71 research outputs found
Large X-ray Flares from LMC X-4: Discovery of Milli-hertz Quasi-periodic Oscillations and QPO-modulated Pulsations
We report the discovery of milli-hertz (mHz) quasi-periodic oscillations
(QPOs) and QPO-modulated pulsations during large X-ray flares from the
high-mass X-ray binary pulsar LMC X-4 using data from the Rossi X-Ray Timing
Explorer (RXTE). The lightcurves of flares show that, in addition to ~74 mHz
coherent pulsations, there exist two more time-varying temporal structures at
frequencies of ~0.65-1.35 and ~2-20 mHz. These relatively long-term structures
appear in the power density spectra as mHz QPOs and as well-developed sidebands
around the coherent pulse frequency as well, indicating that the amplitudes of
the coherent pulsation is modulated by those of the mHz QPOs. One interesting
feature is that, while the first flare shows symmetric sidebands around the
coherent pulse frequency, the second flare shows significant excess emission in
the lower-frequency sidebands due to the ~2-20 mHz QPOs. We discuss the origin
of the QPOs using a combination of the beat-frequency model and a modified
version of the Keplerian-frequency model. According to our discussion, it seems
to be possible to attribute the origin of the ~0.65-1.35 and ~2-20 mHz QPOs to
the beating between the rotational frequency of the neutron star and the
Keplerian frequency of large accreting clumps near the corotation radius and to
the orbital motion of clumps at Keplerian radii of 2-10 times 10^9 cm,
respectively.Comment: 12 pages, including 4 figures; accepted by ApJ Letter
RX J1643.7+3402: a new bright cataclysmic variable
We report the discovery of a new bright (V12.6) cataclysmic variable star identified with the ROSAT X-ray source RX J1643.7+3402. Spectroscopic and photometric observations show it to be a novalike variable sharing some of the characteristics of the SW Sex sub-class of novalike CVs. The spectroscopic period may be either 2\fh575 or 2\fh885, within the period "gap." A photometric modulation with a probable period of 2\fh595 and an amplitude of 0.1 mag in V is present on most nights and could be either a "positive" or a "negative" superhump modulation (depending on the exact spectroscopic period), indicating the presence of a precessing accretion disk in this system. Rapid variations of 0.1 to 0.2 mag amplitude in V repeat with a time scale of 15 min
Evolution of Iron K Line Emission in the Black Hole Candidate GX 339-4
GX 339-4 was regularly monitored with RXTE during a period (in 1999) when its
X-ray flux decreased significantly (from 4.2 erg cm to 7.6 erg cms in the 3--20 keV band),
as the source settled into the ``off state''. Our spectral analysis revealed
the presence of a prominent iron K line in the observed spectrum of
the source for all observations. The line shows an interesting evolution: it is
centered at 6.4 keV when the measured flux is above 5
erg cm, but is shifted to 6.7 keV at lower fluxes. The
equivalent width of the line appears to increase significantly toward lower
fluxes, although it is likely to be sensitive to calibration uncertainties.
While the fluorescent emission of neutral or mildly ionized iron atoms in the
accretion disk can perhaps account for the 6.4 keV line, as is often invoked
for black hole candidates, it seems difficult to understand the 6.7 keV line
with this mechanism, because the disk should be less ionized at lower fluxes
(unless its density changes drastically). On the other hand, the 6.7 keV line
might be due to recombination cascade of hydrogen or helium like iron ions in
an optically thin, highly ionized plasma. We discuss the results in the context
of proposed accretion models.Comment: 18 pages, 2 figures, accepted for publication in the ApJ in v552n2p
May 10, 2001 issu
Timing and spectral studies of LMC X-4 in high and low states with Beppo-SAX: Detection of pulsations in the soft spectral component
We report here detailed timing and spectral analysis of two Beppo-SAX
observations of the binary X-ray pulsar LMC X-4 carried out during the low and
high states of its 30.5 days long super-orbital period. Timing analysis clearly
shows 13.5 s X-ray pulsations in the high state of the super-orbital period
which allows us to measure the mid-eclipse time during this observation.
