203 research outputs found
Updating the orbital ephemeris of Her X-1; rate of decay and eccentricity of the orbit
We present an update of the orbital ephemeris of the binary X-ray pulsar Her
X-1 and determine an improved value for the rate of orbital decay. In addition,
we report the first measurement of the orbital eccentricity. We have analyzed
pulse timing data of Her X-1 from X-ray observations by RXTE (Rossi X-Ray
Timing Explorer) and INTEGRAL over the period 1996-2007. Accurate pulse arrival
times were determined from solar system bary-centered photon arrival times by
generating pulse profiles averaged over appropriately short integration times.
Applying pulse phase connection techniques, it was possible to determine
sufficiently accurate local ephemeris data for seven observation periods
distributed over 12 years. Combining the new local T90 values with historical
values from the literature we update the orbital ephemeris of Her X-1 to T90 =
MJD 46359.871940(6) and Porb = 1.700167590(2) d and measure a continuous change
of the orbital period of dPorb/dt = -(4.85 +/- 0.13) x 10-11 s/s. For the first
time, a value for the eccentricity of the orbit of Her X-1 is measured to be e
= (4.2 +/- 0.8) x 10-4.Comment: 7 pages, 4 figures, accepted by A&A on 30.03.200
Quaternary geology and stratigraphy of Kitsap County, Washington
New radiocarbon dates and stratigraphic evidence indicate correlations between stratigraphic units on Whidbey Island and in Kitsap County. Eight new C14 dates and five previous dates, together with the stratigraphic position of units and similarities in their composition, support the concept that the Double Bluff Drift, Whidbey Formation, and possibly Possession Drift, extend south of Whidbey Island into Kitsap County.
In Kitsap County, fine-grained floodplain deposits of the Whidbey Formation, with radiocarbon dates beyond the limits of conventional laboratory methods, are located at higher elevations than adjacent floodplain deposits of the Olympia nonglacial interval. This stratigraphic relationship suggest that the Whidbey floodplain was extensively eroded prior to deposition of the Olympia floodplain sediments which are unconformable upon this irregular surface. Thus, Molenaar’s belief that the Kitsap Formation was deposited during the Olympia nonglacial interval and his useage of the term Kitsap Formation for these stratified fine-grained sediments of various ages, is incorrect.
The type locality of the Kitsap Formation, near Maplewood in southeast Kitsap County, includes a silt and two thin peat units lying between oxidized gravel of a pre-Vashon glaciation that is overlain by fine-grained Olympia sediments. Molenaar interpreted this sequence as a transition from glacial to nonglacial deposition and included the silt, peat, and oxidized gravel in the Kitsap Formation. However, a recent C14 date obtained from the peat was \u3e40,000 yrs B.P. (WW ), whereas the oldest C14 date obtained by the author from Olympia sediments elsewhere in Kitsap County is 36,235 yrs B.P. (U.W. 446). In addition, the arbitrary contact chosen by Molenaar between glacial and nonglacial sediments at Maplewood is not representative of the contact elsewhere in Kitsap County, and by including the oxidized gravel as part of the Kitsap Formation, he has deviated from the fine-grained nonglacial description originally used in the defining the Formation
Continuous monitoring of pulse period variations in Her X-1 using Swift/BAT
Context: Monitoring of pulse period variations in accreting binary pulsars is
an important tool to study the interaction between the magnetosphere of the
neutron star and the accretion disk. While the X-ray flux of the brightest
X-ray pulsars have been successfully monitored over many years (e.g. with
RXTE/ASM, CGRO/BATSE, Swift/BAT), the possibility to monitor their pulse timing
properties continuously has so far been very limited. Aims: In our work we show
that the Swift/BAT observations can be used to monitor coherent pulsations of
bright X-ray sources and use the Swift archival data to study one of the most
enigmatic X-ray pulsars, Hercules X-1. A quasi-continuous monitoring of the
pulse period and the pulse period derivative of an X-ray pulsar, here Her X-1,
is achieved over a long time (<~ 4 yrs). We compare our observational results
with predictions of accretion theory and use them to test different aspects of
the physical model of the system. Methods: In our analysis we use the data
accumulated with Swift/BAT starting from the beginning of 2005 (shortly after
launch) until the present time. To search for pulsations and for their
subsequent analysis we used the count rate measured by the BAT detector in the
entire field of view. Results: The slope of the correlation between the locally
determined spin-up rate and the X-ray luminosity is measured for Her X-1 and
found to be in agreement with predictions of basic accretion torque theory. The
observed behaviour of the pulse period together with the previously measured
secular decrease of the system's orbital period is discussed in the frame of a
model assuming ejection of matter close to the inner boundary of the accretion
