1,695 research outputs found
Voracious vortexes in cataclysmic variables. A multi-epoch tomographic study of HT Cassiopeia
We present multi-epoch, time-resolved optical spectroscopic observations of
the dwarf nova HT Cas, obtained during 1986, 1992, 1995 and 2005 with the aim
to study the properties of emission structures in the system. We determined
that the accretion disc radius, measured from the double-peaked emission line
profiles, is persistently large and lies within the range of 0.45-0.52a, where
a is the binary separation. This is close to the tidal truncation radius
r_max=0.52a. This result contradicts with previous radius measurements. An
extensive set of Doppler maps has revealed a very complex emission structure of
the accretion disc. Apart from a ring of disc emission, the tomograms display
at least three areas of enhanced emission: the hot spot from the area of
interaction between the gas stream and the disc, which is superposed on the
elongated spiral structure, and the extended bright region on the leading side
of the disc, opposite to the location of the hot spot. The position of the hot
spot in all the emission lines is consistent with the trajectory of the gas
stream. However, the peaks of emission are located in the range of distances
0.22-0.30a, which are much closer to the white dwarf than the disc edge. This
suggests that the outer disc regions have a very low density, allowing the gas
stream to flow almost freely before it starts to be seen as an emission source.
We have found that the extended emission region in the leading side of the disc
is always observed at the very edge of the large disc. Observations of other
cataclysmic variables, which show a similar emission structure in their
tomograms, confirm this conclusion. We propose that the leading side bright
region is caused by irradiation of tidally thickened sectors of the outer disc
by the white dwarf and/or hot inner disc regions.Comment: 15 pages, 12 figures. Minor modifications to match version published
by Astronomy & Astrophysic
Time resolved spectroscopy and photometry of the dwarf nova FS Aurigae in quiescence
We present results of non-simultaneous time resolved photometric and spectroscopic observations of little-studied dwarf nova FS Aur in quiescence. The spectrum of FS Aur shows strong and broad emission lines of hydrogen and HeI, and of weaker HeII 4686 and CIII/NIII blend, similar to other quiescent dwarf novae. All emission lines are single-peaked, however their form varies with an orbital phase. Absorption lines from a late-type secondary are not detected. From the radial velocity measurements of the hydrogen lines H_beta and H_gamma we determined a most probable orbital period P=0.059+-0.002 d. This period agrees well with the 0.0595+-0.0001 estimate by TPST. On the other hand, the period of photometric modulations is longer than spectroscopic period and can be appreciated at least as 3 hours. Longer time-scale coverage during a single night is needed to resolve this problem. Using semi-amplitude of the radial velocities, obtained from measurements of hydrogen and helium lines, and some empirical and theoretical relations we limited basic parameters of the system: a mass ratio q>=0.22, a primary mass M_1=0.34 \div 0.46 M_sun, a secondary mass M_2<=0.1M_sun, and an inclination angle i=51^{\circ }-65^{\circ}. Doppler tomography have shown at least two bright region in accretion disk of FS Aur. The first more bright spot is located at phase about 0.6. The second spot is located opposite to the first one and occupies an extensive area at phases about 0.85-1.15
The origin of seed photons for Comptonization in the black hole binary Swift J1753.5-0127
Aims. The black hole binary SWIFT J1753.5-0127 is providing a unique data set
to study accretion flows. Various investigations of this system and of other
black holes have not, however, led to an agreement on the accretion flow
geometry or on the seed photon source for Comptonization during different
stages of X-ray outbursts. We place constraints on these accretion flow
properties by studying long-term spectral variations of this source. Methods.
We performed phenomenological and self-consistent broad band spectral modeling
of Swift J1753.5-0127 using quasi-simultaneous archived data from
INTEGRAL/ISGRI, Swift/UVOT/XRT/BAT, RXTE/PCA/HEXTE and MAXI/GSC instruments.
Results. We identify a critical flux limit, F \sim 1.5 \times 10^{-8}
erg/cm^2/s, and show that the spectral properties of SWIFT J1753.5-0127 are
markedly different above and below this value. Above the limit, during the
outburst peak, the hot medium seems to intercept roughly 50 percent of the disk
emission. Below it, in the outburst tail, the contribution of the disk photons
reduces significantly and the entire spectrum from the optical to X-rays can be
produced by a synchrotron-self-Compton mechanism. The long-term variations in
the hard X-ray spectra are caused by erratic changes of the electron
temperatures in the hot medium. Thermal Comptonization models indicate
unreasonably low hot medium optical depths during the short incursions into the
soft state after 2010, suggesting that non-thermal electrons produce the
Comptonized tail in this state. The soft X-ray excess, likely produced by the
accretion disk, shows peculiarly stable temperatures for over an order of
magnitude changes in flux. Conclusions. The long-term spectral trends of SWIFT
J1753.5-0127 are likely set by variations of the truncation radius and a
formation of a hot, quasi-spherical inner flow in the vicinity of the black
hole. (abridged)Comment: 16 pages, 8 figures, published in A&
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