1,399 research outputs found
Extensive photometry of the intermediate polar MU Cam: detection of a spin period change
Intermediate polars with known rates of spin period changes are not numerous
because such tasks require measurements performed for a long time. To measure a
spin period change, MU Cam is a good candidate because it has a spin
oscillation with a large amplitude enabling measurements with high precision.
Fortunately, in the past the spin period of MU Cam was measured with high
precision. To measure the spin period anew, in 2014-2015 we performed extensive
photometric observations of MU Cam, spanning a total duration of 208 h within
46 nights. We found that the spin, sideband and orbital periods are equal to
1187.16245+/-0.00047 s, 1276.3424+/-0.0022 s and 4.71942+/-0.00016 h,
respectively. Comparing the measured spin period with the spin period of MU Cam
in the past, we detected the spin period change with dP/dt=-(2.17+/-0.10) X
10^(-10). This rate of the spin period change was not stable and varied in a
time scale of years. During four nights in 2014 April-May MU Cam was fainter
than usual by 0.8 mag, and the amplitude of the sideband oscillation was five
times larger, denoting significant fraction of disc-overflow accretion. The
sideband oscillation showed a double-peaked pulse profile in the normal
brightness state. When the star brightness was decreased by 0.8 mag, the
sideband oscillation showed a single-peaked pulse profile. In contrast, the
spin pulse, which was quasi-sinusoidal, remained remarkably stable both in
profile and in amplitude. Moreover, the spin pulse was also remarkably stable
in a time scale of years and even decades. MU Cam is of great interest because
it represents a distinctive object with a large and unstable rate of the spin
period change and exhibits a distinctive behaviour of the pulse profiles.Comment: 12 pages, 10 figures, accepted for publication in Astrophysics and
Space Scienc
Extensive photometry of the WZ Sge-type dwarf nova V455 And (HS2331+3905): detection of negative superhumps and coherence features of the short-period oscillations
We report the results of photometry of the WZ Sge-type dwarf nova V455 And.
Observations were obtained over 19 nights in 2013 and 2014. The total duration
of the observations was 96 h. We clearly detected three coherent oscillations
with periods of 80.376+/-0.003 min, 40.5431+/-0.0004 min and
67.619685+/-0.000040 s. The 67.619685-s period can be the spin period of the
white dwarf. The 40.5431-minute period is the first harmonic of the orbital
period. The 80.376-minute oscillation can be a negative superhump because its
period is 0.9% less than the orbital period. This oscillation was evident both
in the data of 2013 and in the data of 2014. These results make V455 And a
permanent superhump system which shows negative superhumps. This is also the
first detection of persistent negative superhumps in a WZ Sge-type dwarf nova.
In addition, the analysis of our data revealed incoherent oscillations with
periods in the range 5-6 min, which were observed earlier and accounted for by
non-radial pulsations of the white dwarf. Moreover, we clearly detected an
oscillation with a period of 67.28+/-0.03 s, which was of a low degree of
coherence. This oscillation conforms to the beat between the spin period of the
white dwarf and the 3.5-h spectroscopic period, which was discovered earlier
and accounted for by the free precession of the white dwarf. Because the
67.28-s period is shorter than the spin period and because the free precession
of the white dwarf is retrograde, we account for the 67.28-s oscillation by the
free precession of the white dwarf.Comment: 9 pages, 9 figures, will be published in New Astronom
An extensive photometric study of the recently discovered intermediate polar V647 Aur (1RXS J063631.9+353537)
We report the results of photometry of the intermediate polar V647 Aur.
