119 research outputs found
Testing the cooling flow model in the intermediate polar EX Hydrae
We use the best available X-ray data from the intermediate polar EX Hydrae to
study the cooling-flow model often applied to interpret the X-ray spectra of
these accreting magnetic white dwarf binaries. First, we resolve a
long-standing discrepancy between the X-ray and optical determinations of the
mass of the white dwarf in EX Hya by applying new models of the inner disk
truncation radius. Our fits to the X-ray spectrum now agree with the white
dwarf mass of 0.79 Msun determined using dynamical methods through
spectroscopic observations of the secondary. We use a simple isobaric cooling
flow model to derive the emission line fluxes, emission measure distribution,
and H-like to He-like line ratios for comparison with the 496 ks Chandra High
Energy Transmission Grating observation of EX Hydrae. We find that the H/He
ratios are not well reproduced by this simple isobaric cooling flow model and
show that while H-like line fluxes can be accurately predicted, fluxes of
lower-Z He-like lines are significantly underestimated. This discrepancy
suggests that some extra heating mechanism plays an important role at the base
of the accretion column, where cooler ions form. We thus explored more complex
cooling models including the change of gravitational potential with height in
the accretion column and a magnetic dipole geometry. None of these
modifications to the standard cooling flow model are able to reproduce the
observed line ratios. While a cooling flow model with subsolar (0.1 )
abundances is able to reproduce the line ratios by reducing the cooling rate at
temperatures lower than K, the predicted line-to-continuum
ratios are much lower than observed. We discuss and discard mechanisms such as
photoionization, departures from constant pressure, resonant scattering,
different electron-ion temperatures, and Compton cooling. [Abridged]Comment: Accepted in Astronomy & Astrophysics, modified version after referee
comments and proof correction
X-ray Light Curves and Accretion Disk Structure of EX Hydrae
We present X-ray light curves for the cataclysmic variable EX Hydrae obtained
with the Chandra High Energy Transmission Grating Spectrometer and the Extreme
Ultraviolet Explorer Deep Survey photometer. We confirm earlier results on the
shape and amplitude of the binary light curve and discuss a new feature: the
phase of the minimum in the binary light curve, associated with absorption by
the bulge on the accretion disk, increases with wavelength. We discuss several
scenarios that could account for this trend and conclude that, most likely, the
ionization state of the bulge gas is not constant, but rather decreases with
binary phase. We also conclude that photoionization of the bulge by radiation
originating from the white dwarf is not the main source of ionization, but that
it is heated by shocks originating from the interaction between the inflowing
material from the companion and the accretion disk. The findings in this paper
provide a strong test for accretion disk models in close binary systems.Comment: 19 pages, 4 figures, accepted for publication in the Ap
Recommended from our members
High Spectral Resolution X-ray Observation of Magnetic CVs: EX Hya
In magnetic cataclysmic variables (CVs) the primary is a highly magnetized white dwarf (WD) whose field controls the accretion flow close to the WD, leading to a shock and accretion column that radiate chiefly in X-rays. We present preliminary results from a 500 ks Chandra HETG observation of the brightest magnetic CV EX Hya. From the observational dataset we are able to measure the temperature and density at different points of the cooling accretion column using sensitive line ratios. We also construct line-based light curves to search for rotational modulation of the X-ray emission
Accretion column structure of magnetic cataclysmic variables from X-ray spectroscopy
Using Chandra HETG data we present light curves for individual spectral lines
of Mg XI and Mg XII for EX Hydrae, an intermediate-polar type cataclysmic
variable. The Mg XI light curve, folded on the white dwarf spin period, shows
two spikes that are not seen in the Mg XII or broad-band light curves.
Occultation of the accretion column by the body of the white dwarf would
produce such spikes for an angle between the rotation axis and the accretion
columns of alpha = 18 degrees and a height of the Mg XI emission above the
white dwarf surface of < 0.0004 white dwarf radii or < 4 km. The absence of
spikes in the Mg XII and broad-band light curves could then be explained if the
bulk of its emission forms at much larger height, > 0.004 white dwarf radii or
> 40 km, above the white dwarf surface, although this is not consistent with
the predictions of the standard Aizu model of the accretion column.Comment: 12 pages, 3 figures, accepted for publication in the ApJ Letter
The Ionized Nuclear Environment in NGC 985 as seen by Chandra and BeppoSAX
(Abridged) We investigate the ionized environment of the Seyfert 1 galaxy NGC
985 with a new Chandra-HETGS observation and an archival BeppoSAX observation.
