7510( 6) - 03/27/98 15:28 - [ 1] PROPOSAL FOR HUBBLE SPACE TELESCOPE OBSERVATIONS ST ScI Use Only ID: 7510 Version: 6 Check-in Date: 27-Mar-1998 15:27:53 1.Proposal Title: High-Speed UV Spectroscopy of the Ultracompact Binary 4U 1626--67/KZ TrA ------------------------------------------------------------------------------------ 2. Proposal For 3. Cycle GO 7 ------------------------------------------------------------------------------------ 4. Investigators Contact? PI: DEEPTO CHAKRABARTY MASSACHUSETTS INSTITUTE OF TECHNOLOGY ------------------------------------------------------------------------------------ 5. Abstract The ultracompact X-ray binary 4U 1626--67/KZ TrA has the shortest (42-min) orbital period of any known binary pulsar system and the smallest known mass function (<10^- 6M_\odot) of any stellar binary. The mass donor in the system is thought to be an extremely low-mass hydrogen-depleted star, possibly a helium dwarf or helium-burning star. I will use time-resolved STIS UV spectroscopy to measure the orbital phase variation of the reprocessed X-ray flux, thereby constraining the inclination angle and mass ratio of the binary. In addition, cross-correlation of the time-resolved UV spectra with proposed simultaneous X-ray data from the Rossi X-Ray Timing Explorer (RXTE) will permit detailed study of the accretion disk structure through propagation delay mapping using the 7.7 s X-ray pulsations, the 20-s X-ray quasi- periodic oscillation (QPO), the 42-min binary binary modulation of the X-ray illumination, and the intrinsic flaring behavior of the accreting neutron star. ------------------------------------------------------------------------------------ 7510( 6) - 03/27/98 15:28 - [ 2] Observations Description ------------------------ The program will consist of a single visit of 5 orbits duration. During the first orbit, target acquisition will be followed by two short CCD exposures with the G430L and G750L elements in order to measure a baseline optical spectrum for comparison with the UV spectrum measured in the later orbits. No high signal-to-noise optical spectra of this target have ever been published, and constraining the shape of the overall UV-to-optical continuum is crucial as a probe of the energetics of X-ray reprocessing in the accretion disk of this system. The first orbit will end with a brief NUV-MAMA/TIMETAG exposure with the G230L element. The purpose of this brief initial observation is to keep the total CCD science observing time (including overhead) under 30 minutes for this visit (as per the recently revised STIS operating procedures for MAMA programs), without wasting any source visibility time. The second and third orbits will continue the NUV-MAMA/TIMETAG/G230L observations with one exposure per orbit. The fourth and fifth orbits will be used for FUV-MAMA/TIMETAG exposures with the G140L element. These TIMETAG observations will allow sensitive time-resolved spectra over the entire UV band to be obtained with sampling over multiple 42-min binary orbits of the target. The extinction to the target is not accurately known, but based on the rough relationship between X-ray-measured hydrogen column density and visual extinction (Gorenstein 1975, ApJ, 198, 95), I estimate that A_V~ 1.5. The optical intensity is known from my CTIO photometry, and the CCD exposure times in orbit 1 have been chosen to give a per element SNR of at least 5. The expected UV intensity can be estimated based on standard X-ray irradiated disk theory, existing measurements of the X- ray spectrum, and my optical photometry of the target. This yields an expected UV continuum level of ~ 8* 10^-16 erg cm^-2 s^-1 AA^-1 at 2000 Angstrom (including extinction). The STIS ETC gives a count rate of R=96.32 count s^-1 with the requested setup. To operate in TIMETAG mode, the maximum exposure is T = 6* 10^7/R s, much longer than the total observation request. To be conservative, I estimate the BUFFER-TIME parameter as 20000 seconds for all the requested MAMA/TIMETAG exposures. Real Time Justification ----------------------- None. I am a co-investigator on the ongoing GRO/BATSE Pulsar Key Project, a long-term monitoring program which has made daily pulse phase measurements of the hard X-ray emission of 4U 1626--67 with the GRO/BATSE all-sky monitor continuously since 1991 April (Chakrabarty et al. 1997, ApJ, 474, 414). This program will continue for the duration of the GRO mission (at least through 1998), and guarantees simultaneous measurement of the X-ray pulse phase regardless of when the HST observations are scheduled. In addition, I am a member of the RXTE All-Sky Monitor team, and am proposing for a simultaneous pointed RXTE observation. This would allow precise cross-correlation of the X- ray and ultraviolet flux at millisecond timescales, providing an unprecedented probe of the binary, since the ultraviolet emission is dominated by X-ray reprocessing. Given the rarity of high-time-resolution UV observations of X-ray binary targets and the high mission priority which RXTE places on such coordinated programs, it is extremely likely that a simultaneous RXTE pointing will be approved through either the standard peer-eview or via the RXTE director's