9370( 3) - 05/10/02 10:08 - [ 1] PROPOSAL FOR HUBBLE SPACE TELESCOPE OBSERVATIONS ST ScI Use Only ID: 9370 Version: 3 Check-in Date: 10-May-2002 10:07:47 1.Proposal Title: The Optical Counterpart of an Ultraluminous X-Ray Source ------------------------------------------------------------------------------------ 2. Proposal For 3. Cycle GO 11 ------------------------------------------------------------------------------------ 4. Investigators Contact? PI: Joel Bregman University of Michigan CoI: Dr. Patrick Seitzer University of Michigan Y CoI: Mr. Jifeng Liu University of Michigan N ------------------------------------------------------------------------------------ 5. Abstract Ultraluminous X-ray sources (ULX) are non-nuclear sources in normal disk galaxies that are second only to AGNs in point- source luminosity. These enigmatic objects are thought to be intermediate mass black holes (10^3 -10^4 solar masses), and this can be verified by a direct mass determination, which requires knowledge of the mass of the secondary and the period of the binary system. The first step in determining the mass of ULXs is the optical identification of the secondary and for the ULX in NGC 5204, the optical counterpart is a compact blue object at V = 20.3. This counterpart, with M_V=-8.7, is either a very tight cluster of O stars, a megastar, or a background BL Lac object. We can distinguish between these possibilities with UV spectroscopy and imaging, and for the first time, determine the mass of the secondary in a ULX system. ------------------------------------------------------------------------------------ 9370( 3) - 05/10/02 10:08 - [ 2] Observations Description ------------------------ The first set of observations are imaging, and done with 4 exposures, dithered between exposures to improve resolution. The second set are STIS MAMA spectra. Because this is a crowded field, an initial ACQ is done on a bright star near to the source (the simulations run using WFPC2 data of this field always results in this star being centered even if the ULX source is the starting point). Then an ACQ/PEAK is done to put the object in the slit. PA is chosen to pick up both candidates in a single exposure. Positions are from WFPC2 images taken for GO 8601, u6723901 and u6723902. There are two observational goals, involving spectroscopy and imaging. The goal of the spectroscopy is to determine whether the optical counterpart has an O star spectrum, and if so, to determine the spectral type. The greatest leverage on this issue comes from observations in the FUV region where O stars have broad CIV and SiIV absorption lines (typically 30 Angstroms FWHM but at depths of 10-40%). Also, there are differences between classes of O stars, and as an example, an O5 star has more flux in the 1200-1300 Angstroms band (compared to the 1600-1700 Angstroms band) than an O7 star (by about 20% stronger by about the same amount. We propose to obtain G140L spectra with STIS at a S/N sufficient to detect these absorption lines and to differentiate between classes of O stars. For our proposed 3 orbit observation, the resulting S/N will be 16 per resolution element at 1550 Angstroms (C IV), and 30 at 1402 Angstroms (SiIV), which will permit us to determine the line strengths to a S/N of 10 (C IV) and 30 (Si IV). We will be able to determine the difference in the spectral shape at 1200-1300 Angstrom vs 1600-1700 Angstroms to a relative accuracy of 1%. Not only will this data permit us to determine the mean O star dominating the spectrum (in the event of a cluster of O stars), but it will even permit us to develop models for a mixture of O stars. For the imaging part of the program, the goal is to determine whether this counterpart is a single object or a compact star cluster (and if it is a star cluster, whether there is a dominant or perculiar object). Fortunately, the HRC on the ACS has 0.027" pixels and has good blue coverage, so we believe that the best strategy is to observe at B and in the UV (at 2200 Angstrom, F220W). By observing in these two bands, we obtain colors and magnitudes of the individual blue stars, although the B image will also be sensitive to later type supergiant stars as well. Even with this pixel size (0.027"), the PSF of 0.037" FWHM is not critically sampled at 4300 Angstroms (the F435W filter), so we will use a standard dithering pattern to achieve the best possible resolution, and this should lead to a FWHM of about 0.4". At the shorter wavelength, the telescope has a resolution of 0.019" (2228 Angstroms), and if we use a 3x3 dithering pattern (the dithering patterns have not yet made it into the documentation, so the optimum dithering pattern may be different), then with typical jittering and focus conditions, we will approach a resolution of 0.025" FWHM (the exact limiting FWHM is unclear, but with other instruments, diffraction limited observations are regularly obtained at 3700 Angstrom where the FWHM is 0.034"). The current best image used the WFPC with 0.1" pixels at 6000 Angstroms (no dithering), or an effective resolution of 0.15", so the improvement in angular resolution should be about a factor of 4-6 and will lead to a linear resolution of 0.5-0.8 pc, which is smaller than the half light diameter of globular clusters (as examples of the tightest known star clusters). If the cluster is comprised of about 10 blue stars within a 0.15" region, we will resolve many of them at 