5,558 research outputs found
On the Orbital Period of the Intermediate Polar 1WGA J1958.2+3232
Recently, Norton et al. 2002, on the basis of multiwavelength photometry of
1WGA J1958.2+3232, argued that the -1 day alias of the strongest peak in the
power spectrum is the true orbital period of the system, casting doubts on the
period estimated by Zharikov et al. 2001. We re-analyzed this system using our
photometric and spectroscopic data along with the data kindly provided by Andy
Norton and confirm our previous finding. After refining our analysis we find
that the true orbital period of this binary system is 4.35h.Comment: 4 pages, 5 figures, Accepted for publication in A&A Letter
Comment on ``Superconducting PrBa_2Cu_3O_x''
Recently, Zou et al. (Phys. Rev. Lett. 80, 1074, 1998) reported the
observation of bulk superconductivity (SC) for a PrBa_2Cu_3O_x (Pr123) single
crystal grown by the traveling-solvent floating zone (TSFZ) method. The aim of
this Comment is to show the inconsistency of the value of effective magnetic
moment \mu_{eff} reported by Zou et al. (2.92\mu_B) with their magnetic
susceptibility data. The estimation made directly from their data points gives
a considerably smaller value of \mu_{eff}=2.09\mu_B. At the same time the
values of mu_{eff}=2.9\mu_B and 3.1\mu_B were obtained for our Pr123 single
crystals grown by flux method for H||ab-plane and H||c-axis, respectively. This
suggests that Pr occupies only about a half of the RE sites in TSFZ crystal.
The other half of the RE sites is occupied most probably by the nonmagnetic Ba.
Noteworthy, SC with T_c=43 K was observed earlier for
Pr_{0.5}Ca_{0.5}Ba_2Cu_3O_{7-y} thin films. Ba^{2+} has a larger ionic radius
than Pr^{3+} and so the substitution of Ba for Pr could give a natural
explanation not only for the SC in TSFZ Pr123 but also for the elongation of
the distance between the CuO_2 planes observed by Zou et al.Comment: Slightly extended version of Comment accepted to Phys. Rev. Lett.
(v.81, N24, 1998), tentatevely to be publ. 14Dec98. 1 page, REVTex; 1 EPS fi
Periodicities In The X-Ray Intensity Variations of TV Columbae: An Intermediate Polar
We present results from a temporal analysis of the longest and the most
sensitive X-ray observations of TV Columbae--an intermediate polar. The
observations were carried out with the RXTE PCA, ROSAT PSPC, and ASCA. Data
were analyzed using a 1-dimensional CLEAN and Bayesian algorithms. The presence
of a nearly sinusoidal modulation due to the spin of the white dwarf is seen
clearly in all the data, confirming the previous reports based on the EXOSAT
data. An improved period of 1909.7+/-2.5s is derived for the spin from the RXTE
data.The binary period of 5.5hr is detected unambiguously in X-rays for the
first time. Several side-bands due to the interaction of these periods are
observed in the power spectra, thereby suggesting contributions from both the
disk-fed and the stream-fed accretion for TV Col. The accretion disk could
perhaps be precessing as side-bands due to the influence of 4 day period on the
orbital period are seen. The presence of a significant power at certain
side-bands of the spin frequency indicates that the emission poles are
asymmetrically located. The strong power at the orbital side-bands seen in both
the RXTE and ROSAT data gives an indication for an absorption site fixed in the
orbital frame. Both the spin and the binary modulation are found to be
energy-dependent. Increased hardness ratio during a broad dip in the intensity
at binary phase of 0.75--1.0 confirms the presence of a strong attenuation due
to additional absorbers probably from an impact site of the accretion stream
with the disk or magnetosphere. Hardness ratio variations and the energy
dependent modulation depth during the spin modulation can be explained by
partially covered absorbers in the path of X-ray emission region in the
accretion stream.Comment: 34 pages, including 12 figures, Accepted for publication in
Astronomical Journal, scheduled for January 2004 issue (vol. 127
The X-ray properties of the magnetic cataclysmic variable UUColumbae
Aims. XMM-Newton observations to determine for the first time the broad-band X-ray properties of the faint, high galactic latitude intermediate polar UUCol are presented.
Methods. We performed X-ray timing analysis in different energy ranges of the EPIC cameras, which reveals the dominance of the 863 s white dwarf rotational period. The spin pulse is strongly energy dependent. Weak variabilities at the beat 935 s and at the 3.5 h orbital periods are also observed, but the orbital modulation is detected only below 0.5 keV. Simultaneous UV and optical photometry shows that the spin pulse is anti-phased with respect to the hard X-rays. Analysis of the EPIC and RGS spectra reveals
the complexity of the X-ray emission, which is composed of a soft 50 eV black–body component and two optically thin emission components at 0.2 keV and 11 keV strongly absorbed by dense material with an equivalent hydrogen column density of 1023 cm−2 that partially (50%) covers the X-ray source.