Observations were obtained over 42 nights in 2012 and 2013. The total duration
of the observations was 246 h. We clearly detected three oscillations with
periods of 932.9123+\-0.0011, 1008.30797+\-0.00038 and 1096.955+\-0.004 s,
which may be the white dwarf spin period and two orbital sidebands,
accordingly. The oscillation with a period of 932.9123 s has a quasi-sinusoidal
pulse profile with a slightly changeable semi-amplitude from 10.9 mmag in 2012
to 12.5 mmag in 2013. The oscillation with a period of 1008.30797 s has a
slightly asymmetric pulse profile with a remarkable small hump on the ascending
part. The semi-amplitude of this oscillation is highly changeable both in a
time-scale of days (26-77 mmag) and in a time-scale of years (47 mmag in 2012
and 34 mmag in 2013). The oscillation with a period of 1096.955 s has a highly
asymmetric pulse profile with a semi-amplitude of about 6 mmag. The three
detected oscillations imply an orbital period of 3.46565+\-0.00006 h. By
comparing our data with the data of B. T. Gansicke et al., which were obtained
8 years ago, we discovered that the spin period of the white dwarf in V647 Aur
decreases with dP/dt=(-1.36+\-0.08) X 10^-10. This important result should be
confirmed by future observations. Our oscillation ephemeredes and times of
maxima can be useful for this confirmation.Comment: 9 pages, 8 figures, will be published in MNRAS. arXiv admin note:
text overlap with arXiv:1202.249
Extensive photometry of the intermediate polar V2069 Cyg (RX J2123.7+4217)
To obtain the spin period of the white dwarf in the intermediate polar V2069
Cyg with high precision, we fulfilled extensive photometry. Observations were
performed within 32 nights, which have a total duration of 119 hours and cover
15 months. We found the spin period of the white dwarf, which is equal to
743.40650+/-0.00048 s. Using our precise spin period, we derived the
oscillation ephemeris with a long validity of 36 years. This ephemeris and the
precise spin period can be used for future investigations of spin period
changes of the white dwarf in V2069 Cyg. In addition, for the first time we
detected the sideband oscillation with a period of 764.5125+/-0.0049 s. The
spin and sideband oscillations revealed unstable amplitudes both in a
time-scale of days and in a time-scale of years. On average, the semi-amplitude
of the spin oscillation varied from 17 mmag in 2014 to 25 mmag in 2015. The
semi-amplitude of the sideband oscillation varied from 12 mmag in 2014 to an
undetectable level of less than 7 mmag in 2015. In a time-scale of years, the
optical spin pulse profile revealed essential changes from an asymmetric
double-peaked shape in 2014 to a quasi-sinusoidal shape in 2015. Such drastic
changes of the optical spin pulse profile seem untypical of most intermediate
polars and, therefore, are of great interest. The pulse profile of the sideband
oscillation was quasi-sinusoidal. Moreover, we note that V2069 Cyg possesses
strong flickering with a peak-to-peak amplitude of 0.4-0.6 mag.Comment: 13 pages, 10 figures, accepted for publication in Astrophysics and
Space Scienc
Discovery of deep eclipses in the cataclysmic variable IPHAS J051814.33+294113.0
Performing the photometric observations of the cataclysmic variable IPHAS
J051814.33+294113.0, we discovered very deep eclipses. The observations were
obtained over 14 nights, had a total duration of 56 hours and covered one year.
