10007( 8) - 04/08/04 06:44 - [ 1] PROPOSAL FOR HUBBLE SPACE TELESCOPE OBSERVATIONS ST ScI Use Only ID: 10007 Version: 8 Check-in Date: 08-Apr-2004 06:43:04 1.Proposal Title: The Spectral Energy Distribution of Cen X-4 ------------------------------------------------------------------------------------ 2. Proposal For 3. Cycle 4. Parallel Pointing Tolerance GO 12 ------------------------------------------------------------------------------------ 5. Investigators Contact? PI: Dr. Michael R. Garcia Smithsonian Institution Astrophysical Observatory CoI: Dr. Jeffrey E. McClintock Smithsonian Institution Astrophysical N Observatory CoI: Dr. Jon Matthew Miller Smithsonian Institution Astrophysical N Observatory CoI: Dr. Ronald Remillard Massachusetts Institute of Technology N ------------------------------------------------------------------------------------ 6. Abstract We propose to obtain the first high-quality, wide-band spectral energy distribution (SED) of the neutron star X-ray nova Cen X-4 in quiescence. We will compare this SED to those we have previously obtained for black hole X-ray novae (BHXRN) in quiescence. Our previous comparisons of SEDs have been very powerful in revealing the nature of these exotic objects; for example, they have revealed the best evidence yet for the existence of event horizons. These wide-band, high quality SEDs have revealed the presence of ADAFs in BHXN, and are revealing the presence of winds (ADIOS) and/or convection (CDAFs) in these flows. Previous SEDs of Cen X-4 have suffered from non-simultaneity and have not included the FUV, where the emission may well peak. ------------------------------------------------------------------------------------ 10007( 8) - 04/08/04 06:44 - [ 2] Observations Description ------------------------ We plan 1 orbit of UV spectroscopy followed by 1 orbit of FUV spectroscopy of Cen X-4. The grating/detector/slit combinations to be used are: G230L/NUV- MAMA/52x0.5 to measure the UV (1900-3000 A) spectrum, and the G140L/FUV- MAMA/52x0.5 setup to measure the FUV (1150-1900 A) spectrum. We note that it is necessary to repeat the G230L exposure from HST Cycle 7 because Cen X-4 varies in the optical (and presumably the UV as well) by up to ~1 mag in quiescence. Real Time Justification ----------------------- We request that the 2 HST orbits during the visit be contiguous in order to: a) obtain UV and FUV spectroscopy simultaneous with the 11 ks Chandra observations. b) obtain a near-simultaneous UV spectrum that we can match up sensibly with the FUV spectrum. It would be very desirable if the STIS observations of Cen X-4 could be scheduled during the appropriate observing season and during the dark half of the lunar cycle so that we could conduct ground-based contemporaneous optical observations of the source. Such a schedule was achieved for us during Cycle 7 in Phase II. We will make a concerted effort to carry out ground-based spectroscopic and photometric observations of the source during the time of the HST/Chandra observations. The dark moon windows have been specified with 2 'BETWEEN' constraints. If the specified BETWEENs do not give enough flexibility to actually schedule the HST/Cnandra observations, there is a third and fourth window which could be attempted: 16-Feb-2004 to 23-Feb-2004 and 9-Jul-2004 to 21-Jul-2004. We include these here (rather than in the requirements) because the target visibility from a ground-based telescope will be limited to ~3-5 hours of the observing night. This might make it more difficult to arrange simultaneous optical observations, but even near-simultaneous data is better than non-simultaneous. Calibration Justification ------------------------- We request no special calibrations for our program. Additional Comments ------------------- We request that the PI be notified ASAP after these HST/STIS observations have been scheduled so that he can arrange for contemporaneous ground-based observartions. ------------------------------------------------------------------------------------ 10007( 8) - 04/08/04 06:44 - [ 3] TARGET LIST Fixed Targets ------------------------------------------------------------------------------------------------------------------------------------ Tar| Target | Target | Target |Coord | Radial | Flux data No | Name | Description | Position |Eqnx | Vel. | ------------------------------------------------------------------------------------------------------------------------------------ 1 CENX-4 STAR, ACCRETION DISK, RA=14H58M21.95S +/- 1.0", DEC=- J2000 V = 18.5+/-0.3 LMXB, X-RAY TRANSIENT, 31D40'5.9" +/- 1.0", PLATE- B-V=0.8+/-0.2 X-RAY NOVAE ID=02J0 E(B-V)=0.1+/-0.05 Coordinate Source: GSC_SURVEY_PLATE Comments: The spectrum of Cen X-4 rises steadily with frequency over the entire observed UV band and has an observed flux at 2500 A of 1.5e-16 ergs/cm^2/s/A. An estimated source flux of 2.7e-16 ergs/cm^2/s/A at 1400 A is obtained when we extrapolate the FUV spectrum by assuming a power-law spectrum with energy index=-1.2. 