Interview with Fei Xie

An image of Fei Xie with a yellow sweater and black glasses.
Fei Xie
My name is Fei Xie. I am from China and I got my PhD at the Chinese Academy of Science in Beijing.

What is your field of research and/or what project are you involved in at the OKC?
I mainly work on instrument simulations, like detector optimization, on-orbit background estimation, instrument performance simulation, etc. I am working in the SPHiNX group, a satellite for GRB polarization measurements. Now my interest is in polarization measurements at high energies. It’s an area we don’t know much about at this moment. It is challenging but interesting.

Which of your skills are you most proud of? What new skills would you like to learn in the next year?
My coding and Geant4 are quite good. I would like to learn more about the electronics for a better understanding of the instrument in the future.

What is the biggest obstacle that is slowing down your research field right now?
My research relies on the project, so it is not easy to find a long term and stable position.

What’s your favorite food? Why?
I like spicy food as I was born in a family that are good at cooking spicy foods.

How do you relax after a hard day of work?
Movies and reading are my favorite ways to relax.

Fei is a postdoc in the SPHiNX group at KTH who joined the OKC in May 2016.
Thanks Fei, maybe we should all eat more spicy food to keep warm this winter!

Interview with Hoi-Fung David Yu

Hoi-Fung David Yu stands in front of a chalkboard which says F=ma.I am from Hong Kong. I studied Physics and Astronomy at the University of Hong Kong for my BSc and MPhil degrees. Then I did my PhD study at the Max Planck Institute for Extraterrestrial Physics in Munich, Germany. I chose to be a scientist because I am fascinated by the power of science to describe the physical laws of nature. I like to be a scientist to be involved in the scientific progress. However, I dislike to spend most of my time in the lab / office and being disconnected from other people. That’s why I also spend a lot of time writing science blogs and making science videos on social media to promote science. I co-authored a popular science book on astronomy that was published this July in Hong Kong.

What is your field of research and/or what project are you involved in at the OKC?
My field of research at the OKC is gamma-ray bursts (GRBs). I work at the particle and astroparticle physics group in KTH. The main aim is to identify the dominant emission mechanism of gamma rays from GRBs. I plan to work on a few projects on this topic in Sweden with my supervisor Felix Ryde, who is himself an expert in GRB emission mechanisms.

Which of your skills are you most proud of? What new skills would you like to learn in the next year?
I am proud to be a observer and a data analyst. Observation is the key to science. I plan to learn more about Bayesian data analysis in the next year (although I’ve already been studying this for years). I am glad to have shown that a conventional theory of GRB is incorrect during my PhD studies. I am now excited to look at various possibilities which could explain the data better. I hope that my result is an exciting advancement in the field of GRB research.

What advances or new results are you excited about or looking forward to?
The biggest obstacle slowing down my field of research is that there are no new gamma-ray burst dedicated space missions at the moment. When all the current operating gamma-ray space telescopes are retired we will have no GRB data to continue the research.

What’s your favorite food? Why?
My favourite food is fish, vegetables, and rice, but I don’t know why 🙂

Why did you choose the OKC?
I chose OKC because OKC has a very nice area for research. With all the researchers from different fields staying together in the same building, one could easily get new ideas and get to know the latest advancements across different research fields.

How do you relax after a hard day of work?
I go bouldering after work to relax.

What do you hope to see accomplished scientifically in the next 50 years?
I hope in the next 50 years that humans have solved global warming and begun interplanetary travels.

Hoi-Fung David is a postdoc who came to the OKC in March.
Thanks Hoi-Fung David and keep up the great science communication!

Interview with Mette Friis

​Hej! My name is Mette. I grew up just north of Copenhagen, in a town whose library was surprisingly well equipped on popular astronomy books. I read a lot, and was particularly fascinated by a book series on the Voyager missions. I thought it was so amazing that we were able to send missions to other worlds, and from the age of ~ 9, I knew I wanted to be an astronomer. I ended up never working on planetary physics, but I did go on to study astrophysics.

I started studying at Copenhagen University, and went on to do my PhD at the University of Iceland. It’s a very tiny research group but it’s a beautiful place to live and my advisor always had a lot of time for me.

I did my PhD work on gamma-ray burst afterglows, e.g., studying the gas and dust in the vicinity of the burst using the GRB as a background lighthouse. A specific interest of mine from that time is dust extinction curves at high redshift and how we use them in research (incorrectly accounting for dust extinction can lead to the wrong physical conclusion and I think we, astrophysicists, tend to neglect this contribution).

