806 research outputs found
HT Cas - eclipsing dwarf nova during its superoutburst in 2010
We present results of a world-wide observing campaign of the eclipsing dwarf
nova - HT Cas during its superoutburst in November 2010. Using collected data
we were able to conduct analysis of the light curves and we calculated
diagrams.
The CCD photometric observations enabled us to derive the superhump period
and with the timings of eclipses the orbital period was calculated. Based on
superhump and orbital period estimations the period excess and mass ratio of
the system were obtained
IX Draconis - a curious ER UMa-type dwarf nova
We report results of an extensive world-wide observing campaign devoted to a
very active dwarf nova star - IX Draconis. We investigated photometric
behaviour of the system to derive its basic outburst properties and understand
peculiarities of IX Dra as well as other active cataclysmic variables, in
particular dwarf novae of the ER Uma-type. In order to measure fundamental
parameters of the system, we carried out analyses of the light curve, O-C
diagram, and power spectra. During over two months of observations we detected
two superoutbursts and several normal outbursts. The V magnitude of the star
varied in the range 14.6 - 18.2 mag. Superoutbursts occur regularly with the
supercycle length of 58.5+/-0.5 d. When analysing data over the past 20 years,
we found that the supercycle length is increasing at a rate of P_dot = 1.8 *
10^{-3}. Normal outbursts appear to be irregular, with typical occurrence times
in the range 3.1 - 4.1 d. We detected a double-peaked structure of superhumps
during superoutburst, with the secondary maximum becoming dominant near the end
of the superoutburst. The mean superhump period observed during superoutbursts
equals 0.066982(36) d, which is constant over the last two decades of
observations. Based on the power spectrum analysis, the evaluation of the
orbital period was problematic. We found two possible values: the first one,
0.06641(3) d, which is in agreement with previous studies and our O-C analysis
(0.06646(2) d), and the second one, 0.06482(3) d, which is less likely. The
evolutionary status of the object depends dramatically on the choice between
these two values. A spectroscopic determination of the orbital period is
needed. We updated available information on ER UMa-type stars and present a new
set of their basic statistics. Thereby, we provide evidence that this class of
stars is not uniform.Comment: Accepted for publication in MNRAS; 15 pages, 15 figures, 6 tables;
typo correcte
MN Draconis - peculiar, active dwarf nova in the period gap
Context: We present results of an extensive world-wide observing campaign of
MN Draconis.
Aims: MN Draconis is a poorly known active dwarf nova in the period gap and
is one of the only two known cases of period gap SU UMa objects showing the
negative superhumps. Photometric behaviour of MN Draconis poses a challenge for
existing models of the superhump and superoutburst mechanisms. Therefore,
thorough investigation of peculiar systems, such as MN Draconis, is crucial for
our understanding of evolution of the close binary stars.
Methods: To measure fundamental parameters of the system, we collected
photometric data in October 2009, June-September 2013 and June-December 2015.
Analysis of the light curves, diagrams and power spectra was carried out.
Results: During our three observational seasons we detected four
superoutburts and several normal outbursts. Based on the two consecutive
superoutbursts detected in 2015, the supercycle length was derived P_sc = 74
+/- 0.5 days and it has been increasing with a rate of P_dot = 3.3 x 10^(-3)
during last twelve years. Based on the positive and negative superhumps we
calculated the period excess epsilon = 5.6% +/- 0.1%, the period deficit
epsilon_ = 2.5% +/- 0.6%, and in result, the orbital period P_orb = 0.0994(1)
days (143.126 +/- 0.144 min). We updated the basic light curve parameters of MN
Draconis.
