ASTRAL ODYSSEY: Thesis Journeys Part 2

Next up we have with us the brilliant Prajakta Mane. She has worked extensively on transient astronomical phenomena with special focus on strong lensing on type Ia supernovae. Here's to part 2 of Astral Odyssey.

Q1:What was your thesis topic?

A:I worked on strong lensing on type Ia supernovae (SNe Ia) and methods to find these systems in the upcoming Vera Rubin Observatory's LSST (Legacy Survey of Space and Time) data.

Strong lensing is when the light rays from a source get deflected due to intervening inhomogeneous matter distributions in the universe. This can lead to the formation of multiple images of the same source. For such a multiply-imaged system, the multiple images appear offset in time to the observer. This offset, known as the time delay, is mathematically related to the value of the Hubble Constant ($H_0$). Due to this explicit dependence, lensed sources provide an independent method - time delay cosmography - to test the Hubble tension problem in cosmology. Strongly lensed SNe Ia, specifically, are uniquely suited for time delay cosmography owing to their homogeneous and well-understood light curves and their transient and standard candle nature.

Currently under construction in Chile, the Vera Rubin Observatory will conduct the Legacy Survey of Space and Time, imaging the southern sky in six optical bands for ten years. LSST is anticipated to improve the current sample of multiply-imaged supernovae by order of magnitude, enabling per cent-level constraints on $H_0$. But finding such systems in the ground-based imaging data is a challenge. In my thesis, we explored two strategies to identify strongly lensed SNe Ia in the Rubin LSST.

Firstly, we developed a Difference Imaging (DI) pipeline to identify lensed SNe. Difference Imaging compares two images of the same sky area taken at different times and identifies sources that have varied in brightness. We tested this DI pipeline on the real imaging data from the Subaru Hyper-Cam Survey, a precursor to LSST, injected with a realistically simulated population of lensed SNe Ia. We then studied the properties of the sources detected in the difference images compared to the properties of the injected sources to determine the recovery efficiency of the employed DI pipeline and the factors affecting the efficiency.

Secondly, we investigated the suitability of the colour-magnitude diagrams as a rapid diagnostic tool for screening promising lensed SNe Ia candidates. Lensed SNe Ia, coming primarily from higher redshifts, show redder colours compared to the unlensed SNe due to the evolution of their spectra with the redshift. Using this property, we systematically studied colour-magnitude diagrams of the unlensed and lensed SNe Ia and core-collapse SNe to propose colour-magnitude diagram-based selection criteria to identify lensed SNe Ia.

Q2:Why did you choose it?

A: In the summer of 2022, I started working on this project for my summer internship. My supervisors offered to extend that summer project into my thesis. Since I liked the work, my supervisors, and the environment of IUCAA, I chose to take that opportunity.

Q3:What did you like about it?

I wanted work that involved the study of transient phenomena, sources that vary in their properties on the time scales of a few days to months. Being able to study changes in the objects of astronomical scales on human time scales is what fascinates me about transient objects. Within the transient object classes, SNe Ia are even more fascinating because they are one of the very few known standardizable candles in the universe. Strong lensing of these SNe Ia that forms multiple images adds to the complexity and value of studying such systems. The major part of my interest in my thesis topic comes from my fascination with this source of study. The rarity of occurrence and the cosmological importance of lensed SNe Ia makes me very excited that my work contributes toward the efforts to find more of them!

Apart from that, I like the primarily computational nature of work. The fact that the Vera Rubin Observatory will see its first light soon makes this work more important. During this work, I also got to collaborate and learn from others in the international LSST community, which I think opens doors to new opportunities for me.

 

Q3:Who was your guide?

A:My thesis was supervised by Prof Surhud More and Dr Anupreeta More at the Inter-University Centre for Astronomy and Astrophysics (IUCAA), Pune. Prof Bagla at IISERM supervised it locally.

 

Q4:Was Astronomy always the plan?

A:I always had the naive interest in astronomy that almost every child has at some point in their life, but I was open to other things that I found interesting. My first summer project was in quantum chemistry, inspired by the second-semester quantum chemistry course. Eventually, through Astronomy courses and summer projects, I actually decided to pursue it.

 

Q5: Did you do any internship in this field?

A:My second summer project was with Dr Aru Beri at IISERM, studying tidal disruption events (TDEs). TDEs form when a star passes very close to a supermassive black hole and gets pulled apart by the black hole's tidal force, leading to the stellar material forming an accretion disk around the black hole that emits radiation. I studied the accretion mechanisms in one such system using its spectral data in X-ray.

The third and fourth-year summer projects were parts of my thesis work. So those involved the study of lensed SNe Ia.

I also started to work on spectral analysis of X-ray binaries in the third-year summer with Dr Aru. I am continuing that work at a slower pace along with the thesis.

Q6: What are the prerequisites for this field?

A:Strong lensing theory, simulations of supernova light curves, and technical information on LSST Science Pipelines' software. My thesis serves as a good starting point if you want to learn more and find appropriate references.

 

Q7:Any mistakes that you have made and would recommend the juniors to avoid?

A:Plan your time in the thesis year wisely. I recommend combining the fourth-year summer with the thesis year and spending the summer learning the prerequisites of the thesis topic. Thesis writing also takes a considerable amount of time. Plan the work so that you can dedicate at least one and a half month to just writing. Preparing a short mid-year report around December will also help.

 A few additional points that helped me:

• Try as much as possible to ensure your supervisors are understanding and helpful. Having supportive supervisors will improve the thesis work in every single aspect. Communicate to them about the issues you face during the work and be honest with them.
• If you are doing your thesis externally, choose an understanding and equally supportive local supervisor. Keep them updated about your work. I had a system of weekly updates with my local supervisor. It may feel overwhelming to have new updates each week initially, but penning down each week's work helps keep track of your progress. 
• Interact with the other master's or PhD students in your group. Ask them about their work and discuss your work with them. You will learn a lot, both about their and your topic, through such informal discussions.

 

Q8: What are your future plans?

A: I plan to continue in academia and go for a PhD. I have not yet decided on a PhD position.