10007( 8) - 04/08/04 06:44 - [ 4] Visit: 01 Visit Priority: Visit Requirements: BETWEEN 11-MAY-2004:00:00:00 AND 26-MAY-2004:00:00:00; BETWEEN 11-JUN- 2004:00:00:00 AND 22-JUN-2004:00:00:00 On Hold Comments: Additional Comments: The special requirements on the time of the visit ('BETWEENs') are to insure that simultaneous or near-simultaneous ground-based observations of the target will be possible, i.e. that the HST/Chandra observations occur during the dark half of the lunar cycle (when the sun is far from the target) and when the moon is not near the target. Note that the first BETWEEN is in reality restricted to May 25-26 due to target visibility limitations. Exposures ------------------------------------------------------------------------------------------------------------------------------------ Exp | Target | Instr | Oper. | Aper |Spectral|Central| Optional |Num| Time | Special Num | Name | Config | Mode | or FOV |Element |Waveln.| Parameters |Exp| | Requirements ------------------------------------------------------------------------------------------------------------------------------------ 1 CENX-4 STIS/CCD ACQ F28X50LP MIRROR ACQTYPE=POINT 1 10 S Comments: The recurrence time of an X-ray/optical outburst in Cen X-4 is unpredictable and there exists the possibility that such event occurs before the HST visit. Cen X-4 has shown outbursts in 1969 (estimated peak magnitude V=11.5) and 1979 (observed peak magnitude V=12.8). Assuming for a hypothetical outburst a peak magnitude of V=11.5 and a power-law spectrum with index=-2.3 for the optical spectrum, the HST/Target ACQ calculator predicts that the CCD would be saturated for an exposure of 10 s. It is then necessary to reduce the exposure time below ~1s. For an exposure of 0.5 s, S/N=280 (V=11.5), S/N=150 (V=12.8). If Cen X-4 were to become active previously to the HST visit, RXTE will provide the detection of the X-ray outburst. We and/or our network of collaborators will obtain photometric data of Cen X-4 to estimate its optical brightness during the outburst and its decline. Hence "real-time" planning of the HST/Chandra observations is possible if Cen X-4 undergoes an outburst and it would not impact HST operations in any way. ------------------------------------------------------------------------------------------------------------------------------------ 2 CENX-4 STIS/NUV TIME-TA 52X0.5 G230L 2376 BUFFER-TIME=1100 1 2202 S -MAMA G ------------------------------------------------------------------------------------------------------------------------------------ 3 CENX-4 STIS/FUV TIME-TA 52X0.5 G140L 1425 BUFFER-TIME=1400 1 2844 S -MAMA G ------------------------------------------------------------------------------------------------------------------------------------ 10007( 8) - 04/08/04 06:44 - [ 5] Sub Exposures ------------------------------------------------------------------------------------------------------------------------------------ Visit Id| Exp Id |Primary |Secondary |Iteration |CR-SPLIT |Orbit Num |Duration | |Pattern Pos |Pattern Pos |Num |Num | | ------------------------------------------------------------------------------------------------------------------------------------ Sub Exposures ------------------------------------------------------------------------------------------------------------------------------------ Visit Id| Exp Id |Primary |Secondary |Iteration |CR-SPLIT |Orbit Num |Duration | |Pattern Pos |Pattern Pos |Num |Num | | ------------------------------------------------------------------------------------------------------------------------------------ 01 1 none none none none 1 tbd 01 2 none none none none 1 tbd 01 3 none none none none 2 tbd 10007( 8) - 04/08/04 06:44 - [ 6] Summary Form for Proposal 10007 Item Used in this proposal ------------------------------------------------------------------------------------------------------------------------------------ Apertures F28X50LP 52X0.5 ------------------------------------------------------------------------------------------------------------------------------------ Configurations STIS/CCD STIS/NUV-MAMA STIS/FUV-MAMA ------------------------------------------------------------------------------------------------------------------------------------ Opmodes ACQ TIME-TAG ------------------------------------------------------------------------------------------------------------------------------------ Optional Parameters ACQTYPE=POINT BUFFER-TIME=1100 BUFFER-TIME=1400 ------------------------------------------------------------------------------------------------------------------------------------ Proposal Category GO ------------------------------------------------------------------------------------------------------------------------------------ Special Requirements BETWEEN 11-MAY-2004 00 00 00 AND 26-MAY-2004 00 00 00; BETWEEN 11-JUN-2004 00 00 00 AND 22-JUN-2004 00 00 00 ------------------------------------------------------------------------------------------------------------------------------------ Spectral Elements MIRROR G230L G140L ------------------------------------------------------------------------------------------------------------------------------------ Target Names CENX-4 ------------------------------------------------------------------------------------------------------------------------------------ Wavelengths 2376 1425 ------------------------------------------------------------------------------------------------------------------------------------