7399( 5) - 09/28/98 16:14 - [ 1] PROPOSAL FOR HUBBLE SPACE TELESCOPE OBSERVATIONS ST ScI Use Only ID: 7399 Version: 5 Check-in Date: 28-Sep-1998 16:13:13 1.Proposal Title: Ultraviolet spectroscopy of the supersoft X-ray source RX J0439.8-6809 ------------------------------------------------------------------------------------ 2. Proposal For 3. Cycle GO 7 ------------------------------------------------------------------------------------ 4. Investigators Contact? PI: Andre Van Teeseling Universitaets-Sternwarte Goettingen CoI: Klaus Beuermann Universitaets-Sternwarte Goettingen N CoI: Boris T. Gaensicke Universitaets-Sternwarte Goettingen Y CoI: Klaus Reinsch Universitaets-Sternwarte Goettingen N ------------------------------------------------------------------------------------ 5. Abstract Observationally, supersoft X-ray sources are classified as near-Eddington stellar objects with almost all emission at energies < 0.5 keV. Only 13 supersoft X-ray sources have been optically identified, and of these 11 turn out to be binaries, probably with a shell-burning accreting white dwarf. We have recently identified RX J0439.8-6809 with a V=21.63, very blue star in the LMC. A 3sigma upper limit to the peak-to-peak optical variability is 0.07 mag. Of all optically identified supersoft X-ray sources, RX J0439.8-6809 has the lowest optical-to-X-ray flux ratio. The nature of RX J0439.8-6809 is still unknown. It might be the hottest known pre-white dwarf, suffering a late helium shell flash. Alternatively, RX J0439.8-6809 could be an accreting binary, in which case it might be the first known double-degenerate supersoft X-ray source with a predicted orbital period of only a few minutes. An ultraviolet spectrum is essential to distinguish between these two spectacular possibilities, and to bridge the gap between the X-ray and optical observations. Such a spectrum can only be obtained with the HST STIS. Therefore, we propose to obtain two ultraviolet spectra, which will test the assumption that the optical spectrum is the Rayleigh-Jeans tail of the soft X-ray component, which will determine the spectral energy distribution, and which may provide the first direct evidence for accretion in this source by detecting an excess in the ultraviolet or ultraviolet emission lines like N V Lambda 1240. ------------------------------------------------------------------------------------ 7399( 5) - 09/28/98 16:14 - [ 2] Observations Description ------------------------ We propose to maximize the observing efficiency and to schedule our observations of RX J0439.8-6809 in the continuous viewing zone. We propose to obtain one spectrum with the STIS/NUV-MAMA configuration using the G230L grating, and a second spectrum with the STIS/FUV-MAMA configuration using the G140L grating, both with the 52X0.5 slit. Because of a star 4.3" to the north of the science object, target acquisition (using the F28X50LP longpass filter) and peak-up (with 52X0.2 aperture) will be on an off set star. For the G140L spectrum we propose 32 minutes exposure and for the G230L spectrum 26 minutes exposure. We have calculated the feasibility with the STIS Spectroscopic Exposure Time Calculator, using a blackbody spectrum with T_bb=300 000 K, a normalization of 2*10^-16erg, s^-1Angstrom^-1 at 2000 Angstrom, and E(B- V)=0.07. The wavelength range for which S/N > 10 depends on the rebinning: To determine the continuum flux distribution, we do not need the maximum spectral resolution and will rebin both spectra in the dispersion direction into 10 pixel resolution elements. This corresponds to DeltaLambda ~6 Angstrom for G140L and DeltaLambda ~16 Angstrom for G230L. With 1900 s and 1600 s exposures for the G140L and G230L spectra, respectively, we will obtain a S/N > 10 in the wavelength ranges ~ 1150-1600 Angstrom and ~ 1800-2800 Angstrom. To detect possible emission lines due to accretion, we will rebin the G140L spectrum into smaller resolution elements in the dispersion direction. With a higher resolution of e.g. DeltaLambda ~2.4 Angstrom we will obtain S/N > 10 in the ~ 1180-1450 Angstrom interval. Real Time Justification ----------------------- We have proposed for simultaneous observations with ROSAT. These observations could be scheduled between Dec 14, 1997 and approximately Jan 1999. Because this may be the last opportunity to observe