discretionary time. I request that the HST observation be scheduled after 1-Dec-1997 in order to allow coordination with the RXTE Cycle 3 observing program. Calibration Justification ------------------------- Additional Comments ------------------- ------------------------------------------------------------------------------------ 7510( 6) - 03/27/98 15:28 - [ 3] Data Distribution Media: ELECTRONIC Blocking Factor: 10 Ship To: PI_Address Ship Via: Email: ------------------------------------------------------------------------------------ 7510( 6) - 03/27/98 15:28 - [ 4] TARGET LIST a) Fixed Targets ------------------------------------------------------------------------------------------------------------------------------------ Tar| Target | Target | Target |Coord | Radial | Flux data No | Name | Description | Position |Eqnx | Vel. | ------------------------------------------------------------------------------------------------------------------------------------ 10 KZ-TRA STAR,LMXB,PULSAR RA=16H 32M 16.67S +/- 0.1S,DEC=- J2000 V = 18.7+/-0.2 67D 27' 39.4" +/- 1.0",PLATE- U = 17.5+/-0.3 ID=00JI B-V = 0.0+/-0.3 7510( 6) - 03/27/98 15:28 - [ 5] Visit: 01 Visit Priority: Visit Requirements: BETWEEN 23-Apr-1998:00:00:00 AND 23-Apr-1998:23:59:59 On Hold Comments: Additional Comments: AWAIT REVIEW OF SIMULTANEOUS X-RAY TIMING COVERAGE PROPOSED IN RXTE CYCLE 3 PROGRAM Exposures ------------------------------------------------------------------------------------------------------------------------------------ Exposure| Target |Instr | Oper. | Aper |Spectral|Central| Optional |Num| Time | Special Number | Name |Config| Mode |or FOV |Element |Waveln.| Parameters |Exp| | Requirements ------------------------------------------------------------------------------------------------------------------------------------ 10 KZ-TRA STIS/C ACQ F28X50L MIRROR 1 10.0 S ONBOARD ACQ FOR 20 SEQ 10 CD P ,20,30,40,50 NON-INT ------------------------------------------------------------------------------------------------------------------------------------ 20 KZ-TRA STIS/C ACQ/PEA 52X0.1 MIRROR 1 10.0 S ONBOARD ACQ FOR 30-90 CD K ------------------------------------------------------------------------------------------------------------------------------------ 30 KZ-TRA STIS/C ACCUM 52X0.2 G430L 4300 SIZEAXIS2=80 1 20.0 M CD ------------------------------------------------------------------------------------------------------------------------------------ 40 KZ-TRA STIS/C ACCUM 52X0.2 G750L 7751 SIZEAXIS2=80 1 13.5 M CD ------------------------------------------------------------------------------------------------------------------------------------ 50 CCDFLAT STIS/C ACCUM 0.3X0.0 G750L 7751 2 DEF CD 9 ------------------------------------------------------------------------------------------------------------------------------------ 60 KZ-TRA STIS/N TIME-TA 52X0.2 G230L 2376 BUFFER-TIME=20000 1 47.0 M UV-MAM G A ------------------------------------------------------------------------------------------------------------------------------------ 70 KZ-TRA STIS/N TIME-TA 52X0.2 G230L 2376 BUFFER-TIME=20000 1 49.0 M UV-MAM G A ------------------------------------------------------------------------------------------------------------------------------------ 80 KZ-TRA STIS/F TIME-TA 52X0.2 G140L 1425 BUFFER-TIME=20000 1 45.0 M UV-MAM G A ------------------------------------------------------------------------------------------------------------------------------------ 90 KZ-TRA STIS/F TIME-TA 52X0.2 G140L 1425 BUFFER-TIME=20000 1 45.0 M UV-MAM G A ------------------------------------------------------------------------------------------------------------------------------------ 7510( 6) - 03/27/98 15:28 - [ 6] Summary Form for Proposal 7510 Item Used in this proposal ------------------------------------------------------------------------------------------------------------------------------------ Configurations STIS/CCD STIS/NUV-MAMA STIS/FUV-MAMA ------------------------------------------------------------------------------------------------------------------------------------ Opmodes ACQ ACQ/PEAK ACCUM TIME-TAG ------------------------------------------------------------------------------------------------------------------------------------ Optional Parameters SIZEAXIS2=80 BUFFER-TIME=20000 ------------------------------------------------------------------------------------------------------------------------------------ Proposal Category GO ------------------------------------------------------------------------------------------------------------------------------------ Special Requirements BETWEEN 23-Apr-1998 00 00 00 AND 23-Apr-1998 23 59 59 ONBOARD ACQ FOR 20 SEQ 10,20,30,40,50 NON-INT ONBOARD ACQ FOR 30-90 ------------------------------------------------------------------------------------------------------------------------------------ Spectral Elements MIRROR G430L G750L G230L G140L ------------------------------------------------------------------------------------------------------------------------------------ Target Names KZ-TRA CCDFLAT ------------------------------------------------------------------------------------------------------------------------------------