Photometric Confirmation of MACHO Large Magellanic Cloud Microlensing Events

Abstract

We present previously unpublished photometry of three Large Magellanic Cloud (LMC) microlensing events and show that the new photometry confirms the microlensing interpretation of these events. These events were discovered by the MACHO Project alert system and were also recovered by the analysis of the 5.7 yr MACHO data set. This new photometry provides a substantial increase in the signal-to-noise ratio over the previously published photometry, and in all three cases, the gravitational microlensing interpretation of these events is strengthened. The new data consist of MACHO-Global Microlensing Alert Network (GMAN) follow-up images from the CTIO 0.9 m telescope plus difference imaging photometry of the original MACHO data from the 1.3 m Great Melbourne telescope at Mount Stromlo. We also combine microlensing light-curve fitting with photometry from high-resolution HST images of the source stars to provide further confirmation of these events and to show that the microlensing interpretation of event MACHO LMC-23 is questionable. Finally, we compare our results with the analysis of Belokurov et al., who have attempted to classify candidate microlensing events with a neural network method, and we find that their results are contradicted by the new data and more powerful light-curve fitting analysis for each of the four events considered in this paper. The failure of the Belokurov et al. method is likely to be due to their use of a set of insensitive statistics to feed their neural networks.

Identification of the Microlens in Event MACHO-LMC-20

“…5 This is the second Milky Way disk lens in the MACHO sample, the first being that of event 5, which was closer at ∼600 pc. A total of 0.75 events were expected from the Milky Way thin and thick disks (Bennett et al 2005), and thus these two disk lenses are still consistent with the expected number. Event 20 did not pass the MACHO criteria set "A" and so does not affect the conclusion to the halo fraction made from it.…”

“…However, Bennett et al (2005) pointed out that event 20 is a good disk lens candidate because Figure 7 of Alcock et al (2000) shows that, like event 5, event 20 is quite red for its brightness; i.e., it appears to be blended with a star that is much redder than other LMC stars of similar magnitude.…”

“…Event 20 was not picked as a high-probability self-lensing candidate. The Mancini et al (2004) predictions are tabulated and compared against the MACHO and Belokurov et al (2004) predictions in Bennett et al (2005). Given our measurement of the spectral type of the lens, we estimate its distance from a sample of early M dwarfs using 2MASS and [3.6] photometry provided by M. Schuster JHK s (2006, private communication).…”

Limits on the Macho content of the Galactic Halo from the EROS-2 Survey of the Magellanic Clouds

“…10 (Tisserand 2004). As already noted, Bennett et al (2005) argued that MACHO-LMC-23 was, in any case, a weak candidate and that its variability doesn't call into question the nature of the other MACHO candidates. Indeed, some of the MACHO candidates are very convincing microlensing candidates.…”

Constraints on galaxy structure and evolution from the light of nearby systems

“…MACHO toward the LMC finds fewer events than expected if the total MWg halo mass arose solely from compact objects of mass m. Lensing optical depths τ ∼ 3 × 10 −7 based on ∼ 13 − 17 events suggest that 20% of the MWg halo mass is in 0.5 (Alcock et al 2000, Bennett et al 2005. However, Novati et al (2006) proposes that many MACHO events are actually self-lensing by the LMC halo, thereby lowering the estimated DM halo fraction.…”