[Paper Review] Object DUO 2: A New Binary Lens Candidate

C. Alard, Shude Mao+1 more|arXiv (Cornell University)|Jun 20, 1995

Stellar, planetary, and galactic studies3 citations

TL;DR

This paper presents DUO 2, a microlensing event exhibiting a rapid brightening of over two magnitudes in six days, with achromatic light curves indicating a binary lens system. The best-fit model suggests a low-mass companion—possibly a brown dwarf or Jupiter-mass planet—separated by 1" from the primary lens, confirmed by the first observed centroid shift in microlensing experiments, with direct imaging confirming the separation and position angle.

ABSTRACT

We present the light curve of an unusual variable object, DUO 2, detected during the search for microlensing events by the DUO project. The star remained stable for more than 150 days before it brightened by more than two magnitudes in 6 days in the B and R bands. The light curves are achromatic during the variability. We consider possible explanations of the photometric behavior, with particular emphasis on the binary lens interpretation of the event. The masses of the lenses are quite small, with the companion possibly in the range of a brown dwarf or even a few times of Jupiter. We report evidence of blending of the source by a companion through the first detection of shift in the light centroid among all the microlensing experiments. This shift sets a lower limit of $0.3^{\prime\prime}$ on the separation between the stars. The best lens model obtained requires moderate blending, which was what motivated us to check the centroid shift that was subsequently found. The best lens model predicts a separation of $1^{\prime\prime}$ between the two blended stars. This prediction was recently tested using two CCD images taken under good seeing conditions. Both images show two components. Their separation and position angle are in good agreement with our model.

Motivation & Objective

• To investigate the unusual photometric variability of object DUO 2 detected during the DUO microlensing survey.
• To determine the nature of the lensing system responsible for the rapid brightening event.
• To test the binary lens hypothesis using photometric and astrometric data.
• To detect and quantify blending effects through light centroid shifts, a novel approach in microlensing experiments.
• To validate the lens model predictions using high-resolution imaging.

Proposed method

• Analysis of the B and R band light curves of DUO 2 over a 150-day baseline, identifying a rapid brightening event.
• Modeling the light curve using a binary lens microlensing framework to infer lens masses and separation.
• Calculating the expected centroid shift due to blending, setting a lower limit of 0.3" on the angular separation of the lens components.
• Comparing the predicted lens separation of 1" with high-resolution CCD images taken under good seeing conditions.
• Using astrometric measurements from the images to verify the predicted position angle and separation of the blended stars.
• Applying the microlensing magnification formalism to derive constraints on lens mass and source blending.

Experimental results

Research questions

• RQ1 What caused the rapid, achromatic brightening of DUO 2 over six days?
• RQ2 Can the light curve be explained by a binary lens system rather than a single lens?
• RQ3 What is the minimum angular separation between the lens components, as inferred from centroid shift?
• RQ4 Does high-resolution imaging confirm the predicted lens separation and position angle?
• RQ5 What are the mass constraints on the lens components, particularly the companion?

Key findings

• DUO 2 exhibited a rapid brightening of more than two magnitudes in six days, with no color change, indicating achromatic variability.
• The best-fitting binary lens model implies a companion mass in the range of a brown dwarf or a few times Jupiter's mass.
• A shift in the light centroid was detected, setting a lower limit of 0.3" on the angular separation between the lens components.
• The model predicted a separation of 1" between the two blended stars, which was confirmed by two high-resolution CCD images.
• The images showed two components with a separation and position angle in good agreement with the model prediction.
• The detection of centroid shift marks the first such observation in microlensing experiments, providing direct evidence of blending.