1,071 research outputs found
Phase-resolved spectroscopy and photometry of the eclipsing polar EP Draconis (=H1907+690)
We present phase-resolved optical spectroscopy and CCD photometry of the
faint eclipsing polar EP Dra (H1907+690). A revised ephemeris is derived which
connects all 32000 binary cycles since its discovery by Remillard et al.
(1991). We found no difference between spin and and orbital periods of the
white dwarf. Changes in the light curve morphology are attributed to a
different beaming behaviour which might change on timescales as short as one or
several orbital periods. Optical light curve modelling was used to estimate the
co-latitude of the accretion spot, which must be larger than 40 degrees. We
have detected Zeeman absorption lines of Halpha originating in an accretion
halo in a field of 16MG. The low-resolution spectra reveal no indication of
resolved cyclotron harmonics, which is also suggestive of a relatively low
field strength in the accretion region. The Balmer emission lines contain
significant contributions from the UV-illuminated hemisphere of the companion
star, whereas the HeII4686 emission originates predominantly from the accretion
stream. The emission lines have a multi-component structure and we could single
out a narrow emission line in the Hbeta and Hgamma lines. Its radial velocity
amplitude suggests a low mass for the white dwarf, if the lines are interpreted
as being of reprocessed origin from the whole illuminated hemisphere of the
companion star.Comment: 9 pages, 4 postscript figures Astronomische Nachrichten, in press see
also http://www.aip.de:8080/science/Preprints.htm
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&
Background subtraction and transient timing with Bayesian Blocks
Aims: To incorporate background subtraction into the Bayesian Blocks
algorithm so that transient events can be timed accurately and precisely even
in the presence of a substantial, rapidly variable, background. Methods: We
developed several modifications to the algorithm and tested them on a simulated
XMM-Newton observation of a bursting and eclipsing object. Results: We found
that bursts can be found to good precision for almost all background
subtraction methods, but eclipse ingresses and egresses present problems for
most methods. We found one method that recovered these events with precision
comparable to the interval between individual photons, in which both source and
background region photons are combined into a single list and weighted
according to the exposure area. We have also found that adjusting the Bayesian
Blocks change points nearer to blocks with higher count rate removes a
systematic bias towards blocks of low count rate.Comment: 10 pages, 13 figures, 1 tabl
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
XMM-Newton and optical observations of the eclipsing polar CSS081231:071126+440405
Aims: We aim to study the temporal and spectral behaviour of the eclipsing
polar CSS081231:071126+440405 from the infrared to the X-ray regime.
Methods: We obtained phase-resolved XMM-Newton X-ray observations on two
occasions in 2012 and 2013 in different states of accretion. In 2013 the
XMM-Newton X-ray and UV data were complemented by optical photometric and
spectroscopic observations.
Results: CSS081231 displays two-pole accretion in the high state. The
magnetic fields of the two poles are 36 and 69 MG, indicating a non-dipolar
field geometry. The X-ray spectrum of the main accreting pole with the lower
field comprises a hot thermal component from the cooling accretion plasma,
of a few tens of keV, and a much less luminous blackbody-like
component from the accretion area with 50-100\,eV. The
high-field pole which was located opposite to the mass-donating star accretes
at a low rate and has a plasma temperature of about 4\,keV. At both occasions
the X-ray eclipse midpoint precedes the optical eclipse midpoint by 3.2
seconds. The center of the X-ray bright phase shows accretion-rate dependent
longitudinal motion of ~20 degrees.
Conclusions: CSS081231 is a bright polar that escaped detection in the RASS
survey because it was in a low accretion state. Even in the high state it lacks
the prominent soft component previously thought ubiquitous in polars. Such an
excess may still be present in the unobserved extreme ultraviolet. All polars
discovered in the XMM-Newton era lack the prominent soft component. The
intrinsic spectral energy distribution of polars still awaits characterisation
by future X-ray surveys such as eROSITA. The trajectory taken by material to
reach the second pole is still uncertain.Comment: 12 pages, 14 figure
The complex X-ray spectrum of the isolated neutron star RBS1223
We present a first analysis of a deep X-ray spectrum of the isolated neutron
star RBS1223 obtained with XMM-Newton. Spectral data from four new monitoring
observations in 2005/2006 were combined with archival observations obtained in
2003 and 2004 to form a spin-phase averaged spectrum containing 290000 EPIC-pn
photons. This spectrum shows higher complexity than its predecessors, and can
be parameterised with two Gaussian absorption lines superimposed on a
blackbody. The line centers, E_2 ~ 2E_1, could be regarded as supporting the
cyclotron interpretation of the absorption features in a field B ~ 4 x 10**13
G. The flux ratio of those lines does not support this interpretation. Hence,
either feature might be of truly atomic origin.Comment: 4 pages, 1 figure, to appear in Astrophysics and Space Science, in
the proceedings of "Isolated Neutron Stars: from the Interior to the
Surface", edited by D. Page, R. Turolla and S. Zan
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