0 Assigned ID: 1227 ----- Attempting Submission 1 (Mon May 08 20:53:22 GMT 2017) ----- HST Phase I Proposal 1227 (B1259_Phase1_George_Final.apt) successfully submitted. Receipt: # 1227-1 0 ----- Attempting Submission 1 (Tue May 16 21:58:10 GMT 2017) ----- Optical counterpart of high-speed ejecta from the gamma-ray binary LS 2883 We propose to look for an optical counterpart of a remarkable extended X-ray feature ejected from the famous gamma-ray binary LS 2883 close to the most recent periastron passage. LS 2883 is a colliding-wind binary with a young pulsar B1259-63 in a 3.4-yr eccentric orbit around a fast-rotating massive O9 star. Chandra observations have shown that the feature is moving from the binary with an extremely high speed (0.07c in projection onto the sky), changing its shape and brightness. In the most recent Chandra observation (2017 April 24) the feature showed a hint of acceleration and an unexpected brightening. We detected a similarly fast-moving (but fainter and less extended) X-ray feature during the previous binary orbital cycle. The nature of the feature is puzzling. If it is a clump of the O-star's dense wind, the clump is most likely ablated and pushed away by the highly relativistic pulsar wind. In this case we expect to see both thermal emission from the heated stellar wind matter and nonthermal emission from shocked relativistic electrons. To understand the origin of the feature and the mechanisms driving its extremely fast motion, we need to detect it in the optical. We propose a coronographic HST observation, which should be carried out before the next periastron passage, when the binary system may become too bright to discern the nearby extended feature. The unique capabilities of the HST are required to resolve the faint feature in a close proximity to the bright O star. We are using four STIS/CCD orbits to detect (or set an upper limit on) optical emission from the extended clump of ejecta near the binary LS 2883/B1259-63. The clump's center is about 6 arcsec away from the the binary (GSC2.3 identifier is S7KQ000205; Vmag=10.15, Bmag=10.80) that will be used for acquisition but then will be occulted by the coronograph. We will use WEDGEB2.5 aperture. Observations are organized in four consecutive visits. At the beginning of each orbit the star will be re-acquired (using the F25ND3 filter) and then it will be occulted by the coronograph for the rest of the orbit. Acquisition images are 2 sec long (SNR=68; time to saturation 103 sec). The science exposures are divided into 4 CR-SPLIT subexposures (each 648 s long) to remove cosmic ray events. The first visit is restricted to two pre-determined roll angle (ORIENT) ranges to avoid blocking the science target by the coronograph wedge and to avoid placing it within the diffraction spikes of the bright binary. These restrictions have a margin of 15 degrees to accommodate for the incremental ORIENT increase in the subsequent visits by 3-5 degrees compared to the previous visit. The increments are needed to better separate the faint extended emission from the PSF wings of the bright binary with the aid of differential images. The four visits will happen during consecutive four orbits. Stellar Physics Stellar Populations 0 SUPPORTED STAR OE This object was generated by the target selector and retrieved from the SIMBAD database. The coordinates are taken from the GSC2.3.2 catalog; GSC2.3 identifier is S7KQ000205. J2000 J2000 STIS/CCD Imaging Yes No WEDGEB1.8 Timing link suitability includes constraints ([03 After 02 by 0.0 to 0.1 days]) The scheduling data was generated by CASM 25.0. CASM was run in Phase II mode. /tmp/OPVisit5933210436895076028.xml This is the first visit in a series of four visits in four consecutive orbits. 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