Pluto and its main satellite, Charon, occulted the same star on 2008 June 22. This event was observed from Australia and La Reunion Island, providing the east and north Charon Plutocentric offset in ...the sky plane (J2000): X= + 12,070.5 ? 4 km (+ 546.2 ? 0.2 mas), Y= + 4,576.3 ? 24 km (+ 207.1 ? 1.1 mas) at 19:20:33.82 UT on Earth, corresponding to JD 2454640.129964 at Pluto. This yields Charon's true longitude L= 153.483 ? 0071 in the satellite orbital plane (counted from the ascending node on J2000 mean equator) and orbital radius r= 19,564 ? 14 km at that time. We compare this position to that predicted by (1) the orbital solution of Tholen & Buie (the 'TB97' solution), (2) the PLU017 Charon ephemeris, and (3) the solution of Tholen et al. (the 'T08' solution). We conclude that (1) our result rules out solution TB97, (2) our position agrees with PLU017, with differences of Delta *DL= + 0.073 ? 0071 in longitude, and Delta *Dr= + 0.6 ? 14 km in radius, and (3) while the difference with the T08 ephemeris amounts to only Delta *DL= 0.033 ? 0071 in longitude, it exhibits a significant radial discrepancy of Delta *Dr= 61.3 ? 14 km. We discuss this difference in terms of a possible image scale relative error of 3.35 X 10--3in the 2002-2003 Hubble Space Telescope images upon which the T08 solution is mostly based. Rescaling the T08 Charon semi-major axis, a = 19, 570.45 km, to the TB97 value, a = 19636 km, all other orbital elements remaining the same ('T08/TB97' solution), we reconcile our position with the re-scaled solution by better than 12 km (or 0.55 mas) for Charon's position in its orbital plane, thus making T08/TB97 our preferred solution.
Super-orbital periods in X-ray binaries Sood, R.; Farrell, S.; O’Neill, P. ...
Advances in space research,
2007, 2007-1-00, 20070101, Letnik:
40, Številka:
10
Journal Article
Recenzirano
Super-orbital periods have been reported from more than 30 X-ray binaries, but no consistent picture has emerged as to the underlying mechanism behind these periods. Only four of the sources have ...exhibited stable super-orbital periods over intervals of several years, and three of these are classed as high mass X-ray binaries. The remaining sources are split approximately equally between low mass and high mass binaries. We have searched for correlations with known properties of the binary systems for all the sources, viz. compact object spin periods, orbital periods, accretion disc precession, and companion star properties. We present the results of our investigations.
We have measured the orbital light curve of dwarf nova OY Carinae on eight separate nights between 1997 September and 2005 December. The measurements were made in white light using CCD photometers on ...the Mt Canopus 1-m telescope. The time of eclipse in 2005 December was 168 ± 5 s earlier than that predicted by the Wood et al. ephemeris. Using the times of eclipse from our measurements and the compilation of published measurements by Pratt et al., we find that the observational data are inconsistent with a constant period and indicate that the orbital period is decreasing by 5 ± 1 × 10−12 s s−1. This is too fast to be explained by gravitational radiation emission alone. It is possible that the change is cyclic with a period of ∼35 yr and a fractional period change ΔP/P= 2.6 × 10−7. This is probably due to solar-cycle magnetic activity in the secondary. There are also large systematic deviations, with a time-scale of years, from a sinusoidal modulation.
We present the first microlensing candidate for a free-floating exoplanet-exomoon system, MOA-2011-BLG-262, with a primary lens mass of M sub(host) ~ 4 Jupiter masses hosting a sub-Earth mass moon. ...The argument for an exomoon hinges on the system being relatively close to the Sun. The data constrain the product M sub(L)pi sub(rel) where M sub(L) is the lens system mass and pi sub(rel) is the lens-source relative parallax. If the lens system is nearby (large pi sub(rel)), then M sub(L) is small (a few Jupiter masses) and the companion is a sub-Earth-mass exomoon. The best-fit solution has a large lens-source relative proper motion, mu sub(rel) = 19.6 + or - 1.6 mas yr super(-1), which would rule out a distant lens system unless the source star has an unusually high proper motion. However, data from the OGLE collaboration nearly rule out a high source proper motion, so the exoplanet+exomoon model is the favored interpretation for the best fit model. However, there is an alternate solution that has a lower proper motion and fits the data almost as well. This solution is compatible with a distant (so stellar) host. A Bayesian analysis does not favor the exoplanet+exomoon interpretation, so Occam's razor favors a lens system in the bulge with host and companion masses of M sub(host) = 0.12 super(+0.19) sub(-0.06) M sub(middot in circle) and m sub(comp) = 18 super(+28) sub(-10) M sub(+ in circle), at a projected separation of a sub(perpendicular) = 0.84 super(+0.25) sub(-0.14) AU. The existence of this degeneracy is an unlucky accident, so current microlensing experiments are in principle sensitive to exomoons. In some circumstances, it will be possible to definitively establish the mass of such lens systems through the microlensing parallax effect. Future experiments will be sensitive to less extreme exomoons.
Timing Noise in SGR 1806–20 Woods, P. M; Kouveliotou, C; Finger, M. H ...