Results. The complex X-ray and UV/optical temporal behaviour indicates that accretion occurs predominantly (∼80%) via a disc with a partial contribution (∼20%) directly from the stream. The main accreting pole dominates at high energies whilst the secondary pole mainly contributes in the soft X-rays and at lower energies. The bolometric flux ratio of the soft-to-hard X-ray emissions is found to be consistent with the prediction of the standard accretion shock model. We find the white dwarf in UUCol accretes at a low rate and possesses a low magnetic moment. It is therefore unlikely that UUCol will evolve into a moderate field strength polar, so that the soft X-ray intermediate polars still remain an enigmatic small group of magnetic cataclysmic variables
On the orbital period of the magnetic Cataclysmic Variable HS 0922+1333
Context: The object HS 0922+1333 was visited briefly in 2002 in a mini survey
of low accretion rate polars (LARPs) in order to test if they undergo high
luminosity states similar to ordinary polars. On the basis of that short
observation the suspicion arose that the object might be an asynchronous polar
(Tovmassian et al. 2004). The disparity between the presumed orbital and spin
period appeared to be quite unusual. Aims: We performed follow-up observations
of the object to resolve the problem. Methods: New simultaneous spectroscopic
and photometric observations spanning several years allowed measurements of
radial velocities of emission and absorption lines from the secondary star and
brightness variations due to synchrotron emission from the primary. Results:
New observations show that the object is actually synchronous and its orbital
and spin period are equal to 4.04 hours. Conclusions: We identify the source of
confusion of previous observations to be a high velocity component of emission
line arousing from the stream of matter leaving L1 point.Comment: 5 pages, 5 figures, accepted for publication in Astronomy &
Astrophysic
X-Ray Emission and Optical Polarization of V1432 Aquilae: An Asynchronous Polar
A detailed analysis of X-ray data from ROSAT, ASCA, XMM and RXTE for the
asynchronous polar V1432 Aql along with Stokes polarimetry data from SAAO, is
presented. Power spectra from long-baseline ROSAT data show a spin period of
12150s along with several system related frequency components. However, the
second harmonic of the spin period dominates power spectrum in the XMM data.
For the optical circular polarization, the dominant period corresponds to half
the spin period. The ROSAT data can be explained as due to accretion onto two
hot spots that are not anti-podal. The variations seen in the optical
polarization and the ASCA and XMM data suggest the presence of at least three
accretion foot prints on the white dwarf surface. Two spectral models, a
multi-temperature plasma and a photo-ionized plasma model, are used for
spectral study. The RXTE PCA data are used to constrain the white dwarf mass to
1.20.1 M_odot using the multi-temperature plasma model. A strong soft
X-ray excess (<0.8 keV) in the XMM MOS data is well modeled by a blackbody
component having a temperature of 80-90 eV. The plasma emission lines seen at
6.7 and 7.0 keV are well fitted using the multi-temperature plasma model,
however an additional Gaussian is needed for the 6.4 keV line. The
multi-temperature plasma model requires a homogeneous absorber fully covering
the source and a partial absorber covering 65% of the source. The photo-ionized
plasma model, with a range of Fe column densities, gives a slightly better
overall fit and fits all emission lines. The presence of a strong blackbody
component, a spin period of 12150s, modulation of the 6.4 keV line flux with
spin period, and a very hard X-ray component suggest that V1432 Aql is a polar
with X-ray spectral properties similar to that of a soft intermediate polar.Comment: 46 pages, including 13 figures and 4 tables, To appear in The
Astrophysical Journal, 20 May 2005 issue, vol. 625, Added Report-no and
Journal-ref, no change in the text of the pape
Magnetic interactions in transition metal doped ZnO : An abinitio study
We calculate the nature of magnetic interactions in transition-metal doped
ZnO using the local spin density approximation and LSDA+\textit{U} method of
density functional theory. We investigate the following four cases: (i) single
transition metal ion types (Cr, Mn, Fe, Co, Ni and Cu) substituted at Zn sites,
(ii) substitutional magnetic transition metal ions combined with additional Cu
and Li dopants, (iii) substitutional magnetic transition metal ions combined
with oxygen vacancies and (iv) pairs of magnetic ion types (Co and Fe, Co and
Mn, etc.). Extensive convergence tests indicate that the calculated magnetic
ground state is unusually sensitive to the k-point mesh and energy cut-off, the
details of the geometry optimizations and the choice of the
exchange-correlation functional. We find that ferromagnetic coupling is
sometimes favorable for single type substitutional transition metal ions within
the local spin density approximation. However, the nature of magnetic
interactions changes when correlations on the transition-metal ion are treated
within the more realistic LSDA + \textit{U} method, often disfavoring the
ferromagnetic state. The magnetic configuration is sensitive to the detailed
arrangement of the ions and the amount of lattice relaxation, except in the
case of oxygen vacancies when an antiferromagnetic state is always favored.Comment: 11 pages, 17 figure
The Nature of the Faint Chandra X-ray Sources in the Galactic Centre
Recent Chandra observations have revealed a large population of faint X-ray
point sources in the Galactic Centre. The observed population consists of about
2000 faint sources in the luminosity range ~10^31-10^33 erg/s. The majority of
these sources (70%) are described by hard spectra, while the rest are rather
soft. The nature of these sources still remains unknown. Belczynski & Taam
(2004) demonstrated that X-ray binaries with neutron star or black hole
accretors may account for most of the soft sources, but are not numerous enough
to account for the observed number and X-ray properties of the faint hard
sources. A population synthesis calculation of the Galactic Centre region has
been carried out. Our results indicate that the numbers and X-ray luminosities
of intermediate polars are consistent with the observed faint hard Galactic
Centre population.Comment: resubmitted to ApJ Letters, changes to text (i.e., Discussion) and
Reference
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