832 research outputs found
X-ray Bursts from the Accreting Millisecond Pulsar XTE J1814-338
Since the discovery of the accreting millisecond pulsar XTE J1814-338 a total
of 27 thermonuclear bursts have been observed from the source with the
Proportional Counter Array (PCA) onboard the Rossi X-ray Timing Explorer
(RXTE). Spectroscopy of the bursts, as well as the presence of continuous burst
oscillations, suggests that all but one of the bursts are sub-Eddington. The
remaining burst has the largest peak bolometric flux of 2.64 x E^-8
erg/sec/cm^2, as well as a gap in the burst oscillations, similar to that seen
in Eddington limited bursts from other sources. Assuming this burst was
Eddington limited we obtain a source distance of about 8 kpc. All the bursts
show coherent oscillations at the 314.4 Hz spin frequency. The burst
oscillations are strongly frequency and phase locked to the persistent
pulsations. Only two bursts show evidence for frequency drift in the first few
seconds following burst onset. In both cases the initial drift corresponds to a
spin down of a few tenths of a Hz. The large oscillation amplitude during the
bursts confirms that the burst flux is modulated at the spin frequency. We
detect, for the first time, a significant first harmonic component in burst
oscillations. The ratio of countrate in the first harmonic to that in the
fundamental can be > 0.25 and is, on average, less than that of the persistent
pulsations. If the pulsations result from a single bright region on the
surface, the harmonic strength suggests the burst emission is beamed, perhaps
due to a stronger magnetic field than in non-pulsing LMXBs. Alternatively, the
harmonic content could result from a geometry with two bright regions.Comment: AASTeX, 15 pages, 4 figures. Accepted for publication in the
Astrophysical Journal Letter
Swift/BAT and RXTE Observations of the Peculiar X-ray Binary 4U 2206+54 - Disappearance of the 9.6 Day Modulation
Observations of the high-mass X-ray binary 4U 2206+54 with the Swift Burst
Alert Telescope (BAT) do not show modulation at the previously reported period
of 9.6 days found from observations made with the Rossi X-ray Timing Explorer
(RXTE) All-Sky Monitor (ASM). Instead, the strongest peak in the power spectrum
of the BAT light curve occurs at a period of 19.25 +/- 0.08 days, twice the
period found with the RXTE ASM. The maximum of the folded BAT light curve is
also delayed compared to the maximum of the folded ASM light curve. The most
recent ASM data folded on twice the 9.6 day period show similar morphology to
the folded BAT light curve. This suggests that the apparent period doubling is
a recent secular change rather than an energy-dependent effect. The 9.6 day
period is thus not a permanent strong feature of the light curve. We suggest
that the orbital period of 4U 2206+54 may be twice the previously proposed
value.Comment: Accepted for publication in The Astrophysical Journa
Timing Features of the Accretion--driven Millisecond X-Ray Pulsar XTE J1807--294 in 2003 March Outburst
In order to probe the activity of the inner disk flow and its effect on the
neutron star surface emissions, we carried out the timing analysis of the Rossi
X-Ray Timing Explorer (RXTE) observations of the millisecond X-ray pulsar XTE
J1807--294, focusing on its correlated behaviors in X-ray intensities, hardness
ratios, pulse profiles and power density spectra. The source was observed to
have a serial of broad "puny" flares on a timescale of hours to days on the top
of a decaying outburst in March 2003. In the flares, the spectra are softened
and the pulse profiles become more sinusoidal. The frequency of kilohertz
quasi-periodic oscillation (kHz QPO) is found to be positively related to the
X-ray count rate in the flares. These features observed in the flares could be
due to the accreting flow inhomogeneities. It is noticed that the fractional
pulse amplitude increases with the flare intensities in a range of , comparable to those observed in the thermonuclear bursts of the
millisecond X-ray pulsar XTE J1814--338, whereas it remains at about 6.5% in
the normal state. Such a significant variation of the pulse profile in the
"puny" flares may reflect the changes of physical parameters in the inner disk
accretion region. Furthermore, we noticed an overall positive correlation
between the kHz QPO frequency and the fractional pulse amplitude, which could
be the first evidence representing that the neutron-star surface emission
properties are very sensitive to the disk flow inhomogeneities. This effect
should be cautiously considered in the burst oscillation studies.Comment: Accepted by ApJ, 23 pages, 7 figures, 3 table
The X-ray Position and Optical Counterpart of the Accretion-Powered Millisecond Pulsar XTE J1814-338
We report the precise optical and X-ray localization of the 3.2 ms
accretion-powered X-ray pulsar XTE J1814-338 with data from the Chandra X-Ray
Observatory as well as optical observations conducted during the 2003 June
discovery outburst. Optical imaging of the field during the outburst of this
soft X-ray transient reveals an R = 18 star at the X-ray position. This star is
absent (R > 20) from an archival 1989 image of the field and brightened during
the 2003 outburst, and we therefore identify it as the optical counterpart of
XTE J1814-338. The best source position derived from optical astrometry is R.A.
