180 research outputs found
Period derivative of the M15 X-ray Binary AC211/X2127+119
We have combined Rossi X-ray Timing Explorer observations of X2127+119, the
low-mass X-ray binary in the globular cluster M15, with archival X-ray
lightcurves to study the stability of the 17.1 hr orbital period. We find that
the data cannot be fit by the Ilovaisky (1993) ephemeris, and requires either a
7sigma change to the period or a period derivative Pdot/P~9x10e-7 per year.
Given its remarkably low L_X/L_opt such a Pdot lends support to models that
require super-Eddington mass transfer in a q~1 binary.Comment: 11 pages, 3 figures, to be published in New Astronom
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
ELODIE metallicity-biased search for transiting Hot Jupiters II. A very hot Jupiter transiting the bright K star HD189733
Among the 160 known exoplanets, mainly detected in large radial-velocity
surveys, only 8 have a characterization of their actual mass and radius thanks
to the two complementary methods of detection: radial velocities and
photometric transit. We started in March 2004 an exoplanet-search programme
biased toward high-metallicity stars which are more frequently host extra-solar
planets. This survey aims to detect close-in giant planets, which are most
likely to transit their host star. For this programme, high-precision radial
velocities are measured with the ELODIE fiber-fed spectrograph on the 1.93-m
telescope, and high-precision photometry is obtained with the CCD Camera on the
1.20-m telescope, both at the Haute-Provence Observatory. We report here the
discovery of a new transiting hot Jupiter orbiting the star HD189733. The
planetary nature of this object is confirmed by the observation of both the
spectroscopic and photometric transits. The exoplanet HD189733b, with an
orbital period of 2.219 days, has one of the shortest orbital periods detected
by radial velocities, and presents the largest photometric depth in the light
curve (~ 3%) observed to date. We estimate for the planet a mass of 1.15 +-
0.04 Mjup and a radius of 1.26 +- 0.03 RJup. Considering that HD189733 has the
same visual magnitude as the well known exoplanet host star HD209458, further
ground-based and space-based follow-up observations are very promising and will
permit a characterization of the atmosphere and exosphere of this giant
exoplanet.Comment: 5 pages, submitted to Astronomy & Astrophysic
The Off State of GX 339-4
We report BeppoSAX and optical observations of the black hole candidate GX
339-4 during its X-ray `off' state in 1999. The broad-band (0.8-50 keV) X-ray
emission can be fitted by a single power law with spectral index, \alpha ~1.6.
The observed luminosity is 6.6e33 erg s^{-1} in the 0.5-10 keV band, which is
at the higher end of the flux distribution of black hole soft X-ray transients
in quiescence, comparable to that seen in GS 2023+338 and 4U 1630-47. An
optical observation just before the BeppoSAX observation shows the source to be
very faint at these wavelengths as well (B=20.1, V=19.2). By comparing with
previously reported `off' and low states (LS), we conclude that the `off' state
is actually an extension of the LS, i.e. a LS at lower intensities. We propose
that accretion models such as the advection-dominated accretion flows are able
to explain the observed properties in such a state.Comment: Accepted for publication in MNRA
A new mass-ratio for the X-ray Binary X2127+119 in M15?
The luminous low-mass X-ray binary X2127+119 in the core of the globular
cluster M15 (NGC 7078), which has an orbital period of 17 hours, has long been
assumed to contain a donor star evolving off the main sequence, with a mass of
0.8 solar masses (the main-sequence turn-off mass for M15). We present
orbital-phase-resolved spectroscopy of X2127+119 in the H-alpha and He I 6678
spectral region, obtained with the Hubble Space Telescope. We show that these
data are incompatible with the assumed masses of X2127+119's component stars.
The continuum eclipse is too shallow, indicating that much of the accretion
disc remains visible during eclipse, and therefore that the size of the donor
star relative to the disc is much smaller in this high-inclination system than
the assumed mass-ratio allows. Furthermore, the flux of X2127+119's He I 6678
emission, which has a velocity that implies an association with the stream-disc
impact region, remains unchanged through eclipse, implying that material from
the impact region is always visible. This should not be possible if the
previously-assumed mass ratio is correct. In addition, we do not detect any
spectral features from the donor star, which is unexpected for a 0.8 solar-mass
sub-giant in a system with a 17-hour period.Comment: 6 pages, 4 figures, accepted by A&
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
XMM-Newton observations of UW CrB -- detection of X-ray bursts and evidence for accretion disc evolution
UW CrB (MS1603+2600) is a peculiar short period X-ray binary that exhibits
extraordinary optical behaviour. The optical light curve shape of the system
changes drastically from night to night, without any changes in overall
brightness. Here we report X-ray observations of UW CrB obtained with {\it
XMM-Newton}. We find evidence for several X-ray bursts confirming a neutron
star primary. This considerably strengthens the case that UW CrB is an
Accretion Disc Corona (ADC) system located at a distance of at least 5--7 kpc,
(3--5 kpc above the galactic plane). The X-ray and optical monitor
(UV+optical) light curves show remarkable shape variation from one observing
run to another, which we suggest are due to large scale variations in the
accretion disc shape resulting from a warp which periodically obscures the
optical and soft X-ray emission. This is also supported by the changes in
phase-resolved X-ray spectra.Comment: Accepted for publication in MNRA
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