258 research outputs found
On the iron Kalpha complex in magnetic cataclysmic variables
We present a compilation of spectra of the iron Kalpha region in magnetic
cataclysmic variables, using data from the Chandra-HETG. The H-like, He-like
and fluorescent components are clearly resolved, and there are hints of the
structure within each component. The different shape of the He-like component
in AM Her might be related to greater cyclotron cooling in this star. A
surprising absence of Doppler shifts in the H-like and He-like components
implies that the X-ray emission is predominantly from the denser,
lower-velocity base of the accretion columns. This absence will allow ASTRO-E2
to resolve the structure in each component, leading to temperature diagnostics.
We do not confirm the report that the H-like and He-like components of AO Psc
are Compton broadened; however we do detect a Compton-downshifted shoulder to
the fluorescent line of GK Per. Further, a Doppler-shifted wing of this line
arises in the high-velocity, pre-shock flow.Comment: To appear in MNRAS; 5 page
X-ray and UV emission from the recurrent nova RS Ophiuchi in quiescence: Signatures of accretion and shocked gas
RS Ophiuchi is a recurrent nova system that experiences outbursts every ~20
years, implying accretion at a high rate onto a massive white dwarf. However,
previous X-ray observations of the system in quiescence have detected only
faint emission that is difficult to reconcile with the high accretion rate
predicted by nova theory for such frequent outbursts. Here, we use new Chandra
and XMM-Newton observations obtained 537 and 744 days after the 2006 outburst
to constrain both the accretion rate onto the white dwarf and the properties of
the nova ejecta at these times. We detect low level UV variability with the
XMM-Newton Optical Monitor on day 744 that is consistent with accretion disk
flickering, and use this to place a lower limit on the accretion rate. The
X-ray spectra in both observations are well described by a two component
thermal plasma model. The first component originates in the nova shell, which
can emit X-rays for up to a decade after the outburst. The other component
likely arises in the accretion disk boundary layer, and can be equally well fit
by a single temperature plasma or a cooling flow model. Although the flux of
the single temperature model implies an accretion rate that is 40 times lower
than theoretical predictions for RS Oph, the best fit cooling flow model
implies Mdot < 1.2x10^-8 M_sol/yr 537 days after the outburst, which is within
a factor of 2 of the theoretical accretion rate required to power an outburst
every 20 years. Furthermore, we place an upper limit on the accretion rate
through an optically thick region of the boundary layer of 2.0x10^-8 M_sol/yr.
Thus, the X-ray emission in quiescence is consistent with the accretion rate
expectations of nova theory. Finally, we discuss the possible origins of the
low temperature associated with the accretion component, which is a factor of
10 lower than in T CrB, an otherwise similar recurrent nova.Comment: 16 pages, 6 figures, accepted for publication in Ap
Outbursts of EX Hydrae: mass-transfer events or disc instabilities?
We present the 45-yr record of EX Hya's lightcurve and discuss the
characteristics of its 15 observed outbursts. We then concentrate on the 1998
outburst, reporting the first outburst X-ray observations. We discover an X-ray
beat-cycle modulation, indicating that an enhanced accretion stream couples
directly with the magnetosphere in outburst, confirming our previous
prediction. Optical eclipse profiles late in outburst show that the visible
light is dominated by an enhanced mass-transfer stream overflowing the
accretion disc. We are uncertain whether the enhanced mass transfer is
triggered by a disc instability, or by some other cause. While in outburst, EX
Hya shows some of the characteristics of SW Sex stars.Comment: To appear in MNRAS (8 pages; 9 figs
RS Ophiuchi: Thermonuclear Explosion or Disc Instability?
