13289( 2) - 25-Jun-2013 21:12:12 - [ 1] HUBBLE SPACE TELESCOPE OBSERVING PROGRAM 13289 Version: 2 Check-in Time: 25-Jun-2013 21:12:12 STScI Edit Number: 0 Title RXJ0439.8-6809: the hottest pre-white dwarf, or the first double-degenerate supersoft X-ray source? ------------------------------------------------------------------------------------ Type Cycle GO 21 ------------------------------------------------------------------------------------ Investigators Contact? PI: Prof. Boris T. Gaensicke The University of Warwick Y CoI: Dr. Klaus Reinsch Georg-August-Universitat N CoI: Prof. Klaus Beuermann Universitats-Sternwarte Gottingen N ------------------------------------------------------------------------------------ Abstract The supersoft X-ray source RXJ0439.8-6809, identified with a faint (V=21.7) very blue star in the direction of the LMC is one of the most extreme stellar objects known. Current models require it to be either: (a) the hottest (Teff~300000K) and most massive known pre-white dwarf. In this case, it would allow a real-time observational test of stellar evolution theory, as it will evolve along the white dwarf cooling track by measurable amounts within a decade; or: (b) the first known double degenerate supersoft X-ray source, i.e. an interacting binary with a shell-burning accreting white dwarf and a degenerate donor star, which would make it a very good candidate for a type Ia supernova progenitor. Alas, despite major observational efforts, including an exploratory 1-orbit Cycle 7 HST low-resolution ultraviolet spectrum, it is so far not possible to distinguish between these two scenarii. We propose to invest a modest amount of HST/COS time to finally determine the true nature of RXJ0439.8-6809. Crucially, we will obtain the first high-resolution ultraviolet spectrum of this system to confirm the LMC membership of RXJ0439.8-6809 from the detection of interstellar absorption lines at the LMC systemic velocity. In addition, we will use this spectrum to obtain a second-epoch effective temperature measurement, which we will compare that from Cycle 7. The excellent flux calibration of HST will easily detect a change of a few 10000K, which is predicted for the evolution of a massive white dwarf over the fifteen-year baseline (Cycle 7 to 21) ------------------------------------------------------------------------------------ Observations Description ------------------------ The aim of this program is to obtain high-resolution far-UV spectroscopy of a hot blue star in the LMC, which is either a hot pre-white dwarf or a stable shell-burning white dwarf in a very compact binary. The co-ordinates are good to ~0.5", and the current Phase II implementation includes an ACQ/SEARCH pattern, however, we will endeavour to provide updated and improved ground-based coordinates that will allow to proceed with an ACQ/Image only. The acquisition is followed G130M spectroscopy using all four FP-POS positions. The object is briefly in the CVZ, which greatly improves the efficiency of the observatory. Real Time Justification ----------------------- 13289( 2) - 25-Jun-2013 21:12:12 - [ 2] Calibration Justification ------------------------- Additional Comments ------------------- ------------------------------------------------------------------------------------ 13289( 2) - 25-Jun-2013 21:12:12 - [ 3] TARGET LIST Fixed Targets ------------------------------------------------------------------------------------------------------------------------------------ Tar| Target | Target | Target |Coord | Radial | Flux data No | Name | Description | Position |Eqnx | Vel. | ------------------------------------------------------------------------------------------------------------------------------------ 1 RXJ0439.8-68 STAR, LMXB, NOVA-LIKE RA=04H39M49.5900S +/- 0.3", J2000 V = 21.7 +/- 0.1 F(1300A)=1.5e-15 09 DEC=-68D09'0.60" +/- 0.3" erg/cm2/s/AA, F(1528A)=8.3+-0.9e-15 erg/cm2/s/AA and F(2271A)=2.3+-0.07e-16 erg/cm2/s/AA Reference Frame: ICRS Comments: This target is either a supersoft X-ray binary (stable shell burning white dwarf) or a hot single pre-white dwarf. We observed it with STIS in November 1998 (data sets O55G01010 & O55G01020). The target has also been observed with GALEX in December 2005, finding it at F(1528A)=8.3+-0.9e-15 erg/cm2/s/AA and F(2271A)=2.3+-0.07e-16 erg/cm2/s/AA, consistent with the STIS observations. Epoch of Position RA proper motion (seconds of time/yr) DEC Proper Motion (arcsec/yr) Annual Parallax (arcsec) 2000 0.0 0.0 0.0 ------------------------------------------------------------------------------------------------------------------------------------ 13289( 2) - 25-Jun-2013 21:12:12 - [ 4] Visit: 01 Visit Requirements: CVZ On Hold Comments: Additional Comments: Exposures ------------------------------------------------------------------------------------------------------------------------------------ Exp | Target | Instr | Oper. | Aper |Spectral|Central| Optional |Num| Time | Special Num | Name | Config | Mode | or FOV |Element |Waveln.