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A Close Look at the Transient Sky in a Neighbouring Galaxy

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A Close Look at the Transient Sky in a Neighbouring Galaxy Kiran Tikare Space Engineering, master's level (120 credits) 2020 Luleå University of Technology Department of Computer Science, Electrical and Space Engineering Master Thesis A close look at the transient sky in a neighboring galaxy Kiran Tikare Erasmus Mundus SpaceMaster Program Department of Computer Science, Electrical and Space Engineering Lule˚aUniversity of Technology Kiruna, Sweden Supervisors: Examiner: Prof. Ariel Goobar Prof. Anita Enmark Dr. Rahul Biswas ii To my beloved motherland Bharat and to the ancient and modern Gurus... iii iv Om Asatomaa Sadgamaya Tamasomaa Jyotirgamaya Mrtyormaa Amrtamgamaya Om Shaantih Shaantih Shaantih Lead us from the unreal to the real Lead us from darkness to light Lead us from death to immortality Aum peace, peace, peace! { Brihadaranyaka Upanishad (1.3.28) If I have seen further it is by standing on ye sholders of Giants. { Isaac Newton in a letter to Robert Hooke One thing I have learned in a long life: that all our science, measured against reality, is primitive and childlike | and yet it is the most precious thing we have. { Albert Einstein Bear in mind that the wonderful things you learn in your schools are the work of many generations, produced by enthusiastic effort and infinite labour in every country of the world. All this is put into your hands as your inheritance in order that you may receive it, honour it, add to it, and one day faithfully hand it on to your children. Thus do we mortals achieve immortality in the permanent things which we create in common. If you always keep that in mind you will find a meaning in life and work and acquire the right attitude toward other nations and ages. { Albert Einstein, Ideas and Opinions v vi Abstract Study of the time variable sources and phenomena in Astrophysics provides us with im- portant insights into the stellar evolution, galactic evolution, stellar population studies and cosmological studies such as number density of dark massive objects. Study of these sources and phenomena forms the basis of Time Domain surveys, where the telescopes while scanning the sky regularly for a period of time provides us with positional and tem- poral data of various Astrophysical sources and phenomena happening in the Universe. Our vantage point within the Milky Way galaxy greatly limits studying our galaxy in its entirety. In such a scenario our nearest neighbour The Andromeda galaxy (M31) proves to be an excellent choice as its proximity and inclination allows us to resolve millions of stars using space based telescopes. Zwicky Transient Facility (ZTF) is a new optical time domain survey at Palomar Observatory, which has collected data in the direction of M31 for over 6 months using multiple filters. This Thesis involves exploitation of this rich data set. Stars in M31 are not resolved in ZTF as it is a ground based facility. This requires us to use the large public catalogue of stars observed with Hubble Space Telescope (HST): The Panchromatic Hubble Andromeda Treasury (PHAT). The PHAT catalogue provides us with stellar co- ordinates and observed brightness for millions of resolved stars in the direction of the M31 in multiple filters. Processing of the large volumes of data generated by the time domain surveys, requires us to develop new data processing pipelines and utilize statistical techniques for deter- mining various statistical features of the data and using machine learning algorithms to classify the data into different categories. End result of such processing of the data is the astronomical catalogues of various astrophysical sources and phenomena and their light curves. In this thesis we have developed a data processing and analysis pipeline based on Forced Aperture Photometry Technique. Since the stars are not resolved in ZTF, we per- formed photometry at pixel level. Only small portion of the ZTF dataset has been an- alyzed and photometric light curves have been generated for few interesting sources. In our preliminary investigations we have used a machine learning algorithm to classify the resulting time series data into different categories. We also performed cross comparison with data from other studies in the region of the Andromeda galaxy. vii viii Contents List of Figures1 List of Tables3 Chapter 1 { Introduction 5 1.1 Our work . .5 1.2 Outline of the Thesis . .6 1.3 Scientific Context . .7 1.3.1 The Realm of the Nebulae - Island Universe . .9 1.3.2 Golden Era of Sky Surveys and Big Data . 10 1.3.3 Statistical and Machine Learning Techniques . 13 1.3.4 Time Domain Astronomy . 13 Stellar Variability . 14 Transients . 15 Gravitational Lensing . 16 Strong Lensing . 19 Weak Lensing . 19 Microlensing . 20 Applications of Gravitational Lensing . 22 1.4 Our Testbed - The Andromeda Galaxy . 24 Chapter 2 { Studying Stars 29 2.1 Magnitude System . 