8692( 2) - 02/28/00 13:37 - [ 1] PROPOSAL FOR HUBBLE SPACE TELESCOPE OBSERVATIONS ST ScI Use Only ID: 8692 Version: 2 Check-in Date: 28-Feb-2000 13:36:48 1.Proposal Title: Search for an Optical Counterpart to the Central X-ray Point Source in Cas A ------------------------------------------------------------------------------------ 2. Proposal For 3. Cycle GO 9 ------------------------------------------------------------------------------------ 4. Investigators Contact? PI: Robert Fesen Dartmouth College CoI: Dr. Roger Chevalier University of Virginia N CoI: Dr. Stephen Holt NASA/Goddard N CoI: Dr. Harvey Tananbaum Harvard-Smithsonian CfA N ------------------------------------------------------------------------------------ 5. Abstract First-light Chandra X-ray Observatory images have uncovered a central point-like source in Cas A, the Galaxy's youngest known supernova remnant (SN 1680). The object lies within 5'' of the remnant's estimated expansion center and is presumably either the remnant's neutron star or black hole. Preliminary analysis of the source's ACIS spectrum indicates Si (and possibly S) emission, not unlike that seen in the remnant's ejecta, suggesting possibly an accretion disk of fall-back SN debris. If this spectral analysis holds up, there is then a good chance of detecting an optical emission line counterpart. We propose to obtain deep `white light' STIS images of the Cas A center during five LOW background orbits to search for an optical candidate down to ~ 29.5 mag. Complementary WFPC2 images of Cas A which are sensitive to ejecta line emission and already scheduled as part of a Cycle 8 program will be combined to yield spectral information on all but the faintest sources. These data will constrain the point source's nature through limits on its X- ray/optical flux ratio, color properties, and the presence of any extended emission structure. If a sufficiently bright optical source is found coincident with the X- ray position, five additional orbits (TOO) are requested for low-dispersion STIS spectra to search for optical line emission. ------------------------------------------------------------------------------------ 8692( 2) - 02/28/00 13:37 - [ 2] Observations Description ------------------------ Due to its lower read noise, lower dark current, and higher throughput, the STIS CCD has a significantly higher sensitivity to faint sources than WFPC2 (STIS Manual, June 1998). Because of this and the already scheduled Cycle 8 R- band observations (GO 8281), we have opted for the deepest census possible of optical sources near the X-ray point source using STIS with the clear filter (`50CCD'). Fortunately, the central region in Cas A around both the center of expansion and X-ray point source is void of bright stars in an area ~eq 60 arcsec in diameter, thus larger than the STIS CCD imaging FOV of 52''. To go as deep as possible, we have proposed using only LOW background exposures, meaning low zodiacal light and Earthshine OTA light scattering requirements. At Cas A's declination of 58.8 degrees, this limits object visibility to 46 minutes per orbit. Adopting 40 minutes as an estimate of usable exposure time each orbit, one can obtain two 1200 sec exposures and hence a total of 12, 000 sec in five LOW orbits. Using the on-line STIS image calculator, these data will have a limiting equivalent R magnitude around 29.5 with a S/N of 2.5--4 depending on the exact object spectrum adopted. The ten individual images will be sub-pixel dithered in sets of two to eliminate both hot pixels and cosmic rays hits. If a potential emission line candidate turns up at the X-ray point source's position, we will request five additional LOW background orbits to get a spectrum of it using STIS in low dispersion mode. Because most of the emission lines that are likely from core material lie in the red (see Fig. 3), we have chosen the G750L grating. Using the on-line STIS spectral exposure calculator, we estimate that we can detect line emissions from point sources in 12, 000 sec (= 5 orbits; 5 sets of 2 x 1200 sec exposures) down to about 5 * 10^-19 erg cm^ -2 s^-1 with S/N > 4. This is substantially fainter by more than two orders of magnitude than has been done on Cas A's faintest outlying knots (Fesen & Gunderson 1996). The resulting dispersion resolution of ~eq 10 Angstrom \ should be sufficient to measure radial velocities with accuracies of +/- 100 km s^-1. Like in the case of the STIS CCD imaging, these STIS spectra would be dithered across the slit to