Astrophysical journal/The Astrophysical journal,
05/2000, Letnik:
535, Številka:
1
Journal Article
Recenzirano
Odprti dostop
We have phase-connected a sequence of Rossi X-Ray Timing Explorer Proportional Counter Array observations of SGR 1806-20 covering 178 days. We find that a simple secular spin-down model does not ...adequately fit the data. The period derivative varies gradually during the observations between 8.1x10-11 and 11.7x10-11 s s(-1) (at its highest, approximately 40% larger than the long-term trend), while the average burst rate as seen with the Burst and Transient Source Experiment drops throughout the time interval. The phase residuals give no compelling evidence for periodicity, but more closely resemble timing noise as seen in radio pulsars. The magnitude of the timing noise, however, is large relative to the noise level typically found in radio pulsars (Delta8=4.8; frequency derivative average power approximately 7x10-20 cycles(2) s(-3)). Combining these results with the noise levels measured for some anomalous X-ray pulsars, we find that all magnetar candidates have Delta(8) values larger than those expected from a simple extrapolation of the correlation found in radio pulsars. We find that the timing noise in SGR 1806-20 is greater than or equal to the levels found in some accreting systems (e.g., Vela X-1, 4U 1538-52, and 4U 1626-67), but the spin-down of SGR 1806-20 has thus far maintained coherence over 6 yr. Alternatively, an orbital model with a period Porb=733 days provides a statistically acceptable fit to the data. If the phase residuals are created by Doppler shifts from a gravitationally bound companion, then the allowed parameter space for the mass function (small) and orbital separation (large) rule out the possibility of accretion from the companion sufficient to power the persistent emission from the SGR.
We present a systematic, homogeneous analysis of all the EXOSAT ME, high time resolution data on Sco X-1. We investigate, for the first time, all power spectral properties of the <100 Hz ...quasi-periodic oscillations (QPO) and noise of Sco X-1 as a function of position on the Z-shaped track traced out in the X-ray colour—colour diagram. For this purpose, we introduce a new generally applicable method for parametrizing the position of a source on its track in a colour—colour or hardness—intensity diagram. Generally, the properties of Sco X-1 vary smoothly as a function of position along the Z track. However, some variability parameters change abruptly at either of the vertices of the Z track, indicating that the branches of the Z track represent distinct source states not only in spectral state but also in rapid variability characteristics. All variability components are found to have energy spectra harder than the average flux. We show that the very low-frequency noise (VLFN) is consistent with being solely the result of motion along the Z track. The power spectra of the X-ray intensity as well as source position along the Z track extend, unbroken, to time-scales of nearly 1 day. We study the high-frequency noise (HFN) component for the first time in sufficient detail to show that there are changes in the HFN cut-off frequency with position on the Z track. It changes abruptly from |75 to |35 Hz at the normal/flaring branch vertex. The HFN is found to extend out to about 300 Hz. The QPO show a remarkably rapid change in frequency at or just before the normal-branch/flaring-branch (NB/FB) vertex. This transition happens within 1.5 per cent of the entire extent of the Z track. The QPO themselves are visible for 17 per cent of the Z. We find a new type of behaviour near the NB/FB vertex, i.e., rapid excursions from the NB into the FB and back again taking only a few minutes. We find several indications that position on the Z track is not the only parameter governing the behaviour of Sco X-1. The most dramatic examples of this are two brief episodes where the QPO frequency changed rapidly without the usual changes in colours and intensity that accompany a change of source state. In one case the frequency rapidly rose from 6 to 16 Hz, and then returned to 6 Hz, while Sco X-1 apparently remained unmoved on the normal branch in the colour—colour diagram. In the second case the QPO frequency changed from 16 to 7 Hz without the usual indications of a passage through the normal/flaring branch vertex (simultaneous dips in the count rate and colours as the frequency changes through 8 Hz). Thus it seems that deviations from the usual one-to-one correspondence between QPO behaviour and spectral state, perhaps caused QPO frequency mode switching, do occasionally occur.
We present the analysis of the microlensing event MOA-2010-BLG-117, and show that the light curve can only be explained by the gravitational lensing of a binary source star system by a star with a ...Jupiter-mass ratio planet. It was necessary to modify standard microlensing modeling methods to find the correct light curve solution for this binary source, binary-lens event. We are able to measure a strong microlensing parallax signal, which yields the masses of the host star, M* = 0.58 0.11 M , and planet, mp = 0.54 0.10MJup, at a projected star-planet separation of a = 2.42 0.26 au, corresponding to a semimajor axis of au. Thus, the system resembles a half-scale model of the Sun-Jupiter system with a half-Jupiter0mass planet orbiting a half-solar-mass star at very roughly half of Jupiter's orbital distance from the Sun. The source stars are slightly evolved, and by requiring them to lie on the same isochrone, we can constrain the source to lie in the near side of the bulge at a distance of DS = 6.9 0.7 kpc, which implies a distance to the planetary lens system of DL = 3.5 0.4 kpc. The ability to model unusual planetary microlensing events, like this one, will be necessary to extract precise statistical information from the planned large exoplanet microlensing surveys, such as the WFIRST microlensing survey.
PDF
