1,071 research outputs found
Analysis of variability in the burst oscillations of the accreting millisecond pulsar XTE J1814-338
The accreting millisecond pulsar XTE J1814-338 exhibits oscillations at the
known spin frequency during Type I X-ray bursts. The properties of the burst
oscillations reflect the nature of the thermal asymmetry on the stellar
surface. We present an analysis of the variability of the burst oscillations of
this source, focusing on three characteristics: fractional amplitude, harmonic
content and frequency. Fractional amplitude and harmonic content constrain the
size, shape and position of the emitting region, whilst variations in frequency
indicate motion of the emitting region on the neutron star surface. We examine
both long-term variability over the course of the outburst, and short-term
variability during the bursts. For most of the bursts, fractional amplitude is
consistent with that of the accretion pulsations, implying a low degree of fuel
spread. There is however a population of bursts whose fractional amplitudes are
substantially lower, implying a higher degree of fuel spread, possibly forced
by the explosive burning front of a precursor burst. For the first harmonic,
substantial differences between the burst and accretion pulsations suggest that
hotspot geometry is not the only mechanism giving rise to harmonic content in
the latter. Fractional amplitude variability during the bursts is low; we
cannot rule out the hypothesis that the fractional amplitude remains constant
for bursts that do not exhibit photospheric radius expansion (PRE). There are
no significant variations in frequency in any of the bursts except for the one
burst that exhibits PRE. This burst exhibits a highly significant but small
(Hz) drop in frequency in the burst rise. The timescale of the
frequency shift is slower than simple burning layer expansion models predict,
suggesting that other mechanisms may be at work.Comment: 20 pages, 20 figures, accepted for publication in ApJ. Uses
emulateapj.cl
Aircraft woods: their properties, selection, and characteristics
Strength values of various woods for aircraft design for a 15 per cent moisture condition of material and a 3-second duration of stress are presented, and also a discussion of the various factors affecting the values. The toughness-test method of selecting wood is discussed, and a table of acceptance values for several species is given
Secular spin-down of the AMP XTE J1751-305
Context. Of the 13 known accreting millisecond pulsars, only a few showed
more than one outburst during the RXTE era. XTE J1751-305 showed, after the
main outburst in 2002, other three dim outbursts. We report on the timing
analysis of the latest one, occurred on October 8, 2009 and serendipitously
observed from its very beginning by RXTE. Aims. The detection of the pulsation
during more than one outburst permits to obtain a better constraint of the
orbital parameters and their evolution as well as to track the secular spin
frequency evolution of the source. Methods. Using the RXTE data of the last
outburst of the AMP XTE J1751-305, we performed a timing analysis to improve
the orbital parameters. Because of the low statistics, we used an epoch folding
search technique on the whole data set to improve the local estimate of the
time of ascending node passage. Results. Using this new orbital solution we
epoch folded data obtaining three pulse phase delays on a time span of 1.2
days, that we fitted using a constant spin frequency model. Comparing this
barycentric spin frequency with that of the 2002 outburst, we obtained a
secular spin frequency derivative of -0.55(12) x 10^{-14} Hz s^{-1}. In the
hypothesis that the secular spin-down is due to a rotating magneto-dipole
emission, consistently with what is assumed for radio pulsars, we estimate the
pulsar's magnetic dipole value. We derive an estimate of the magnetic field
strength at the polar cap of B_{PC} = 4.0(4) x 10^8 Gauss, for a neutron star
mass of 1.4M\odot, assuming the Friedman Pandharipande Skyrme equation of
state.Comment: 7 pages, 4 figures, accepted for publication on A&
Using Exploratory Data Analytics to Identify Deficiencies in mCherry Red Fluorescent Protein and Suggest Improvements
Fitxes dels barri
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
Visualization of Spatially Controlled Glucokinase Activation in Living Pancreatic Beta Cells using an Optimized FRET-Based Biosensor
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
Search for pulsations at high radio frequencies from accreting millisecond X-ray pulsars in quiescence
It is commonly believed that millisecond radio pulsars have been spun up by
transfer of matter and angular momentum from a low-mass companion during an
X-ray active mass transfer phase. A subclass of low-mass X-ray binaries is that
of the accreting millisecond X-ray pulsars, transient systems that show periods
of X-ray quiescence during which radio emission could switch on. The aim of