31 2.1.1 Apparent Magnitude - m ....................... 31 2.1.2 Absolute Magnitude - M ....................... 31 2.1.3 Bolometric Magnitude . 32 2.2 Photon Detection using Charge Coupled Devices (CCDs) . 33 2.2.1 Key parameters of CCDs . 34 2.3 Errors and Noise . 35 2.3.1 Dark Current . 35 2.3.2 Photon Noise . 35 2.3.3 Readout Noise . 35 2.3.4 Pixel Saturation . 36 2.3.5 Cosmic Rays . 36 2.3.6 Sky Background . 36 2.3.7 Other sources of Noise . 36 ix 2.3.8 Crowding . 36 2.4 Photometric Filter System . 37 2.5 Stellar Photometry . 38 2.5.1 Aperture Photometry . 39 Extinction correction and Photometric Calibration . 41 Uncertainties . 41 2.5.2 Light Curves . 41 Chapter 3 { Observational Datasets 43 3.1 Panchromatic Hubble Andromeda Treasury(PHAT) . 43 3.2 Zwicky Transient Facility (ZTF) . 47 Chapter 4 { Data Processing Approach and Pipeline Development 51 4.1 Software Tool Chain . 51 4.1.1 Python Programming Language . 51 4.1.2 Python Packages . 52 4.1.3 Git as Version Control System . 52 4.1.4 Imaging and data visualization . 52 4.2 HPC Computing Resource - NSC's Tetralith Supercomputer . 52 4.3 Exploratory Data Analysis . 53 4.3.1 Data used for processing and Analysis . 53 4.3.2 Quality Filtering of ZTF images . 56 4.4 Our Approach . 57 4.4.1 Pixel Photometry . 57 4.4.2 Source Measurement and Estimation . 58 4.4.3 Background Measurement and Estimation . 58 4.5 Methodology and Workflow . 59 4.6 Use of Machine Learning Algorithm for Classification . 63 4.6.1 Principal Component Analysis (PCA) . 65 4.6.2 Random Forest Algorithm (RFA) . 65 4.7 Validating the Pipeline . 66 Chapter 5 { Results and Data Analysis 67 5.1 Time Series Data Analysis . 67 5.1.1 Visual Assessment . 67 5.1.2 Lomb Scargle Periodogram Analysis for Variability . 67 5.2 Machine Learning Classification . 68 5.2.1 Microlensing Detections . 68 5.2.2 Variable Star Detections . 69 5.2.3 Cataclysmic Variable Detections . 71 5.2.4 Constant Source Detections . 72 5.3 Cross Comparisons . 73 5.3.1 Comparison with Hubble Catalog of Variables (HCV) . 73 5.3.2 Comparison with ZTF Caltech team . 73 5.3.3 Comparison with Caldwell Star Catalogue in M31 . 74 x 5.4 Summary and Conclusion . 75 5.5 Outlook and Future Work . 76 Appendix { A - Terminology 79 Appendix { B - zwindromeda - python pipeline 83 Python Packages . 83 7.1 Zwindromeda - Pipeline . 83 References 85 xi xii Acknowledgments It was in 1999 I first came to know about Gravitational Lensing during one of the thought- provoking classes taught by my physics teacher Shri Arun Pujer. I remember it even after 20 years and that is a testimony to the impression it had created on me and the subsequent curiosity about the phenomena. Coming back to 2019, while looking for a Master Thesis I came across Caltech webpage of Prof. Shrinivas Kulkarni, about whom I had read during my school and college days in various Indian regional and national print media. I was excited to be writing to him to seek his guidance for my Master Thesis. Subsequent to our email exchange, Prof. Shrinivas Kulkarni suggested me to do my Master Thesis under his collaborator Prof. Ariel Goobar at The Oskar Klein Center for Cosmoparticle Physics, Stockholm University in Stockholm, Sweden. I readily accepted the Thesis topic proposed by Prof. Ariel Goobar as I was excited to work on the topic. Coincidentally 2019 also marked the 100th anniversary of observation of deflection of light by gravity, which was predicted by Albert Einstein in his General Theory of Relativity and was observationally confirmed by Sir Arthur Eddington during a Solar Eclipse on 29 May 1919. Apart from this there are four other important historical anniversaries which makes it bit exciting period, namely, the 100th anniversary of Saha Equation (named after Meghnad Saha), 50th anniversary of discovery of CCD, 100th anniversary of Great Debate (April 26, 1920) between Astronomers Harlow Shapley and Heber Curtis. And without those works I would not be doing this thesis work today. Every thesis work reflects the subject matter understanding (at the time of writing the thesis) of the author or the lack there of. As this work is in the domain of science ideas are put to test rigorously and any lack in understandings or wrong directions or misinterpretations will be found out with subsequent works by the author or the other researcher working on same problems and may require revision in some point in time after the thesis has been published. As one takes a look at History of Science, it is evident that scientific explorations and the insights gained are not linear, it has undergone rigorous tests and revisions and modifications, only those works remain which agree with the experiments.1 As one goes through this thesis, one may notice some mention of historical developments and some quotes from the works which I have read, and which had an impact on me 1It doesn't matter how beautiful your theory is, it doesn't matter how smart you are.
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