2200 Angstroms. The imaging can be accomplished with one orbit per filter. If the cluster is composed of O stars, it would require at least 6 such stars, each with V=22.2. To be conservative, we use V=23 and for a 3x3 dithering pattern at B, each image will be 240 sec, leading to a S/N of 30 per star (or a final S/N per star of 90). At 2200 Angstroms , for the same dithering pattern, the S/N will be 20 per image (60 for the combined set). Standard reconstruction described by Lauer and Fruchter will be carried out. Real Time Justification ----------------------- Calibration Justification ------------------------- Additional Comments ------------------- ------------------------------------------------------------------------------------ 9370( 3) - 05/10/02 10:08 - [ 3] TARGET LIST Fixed Targets ------------------------------------------------------------------------------------------------------------------------------------ Tar| Target | Target | Target |Coord | Radial | Flux data No | Name | Description | Position |Eqnx | Vel. | ------------------------------------------------------------------------------------------------------------------------------------ 1 NGC5204-ULX EXT-STAR, X-RAY RA=13H 29M 38.5581S +/- 0.1", J2000 V = 20.305+/-0.01 TRANSIENT DEC=58D 25' 5.602" +/- 0.1", PLATE-ID=01R2 Coordinate Source: HST_IMAGE 2 OFFSET-STAR STAR RA=13H 29M 39.2468S +/- 0.1", J2000 V = 18.0+/-0.2 DEC=58D 25' 9.518" +/- 0.1", PLATE-ID=01R2 Coordinate Source: HST_IMAGE Comments: offset star for initial ACQ 9370( 3) - 05/10/02 10:08 - [ 4] Patterns ------------------------------------------------------------------------------------------------------------------------------------ Pattern_Number: 1 Primary_Pattern Secondary_Pattern Pattern_Type ACS-HRC-DITHER-BOX Pattern_Purpose DITHER Number_Of_Points 4 Point_Spacing 0.150 Coordinate_Frame POS-TARG Pattern_Orient 19.89 Center_Pattern NO ------------------------------------------------------------------------------------------------------------------------------------ 9370( 3) - 05/10/02 10:08 - [ 5] Visit: 01 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 NGC5204-ULX ACS/HR ACCUM HRC F435W CR-SPLIT=NO 1 650 S PATTERN 1 10 C ------------------------------------------------------------------------------------------------------------------------------------ 20 NGC5204-ULX ACS/HR ACCUM HRC F220W CR-SPLIT=NO 1 680 S PATTERN 1 20 C ------------------------------------------------------------------------------------------------------------------------------------ 9370( 3) - 05/10/02 10:08 - [ 6] Visit: 02 Visit Priority: Visit Requirements: ORIENT 141.5D TO 143.5D ORIENT 321.5D TO 323.5D 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 ------------------------------------------------------------------------------------------------------------------------------------ 1 OFFSET-STAR STIS/C ACQ F28X50L MIRROR ACQTYPE=POINT 1 8 S CD P Comments: for ACQ in crowded field ------------------------------------------------------------------------------------------------------------------------------------ 20 NGC5204-ULX STIS/F ACCUM 52X0.2 G140L 1425 1 2500 S UV-MAM A ------------------------------------------------------------------------------------------------------------------------------------ 30 NGC5204-ULX STIS/F ACCUM 52X0.2 G140L 1425 1 3100 S UV-MAM A ------------------------------------------------------------------------------------------------------------------------------------ 40 NGC5204-ULX STIS/F ACCUM 52X0.2 G140L 1425 1 3100 S UV-MAM A ------------------------------------------------------------------------------------------------------------------------------------ 9370( 3) - 05/10/02 10:08 - [ 7] Summary Form for Proposal 9370 Item Used in this proposal ------------------------------------------------------------------------------------------------------------------------------------ Apertures HRC F28X50LP 52X0.2 ------------------------------------------------------------------------------------------------------------------------------------ Configurations ACS/HRC STIS/CCD STIS/FUV-MAMA ------------------------------------------------------------------------------------------------------------------------------------ Opmodes ACCUM ACQ ------------------------------------------------------------------------------------------------------------------------------------ Optional Parameters CR-SPLIT=NO ACQTYPE=POINT ------------------------------------------------------------------------------------------------------------------------------------ Proposal Category GO ------------------------------------------------------------------------------------------------------------------------------------ Special Requirements PATTERN 1 10 PATTERN 1 20 ORIENT 141.5D TO 143.5D ORIENT 321.5D TO 323.5D ------------------------------------------------------------------------------------------------------------------------------------ Spectral Elements F435W F220W MIRROR G140L ------------------------------------------------------------------------------------------------------------------------------------ Target Names NGC5204-ULX OFFSET-STAR ------------------------------------------------------------------------------------------------------------------------------------ Wavelengths 1425 ------------------------------------------------------------------------------------------------------------------------------------