6429( 5) - 08/15/96 13:54 - [ 0] PROPOSAL FOR HUBBLE SPACE TELESCOPE OBSERVATIONS ST ScI Use Only ID: 6429 Version: 5 Check-in Date: 15-Aug-1996 13:52:21 1.Proposal Title: Parallax, Proper Motion, and Spectral Energy Distribution of an Isolated Old Neutron Star ------------------------------------------------------------------------------------ 2. Scientific Category 3. Proposal For 4. Cycle HOT STARS GO 6 ------------------------------------------------------------------------------------ 5. Investigators Contact? PI: Frederick M. Walter STATE UNIVERSITY OF NEW YORK CoI: James Lattimer State University of New York N CoI: Madappa Prakash State University of New York N CoI: Scott Wolk State University of New York N CoI: Lynn Matthews State University of New York N ------------------------------------------------------------------------------------ 6. Abstract We propose to determine the parallax, proper motion, and spectral energy distribution of a nearby isolated, old neutron star. The target is a bright soft X- ray source in the direction of the R CrA molecular cloud. The X-ray spectrum is that of a kT=57 eV black body; the absorption column places it foreground to the cloud, and closer than 130 pc. There is no optical counterpart to V=23. The emitting area of 480 km^2 suggests emission from nearly the entire surface of a neutron star. It is not a pulsar, nor is it in an accreting binary system. It is the best candidate yet found for a nearby isolated old neutron star, and hence affords a unique opportunity to examine a neutron star surface. The predicted V magnitude is 27.4. We propose to measure the distance to better than 20% and can measure the transverse space velocity if it exceeds 10 km/s. We will measure the broadband spectral energy distribution with 25% accuracy, permitting comparison with a black body and testing of proposed surface cooling mechanisms. We expect to determine the radius of this neutron star to within 25%, yielding important constraints on the equation of state of the interior. ------------------------------------------------------------------------------------ Observations Description ------------------------ This field is in the galactic bulge, and has a fair number of stars down to V=22. The field may be crowded at V=27.4, with more than one candidate within the 1.5 arcsec radius error circle. We can only identify the counterpart based on its color. Since the spectrum is a black body (or nearly so) with low reddening, we expect the counterpart to have B- V=-0.3 and U-V=-1.7. We require three visits to the target: Visit 1 must occur in early April or early October, to maximize the parallactic shift. We will obtain a deep V (F606W) image to obtain an accurate baseline position for measuring the parallax and proper motion (2 orbits). We will also observe the star through the F300W (wide U) filter to obtain a color (1 orbit). These color is required in order to identify the target (which will have a U-V color of -1.7). Visit 2 must occur 6 months after Visit 1, to maximize the parallactic shift. We will obtain a wide- V (F606W) image to measure the stellar position (3 orbits). Visit 3 must occur 1 year after visit 1 to minimize the parallactic shift and reveal the proper motion. It will therefore take place during HST cycle 7. We will obtain a wide- V (F606W) image to measure the stellar position (3 orbits). (With the time awarded, visits 2 and 3 will be the subject of a cycle 7 proposal.) Exposure times and astrometric performance: The target is a black body of temperature 660,000K, with V=27.4 and A_V=0.1 mag. We estimate S/N as prescribed in the WFPC2 handbook, with results in Table 1. We plan to CR-SPLIT the images, and to dither the camera between exposures, to improve the S/N, as recommended by the WFPC-2 team (J. Gallagher, private communication). We will measure the parallax and proper motions relative to the nearby stars in the frames. These stars (at least to V=23) have the colors of reddened K giants, and are likely background galactic bulge stars, which should provide a stable frame of reference. There are at least 8 stars visible in Figure 1 which will fall on the PC image, and we expect many more at V>23. We simulated the astrometric performance of the PC by generating simulated images, using the PC PSF, predicted source and background counts, and folding in Poisson statistics. While the highest S/N is in the wide-B filter, because the background is lower, the best throughput is in the wide-V filter. We get the best astrometric accuracy with F606W. The simulations show that in 3 orbits (834 source counts, 254 background counts) we can centroid the target to better than 3 mas at 99% confidence. The parallactic shift is +/-8 mas, so we will be able to determine the distance (at 130 pc) to better than 20% We can measure a transverse velocity of 10 km/s at 5 sigma confidence. At U the wide bandpass yields 2-3 times higher S/N than does the Johnson U filter; at V the difference is small because of the higher sky background in the wider and redder F606W. We use the prescription in the WFPC2 handbook to estimate source and background count rates. The uncertainty on the flux measurement is larger than that for detection alone. Folding the expected counts through the PC PSF and including uncertainties in the background, we estimate that we require 2 