9140( 3) - 05/03/01 09:13 - [ 1] PROPOSAL FOR HUBBLE SPACE TELESCOPE OBSERVATIONS ST ScI Use Only ID: 9140 Version: 3 Check-in Date: 03-May-2001 09:13:17 1.Proposal Title: Is GRO J1655-40 a runaway Black Hole? ------------------------------------------------------------------------------------ 2. Proposal For 3. Cycle GO 10 ------------------------------------------------------------------------------------ 4. Investigators Contact? PI: Felix Mirabel Commissariat a l'Energie Atomique (CEA) CoI: Dr. Roberto Mignani ESO N CoI: Dr. Luis Rodriguez UNAM N CoI: Dr. Fabrizia Guglielmetti Space Telescope Science Institute N ------------------------------------------------------------------------------------ 5. Abstract The formation of black holes is one of the most intriguing problems in astronomy. In the case of stellar-mass black holes, several theories propose that they are born with large space velocities. To test this idea we wish to determine the space velocity of X-ray Nova Scorpii 1994 (GRO J16155-40), which is one of the best black hole candidates of stellar mass, and a source of jets with apparent superluminal motions. This intriguing object is located ~eq 2.5^degrees above the galactic plane and the centre of mass of the system is known to be moving with a large anomalous radial velocity (~eq -114 km s^-1) with respect to the Local Standard of Rest. Since GRO J16155-40 has already been imaged with the WFPC2 in 1995, one more image with the same instrumental set-up could allow us to measure its expected proper motion with an accuracy of a few percent. The observation proposed here may provide the first unambigous evidence for a black hole formed in the Milky Way that is migrating into intergalactic space. ------------------------------------------------------------------------------------ 9140( 3) - 05/03/01 09:13 - [ 2] Observations Description ------------------------ beginfigure centerline figure=figures/groj1655_HST.ps, height=10.5cm, clip= vspace1.0cm captionImage of the field around GRO J1655-40 taken with the PC chip of the WFPC2 in the 675W filter (Tavani et al. 1996). The optical counterpart of the black hole is marked by the arrow. endfigure We plan to perform precise, high resolution, imaging of the companion star of the black hole binary GRO J1655-40 using the WFPC2. In order to exploit the maximum possible spatial resolution of the instrument (45 mas/px), the target should be located at the center of the Planetary Camera (PC) chip. For this observation we plan to use the `` R-band'' F675W filter (Lambda=6717 Angstrom; Delta Lambda=736 Angstrom), i.e., the same used in a previous WFPC2 observation of the source originally aimed at detecting the optical counterparts of the radio jets (Tavani et al. 1996). The selection of the same instrument configuration and the same filter set-up has been planned to make a direct comparison between the image datasets easier. This will allow to measure the relative target position at the two epochs and to search for the expected displacement. , The procedure to carry out extremely precise relative astrometry with the WFPC2 has been well tested and successfully experimented in the cases of the measurements of the proper motion and parallax of Geminga (Caraveo et al. 1996), of the proper motion of the Crab (Caraveo and Mignani 1999) and Vela (Deluca, Mignani and Caraveo 2000a) pulsars and more recently of PSR B0656+14 (Mignani, Deluca and Caraveo 2000). Although all of them, Crab apart, are definitely fainter than our target (with magnitudes between 23.5 and 25.5), we could achieve similar proper motion accuracies over a 1-2 years time span. , The details of the whole astrometric procedures as well as a discussion of the known systematics can be found in Deluca, Mignani and Caraveo (2000b). Briefly, the procedure consists of (i) primarily determining a well-suited grid of reference objects, which are neither saturated nor extended and are randomly distributed across the field, (ii) computing their pixel coordinates through 2-D PSF fitting using an optimized centering area, (iii) correcting the centroids for the geometric distorsion of the PC and (iv) registering the two reference grids through a five parameter (2 offset factors in X and Y, 2 scale factors and a rotation angle) fit. As it is seen from Fig.1, the WFPC2 image of Tavani et al. largely meets the requirements. , Once the target position has been translated to a unique reference frame, we finally compute the expected displacement from the relative offset between the centroids measured at the two epochs. Since our target is relatively bright with R=16.2 in quiescence (Tavani et al.1996), its centroid can be measured with an accuracy better than 0.05 PC pixels (i.e. ~ 2 mas). After accounting for a comparable error associated to the frame registration, we conservatively estimate an overall uncertainty of about 4 mas on the expected target displacement of ~ 45 mas. Diluted over a time span >= 6 years, this corresponds to an error of <= 1 mas yr^-1 on the expected source proper motion of >~ 7 mas yr^-1. , The exposure time required for our observations has been estimated taking as a reference the target brightness in quiescence. To summarize, we plan to split our observation in a sequence of 16 exposures of 40 s each, for a total of 640 s. The planned observation, inclusive of overheads, can thus easily fit one single HST orbit. The observing strategy has been choosen both to avoid saturation problems with our relatively bright target, as well as with the reference stars, and to allow for a more efficient cosmic rays rejection. At the same time, a sequence of short exposures would allow us to monitor accurately the telescope jitter and thus to identify possible additional errors in the centering procedure. beginthebibliography bibitem Bekenstein, J.D. 1973, ApJ 183, 657 bibitem Brandt, W.N. et al. 1995, MN 277, L35 bibitem Brown, G.E. & Bethe, H. 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Calibration Justification ------------------------- Additional Comments ------------------- ------------------------------------------------------------------------------------ 9140( 3) - 05/03/01 09:13 - [ 3] TARGET LIST Fixed Targets ------------------------------------------------------------------------------------------------------------------------------------ Tar| Target | Target | Target |Coord | Radial | Flux data No | Name | Description | Position |Eqnx | Vel. | ------------------------------------------------------------------------------------------------------------------------------------ 1 STAR-1654- STAR, LMXB RA=16H 54M 0.137S +/- 1", DEC=- J2000 V = 17.3 3950 39D 50' 44.9" +/- 1", PLATE- B-V = 0 ID=06Q2 Coordinate Source: GUIDE_STAR_CATALOG Epoch of Position RA proper motion (seconds of time/yr) DEC Proper Motion (arcsec/yr) Annual Parallax (arcsec) 1995.7 0.0 0.0 0.0 9140( 3) - 05/03/01 09:13 - [ 4] Visit: 01 Visit Priority: Visit Requirements: PCS MODE FINE 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 STAR-1654-3 WFPC2 IMAGE PC1-FIX F675W 6717 18 40 S 950 ------------------------------------------------------------------------------------------------------------------------------------ 9140( 3) - 05/03/01 09:13 - [ 5] Summary Form for Proposal 9140 Item Used in this proposal ------------------------------------------------------------------------------------------------------------------------------------ Apertures PC1-FIX ------------------------------------------------------------------------------------------------------------------------------------ Configurations WFPC2 ------------------------------------------------------------------------------------------------------------------------------------ Opmodes IMAGE ------------------------------------------------------------------------------------------------------------------------------------ Proposal Category GO ------------------------------------------------------------------------------------------------------------------------------------ Special Requirements PCS MODE FINE ------------------------------------------------------------------------------------------------------------------------------------ Spectral Elements F675W ------------------------------------------------------------------------------------------------------------------------------------ Target Names STAR-1654-3950 ------------------------------------------------------------------------------------------------------------------------------------ Wavelengths 6717 ------------------------------------------------------------------------------------------------------------------------------------