60 research outputs found
Resonant Scattering and Recombination in CAL 87
The eclipsing supersoft X-ray binary CAL 87 has been observed with Chandra on
August 13/14, 2001 for nearly 100 ksec, covering two full orbital cycles and
three eclipses. The shape of the eclipse light curve derived from the
zeroth-order photons indicates that the size of the X-ray emission region is
about 1.5 solar radii. The ACIS/LETG spectrum is completely dominated by
emission lines without any noticeable continuum. The brightest emission lines
are significantly redshifted and double-peaked, suggestive of emanating in a
2000 km/s wind. We model the X-ray spectrum by a mixture of recombination and
resonant scattering. This allows us to deduce the temperature and luminosity of
the ionizing source to be kT = 50-100 eV and L_X = 5E37 erg/s.Comment: To appear in Proceedings of IAU Coll. 194 "Compact binaries in the
Galaxy and beyond" (Rev. Mex. A&A Conf. Series), eds. G. Tovmassian and E.
Sio
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
Neutral absorber dips in the periodic burster LMXB XB 1323-619 from Suzaku
We present results of an observation with Suzaku of the dipping, periodic
bursting low mass X-ray binary XB 1323-619. Using the energy band 0.8 - 70 keV,
we show that the source spectrum is well-described as the emission of an
extended accretion disk corona, plus a small contribution of blackbody emission
from the neutron star. The dip spectrum is well-fitted by the progressive
covering model in which the extended ADC is progressively overlapped by the
absorbing bulge of low ionization state in the outer accretion disk and that
dipping is basically due to photoelectric absorption in the bulge. An
energy-independent decrease of flux at high energies (20 - 70 keV) is shown to
be consistent with the level of Thomson scattering expected in the bulge. An
absorption feature at 6.67 keV (Fe XXV) is detected in the non-dip spectrum and
other possible weak features. In dipping, absorption lines of medium and highly
ionized states are seen suggestive of absorption in the ADC but there is no
evidence that the lines are stronger than in non-dip. We show that the
luminosity of the source has changed substantially since the Exosat observation
of 1985, increasing in luminosity between 1985 and 2003, then in 2003 - 2007
falling to the initial low value. X-ray bursting has again become periodic,
which it ceased to do in its highest luminosity state, and we find that the
X-ray bursts exhibit both the fast decay and later slow decay characteristic of
the rp burning process. We present arguments against the recent proposal that
the decrease of continuum flux in the dipping LMXB in general can be explained
as absorption in an ionized absorber rather than in the bulge in the outer disk
generally accepted to be the site of absorption.Comment: 12 pages, 6 figures, Astronomy and Astrophysics in pres
Development of thermally formed glass optics for astronomical hard x-ray telescopes
The next major observational advance in hard X-ray/soft gamma-ray astrophysics will come with the implementation of telescopes capable of focusing 10-200 keV radiation. Focusing allows high signal-to-noise imaging and spectroscopic observations of many sources in this band for the first time. The recent development of depth-graded multilayer coatings has made the design of telescopes for this bandpass practical, however the ability to manufacture inexpensive substrates with appropriate surface quality and figure to achieve sub-arcminute performance has remained an elusive goal. In this paper, we report on new, thermally-formed glass micro-sheet optics capable of meeting the requirements of the next-generation of astronomical hard X-ray telescopes
Identification of an Extended Accretion Disk Corona in the Hercules X-1 Low State: Moderate Optical Depth, Precise Density Determination, and Verification of CNO Abundances
We identify an accretion disk atmosphere and corona from the high resolution
X-ray spectrum of Hercules X-1, and we determine its detailed physical
properties. More than two dozen recombination emission lines (from Fe XXVI at
1.78 A to N VI at 29.08 A) and Fe K-alpha, K-beta fluorescence lines were
detected in a 50 ks observation with the Chandra High-Energy Transmission
Grating Spectrometer (HETGS). They allow us to measure the density,
temperature, spatial distribution, elemental composition, and kinematics of the
plasma. We exclude HZ Her as the source of the recombination emission. We
compare accretion disk model atmospheres with the observed spectrum in order to
constrain the stratification of density and ionization, disk atmosphere area,
elemental composition, and energetics. The atmospheric spectrum observed during
the low state is photoionized by the main-on X-ray continuum, indicating that
the disk is observed edge-on during the low state. We infer the mean number of
scatterings N of Ly-alpha and Ly-beta line photons from H-like ions. We derive
N < 69 for O VIII Ly_alpha_1, which rules out the presence of a mechanism
modeled by Sako (2003) to enhance N VII emission via a line overlap with O
VIII. The line optical depth diagnostics are consistent with a flattened
atmosphere. Our spectral analysis, the disk atmosphere model, and the presence
of intense N VII and N VI lines (plus N V in the UV), confirm the
over-abundance of nitrogen relative to other metals, which was shown to be
indicative of CNO cycle processing in a massive progenitor.Comment: 38 pages, 14 figures, accepted for publication in Ap
Swift monitoring of Cygnus X-2: investigating the NUV-X-ray connection
The neutron star X-ray binary (NSXRB) Cygnus X-2 was observed by the Swift
satellite 51 times over a 4 month period in 2008 with the XRT, UVOT, and BAT
instruments. During this campaign, we observed Cyg X-2 in all three branches of
the Z track (horizontal, normal, and flaring branches). We find that the NUV
emission is uncorrelated with the soft X-ray flux detected with the XRT, and is
anticorrelated with the BAT X-ray flux and the hard X-ray color. The observed
anticorrelation is inconsistent with simple models of reprocessing as the
source of the NUV emission. The anticorrelation may be a consequence of the
high inclination angle of Cyg X-2, where NUV emission is preferentially
scattered by a corona that expands as the disk is radiatively heated.
Alternatively, if the accretion disk thickens as Cyg X-2 goes down the normal
branch toward the flaring branch, this may be able to explain the observed
anticorrelation. In these models the NUV emission may not be a good proxy for
in the system. We also discuss the implications of using Swift/XRT to
perform spectral modeling of the continuum emission of NSXRBs.Comment: 10 pages, 8 figures. ApJ Accepte
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