orbits (5.7 ksec) to determine the U and B fluxes to better than 20%. In 3 orbits (8.9 ksec), we expect to determine the V band flux to comparable accuracy, and to determine the 1700A flux to about 30%. Table 1 Predicted Count Rates for V=27.4, filter ksec counts S/N source back F170W 8.9 123 54 4.2 F300W 5.7 247 45 9.3 F450W 5.7 334 69 10.0 F606W 8.9 834 254 12.1 Notes for Table 1: source: the total number of source counts spread over the instrumental PSF. back: the number of background counts expected per pixel. S/N: signal-to-noise ratio for a source at the corner of a pixel. Real Time Justification ----------------------- This is a time-critical observation. We require observations in early April or early October, in order to maximize the parallactic shift. Supporting observations have been proposed. A deep VLA pointing to look for an H II region surrounding the IONS has been scheduled. The ASCA observation, accepted but not yet scheduled, will obtain a better X-ray spectrum, to search for departures from the Wien tail of a black body. We will use the ASCA spectrum to search for the expected 5-15 keV cyclotron line from a magnetized neutron star. The EUVE program is an accepted RAP observation, to pin down the absorption column. We also now have an accepted SAX observation of this target. Calibration Justification ------------------------- Additional Comments ------------------- We specify that these images be obtained under LOW-SKY conditions, because of the faintness of the target. Minimizing the background will help with the detection and measurement of the position. We will use a 2-step dither because this is adequate for PC observations (A. Fruchter, private communication). ------------------------------------------------------------------------------------ Data Distribution Media: 8MM Blocking Factor: 10 Ship To: ESS Dept, Z=2100*Stony Brook NY*11794- 2100 Ship Via: UPS Email: fwalter@astro.sunysb.edu , ------------------------------------------------------------------------------------ 6429( 5) - 08/15/96 13:54 - [ 1] TARGET LIST a) Fixed Targets ------------------------------------------------------------------------------------------------------------------------------------ Tar| Target | Target | Target |Coord | Radial | Flux data No | Name | Description | Position |Eqnx | Vel. | ------------------------------------------------------------------------------------------------------------------------------------ 1 RXJ185635- STAR, Neutron Star RA=18H 56M 35.35S +/- 0.12S, J2000 V = 27.4+/-0.5 3754 DEC=-37D 54'32.0" +/- 1.5" B-V = -0.3+/-0.1 Comments: optical fluxes are predictions Epoch of Position RA proper motion (seconds of time/yr) DEC Proper Motion (arcsec/yr) Annual Parallax (arcsec) 1994 0.0 0.0 0.0 6429( 5) - 08/15/96 13:54 - [ 2] Visit: 01 Visit Requirements: BETWEEN 15-SEP-96 AND 15-OCT-96 On Hold Comments: Additional Comments: Maximum parallactic shift on 7 October Exposures ------------------------------------------------------------------------------------------------------------------------------------ Exposure| Target |Instr | Oper. | Aper |Spectral|Central| Optional |Num| Time | Special Number | Name |Config| Mode |or FOV |Element |Waveln.| Parameters |Exp| | Requirements ------------------------------------------------------------------------------------------------------------------------------------ 10 RXJ185635-3 WFPC2 IMAGE PC1 F606W CR-SPLIT=DEF,DITHER-TYPE=LINE 1 2000 S LOW-SKY EXPAND 754 ------------------------------------------------------------------------------------------------------------------------------------ 20 RXJ185635-3 WFPC2 IMAGE PC1 F300W CR-SPLIT=DEF 1 2400 S LOW-SKY 754 ------------------------------------------------------------------------------------------------------------------------------------ 6429( 5) - 08/15/96 13:54 - [ 3] Summary Form for Proposal 6429 Item Used in this proposal ------------------------------------------------------------------------------------------------------------------------------------ Configurations WFPC2 ------------------------------------------------------------------------------------------------------------------------------------ Opmodes IMAGE ------------------------------------------------------------------------------------------------------------------------------------ Optional Parameters CR-SPLIT=DEF DITHER-TYPE=LINE ------------------------------------------------------------------------------------------------------------------------------------ Proposal Category GO ------------------------------------------------------------------------------------------------------------------------------------ Scientific Category HOT STARS ------------------------------------------------------------------------------------------------------------------------------------ Special Requirements BETWEEN 15-SEP-96 AND 15-OCT-96 LOW-SKY EXPAND LOW-SKY ------------------------------------------------------------------------------------------------------------------------------------ Spectral Elements F606W F300W ------------------------------------------------------------------------------------------------------------------------------------ Target Names RXJ185635-3754 ------------------------------------------------------------------------------------------------------------------------------------