the source with ROSAT, it is preferred that the HST observations will be obtained in this time interval. Calibration Justification ------------------------- Additional Comments ------------------- ------------------------------------------------------------------------------------ Data Distribution Media: DAT Blocking Factor: 10 Ship To: PI_Address Ship Via: Email: ------------------------------------------------------------------------------------ 7399( 5) - 09/28/98 16:14 - [ 3] TARGET LIST a) Fixed Targets ------------------------------------------------------------------------------------------------------------------------------------ Tar| Target | Target | Target |Coord | Radial | Flux data No | Name | Description | Position |Eqnx | Vel. | ------------------------------------------------------------------------------------------------------------------------------------ 1 RXJ0439.8- EXT-STAR, K III-I RA=4H 39M 53.39S +/- 0.02S, DEC= J2000 V = 17.2+/-0.1 6809-OFFSET -68D 8' 57.62" +/- 0.1", PLATE- B-V = 1.6 ID=0186 E(B-V) = 0.07 2 RXJ0439.8- EXT-STAR, Interacting RA-OFF = -3.722S +/- 0.004S, DEC J2000 V = 21.63+/-0.05 6809 Binary, LMXB, Accretion -OFF = -4.05" +/- 0.02", FROM 1 U = 20.4+/-0.1 Disk E(B-V) = 0.07 F-CONT(1600)=6e-16 F-CONT(2000)=2e-16 Comments: approximately blackbody with T=300000K 7399( 5) - 09/28/98 16:14 - [ 4] Visit: 01 Visit Priority: Visit Requirements: CVZ BETWEEN 1-NOV-98 AND 1-DEC-98 On Hold Comments: Additional Comments: Exposures ------------------------------------------------------------------------------------------------------------------------------------ Exposure| Target |Instr | Oper. | Aper |Spectral|Central| Optional |Num| Time | Special Number | Name |Config| Mode |or FOV |Element |Waveln.| Parameters |Exp| | Requirements ------------------------------------------------------------------------------------------------------------------------------------ 1 RXJ0439.8-6 STIS/C ACQ F28X50L MIRROR 1 6.0 S ONBOARD ACQ FOR 2, 3 809-OFFSET CD P ------------------------------------------------------------------------------------------------------------------------------------ 2 RXJ0439.8-6 STIS/F TIME-TA 52X0.5 G140L 1425 BUFFER-TIME=2200 1 2100 S 809 UV-MAM G A ------------------------------------------------------------------------------------------------------------------------------------ 3 RXJ0439.8-6 STIS/N TIME-TA 52X0.5 G230L 2376 BUFFER-TIME=1800 1 1700 S 809 UV-MAM G A ------------------------------------------------------------------------------------------------------------------------------------ 7399( 5) - 09/28/98 16:14 - [ 5] Summary Form for Proposal 7399 Item Used in this proposal ------------------------------------------------------------------------------------------------------------------------------------ Apertures F28X50LP 52X0.5 ------------------------------------------------------------------------------------------------------------------------------------ Configurations STIS/CCD STIS/FUV-MAMA STIS/NUV-MAMA ------------------------------------------------------------------------------------------------------------------------------------ Opmodes ACQ TIME-TAG ------------------------------------------------------------------------------------------------------------------------------------ Optional Parameters BUFFER-TIME=2200 BUFFER-TIME=1800 ------------------------------------------------------------------------------------------------------------------------------------ Proposal Category GO ------------------------------------------------------------------------------------------------------------------------------------ Special Requirements CVZ BETWEEN 1-NOV-98 AND 1-DEC-98 ONBOARD ACQ FOR 2, 3 ------------------------------------------------------------------------------------------------------------------------------------ Spectral Elements MIRROR G140L G230L ------------------------------------------------------------------------------------------------------------------------------------ Target Names RXJ0439.8-6809-OFFSET RXJ0439.8-6809 ------------------------------------------------------------------------------------------------------------------------------------ Wavelengths 1425 2376 ------------------------------------------------------------------------------------------------------------------------------------