10099( 3) - 06/15/04 11:53 - [ 1] PROPOSAL FOR HUBBLE SPACE TELESCOPE OBSERVATIONS ST ScI Use Only ID: 10099 Version: 3 Check-in Date: 15-Jun-2004 15:51:08 1.Proposal Title: The Parallax of Geminga ------------------------------------------------------------------------------------ 2. Proposal For 3. Cycle 4. Parallel Pointing Tolerance GO 13 ------------------------------------------------------------------------------------ 5. Investigators Contact? PI: Dr. Frederick M. Walter State University of New York at Stony Brook CoI: Dr. George G. Pavlov The Pennsylvania State University N CoI: Dr. James M. Lattimer State University of New York at Stony N Brook CoI: Dr. Madappa Prakash State University of New York at Stony N Brook ------------------------------------------------------------------------------------ 6. Abstract The distances to neutron stars are a fundamental parameter that impacts estimates of radii and luminosities, both of which have important ramifications for neutron star heating and cooling and for the equation of state of nuclear matter. A radius measured to 5% accuracy will provide useful astrophysical constraints on the equation of matter at nuclear densities. This requires an accurate measurment of the distance. Caraveo et al. have reported a distance to the isolated pulsar Geminga based on three WFPC2 images. We have not been able to reproduce that result. We request time with the ACS/WFC over two years to obtain 4 images in order to determine the parallax of Geminga. In addition to the general importance of understanding neutron star radii and luminosities, this will provide specific insights into the history of gamma ray pulsars in general, and of Geminga itself. We have obtained the first two images (cycle 12); these two cycle 13 images will complete the program. ------------------------------------------------------------------------------------ 10099( 3) - 06/15/04 11:53 - [ 2] Observations Description ------------------------ This is the second year of a two year program to determine the parallax of the nearby neutron star Geminga. We require ACS/WFC/F555W observations of Geminga on or about 30 September 2004, and 27 March 2005. There is about a +/- 2 week tolerance on the dates of observation. We have been awarded 6 orbit s during cycle 13. We will obtain a 3 orbit exposure at each epoch. The exposure will be dithered and CR-split; 12 images will be obtained at each epoch.-- Observing description from the proposal -- As viewed from the Earth, stars annually trace small ellipses in the sky, reflecting the Earth's orbit around the Sun. The shape of the ellipse is the projection of the Earth's orbit as seen from the star; the semimajor axis of the ellipse is the parallax. The maximal displacement from the mean position occurs twice a year, when the Earth, Sun, and star form a right triangle. By observing twice, on these days of maximal displacement, we can in principle determine the parallax. However, stars move. Separating the proper motions from the parallax requires a minimum of three observations. Two observations separated by an integral number of years suffice to yield the proper motion, since the parallactic shifts will be the same. A third observation, offset by 6 months, can then yield the parallax. Since the parallax is small (a sub-pixel shift in the detector), we request a fourth observation to confirm the parallax, and reduce its uncertainties. This program requires a minimum of 18 months to complete, and so becomes a long term program, requiring time in the next cycle. In order to measure the parallax efficiently, we need to observe near the days of maximum parallactic shift (March 27 and September 30), so this is a time critical observation. At the position of Geminga (ecliptic latitude -5 deg), the parallactic ellipse is nearly linear, with a major-to-minor axis ratio of 20. The semimajor axis of the ellipse is 10 (100 pc/d) mas. At the nominal 160 pc distance, the total shift over six months is 12.5 mas, or about 1/4 pixel in the ACS/WFC. The WFC is the preferred camera because it offers the best sensitivity and the widest field of view. There are about 8 astrometric reference stars in the HRC field, based on the WFPC2 images. The larger grid afforded by the larger WFC field of view provides for a more robust astrometric grid, with commensurably smaller systematic uncertainties. To measure the parallax of an object at the nominal 160pc distance with 5 sigma confidence we will need to