7408( 12) - 08/17/99 10:50 - [ 1] PROPOSAL FOR HUBBLE SPACE TELESCOPE OBSERVATIONS ST ScI Use Only ID: 7408 Version: 12 Check-in Date: 17-Aug-1999 10:49:59 1.Proposal Title: Parallax, Proper Motion, and Spectral Energy Distribution of an Isolated Old Neutron Star ------------------------------------------------------------------------------------ 2. Proposal For 3. Cycle GO 7 ------------------------------------------------------------------------------------ 4. Investigators Contact? PI: Frederick Walter State University of New York at Stony Brook CoI: James Lattimer State University of New York N CoI: Madappa Prakash State University of New York N CoI: Lynn Matthews State University of New York N CoI: Peng-hui An State University of New York N CoI: Ralph Neuhaeuser Max-Planck-Institut fuer N Extraterrestrische Physik ------------------------------------------------------------------------------------ 5. 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. Last year we identified the optical counterpart with a blue object at V=25.8 (HST program 6429). 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. As a candidate nearby isolated old neutron star, it affords a unique opportunity to examine a neutron star surface. We obtained the optical ID and baseline position with WFPC2 during cycle 6. With two observations in cycle 7, we will be able to measure the parallax to better than 20% and the transverse space velocity if it exceeds 10 km s^-1. We will measure the broadband spectral energy distribution with 25% accuracy, permitting comparison with a black body and testing of proposed surface cooling mechanisms. By determining the radius of this neutron star to within 25%, we obtain important constraints on the equation of state of the interior. ------------------------------------------------------------------------------------ 7408( 12) - 08/17/99 10:50 - [ 2] Observations Description ------------------------ We will use WFPC2 F606W observations to determine the position of the IONS candidate. Changes in the position over one year, relative to a grid of (presumably background) stars, will be used to determine the parallax and proper motion. Observations at other bands will be used to construct the SED in the optical and near-UV. We identified the counterpart during our cycle 6 observations, based on a very blue color (U-V~--1.2) and positional coincidence with the X- ray position. Determination of the parallax and proper motion requires two more visits to the target. These visits must occur 6 months after and one year after the cycle 6 observation (modulo an integral number of years). Visit after 1 year: At this epoch the parallactic shift cancels out, and the shift in position is the annual proper motion. We will obtain a single 3 orbit F606W image. Visit after 6 months: At this epoch the positional shift is the sum of the parallactic shift and half the annual proper motion. Knowing the annual proper motion, it is fairly easy to determine the parallax. At this visit we will also obtain a single 3 orbit (F606W) image. We will also observe the star through the F170W, F300W (wide U), and F450W (wide B) filters to obtain the spectral energy distribution (8 orbits). These observations are not time critical, and can be scheduled at any time. We distribute these observations between the two visits so as not to exceed the SAA constraints. ************** REVISION In response to a request not to exceed 6 orbits per visit, we have redesigned the program. Visits 1 and 2 are time critical. Visit 3, the F170W observation (4 orbits), is not time critical, and should occur shortly after a WFPC2 decontamination. ************** Exposure times and astrometric performance: The target is a 660, 000K blackbody, with A_V=0.1 mag and V=25.8. 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 have the colors of reddened K giants, and are likely background galactic bulge stars, which should provide a stable frame of reference. There are 38 stars to V=25 within 30 arcsec of the nominal IONS position on a deep V-band image taken in July 1996 at the ESO-MPI 2.2m telescope (8 are visible to V=23 in Figure 1). 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 (1400 source counts, 230 background counts) we can centroid the target to about 3 mas at 99% confidence. Since there are at least 38 possible astrometric reference stars in the field, we assume that the uncertainty in the distortion solution will not dominate the errors. In a single orbit the centroiding accuracy at 99% parallactic shift is >+/-8 mas, so we will be able to determine the distance (at 130 pc) to better than 20%. We can detect and measure a transverse velocity of 10(D/100 pc) km/s at 5 Sigma confidence. We will measure the SED in the wide U, wide B, and wide V bandpasses, and in the far UV (F170W). The target is very blue and the red leak in F170W is not a concern; the target is too faint to observe at R and I. At U and B the wide bandpasses yield 2-3 times higher S/N than do the Johnson U and B filters; 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 (5.5 ksec) to determine the U and B fluxes to better than 20%. In 3 orbits (8 ksec), we expect to determine the V band flux to better than 10%. Table 1 Predicted Count Rates for V=26.7 filter ksec counts S/N source back F170W 8.0 208 51 7.3 F300W 5.5 453 44 17.2 F450W 5.5 612 67 18.6 F606W 8.0 1417 230 21.7 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. note: under low-sky conditions, available in October, the F606W background is significantly lower. Real Time Justification ----------------------- These are time-critical observations. We require observations in early April and early October, in order to maximize the parallactic shift. The first observation was obtained on 6 October 1996 We will reobserve the IONS with ROSAT in October 1997, to look for a proper motion in X-rays (at about .002 the spatial resolution of the HST) and for long term stability of the