A totally new type of optical detector has been used on the William Herschel Telescope to directly measure intensity and colour changes in a faint, rapidly variable binary star system, UZ Fornacis, for the first time. A team from the Space Science Department of the European Space Agency's Research and Technology Centre in the Netherlands (ESTEC), who have developed the superconducting detector, were joined by astronomers from Mullard Space Science Laboratory (MSSL), to exploit this novel technology.

With conventional optical CCD detectors, very rapid changes in light intensity cannot be measured. Furthermore, the energy or wavelength of the arriving photons can only be measured by introducing a filter or spectrograph into the optical light path, degrading efficiency. With the new instrument, the so-called S-Cam camera, advanced detector elements based on superconducting technology register the arrival of each photon individually, and measure its energy directly.

"This cryogenic detector is very efficient, very sensitive, and very fast; and there is no dark current or readout noise" explains Tone Peacock, who has led the development of the instrument from ESA's Astrophysics Division. The instrument is cooled to below 1K to minimise all possible noise, resulting in an almost perfect detector.

Larger and larger telescopes are being built on ground and in space to study fainter and more distant objects, and more efficient detectors can contribute enormously to astronomers' progress in understanding. "We have a very powerful instrument for looking at faint astronomical sources which vary rapidly, for example pulsars or binary star systems. It is of particular interest when the light changes its energy distribution at the same time", said Michael Perryman, who has led this astronomical investigation.

In the particular binary star system studied, one of the two stars is a so-called white dwarf, a star in an advanced state of stellar evolution that collapses slowly under its own gravity. This white dwarf tears gas from the surface of its nearby companion, which is then engulfed by the white dwarf's powerful gravitational field. The material is channelled down highly intense magnetic field lines onto the surface of the white dwarf, where it emits prolific amounts of optical, ultraviolet, and X-ray radiation as it cools.

Many clues are contained in the very short interval of time in which the intense light emitted by the infalling material is eclipsed by its larger but fainter companion. The results show that the diameter of the accretion spot, where material hits the white dwarf surface, is less than about 100 km. Mark Cropper, from Mullard Space Science Laboratory, has been studying this type of binary system, known as a magnetic cataclysmic variable, for more than a decade. "This new detector allows us to look at changes in the light from the system as it changes over small fractions of a second. And this is the first time we have been able to study the rapid colour changes which occur at the same time."

The results of the observations, performed by an ESA technical team led by Nicola Rando in support of the ESA/MSSL science teams, were made at the William Herschel Telescope in December 1999, and are reported in the scientific journal Monthly Notices of the Royal Astronomical Society, October 2000.

ESA scientists Michael Perryman, Clare Foden and Tone Peacock published the theoretical ideas underlying the new detector in 1993, the first instrument able to detect the energy of optical photons directly. Tone Peacock leads the group which reported the first detection of optical photons using this technique in 1996. The instrument making use of these principles was commissioned at the William Herschel Telescope in February 1999 (see ING Press Release ING 0/99).

The Isaac Newton Group of Telescopes (ING) is an establishment of the Particle Physics and Astronomy Research Council (PPARC) of the United Kingdom and the Nederlandse Organisatie voor Wetenschappelijk Onderzoek (NWO) of the Netherlands. The ING operates the 4.2 metre William Herschel Telescope, the 2.5 metre Isaac Newton Telescope, and the 1.0 metre Jacobus Kapteyn Telescope. The telescopes are located in the Spanish Observatorio del Roque de Los Muchachos on La Palma which is operated by the Instituto de Astrofísica de Canarias (IAC).

PICTURES

Caption: In order to demonstrate the capabilities of S-Cam, we have created an animation showing an observation of an entire eclipse of UZ For. This was created by splicing together two integrations, with the join between them near the mid-point of the eclipse. Each step in the animation corresponds to 3 seconds of actual data. The lightcurve in the top of the plot shows the evolution of the total source intensity across the complete spectral range. In order to illustrate the energy-sensitivity of the device, however, the 36 array pixels in the lower image are first assigned colours based on the V/R ratio obtained from two energy bands (ranging from blue to red on a spectral scale), and are then assigned intensities based on the total counts in that pixel over the 3 second time period.
Picture credit: Astrophysics Division, ESTEC.
Available formats: AVI (9,072 K) | GIF (499 K)

Caption: Picture of S-Cam.
Picture credit:
Available formats:

Caption: The William Herschel Telescope.
Picture credit: Rainer Girnstein.
Available formats: JPEG (331 K) | TIFF (11,241 K)

Caption: The William Herschel Telescope in Moon light.
Picture credit: Nik Szymanek and Ian King.
Available formats: GIF (122 K) | TIFF (6,186 K)

FOR FURTHER INFORMATION PLEASE CONTACT:

Dr. Michael Perryman
Astrophysics Division, ESTEC
2200AG Noordwijk
The Netherlands
Tel: +31-71-5653615
Fax: +31-71-5654690
E-mail: mperryma@astro.estec.esa.nl

Dr. Mark Cropper
Mullard Space Science Laboratory,
University College London,
Holmbury St Mary,
Dorking, Surrey RH5 6NT
United Kingdom.
Tel: +44 1483 204155
Fax: +44 1483 278312
E-mail: msc@mssl.ucl.ac.uk

Mr. Javier Méndez
Public Relations Officer
Isaac Newton Group of Telescopes
Phone: +34 922 425464, +34 616 464111
Fax: +34 922 425401
E-mail: jma@ing.iac.es

Other web pages of interest:

The Superconducting Tunnel Junction Page at ESTEC

Astrophysics Division, Space Science Department of ESA, ESTEC

Mullard Space Science Laboratory

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