centroid the position of Geminga and the astrometric comparison stars to about 0.05 pixel accuracy. This is easily accomplished for sufficient S/N in the star. Geminga is brighter in V than in B or R (Bignami et al. 1996), so we choose to observe in the V filter. There are a sufficient number of astrometric comparison stars in the V band. We simulated the accuracy to which we could recover the position of a source in the WFC, using the 5x5 pixel PSF in the ACS Instrument Handbook. We ran the ACS ETC for the WFC/F555W for a hot continuum source with a V magnitude of 25.5, for exposure times corresponding to integral numbers of orbits. We generated a test image using the source and background counts from the ETC, added Poisson noise, and measured the position. We find that for a 3 orbit (8 ksec) observation, we can measure the position with a 1 sigma statistical uncertainty of 0.04 pix. We repeated this exercise with the HRC and its smaller pixels. For a star this faint we find no advantage in going to the HRC at these wavelengths. This 0.04 pixel uncertainty corresponds to 2 mas in the WFC. Since the parallax is half the total displacement, we expect to reach 1 sigma confidence levels of about 1 mas. If the distance is comparable to the 160 pc reported, we will confirm the result at 6 sigma confidence; we should detect 10099( 3) - 06/15/04 11:53 - [ 3] the parallax at > 3 sigma confidence so long as the distance is less than about 333 pc, or twice the current estimate. In order to reach these levels, we will require 4 observations, one every six months, to image Geminga and determine its parallax. Each of the four observations is identical: a 3 orbit (~8 ksec) integration using ACS/WFC through the F555W filter. We will dither during each orbit using the default dither, to improve the spatial resolution of the images. We require 6 orbits in Cycle 12, and 6 orbits in Cycle 13. The coordinates supplied are those from Caraveo et al. (1998), with proper motion to 2004 applied. Real Time Justification ----------------------- Calibration Justification ------------------------- Additional Comments ------------------- ------------------------------------------------------------------------------------ 10099( 3) - 06/15/04 11:53 - [ 4] TARGET LIST Fixed Targets ------------------------------------------------------------------------------------------------------------------------------------ Tar| Target | Target | Target |Coord | Radial | Flux data No | Name | Description | Position |Eqnx | Vel. | ------------------------------------------------------------------------------------------------------------------------------------ 1 GEMINGA STAR, NEUTRON STAR RA=06H33M54.23S +/- 0.1", J2000 V = 25.5+/-0.2 DEC=+17D46'13.7" +/- 0.1" Coordinate Source: HST_IMAGE 10099( 3) - 06/15/04 11:53 - [ 5] Patterns ------------------------------------------------------------------------------------------------------------------------------------ Pattern_Number: 1 Primary_Pattern Secondary_Pattern Pattern_Type ACS-WFC-DITHER-LINE Pattern_Purpose DITHER Number_Of_Points 2 Point_Spacing 3.011 Line_Spacing Coordinate_Frame POS-TARG Pattern_Orient 85.28 Angle_Between_Sides Center_Pattern NO ------------------------------------------------------------------------------------------------------------------------------------ 10099( 3) - 06/15/04 11:53 - [ 6] Visit: 01 Visit Priority: Visit Requirements: PCS MODE FINE; ORIENT 276.0D TO 277.0 D; BETWEEN 16-SEP-2004:00:00:00 AND 14-OCT-2004:00:00:00 On Hold Comments: Additional Comments: Must be observed within 2 weeks of maximum parallactic shift on Sept 30. Exposures ------------------------------------------------------------------------------------------------------------------------------------ Exp | Target | Instr | Oper. | Aper |Spectral|Central| Optional |Num| Time | Special Num | Name | Config | Mode | or FOV |Element |Waveln.