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Lewin, W. H. G.

Publications and source records attributed to Lewin, W. H. G..

At least 37 records · Page 2
All sourcesNASA NTRSDOE OSTI
NASA NTRS1994

Spectral and correlated timing behaviour of GX 5-1

We have analyzed the X-ray spectral and fast timing behavior of the Z-source GX 5-1 using X-ray color-color diagrams (CDs), hardness-intensity diagrams (HIDs) and power spectra. We used all EXOSAT ME data on GX 5-1, a total of seven observations during 1983-1985. We detect, for the first time in GX 5-1, secular motion of the well-known 'Z' pattern through the CDs and HIDs. This behavior resembles Cyg X-2, but in GX 5-1 has a smaller amplitude. We find that the power spectra do not change when the 'Z' moves, and are only dependent on the position inside the 'Z'. We also report, for the first time in GX 5-1, indications of flaring-branch (FB) like behavior, both during one of our EXOSAT observations, and during observations done with Ginga. As the secular changes of the position of the 'Z' pattern in the CD and HID are relatively small we investigated in detail the systematic changes in the EXOSAT ME detectors using the only bright and steady X-ray 'standard star', the Crab Nebula. Finally, we determined an intrinsic CD of GX 5-1 (i.e. a soft photon flux-ratio versus hard photon flux-ratio diagram) in a model independent way, by applying a first order correction for the detector efficiency. It is shown, using data of the Crab Nebula, that this method works reasonably well for the determination of intrinsic colors.

Kuulkers, E.↗
NASA NTRS1994

The time-delay spectrum of GX 5-1 in its horizontal branch

Using a cross-spectral technique we investigate time delays between intensity variations of GX 5-1 in 10 X-ray spectral channels. The data were taken during a 1989 Ginga observation during which the source was in its horizontal-branch spectral state. We develope a new method to measure 'time-delay spectra' in fixed Fourier frequency ranges and use it to determine the energy and intensity dependence of time delays in the low-frequency noise (nu less than 2 Hz), the horizontal branch quasi-periodic oscillations (QPO), and the QPO second harmonic. These are the first time-delay spectra of a Z-source in its horizontal branch, and the first detection of time delays in the second harmonic. We consider two mechanisms for the production of the time lags: Comptonization and evolving shots. We perform Monte Carlo simulations of Compton scattering in a homogeneous, isotropic, central corona and show that it qualitatively explain the observed energy and time-delay spectra, but that it cannot explain the differences in the QPO first and second harmomnic time-delay spectra, nor the observed dependence of the QPO fractional rms variability upon energy. We consider implications of our results for millisecond pulsar searches in low-mass X-ray binaries.

Vaughan, B.↗
NASA NTRS1994

Simultaneous radio and X-ray observations of GX 340+0

We present the results of an analysis of simultaneous X-ray (Ginga) and radio Very Large Array (VLA) observations of the Z source GX 340+0. We find GX 340+0 in different X-ray states at the times of the radio observations. We find some evidence for a global correlation between X-ray state and radio flux on a time scale of days; however, this correlation does not hold on shorter time scales.

Oosterbroek, T.↗
NASA NTRS1993

Further ROSAT measurements of the period of 4U 1820-30

We have made two new observations of 4U 1820-30 with the ROSAT position sensitive proportional counter (PSPC). The measurements do not provide further evidence for a secular period decrease of this source. Instead, after inclusion of our new arrival time measurements the significance of the orbital period derivative drops (from 5.5 to 4.6 sigma), its best-fit value decreases (to P-dot/P =(-5.3 +/- 1.1) x 10(exp -8)/yr) and the formal probability that statistical fluctuations have caused the observed P-dot increases to 0.5% (from 0.003%). We detect light curve changes that are large enough to be the cause of some (or all) of the period changes seen in 4U 1820-30, but not large enough to explain the discrepancy of the observed period derivative with that theoretically predicted. We explore the possibility that the secular period change observed during 1976-1991 could have been dominated by random or systematic changes in the position and shape of an occulting bulge on the disk rim and conclude that phase shifts caused by this mechanism could in principle explain the discrepancy entirely.

Klis, M. Van Der↗
NASA NTRS1993

The radio counterpart of the Z source GX 340+0

We report the detection of a variable radio source at the position of the low-mass X-ray binary GX 340+0. This detection confirms the expectation that all six Z sources are variable radio sources.

