28 research outputs found
Rapid Optical Fluctuations in the Black Hole Binary, V4641 Sgr
We report on unprecedented short-term variations detected in the optical flux
from the black hole binary system, V4641 Sgr. Amplitudes of the optical
fluctuations were larger at longer time scales, and surprisingly reached ~60%
around a period of ~10 min. The power spectra of fluctuations are characterized
by a power law. It is the first case in black hole binaries that the optical
emission was revealed to show short-term and large-amplitude variations given
by such a power spectrum. The optical emission from black hole binaries is
generally dominated by the emission from the outer portion of an accretion
disc. The rapid optical fluctuations however indicate that the emission from an
inner accretion region significantly contributes to the optical flux. In this
case, cyclo-synchrotron emission associated with various scales of magnetic
flares is the most promising mechanism for the violently variable optical
emission.Comment: 5 pages, 2 figures, accepted for publication in PAS
Photometric Studies of New Southern SU UMa-type dwarf novae, FL Triangulum Australe and CTCV J0549-4921
We report time-resolved optical CCD photometry on newly discovered SU
UMa-type dwarf novae, FL TrA and CTCV J0549-4921. During the 2006 August
outburst, we detected superhumps with a period of 0.59897(11) days for FL TrA,
clarifying the SU UMa nature of the system. On the first night of our
observations on FL TrA, the object showed no superhumps. This implies that it
takes a few days for full development of superhumps. The superhump period
variation diagram of FL TrA was similar to that observed in some WZ Sge stars
and short period SU UMa-type stars. This indicates that the system is closely
related to WZ Sge stars and SU UMa stars having short orbital periods. For CTCV
J0549-4921, the candidates of the mean superhump period are 0.083249(10) days
and 0.084257(8) days, respectively. Due to a lack of the observations, we
cannot determine the true superhump period, but the latter period is favorable.
Using the ASAS-3 archive, it turned out that the system shows only four
outbursts over the past 6 years. The outburst amplitude of CTCV J0549-4921 was
relatively small, with about 4.5 mag. One possibility is that mass evaporation
may play a role during quiescence.Comment: 7 pages, 8 figures, published for PASJ vol.6
The 2011 Eruption of the Recurrent Nova T Pyxidis; the Discovery, the Pre-eruption Rise, the Pre-eruption Orbital Period, and the Reason for the Long Delay
We report the discovery by M. Linnolt on JD 2455665.7931 (UT 2011 April
14.29) of the sixth eruption of the recurrent nova T Pyxidis. This discovery
was made just as the initial fast rise was starting, so with fast notification
and response by observers worldwide, the entire initial rise was covered (the
first for any nova), and with high time resolution in three filters. The speed
of the rise peaked at 9 mag/day, while the light curve is well fit over only
the first two days by a model with a uniformly expanding sphere. We also report
the discovery by R. Stubbings of a pre-eruption rise starting 18 days before
the eruption, peaking 1.1 mag brighter than its long-time average, and then
fading back towards quiescence 4 days before the eruption. This unique and
mysterious behavior is only the fourth known anticipatory rise closely spaced
before a nova eruption. We present 19 timings of photometric minima from 1986
to February 2011, where the orbital period is fast increasing with
P/dot{P}=313,000 yrs. From 2008-2011, T Pyx had a small change in this rate of
increase, so that the orbital period at the time of eruption was
0.07622950+-0.00000008 days. This strong and steady increase of the orbital
period can only come from mass transfer, for which we calculate a rate of
1.7-3.5x10^-7 Mo/yr. We report 6116 magnitudes between 1890 and 2011, for an
average B=15.59+-0.01 from 1967-2011, which allows for an eruption in 2011 if
the blue flux is nearly proportional to the accretion rate. The
ultraviolet-optical-infrared spectral energy distribution is well fit by a
power law with flux proportional to nu^1.0, although the narrow ultraviolet
region has a tilt with a fit of \nu^{1/3}. We prove that most of the T Pyx
light is not coming from a disk, or any superposition of blackbodies, but
rather is coming from some nonthermal source.Comment: ApJ submitted, 62 pages, 8 figures; much added data, updated analysi
Outbursts of EX Hydrae: mass-transfer events or disc instabilities?
We present the 45-yr record of EX Hya's lightcurve and discuss the
characteristics of its 15 observed outbursts. We then concentrate on the 1998
outburst, reporting the first outburst X-ray observations. We discover an X-ray
beat-cycle modulation, indicating that an enhanced accretion stream couples
directly with the magnetosphere in outburst, confirming our previous
prediction. Optical eclipse profiles late in outburst show that the visible
light is dominated by an enhanced mass-transfer stream overflowing the
accretion disc. We are uncertain whether the enhanced mass transfer is
triggered by a disc instability, or by some other cause. While in outburst, EX
Hya shows some of the characteristics of SW Sex stars.Comment: To appear in MNRAS (8 pages; 9 figs