| Parameters |Exp| | Requirements ------------------------------------------------------------------------------------------------------------------------------------ 1 GEMINGA ACS/WFC ACCUM WFC F555W 1 1090 S SEQ 1 NON-INT; PATTERN 1 1 ------------------------------------------------------------------------------------------------------------------------------------ 2 GEMINGA ACS/WFC ACCUM WFC F555W 1 1165 S SEQ 2 NON-INT; PATTERN 1 2 ------------------------------------------------------------------------------------------------------------------------------------ 3 GEMINGA ACS/WFC ACCUM WFC F555W 1 1160 S SEQ 3 NON-INT; PATTERN 1 3 ------------------------------------------------------------------------------------------------------------------------------------ Sub Exposures ------------------------------------------------------------------------------------------------------------------------------------ Visit Id| Exp Id |Primary |Secondary |Iteration |CR-SPLIT |Orbit Num |Duration | |Pattern Pos |Pattern Pos |Num |Num | | ------------------------------------------------------------------------------------------------------------------------------------ 01 1 1 none none 1 1 548.0 01 1 1 none none 2 1 548.0 01 1 2 none none 1 1 548.0 01 1 2 none none 2 1 548.0 01 2 1 none none 1 2 tbd 01 2 1 none none 2 2 tbd 01 2 2 none none 1 2 tbd 01 2 2 none none 2 2 tbd 01 3 1 none none 1 3 584.0 01 3 1 none none 2 3 584.0 01 3 2 none none 1 3 584.0 01 3 2 none none 2 3 584.0 10099( 3) - 06/15/04 11:53 - [ 7] Visit: 02 Visit Priority: Visit Requirements: PCS MODE FINE; ORIENT 94.6D TO 95.6 D; BETWEEN 14-MAR-2005:00:00:00 AND 11- APR-2005:00:00:00 On Hold Comments: Additional Comments: Must be observed within 2 weeks of maximum parallactic shift on March 27 Exposures ------------------------------------------------------------------------------------------------------------------------------------ Exp | Target | Instr | Oper. | Aper |Spectral|Central| Optional |Num| Time | Special Num | Name | Config | Mode | or FOV |Element |Waveln.| Parameters |Exp| | Requirements ------------------------------------------------------------------------------------------------------------------------------------ 1 GEMINGA ACS/WFC ACCUM WFC F555W 1 1090 S SEQ 1 NON-INT; PATTERN 1 1 ------------------------------------------------------------------------------------------------------------------------------------ 2 GEMINGA ACS/WFC ACCUM WFC F555W 1 1165 S SEQ 2 NON-INT; PATTERN 1 2 ------------------------------------------------------------------------------------------------------------------------------------ 3 GEMINGA ACS/WFC ACCUM WFC F555W 1 1165 S SEQ 3 NON-INT; PATTERN 1 3 ------------------------------------------------------------------------------------------------------------------------------------ Sub Exposures ------------------------------------------------------------------------------------------------------------------------------------ Visit Id| Exp Id |Primary |Secondary |Iteration |CR-SPLIT |Orbit Num |Duration | |Pattern Pos |Pattern Pos |Num |Num | | ------------------------------------------------------------------------------------------------------------------------------------ 02 1 1 none none 1 1 548.0 02 1 1 none none 2 1 548.0 02 1 2 none none 1 1 548.0 02 1 2 none none 2 1 548.0 02 2 1 none none 1 2 tbd 02 2 1 none none 2 2 tbd 02 2 2 none none 1 2 tbd 02 2 2 none none 2 2 tbd 02 3 1 none none 1 3 tbd 02 3 1 none none 2 3 tbd 02 3 2 none none 1 3 tbd 02 3 2 none none 2 3 tbd 10099( 3) - 06/15/04 11:53 - [ 8] Summary Form for Proposal 10099 Item Used in this proposal ------------------------------------------------------------------------------------------------------------------------------------ Apertures WFC ------------------------------------------------------------------------------------------------------------------------------------ Configurations ACS/WFC ------------------------------------------------------------------------------------------------------------------------------------ Opmodes ACCUM ------------------------------------------------------------------------------------------------------------------------------------ Proposal Category GO ------------------------------------------------------------------------------------------------------------------------------------ Special Requirements PCS MODE FINE; ORIENT 276.0D TO 277.0 D; BETWEEN 16-SEP-2004 00 00 00 AND 14-OCT-2004 00 00 00 SEQ 1 NON-INT; PATTERN 1 1 SEQ 2 NON-INT; PATTERN 1 2 SEQ 3 NON-INT; PATTERN 1 3 PCS MODE FINE; ORIENT 94.6D TO 95.6 D; BETWEEN 14-MAR-2005 00 00 00 AND 11-APR-2005 00 00 00 ------------------------------------------------------------------------------------------------------------------------------------ Spectral Elements F555W ------------------------------------------------------------------------------------------------------------------------------------ Target Names GEMINGA ------------------------------------------------------------------------------------------------------------------------------------