320 research outputs found
The serendipituous discovery of a short-period eclipsing polar in 2XMMp
We report the serendipituous discovery of the new eclipsing polar 2XMMp
J131223.4+173659. Its striking X-ray light curve attracted immediate interest
when we were visually inspecting the source products of the 2XMMp catalogue.
This light curve revealed its likely nature as a magnetic cataclysmic variable
of AM Herculis (or polar) type with an orbital period of ~92 min, which was
confirmed by follow-up optical spectroscopy and photometry. 2XMMp
J131223.4+173659 probably has a one-pole accretion geometry. It joins the group
of now nine objects that show no evidence of a soft component in their X-ray
spectra despite being in a high accretion state, thus escaping ROSAT/EUVE
detection. We discuss the likely accretion scenario, the system parameters, and
the spectral energy distribution.Comment: Accepted for publication in A&
Probing the accretion processes in soft X-ray selected polars
High-energy data of accreting white dwarfs give access to the regime of the
primary accretion-induced energy release and the different proposed accretion
scenarios. We perform XMM-Newton observations of polars selected due to their
ROSAT hardness ratios close to -1.0 and model the emission processes in
accretion column and accretion region. Our models consider the
multi-temperature structure of the emission regions and are mainly determined
by mass-flow density, magnetic field strength, and white-dwarf mass. To
describe the full spectral energy distribution from infrared to X-rays in a
physically consistent way, we include the stellar contributions and establish
composite models, which will also be of relevance for future X-ray missions. We
confirm the X-ray soft nature of three polars.Comment: Accepted for publication in Acta Polytechnica, Proceedings of "The
Golden Age of Cataclysmic Variables and Related Objects II
A long-term optical and X-ray ephemeris of the polar EK Ursae Majoris
We searched for long-term period changes in the polar EK UMa using new
optical data and archival X-ray/EUV data. An optical ephemeris was derived from
data taken remotely with the MONET/N telescope and compared with the X-ray
ephemeris based on Einstein, Rosat, and EUVE data. A three-parameter fit to the
combined data sets yields the epoch, the period, and the phase offset between
the optical minima and the X-ray absorption dips. An added quadratic term is
insignificant and sets a limit to the period change. The derived linear
ephemeris is valid over 30 years and the common optical and X-ray period is
P=0.0795440225(24) days. There is no evidence of long-term O-C variations or a
period change over the past 17 years Delta P = -0.14+-0.50 ms. We suggest that
the observed period is the orbital period and that the system is tightly
synchronized. The limit on Delta P and the phase constancy of the bright part
of the light curve indicate that O-C variations of the type seen in the polars
DP Leo and HU Aqr or the pre-CV NN Ser do not seem to occur in EK UMa. The
X-ray dips lag the optical minima by 9.5+-0.7 deg in azimuth, providing some
insight into the accretion geometry.Comment: 4 pages, 2 Postscript figures, accepted for publication in Astronomy
& Astrophysic
Multi-epoch Doppler tomography and polarimetry of QQ Vul
We present multi-epoch high-resolution spectroscopy and photoelectric polarimetry of the long-period polar (AM Herculis star) QQ Vul. The blue emission lines show several distinct components, the sharpest of which can unequivocally be assigned to the illuminated hemisphere of the secondary star and used to trace its orbital motion. This narrow emission line can be used in combination with Nai-absorption lines from the photosphere of the companion to build a stable long-term ephemeris for the star: inferior conjunction of the companion occurs at HJD = 244 8446.4710(5)+E×0. d 15452011(11). The polarization curves are dissimilar at different epochs, thus supporting the idea of fundamental changes of the accretion geometry, e.g. between one- and two-pole accretion modes. The linear polarization pulses display a random scatter by 0.2 phase units and are not suitable for the determination of the binary period. The polarization data suggest that the magnetic (dipolar) axis has a co-latitude of 23 ◦ , an azimuth of −50 ◦, and an orbital inclination between 50 ◦ and 70 ◦. Doppler images of blue emission and red absorption lines show a clear separatio
The high-field polar RX J1007.5-2017
We report optical and X-ray observations of the high-field polar
RXJ1007.5-2017 performed between 1990 and 2012. It has an orbital period of
208.60 min determined from the ellipsoidal modulation of the secondary star in
an extended low state. The spectral flux of the dM3- secondary star yields a
distance of 790+-105 pc. At low accretion levels, \RX{} exhibits pronounced
cyclotron emission lines. The second and third harmonic fall in the optical
regime and yield a field strength in the accretion spot of 94 MG. The source is
highly variable on a year-to-year basis and was encountered at visual
magnitudes between V \sim 20 and V \sim 16. In the intermediate state of 1992
