9397( 2) - 02/13/02 12:25 - [ 1] PROPOSAL FOR HUBBLE SPACE TELESCOPE OBSERVATIONS ST ScI Use Only ID: 9397 Version: 2 Check-in Date: 13-Feb-2002 12:24:47 1.Proposal Title: Unmasking the optical counterpart to the ultraluminous X-ray source, NGC 5204 X-1 ------------------------------------------------------------------------------------ 2. Proposal For 3. Cycle GO 11 ------------------------------------------------------------------------------------ 4. Investigators Contact? PI: Timothy Roberts University of Leicester CoI: Dr. Paulina Lira University of Leicester N CoI: Prof. Andrew King University of Leicester N CoI: Dr. Michael Goad University of Southampton N CoI: Prof. Martin Ward University of Leicester N CoI: Prof. Robert Warwick University of Leicester N CoI: Dr. Paul O'Brien University of Leicester N ------------------------------------------------------------------------------------ 5. Abstract We propose HST/STIS near- and far-UV spectroscopy of the recently identified optical counterpart to the ultraluminous X-ray source (ULX) NGC 5204 X-1 (Roberts et al. 2001). This source has since been resolved with HST/WFPC2 (data just public) into two sources with V magnitudes of 20.5 and 22.0 respectively, separated by 0.4 arcsecs (10 pc at the distance of NGC 5204). Existing low resolution ground-based data are dominated by the brighter source, the spectrum of which appears blue and apparently featureless, consistent with either (i) at least one unusually bright supergiant O star (M_v<=-7.8), or (ii) an O star association, although the data cannot presently rule out the presence of a background BL-Lac object. Ground-based data cannot unambiguously type both the new sources, due to their small separation (there are no bright nearby stars necessary for adaptive optics), and the absence of strong features in the optical spectra of O stars. By observing with the HST/STIS far- and near-UV MAMA we will obtain spectra in a waveband rich in O star absorption features, with sufficient S/N (10--15) and resolution (0.6- 1.5Angstrom) to identify the sources, and reveal the nature of the first optical counterpart to an ULX. This result will be an important step in determining whether most ULX systems contain a new, 10^2 - 10^3 M_\odot `intermediate-mass' class of black hole. ------------------------------------------------------------------------------------ 9397( 2) - 02/13/02 12:25 - [ 2] Observations Description ------------------------ The scientific requirements for this experiment are : moderate resolution, high S/N far- and near-UV spectroscopy of two faint sources separated by ~ 0.4''. We will observe with the STIS/MAMA at the prime tilt of the G140L and G230L gratings, covering the wavelength range LambdaLambda1150- 3180Angstrom\ at a resolution of 0.6 and 1.58Angstrom\ per pixel respectively. We adopt a moderate slit width (52x0.5) as a compromise between high throughput and low starlight contamination from the host galaxy. Below we give detailed exposure time calculations based on the HST/STIS ETC for a template OV5 star with V magnitudes appropriate for our targets. For our source declination (+58 deg) we have a maximum of 56 mins visibility on target. The breakdown for our observations is as follows. In orbit 1 we will need 12 mins for guide star + target acquisition, followed by 44 mins observing with the G230L grating (this includes an 8 min overhead for the first STIS exposure). We require 5 mins guide star reacquisition in orbit 2, followed by 51 mins observing with the G140L grating (including an 8 min overhead for a change of grating). For our fainter target (V=22) we will achieve a S/N of ~6.6 and 8.5 per 2 pixel resolution element in the G230L and G140L gratings respectively (assumes E(b- v)=0.012 mag, and average zodiacal light). Similarly, for our brighter source we will achieve a S/N of ~14.5 and 16.9 per 2 pixel resolution element in the same two gratings. These moderate S/N spectra will enable us to clearly identify the strong narrow absorption features present in the UV spectra of typical O stars, and thus allow us to identify the nature of the counterpart to the ULX. Real Time Justification ----------------------- This observation will be a time-critical observation, due to the need to align the two candidate sources such that they fall down a single slit. This will require the slit to be set at an angle ~ 95^degrees East of North. Considering the alignment of the slit with the U3 spacecraft axis implies a position angle of ~ 140 or 320^degrees for the observation. Note that the precise orientation on the sky is reasonably flexible due to the small source separation of 0.4'' and the large adopted slit width (52*0.5). None. Calibration Justification ------------------------- Additional Comments ------------------- ------------------------------------------------------------------------------------ 9397( 2) - 02/13/02 12:25 - [ 3] TARGET LIST Fixed Targets ------------------------------------------------------------------------------------------------------------------------------------ Tar| Target | Target | Target |Coord | Radial | Flux data No | Name | Description | Position |Eqnx | Vel. | ------------------------------------------------------------------------------------------------------------------------------------ 1 NGC5204-X-1 EXT-STAR, MXB, RA=13H 29M 38.56S +/- 0.2", J2000 Z = V = 20.5+/-0.5 SUPERGIANT O, EXT- DEC=58D 25' 5.7" +/- 0.2", PLATE 0.00067 B-V = -0.3+/-0.1 CLUSTER, OB ASSOCIATION -ID=01R2 Coordinate Source: HST_IMAGE Comments: Colour estimate assumes an O5-7 star. 9397( 2) - 02/13/02 12:25 - [ 4] Visit: 01 Visit Priority: Visit Requirements: ORIENT 120D TO 160D ORIENT 300D TO 340D On Hold Comments: Additional Comments: Orientation calculated by requiring slit to lie along PA 95D E of N (U3 axis is this value plus 45D). Toleration of 20D either way since two interesting sources are separated by 0.4 arcsec (centred on bright source), slit width is 0.5 arcsec. 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-X-1 STIS/C ACQ F28X50L MIRROR 1 80 S CD P ------------------------------------------------------------------------------------------------------------------------------------ 20 NGC5204-X-1 STIS/N TIME-TA 52X0.5 G230L 2376 BUFFER-TIME=300 1 36 M UV-MAM G A ------------------------------------------------------------------------------------------------------------------------------------ 30 NGC5204-X-1 STIS/F TIME-TA 52X0.5 G140L 1425 BUFFER-TIME=300 1 51 M UV-MAM G A ------------------------------------------------------------------------------------------------------------------------------------ 9397( 2) - 02/13/02 12:25 - [ 5] Summary Form for Proposal 9397 Item Used in this proposal ------------------------------------------------------------------------------------------------------------------------------------ Apertures F28X50LP 52X0.5 ------------------------------------------------------------------------------------------------------------------------------------ Configurations STIS/CCD STIS/NUV-MAMA STIS/FUV-MAMA ------------------------------------------------------------------------------------------------------------------------------------ Opmodes ACQ TIME-TAG ------------------------------------------------------------------------------------------------------------------------------------ Optional Parameters BUFFER-TIME=300 ------------------------------------------------------------------------------------------------------------------------------------ Proposal Category GO ------------------------------------------------------------------------------------------------------------------------------------ Special Requirements ORIENT 120D TO 160D ORIENT 300D TO 340D ------------------------------------------------------------------------------------------------------------------------------------ Spectral Elements MIRROR G230L G140L ------------------------------------------------------------------------------------------------------------------------------------ Target Names NGC5204-X-1 ------------------------------------------------------------------------------------------------------------------------------------ Wavelengths 2376 1425 ------------------------------------------------------------------------------------------------------------------------------------