133 research outputs found
The Fe XXII I(11.92 A)/I(11.77 A) Density Diagnostic Applied to the Chandra High Energy Transmission Grating Spectrum of EX Hydrae
Using the Livermore X-ray Spectral Synthesizer, which calculates spectral
models of highly charged ions based primarily on HULLAC atomic data, we
investigate the temperature, density, and photoexcitation dependence of the
I(11.92 A)/I(11.77 A) line ratio of Fe XXII. We find that this line ratio has a
critical density n_c \approx 5x10^13 cm^-3, is approximately 0.3 at low
densities and 1.5 at high densities, and is very insensitive to temperature and
photoexcitation, so is a useful density diagnostic for sources like magnetic
cataclysmic variables in which the plasma densities are high and the efficacy
of the He-like ion density diagnostic is compromised by the presence of a
bright ultraviolet continuum. Applying this diagnostic to the Chandra High
Energy Transmission Grating spectrum of the intermediate polar EX Hya, we find
that the electron density of its T_e \approx 12 MK plasma is n_e =
1.0^{+2.0}_{-0.5} x 10^14 cm^-3, orders of magnitude greater than that
typically observed in the Sun or other late-type stars.Comment: 11 pages including 3 encapsulated postscript figures; LaTeX format,
uses aastex.cls; accepted on 2003 April 3 for publication in The
Astrophysical Journa
Atomic X-Ray Spectra of Accretion Disk Atmospheres in the Kerr Metric
We calculate the atmospheric structure of an accretion disk around a Kerr
black hole and obtain its X-ray spectrum, which exhibits prominent atomic
transitions under certain circumstances. The gravitational and Doppler
(red)shifts of the C V, C VI, O VII, O VIII, and Fe I-XXVI emission lines are
observable in active galaxies. We quantify the line emissivities as a function
of radius, to identify the effects of atmospheric structure, and to determine
the usefulness of these lines for probing the disk energetics. The line
emissivities do not always scale linearly with the incident radiative energy,
as in the case of Fe XXV and Fe XXVI. Our model incorporates photoionization
and thermal balance for the plasma, the hydrostatic approximation perpendicular
to the plane of the disk, and general relativistic tidal forces. We include
radiative recombination rates, fluorescence yields, Compton scattering, and
photoelectric opacities for the most abundant elements.Comment: 4 pages, 1 figure, to appear in the Proc. of the 10th Marcel
Grossmann Meeting on General Relativity, World Scientific, Rio de Janeiro,
July 20-26, 200
Correlation of the Quasi-Periodic Oscillation Frequencies of White Dwarf, Neutron Star, and Black Hole Binaries
Using data obtained in 1994 June/July with the Extreme Ultraviolet Explorer
deep survey photometer and in 2001 January with the Chandra X-ray Observatory
Low Energy Transmission Grating Spectrograph, we investigate the
extreme-ultraviolet (EUV) and soft X-ray oscillations of the dwarf nova SS Cyg
in outburst. We find quasi-periodic oscillations (QPOs) at nu_0 ~ 0.012 Hz and
nu_1 ~ 0.13 Hz in the EUV flux and at nu_0 ~ 0.0090 Hz, nu_1 ~ 0.11 Hz, and
possibly nu_2 ~ nu_0 + nu_1 ~ 0.12 Hz in the soft X-ray flux. These data,
combined with the optical data of Woudt & Warner for VW Hyi, extend the
Psaltis, Belloni, & van der Klis nu_high-nu_low correlation for neutron star
and black hole low-mass X-ray binaries (LMXBs) nearly two orders of magnitude
in frequency, with nu_low ~ 0.08 nu_high. This correlation identifies the
high-frequency quasi-coherent oscillations (so-called ``dwarf nova
oscillations'') of cataclysmic variables (CVs) with the kilohertz QPOs of
LMXBs, and the low-frequency QPOs of CVs with the horizontal branch
oscillations (or the broad noise component identified as such) of LMXBs.
Assuming that the same mechanisms produce the QPOs in white dwarf, neutron
star, and black hole binaries, we find that the data exclude the relativistic
precession model and the magnetospheric and sonic-point beat-frequency models
(as well as any model requiring the presence or absence of a stellar surface or
magnetic field); more promising are models that interpret QPOs as
manifestations of disk accretion onto any low-magnetic field compact object.Comment: 15 pages including 4 encapsulated postscript figures; LaTeX format,
uses aastex.cls; accepted on 2002 July 23 for publication in The
Astrophysical Journa
The X-ray transient XTE J1118+480: Multiwavelength observations of a low-state mini-outburst
We present multiwavelength observations of the newly discovered X-ray
transient XTE J1118+480 obtained in the rising phase of the 2000 April
outburst. This source is located at unusually high Galactic latitude and in a
very low absorption line of sight. This made the first EUVE spectroscopy of an
X-ray transient outburst possible. Together with our HST, RXTE, and UKIRT data
this gives unprecedented spectral coverage. We find the source in the low hard
state. The flat IR-UV spectrum appears to be a combination of optically thick
disk emission and flat spectrum emission, possibly synchrotron, while at higher
energies, including EUV, a typical low hard state power-law is seen. EUVE
observations reveal no periodic modulation, suggesting an inclination low
enough that no obscuration by the disk rim occurs. We discuss the nature of the
source and this outburst and conclude that it may be more akin to
mini-outbursts seen in GRO J0422+32 than to a normal X-ray transient outburst.Comment: 4 pages, 3 figures, replaced with accepted version. Uses
emulateapj5.st
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