What is your field of research and/or what project are you involved in at the OKC?
At the OKC I am currently working with the PoGO+ project, finishing up the interpretation of our observations. PoGO stands for Polarized Gamma-ray Observer and is a Compton polarimeter built here in Stockholm and flown on a balloon. The PoGO measurements are the first observations of polarization in the hard X-ray band (energy range 20-160 keV) and provide new data for modelling the geometry of the Crab pulsar and nebula as well as the black-hole binary Cygnus X-1. Our Crab results have been published (see the KTH press release) and I very much hope that I can say the same about Cygnus X-1 soon. I can promise some very interesting results!

What do you like/dislike about being a scientist?
I do not have much time left of my contract in Sweden, so my overall goal these days is to get the paper submitted and then I am really focused on improving my teaching skills, a part of this job which I have found I really like. I am giving some lectures in the KTH course on astro-particle physics. Speaking of leaving Sweden soon, this is definitely the part I dislike about being a scientist, the many years of job insecurity, moving from place to place. I think I am ready to call some place home!

Which of your skills are you most proud of? What new skills would you like to learn in the next year?
I am actually very proud of my ability to lead a ‘job free’ life when I am not at work. I think it is very important for the brain to leave work behind and also focus on other parts of life and I think it makes me a better scientist. I also think I am quite good at forming the big picture in my head of a particular problem or task, to see what is important and what is less so. Joining the PoGO team just 6 months before launch last year really trained my ability to hit the ground running and sort through information getting to the heart of the problem.

I would really love to be a better programmer, but most of all I want to learn to like programming! I am sure I could grow to have a better relationship with my code if I just find the right approach.

What new results are you excited about or looking forward to?
I am looking forward to the launch (and results) of future high-energy polarimeters such as IXPE! PoGO really just scratched the surface of what we can discover, for instance, about supermassive black holes. As my colleague loves to say, polarimetry opens a whole new window onto the sky, and while this has been open for a while in other energy ranges, we have so little data at X-ray energies.

What’s your favorite food?
My favorite food is most things my father cooks, especially his saltimbocca (any Italian will probably say he cooks it all wrong, but I think it is the best). I also have a very large sweet tooth, and love cake and ice cream (I also love to bake!).

Mette is a postdoc in the PoGO+ group at KTH.
Thanks Mette and enjoy the sweets!

Interview with Luca Visinelli

Hej! My name is Luca, 34, Italian. I’ve wanted to be a scientist since I was 5, at that time I was fascinated by prehistoric animals, to the extent that in high school I chose the scientific specialization (yes, in Italy you make such life choices at 14!!) to become a biologist. Then, two things occurred: 1) I had a great high-school physics teacher that made me love it, and 2) at biology classes I always pushed to get the reason behind things, but the reason for biology is chemistry, whose reason is physics, so at the end of high school the choice of the University major was “only natural.” I studied physics both at the University of Bologna (bachelor) and at the University of Utah (master-PhD).

What is your field of research and/or what project are you involved in at the OKC?
With colleagues, I’m broadly working on various aspects at the interface of particle physics and cosmology, from the theoretical physics perspective. I work on models of dark matter, inflation, and recently also on primordial black holes. In the past, I have also studied some aspects of neutrino oscillations.

What do you like/dislike about being a scientist?
Being a scientist is the best choice I could take. It’s so satisfying to have the freedom to pursue personal interests and to get in touch with great people from the whole world. On the other side of the coin, the job market is narrow and the job is very time-demanding, although this second point is relieved by the fact that you pursue what you like.

What are your research plans for your time in Sweden?
Get out as many papers as possible and get in touch with the largest number of people. Sweden, OKC, and my supervisor Katherine Freese have given me a great opportunity to meet the best people in the world that actively work on my topics of interest, so I am both working on the interests I mentioned before and getting new ideas to tackle together with old and new colleagues.

Which of your skills are you most proud of?
Let me mention two, stubbornness and being very good at solving equations. So far, I have always managed to combine these two skills to find a strategy to solve most given problem. I stress that stubbornness is a very precious quality, since problems might take weeks or even months before they are solved.

What’s your favorite food?
Pizza wins, but honorable mentions go to räksmörgås and anything that accompanies the fikapaus.

What advances and/or new results are you excited about?
Even if I don’t work on it, I am thrilled when I read about the steady progresses in neutrino physics. Neutrinos were postulated in the 30’s and discovered in the 50’s, yet we still know so little about them, for example we don’t know their mass and we can’t even answer the question “are neutrinos their own antiparticle?” We could soon be able to answer such a question, and the solution might even carry new physics with it.

Why did you choose the OKC?
I loved the idea of working with my current supervisor on current topics, plus I loved Stockholm since I visited it during high school (I have a Vasa replica at home in Italy from the period). I am very lucky since all of the students and postdocs working in my group are excellent colleagues and friends. It’s a great environment and coming to work is joyful every day.

What do you hope to see accomplished scientifically in the next 50 years?
Frankly, I hope for clean environment solutions for our energy needs, in a place where self-driving cars have drastically reduced accidents, meat mass production has lowered, diseases are checkable way beforehand and possibly cured.

