(as published in Nova Notes - Jan/Feb 2017)

Introduction

A gravitational what? As predicted by Einstein’s general theory of relativity, high mass objects can bend the path of light passing close by. Observations of background stars near the limb of the Sun during a solar eclipse in 1919 were the first confirmation of this phenomenon.

It was later suggested that light from an object located directly behind a compact massive object (such as a black hole or neutron star) would be lensed – the result being that the distant object would be brighter than expected. Normally light intensity decreases by the inverse-square law where each doubling of the distance results in an intensity drop of a factor of four. The “lens” disrupts that process making the distant object brighter than it would otherwise be – this is because the “lens” allows the observer to receive light from multiple paths.

Figure 1: A “cartoon” model of the gravitational micro-lensing of a distant star.

There are two main types of gravitational lenses:

  • Large scale: this is where a distant galaxy (or quasar) is located far behind a foreground massive galaxy or galaxy cluster. The foreground “mass” creates a “lens” for the light emitted by the distant object and it focuses its light, forming a brighter image of the distant object. Because the lens is not perfect, the images formed are rather poor! Visually, they often appear as multiple single images or arc-shapes surrounding the “lens”.
  • Small scale: these are called gravitational micro-lenses (the subject of this article) and are believed to be made when a compact massive object (such as a black hole) passes in front of a background object such as a star. Because both the source and lens are not resolved, a normally invisible object is “lensed” and becomes bright enough to observe or a known object becomes brighter. This type was first detected in 1989.

In the micro-lens type, because the relative distances between the light source, lens, and observer is small and due to their relative motions, objects can pass in and out of being “lensed” on a short time scale. Since many are discovered each year, this makes them quite useful for studying a variety of astrophysical phenomenon (eg. constraining the number of stellar black holes – aka dark matter).

An even more bizarre variation is when there two compact massive objects in close orbit with a separation near the Einstein radius (the angle of the lensed ring around the object)! The resulting geometry can be complex and their “warped” gravitational fields create what are called “caustic events” that cause the lensed brightness to rapidly change. These are called binary micro-lenses.

Discovery

This story begins on August 5, 2016 when the Gaia space telescope’s operating programs detected an increase in brightness of a previously stable star by over one magnitude. It was then named Gaia15aye. This triggered an alert which resulted in ground based follow-up observations lead by the Gaia Photometric Science Alerts Team. The star continued to brighten and, but instead of a rise and fall at the same rate – the typical signature of a micro-lens event - the fall was very rapid.

When the light curve (a fancy term for a graph of a star’s brightness vs. time) was studied and more data came in, it was realized that this event was likely caused by a binary micro-lens! A “model” of the system from the observed light curve predicted a second pair of events was predicted to occur in mid-September and late November.

Figure 2: The accumulated light curve of Gaia15aye from late April until just after the last caustic event in late November. The solid line is the model computed by Przemek Mróz, a PhD student at the University of Warsaw. The chart did not indicate the filter used – the magnitude scale is not V-band for sure.

More Data Needed

The American Association of Variable Star Observers (AAVSO) mobilizes campaigns to observe stellar objects of interest to professional astronomers who, due to limited telescope resources, could not possibly dedicate the resources that a team of mostly amateur astronomers could.

After the mid-September caustic event took place (confirming the object’s nature), Dr. Kirill Sokolovsky (National Observatory of Athens and Sternberg Astronomical Institute, Moscow State University) requested that the AAVSO initiate a campaign urgently requesting observations of Gaia15aye. It was clear in Halifax the evening after the campaign notice was published and my data was the second observer to report in.

Figure 3: My first image (cropped) of Gaia15aye (marked by the arrow) taken on the evening of September 20 when it was micro-lensed and nearly 2 magnitudes brighter than normal.

I continued to make measurements of Gaia15aye every clear night from either the Burke-Gaffney Observatory (BGO) or my own Abbey Ridge Observatory. Up to December 6th, I submitted 248 V-band observations on 28 nights (who says the weather has been bad this fall?). Unfortunately, it was cloudy when the final caustic event took place, but it was clear on the next evening so those observations would help constrain the fall time.

 

Figure 4: The light curve of V-band (green filter) observations submitted to the AAVSO’s database. The orange “cross” symbols mark the BGO and ARO observations which were taken on 28 separate nights.

In all, about 5,000 brightness measurements were made in five filters by 28 observers from 14 countries – citizen science in action!

Conclusion

Contributing to this project was fun and exciting, especially leading up the final caustic event when it was uncertain when it would take place. It was also helpful that the professional astronomers were engaged and appreciative of the work done by the AAVSO observers.

This sort of project could be done by any of you. Don’t have the equipment? Use these Twitter-controlled telescopes yourself – see my article in the June issue of Nova Notes!

References

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