this work is to search for millisecond pulsations from three accreting
millisecond X-ray pulsars, XTE J1751-305, XTE J1814-338, and SAX J1808.4-3658,
observed during their quiescent X-ray phases at high radio frequencies (5 - 8
GHz) in order to overcome the problem of the free-free absorption due to the
matter engulfing the system. A positive result would provide definite proof of
the recycling model, providing the direct link between the progenitors and
their evolutionary products. The data analysis methodology has been chosen on
the basis of the precise knowledge of orbital and spin parameters from X-ray
observations. It is subdivided in three steps: we corrected the time series for
the effects of (I) the dispersion due to interstellar medium and (II) of the
orbital motions, and finally (III) folded modulo the spin period to increase
the signal-to-noise ratio. No radio signal with spin and orbital
characteristics matching those of the X-ray sources has been found in our
search, down to very low flux density upper limits. We analysed several
mechanisms that could have prevented the detection of the signal, concluding
that the low luminosity of the sources and the geometric factor are the most
likely reasons for this negative result.Comment: 5 pages, 3 figures. Accepted for publication by A&
The optical counterparts of Accreting Millisecond X-Ray Pulsars during quiescence
Eight Accreting Millisecond X-ray Pulsars (AMXPs) are known to date. Optical
and NIR observations carried out during quiescence give a unique opportunity to
constrain the nature of the donor star and to investigate the origin of the
observed quiescent luminosity at long wavelengths. Using data obtained with the
ESO-Very Large Telescope, we performed a deep optical and NIR photometric study
of the fields of XTE J1814-338 and of the ultracompact systems XTE J0929-314
and XTE J1807-294 during quiescence in order to look for the presence of a
variable counterpart. If suitable candidates were found, we also carried out
optical spectroscopy. We present here the first multi-band (VR) detection of
the optical counterpart of XTE J1814-338 in quiescence together with its
optical spectrum. The optical light curve shows variability in both bands
consistent with a sinusoidal modulation at the known 4.3 hr orbital period and
presents a puzzling decrease of the V-band flux around superior conjunction
that may be interpreted as a partial eclipse. The marginal detection of the
very faint counterpart of XTE J0929-314 and deep upper limits for the
optical/NIR counterpart of XTE J1807-294 are also reported. We also briefly
discuss the results reported in the literature for the optical/NIR counterpart
of XTE J1751-305. Our findings are consistent with AMXPs being systems
containing an old, weakly magnetized neutron star, reactivated as a millisecond
radio pulsar during quiescence which irradiates the low-mass companion star.
The absence of type I X-ray bursts and of hydrogen and helium lines in outburst
spectra of ultracompact (P_orb < 1 hr) AMXPs suggests that the companion stars
are likely evolved dwarf stars.Comment: Accepted for publication by A&A; 12 pages, 12 figure
An Analysis of the Shapes of Interstellar Extinction Curves. VI. The Near-IR Extinction Law
We combine new HST/ACS observations and existing data to investigate the
wavelength dependence of NIR extinction. Previous studies suggest a power-law
form, with a "universal" value of the exponent, although some recent
observations indicate that significant sight line-to-sight line variability may
exist. We show that a power-law model provides an excellent fit to most NIR
extinction curves, but that the value of the power, beta, varies significantly
from sight line-to-sight line. Therefore, it seems that a "universal NIR
extinction law" is not possible. Instead, we find that as beta decreases, R(V)
[=A(V)/E(B-V)] tends to increase, suggesting that NIR extinction curves which
have been considered "peculiar" may, in fact, be typical for different R(V)
values. We show that the power law parameters can depend on the wavelength
interval used to derive them, with the beta increasing as longer wavelengths
are included. This result implies that extrapolating power law fits to
determine R(V) is unreliable. To avoid this problem, we adopt a different
functional form for NIR extinction. This new form mimics a power law whose
exponent increases with wavelength, has only 2 free parameters, can fit all of
our curves over a longer wavelength baseline and to higher precision, and
produces R(V) values which are consistent with independent estimates and
commonly used methods for estimating R(V). Furthermore, unlike the power law
model, it gives R(V)'s that are independent of the wavelength interval used to
derive them. It also suggests that the relation R(V) = -1.36 E(K-V)/E(B-V) -
0.79 can estimate R(V) to +/-0.12. Finally, we use model extinction curves to
show that our extinction curves are in accord with theoretical expectations.Comment: To appear in the Astrophysical Journa
- …