1,776 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
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&
High Speed Phase-Resolved 2-d UBV Photometry of the Crab pulsar
We report a phase-resolved photometric and morphological analysis of UBV data
of the Crab pulsar obtained with the 2-d TRIFFID high speed optical photometer
mounted on the Russian 6m telescope. By being able to accurately isolate the
pulsar from the nebular background at an unprecedented temporal resolution (1
\mu s), the various light curve components were accurately fluxed via
phase-resolved photometry. Within the range, our datasets are consistent
with the existing trends reported elsewhere in the literature. In terms of flux
and phase duration, both the peak Full Width Half Maxima and Half Width Half
Maxima decrease as a function of photon energy. This is similarly the case for
the flux associated with the bridge of emission. Power-law fits to the various
light curve components are as follows; \alpha = 0.07 \pm 0.19 (peak 1), \alpha
= -0.06 \pm 0.19 (peak 2) and \alpha = -0.44 \pm 0.19 (bridge) - the
uncertainty here being dominated by the integrated CCD photometry used to
independently reference the TRIFFID data. Temporally, the main peaks are
coincident to \le 10 \mu s although an accurate phase lag with respect to the
radio main peak is compromised by radio timing uncertainties. The plateau on
the Crab's main peak was definitively determined to be \leq 55 \mu s in extent
and may decrease as a function of photon energy. There is no evidence for
non-stochastic activity over the light curves or within various phase regions,
nor is there evidence of anything akin to the giant pulses noted in the radio.
Finally, there is no evidence to support the existence of a reported 60 second
modulation suggested to be as a consequence of free precession.Comment: 13 pages, 12 figures, accepted for publication in Astronomy &
Astrophysic
- …