First Observation of Parallax in a Gravitational Microlensing Event

We present the first detection of parallax effects in a gravitational microlensing event. Parallax in a gravitational microlensing event observed only from the Earth appears as a distortion of the lightcurve due to the motion of the Earth around the Sun. This distortion can be detected if the event duration is not much less than a year and if the projected velocity of the lens is not much larger than the orbital velocity of the Earth about the Sun. The event presented here has a duration of 220 days and clearly shows the distortion due to the Earth's motion. We find that the projected velocity of the lens is 75+/-5 km/s at an angle of 28+/-4 deg from the direction of increasing galactic longitude, as expected for a lens in the galactic disk. A likelihood analysis yields estimates of the distance to and mass of the lens: D_{lens} = 1.7 (+1.1/-0.7) kpc and M = 1.3 (+1.3/-0.6) Msun, suggesting that the lens is a remnant such as a white dwarf or neutron star. A less likely possibility is that the lens is a main sequence star. If so, we can add our upper limit on the observed flux from the lens to the analysis. This modifies the estimates to: D_{lens} = 2.8 (+1.1/-0.6) kpc, and M = 0.6 (+0.4/-0.2) Msun.Comment: 11 pages, 3 figs in uuencoded, compressed, tared postscript file

The MACHO Project LMC Microlensing Results from the First Two Years and the Nature of the Galactic Dark Halo

The MACHO Project is a search for dark matter in the form of massive compact halo objects (Machos). Photometric monitoring of millions of stars in the Large Magellanic Cloud (LMC), Small Magellanic Cloud (SMC), and Galactic bulge is used to search for gravitational microlensing events caused by these otherwise invisible objects. Analysis of the first 2.1 years of photometry of 8.5 million stars in the LMC reveals 8 candidate microlensing events. This is substantially more than the number expected ($\sim 1.1$) from lensing by known stellar populations. The timescales (\that) of the events range from 34 to 145 days. We estimate the total microlensing optical depth towards the LMC from events with 2 < \that < 200 days to be \tau_2^{200} = 2.9 ^{+1.4}_{-0.9} \ten{-7} based upon our 8 event sample. This exceeds the optical depth, \tau_{\rm backgnd} = 0.5 \ten{-7}, expected from known stars, and the difference is to be compared with the optical depth predicted for a ``standard" halo composed entirely of Machos: \tau_{halo} = 4.7\ten{-7}. Likelihood analysis gives a fairly model independent estimate of the halo mass in Machos within 50 kpc of 2.0^{+1.2}_{-0.7} \ten{11} \msun, about half of the ``standard halo" value. We also find a most probable Macho mass of 0.5^{+0.3}_{-0.2}\msun, although this value is strongly model dependent. Additionally, the absence of short duration events places stringent upper limits on the contribution of low-mass Machos: objects from 10^{-4} \msun to 0.03 \msun contribute \simlt 20\% of the ``standard" dark halo.Comment: Latex, 54 pages, uses aas2pp4.sty and astrobib.sty, with 24 out of 26 Postscript figures in gzipped tar file. 2 extra greyscale figures and/or full paper available from ftp://igpp.llnl.gov/pub/macho/LMC2/ Submitted to ApJ, June 199