Penninx, W.↗
NASA NTRS1992

BV RI CCD photometry of 361,281 objects in the field of M 31

Deep BV RI CCD photometry was performed on a 1 sq deg region of M 31. A catalog of photometry and astrometry of a total of 361,281 stars is presented, with typical completion limits of BV RI = (22.3, 22.2, 22.2, 20.9). Photometric accuracy is about 2 percent at V = 19. This catalog allows detailed studies of stellar populations and reddening. The data are currently being used to assist in finding the optical counterparts of Einstein and ROSAT X-ray sources.

Magnier, E. A.↗
NASA NTRS1991

Searches for millisecond pulsations in low-mass X-ray binaries

High-sensitivity search techniques for millisecond periods are presented and applied to data from the Japanese satellite Ginga and HEAO 1. The search is optimized for pulsed signals whose period, drift rate, and amplitude conform with what is expected for low-class X-ray binary (LMXB) sources. Consideration is given to how the current understanding of LMXBs guides the search strategy and sets these parameter limits. An optimized one-parameter coherence recovery technique (CRT) developed for recovery of phase coherence is presented. This technique provides a large increase in sensitivity over the method of incoherent summation of Fourier power spectra. The range of spin periods expected from LMXB phenomenology is discussed, the necessary constraints on the application of CRT are described in terms of integration time and orbital parameters, and the residual power unrecovered by the quadratic approximation for realistic cases is estimated.

Wood, K. S.↗
NASA NTRS1990

Quasi-periodic oscillations in GX 17 + 2

X-ray observations of GX 17 + 2 were made in March and April, 1988 with the Large Area Counter of the Ginga satellite. The source was observed in the flaring, normal, and horizontal branches. Quasi-periodic oscillations (QPOs) were oberved in all three of these branches. The relationship between QPO behavior and the X-ray spectral properties of GX 17 + 2 is examined. Continuous variation of QPO behavior is observed as GX 17 + 2 moves from the normal to flaring branch, indicating that the normal-branch QPO and the flaring-branch QPO are probably one physical phenomenon. This QPO behavior is similar to that observed in Sco X-1. Also, it is found that GX 17 + 2 is an example of the Z-type sources defined by Hasinger et al. (1989).

Penninx, W.↗
NASA NTRS1989

Connections between X-ray and optical variability in the low mass X-ray binary 1735-444

The results of a long duration (4 day) simultaneous optical and X-ray observation of the low mass X-ray binary 1735-444 are presented. The observed X-ray and optical fluxes are correlated; the strength of this correlation is increased when allowance is made for the relatively large orbital modulation of the optical light. A simple interpretation of the optical radiation as reprocessed X-rays in a blackbody disk leads to an implausibly low disk temperature if the disk is assumed to have constant geometry. 1735-444 exhibits bimodal behavior having an X-ray spectral hardness ratio versus source intensity which is similar to that previously seen in sources such as Cyg X-2.

Corbet, R. H. D.↗
NASA NTRS1989

Exosat observations of the X-ray burst source 4U 1608-52

The results of an analysis of two Exosat observations (July 1984, May 1986) of the highly variable X-ray burst source 4U 1608-52 are presented. During both observations the persistent X-ray flux was low (quiescence), and the persistent X-ray spectrum could be well fitted with a power-law model, with approximately the same index, but a somewhat different low-energy cut off. During both observations one type 1 burst was seen. It is shown that the relation between color temperature and effective temperature differs markedly from simple relations derived from theoretical models. A dip in the bolometric flux occurred near the peak of the 1986 burst. Possible models for this dip are discussed.

Penninx, W.↗
NASA NTRS1988

On the relation between X-ray burst properties and the persistent X-ray luminosity

Published data on X-ray bursts is analyzed, assuming that the peak luminosity of X-ray bursts with radius expansion is a standard candle and that the ratio of the anisotropy factors, xi(b) and xi(p) (Sztajno et al., 1987) is the same for all burst sources. It is shown that the effective burst duration, defined as the ratio of burst fluence to maximum burst flux, is strongly anticorrelated with the persistent luminosity. A positive correlation with the persistent luminosity of the ratio of the average persistent flux to the time-averaged burst flux is found. It is concluded that, independent of the accretion rate, after a given waiting time burst sources produce bursts with approximately the same (average) energy. This suggests that continuous stable burning of a sizeable fraction of the nuclear fuel is a general phenomenon on the surface of accreting neutron stars. It is suggested that this fraction is a gradually increasing function of the accretion rate.

