9073( 2) - 02/16/01 17:54 - [ 1] PROPOSAL FOR HUBBLE SPACE TELESCOPE OBSERVATIONS ST ScI Use Only ID: 9073 Version: 2 Check-in Date: 16-Feb-2001 17:53:46 1.Proposal Title: Ultraluminous X-Ray Sources and Intermediate Mass Black Holes ------------------------------------------------------------------------------------ 2. Proposal For 3. Cycle GO 10 ------------------------------------------------------------------------------------ 4. Investigators Contact? PI: Joel Bregman University of Michigan CoI: Mr. Jifeng Liu University of Michigan N CoI: Dr. Patrick Seitzer University of Michigan N ------------------------------------------------------------------------------------ 5. Abstract Ultraluminous X-ray sources (ULX) are second only to AGNs in point-source luminosity yet they have no known optical counterparts. These extraordinary X-ray objects, which radiate at the Eddington luminosity of a 30-100 solar mass object, are non- nuclear sources in normal disk galaxies. One explanation is that they are binary systems where the accreting black hole has a mass of 30-100 solar masses, although this challenges stellar evolutionary models, which do not predict remnants larger than about 10 solar masses. Another possibility is that ULXs are intermediate mass black holes (10^3-10^4 solar masses), acting as micro-quasars in galaxy disks and representing the link between stellar black holes (4-10 solar masses) and supermassive black holes (10^6-10^10 solar masses). We can distinguish between the models by identifying the optical counterparts of ULXs and measuring their colors. This would be a fundamental step in gaining an understanding of these enigmatic and possibly new astronomical objects. ------------------------------------------------------------------------------------ 9073( 2) - 02/16/01 17:54 - [ 2] Observations Description ------------------------ We propose to obtain broad-band observations in the B (broad; F450W), V (F555W), and I (F814W) filters on WFPC2 to (1) detect the optical counterpart of ULXs and (2) to place them on color-color and color-magnitude diagrams to determine if their spectral energy distributions are stellar or non- stellar. If the counterpart is an OB star or a red giant, it typically has an absolute magnitude of M(V)=-2 to -6 mag. Given the distance moduli of the target galaxies (about 28), this requires we reach to a limit of V=26 (worst case) and obtain a color to 10% need to distinguish between an OB star and other types of objects, such as red giants). If the ULX were a micro-AGN with a normal optical to X-ray flux ratio, it would have V=18 mag and would have been identified already. If it is 1000 times fainter in the optical band than a normal AGN, we still can detect the counterpart (at V=25.5). AGNs have different colors than stars, which make them distinctive. If only 0.1% of the power emerges in the optical region, the WFPC2 can detect these objects easily, so we believe that our chance of success is high. To calculate the exposure times, we use the known galactic surface brightness at the locations of the ULXs. We used the WFPC2 exposure calculator to determine the exposure times necessary. The length of exposure in the three different bands depends upon whether the object is an O star, a K giant, or has an AGN-like spectral energy distribution. For most types of objects with V = 26, the average S/N in the three bands will exceed 10 if we devote one full orbit per object for each of the filters; the observing time per target is 3 orbits (with three 1200 second exposures per orbit to reject cosmic rays); the target will be centered in the PC chip. Our highest priority target is in M81, the nearest galaxy and the one with the most extensive X-ray data. \ The remaining targets are prioritized by distance. We have examined similar parts of disks of other nearby galaxies to examine whether small-scale galaxy fluctuations will cause problems. We studied the region near spiral arms in M101 (archival data; d=7.4 Mpc) and find that the random chance of finding a 26 mag object in an error circle of radius 0.1- 0.2'' is very small (<3% by assuming that the present day luminosity function of stars seen in the Milky Way (e.g., Quirk and Tinsley 1973) applies to spirals such as M101. For an error circle with a diameter of 0.2'', the chance of finding a star brighter than -2 mag is 5%. HST is essential for this program, because if one had an error circle of 1'' in radius, there would be about 5 objects brighter than -2 mag in the