disk.Comment: 7 pages, 5 figures, accepted for publication in Astronomy &
Astrophysic
Giant outburst of EXO 2030+375: pulse-phase resolved analysis of INTEGRAL data
In June-September 2006 the Be/X-ray binary EXO 2030+375 experienced the
second giant outburst since its discovery. The source was shown to have a
complicated pulse-averaged X-ray spectral continuum with possible evidence of
cyclotron absorption features. In this paper we present the first pulse-phase
resolved analysis of the broad band X-ray spectra of EXO 2030+375 obtained with
the INTEGRAL observatory close to the maximum and during the decay phase of the
giant outburst. We report a strong variability of the spectrum with pulse
phase. Alternative spectral continuum models are discussed. The dependence of
the spectral parameters on pulse phase during the maximum of the outburst and
the evolution of the pulse profiles with time are qualitatively consistent with
the pulsar's emission diagram changing from the fan-beam geometry close to the
maximum of the outburst to a combination of pencil and fan beams (of comparable
intesities) at the end of the decay phase. Evidence of a cyclotron absorption
line around 63 keV at the pulse phase interval preceeding the main peak of the
pulse profile is present in the spectrum obtained close to the maximum of the
outburst.Comment: 8 pages, 10 figures, accepted for publication in A&
Two ~35 day clocks in Her X-1: evidence for neutron star free precession
We present evidence for the existence of two ~35 day clocks in the Her X-1/HZ
Her binary system. ~35 day modulations are observed 1) in the Turn-On cycles
with two on- and two off-states, and 2) in the changing shape of the pulse
profiles which re-appears regularly. The two ways of counting the 35 day cycles
are generally in synchronization. This synchronization did apparently break
down temporarily during the long Anomalous Low (AL3) which Her X-1 experienced
in 1999/2000, in the sense that there must have been one extra Turn-On cycle.
Our working hypothesis is that there are two clocks in the system, both with a
period of about ~35 days: precession of the accretion disk (the less stable
"Turn-On clock") and free precession of the neutron star (the more stable
"Pulse profile clock"). We suggest that free precession of the neutron star is
the master clock, and that the precession of the accretion disk is basically
synchronized to that of the neutron star through a feed-back mechanism in the
binary system. However, the Turn-On clock can slip against its master when the
accretion disk has a very low inclination, as is observed to be the case during
AL3. We take the apparent correlation between the histories of the Turn-Ons, of
the Anomalous Lows and of the pulse period evolution, with a 5 yr
quasi-periodicity, as evidence for strong physical interaction and feed-back
between the major components in the system. We speculate that the 5 yr (10 yr)
period is either due to a corresponding activity cycle of HZ Her or a natural
ringing period of the physical system of coupled components. The question
whether free precession really exists in neutron stars is of great importance
for the understanding of matter with supra-nuclear density.Comment: 6 pages, 5 figures, accepted for publication by A&
The mass of the neutron star in Vela X-1 and tidally induced non-radial oscillations in GP Vel
We report new radial velocity observations of GP Vel/HD77581, the optical
companion to the eclipsing X-ray pulsar Vela X-1. Using data spanning more than
two complete orbits of the system, we detect evidence for tidally induced
non-radial oscillations on the surface of GP Vel, apparent as peaks in the
power spectrum of the residuals to the radial velocity curve fit. By removing
the effect of these oscillations (to first order) and binning the radial
velocities, we have determined the semi-amplitude of the radial velocity curve
of GP Vel to be K_o=22.6+/-1.5 km/s. Given the accurately measured
semi-amplitude of the pulsar's orbit, the mass ratio of the system is
0.081+/-0.005. We are able to set upper and lower limits on the masses of the
component stars as follows. Assuming GP Vel fills its Roche lobe then the
inclination angle of the system, i=70.1+/-2.6 deg. In this case we obtain the
masses of the two stars as M_x=2.27 +/-0.17 M_sun for the neutron star and
M_o=27.9+/-1.3 M_sun for GP Vel. Conversely, assuming the inclination angle is
i=90 deg, the ratio of the radius of GP Vel to the radius of its Roche lobe is
beta=0.89+/-0.03 and the masses of the two stars are M_x=1.88+/-0.13 M_sun and
M_o=23.1+/-0.2 M_sun. A range of solutions between these two sets of limits is
also possible, corresponding to other combinations of i and beta. In addition,
we note that if the zero phase of the radial velocity curve is allowed as a
free parameter, rather than constrained by the X-ray ephemeris, a significantly
improved fit is obtained with an amplitude of 21.2+/-0.7 km/s and a phase shift
of 0.033+/-0.007 in true anomaly. The apparent shift in the zero phase of the
radial velocity curve may indicate the presence of an additional radial
