2 The proper motion of GRO J1655-40

On April 1996 the field of GRO J1655-40 was observed for the first time with the WFPC2 on the Hubble Space Telescope (Tavani et al. 1996). In quiescence, the secondary star has an apparent magnitude of m_V$\sim$ 17.2. The target had been placed at the center of the Planetary Camera (PC) chip, with a pixel size of $0\hbox{$.\!\!^{\prime\prime}$ }045$. Observations were obtained through the 675W filter ( $\lambda = 6717$ Å; $\Delta \lambda = 736$ Å). A total of 16 exposures of 40 s each were acquired on April 26th and the same sequence was repeated on April 27th but with a different telescope roll angle. Both sets of exposures were taken in groups of 4 and each group was dithered by a few pixels in both Right Ascension and Declination with respect to the others.

On June 20th 2001 a total of 18 exposures of 40 s each were acquired by us, with the same observational set-up as in April 1996, but without any dithering between single exposures. After a pre-reduction with the standard HST pipeline reduction (debiasing, dark removal, flatfielding), groups of well-aligned exposures (i.e. with relative shift smaller than 0.01 pixel) were combined with a median filter, stacked and cosmic ray hits filtered out.

We finally ended up with 4 images for each of the two April 1996 observations and one image for the June 2001 one. All the 1996 images were then registered on the 2001 one, previously aligned in Right Ascension and Declination, by fitting the coordinate transformation between grids of reference objects. The correction for the WFPC2 geometrical distortions, optimized for the filter used (Trauger et al. 1995) was taken into account. The whole procedure was iterated until the transformation residuals were all below $\simeq$ $1.5~\sigma$. We finally computed the relative displacements of our target for each of the 8 independent pairs of 1996/2001 images and we averaged the results. The computed proper motion is $\mu_{\alpha} = -3.3 \pm 0.5$ mas yr^-1 and  $\mu_{\delta} = -4.0 \pm 0.4$ mas yr^-1, corresponding to an overall yearly displacement $\mu = 5.2 \pm 0.5$ mas yr^-1 along a Position Angle of $220^{\circ} \pm 5^{\circ}$. At a distance D(kpc) this proper motion corresponds to a transverse velocity on the plane of the sky of (25 $\pm$ 3) D(kpc) km s^-1. In Fig. 1 we show the path of the black hole binary on a R-band image of the Digitized Palomar Observatory Sky Survey II (POSS II). NGC 6242 is a well-studied open cluster at a distance of $1020\pm 100$ pc from the Sun (Glushkova et al. 1997). Figure 1 shows that GRO J1655-40 is close to the boundary of a dark cloud (cataloged as DC344.9+2.6 in Hartley et al. 1986), which could explain the relatively large reddening of the secondary star in GRO J1655-40.