Extreme Microlensing toward the Galactic Bulge

Extreme microlensing events, defined as events with maximum magnification Amax 200, are a potentially powerful probe of the mass spectrum and spatial distribution of objects along lines of sight toward the Galactic bulge. About 30 yr-1 such events are expected for main-sequence sources with I0 < 19. For many of these it is possible to measure both a "proper motion" and a "parallax," that together would yield individual mass, distance, and transverse-speed determinations of the lensing object. The proper motion is determined from finite-source effects when the lens transits, or nearly transits, the source. The parallax is determined by observing the difference in the light curve as seen from two Earth observatories separated by about 1 Earth radius, R⊕. The size of the parallax effect is ~Amax R⊕/e, where e is the projected Einstein radius and can be of order 1%. Detection of candidate events requires a pixel-lensing search of the entire bulge once per day, preferably by at least two observatories on different continents. Follow-up observation must be carried out using optical/infrared photometry, with short (e.g., 1 minute) exposures on small ( 1 m) telescopes. Extreme microlensing observations toward the Large Magellanic Cloud do not appear feasible at the present time.

[1]  Bohdan Paczynski,et al.  Gravitational microlensing of the Galactic bulge stars , 1991 .

[2]  S. Mao,et al.  Can lensed stars be regarded as pointlike for microlensing by MACHOs , 1994 .

[3]  C. Alard First results of the DUO program , 1996 .

[4]  Andrew Gould,et al.  Proper Motions of MACHOs , 1994 .

[5]  Andrew Gould,et al.  Disk M Dwarf Luminosity Function From HST Star Counts , 1995 .

[6]  M. J. Lehner,et al.  First Observation of Parallax in a Gravitational Microlensing Event , 1995, astro-ph/9506114.

[7]  B. Peterson,et al.  Gravitational microlensing as a method of detecting disk dark matter and faint disk stars , 1991 .

[8]  R. Gilliland,et al.  LIMITS TO CCD ENSEMBLE PHOTOMETRY PRECISION, AND PROSPECTS FOR ASTEROSEISMOLOGY , 1992 .

[9]  A. Gould,et al.  Statistical Determination of the MACHO Mass Spectrum , 1995, astro-ph/9504078.

[10]  Bohdan Paczynski,et al.  Gravitational microlensing by the galactic halo , 1986 .

[11]  Andrew Gould MACHO Parallaxes from a Single Satellite , 1995 .

[12]  D. Spergel,et al.  Microlensing by the galactic bar , 1994, astro-ph/9409022.

[13]  B. Paczynski,et al.  The Optical Gravitational Lensing Experiment. The Optical Depth to Gravitational Microlensing in the Direction of the Galactic Bulge , 1994 .

[14]  A. Gould,et al.  The Mass Spectrum Of Machos From Parallax Measurements , 1994, astro-ph/9409036.

[15]  Andrew Gould,et al.  MACHO Velocities from Satellite-based Parallaxes , 1994 .

[16]  A. Gould,et al.  Einstein Radii from Binary-Source Lensing Events , 1996, astro-ph/9604031.

[17]  R. Nemiroff,et al.  Finite source sizes and the information content of macho-type lens search light curves , 1994, astro-ph/9401005.

[18]  B. Paczyński,et al.  On the cosmological origin of gamma-ray bursts , 1992 .

[19]  D. Holz,et al.  Photon Statistics Limits for Earth-Based Parallax Measurements of MACHO Events , 1995, astro-ph/9503039.

[20]  Real-Time Detection of Gravitational Microlensing , 1995, astro-ph/9508039.

[21]  Arlin P. S. Crotts,et al.  M31: A Unique Laboratory for Gravitational Microlensing , 1992 .

[22]  H. Witt The Effect of the Stellar Size on Microlensing at the Baade Window , 1995 .

[23]  Andrew Gould,et al.  Extending the MACHO Search to approximately 10 6 M sub sun , 1992 .

[24]  Andrew Gould Theory of Pixel Lensing , 1995 .

[25]  A. Gould MICROLENSING AND THE STELLAR MASS FUNCTION , 1996, astro-ph/9604014.

[26]  S. Refsdal,et al.  On the Possibility of Determining the Distances and Masses of Stars from the Gravitational Lens Effect , 1966 .

[27]  K. Griest,et al.  Effect of binary sources on the search for massive astrophysical compact halo objects via microlensing , 1992 .

[28]  Andrew Gould,et al.  Satellite Parallaxes of Lensing Events toward the Galactic Bulge , 1996 .

[29]  Andrew Gould,et al.  Discovering Planetary Systems through Gravitational Microlenses , 1992 .

[30]  Austin B. Tomaney,et al.  Expanding the Realm of Microlensing Surveys with Difference Image Photometry , 1996 .

[31]  M. J. Lehner,et al.  The Macho Project: 45 Candidate Microlensing Events from the First Year Galactic Bulge Data , 1997 .

[32]  A. Gould Analytic Error Estimates , 1994, astro-ph/9406059.