Van Paradijs, J.↗
NASA NTRS1988

Quasi-periodic oscillations in the bright Galactic bulge X-ray source GX 340 + 0

Quasi-periodic oscillations (QPOs) have been detected in the X-ray flux of the bright Galactic bulge X-ray source GX 340+0. The QPO, the associated red noise, and their relation to the spectral state of the source are described. The QPO behavior is found to be similar to that seen in Cyg X-2 and GX 5-1.

Van Paradijs, J.↗
NASA NTRS1988

The recurrence behaviour of X-ray bursts from 4U/MXB 1735-44

During about 80 h of uninterrupted observations of the X-ray burst source 4 U/MXB 1735-44, made in August 1985 with Exosat, five bursts were observed. The bursts occur at irregular intervals, ranging between 30 min and 36 h. The burst sizes do not scale with the intervals preceding the bursts; in particular the energy in a burst which occurred after an interval of 50 min was only 30 percent less than that of a burst which occurred after more than 20 h. Due to the long uninterrupted observations possible with Exosat, this is the first time for 1735-44 that the burst intervals and the burst characteristics (e.g., fluence) can be compared. The observations show that the model recently proposed by Fujimoto et al. (1987) to explain irregular burst intervals, is not applicable to 1735-44. The amplitude of periodic X-ray flux variations at the 4.65 h orbital period is less than 4 percent.

Van Paradijs, J.↗
NASA NTRS1988

Quasi-periodic oscillations in the X-ray flux of the rapid burster (MXB 1730-335)

The results of an Exosat observation of the 'Rapid Burster' in August 1985 during an active state are presented. Forty 1-10 minute long type 2 bursts were detected together with constant emission. Quasi-periodic oscillations (QPOs) were observed in 23 bursts with frequencies in the range of 2-5 Hz. Obscuration of the central X-ray source by an oscillating accretion disk rim is suggested as a possible scenario for the generation of the QPO from the Rapid Burster.

Stella, L.↗
NASA NTRS1987

Multipeaked X-ray bursts from 4U/MXB 1636-53 - Evidence against burst-induced accretion disk coronae

The burst-induced accretion-disk corona (BIADC) model proposed by Melia (1987) to explain the multiple-peak burst profiles of 4U/MXB 1636-53 in terms of direct and scattered components is examined critically. Published observational data (Sztajno et al., 1986, and Lewin et al., 1987) are presented in tables and graphs and analyzed. A number of possible BIADC scenarios are discussed, and it is argued that the observed characteristics of the 1636-53 bursts are not well accounted for by the BIADC model.

Penninx, W.↗
NASA NTRS1987

Energy dependent delay measurements of quasi-periodic oscillations in low-mass X-ray binaries

Results of Monte Carlo simulations of the Comptonization of photons in a finite spherical Comptonizing cloud are presented. The energy dependence of the arrival times of the Comptonized photons is very different for different input photon spectra. If the delays observed in the high-frequency QPO of Cyg X-2 and GX5-1 are the result of Comptonization, multichannel time-delay measurements may constrain the energy of the input photons and thus provide important information on the accretion process in low-mass X-ray binaries.

Wijers, R. A. M. J.↗
NASA NTRS1987

Exosat observations of 4U/MXB 1636-53 - On the relation between the amount of accreted fuel and the strength of an X-ray burst

During Exosat observations of 4U/MXB 1636-53 in August 1985, three bursts were observed in 6 h and 24 bursts in 79 h of uninterrupted observation. The burst times, burst fluences, peak burst fluxes, burst rise times, decay times, burst intervals, mean persistent fluxes, and the product of the latter two are listed, analyzed, and discussed. The implications of the results for thermonuclear flash models are examined. A model which fits the data well involves bursts which suffer from nuclear energy losses due to stable hydrogen burning between bursts. Thus, the percentage of 'lost' energy increases with increasing burst intervals. For individual bursts, however, there are significant deviations from the approximate linear relationship.

Lewin, W. H. G.↗