Sokoloski et al (2008) have recently reported evidence that the recurrent
nova RS Ophiuchi produced a pair of highly collimated radio jets within days of
its 2006 outburst. This suggests that an accretion disc must be present during
the outburst. However in the standard picture of recurrent novae as
thermonuclear events, any such disc must be expelled from the white dwarf
vicinity, as the nuclear energy yield greatly exceeds its binding energy. We
suggest instead that the outbursts of RS Oph are thermal--viscous instabilities
in a disc irradiated by the central accreting white dwarf. The distinctive
feature of RS Oph is the very large size of its accretion disc. Given this, it
fits naturally into a consistent picture of systems with unstable accretion
discs. This picture explains the presence and speed of the jets, the brightness
and duration of the outburst, and its rise time and linear decay, as well as
the faintness of the quiescence. By contrast, the hitherto standard picture of
recurrent thermonuclear explosions has a number of severe difficulties. These
include the presence of jets, the faintness of quiescence, and the fact the the
accretion disc must be unstable unless it is far smaller than any reasonable
estimate.Comment: MNRAS, in pres
A burst from the direction of UZ Fornacis with XMM-Newton
The XMM-Newton pointing towards the magnetic cataclysmic variable UZ For
finds the source to be a factor > 10^3 fainter than previous EXOSAT and ROSAT
observations. The source was not detected for the majority of a 22 ksec
exposure with the EPIC cameras, suggesting that the accretion rate either
decreased, or stopped altogether. However a 1.1 ksec burst was detected from UZ
For during the observation. Spectral fits favour optically thin, kT = 4.4 keV
thermal emission. Detection of the burst by the on-board Optical Monitor
indicates that this was most probably an accretion event. The 0.1-10 keV
luminosity of 2.1 x 10^30 erg/s is typical for accretion shock emission from
high state polars and would result from the potential energy release of ~ 10^16
g of gas. There is no significant soft excess due to reprocessing in the white
dwarf atmosphere.Comment: 7 pages, 2 postscript figures, ApJL, in pres
A new soft X-ray spectral model for polars with an application to AM Herculis
We present a simple heuristic model for the time-averaged soft X-ray
temperature distribution in the accretion spot on the white dwarf in polars.
The model is based on the analysis of the Chandra LETG spectrum of the
prototype polar AM Her and involves an exponential distribution of the emitting
area vs. blackbody temperature a(T) = a0 exp(-T/T0). With one free parameter
besides the normalization, it is mathematically as simple as the single
blackbody, but is physically more plausible and fits the soft X-ray and
far-ultraviolet spectral fluxes much better. The model yields more reliable
values of the wavelength-integrated flux of the soft X-ray component and the
implied accretion rate than reported previously.Comment: 5 pages, 3 figures, accepted for publication in Astronomy &
Astrophysic
The Far-Ultraviolet Spectrum and Short Timescale Variability of AM Herculis from Observations with the Hopkins Ultraviolet Telescope
Using the Hopkins Ultraviolet Telescope (HUT), we have obtained 850-1850
angstrom spectra of the magnetic cataclysmic variable star AM Her in the high
state. These observations provide high time resolution spectra of AM Her in the
FUV and sample much of the orbital period of the system. The spectra are not
well-modelled in terms of simple white dwarf (WD) atmospheres, especially at
wavelengths shortward of Lyman alpha. The continuum flux changes by a factor of
2 near the Lyman limit as a function of orbital phase; the peak fluxes are
observed near magnetic phase 0.6 when the accreting pole of the WD is most
clearly visible. The spectrum of the hotspot can be modelled in terms of a 100
000 K WD atmosphere covering 2% of the WD surface. The high time resolution of
the HUT data allows an analysis of the short term variability and shows the UV
luminosity to change by as much as 50% on timescales as short as 10 s. This
rapid variability is shown to be inconsistent with the clumpy accretion model
proposed to account for the soft X-ray excess in polars. We see an increase in
narrow line emission during these flares when the heated face of the secondary
is in view. The He II narrow line flux is partially eclipsed at secondary
conjunction, implying that the inclination of the system is greater than 45
degrees. We also present results from models of the heated face of the
