6831( 8) - 09/24/96 09:20 - [ 0] PROPOSAL FOR HUBBLE SPACE TELESCOPE OBSERVATIONS ST ScI Use Only ID: 6831 Version: 8 Check-in Date: 24-Sep-1996 09:15:27 1.Proposal Title: High-Speed Photometry of the UV Counterpart to 4U 1820-30 ------------------------------------------------------------------------------------ 2. Scientific Category 3. Proposal For 4. Cycle BINARIES AND STAR FOR GO 6 ------------------------------------------------------------------------------------ 5. Investigators Contact? PI: ANDREW FRUCHTER SPACE TELESCOPE SCIENCE INSTITUTE CoI: Lars Bildsten University of California at Berkeley N CoI: Charles Keyes STSCI N CoI: Ivan King University of California at Berkeley N CoI: Jonathan Grindlay Harvard College Observatory N ------------------------------------------------------------------------------------ 6. Abstract 4U 1820-30, the 11 minute binary in the globular cluster NGC 6624, is one of the most extraordinary objects in the galaxy; it is also an exemplary low-mass x-ray binary, for it displays nearly all of the behaviors that have been observed in these objects. The x-ray flux of this binary is modulated at its 11- minute orbital period, and cycles between high and low luminosity states at an approximate period of 176 days. While in its high-state the x-ray flux shows high frequency variability similar to that seen in QPO's; in the low-state x- ray bursts recur on periods of hours and high-frequency noise is strong but flat. The binary is also a luminous UV source due to the reprocessing of the x-ray emission by the accretion disk. We propose to use the FOS in a novel mode to study the time variability of the UV radiation and through it, the geometry of the accretion disk. By replacing the FOS grating with the acquisition mirror and increasing the FOS sampling speed to 30 Hz we will gain both the quantum efficiency and rapid sampling necessary to measure the transmission function of x-ray to UV time variability. This function will in turn provide a direct estimate of the size and geometry of the disk. The reprocessing of x-rays on the face of the companion should also produce a detectable modulation of the UV light the measurement of which will determine the inclination of the orbit on the sky, the time of ascending node, and the orbital phase of the x-ray variations. ------------------------------------------------------------------------------------ Observations Description ------------------------ sectionRapid Mode We plan to perform high-speed photometry on the x-ray binary 4U 1820-30 using the FOS blue channel with the acquisition mirror in place as the spectral element. We will read the FOS at its maximum rate. Each sample will be composed of 0.02 s live time, 0.01 s dead time, and 0.003 s readout, for a total sample time of 0.033 s and an obervational efficiency of 60\\%. Use of this mode will enable the high telemetry rate. We will therefore need to either use both tape recorders with frequent TDRS access to allow the downloading of the data as the tape recorders fill, or a direct TDRS to ground- station link during the observations. The latter is preferable because a ~ 0.25 s jitter has been found in the absolute phase of FOS observations. A direct down-link would allow keep our observation phase coherent throughout an entire orbit. However either method is acceptable, as we will obtain signal -to-noise greater than one in each spectral element in less than a single orbit, and therefore the power spectra can be added incoherently. If necessary the interval between the observations may be varied to allow TDRS access. We also understand that it is possible that on one or more occasions limitations in TDRS availability may limit the fast sampling mode to slightly less than the requested four orbits. We have discussed the technical demands of these observations in detail with other members of the FOS group as well as with members of the Institute who would be responsible for scheduling TDRS access, including Roger Doxsey and Merle Reinhart. None of these experts sees any technical impediment to the successful conclusion of these observations. sectionSpectral Observations We will begin and end each set of observations with short G160L spectra of 4U 1820-30. These will provide a flux calibrated measure of the magnitude and effective temperature of the source at each observing period. sectionAcquisition Each visit to 4U 1820-30 will require five orbits, as the