Combining this with two other mid-eclipse times derived earlier with the ASCA,
we derived a new estimate of the orbital period derivative. Pulse-phase
averaged spectroscopy in the high and low states shows that the energy spectrum
in the 0.1 - 10 keV band comprises of a hard power-law, a soft excess, and a
strong iron emission line. The continuum flux is found to decrease by a factor
of ~ 60 in the low state while the decrease in the iron line flux is only by a
factor of ~ 12, suggesting a different site for the production of the line
emission. In the low state, we have not found any significant increase in the
absorption column density. The X-ray emission is found to come from a very
large region, comparable to the size of the companion star. Pulse phase
resolved spectroscopy in the high state shows a pulsating nature of the soft
spectral component with some phase offset compared to the hard X-rays, as is
known in some other binary X-ray pulsars.Comment: 13 pages, 10 figures, Accepted for publication in The Astrophysical
Journa
Multiwavelength Observations of GX 339-4 in 1996. I. Daily Light Curves and X-ray and Gamma-Ray Spectroscopy
As part of our multiwavelength campaign of GX 339-4 observations in 1996 we
present our radio, X-ray, and gamma-ray observations made in July, when the
source was in a hard state (= soft X-ray low state). The radio observations
were made at the time when there was a possible radio jet. We show that the
radio spectrum was flat and significantly variable, and that the radio spectral
shape and amplitude at this time were not anomalous for this source. Daily
light curves from our pointed observation July 9-23 using OSSE, from BATSE, and
from the ASM on RXTE also show that there was no significant change in the X-
and gamma-ray flux or hardness during the time the possible radio jet-like
feature was seen. The higher energy portion of our pointed RXTE observation
made July 26 can be equally well fit using simple power law times exponential
(PLE) and Sunyaev-Titarchuk (ST) functions. An additional soft component is
required, as well as a broad emission feature centered on 6.4 keV. This may be
an iron line that is broadened by orbital Doppler motions and/or scattering off
a hot medium. Its equivalent width is 600 eV. Our simplistic continuum fitting
does not require an extra reflection component. Both a PLE and a ST model also
fit our OSSE spectrum on its own. Although the observations are not quite
simultaneous, combining the RXTE and CGRO spectra we find that the PLE model
easily fits the joint spectrum. However, the ST model drops off too rapidly
with increasing energies to give an acceptable joint fit.Comment: Submitted to Astrophysical Journal. 25 pages. 11 figure
A Transition to the Soft State in GRS 1758-258
Near the end of 2001 February, the black-hole candidate (BHC) GRS 1758-258
made an abrupt transition from a standard hard (low) state to a soft state.
Unlike Cyg X-1 and other BHCs, whose luminosity increases during this
transition, GRS 1758-258 was dimmer after the transition. We present
observations with the Proportional Counter Array on the Rossi X-ray Timing
Explorer and interpret the phenomenon in the context of a ``dynamical'' soft
state model. Using this model we predicted that mass transfer from the
companion had ceased, and that the luminosity should decay on a timescale of a
few weeks. The most recent data support this prediction, being consistent with
a decay time of 34 dy. The current state is consistent with the ``off'' state
of GRS 1758-258 reported by GRANAT/Sigma in 1991-1992.Comment: 13 pages, 5 figures, accepted for publication in The Astrophysical
Journal Letters Accepted version has only minor changes, plus extra data
showing more of the deca
The Evolution Of LMC X-4 Flares: Evidence For Super-Eddington Radiation Oozing Through Inhomogeneous Polar Cap Accretion Flows ?
We present the results of two extensive Rossi X-ray Timing Explorer
observations of large X-ray flaring episodes from the high-mass X-ray binary
pulsar LMC X-4. Light curves during the flaring episodes comprise bright peaks
embedded in relatively fainter regions, with complex patterns of recurrence and
clustering of flares. We identify precursors preceding the flaring activity.
Pulse profiles during the flares appear to be simple sinusoids, and pulsed
fractions are proportional to the flare intensities. We fit Gaussian functions
to flare peaks to estimate the mean full-width-half-maximum to be 68 s.
Significant rapid aperiodic variability exists up to a few hertz during the
flares, which is related to the appearance of narrow, spiky peaks in the light
curves. While spectral fits and softness ratios show overall spectral softening
as the flare intensity increases, the narrow, spiky peaks do not follow this
trend. The mean fluence of the flare peaks is (3.1 2.9)
10 ergs in the 2.5--25 keV energy range, with its maximum at 1.9
10 ergs. The flare peak luminosity reaches up to (2.1
0.2) 10 ergs s, far above the Eddington luminosity of a
neutron star. We discuss possible origins of the flares, and we also propose
that inhomogeneous accretion columns onto the neutron star polar caps are
responsible for the observed properties.Comment: 39 pages (including figures and tables), accepted for publication in
Ap
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
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