improve cosmic ray and hot pixel rejection. Real Time Justification ----------------------- The special requirement of LOW background sky is key for this deep imaging and spectral study. For example, the S/N for a 29.5 mag M star for low, average, and bright zodiacal and Earthshine backgrounds are 3.0, 1.6, and 1.2 respectively. The TOO requirement for the STIS low-dispersion spectra is requested in case a viable optical emission line candidate is discovered. The probability of this occurrence is highly speculative at this time but could be as large as 50\ if the Chandra ACIS spectrum of the central point source is confirmed to show emission lines. Cycle 8 WFPC2 observations (GO 8281) will provide important supporting data for this program. We have already discussed the possible expedited execution of the proposed STIS observations with the STScI Director so as to be done around the Cycle 8's current planned 22--25 January 2000 observing timeline and this appears feasible. Calibration Justification ------------------------- Additional Comments ------------------- ------------------------------------------------------------------------------------ 8692( 2) - 02/28/00 13:37 - [ 3] TARGET LIST Fixed Targets ------------------------------------------------------------------------------------------------------------------------------------ Tar| Target | Target | Target |Coord | Radial | Flux data No | Name | Description | Position |Eqnx | Vel. | ------------------------------------------------------------------------------------------------------------------------------------ 1 CAS-A-CENTER ISM, SNR RA=23H 23M 27.94S +/- 0.15S, J2000 V = 28.0+/-2.0 DEC=58D 48' 42.4" +/-2" Comments: E(B-V)=1.60 +/-0.50 8692( 2) - 02/28/00 13:37 - [ 4] Patterns ------------------------------------------------------------------------------------------------------------------------------------ Pattern_Number: 1 Primary_Pattern Secondary_Pattern Pattern_Type STIS-CCD-BOX Pattern_Purpose DITHER Number_Of_Points 4 Point_Spacing 0.567 Coordinate_Frame POS-TARG Pattern_Orient 26.6 Center_Pattern NO ------------------------------------------------------------------------------------------------------------------------------------ 8692( 2) - 02/28/00 13:37 - [ 5] Visit: 01 Visit Priority: Visit Requirements: ORIENT 15D TO 50D ORIENT 195D TO 230D 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 ------------------------------------------------------------------------------------------------------------------------------------ 101 CAS-A-CENTE STIS/C ACCUM 50CCD MIRROR CR-SPLIT=NO 3 820 S PATTERN 1; LOW-SKY R CD ------------------------------------------------------------------------------------------------------------------------------------ 102 CAS-A-CENTE STIS/C ACCUM 50CCD MIRROR CR-SPLIT=NO 3 820 S POS TARG -0.2, -0.2; R CD LOW-SKY ------------------------------------------------------------------------------------------------------------------------------------ 8692( 2) - 02/28/00 13:37 - [ 6] Summary Form for Proposal 8692 Item Used in this proposal ------------------------------------------------------------------------------------------------------------------------------------ Apertures 50CCD ------------------------------------------------------------------------------------------------------------------------------------ Configurations STIS/CCD ------------------------------------------------------------------------------------------------------------------------------------ Opmodes ACCUM ------------------------------------------------------------------------------------------------------------------------------------ Optional Parameters CR-SPLIT=NO ------------------------------------------------------------------------------------------------------------------------------------ Proposal Category GO ------------------------------------------------------------------------------------------------------------------------------------ Special Requirements ORIENT 15D TO 50D ORIENT 195D TO 230D PATTERN 1; LOW-SKY POS TARG -0.2, -0.2; LOW-SKY ------------------------------------------------------------------------------------------------------------------------------------ Spectral Elements MIRROR ------------------------------------------------------------------------------------------------------------------------------------ Target Names CAS-A-CENTER ------------------------------------------------------------------------------------------------------------------------------------