Observing the parallax effect due to gravitational lensing with OSIRIS Martin Burgdorf Martin Dominik Björn Grieger Michael Küppers Horst Uwe Keller Joachim Wambsganß Daniel Kubas

Extragalactic Source Quasar or galaxy Lensing galaxy Observer

Stellar Source Star (in most cases close to the galactic centre) Lensing star (most likely an M-dwarf) Observer

The Einstein Ring what are M DLS etc? Typical size of the Einstein ring for a stellar source close to the galactic centre: ΘE several 10-4 arcsec scales with (M DLS/(DLDS))1/2 RE several AU scales with (M DLDS/DLS)1/2

Animation of an Event 1: Perfect alignment

Animation of an Event 2: Slight offset

Space Telescope Observation

Example light curves what are the axes? magnitude vs. days?

Scientific objective of observation of parallex effect Parallaxes greatly enhance microlensing as a probe of the stellar mass function. Particularly interesting: frequency of disk brown dwarfs (BDs) Only young BDs can be detected with other techniques. If a planet is involved in a microlensing event, its mass will be measured with 10 % accuracy Microlensing is currently the only technique sensitive to Earth mass extrasolar planets

Simultaneous Observations

Geometry

Expected brightness difference between Rosetta and Earth based observations Gradient in light curves tenths of mags or mags per week. Typical transverse velocity: 150 km/s. → In a week the observer travels 0.6 AU. → We always expect more than 0.1 mag brightness difference per 0.6 AU. OSIRIS has performed photometry with a relative accuracy of 0.02 mag for Asteroid Steins (magnitude 16.6) with 5 minute exposures → The observations are feasible A serendipitious OSIRIS image of a field close to the galactic centre isolates individual stars down to magnitude 16 - Even fainter sources can be measured with adequate background subtraction → Source confusion is not a show stopper

Possible time slots Criteria: Projected distance between Rosetta and Earth at least 0.7 AU Solar elongation of galactic bulge larger than 90°(straylight minimization) and smaller than 140° (165) (spacecraft constraint). Galactic centre region must be observable from Earth Proposed time Distance [AU] Solar Elongation [deg.] 2008-Jul (Active checkout PC8)        1.6 150 2008-Sep (Steins flyby)    1.7 160 2009-May (-)                1.9 2010-Jun (after active checkout PC12) 2.1 90 2010-Jul (Lutetia flyby)   3.0 100 2010-Sep (-)               4.0 110 2011-May (-)               2.8 130 what is difference of 140 or 165 for elongation angle?, spalte velocity weglassen

OSIRIS Observations OSIRIS NAC (2 x 2 deg. Field of View) will scan the galactic bulge with an 8 point raster – altogether 56 images Only one image per field needed Raster takes about 4 hours and will be performed 7 times in 25 days Same operational sequence in each repetition - Limited operational effort OSIRIS image 8 deg from galactic centre how many obs.? raster 8x8 or only 8? total 56 Note the magnitude scale!

Ground-Based microlensing surveys Monitor 108 stars of Gal. Bulge OGLE: 1.3-m telescope at Las Campanas, Chile MOA: 1.8-m at Mount John, NZ ~700 events each season > ~ Gal. Bulge in I///

Follow-up: PLANET/RoboNet... PLANET/RoboNet, MicroFUN search events for anomalies Several mid-size tels. @ diff places respond within minutes to target requests hunt for anomalies and sample these quasi-continuously 5 exoplanets found

Conclusions This is a first! Simultaneous measurements with OSIRIS and ground based telescopes will provide accurate masses for dozens of lens stars based on parallax observations of gravitational lensing possible determination of exoplanet properties OSIRIS has proven the capability to perform the observations The operational burden is limited Observations should be done as soon as possible Similar program with Spitzer expected to start in 2009 This is a first! a first!