 | Dr Jane Lixin DAI Assistant Professor of Department of Physics
|
Q: What do you think are your most significant research accomplishments, and what has been the impact of your research?
A: I have been mainly working on black hole astrophysics and in the past few years focusing on an astronomical phenomenon called tidal disruption events. In these events, stars are torn apart by the gravity of black holes, and very luminous flares are produced as the stellar materials are swallowed by the black holes. Astronomers are very interested in these events, since they can give us precious chances to observe the supermassive black holes which are usually too dark to be seen.
I have been developing theories and conducting numerical simulations to explain these events. In particular, I have proposed a ‘unified model’ giving a solution to one of the biggest puzzles in the field : why some such events shine brightly in optical wavebands while the others only shine in X-rays. I found it is like what is described in the ancient story about ’the blind men and the elephant’. From certain viewing angles, we can see the X-rays produced at the very center as stellar gas falls into the black hole. But from other angles, the accretion site is veiled by a lot of winds, so the X-rays are converted to optical emissions. These works are useful for answering many questions posed by the observations of these events and understanding their underlying physical mechanisms. They are also useful for studying black hole astrophysics in general, such as how black holes grow and, in the process, release huge amounts of energy in forms of light, wind and relativistic jets.
Q: Please give a brief description of 1 - 2 ongoing research projects that best reflect your visions in the scientific field.
A: In the old days, theoretical astrophysicists mainly conducted calculations using pen and paper, which allowed them to give good, qualitative or first-order descriptions of many astronomical phenomena. These days, for many ‘theorists’ such as myself, we often rely on numerical simulations to obtain more detailed results or deal with problems which are too complicated to be calculated analytically. One main project of mine is to develop and utilise a state-of-the-art numerical code to calculate how gas, radiation, and magnetic fields interact close to black holes under general relativity. This work allows me to calculate the structure and geometry of the gaseous accretion disks in accreting black hole systems such as tidal disruption events, X-ray binaries, and exotic transients.
I also realise that we theoretical astrophysicists can sometimes spend too much time designing more and more complex simulations but forget that it is very important that our results need to be tested by observations. Therefore, these days I also put more attention to predicting the emissions produced from my simulations. For example, I have been working in a newly developed area called X-ray reverberation, using which we can probe the structure of the disk and the parameters of the black hole from the ’echoes’ produced when X-rays are reflected by accretion disks. With this technique I can then make predictions of my disk simulations that X-ray observers can directly test with powerful, modern telescopes.
Q: What is the most important question you want to address?
A: Recently a few big discoveries have been made about black holes, such as the detection of gravitational waves by LIGO and the black hole shadow image taken by the Event Horizon Telescope. These discoveries are very exciting. At the same time, black hole astrophysicists know what has been tested and understood so far is just the tip of the iceberg. There is still so much about black holes that we do not understand.
I am particularly interested in performing numerical simulation to study the the realistic structure and emission of black hole accretion disks. Early theories say the gas either just forms a paper-thin annulus or some simple torus structure around the black holes. Nowadays we know these simple models do not give the complete picture. For example, some black hole accreting systems also produce coronae, which are X-ray photons with fast variability in energy bands much higher than expected while some accretion disks also eject a lot of materials and launch winds. All of these important features are not captured yet in the existing black hole accretion disk models. I hope that I can push forward the theoretical understanding in this area and test my theoretical work with the new observations from the next-generation telescopes.
Q: Where do you see yourself in five years/ten years? What do you want to accomplish the most?
A: Among the universities in Hong Kong, HKU has the most renowned astronomy programme and has a tradition of being very strong in high-energy astrophysics. I would like to contribute to reinforce this strength by building up a bigger and stronger group performing cutting-edge research. Regarding the research areas, so far I have become a well-established theorist in the field of tidal disruption events. I will work towards making a broader impact in black hole astrophysics and other topics in astronomy.
Q: What are the challenges you are facing?
A: As scientists we constantly need to listen to talks, collaborate with people and broadcast our results to make a bigger impact. It is a bit challenging to do this in Hong Kong, since Hong Kong is a small place with only a few astronomers and astrophysicists, and we generally do not have a lot of visitors. I have been trying hard to actively initiate and maintain collaborations. This is not easy, especially when the pandemic makes it impossible to pay physical visits to my collaborators. Another challenge that I have experienced is recruiting the best graduate students and postdocs. Maybe one good strategy that can benefit everyone is to establish more prize fellowships. For example, the new HKU-PS PhD Fellowship is a great one and I already have a PhD student attracted to HKU under this scheme.
Q: Who has influenced you the most?
A: It is hard to pinpoint one name. There are a few senior mentors and collaborators who have influenced how I do science, including my PhD supervisor Professor Roger BLANDFORD at Stanford University (who is also a Shaw Laureate). Being a part of the astronomical community has also shaped me as a person. Most of the astronomers I have interacted with are proactive, hard-working, open-minded and extremely resilient. I think we can always learn a bit from everyone, sometimes even in unexpected ways.
Q: How would you go about motivating yourself when you are going through a low point?
A: Actually I do not have a fixed set of strategies and I am not even sure I am particularly good at dealing with low points. Sometimes I spend time in nature, walking or swimming. Sometimes I talk to my collaborators or my group members. I also think that it is quite natural for scientists that our ideas often do not work out or some project can get stuck sometimes. On such occasions there is really not much one can do but to stay true to one’s own feelings, and think about possible ways to make things work or switch to a new project.
Q: Can you tell us more about your research group? What are the roles and the missions?
A: I am fortunate to be able to form a sizable group of good students and researchers, especially given I started at HKU not long before COVID-19 started. Currently I am hosting a prize postdoctoral fellow (jointly with the Niels Bohr Institute), 3 PhD students all under HKPF/HKU-PS Fellowships, and 2 MPhil students. The group is also very international with members from HK, mainland China, Denmark, and France. Soon we will have another postdoc from India joining us. It has been a fulfilling experience for me to mentor and interact with them.
My group members are working on different yet complementary topics. For example, the postdoc works on cosmology and dynamics of stars. Some of my students work on simulating black hole accretion disks and jets, and some work on translating the simulations to what we can observe. Now my group members have developed their own unique skill sets, so they also effectively collaborate together on some projects. They also share their progress, achievements and frustrations with each other like a support group. I think this is important to keep the momentum going. As one of my mentors said before, ’people have to be happy and feel supported before they can be productive.’