= 18h13m39.s04, Dec.= -33d46m22.3s (J2000). The featureless X-ray spectrum of
the pulsar in outburst is best fit by an absorbed power-law (with photon index
= 1.41 +- 0.06) plus blackbody (with kT = 0.95 +- 0.13 keV) model, where the
blackbody component contributes approximately 10% of the source flux. The
optical broad-band spectrum shows evidence for an excess of infrared emission
with respect to an X-ray heated accretion disk model, suggesting a significant
contribution from the secondary or from a synchrotron-emitting region. A
follow-up observation performed when XTE J1814-338 was in quiescence reveals no
counterpart to a limiting magnitude of R = 23.3. This suggests that the
secondary is an M3 V or later-type star, and therefore very unlikely to be
responsible for the soft excess, making synchroton emission a more reasonable
candidate.Comment: Accepted for publication in ApJ. 6 pages; 3 figure
A fast search strategy for gravitational waves from low-mass X-ray binaries
We present a new type of search strategy designed specifically to find
continuously emitting gravitational wave sources in known binary systems based
on the incoherent sum of frequency modulated binary signal sidebands. The
search pipeline can be divided into three stages: the first is a wide
bandwidth, F-statistic search demodulated for sky position. This is followed by
a fast second stage in which areas in frequency space are identified as signal
candidates through the frequency domain convolution of the F-statistic with an
approximate signal template. For this second stage only precise information on
the orbit period and approximate information on the orbital semi-major axis are
required apriori. For the final stage we propose a fully coherent Markov chain
monte carlo based follow up search on the frequency subspace defined by the
candidates identified by the second stage. This search is particularly suited
to the low-mass X-ray binaries, for which orbital period and sky position are
typically well known and additional orbital parameters and neutron star spin
frequency are not. We note that for the accreting X-ray millisecond pulsars,
for which spin frequency and orbital parameters are well known, the second
stage can be omitted and the fully coherent search stage can be performed. We
describe the search pipeline with respect to its application to a simplified
phase model and derive the corresponding sensitivity of the search.Comment: 13 pages, 3 figures, to appear in the GWDAW 11 conference proceeding
Monitoring and Discovering X-ray Pulsars in the Small Magellanic Cloud
Regular monitoring of the SMC with RXTE has revealed a huge number of X-ray
pulsars. Together with discoveries from other satellites at least 45 SMC
pulsars are now known. One of these sources, a pulsar with a period of
approximately 7.8 seconds, was first detected in early 2002 and since discovery
it has been found to be in outburst nine times. The outburst pattern clearly
shows a period of 45.1 +/- 0.4 d which is thought to be the orbital period of
this system. Candidate outburst periods have also been obtained for nine other
pulsars and continued monitoring will enable us to confirm these. This large
number of pulsars, all located at approximately the same distance, enables a
wealth of comparative studies. In addition, the large number of pulsars found
(which vastly exceeds the number expected simply by scaling the relative mass
of the SMC and the Galaxy) reveals the recent star formation history of the SMC
which has been influenced by encounters with both the LMC and the Galaxy.Comment: 5 pages, 4 figures, AIP conference proceedings format. Contribution
to "X-ray Timing 2003: Rossi and Beyond." meeting held in Cambridge, MA,
November, 200
Type I X-ray bursts and burst oscillations in the accreting millisecond X-ray pulsar IGR J17511-3057
We report the discovery of burst oscillations at the spin frequency in ten
thermonuclear bursts from the accreting millisecond X-ray pulsar (AMXP) IGR
J17511-3057. The burst oscillation properties are, like those from the
persistent AMXPs SAX J1808.4-3658 and XTE J1814-338, anomalous compared to
burst oscillations from intermittent pulsars or non-pulsing LMXBs. Like SAX
J1808.4-3658 they show frequency drifts in the rising phase rather than the
tail. There is also evidence for harmonic content. Where IGR J17511-3057 is
unusual compared to the other two persistent pulsars is that oscillations are
not detected throughout all bursts. As accretion rate drops the bursts get
brighter and their rise/decay time scales become shorter, while the oscillation
amplitude falls below the detection threshold: first in the burst peak and then
also in the rise. None of the bursts from IGR J17511-3057 show evidence for
photospheric radius expansion (which might be expected to suppress oscillation
amplitude) which allow us to set an upper limit to the distance of 6.9 kpc. We
discuss the implications of our results for models of the burst oscillation
mechanism.Comment: 11 pages, 5 figures, MNRAS in pres
Comparison between Windowed FFT and Hilbert-Huang Transform for Analyzing Time Series with Poissonian Fluctuations: A Case Study
Hilbert-Huang Transform (HHT) is a novel data analysis technique for
nonlinear and non-stationary data. We present a time-frequency analysis of both
simulated light curves and an X-ray burst from the X-ray burster 4U 1702-429
with both the HHT and the Windowed Fast Fourier Transform (WFFT) methods. Our
results show that the HHT method has failed in all cases for light curves with
Poissonian fluctuations which are typical for all photon counting instruments
used in astronomy, whereas the WFFT method can sensitively detect the periodic
signals in the presence of Poissonian fluctuations; the only drawback of the
WFFT method is that it cannot detect sharp frequency variations accurately.Comment: 10 pages, 12 figure
Suzaku Observations of Four Heavily Absorbed HMXBs
We report on Suzaku observations of four unidentified sources from the
INTEGRAL and Swift BAT Galactic plane surveys. All the sources have a large
neutral hydrogen column density and are likely members of an emerging class of
heavily absorbed high mass X-ray binary (HMXB) first identified in INTEGRAL
observations. Two of the sources in our sample are approximately constant flux
sources, one source shows periodic variation and one source exhibited a short,
bright X-ray outburst. The periodicity is transient, suggesting it is produced
by a neutron star in an elliptical orbit around a stellar wind source. We
analyze the flaring source in several segments to look for spectral variation
and discuss the implications of the findings for the nature of the source. We
conclude that all four sources in our sample can be identified with the
emerging class of highly absorbed HMXBs, that one is a newly identified
transient X-ray pulsar and that at least one is a newly identified supergiant
fast X-ray transient (SFXT).Comment: 22 pages, 5 figures, submitted to Ap
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
- âŠ