flux (expected if the source is internally heated, but not necessarily so if due to accretion from the ISM). We have time on SAX to look for a hard X-ray tail, expected from either cyclotron radiation in a strong magnetic field or an accretion shock. We will observe with ASCA in October 1997 to to obtain a better X-ray spectrum, to search for departures from the Wien tail of a black body as well as to search for the 5-15 keV cyclotron line expected from a magnetized neutron star. We have also obtained EUVE SWP spectra to further constrain the absorption column. None of these observations need be coordinated, as the target is not a variable source. Calibration Justification ------------------------- Additional Comments ------------------- ------------------------------------------------------------------------------------ 7408( 12) - 08/17/99 10:50 - [ 3] Data Distribution Paper Products: Media: DAT Blocking Factor: 10 Ship To: PI_Address Ship Via: Email: ------------------------------------------------------------------------------------ 7408( 12) - 08/17/99 10:50 - [ 4] TARGET LIST 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 = 25.8+/-0.3 3754 DEC=-37D 54'32.0" +/- 1.5" B-V = -0.3+/-0.1 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 7408( 12) - 08/17/99 10:50 - [ 5] Visit: 01 Visit Priority: Visit Requirements: BETWEEN 30-MAR-1999 AND 15-APR-1999 On Hold Comments: Additional Comments: Maximum parallactic shift on 7 April Exposures ------------------------------------------------------------------------------------------------------------------------------------ Exposure| Target |Instr | Oper. | Aper |Spectral|Central| Optional |Num| Time | Special Number | Name |Config| Mode |or FOV |Element |Waveln.| Parameters |Exp| | Requirements ------------------------------------------------------------------------------------------------------------------------------------ 20 RXJ185635-3 WFPC2 IMAGE PC1 F606W CR-SPLIT=NO ,DITHER-TYPE=LINE 1 1100 S 754 ------------------------------------------------------------------------------------------------------------------------------------ 21 RXJ185635-3 WFPC2 IMAGE PC1 F606W CR-SPLIT=DEF ,DITHER-TYPE=LINE 1 2600 S 754 ------------------------------------------------------------------------------------------------------------------------------------ 7408( 12) - 08/17/99 10:50 - [ 6] Visit: 02 Visit Priority: Visit Requirements: BETWEEN 15-SEP-1999 AND 30-OCT-1999 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 ------------------------------------------------------------------------------------------------------------------------------------ 30 RXJ185635-3 WFPC2 IMAGE PC1 F300W CR-SPLIT=DEF 1 2600 S 754 ------------------------------------------------------------------------------------------------------------------------------------ 40 RXJ185635-3 WFPC2 IMAGE PC1 F606W CR-SPLIT=DEF ,DITHER-TYPE=LINE 1 2600 S 754 ------------------------------------------------------------------------------------------------------------------------------------ 7408( 12) - 08/17/99 10:50 - [ 7] Visit: 03 Visit Priority: Visit Requirements: On Hold Comments: Additional Comments: SHOULD CLOSELY FOLLOW WFPC2 DECONTAMINATION Exposures ------------------------------------------------------------------------------------------------------------------------------------ Exposure| Target |Instr | Oper. | Aper |Spectral|Central| Optional |Num| Time | Special Number | Name |Config| Mode |or FOV |Element |Waveln.| Parameters |Exp| | Requirements ------------------------------------------------------------------------------------------------------------------------------------ 50 RXJ185635-3 WFPC2 IMAGE PC1 F170W CR-SPLIT=DEF 2 5400 S 754 ------------------------------------------------------------------------------------------------------------------------------------ 7408( 12) - 08/17/99 10:50 - [ 8] Visit: 04 Visit Priority: 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 RXJ185635-3 WFPC2 IMAGE PC1 F450W CR-SPLIT=NO ,DITHER-TYPE=LINE 1 1100 S 754 ------------------------------------------------------------------------------------------------------------------------------------ 11 RXJ185635-3 WFPC2 IMAGE PC1 F450W CR-SPLIT=DEF ,DITHER-TYPE=LINE 1 2600 S 754 ------------------------------------------------------------------------------------------------------------------------------------ 7408( 12) - 08/17/99 10:50 - [ 9] Summary Form for Proposal 7408 Item Used in this proposal ------------------------------------------------------------------------------------------------------------------------------------ Apertures PC1 ------------------------------------------------------------------------------------------------------------------------------------ Configurations WFPC2 ------------------------------------------------------------------------------------------------------------------------------------ Opmodes IMAGE ------------------------------------------------------------------------------------------------------------------------------------ Optional Parameters CR-SPLIT=NO DITHER-TYPE=LINE CR-SPLIT=DEF ------------------------------------------------------------------------------------------------------------------------------------ Proposal Category GO ------------------------------------------------------------------------------------------------------------------------------------ Special Requirements BETWEEN 30-MAR-1999 AND 15-APR-1999 BETWEEN 15-SEP-1999 AND 30-OCT-1999 ------------------------------------------------------------------------------------------------------------------------------------ Spectral Elements F606W F300W F170W F450W ------------------------------------------------------------------------------------------------------------------------------------ Target Names RXJ185635-3754 ------------------------------------------------------------------------------------------------------------------------------------