How do you relax after a hard day of work?
Albanova gym, the training staff is very passionate and helpful. I also enjoy walking around Stockholm.

Luca is a researcher (forskare) who came to the OKC in September 2016.
Thanks Luca and enjoy the fikabröd!

Extreme-Gravity Stars

The lives of massive stars are characterized by companionship: these stars are almost always found in gravitationally bound pairs. As such massive binaries evolve further, their cores run out of nuclear fuel and the stars can explode as supernovae, leaving behind in their centers either a neutron star or a black hole. In most cases such an explosion would be fatal for the binary, and disrupt it. In some cases, however, the final phases of binary stellar evolution can produce two compact objects -either white dwarfs, neutron stars or black holes– in tight binary systems.

Compact objects in binary pairs are driven closer and closer together as their system loses energy through interactions and gravitational radiation. The resulting merger of these types of objects is thought to be responsible for some of the most energetic events in the Universe including Type Ia supernovae and short duration gamma ray bursts.

The Laser Interferometer Gravitational-wave Observatory (LIGO) has made it possible to observe the gravitational waves emitted during mergers of compact objects. Since the beginning of data collection with the advanced instrument the LIGO observatory has detected gravitational waves from three different mergers of black hole pairs. Each of these detections was surprising because they involved a population of black holes that had not been observed before : black holes with masses of a few tens of solar masses. Scientists are also anticipating that LIGO will detect merging neutron stars.

A photo of Stephan Rosswog
Stephan Rosswog, Oskar Klein Centre, Stockholm University

Compact binary systems are really a cornerstone of modern astrophysics. Once the merger of a neutron star binary is detected in gravitational waves and electromagnetically, this will tell us about General Relativity, give us hints on how and where such binary systems form and —maybe most surprisingly— it may answer questions about nuclear physics that cannot be answered otherwise. This is maybe the most fascinating part of this rich story. — Stephan Rosswog

Connecting the sources of gravitational waves with phenomena that scientists are already familiar with, like supernovae and gamma ray bursts, requires that we observe the electromagnetic counterpart to the gravitational wave event. This is a challenging task with LIGO in its current state because the detector isn’t able to localize an event very precisely so follow-up searches with optical (and other wavelength) telescopes must search large areas of the sky for a new transient source.

A conference on The Physics of Extreme-Gravity Stars took place in Stockholm in June 2017.

Header image is from a simulation of two neutron stars merging, credit to Stephan Rosswog.

Results released from first 34 days of the XENON1T experiment

Understanding all the detailed physics going on inside the world’s most sensitive dark matter detector is a great challenge to work on. — Bart Pelssers, PhD student, Stockholm University, Oskar Klein Centre

The first results have just been released from XENON1T (“Xenon One Ton”), the most sensitive dark matter detection experiment in the world. Dark matter has not yet been detected but these results push the limits for the detection of a specific kind of dark matter particle (Weakly Interacting Massive Particles) lower than those from previous experiments.  This is after collecting data for just 34 days.

This experiment consists of a liquid xenon central detector surrounded by ultra-pure water which shields the detector. When particles collide with the xenon nucleus it emits light and electrons that the experiment can detect. Xenon can also be made very pure and it is dense enough that the inner part of the xenon nucleus is almost completely isolated from radioactivity from the detector itself as well as outside.  The XENON1T experiment is located underneath a mountain in the Gran Sasso Underground Laboratory in Italy in order to shield the detector from cosmic rays.

The XENON collaboration contains scientists from 10 different countries, including a number of Oskar Klein Centre researchers. Given these results the dark matter community will be watching this experiment closely to see what it finds after analyzing a larger amount of data. In fact, more than 60 new days of data have already been recorded!

Note how quickly the sensitivity of these experiments increases — the last result from XENON100 was made with 100kg of xenon and a detector that could fit in a living room while the new one is suspended in a three-story tank of water. One should also take a second to look at the green band– that shows the region the upper limit would be in 68% of the time. It is huge! That is because we are looking for very rare events, and so any statistical fluctuation or misestimated background could move our result a lot. That we find a result like the one we show is a good indication that we already understand our detector quite well. — Knut Morå, PhD student, Stockholm University, Oskar Klein Centre

The spin-independent WIMP-nucleon cross section sensitivity limits as a function of WIMP mass at 90% confidence level for this run of XENON1T (in black). Results from previous experiments are shown.  The 1- and 2σ sensitivity bands are shown in green and yellow.
The spin-independent WIMP-nucleon cross section sensitivity limits as a function of WIMP mass at 90% confidence level for this run of XENON1T (in black). Results from previous experiments are shown. The 1- and 2σ sensitivity bands are shown in green and yellow.

