arXiv subjects

Carole A. Haswell

Publications and source records attributed to Carole A. Haswell.

26 records · Page 2Linked to original sources

arXiv2006

Three dimensional SPH simulations of radiation-driven warped accretion discs

We present three dimensional smoothed particle hydrodynamics (SPH) calculations of warped accretion discs in X-ray binary systems. Geometrically thin, optically thick accretion discs are illuminated by a central radiation source. This illumination exerts a non-axisymmetric radiation pressure on the surface of the disc resulting in a torque that acts on the disc to induce a twist or warp. Initially planar discs are unstable to warping driven by the radiation torque and in general the warps also precess in a retrograde direction relative to the orbital flow. We simulate a number of X-ray binary systems which have different mass ratios using a number of different luminosities for each. Radiation-driven warping occurs for all systems simulated. For mass ratios q ~ 0.1 a moderate warp occurs in the inner disc while the outer disc remains in the orbital plane (c.f. X 1916-053). For less extreme mass ratios the entire disc tilts out of the orbital plane (c.f. Her X-1). For discs that are tilted out of the orbital plane in which the outer edge material of the disc is precessing in a prograde direction we obtain both positive and negative superhumps simultaneously in the dissipation light curve (c.f. V603 Aql).

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arXiv2004

Multi-epoch Spectroscopy of IY UMa: Quiescence, Rise, Normal Outburst & Superoutburst

We exploit rare observations covering the time before and during a normal outburst in the deeply-eclipsing SU UMa system IY UMa to study the dramatic changes in the accretion flow and emission at the onset of outburst. Through Doppler tomography we study the emission distribution, revealing classic accretion flow behaviour in quiescence, with the stream-disc impact ionizing the nearby accretion disc. We observe a delay of hours to a couple of days between the rise in continuum and the rise in the emission lines at the onset of the outburst. From line profiles and Doppler maps during normal and superoutburst we conclude that reprocessing of boundary layer radiation is the dominant emission line mechanism in outburst, and that the normal outburst began in the outer disc. The stream-disc impact feature (the `orbital hump') in the H alpha line flux light curve disappears before the onset of the normal outburst, and may be an observable signal heralding an impending outburst.

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arXiv2002

The detection of the donor star in IY UMa

We present the results of a search for the donor star in the high inclination SU UMa type cataclysmic variable IY UMa. We detect absorption features in the near infrared consistent with an M type dwarf donor star. Using the skew mapping technique to exploit the velocity information provided by the 8183-8194 Angstrom Na I absorption doublet, we locate the absorption at the expected donor velocity of IY UMa.

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arXiv2002

Spectral mapping of the spiral structures in IP Pegasi on the decline from an outburst

(abridged) We report eclipse mapping of time resolved spectroscopy of the dwarf nova IP Pegasi on the late decline from the May 1993 outburst. The spiral arms are still visible some 8 days after the onset of the outburst. Their fractional contribution to the continuum emission is similar to that measured close to outburst maximum, whereas their orientation is rotated by 58 degrees with respect to the spirals seen in the eclipse map at outburst maximum. Velocity-resolved light curves across the Halpha and the HeI lines show the classical rotational disturbance, with the blue side of the line being eclipsed earlier than the red side. The differences between the Halpha and the HeI maps are significant. The spiral arms are clearly seen in the HeI maps, with the receding arm being stronger in the red side while the approaching arm is stronger in the blue side of the line. The analysis of the Halpha maps suggests that this emission arises mainly from a large and vertically-extended region which we interpret as an outflowing (and spiraling) disc wind. The spectrum of the uneclipsed light is dominated by a strong, blueshifted and narrow Halpha emission line superimposed on a red continuum and can be understood as a combination of emission from an M5V secondary star plus optically thin emission from the outer parts of the vertically-extended disc wind. The inner disc regions show an emission line spectrum with a strong and broad Halpha component superimposed on a flat continuum. This is in marked contrast with the results from the spectral mapping of nova-like variables of comparable inclination and mass ratio and suggests that intrinsically different physical conditions hold in the inner disc regions of outbursting dwarf novae and nova-like systems.

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arXiv2000

Late superhumps and the stream-disc impact in IY UMa

We use the hot spot eclipse times of the newly discovered deeply-eclipsing dwarf nova IY UMa to trace out the shape of its disc during the late superhump era. We find an eccentric disc. We show that the brightness of the stream-disc impact region varies as expected with |deltaV|^2, where deltaV is the differential velocity of the stream with respect to the velocity of the disc at the impact point. We conclude that the hot spot is the source of late superhump light.

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arXiv2000

Superhumps in Low-Mass X-Ray Binaries

We propose a mechanism for the superhump modulations observed in optical photometry of at least two black hole X-ray transients (SXTs). As in extreme mass-ratio cataclysmic variables (CVs), superhumps are assumed to result from the presence of the 3:1 orbital resonance in the accretion disc. This causes the disc to become non-axisymmetric and precess. However the mechanism for superhump luminosity variations in low mass X-ray binaries (LMXBs) must differ from that in CVs, where it is attributed to a tidally-driven modulation of the disc's viscous dissipation, varying on the beat between the orbital and disc precession period. By contrast in LMXBs, tidal dissipation in the outer accretion disc is negligible: the optical emission is overwhelming dominated by reprocessing of intercepted central X-rays. Thus a different origin for the superhump modulation is required. Recent observations and numerical simulations indicate that in an extreme mass-ratio system the disc area changes on the superhump period. We deduce that the superhumps observed in SXTs arise from a modulation of the reprocessed flux by the changing area. Therefore, unlike the situation in CVs, where the superhump amplitude is inclination-independent, superhumps should be best seen in low-inclination LMXBs, whereas an orbital modulation from the heated face of the secondary star should be more prominent at high inclinations. Modulation at the disc precession period (10s of days) may indicate disc asymmetries such as warping. We comment on the orbital period determinations of LMXBs, and the possibility and significance of possible permanent superhump LMXBs.

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arXiv2000

Doppler Tomography of Dwarf Nova IY UMa during Quiescence

Quiescent Doppler tomography of the newly discovered deeply-eclipsing SU UMa system IY UMa reveals properties of the region where the accretion stream from the donor impacts the edge of the disc. A very strong bright spot is produced and the Keplerian disc emission in the impact region is disrupted or obscured. The differing properties of Halpha, Hbeta and He I emission will allow physical parameters of the converging flow region to be studied.

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arXiv2000

Superhumps in V348 Pup

The eclipsing novalike cataclysmic variable star V348 Pup exhibits a persistent luminosity modulation with a period 6 per cent longer than its 2.44 hour orbital-period (Porb). This has been interpreted as a `positive superhump' resulting from a slowly precessing non-axisymmetric accretion disc gravitationally interacting with the secondary. We find a clear modulation of mid-eclipse times on the superhump period, which agrees well with the predictions of a simple precessing eccentric disc model. Our modelling shows that the disc light centre is on the far side of the disc from the donor star when the superhump reaches maximum light. This phasing suggests a link between superhumps in V348 Pup and late superhumps in SU UMa systems. Modelling of the full lightcurve and maximum entropy eclipse mapping both show that the disc emission is concentrated closer to the white dwarf at superhump maximum than at superhump minimum. We detect additional signals consistent with the beat periods between the implied disc precession period and both (1/2)Porb and (1/3)Porb.

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