error circle, which would render unique identifications impossible. An important aspect of this program is establishing accurate astrometry between HST and Chandra. The internal relative positions across the WFPC2 chips can be determined to better than 0.1" (WFPC2 FAQ), but the absolute positions are only good to 0.5" due to uncertainties in the Guide Star positions. Similarly, the relative positions within a Chandra field are good to 0.1'', but the uncertainty in the absolute position is 1''. Therefore, the two fields need to be aligned and we will use two approaches. The first method will be to identify background AGNs both optically and in X- rays that fall in the WFPC2 footprint. Based upon existing work, there will be 2-3 such objects per field. As an independent method, we will take supplemental groundbased CCD data (at MDM) that covers the Chandra and HST fields and tie it to the USNO-A2.0 frame; this will lead to absolute photometric positions better than 0.1''. There will be at least 10 optical counterparts to X- ray sources in each Chandra field, which will define it's absolute astrometry to better than 0.1''. The Chandra data have already been obtained (3/2000) for M81 and M51, and a Guaranteed Time Observer (GTO) observation of NGC 6946 is approved for Cycle 2, which begins December 2000 (it's long visibility window begins at the same time so it is likely to be observed in the first 6 months of Cycle 2). The Chandra data for M51 and M81 will enter the archive in 3/2001 and for NGC 6946 probably 6-9 months later, so they will be publicly available either before or about at the time that the HST data will be obtained. The galaxy NGC 4559 is not scheduled to be observed by Chandra, so here we request that it be observed as part of this program. Since the ULX source is bright (0.018 cnt/sec in the ROSAT HRI), only a brief Chandra observation is required. The proposed Chandra observation should be carried out with the High Resolution Camera (HRC-I) with the ULX at the aimpoint (an ACIS chip would be fine but there are minor advantages to using the HRC for this). Observations shorter than 3 ksec are not guaranteed by the CSC, so we request a 3 ksec observation, which will result in 250 photons defining the position of the ULX, more than adequate to obtain a high-accuracy X-ray centroid. The X-ray background will be negligible and the observation is not coordinated with the HST. If we identify the optical counterpart of ULXs, there will be a wealth of follow-up observations, the nature of which will depend upon the brightness and colors of the source. However, the first observation that we would pursue will be to obtain optical spectra (with Keck or Magellan) if any of our optical identifications are sufficiently bright (< 24 mag). Real Time Justification ----------------------- The single requested Chandra observation does not need to be coordinated with the HST observation. Calibration Justification ------------------------- Additional Comments ------------------- ------------------------------------------------------------------------------------ 9073( 2) - 02/16/01 17:54 - [ 3] TARGET LIST Fixed Targets ------------------------------------------------------------------------------------------------------------------------------------ Tar| Target | Target | Target |Coord | Radial | Flux data No | Name | Description | Position |Eqnx | Vel. | ------------------------------------------------------------------------------------------------------------------------------------ 1 M81-ULX UNIDENTIFIED, X-RAY RA=9H 55M 33.28S +/- 0.1", J2000 V = 25.00+/-1.0 EMITTER DEC=69D 0' 33.7" +/- 0.1" Coordinate Source: OTHER_SOURCE 2 NGC6946-ULX UNIDENTIFIED, X-RAY RA=20H 35M 01.0S +/- 0.1", J2000 V = 26+/-1.0 EMITTER DEC=60D 11' 31.4" +/- 0.1" Coordinate Source: OTHER_SOURCE 3 M51-ULX1 UNIDENTIFIED, X-RAY RA=13H 30M 7.8S +/- 0.1", J2000 V = 26+/-1.0 EMITTER DEC=47D 11' 05" +/- 0.1" Coordinate Source: OTHER_SOURCE 4 M51-ULX2 UNIDENTIFIED, X-RAY RA=13H 30M 1.1S +/- 0.1", J2000 V = 26+/-1.0 EMITTER DEC=47D 13' 43" +/- 0.1" Coordinate Source: OTHER_SOURCE 5 NGC4559-ULX UNIDENTIFIED, X-RAY RA=12H 35M 51.7S +/- 0.1", J2000 V = 26+/-1.0 EMITTER DEC=27D 56' 5" +/- 0.1" Coordinate Source: OTHER_SOURCE 9073( 2) - 02/16/01 17:54 - [ 4] 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 M81-ULX WFPC2 IMAGE PC1 F555W ATD-GAIN=7,CR-SPLIT=NO 4 500 S ------------------------------------------------------------------------------------------------------------------------------------ 