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Charles, P. A.

Publications and source records attributed to Charles, P. A..

At least 19 records
All sourcesNASA NTRSDOE OSTI
NASA NTRS2002

Long-Term Properties of Accretion Discs in X-ray Binaries: The Variable Third Period in SMC X-1 - 1

Long term X-ray monitoring data from the RXTE All Sky Monitor (ASM) reveal that the third (superorbital) period in SMC X-1 is not constant but varies between 40-60 days. A dynamic power spectrum analysis indicates that the third period has been present continuously throughout the five years of ASM observations. This period changed smoothly from 60 days to 45 days and then returned to its former value, on a timescale of approximately 1600 days. During the nearly 4 years of overlap between the CGRO & RXTE missions, the simultaneous BATSE hard X-ray data confirm this variation in SMC X-1. Sources of systematic error and possible artefacts are investigated and found to be incapable of reproducing the results reported here. Our disco cry of such an instability in the superorbital period of SMC X-1 is interpreted in the context of recent theoretical studies of warped, precessing accretion discs. We find that the behaviour of SMC X-1 is consistent with a radiation - driven warping model.

Charles, P. A.↗
NASA NTRS1998

Period Derivative of the M15 X-Ray Binary AC211/X2127+119

We have combined Rossi X-ray Timing Explorer observations of X2127+119, the low-mass X-ray binary in the globular cluster M15, with archival X-ray lightcurves to study the stability of the 17.1 hr orbital period. We find that the data cannot be fit by the Ilovaisky ephemeris, and requires either a 7.sigma change to the period or a period derivative P prime/P approximately 9 x 10(exp -7)/yr. Given its remarkably low L(sub X)/L(sub opt) such a P prime lends support to models that require super-Eddington mass transfer in a q approximately 1 binary.

Homer, L.↗
NASA NTRS1997

T Pyxidis: The First Cataclysmic Variable with a Collimated Jet

We present the first observational evidence for a collimated jet in a cataclysmic variable system; the recurrent nova T Pyxidis. Optical spectra show bipolar components of H(alpha) with velocities approx. 1400 km/s, very similar to those observed in the supersoft X-ray sources and in SS 433. We argue that a key ingredient of the formation of jets in the supersoft X-ray sources and T Pyx (in addition to an accretion disk threaded by a vertical magnetic field), is the presence of nuclear burning on the surface of the white dwarf.

Shahbaz, T.↗
NASA NTRS1997

Multi-Wavelength Monitoring of GRS 1915+105

Since its discovery in 1992, the superluminal X-ray transient GRS 1915+105 has been extensively observed in an attempt to understand its behaviour. We present here preliminary results from a multi-wavelength campaign undertaken from July to September 1996. This study includes X-ray data from the RXTE All Sky Monitor and BATSE, two-frequency data from the Nancay radio telescope, and infrared photometry from the 1.8m Perkins telescope at Lowell Observatory. The K-band data presented herein provide the first long-term well-sampled IR light curve of GRS 1915+105. We compare the various light curves, searching for correlations in the behaviour of the source at differing wavelengths and for possible periodicities.

Bandyopadhyay, R.↗
NASA NTRS1997

Multi-Wavelength Monitoring of GRS 1915+105

Since its discovery in 1992, the superluminal X-ray transient GRS 1915+105 has been extensively observed in an attempt to understand its behaviour. We present here first results from a multi-wavelength campaign undertaken from July to September 1996. This study includes X-ray data from the RXTE All Sky Monitor and BATSE, two-frequency data from the Nancay radio telescope, and infrared photometry from the 1.8 m Perkins telescope at Lowell Observatory. The first long-term well-sampled IR light curve of GRS 1915+105 is presented herein and is consistent with the interpretation of this source as a long-period binary. We compare the various light curves, searching for correlations in the behaviour of the source at differing wavelengths and for possible periodicities.