The large time span, during which we observed the eclipses, allowed us to
measure the orbital period in IPHAS J051814.33+294113.0 with high precision,
Porb=0.20603098+/-0.00000025 d. The prominent parts of the eclipses lasted
0.1+/-0.01 phases or 30+/-3 min. The depth of the eclipses was variable in the
range 1.8-2.9 mag. The average eclipse depth was equal to 2.42+/-0.06 mag. The
prominent parts of the eclipses revealed a smooth and symmetric shape. We
derived the eclipse ephemeris, which, according to the precision of the orbital
period, has a formal validity time of 500 years. This ephemeris can be useful
for future investigations of the long-term period changes. During the latter
four observational nights in 2017 January, we observed the sharp brightness
decrease of IPHAS J051814.33+294113.0 by 2.3 mag. This brightness decrease
imitated the end of the dwarf nova outburst. However, the long-term light curve
of IPHAS J051814.33+294113.0 obtained in the course of the Catalina Sky Survey
during 8 years showed no dwarf nova outbursts. From this we conclude that IPHAS
J051814.33+294113.0 is a novalike variable. Moreover, the sharp brightness
decrease, which we observed in IPHAS J051814.33+294113.0, suggests that this
novalike variable belongs to the VY Scl-subtype. Due to very deep eclipses,
IPHAS J051814.33+294113.0 is suitable to study the accretion disc structure
using eclipse mapping techniques. Because this novalike variable has the long
orbital period, it is of interest to determine the masses of the stellar
components from radial velocity measurements. Then, our precise eclipse
ephemeris can be useful to the phasing of spectroscopic data.Comment: 11 pages, 8 figures, accepted for publication in Astrophysics and
Space Scienc
Neutron resonances in planar waveguides
Results of experimental investigations of a neutron resonances width in
planar waveguides using the time-of-flight reflectometer REMUR of the IBR-2
pulsed reactor are reported and comparison with theoretical calculations is
presented. The intensity of the neutron microbeam emitted from the waveguide
edge was registered as a function of the neutron wavelength and the incident
beam angular divergence. The possible applications of this method for the
investigations of layered nanostructures are discussed
Wannier functions and exchange integrals: The example of LiCuO
Starting from a single band Hubbard model in the Wannier function basis, we
revisit the problem of the ligand contribution to exchange and derive explicit
formulae for the exchange integrals in metal oxide compounds in terms of atomic
parameters that can be calculated with constrained LDA and LDA+U. The analysis
is applied to the investigation of the isotropic exchange interactions of
LiCuO, a compound where the Cu-O-Cu angle of the dominant exchange
path is close to 90. Our results show that the magnetic moments are
localized in Wannier orbitals which have strong contribution from oxygen atomic
orbitals, leading to exchange integrals that considerably differ from the
estimates based on kinetic exchange only. Using LSDA+U approach, we also
perform a direct {\it ab-initio} determination of the exchange integrals
LiCuO. The results agree well with those obtained from the Wannier
function approach, a clear indication that this modelization captures the
essential physics of exchange. A comparison with experimental results is also
included, with the conclusion that a very precise determination of the Wannier
function is crucial to reach quantitative estimates.Comment: 8 pages, 8 figure
Electronic correlations and crystal structure distortions in BaBiO3
BaBiO3 is a material where formally Bi4+ ions with the half-filled 6s-states
form the alternating set of Bi3+ and Bi5+ ions resulting in a charge ordered
insulator. The charge ordering is accompanied by the breathing distortion of
the BiO6 octahedra (extension and contraction of the Bi-O bond lengths).
Standard Density Functional Theory (DFT) calculations fail to obtain the
crystal structure instability caused by the pure breathing distortions.
Combining effects of the breathing distortions and tilting of the BiO6
octahedra allows DFT to reproduce qualitatively experimentally observed
insulator with monoclinic crystal structure but gives strongly underestimate
breathing distortion parameter and energy gap values. In the present work we
reexamine the BaBiO3 problem within the GGA+U method using a Wannier functions
basis set for the Bi 6s-band. Due to high oxidation state of bismuth in this
material the Bi 6s-symmetry Wannier function is predominantly extended
spatially on surrounding oxygen ions and hence differs strongly from a pure
atomic 6s-orbital. That is in sharp contrast to transition metal oxides (with
exclusion of high oxidation state compounds) where the major part a of d-band
Wannier function is concentrated on metal ion and a pure atomic d-orbital can
serve as a good approximation. The GGA+U calculation results agree well with
experimental data, in particular with experimental crystal structure parameters
and energy gap values. Moreover, the GGA+U method allows one to reproduce the
crystal structure instability due to the pure breathing distortions without
octahedra tilting
Neutron methods for the direct determination of the magnetic induction in thick films
We review different neutron methods which allow extracting directly the value
of the magnetic induction in thick films: Larmor precession, Zeeman spatial
beam-splitting and neutron spin resonance. Resulting parameters obtained by the
neutron methods and standard magnetometry technique are presented and compared.
The possibilities and specificities of the neutron methods are discussed
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