Contamination in the MACHO dataset and the puzzle of LMC Microlensing

• Main Authors: Griest, Kim, Thomas, Christian L
• Format: Journal Article
• Language: English
• Published: 16-12-2004
• Subjects: Physics - Astrophysics of Galaxies; Physics - Cosmology and Nongalactic Astrophysics; Physics - Earth and Planetary Astrophysics; Physics - High Energy Astrophysical Phenomena; Physics - Instrumentation and Methods for Astrophysics; Physics - Solar and Stellar Astrophysics
• DOI: 10.48550/arxiv.astro-ph/0412443

Mon.Not.Roy.Astron.Soc. 359 (2005) 464-468 In a recent series of three papers, Belokurov, Evans, and Le Du, and Evans
and Belokurov, reanalysed the MACHO collaboration data and gave alternative
sets of microlensing events and an alternative optical depth to microlensing
toward the Large Magellanic Cloud (LMC). Even though they examined less than
0.2% of the data they claimed that by using a neural net program they had
reliably selected a better (and smaller) set of microlensing candidates.
Estimating the optical depth from this smaller set, they claim that the MACHO
collaboration overestimated the optical depth by a significant factor and that
the MACHO microlensing experiment is consistent with lensing by known stars in
the Milky Way and LMC. As we show below, the analysis by these authors contains
several errors which render their conclusions meaningless. Their efficiency
analysis is clearly in error, and since they did not search through the entire
MACHO dataset, they do not know how many microlensing events their neural net
would find in the data or what optical depth their method would give.
Examination of their selected events suggests that their method misses low S/N
events and thus would have lower efficiency than the MACHO selection criteria.
In addition, their method is likely to give many more false positives
(non-lensing events identified as lensing). Both effects would increase their
estimated optical depth. Finally, we note that the EROS discovery that LMC
event-23 is a variable star reduces the MACHO collaboration estimates of
optical depth and Macho halo fraction by around 8%, and does open the question
of additional contamination.