J-PEX

The Joint Astrophysical Plasmadynamic Experiment

University of Leicester : M.A. Barstow, N.P. Bannister, G.W. Fraser

Naval Research Laboratory : R. Cruddace, M. Kowalski, G. Fritz, T. Barbee (LLNL)

Mullard Space Science Laboratory : J. Lapington, J. Tandy, B. Sanderson

Abstract Operating near normal incidence and incorporating the most recent developments in multilayer reflective coatings, ion-etched gratings, and detectors, the J-PEX instrument will deliver a resolution and effective area 10 times that achieved by EUVE in the 225-245 Å band. The first flight of JPEX at the beginning of 1999, on board a sounding rocket, will observe the hot DA white dwarf G191-B2B. The aim is to search for the presence of HeII along the line-of-sight and, if found, determine whether it resides in the photosphere or interstellar medium.

TARGET: G191-B2B As one of the brightest DA white dwarfs, G191-B2B is the subject of intensive study, and has been observed by instruments including IUE, ROSAT, HST and EUVE. Optical spectra have been used to derive the basic stellar parameters, giving Teff=53,700 K and log g=7.3. The observed flux in the J-PEX band is 0.4 ph cm-2 s-1 Å-1.

GRATINGS

J-PEX employs near-normal incidence optics. The conventional grazing-incidence mirrors of earlier instruments, with their limited collecting area, are replaced by dual purpose SiO2 multilayer coated gratings which act as both the dispersion and light collection elements. This arrangement achieves the effective area performance shown (left), with Aeff = 9cm² @ 235 Å (10 x EUVE).
	Gratings are manufactured by Carl Zeiss, using multilayer coatings developed at NRL and Lawrence Livermore National Laboratory. The gratings have a total area of 512 cm², and have a spherical figure, with a 2.2m focal length.

Flight collimator (above)

DETECTOR

Vernier conductive anode

Built at the University of Leicester, the detector consists of 2 x 37mm square microchannel plates in a chevron configuration, with a progressive geometry vernier conductive anode readout (above), developed and supplied by MSSL, who have also constructed the electronics and HV supply.

The assembled detector

The front MCP is coated with a CsI photocathode, and the detector mounted in its own vacuum chamber with integral ion pump. The spatial resolution of the detector will be better than 20 microns. An Al filter rejects geocoronal EUV background radiation.

Scientific Goal

The EUV count spectrum of G191-B2B above (green) is compared with the predictions of a best fit non-LTE model (red), including the effects of interstellar absorption from HI, HeI and HeII. The discontinuity near 320 Å reflects different spectrometer effective areas for MW and LW ranges. An excess flux predicted by the model at wavelengths below 228 Å can only be suppressed by the introduction of significant HeII absorption. If interstellar / circumstellar, this implies HI, HeI and HeII column densities of 2.1 x 1018, 1.8 x 1017 & 7.9 x 1017 cm² respectively. If entirely photospheric in origin, the required He/H abundance is 5.5 x 10-5. [Lanz, Barstow, Hubeny & Holberg, ApJ, 1996, 473, pp.1089-1093].

Although a consistent interpretation of the combined EUV / FUV / optical spectra required the inclusion of a significant HeII component, the signature of HeII cannot be unambiguously identified in the G191-B2B EUVE spectrum. It is possible that He may be blended with the huge numbers of heavy element lines present, that EUVE is unable to resolve. The EUVE spectrum of G191-B2B is shown above, compared with a simulation of the higher resolution data expected from J-PEX. In the latter spectrum, heavy element lines are resolved and the distinct signatures of photospheric and interstellar HeII are seen. J-PEX will allow us to demonstrate whether or not He is really present in the spectrum of this star and, if found, determine its location.

J-PEX will be flown from White Sands Missile Range on a Black Brandt VC sounding rocket, scheduled to take place at the beginning of 1999.

Web page created by Louise Twist.

Longslade Community College.

Last updated 26/5/00