Alert Notice 552: Gaia16aye microlensing event monitoring urgently requested

September 20, 2016 : Dr. Kirill Sokolovsky (National Observatory of Athens and Sternberg Astronomical Institute, Moscow State University), has requested AAVSO assistance in monitoring the ongoing microlensing event Gaia16aye beginning immediately to catch transient behavior occurring now and continuing through the end of October (or until further notice).

Dr. Sokolovsky writes: "Gaia16aye was first identified as an unusual variable object by Gaia on 2016 August 5 (V. Bakis et al., ATel #9376) and it was later realized that its achromatic light variations are consistent with being caused by microlensing of a distant M-type giant star by a binary star system (possibly consisting of main sequence dwarfs) crossing the line of sight. The complex geometry of the lens causes multiple brightness peaks corresponding to caustic crossings. According to preliminary modeling, the object's variability was noticed during decline after the first caustic crossing. The second brightness peak was observed on August 13 and the third peak on September 19. The source is expected to decline quickly in next days from its current magnitude V~14 to reach the plateau around V~16 between the  caustics, in order to start rising again to cross the caustic in one or couple of weeks (L. Wyrzykowski et al., ATel #9507). A well-sampled lightcurve of the microlensing event, especially during the caustic crossing, is needed to constrain geometry and some physical parameters of the lens and the lensed star.

"CCD observations of Gaia16aye using any subset of B, V, Rc, Ic, g, r, i filters are requested starting immediately and continuing for about four weeks (until the decline from the expected next caustic crossing). If the object is at its bright state around V~14, time series observations are needed. If the object is found at the plateau level around V~16, one-two observations per night are sufficient. Unfiltered CCD observations are also useful if photometric measurements are collected during multiple nights (otherwise it would be difficult to match the unfiltered measurements to the data collected by other observers).

"Observers are asked to... [reduce their data in their usual way using] UCAC4 601-089811 as the comparison star and submit observations to the AAVSO International Database using the standard procedure." UCAC4 601-089811 is the 145 comparison star in the AAVSO sequence for Gaia16aye.

Observers are also requested to upload their calibrated (dark and flat-field corrected) images to the anonymous FTP server  for semi-automated processing with the Cambridge Photometry Calibration Server pipeline.

Coordinates (2000.0): R.A. 19 40 01.13  Dec. +30 07 53.4

Charts with a comparison star sequence for Gaia16aye may be created using the AAVSO Variable Star Plotter (VSP). Creating an 'e' or 'f' scale chart is recommended.

Please submit observations to the AAVSO International Database using the name Gaia16aye (there are no spaces within the name).

This campaign is being followed on the AAVSO Observing Campaigns webpage. A thread has
been created on the Campaigns and Observation Reports forum at (

This AAVSO Alert Notice was compiled by Elizabeth O. Waagen.


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