72 research outputs found
Assisted stellar suicide in V617 Sgr
V617 Sgr is a V Sagittae star - a group of binaries thought to be the
galactic counterparts of the Compact Binary Supersoft X-ray Sources - CBSS. To
check this hypothesis, we measured the time derivative of its orbital period.
Observed timings of eclipse minima spanning over 30,000 orbital cycles are
presented. We found that the orbital period evolves quite rapidly: P/Pdot = 1.1
x 10^{6} years. This is consistent with the idea that V617 Sgr is a wind driven
accretion supersoft source. As the binary system evolves with a time-scale of
about one million years, which is extremely short for a low mass evolved
binary, it is likely that the system will soon end either by having its
secondary completely evaporated or by the primary exploding as a supernova of
type Ia.Comment: 4 pages, accepted for publication in A&A Letter
Discovery of optical pulsations in V2116 Ophiuchi/GX 1+4
We report the detection of pulsations with s period in V2116 Oph,
the optical counterpart of the low-mass X-ray binary GX 1+4. The pulsations are
sinusoidal with modulation amplitude of up to 4% in blue light and were
observed in ten different observing sessions during 1996 April-August using a
CCD photometer at the 1.6-m and 0.6-m telescopes of Laborat\'orio Nacional de
Astrof\'{\i}sica, in Brazil. The pulsations were also observed with the
fast photometer. With only one exception the observed optical periods are
consistent with those observed by the BATSE instrument on board the Compton
Gamma Ray Observatory at the same epoch. There is a definite correlation
between the observability of pulsations and the optical brightness of the
system: V2116~Oph had magnitude in the range when the pulsed
signal was detected, and when no pulsations were present. The
discovery makes GX 1+4 only the third of accretion-powered X-ray
pulsars to be firmly detected as a pulsating source in the optical. The
presence of flickering and pulsations in V2116 Oph adds strong evidence for an
accretion disk scenario in this system. The absolute magnitude of the pulsed
component on 1996 May 27 is estimated to be . The implied
dimensions for the emitting region are 1.1 R_{\sun}, 3.2 R_{\sun}, and 7.0
R_{\sun}, for black-body spectral distributions with K, K, and K, respectively.Comment: 9 pages, 3 figures in PostScript, latex, accepted for publication on
the Astrophysical Journal Letter
SPARC4: A Simultaneous Polarimeter and Rapid Camera in 4 Bands
We present the basic concept of a new astronomical instrument: SPARC4 -
Simultaneous Polarimeter and Rapid Camera in 4 bands. SPARC4 combines in one
instrument: (i) photometric and polarimetric modes; (ii) sub-second
time-resolution in photometric mode and excellent time-resolution in
polarimetric mode; (iii) simultaneous imaging in four broad-bands for both
modes. This combination will make SPARC4 a unique facility for ground-based
optical observatories. Presently, the project is in its conceptual design
phase.Comment: Poster presented at "Stellar Polarimetry: From birth to death". This
article has been submitted to AIP Conference Proceedings. After it is
published, it will be found at http://www.aip.or
Spectroscopic Coronal Observations during the Total Solar Eclipse of 11 July 2010
The flash spectrum of the solar chromosphere and corona was measured with a
slitless spectrograph before, after, and during the totality of the solar
eclipse, of 11 July 2010, at Easter Island, Chile. This eclipse took place at
the beginning of the Solar Cycle 24, after an extended minimum of solar
activity. The spectra taken during the eclipse show a different intensity ratio
of the red and green coronal lines compared with those taken during the total
solar eclipse of 1 August 2008, which took place towards the end of the Solar
Cycle 23. The characteristic coronal forbidden emission line of forbidden Fe
XIV (5303 {\AA}) was observed on the east and west solar limbs in four areas
relatively symmetrically located with respect to the solar rotation axis.
Subtraction of the continuum flash-spectrum background led to the
identification of several extremely weak emission lines, including forbidden Ca
XV (5694 {\AA}), which is normally detected only in regions of very high
excitation, e.g., during flares or above large sunspots. The height of the
chromosphere was measured spectrophotometrically, using spectral lines from
light elements and compared with the equivalent height of the lower
chromosphere measured using spectral lines from heavy elements.Comment: 14 pages, 8 figures, 1 table; Solar Physics, 2012, Februar
Mira Variables in the OGLE Bulge fields
(abridged) The 222000 I-band light curves of variable stars detected by the
OGLE-II survey in the direction of the Galactic Bulge have been fitted and have
also been correlated with the DENIS and 2MASS databases. Results are presented
for 2691 objects with I-band semi-amplitude larger than 0.45 magnitude,
corresponding to classical Mira variables. The Mira period distribution of 6
fields at similar longitude but spanning latitudes from -1.2 to -5.8 degrees
are statistically indistinguisable indicating similar populations with initial
masses of 1.5-2 \msol (corresponding to ages of 1-3 Gyr). A field at similar
longitude at b = -0.05 from Glass et al. (2001) does show a significantly
different period distribution, indicating the presence of a younger population
of 2.5-3 \msol and ages below 1 Gyr. The K-band period-luminosity relation is
presented for the whole sample, and for sub-fields. The zero point depends on
Galactic longitude. Simulations are carried out to show that the observed
dependence of the zero point with , and the number of stars per field are
naturally explained using the model of disk and bulge stars of Binney et al.
(1997), for a viewing angle of 43 +/- 17 degrees. A comparison is made with
similar objects in the Magellanic Clouds, studied in a previous paper. The
slope of the PL-relation in the Bulge and the MCs agree within the errorbars. A
distance to the Galactic Centre between 8.6 and 9.0 kpc is derived (for an
assumed DM of 18.50 to the LMC).Comment: accepted for publication in Astronomy and Astrophysic
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