20 M81-ULX WFPC2 IMAGE PC1 F450W ATD-GAIN=7,CR-SPLIT=NO 4 500 S ------------------------------------------------------------------------------------------------------------------------------------ 30 M81-ULX WFPC2 IMAGE PC1 F814W ATD-GAIN=7,CR-SPLIT=NO 4 500 S ------------------------------------------------------------------------------------------------------------------------------------ 9073( 2) - 02/16/01 17:54 - [ 5] Visit: 02 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 NGC6946-ULX WFPC2 IMAGE PC1 F555W ATD-GAIN=7,CR-SPLIT=NO 4 500 S ------------------------------------------------------------------------------------------------------------------------------------ 20 NGC6946-ULX WFPC2 IMAGE PC1 F450W ATD-GAIN=7,CR-SPLIT=NO 4 500 S ------------------------------------------------------------------------------------------------------------------------------------ 30 NGC6946-ULX WFPC2 IMAGE PC1 F814W ATD-GAIN=7,CR-SPLIT=NO 4 500 S ------------------------------------------------------------------------------------------------------------------------------------ 9073( 2) - 02/16/01 17:54 - [ 6] Visit: 03 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 M51-ULX1 WFPC2 IMAGE PC1 F555W ATD-GAIN=7,CR-SPLIT=NO 4 500 S ------------------------------------------------------------------------------------------------------------------------------------ 20 M51-ULX1 WFPC2 IMAGE PC1 F450W ATD-GAIN=7,CR-SPLIT=NO 4 500 S ------------------------------------------------------------------------------------------------------------------------------------ 30 M51-ULX1 WFPC2 IMAGE PC1 F814W ATD-GAIN=7,CR-SPLIT=NO 4 500 S ------------------------------------------------------------------------------------------------------------------------------------ 9073( 2) - 02/16/01 17:54 - [ 7] 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 M51-ULX2 WFPC2 IMAGE PC1 F555W ATD-GAIN=7,CR-SPLIT=NO 4 500 S ------------------------------------------------------------------------------------------------------------------------------------ 20 M51-ULX2 WFPC2 IMAGE PC1 F450W ATD-GAIN=7,CR-SPLIT=NO 4 500 S ------------------------------------------------------------------------------------------------------------------------------------ 30 M51-ULX2 WFPC2 IMAGE PC1 F814W ATD-GAIN=7,CR-SPLIT=NO 4 500 S ------------------------------------------------------------------------------------------------------------------------------------ 9073( 2) - 02/16/01 17:54 - [ 8] Visit: 05 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 NGC4559-ULX WFPC2 IMAGE PC1 F555W ATD-GAIN=7,CR-SPLIT=NO 4 500 S ------------------------------------------------------------------------------------------------------------------------------------ 20 NGC4559-ULX WFPC2 IMAGE PC1 F450W ATD-GAIN=7,CR-SPLIT=NO 4 500 S ------------------------------------------------------------------------------------------------------------------------------------ 30 NGC4559-ULX WFPC2 IMAGE PC1 F814W ATD-GAIN=7,CR-SPLIT=NO 4 500 S ------------------------------------------------------------------------------------------------------------------------------------ 9073( 2) - 02/16/01 17:54 - [ 9] Summary Form for Proposal 9073 Item Used in this proposal ------------------------------------------------------------------------------------------------------------------------------------ Apertures PC1 ------------------------------------------------------------------------------------------------------------------------------------ Configurations WFPC2 ------------------------------------------------------------------------------------------------------------------------------------ Opmodes IMAGE ------------------------------------------------------------------------------------------------------------------------------------ Optional Parameters ATD-GAIN=7 CR-SPLIT=NO ------------------------------------------------------------------------------------------------------------------------------------ Proposal Category GO ------------------------------------------------------------------------------------------------------------------------------------ Spectral Elements F555W F450W F814W ------------------------------------------------------------------------------------------------------------------------------------ Target Names M81-ULX NGC6946-ULX M51-ULX1 M51-ULX2 NGC4559-ULX ------------------------------------------------------------------------------------------------------------------------------------