1. 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

3. Extragalactic Source
Quasar or galaxy
Lensing galaxy
Observer

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

5. 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 arcsec scales with (M DLS/(DLDS))1/2
RE several AU scales with (M DLDS/DLS)1/2

6. Animation of an Event 1: Perfect alignment

7. Animation of an Event 2: Slight offset

8. Space Telescope Observation

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

10. 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

11. Simultaneous Observations

12. Geometry

13. 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

14. 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

15. 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!

16. Ground-Based microlensing surveys
Monitor 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///

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

18. 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!