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Hassall, B. J. M.

Publications and source records attributed to Hassall, B. J. M..

12 records
All sourcesNASA NTRSDOE OSTI
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

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 NTRS1991

A study of the flaring and quiescent X-ray and UV emission from II Pegasi

An investigation has been conducted of the rotational modulation of the transition-region UV and coronal X-ray emission for the RS CVn system II Pegasi. The X-ray light curve is dominated by a strong flare detected at orbital phase, where the minimum of the photometric wave occurred. The flare parameters derived show that the flare originates with a height greater than half the stellar radius. The characteristics of the flare are similar to those of a solar two-ribbon flare; a comparison of the midtransition region density with that in the coronal region shows a very steep pressure gradient.

Tagliaferri, G.↗
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 NTRS1988

IUE observations of the dwarf nova BV Centauri

The IUE observations of the dwarf nova BV Cen in quiescence are summarized. The reprocessed spectra from January 1979 are analyzed to show that the maximum temperature of the white dwarf primary star, based on its brightness at 1375 A, ranges from 42,000 to about 125,000 K depending upon the adopted values of interstellar reddening, the mass of the white dwarf, and the distance to the system. Physical arguments based on the likely state of the quiescent accretion disk in the BV Cen system suggest that the effective temperature of the white dwarf is significantly lower than this maximum temperature. The disk is more likely to be optically thick than optically thin. The impact of these findings on models for dwarf nova outbursts is discussed, but no firm conclusions can be drawn. The ultraviolet emission-line spectrum from observations obtained in August 1985 is described in detail.

Williger, G.↗
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

Exosat and IUE observations of SW UMa during superoutburst

IUE and Exosat observations obtained during the March 1986 superoutburst of the SU UMa type dwarf nova SW UMa are presented. The observations show the presence of a hot, extensive, optically thick disk. The UV fluxes show an increase of 1000 over the quiescent values, implying a large increase in mass accretion rate. The Exosat obserations show an increase in the soft X-ray count rate by a factor of 44 over quiescence, while the medium-energy flux remains undetected. The most surprising result is that the softness ratio in superoutburst is similar to the quiescent value of 0.3. No variation at the orbital or spin periods or at the superhump period are evident in the superoutburst X-ray data. A transient variation is observed at 5.1 minutes similar to that observed in the optical a few days after the X-ray observations.

Szkody, Paula↗
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.↗