| Parameters |Exp| (Total) | Requirements ------------------------------------------------------------------------------------------------------------------------------------ 1 RXJ0439.8-6 COS/NUV ACQ/SEA PSA MIRRORA STEP-SIZE=1.767, 1 122 S (122 809 RCH CENTER=FLUX-WT-FLR, S) SCAN-SIZE=3 ------------------------------------------------------------------------------------------------------------------------------------ 2 RXJ0439.8-6 COS/NUV ACQ/IMA PSA MIRRORA 1 40 S (40 809 GE S) ------------------------------------------------------------------------------------------------------------------------------------ 3 RXJ0439.8-6 COS/FUV TIME-TA PSA G130M 1291 BUFFER-TIME=3480, FLASH=YES, 1 3600 S 809 G FP-POS=1 (3600 S) ------------------------------------------------------------------------------------------------------------------------------------ 4 RXJ0439.8-6 COS/FUV TIME-TA PSA G130M 1291 BUFFER-TIME=3480, FLASH=YES, 1 3600 S 809 G FP-POS=2 (3600 S) ------------------------------------------------------------------------------------------------------------------------------------ 5 RXJ0439.8-6 COS/FUV TIME-TA PSA G130M 1291 BUFFER-TIME=3480, FLASH=YES, 1 3600 S 809 G FP-POS=3 (3600 S) ------------------------------------------------------------------------------------------------------------------------------------ 6 RXJ0439.8-6 COS/FUV TIME-TA PSA G130M 1291 BUFFER-TIME=3480, FLASH=YES, 1 3598 S 809 G FP-POS=4 (3598 S) ------------------------------------------------------------------------------------------------------------------------------------ Sub Exposures ------------------------------------------------------------------------------------------------------------------------------------ Target | Exp |Instr | Oper. | Aper |Spectral|Cent.|Primary |Secondary |Iteration |CR-SPLIT |Orbit |Duration Name | Num |Config | Mode | or FOV |Element |Wave.|Pattern Pos |Pattern Pos |Num |Num |Number | ------------------------------------------------------------------------------------------------------------------------------------ RXJ0439.8-6809 1 COS/NUV ACQ/SEA PSA MIRRORA none none none none 1 N/A RCH RXJ0439.8-6809 2 COS/NUV ACQ/IMA PSA MIRRORA none none none none 1 N/A GE RXJ0439.8-6809 3 COS/FUV TIME-TA PSA G130M 1291 none none none none 1 N/A G RXJ0439.8-6809 4 COS/FUV TIME-TA PSA G130M 1291 none none none none 2 N/A G RXJ0439.8-6809 5 COS/FUV TIME-TA PSA G130M 1291 none none none none 2 N/A G RXJ0439.8-6809 6 COS/FUV TIME-TA PSA G130M 1291 none none none none 3 N/A G 13289( 2) - 25-Jun-2013 21:12:12 - [ 5] Summary Form for Proposal 13289 Item Used in this proposal ------------------------------------------------------------------------------------------------------------------------------------ Apertures PSA ------------------------------------------------------------------------------------------------------------------------------------ Configurations COS/FUV, COS/NUV ------------------------------------------------------------------------------------------------------------------------------------ Opmodes ACQ/IMAGE, ACQ/SEARCH, TIME-TAG ------------------------------------------------------------------------------------------------------------------------------------ Optional Parameters BUFFER-TIME=3480, CENTER=FLUX-WT-FLR, FLASH=YES, FP-POS=1, FP-POS=2, FP-POS=3, FP-POS=4, SCAN-SIZE=3, STEP-SIZE=1.767 ------------------------------------------------------------------------------------------------------------------------------------ Special Requirements CVZ ------------------------------------------------------------------------------------------------------------------------------------ Spectral Elements G130M, MIRRORA ------------------------------------------------------------------------------------------------------------------------------------ Target Names RXJ0439.8-6809 ------------------------------------------------------------------------------------------------------------------------------------