secondary. These models show that reprocessing on the face of the secondary
star of X-ray/EUV emission from the accretion region near the WD can account
for the intensities and kinematics of most of the narrow line components
observed.Comment: 19 pp., 12 fig., 3 tbl. To appear in The Astrophysical Journal. Also
available at http://greeley.pha.jhu.edu/papers/amherpp.ps.g
The emission-line pulse pattern in the intermediate polar RX J0558
We observed the intermediate polar RX J0558+53 with the 4.2m WHT and find in
the pulsed emission lines, a ``corkscrew'' pattern, which indicates a two-pole
white dwarf accretion. The ``corkscrew'' pattern consists of two emission-line
pulses, separated by half the white dwarf spin period, and moving from red to
blue velocities. The detected emission-line pulsations have an amplitude of
1.1--2.7 per cent in the He II and Balmer emission lines on the 545-s spin
period of the white dwarf which compare to 3.5-4.8 per cent for the continuum
double-peak pulsations. We image the emission-line pulse pattern and is shown
to lag the continuum pulse by 0.12 spin cycles. We interpret the pattern by
invoking an accretion curtain from the disrupted, inner disc to the two poles
of the magnetic white dwarf. The semi-amplitude of the He II pulse of 408+-35
km/s can be used to constrain the size of the magnetosphere, R ~ 4.1 x 10^4 km,
and the magnetic moment of the white dwarf (~2.4 10^32 G cm^3). Power spectra
show dominant frequencies at 2 \omega and 2 (\omega-\Omega) which suggest
reprocessing of the white dwarf's illuminating beams in the accretion disc.
Finally, the steady He II emission line shows a strong sinusoidal component
moving from red to blue on the orbital period, with a width similar to that
expected from irradiation of the secondary star. Imaging of the emission lines
indicate illuminated locations at the inner side of the red star and the back
side of the accretion disc.Comment: 12 pages, 11 figures, MNRAS journal paper. in pres
Syzygies of torsion bundles and the geometry of the level l modular variety over M_g
We formulate, and in some cases prove, three statements concerning the purity
or, more generally the naturality of the resolution of various rings one can
attach to a generic curve of genus g and a torsion point of order l in its
Jacobian. These statements can be viewed an analogues of Green's Conjecture and
we verify them computationally for bounded genus. We then compute the
cohomology class of the corresponding non-vanishing locus in the moduli space
R_{g,l} of twisted level l curves of genus g and use this to derive results
about the birational geometry of R_{g, l}. For instance, we prove that R_{g,3}
is a variety of general type when g>11 and the Kodaira dimension of R_{11,3} is
greater than or equal to 19. In the last section we explain probabilistically
the unexpected failure of the Prym-Green conjecture in genus 8 and level 2.Comment: 35 pages, appeared in Invent Math. We correct an inaccuracy in the
statement of Prop 2.
XMM-Newton and Swift observations of WZ Sge: spectral and timing analysis
WZ Sagittae is the prototype object of a subclass of dwarf novae, with rare
and long (super)outbursts, in which a white dwarf primary accretes matter from
a low mass companion. High-energy observations offer the possibility of a
better understanding of the disk-accretion mechanism in WZ Sge-like binaries.
We used archival XMM-Newton and Swift data to characterize the X-ray spectral
and temporal properties of WZ Sge in quiescence. We performed a detailed timing
analysis of the simultaneous X-ray and UV light curves obtained with the EPIC
and OM instruments on board XMM-Newton in 2003. We employed several techniques
in this study, including a correlation study between the two curves. We also
performed an X-ray spectral analysis using the EPIC data, as well as Swift/XRT
data obtained in 2011. We find that the X-ray intensity is clearly modulated at
a period of about 28.96 s, confirming previously published preliminary results.
We find that the X-ray spectral shape of WZ Sge remains practically unchanged
between the XMM-Newton and Swift observations. However, after correcting for
inter-stellar absorption, the intrinsic luminosity is estimated to be about
2.65X10^ 30 erg/s/cm^2 and 1.57X10^30 erg/s/cm^2 in 2003 and 2011,
respectively. During the Swift/XRT observation, the observed flux is a factor
of about 2 lower than that observed by XMM-Newton, but is similar to the
quiescent levels observed various times before the 2001 outburst.Comment: Accepted for publication on A&A.10 pages, 9 figure
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