first orbit will be used for target acquisition. We have obtained a U band PC2 image of the core of NGC 6624 taken by Brian Yanny. We have found a bright, isolated star about 20" from the core which will serve well as an offset star. sectionSignal-to-Noise We have used the photometry of King Etal (1993) and the HST simulation program, xcalc, to estimate our sensitivity. Assuming a mirror reflectance of 80\\% we predict 600 photons per second will be detected by the diodes; 60\\% of these will be detected during the live time for a total count rate of 360 counts per second. (We note that using our assumption of an 80\\% reflectance for the acquisition mirror, xcalc correctly reproduced the factor of five increase in throughput between the mirror over the zeroth mode of G160L typically observed during guide star acquisitions.) The r.m.s. power between 0.01 and 100 Hz varies from about 6\\% in the x-ray high-state to 12\\% in the low-state citehk89b. This variability has typically been detected with S/N ratios between 10 and 50 by Exosat observations of length 2 - 8 hours. Our expected count rate is nearly identical to that of Exosat during the low-state. We therefore expect S/N ratios in 4- orbit long observing sessions to vary between four and ten, depending upon the strength of the signal at the time of observation. This S/N will allow us to detect the effect of the transmission function in a single observing session. However, we are asking for 3*4-orbit observing sessions (plus one orbit each session for acquisition) to be taken with separations of about 45 days (though the exact interval can be varied if necessary to permit TDRS access) to allow us to study the source at different x-ray luminosities, and to further refine the measurement of the average transmission function. Our ability to detect orbital period modulation is discussed in detail in the Scientific Justification. sectionAppendix: Calibration We will be using the acquisition mirror rather than a grating to observe 4U 1820-30. No calibration has yet been done on the acquisition mirror, though it is well known from observation of acquisition stars that the throughput of the mirror is about a factor five greater than that of the G160L zeroth order. Nonetheless, the data presently exist in the archive to calibrate the acquisition mirror and determine its transmission as a function of wavelength. Many objects of varying spectra have been observed first by the acquisition mirror and then by well-calibrated spectrophotmetric gratings. A comparison of count rates as a function of spectral type will allow us to determine the wavelength dependence of the reflectance of the acquisition mirror. Real Time Justification ----------------------- The rapid read-out of the FOS planned here requires the use of the high telemetry rate which will exhaust the two tape recorders in 36 minutes of oberving. As discussed in the Description of the Observations this program will require approximately four consecutive orbits of TDRS access either for dumping of the tape drives or direct transmission of the data to a ground station (the latter is preferred). The Description of the Observations provides a detailed discussion of the TDRS access. We are requesting that the three visits be scheduled approximately 45 days apart. This requirement may be loosened if necessary to obtain TDRS access. Calibration Justification ------------------------- Additional Comments ------------------- ------------------------------------------------------------------------------------ Data Distribution Media: 8MM Blocking Factor: 10 Ship To: PI_Address Ship Via: UPS Email: , ------------------------------------------------------------------------------------ 6831( 8) - 09/24/96 09:20 - [ 1] TARGET LIST a) Fixed Targets ------------------------------------------------------------------------------------------------------------------------------------ Tar| Target | Target | Target |Coord | Radial | Flux data No | Name | Description | Position |Eqnx | Vel. | ------------------------------------------------------------------------------------------------------------------------------------ 1 4U1820-30- CALIBRATION,FLAT RA=18H 23M 41.651S +/- 0.010S, J2000 V = 13.97+/-0.02 OFFSET FIELD,PHOTOMETRIC, DEC=-30D 21' 40.6" +/- 0.20", B = 15.95+/-0.02 PLATE-ID=060E U = 17.56+/-0.02 Comments: Offset star - coord and colors from WFPC2 imagery taken in early 1994 2 4U1820-30 CALIBRATION,FLAT