and 2000, the soft X-ray luminosity exceeds the sum of the luminosities of the
cyclotron source, the hard X-ray source, and the accretion stream by an order
of magnitude. An X-ray high state, corresponding to the brightest optical
level, has apparently not been observed so far.Comment: To be published in A&
Evidence for an oscillation of the magnetic axis of the white dwarf in the polar DP Leonis
From 1979 to 2001, the magnetic axis of the white dwarf in the polar DP Leo
slowly rotated by 50 deg in azimuth, possibly indicating a small asynchronism
between the rotational and orbital periods of the magnetic white dwarf. We have
obtained phase-resolved orbital light curves between 2009 and 2013, which show
that this trend has not continued in recent years. Our data are consistent with
the theoretically predicted oscillation of the magnetic axis of the white dwarf
about an equilibrium orientation, which is defined by the competition between
the accretion torque and the magnetostatic interaction of the primary and
secondary star. Our data indicate an oscillation period of ~60 yr, an amplitude
of about 25 deg, and an equilibrium orientation leading the connecting line of
the two stars by about 7 deg.Comment: Accepted by A&
Phase-resolved high-resolution spectrophotometry of the eclipsing polar HU Aquarii
We present phase-resolved spectroscopy of the bright, eclipsing polar HU Aqr obtained with high time (~30sec) and spectral (1.6 A) resolution when the system was in a high accretion state. The trailed spectrograms reveal clearly the presence of three different line components with different width and radial velocity variation. By means of Doppler tomography their origin could be located unequivocally (a) on the secondary star, (b) the ballistic part of the accretion stream (horizontal stream), and (c) the magnetically funnelled part of the stream (vertical stream). For the first time we were able to derive a (near-)complete map of the stream in a polar. We propose to use Doppler tomography of AM Herculis stars as a new tool for the mass determination of these binaries. This method, however, still needs to be calibrated by an independent method. The asymmetric light curve of the narrow emission line originating on the mass-donating companion star reveals evidence for significant shielding of 60% of the leading hemisphere by the gas between the two stars
On the compactness of the isolated neutron star RX J0720.4-3125
The data from all observations of RX J0720.4-3125 conducted by XMM-Newton
EPIC-pn with the same instrumental setup in 2000-2012 were reprocessed to form
a homogenous data set of solar barycenter corrected photon arrival times
registered from RX J0720.4-3125. A Bayesian method for the search, detection,
and estimation of the parameters of an unknown-shaped periodic signal was
employed as developed by Gregory & Loredo (1992).
A number of complex models (single and double peaked) of light curves from
pulsating neutron stars were statistically analyzed. The distribution of phases
for the registered photons was calculated by folding the arrival times with the
derived spin-period and the resulting distribution of phases approximated with
a mixed von Mises distribution, and its parameters were estimated by using the
Expected Maximization method. Spin phase-resolved spectra were extracted, and a
number of highly magnetized atmosphere models of an INS were used to fit
simultaneously, the results were verified via an MCMC approach. The
phase-folded light curves in different energy bands with high S/N ratio show a
high complexity and variations depending on time and energy.
They can be parameterized with a mixed von Mises distribution, i.e. with
double-peaked light curve profile showing a dependence of the estimated
parameters (mean directions, concentrations, and proportion) upon the energy
band, indicating that radiation emerges from at least two emitting areas.
The genuine spin-period of the isolated neutron star RX J0720-3125 derived as
more likely is twice of that reported in the literature (16.78s instead of
8.39s).
The gravitational redshift of RX J0720.4-3125 was determined to
and the compactness was estimated to
.Comment: Comments: 19 pages, 15 figures and 5 tables, Astronomy and
Astrophysics accepted. arXiv admin note: text overlap with arXiv:1108.389
Probing the Accretion Processes in Soft X-Ray Selected Polars
High-energy data of accreting white dwarfs give access to the regime of the primary accretion-induced energy release and the different proposed accretion scenarios. We perform XMM-Newton observations of polars selected due to their ROSAT hardness ratios close to -1.0 and model the emission processes in accretion column and accretion region. Our models consider the multi-temperature structure of the emission regions and are mainly determined by mass-flow density, magnetic field strength, and white-dwarf mass. To describe the full spectral energy distribution from infrared to X-rays in a physically consistent way, we include the stellar contributions and establish composite models, which will also be of relevance for future X-ray missions. We confirm the X-ray soft nature of three polars
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