Conclusions: MN Draconis is the first discovered SU UMa system in the period
gap with increasing supercycle length.Comment: 14 pages, 20 figures, 8 tables, accepted for publication in Astronomy
and Astrophysic
The impact of skull bone intensity on the quality of compressed CT neuro images
International audienceThe increasing use of technologies such as CT and MRI, along with a continuing improvement in their resolution, has contributed to the explosive growth of digital image data being generated. Medical communities around the world have recognized the need for efficient storage, transmission and display of medical images. For example, the Canadian Association of Radiologists (CAR) has recommended compression ratios for various modalities and anatomical regions to be employed by lossy JPEG and JPEG2000 compression in order to preserve diagnostic quality. Here we investigate the effects of the sharp skull edges present in CT neuro images on JPEG and JPEG2000 lossy compression. We conjecture that this atypical effect is caused by the sharp edges between the skull bone and the background regions as well as between the skull bone and the interior regions. These strong edges create large wavelet coefficients that consume an unnecessarily large number of bits in JPEG2000 compression because of its bitplane coding scheme, and thus result in reduced quality at the interior region, which contains most diagnostic information in the image. To validate the conjecture, we investigate a segmentation based compression algorithm based on simple thresholding and morphological operators. As expected, quality is improved in terms of PSNR as well as the structural similarity (SSIM) image quality measure, and its multiscale (MS-SSIM) and informationweighted (IW-SSIM) versions. This study not only supports our conjecture, but also provides a solution to improve the performance of JPEG and JPEG2000 compression for specific types of CT images
FIRST "Winged" and X-shaped Radio Source Candidates: II. New Redshifts
We report optical spectroscopic observations of X-shaped radio sources with
the Hobby-Eberly Telescope and Multiple-Mirror Telescope, focused on the sample
of candidates from the FIRST survey presented in Paper I (Cheung 2007). A total
of 27 redshifts were successfully obtained, 21 of which are new, including that
of a newly identified candidate source of this type which is presented here.
With these observations, the sample of candidates from Paper I is over 50%
spectroscopically identified. Two new broad emission-lined X-shaped radio
sources are revealed, while no emission lines were detected in about one third
of the observed sources; a detailed study of the line properties is deferred to
a future paper. Finally, to explore their relation to the Fanaroff-Riley
division, the radio luminosities and host galaxy absolute magnitudes of a
spectroscopically identified sample of 50 X-shaped radio galaxies are
calculated to determine their placement in the Owen-Ledlow plane.Comment: emulateapj style, 10 pages, 4 figures, 2 tables; ApJS accepted with
minor revision from submitted version (v1
The magnetic field and geometry of the oblique shock in the jet of 3C 346
We investigate the brightest regions of the kpc-scale jet in the powerful
radio galaxy 3C 346, using new optical HST ACS/F606W polarimetry together with
Chandra X-ray data and 14.9 GHz and 22.5 GHz VLA radio polarimetry. The jet
shows a close correspondence in optical and radio morphology, while the X-ray
emission shows an 0.80 +/- 0.17 kpc offset from the optical and radio peak
positions. Optical and radio polarimetry show the same apparent magnetic field
position angle and fractional polarization at the brightest knot, where the jet
undergoes a large kink of almost 70 degrees in the optical and radio images.
The apparent field direction here is well-aligned with the new jet direction,
as predicted by earlier work that suggested the kink was the result of an
oblique shock. We have explored models of the polarization from oblique shocks
to understand the geometry of the 3C 346 jet, and find that the upstream flow
is likely to be highly relativistic (0.91 +0.05 / -0.07 c), where the plane of
the shock front is inclined at an angle of 51 (+/- 11) degrees to the upstream
flow which is at an angle 14 (+8 / -7) degrees to our line of sight. The actual
deflection angle of the jet in this case is only 22 degrees.Comment: 11 pages, 5 figures. Accepted by MNRA
Observation of the first gravitational microlensing event in a sparse stellar field : the Tago event
We report the observation of the first gravitational microlensing event in a
sparse stellar field, involving the brightest (V=11.4 mag) andclosest (~ 1 kpc)
source star to date. This event was discovered by an amateurastronomer, A.
Tago, on 2006 October 31 as a transient brightening, by ~4.5 mag during a ~15
day period, of a normal A-type star (GSC 3656-1328) in the Cassiopeia
constellation. Analysis of both spectroscopic observations and the light curve
indicates that this event was caused by gravitational microlensing rather than
an intrinsically variable star. Discovery of this single event over a 30 year
period is roughly consistent with the expected microlensing rate for the whole
sky down to V = 12 mag stars. However, the probability for finding events with
such a high magnification (~ 50) is much smaller, by a factor ~1/50, which
implies that the true event rate may be higher than expected. This discovery
indicates the potential of all sky variability surveys, employing frequent
sampling by telescopes with small apertures and wide fields of view, for
finding such rare transient events, and using the observations to explore
galactic disk structure and search for exo-planets.Comment: 13 pages, 2 tables, 3 figures, accepted by Ap
- …