430 research outputs found
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 () 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
- …