Bandyopadhyay, R.↗
NASA NTRS1994

Multiwavelength observations of Hercules X-1/HZ Herculis

We present first results from a multiwavelength campaign to observe the binary pulsar system Her X-1/ HZ Her. The campaign spanned 4 days during 1993 August: observations were taken with five satellites (IUE, EUVE, ROSAT, ASCA, and CGRO/BATSE) and several ground-based facilities. A substantial, unexpected drop in flux at X-ray energies, with no change in absorbing column density, was observed. The pulse period has increased from the previous measurement, contrary to the usual spin-up, and no pulsed emission is detected above 0.9 keV in the low state. The optical and UV fluxes continued to show 1.7 day modulation attributed to X-ray heating of the companion star. However, the significant reduction in UV flux observed around eclipse implies an absence of the normally observed excess attributed to X-ray heating of the disk. We conclude that we have observed an anomalous low state, seen only once before (Parmar et al. 1985), in which the X-ray flux is not redistributed but obscured. We suggest explanations for the behavior of the flux at different wavelengths.

Vrtilek, S. D.↗
NASA NTRS1992

UV spectroscopy of Z Chamaeleontis. II - The 1988 January normal outburst

IUE observations taken during the 1988 January normal outburst of Z Cha are presented and a detailed comparison with the 1987 April superoutburst is made. The most important difference from the superoutburst is that the normal outburst continuum flux shows less than 10 percent orbital variation away from the eclipse, implying that there is no 'cool' bulge on the disk to occult the brighter inner disk periodically. The implications for the outburst mechanism in the types of outburst are discussed. The evolution of the continuum flux distribution and emission-line fluxes, the modulation of the continuum and line fluxes with orbital phase, and the behavior of the mideclipse spectral during normal outburst are investigated.

Harlaftis, E. T.↗
NASA NTRS1992

The 14.8-h orbital period of GX339-4

We present the results of photometric observations of the black hole candidate GX339-4, obtained while the system was in an 'off' state. We show that a 14.8-h modulation was present, and provide evidence for a similar periodicity in the 'high' state from a reanalysis of previously published photometry and spectroscopy. The presence of the same period in both states implies that it is likely to be the orbital period of the system. The spectroscopy analysis provides evidence for an apparent change in the systemic velocity of the system. The amplitude of the observed radial velocity variations, however, permits only crude limits to be placed on the mass of the compact object. Only absorption-line spectroscopy of the secondary in the 'off' state will provide a convincing mass determination.

Callanan, P. J.↗
NASA NTRS1992

An observational case against nova hibernation

We use WHT spectroscopy and imaging to show that nova Vul 1670 (= CK Vul) has been incorrectly identified, and thus its luminosity cannot be used as evidence that novae fade into a 'hibernation' phase within 300 yr of their outbursts. INT spectroscopy is used to correct the magnitude of nova Sge 1783 (= WY Sge) for inclination, this result also implying that novae do not fade significantly. We therefore suggest that, while novae decline in the first 60 yr after outburst, thereafter their luminosity remains constant, and they never undergo a 'hibernation' phase. We show that this idea is consistent with the space density of novae and novalike variables, the outburst interval of SS Cyg and the current luminosities of old novae.

Naylor, T.↗
NASA NTRS1992

UV spectroscopy of Z Chamaeleontis. I - Time dependent dips in superoutburst

Extensive IUE observations of the dwarf nova Z Cha during the 1987 April superoutburst and IUE-Exosat observations during the 1985 July superoutburst are presented. The UV light curve shows two dips when folded on the orbital period. One dip, at orbital phase 0.8 becomes shallower as the superoutburst progresses and can be associated with decreasing mass transfer rate from the secondary star. The other dip, at orbital phase 0.15, appears after the development of the superhump and some days after the occurrence of the largest dip at phase 0.8. The continuum flux distribution during superoutbursts is fainter and redder than in low-inclination dwarf novae during superoutbursts. This is interpreted in terms of the extended vertical disk structure which occults the hot inner parts of the disk with the development of a 'cool' bulge on the edge of the disk at orbital phase 0.75. Details of the behaviour of the UV emission lines during eclipse and away from eclipse are discussed.

