J-PEX

Joint Astrophysical Plasmadynamic Experiment

 

High Resolution EUV Spectroscopy of Hot White Dwarf Stars

A major outstanding question in the study of white dwarf stars concerns the relationship between the hydrogen and helium dominated groups and the interaction of these with the insterstellar medium. Of particular importance is the role He plays in the H-rich DA white dwarfs as they cool. Although He can be observed in the UV and optical bands, when present in very small quantities it is only detectable in the EUV. Furthermore, an interstellar He component can be revealed by observing its shadowing effect on a stellar EUV spectrum. However, until now, EUV instrumentation has lacked the spectral resolution capable of separating the signature of HeII from the large number of other lines (mainly Fe and Ni) present in the white dwarf spectra and distinguishing the source of HeII (photospheric or interstellar), if present.

J-PEX is a sounding rocket-borne normal incidence high resolution EUV spectrometer. Its objective is to obtain high resolution EUV spectra of white dwarf stars at EUV wavelengths. Covering the spectral range 225-245A it has a theoretical resolving power of 5000, 10 times better than EUVE. The mission is a collaboration led by the US Naval Research Laboratory, involving the University of Leicester MSSL and the US Lawrence Livermore National Laboratory. The UK effort was to supply the high spatial resolution imaging detector together with its associated readout electronics. This MCP detector stack, constructed at Leicester, was based on newly-developed small pore (6micron channel diameter) plates and was coupled to a vernier anode readout provided by MSSL. Leicester also had responsibility for defining the scientific objectives of the first flight of the instrument, to observe the hot H-rich white dwarf G191-B2B, with the aim of determining whether or not helium is really present in the star and, if found, where it is located.

The telescope aperture on J-PEX.

The mission was approved by NASA in 1996 and development of the payload began in January 1997. Technical difficulties with the small-pore MCPs supplied by Photonis, which produced unexpectedly low quantum efficiencies and problems with the instrument stability during calibration, delayed the first flight of the spectrometer until February 2000. The Photonis MCPs were replaced with larger-pore devices from earlier stock, which met the QE requirements but with a consequent small degradation of the imaging performance and, as a result, spectral resolution. Further work with Photonis on the problem has not yet produced a solution to the QE problem. The instrument stability problems were produced by internal heating of the payload during the long test exposures, required to build up photon counting statistics, causing some flexure of the spectrometer. This was not expected to be a problem during a short duration flight and the measured spectral resolving power of 2500-3000 was believed to be a lower limit on the true value.

J-PEX was first flown from White Sands Missile Range, New Mexico, on board a Terrier-boosted Black Brant Vc sounding rocket on 25th February 2000. Unfortunately, the rocket drifted outside the boundary of the range and the flight was terminated from the ground 2 seconds before burnout. The explosion from the termination charge caused the payload to tumble and, although stability was restored by the ACS no gas remained for pointing the telescope and no science data were obtained. However, the detector, payload electronics and mechanisms all functioned as anticipated and the experiment was recovered in good condition after deployment of the parachute. The flight problem was later attributed to highly variable upper atmosphere winds and overcorrection of the launcher azimuth setting for the actual conditions during flight. A compensatory second launch was offered by NASA and new procedures were implemented for the wind-weighting.

The detector was refurbished for the second J-PEX flight and, following reintegration in the payload and recalibration of the spectrometer, the instrument was launched on February 22nd 2001 (see Cover). On this occasion, the boost phase was nominal and G191-B2B was acquired without any problems and observed for 303s before the payload was shut down for re-entry. Real time images from the spectrometer, EUV tracking telescope and CCD optical camera showed that the target was well-centred and tracked with low drift-rate by the Mark VID ACS (necessary to achieve high spectral resolution). The four spectra (one from each of 4 gratings, see image below) were clearly visible within a few seconds. Since the flight, Leicester has had the responsibility of reducing and analysing the spectral data with assistance from NRL and MSSL. We are still in the early phase of the scientific analysis, but the composite spectrum (sum of the 4 gratings, see spectrum) shows a number of clear features which we can identify with (variously) HeII, OVI and FeV (to be updated later). The spectral resolving power achieved is ~4000, making this the highest resolution spectrum ever obtained in soft X-ray and EUV wavebands.

Image containing the four spectra of G191-B2B recorded during the flight of J-PEX. The bright spot at location [X=220, Y=210] is the image of G191-B2B produced by the co-aligned EUV mirror. All photon positions have been corrected for the effects of small ACS drift motions, by using data from this mirror and the optical telescope. (Above) High resolution EUV spectrum of G191-B2B, obtained with the J-PEX spectrometer, spanning the wavelength range 221-244 Angstroms (error bars). The histogram is the best-fit theoretical model of the star and ISM, as described in the text. The strongest predicted lines of He, C, N, O, and P are labeled with their ionization state and wavelength. Lines of Fe and Ni are too numerous to include here an account for the unlabelled individual features and broader absorption structures. (Right)