RA=18H 23M 40.486S +/- 0.010S, J2000 V = 16.58+/-0.02 FIELD,PHOTOMETRIC, DEC= -30D 21' 39.14" +/- 0.20", B = 17.42+/-0.02 PLATE-ID=060E U = 17.13+/-0.02 Comments: coord from metric on WFPC2 imagery taken in early 1994 6831( 8) - 09/24/96 09:20 - [ 2] Visit: 01 Visit Requirements: 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 ------------------------------------------------------------------------------------------------------------------------------------ 10 4U1820-30-O FOS/BL ACQ/BIN 4.3 MIRROR 1 120 S ONBOARD ACQ FOR 15 FFSET ARY ------------------------------------------------------------------------------------------------------------------------------------ 15 4U1820-30-O FOS/BL ACQ/PEA 0.25-PA MIRROR SCAN-STEP-Y=0.11,SCAN-STEP-X=0.11 1 20 S ONBOARD ACQ FOR 18 FFSET K IR-A ,SEARCH-SIZE-Y=4,SEARCH-SIZE-X=4 Comments: Pattern P4 to achieve 0.08 pointing (0.06 each coord) prior to offset. ------------------------------------------------------------------------------------------------------------------------------------ 18 4U1820-30 FOS/BL ACQ/PEA 0.25-PA MIRROR SCAN-STEP-Y=0.11,SCAN-STEP-X=0.11 1 15 S ONBOARD ACQ FOR 20 K IR-A ,SEARCH-SIZE-Y=3,SEARCH-SIZE-X=3 Comments: Achieve 0.08 pting on target. ------------------------------------------------------------------------------------------------------------------------------------ 20 4U1820-30 FOS/BL ACQ/PEA 0.25-PA MIRROR SCAN-STEP-Y=0.052, 1 15 S ONBOARD ACQ FOR 50-58 K IR-A SCAN-STEP-X=0.052,SEARCH-SIZE-Y=3 ,SEARCH-SIZE-X=3 Comments: Achieve 0.04 pting on target. ------------------------------------------------------------------------------------------------------------------------------------ 50 4U1820-30 FOS/BL RAPID 0.25-PA G160L SUB-STEP=4,COMB=YES,READ-TIME=6.5 1 12 M MAX DUR 12M IR-A ,STEP-PATT=SINGLE Comments: first half of orbit 2 ------------------------------------------------------------------------------------------------------------------------------------ 51 4U1820-30 FOS/BL RAPID 0.25-PA MIRROR SUB-STEP=1,COMB=NO,READ-TIME=.036 1 970 S MAX DUR 970S; NO SPLIT IR-A ,STEP-PATT=SINGLE,STEP-TIME=.003 Comments: DUE TO TRANS deficiency, we cannot specify restricted diode readout range for this line; the PMDB will have to be manually edited to provide a starting diode and number of diodes to be read out and this will remove RPS2 warning about minimum read-time. use tape recorder; second half orbit 2 ------------------------------------------------------------------------------------------------------------------------------------ 52 4U1820-30 FOS/BL RAPID 0.25-PA MIRROR SUB-STEP=1,COMB=NO,READ-TIME=.036 1 32 M RT ANALYSIS; MAX DUR 35 IR-A ,STEP-PATT=SINGLE,STEP-TIME=.003 M; NO SPLIT Comments: Same manual PMDB entries as for 51; no tape backup ------------------------------------------------------------------------------------------------------------------------------------ 55 4U1820-30 FOS/BL RAPID 0.25-PA MIRROR SUB-STEP=1,COMB=NO,READ-TIME=.036 1 33 M RT ANALYSIS; MAX DUR 35 IR-A ,STEP-PATT=SINGLE,STEP-TIME=.003 M; NO SPLIT Comments: Same manual PMDB entries as for 51; no tape backup ------------------------------------------------------------------------------------------------------------------------------------ 58 4U1820-30 FOS/BL RAPID 0.25-PA MIRROR SUB-STEP=1,COMB=NO,READ-TIME=.036 1 25 M RT ANALYSIS; MAX DUR 25M; IR-A ,STEP-PATT=SINGLE,STEP-TIME=.003 NO SPLIT Comments: Same manual PMDB entries as for 51; no tape backup ------------------------------------------------------------------------------------------------------------------------------------ 6831( 8) - 09/24/96 09:20 - [ 3] Visit: 02 Visit Requirements: AFTER 01 BY 20 DAYS 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 ------------------------------------------------------------------------------------------------------------------------------------ 10 4U1820-30-O FOS/BL ACQ/BIN 4.3 MIRROR FAINT=1100 1 90 S ONBOARD ACQ FOR 15 FFSET ARY ------------------------------------------------------------------------------------------------------------------------------------ 15 4U1820-30-O FOS/BL ACQ/PEA 0.25-PA MIRROR SCAN-STEP-Y=0.11,SCAN-STEP-X=0.11 1 20 S ONBOARD ACQ FOR 18 FFSET K IR-A ,SEARCH-SIZE-Y=4,SEARCH-SIZE-X=4 Comments: Pattern P4 to achieve 0.08 pointing (0.06 each coord) prior to offset. ------------------------------------------------------------------------------------------------------------------------------------ 18 4U1820-30 