108 research outputs found
Independent Confirmation of the Pioneer 10 Anomalous Acceleration
I perform an independent analysis of radio Doppler tracking data from the
Pioneer 10 spacecraft for the time period 1987-1994. All of the tracking data
were taken from public archive sources, and the analysis tools were developed
independently by myself. I confirm that an apparent anomalous acceleration is
acting on the Pioneer 10 spacecraft, which is not accounted for by present
physical models of spacecraft navigation. My best fit value for the
acceleration, including corrections for systematic biases and uncertainties, is
(8.60 +/- 1.34) x 10^{-8} cm s^{-2}, directed towards the Sun. This value
compares favorably to previous results. I examine the robustness of my result
to various perturbations of the analysis method, and find agreement to within
+/- 5%. The anomalous acceleration is reasonably constant with time, with a
characteristic variation time scale of > 70 yr. Such a variation timescale is
still too short to rule out on-board thermal radiation effects, based on this
particular Pioneer 10 data set.Comment: RevTeX, 29 pages, 5 figures, submitted to Phys Rev
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
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
The Symbiotic System SS73 17 Seen with Suzaku
We observed with Suzaku the symbiotic star SS73 17, motivated by the
discovery by the INTEGRAL satellite and the Swift BAT survey that it emits hard
X-rays. Our observations showed a highly-absorbed X-ray spectrum with NH >
10^23 cm-2, equivalent to A_V > 26, although the source has B magnitude 11.3
and is also bright in UV. The source also shows strong, narrow iron lines
including fluorescent Fe K as well as Fe xxv and Fe xxvi. The X-ray spectrum
can be fit with a thermal model including an absorption component that
partially covers the source. Most of the equivalent width of the iron
fluorescent line in this model can be explained as a combination of
reprocessing in a dense absorber plus reflection off a white dwarf surface, but
it is likely that the continuum is partially seen in reflection as well. Unlike
other symbiotic systems that show hard X-ray emission (CH Cyg, RT Cru, T CrB,
GX1+4), SS73 17 is not known to have shown nova-like optical variability, X-ray
flashes, or pulsations, and has always shown faint soft X-ray emission. As a
result, although it is likely a white dwarf, the nature of the compact object
in SS73 17 is still uncertain. SS73 17 is probably an extreme example of the
recently discovered and relatively small class of hard X-ray emitting symbiotic
systems.Comment: 6 pages, accepted by PASJ for 2nd Suzaku Special Issu
Discovery of the Spin Frequency of 4U 0614+09 with SWIFT/BAT
We report the discovery of burst oscillations at 414.7 Hz during a
thermonuclear X-ray burst from the low mass X-ray binary (LMXB) 4U 0614+091
with the Burst Alert Telescope (BAT) onboard SWIFT. In a search of the BAT
archive, we found two burst triggers consistent with the position of 4U
0614+091. We searched both bursts for high frequency timing signatures, and
found a significant detection at 414.7 Hz during a 5 s interval in the cooling
tail of the brighter burst. This result establishes the spin frequency of the
neutron star in 4U 0614+091 as 415 Hz. The oscillation had an average amplitude
(rms) of 14%, These results are consistent with those known for burst
oscillations seen in other LMXBs. The inferred ratio of the frequency
difference between the twin kHz QPOs, and the spin frequency in this source is
strongly inconsistent with either 0.5 or 1, and tends to support the recent
suggestions by Yin et al., and Mendez & Belloni, that the kHz QPO frequency
difference may not have a strong connection to the neutron star spin frequency.Comment: 10 pages, 3 figures. AASTeX. Accepted for publication in the
Astrophysical Journal Letter
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
The ASCA Spectrum of the Vela Pulsar Jet
ROSAT observations of the Vela pulsar and its surroundings revealed a
collimated X-ray feature almost 45' in length (Markwardt & Ogelman 1995),
interpreted as the signature ``cocoon'' of a one-sided jet from the Vela
pulsar. We report on a new ASCA observation of the Vela pulsar jet at its head,
the point where the jet is believed to interact with the supernova remnant. The
head is clearly detected, and its X-ray spectrum is remarkably similar to the
surrounding supernova remnant spectrum, extending to X-ray energies of at least
7 keV. A ROSAT+ASCA spectrum can be fit by two-component emission models but
not standard one-component models. The lower energy component is thermal and
has a temperature of 0.29+/-0.03 keV (1 sigma); the higher energy component can
be fit by either a thermal component of temperature ~4 keV or a power law with
photon index ~2.0. Compared to the ROSAT-only results, the mechanical
properties of the jet and its cocoon do not change much. If the observed
spectrum is that of a hot jet cocoon, then the speed of the jet is at least 800
km s^-1, depending on the angle of inclination. The mechanical power driving
the jet is >10^36 erg s^-1, and the mass flow rate at the head is > 10^-6 M_sun
yr^-1. We conclude that the jet must be entraining material all along its
length in order to generate such a large mass flow rate. We also explore the
possibility that the cocoon emission is synchrotron radiation instead of
thermal.Comment: 12 pages, LaTeX in AAS v4.0 preprint style, two PS figures, accepted
for publication in the ApJ Letter
- …