Monitoring the Variable Stellar Universe

Morrell, Meredith Vivian (2021). Monitoring the Variable Stellar Universe. PhD thesis The Open University.



Monitoring the Variable Stellar Universe is a demonstration of the use of a small aperture, autonomous telescope as a tool to follow up transient variable stars. I used the Open University robotic telescope PIRATE, a relatively low cost facility built with off-the-shelf components, located in Tenerife, to observe targets selected from the Gaia Science Alerts stream which uses Gaia space observatory observations to identify potential transient objects. A total of 45,000 images were collected by me for over 2,700 hours of exposure time. A subset of 22,000 of those observations are discussed here.

To match the pace of data acquisition of the automated telescope, I used Python to construct both a data reduction pipeline, and a data analysis pipeline which uses ensemble photometry and string length minimisation to generate photometric light curves. I compared the light curves generated by my data analysis pipeline to the those generated using the Cambridge Photometry Calibration Server, which produced similar results.

The largest single data set for a target was Gaia16aye, a binary microlensing event which began in July 2016, and peaked five times over the next 18 months. The data I collect was part of a collaborative effort by over 100 observatories to follow up and model the system. I used the PIRATE data to investigate the colour index variation over time, and found that the event was achromatic as expected for a microlensing event. In addition to Gaia16aye, 77 other targets were observed during the main acquisition period between May 2017 and January 2019, including microlensing events, supernovae, cataclysmic variables, active galactic nuclei and several targets with unknown classification. The light curves generated for these targets are also discussed here.

PIRATE proved to be an effective example of a small, autonomous telescope collecting a large quantity of data for a relatively small investment of staff time and financial resource. The Gaia Science Alerts stream used in conjunction with PIRATE proved to be a slight mismatch, with many alerts being too faint for PIRATE to follow. A larger telescope would be preferable to exclusively follow the Gaia alerts stream, and regular data quality monitoring through daily light curve generation would make PIRATE target selection for longer term follow up better. But PIRATE as it is now is very capable of transient follow up and would be used more effectively by branching out to other alert streams such as ASAS-SN, which is more closely matched to current PIRATE setup in terms of observing magnitude.

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