Two absorption components are clearly required to fit absorption features
observed in the Chandra spectrum. The components have a difference of 29 in
ionization parameter and 3 in column density. The presence of the low
ionization component is evidenced by an Fe M-shell unresolved transition array
(UTA) produced by charge states VII to XIII. The high ionization phase is
required by the presence of broad absorption features arising from several
blends of Fe L-shell transitions (Fe XVII-XXII). A third highly ionized
component might also be present, but the data does not allow to constrain its
properties. An X-ray luminosity variation by a factor 2.3 is observed between
the BeppoSAX and Chandra observations (separated by almost 3 years).
Variability in the opacity of the absorbers is detected in response to the
continuum variation, but while the colder component is consistent with a simple
picture of photoionization equilibrium, the ionization state of the hotter
component seems to increase while the continuum flux drops. The most striking
result in our analysis is that during both the Chandra and the BeppoSAX
observations, the two absorbing components appear to have the same pressure.
Thus, we suggest that the absorption arises from a multi-phase wind. Such
scenario can explain the change in opacity of both absorption components during
the observations, but requires that a third hotter component is
pressure-confining the two phases. Hence, our analysis points to a 3-phase
medium similar to the wind found in NGC 3783, and further suggests that such a
wind might be a common characteristic in AGN.Comment: Accepted for publication in ApJ. 6 tables and 17 figure
TW Hya: Spectral Variability, X-Rays, and Accretion Diagnostics
The nearest accreting T Tauri star, TW Hya was observed with spectroscopic
and photometric measurements simultaneous with a long se gmented exposure using
the CHANDRA satellite. Contemporaneous optical photometry from WASP-S indicates
a 4.74 day period was present during this time. Absence of a similar
periodicity in the H-alpha flux and the total X-ray flux points to a different
source of photometric variations. The H-alpha emission line appears
intrinsically broad and symmetric, and both the profile and its variability
suggest an origin in the post-shock cooling region. An accretion event,
signaled by soft X-rays, is traced spectroscopically for the first time through
the optical emission line profiles. After the accretion event, downflowing
turbulent material observed in the H-alpha and H-beta lines is followed by He I
(5876A) broadening. Optical veiling increases with a delay of about 2 hours
after the X-ray accretion event. The response of the stellar coronal emission
to an increase in the veiling follows about 2.4 hours later, giving direct
evidence that the stellar corona is heated in part by accretion. Subsequently,
the stellar wind becomes re-established. We suggest a model that incorporates
this sequential series of events: an accretion shock, a cooling downflow in a
supersonically turbulent region, followed by photospheric and later, coronal
heating. This model naturally explains the presence of broad optical and
ultraviolet lines, and affects the mass accretion rates determined from
emission line profiles.Comment: 61 pages; 22 figures; to appear in The Astrophysical Journa
Soft X-ray emission lines of Fe XV in solar flare observations and the Chandra spectrum of Capella
Recent calculations of atomic data for Fe XV have been used to generate
theoretical line ratios involving n = 3-4 transitions in the soft X-ray
spectral region (52-83 A), for a wide range of electron temperatures and
densities applicable to solar and stellar coronal plasmas. A comparison of
these with solar flare observations from a rocket-borne spectrograph (XSST)
reveals generally good agreement between theory and experiment. In particular,
the 82.76 A emission line in the XSST spectrum is identified, for the first
time to our knowledge in an astrophysical source. Most of the Fe XV transitions
which are blended have had the species responsible clearly identified, although
there remain a few instances where this has not been possible. The line ratio
calculations are also compared with a co-added spectrum of Capella obtained
with the Chandra satellite, which is probably the highest signal-to-noise
observation achieved for a stellar source in the 25-175 A soft X-ray region.
Good agreement is found between theory and experiment, indicating that the Fe
XV lines are reliably detected in Chandra spectra, and hence may be employed as
diagnostics to determine the temperature and/or density of the emitting plasma.
However the line blending in the Chandra data is such that individual emission
lines are difficult to measure accurately, and fluxes may only be reliably
determined via detailed profile fitting of the observations. The co-added
Capella spectrum is made available to hopefully encourage further exploration
of the soft X-ray region in astronomical sources.Comment: 27 pages, 10 figures, Astrophysical Journal, in pres
- âŠ