Paper: astro-ph/0507286
Date: Tue, 12 Jul 2005 14:25:20 GMT (958kb)

Title: MACHOs in M31? Absence of evidence but not evidence of absence

Authors: Jelte T. A. de Jong, Lawrence M. Widrow, Patrick Cseresnjes, Konrad
Kuijken, Arlin P. S. Crotts, Alexander Bergier, Edward A. Baltz, Geza Gyuk,
Penny D. Sackett, Robert R. Uglesich, Will J. Sutherland, the MEGA
collaboration
Comments: 38 pages, 49 figures, submitted to Astronomy and Astrophysics
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We present results of a microlensing survey toward the Andromeda Galaxy (M31)
carried out during four observing seasons at the Isaac Newton Telescope (INT).
This survey is part of the larger microlensing survey toward M31 performed by
the Microlensing Exploration of the Galaxy and Andromeda (MEGA) collaboration.
Using a fully automated search algorithm, we indentify 14 candidate
microlensing events, three of which are reported here for the first time.
Observations obtained at the Mayall telescope are combined with the INT data to
produce composite lightcurves for these candidates. The results from the survey
are compared with theoretical predictions for the number and distribution of
events. These predictions are based on a Monte Carlo calculation of the
detection efficiency and disk-bulge-halo models for M31. The models provide the
full phase-space distribution functions for the lens and source populations and
are motivated by dynamical and observational considerations. They include
differential extinction and span a wide range of parameter space characterized
primarily by the mass-to-light ratios for the disk and bulge. For most models,
the observed event rate is consistent with the rate predicted for self-lensing
-- a MACHO halo fraction of 30% or higher can be ruled at the 95% confidence
level. The event distribution does show a large near-far asymmetry hinting at a
halo contribution to the microlensing signal. Two candidate events are located
at particularly large projected radii on the far side of the disk. These events
are difficult to explain by self lensing and only somewhat easier to explain by
MACHO lensing. A possibility is that one of these is due to a lens in a giant
stellar stream.

\\ ( http://arXiv.org/abs/astro-ph/0507286 , 958kb)

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Paper: astro-ph/0507287
Date: Tue, 12 Jul 2005 13:46:02 GMT (74kb)

Title: HST Imaging of MEGA Microlensing Candidates in M31

Authors: Patrick Cseresnjes, Arlin P.S. Crotts, Jelte T.A. de Jong, Alex
Bergier, Edward A. Baltz, Geza Gyuk, Konrad Kuijken, Lawrence M. Widrow

Comments: Submitted to ApJL. Higher resolution version available at
http://www.astro.columbia.edu/~patrick/hst/hst_ml.pdf
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We investigate HST/ACS and WFPC2 images at the positions of five candidate
microlensing events from a large survey of variability in M31 (MEGA). Three
closely match unresolved sources, and two produce only flux upper limits. All
are confined to regions of the color-magnitude diagram where stellar
variability is unlikely to be easily confused with microlensing. Red variable
stars cannot explain these events (although background supernova are possible
for two). If these lenses arise in M31's halo, they are due to masses 0.08 < m
/ M_Sun < 0.85 (95% certainty, for a delta-function mass distribution), brown
dwarfs for disk/disk, and stellar masses for disk/bulge ``self-lensing''.

\\ ( http://arXiv.org/abs/astro-ph/0507287 , 74kb)