that the soft disk spectrum is only very mildly absorbed with N_H =1-2 X
10^{21} cm^-2. These data additionally reveal 13.449 A NeIX absorption
consistent with the warm/hot phase of the interstellar medium. The fitted disk
model implies a highly inclined disk around a low mass black hole rapidly
rotating with normalized spin a*~1. We show, however, that pure Schwarzschild
black hole models describe the data extremely well, albeit with large disk
atmosphere ``color-correction'' factors. Standard color-correction factors can
be attained if one additionally incorporates mild Comptonization. We find that
the Chandra observations do not uniquely determine spin. Similarly, XMM/RXTE
observations, taken only six weeks later, are equally unconstraining. This lack
of constraint is partly driven by the unknown mass and unknown distance of
4U1957+11; however, it is also driven by the limited bandpass of Chandra and
XMM. We therefore present a series of 48 RXTE observations taken over the span
of several years and at different brightness/hardness levels. These data prefer
a spin of a*~1, even when including a mild Comptonization component; however,
they also show evolution of the disk atmosphere color-correction factors. If
the rapid spin models with standard atmosphere color-correction factors of
h_d=1.7 are to be believed, then the RXTE observations predict that 4U1957+11
can range from a 3 M_sun black hole at 10 kpc with a*~0.83 to a 16 M_sun black
hole at 22 kpc with a* ~ 1, with the latter being statistically preferred.Comment: 16 pages (emulateapj style). Accepted for Publication in the
Astrophysical Journa
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