velocity component at the orbital period.Comment: Accepted for publication in Astronomy & Astrophysic
INTEGRAL observations of the variability of OAO 1657-415
The Galactic Plane Scan (GPS) was one of the core observation programmes of
the INTEGRAL satellite. The highly variable accreting pulsar OAO 1657-415 was
frequently observed within the GPS. We investigate the spectral and timing
properties of OAO 1657-415 and their variability on short and long time scales
in the energy range 6-160 keV. During the time covered by the INTEGRAL
observations, the pulse period evolution shows an initial spin-down, which is
followed by an equally strong spin-up. In combining our results with historical
pulse period measurements (correcting them for orbital variation) and with
stretches of continuous observations by BATSE, we find that the long-term
period evolution is characterised by a long-term spin-up overlayed by sets of
relative spin-down/spin-up episodes, which appear to repeat quasi-periodically
on a 4.8 yr time scale. We measure an updated local ephemeris and confirm the
previously determined orbital period with an improved accuracy. The spectra
clearly change with pulse phase. The spectrum measured during the main peak of
the pulse profile is particularly hard. We do not find any evidence of a
cyclotron line, wether in the phase-averaged spectrum or in phase-resolved
spectra.Comment: 12 pages, 11 figure
The influence of boreal biomass burning emissions on the distribution of tropospheric ozone over North America and the North Atlantic during 2010
We have analysed the sensitivity of the tropospheric ozone distribution over North America and the North Atlantic to boreal biomass burning emissions during the summer of 2010 using the GEOS-Chem 3-D global tropospheric chemical transport model and observations from in situ and satellite instruments. We show that the model ozone distribution is consistent with observations from the Pico Mountain Observatory in the Azores, ozonesondes across Canada, and the Tropospheric Emission Spectrometer (TES) and Infrared Atmospheric Sounding Instrument (IASI) satellite instruments. Mean biases between the model and observed ozone mixing ratio in the free troposphere were less than 10 ppbv. We used the adjoint of GEOS-Chem to show the model ozone distribution in the free troposphere over Maritime Canada is largely sensitive to NO<sub>x</sub> emissions from biomass burning sources in Central Canada, lightning sources in the central US, and anthropogenic sources in the eastern US and south-eastern Canada. We also used the adjoint of GEOS-Chem to evaluate the Fire Locating And Monitoring of Burning Emissions (FLAMBE) inventory through assimilation of CO observations from the Measurements Of Pollution In The Troposphere (MOPITT) satellite instrument. The CO inversion showed that, on average, the FLAMBE emissions needed to be reduced to 89% of their original values, with scaling factors ranging from 12% to 102%, to fit the MOPITT observations in the boreal regions. Applying the CO scaling factors to all species emitted from boreal biomass burning sources led to a decrease of the model tropospheric distributions of CO, PAN, and NO<sub>x</sub> by as much as −20 ppbv, −50 pptv, and −20 pptv respectively. The modification of the biomass burning emission estimates reduced the model ozone distribution by approximately −3 ppbv (−8%) and on average improved the agreement of the model ozone distribution compared to the observations throughout the free troposphere, reducing the mean model bias from 5.5 to 4.0 ppbv for the Pico Mountain Observatory, 3.0 to 0.9 ppbv for ozonesondes, 2.0 to 0.9 ppbv for TES, and 2.8 to 1.4 ppbv for IASI
X-ray variation statistics and wind clumping in Vela X-1
We investigate the structure of the wind in the neutron star X-ray binary
system Vela X-1 by analyzing its flaring behavior. Vela X-1 shows constant
flaring, with some flares reaching fluxes of more than 3.0 Crab between 20-60
keV for several 100 seconds, while the average flux is around 250 mCrab. We
analyzed all archival INTEGRAL data, calculating the brightness distribution in
the 20-60 keV band, which, as we show, closely follows a log-normal
distribution. Orbital resolved analysis shows that the structure is strongly
variable, explainable by shocks and a fluctuating accretion wake. Analysis of
RXTE ASM data suggests a strong orbital change of N_H. Accreted clump masses
derived from the INTEGRAL data are on the order of 5 x 10^19 -10^21 g. We show
that the lightcurve can be described with a model of multiplicative random
numbers. In the course of the simulation we calculate the power spectral
density of the system in the 20-100 keV energy band and show that it follows a
red-noise power law. We suggest that a mixture of a clumpy wind, shocks, and
turbulence can explain the measured mass distribution. As the recently
discovered class of supergiant fast X-ray transients (SFXT) seems to show the
same parameters for the wind, the link between persistent HMXB like Vela X-1
and SFXT is further strengthened.Comment: 8 pages, 6 figures, accepted for publication in A&
- …