Harlaftis, E. T.↗
NASA NTRS1990

WHT observations of cataclysmic variables in globular clusters. II - The CV candidates in M71

Results are presented from photometric and spectroscopic observations of the six UV-bright objects in M71 which Richer and Fahlman (1988) proposed as candidate cataclysmic variables. It is found that the two brightest of these candidates are probably hot subdwarfs. It is suggested that the remaining candidates are more likely to be white dwarfs and subdwarfs than cataclysmic variables.

Machin, G.↗
NASA NTRS1989

WHT observations of cataclysmic variables in globular clusters. I - V101 in M5 during quiescence and outburst

WHT spectroscopy and photometry are used to investigate the structure of the cataclysmic vaiable V101, in the globular cluster M5. An encouraging consistency is found between the properties derived from the color, magnitude, radial-velocity variations and outburst light curve. All these indicate that V101's orbital period lies above the period gap, is at the distance of M5, and is probably a dwarf nova.

Naylor, T.↗
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 NTRS1988

IUE observations of the 1987 superoutburst of the dwarf nova Z Cha

Low resolution IUE observations of the dwarf nova Z Cha during superoutburst are presented. These cover most of the development of the outburst and have sufficient time resolution to probe continuum and line behavior on orbital phase. The observed modulation on this phase is very similar to that observed in the related object OY Car. The results imply the presence of a cool spot on the edge of the edge of the accretion disk, which periodically occults the brighter inner disk. Details of the line behavior suggest that the line originated in an extended wind-emitting region. In contrast to archive spectra obtained in normal outburst, the continuum is fainter and redder, indicating that the entire superoutburst disk may be geometrically thicker than during a normal outburst.

Harlaftis, E.↗
NASA NTRS1988

The 1985 May superoutburst of the dwarf nova OY Carinae. II - IUE and Exosat observations

Ultraviolet (IUE) and X-ray (Exosat) observations of the eclipsing dwarf nova OY Carinae during the superoutburst of May 1985 are presented. From the lack of X-ray eclipse and UV behavior, it is deduced that the X-ray flux originates in an optically thin corona comparable in size to the Roche lobe, and not directly from the white dwarf or boundary layer. The asymmetric UV line emission originates partly in the accretion disk and partly in a wind. There is a strong modulation of the UV continuum flux that is thought to be caused by extended vertical disk structure shadowing the inner regions.

Naylor, T.↗
NASA NTRS1988

The dwarf nova dippers

Previous papers have shown that the dwarf nova OY Car has an extensive vertical disk structure during its superoutbursts. This structure is analogous to that seen in the low mass X-ray binaries (LMXBs), in that the whole disk flares toward its edge, and there is a bulge or 'dark spot' where the mass transfer stream meets the disk edge. The paper presents evidence suggesting that two more dwarf novae may have such a stream impact bulge, which manifests itself as a dip in the UV light curve, analogous to those seen in the X-ray light curves of the LMXB dippers.

Naylor, T.↗
NASA NTRS1986

IUE observations of the dwarf novae OY Car in superoutburst

The May 1985 superoutburst of OY Car, an eclipsing member of the SU UMa subclass of dwarf novae, was observed extensively with IUE to obtain phase-resolved spectra throughout several orbital cycles, when the superhump was well developed and lay close in phase to the eclipse. The UV lines and continuum vary with orbital phase and show a strong UV eclipse coincident with the optical eclipse. This implies that the superhump source at eclipse is not a strong contributor in the ultraviolet. The UV emission line spectrum and lack of X-ray eclipse suggest the presence of an extended coronal region. The flux evolution observed is unusual in that gross changes occur during the decline from superoutburst. The later spectra actually appear hotter than spectra earlier in the decline, and suggest that, early on, the inner disk is obscured from view.

Hassall, B. J. M.↗