FOS/BL ACQ/PEA 0.25-PA MIRROR SCAN-STEP-Y=0.11,SCAN-STEP-X=0.11 1 15 S ONBOARD ACQ FOR 20 K IR-A ,SEARCH-SIZE-Y=3,SEARCH-SIZE-X=3 Comments: Achieve 0.08 pting on target. ------------------------------------------------------------------------------------------------------------------------------------ 20 4U1820-30 FOS/BL ACQ/PEA 0.25-PA MIRROR SCAN-STEP-Y=0.052, 1 15 S ONBOARD ACQ FOR 50-58 K IR-A SCAN-STEP-X=0.052,SEARCH-SIZE-Y=3 ,SEARCH-SIZE-X=3 Comments: Achieve 0.04 pting on target. ------------------------------------------------------------------------------------------------------------------------------------ 50 4U1820-30 FOS/BL RAPID 0.25-PA G160L SUB-STEP=4,COMB=YES,READ-TIME=6.5 1 12 M MAX DUR 12M IR-A ,STEP-PATT=SINGLE Comments: first half of orbit 2 ------------------------------------------------------------------------------------------------------------------------------------ 51 4U1820-30 FOS/BL RAPID 0.25-PA MIRROR SUB-STEP=1,COMB=NO,READ-TIME=.036 1 970 S MAX DUR 970S; NO SPLIT IR-A ,STEP-PATT=SINGLE,STEP-TIME=.003 Comments: DUE TO TRANS deficiency, we cannot specify restricted diode readout range for this line; the PMDB will have to be manually edited to provide a starting diode and number of diodes to be read out and this will remove RPS2 warning about minimum read-time. use tape recorder; second half orbit 2 ------------------------------------------------------------------------------------------------------------------------------------ 52 4U1820-30 FOS/BL RAPID 0.25-PA MIRROR SUB-STEP=1,COMB=NO,READ-TIME=.036 1 32 M RT ANALYSIS; MAX DUR 35 IR-A ,STEP-PATT=SINGLE,STEP-TIME=.003 M; NO SPLIT Comments: Same manual PMDB entries as for 51; no tape backup ------------------------------------------------------------------------------------------------------------------------------------ 55 4U1820-30 FOS/BL RAPID 0.25-PA MIRROR SUB-STEP=1,COMB=NO,READ-TIME=.036 1 33 M RT ANALYSIS; MAX DUR 35 IR-A ,STEP-PATT=SINGLE,STEP-TIME=.003 M; NO SPLIT Comments: Same manual PMDB entries as for 51; no tape backup ------------------------------------------------------------------------------------------------------------------------------------ 58 4U1820-30 FOS/BL RAPID 0.25-PA MIRROR SUB-STEP=1,COMB=NO,READ-TIME=.036 1 25 M RT ANALYSIS; MAX DUR 25M; IR-A ,STEP-PATT=SINGLE,STEP-TIME=.003 NO SPLIT Comments: Same manual PMDB entries as for 51; no tape backup ------------------------------------------------------------------------------------------------------------------------------------ 6831( 8) - 09/24/96 09:20 - [ 4] Summary Form for Proposal 6831 Item Used in this proposal ------------------------------------------------------------------------------------------------------------------------------------ Configurations FOS/BL ------------------------------------------------------------------------------------------------------------------------------------ Opmodes ACQ/BINARY ACQ/PEAK RAPID ------------------------------------------------------------------------------------------------------------------------------------ Optional Parameters SCAN-STEP-Y=0.11 SCAN-STEP-X=0.11 SEARCH-SIZE-Y=4 SEARCH-SIZE-X=4 SEARCH-SIZE-X=3 SEARCH-SIZE-Y=3 STEP-PATT=SINGLE STEP-TIME=.003 READ-TIME=.036 COMB=NO SUB-STEP=1 FAINT=1100 SCAN-STEP-X=0.052 READ-TIME=6.5 ------------------------------------------------------------------------------------------------------------------------------------ Proposal Category GO ------------------------------------------------------------------------------------------------------------------------------------ Scientific Category BINARIES AND STAR FORMATION ------------------------------------------------------------------------------------------------------------------------------------ Special Requirements ONBOARD ACQ FOR 15 ONBOARD ACQ FOR 18 ONBOARD ACQ FOR 20 ONBOARD ACQ FOR 50-58 MAX DUR 12M MAX DUR 970S; NO SPLIT RT ANALYSIS; MAX DUR 35 M; NO SPLIT RT ANALYSIS; MAX DUR 25M; NO SPLIT AFTER 01 BY 20 DAYS ------------------------------------------------------------------------------------------------------------------------------------ Spectral Elements MIRROR G160L ------------------------------------------------------------------------------------------------------------------------------------ Target Names 4U1820-30-OFFSET 4U1820-30 ------------------------------------------------------------------------------------------------------------------------------------