Ordinary Meeting, 2005 January 26

Clocking a Spinning White Dwarf

In his talk, Dr Boyd outlined his observations of the variable star DO Draconis (Dra) during its outburst of 2004 January. He opened by explaining that this system was a member of a class of cataclysmic variables known as intermediate polars. Such systems consisted of a pair of stars, orbiting about their common centre of gravity, where the more massive star had exhausted its fuel and become a white dwarf, while its less massive companion continued to burn hydrogen. Upon reaching the red giant phase of its life, the smaller secondary star would expand, pushing its outer layers towards its companion, such that some of its outer envelope entered the strong gravitational field surrounding the white dwarf – a process called Roche-lobe overflow. This gas, having no escape from the white dwarf, would spiral in towards it, forming an accretion disc.

From his observations of the variations in the brightness of DO Dra, the speaker explained that he had found it possible to measure what he believed to be the spin-rate of the white dwarf in the system. He added that this had been possible as a consequence of the behaviour of the inner-edge of the accretion disc. It was well established that white dwarfs were surrounded by intense magnetic fields, essentially the result of compressing the magnetic field of an entire star into the tiny volume of such an object – comparable to that of the Earth. This was relevant, as the material falling onto the accretion disc would rapidly become ionised, a result of the tremendous temperatures generated by viscous friction within the disc, fuelled by the acceleration of material to velocities of several thousand km/s upon falling in towards the white dwarf. However, such ionised material was known to follow the direction of magnetic fields, and so the strong field in the vicinity of the white dwarf would be expected to alter the dynamics of the inner parts of the accretion disc, drawing material out of its plane, along the field lines emanating from the star's magnetic poles, and ultimately onto the surface of these poles.

However, the speaker explained that there was no reason why the white dwarf's magnetic poles should be aligned with its rotation axis, and so as it spun, the magnetic poles would rotate, and the geometry of the inflow onto them change with respect to our line of sight. The result would be some periodic modulation of the observed brightness of the system, typically with two brightenings per rotation, on account of seeing both poles alternately crossing its disk.

As a historical note, Dr Boyd added that DO Dra had not been identified as an intermediate polar until it had been realised to be a significant source of X-ray radiation, suggestive that accretion onto a compact object was involved in its variability. Prior to this, however, in 1934, an eclipsing binary, catalogued YY Dra, had been identified very close nearby, though it was apparently no longer observable. Recent literature argued that it seemed likely that this was a case of misclassification, and both were in fact the same object.

Previous observations taken over the course of the 1990s by ROSAT in X-rays, and by the Faint Object Spectrograph (FOS) aboard the Hubble Space Telescope (HST) in the ultraviolet, had identified two periodic variations in the brightness of DO Dra, with periods of 530s and 265s, interpreted as being due to the spin period of the white dwarf and its first harmonic respectively. The detection of brightness modulation at the first harmonic of the spin period fitted well with the suggestion that there should be two brightenings per rotation, corresponding to accretion onto each of the two magnetic poles. Previous observations in the V-band (visible light), yielded a marginal detection of the 530s modulation, but no detection of its first harmonic.

Moving on to discuss his own observations, the speaker explained that these had been made during the outburst of 2004 January. Normally DO Dra varied in the range mag 15-15.5, but on 2004 January 23, Mike Simonsen had reported that it appeared brighter in his images, around mag 14.5. The subsequent brightening was rapid, and the speaker's photometry the following night found it closer to mag 11.5. During the course of the night of January 24/25, he had set his CCD to take automatic 30s integrations over a period of 7.5 hours. In each of the resulting frames he had later been able to use a reference star in the same field to estimate the brightness of DO Dra. He had repeated the same procedure with a second reference star to estimate the error in his photometry. More details of the data analysis can be found in his paper, Boyd (2005)¹.

When the frequency spectrum of the brightness variations over the night was calculated, significant modulation with a period of 527.8±1.8s was identified, though the first harmonic, at half this period, was not observed. This value was compatible with the best professionally measured value of the spin period of the white dwarf from the HST, of 529.42±0.1s. In conclusion, the speaker believed he had measured the spin-rate of a white dwarf using only a 10-inch aperture amateur telescope – he found this rare opportunity to plot his personal observations alongside HST data rather satisfying. In addition, he had uncovered something of a mystery: why he had only detected only one brightening in the V-band on each rotation, as previous observations in that band had also hinted, and yet two brightenings per rotation were observed in X-rays and the ultraviolet. More observations would be required to answer this question, and the speaker felt concurrent monitoring in both the V-band and at shorter wavelengths would be valuable to ensure that the explanation did not simply lie in a change in the behaviour of the object between the observations at different wavelengths.

Following the applause, the President congratulated Dr Boyd on undertaking observations of genuine scientific value, and expressed his hope that his success might encourage other amateurs to pursue similar projects. A member asked the speaker why he had chosen to observe in the V-band, using a green filter, rather than observing in white light with an unfiltered CCD. The speaker explained that this made it easier to compare his results with those of others, as the wavelength response of unfiltered CCDs varied considerably. A member asked if Dr Boyd would be willing to speculate as to what physical mechanism caused the difference between his V-band observations and those at shorter wavelengths. The speaker replied that he was reluctant to do so, as he was not aware of any well-established explanation having appeared in the professional literature.

Finally, Mrs Hazel McGee asked whether the previous X-ray/UV observations had been taken at times of quiescence or outburst, whether the speaker had made any V-band observations during quiescence, and whether any discrepancy here might account for the different behaviour of the object. The speaker replied that the previous observations had mostly been taken during quiescence, but his own attempts to make V-band observations at such times had been plagued by large error-bars, a result of the object's faintness.²

The President then welcomed the next two speakers, who together would be discussing a new design of telescope optics. Mr Peter Wise, the telescope-maker behind the idea, would explain the design of the instrument, whilst Mr Martin Morgan-Taylor would be presenting some of his results obtained using a telescope with such optics.