Dr. Garrelt Mellema elected to the Royal Swedish Academy of Sciences

A picture of Garrelt Mellema smiling.
Garrelt Mellema. Photo: Stockholm University

Dr. Garrelt Mellema, Professor of Astronomy at Stockholm University, has been elected to the Royal Swedish Academy of Sciences as a Swedish member in the class for astronomy and space science. Members of the Academy of Sciences are chosen for their outstanding research contributions. Dr. Mellema’s research seeks to understand how light from the first stars and galaxies spread through the early Universe and ionized the gas it contained.

Garrelt says, “I feel honoured and am very excited to join the Royal Swedish Academy of Sciences and to participate in their activities.”

The Royal Swedish Academy of Sciences is an organization dedicated to promoting the sciences and strengthening their influence in society. The Academy maintains 450 members from Swedish institutions and 175 members from institutions outside Sweden.  Academy members’ expertise spans the range of natural and social sciences.

First multiply-imaged Type Ia supernova discovered

Gravitational lensing, the bending of light which travels near massive objects, can result in distortion, magnification, and even multiple images of the original light source. The closer the light travels to the massive object the stronger the lensing effect. The magnification that results from this process allows scientists to observe objects which would normally not be visible due to their distance and brightness.

Researchers at the Oskar Klein Centre, and their international collaborators, have observed for the first time a type Ia supernova that is strongly gravitationally lensed producing multiple images. The light from the original supernova explosion (at a redshift z=0.4) is bent as it travels through a galaxy (at a redshift z=0.2) which is almost perfectly aligned between the supernova and the Earth. This alignment produced a magnification of the light from the supernova as well as four images of the source.

This composite image shows the gravitationally lensed type Ia supernova iPTF16geu, as seen with different telescopes. The background image shows a wide-field view of the night sky as seen with the Palomar Observatory located on Palomar Mountain, California. The leftmost image shows observations made with the Sloan Digital Sky Survey (SDSS). The central image was taken by the NASA/ESA Hubble Space Telescope and shows the lensing galaxy SDSS J210415.89-062024.7. The rightmost image was also taken with Hubble and depicts the four lensed images of the supernova explosion, surrounding the lensing galaxy.
This composite image shows the gravitationally lensed type Ia supernova iPTF16geu, as seen with different telescopes. The background image shows a wide-field view of the night sky as seen with the Palomar Observatory located on Palomar Mountain, California. The leftmost image shows observations made with the Sloan Digital Sky Survey (SDSS). The central image was taken by the NASA/ESA Hubble Space Telescope and shows the lensing galaxy SDSS J210415.89-062024.7. The rightmost image was also taken with Hubble and depicts the four lensed images of the supernova explosion, surrounding the lensing galaxy. Credit: ESA/Hubble, NASA, Sloan Digital Sky Survey, Palomar Observatory/California Institute of Technology

Type Ia supernovae are thought to always have the same brightness when they explode and thus have been used to measure distances in the Universe. Distance measurements to a large number of Type Ia supernovae led to the discovery that the expansion of the Universe is accelerating. The team will now use the difference in the time it takes the light from each of the four images of the single supernova to measure the current expansion rate of the Universe, the Hubble Constant.

These results are published in the journal Science in an article titled, iPTF16geu: A multiply-imaged gravitationally lensed Type Ia supernova.

For more information please contact:
Ariel Goobar ( ariel@fysik.su.se)
Rahman Amanullah (rahman@fysik.su.se)

VR Grant to Project “GREAT”

In September 14, 2015 gravitational waves (GWs) were detected for the first time: the LIGO-VIRGO team monitored the “chirping signal” and subsequent ring-down from the merger of two black holes. This watershed event confirmed a 100-year-old prediction of Einstein’s General Theory of Relativity; even more importantly, it opened up a completely new channel to observe the Universe. GW astronomy allows us to probe how the strongest gravitational fields warp space-time, how ultra-gravity binary stars contribute to the heaviest elements in Nature, how such binary systems form and, eventually, how their mergers can be used as probes of the expansion history of the Universe. These exciting prospects, however, hinge on the detection of the electromagnetic counterparts of the GW sources.

Bidrag till forskningsmiljöer (or Research Environment Grant) is a new Vetenskapsrådet (VR) initiative which aims to encourage larger research collaborations than are normally supported. Joining together for this proposal were Ariel Goobar and Hiranya Peiris from the Stockholm University Physics department along with Jesper Sollerman and Stephan Rosswog from the Astronomy department.

Hiranya Peiris, Jesper Sollerman, Stephan Rosswog, and Ariel Goobar are standing on one of the walking bridges between the two wings in the AlbaNova building.
From left to right, Hiranya Peiris, Jesper Sollerman, Stephan Rosswog, and Ariel Goobar.

With this newly VR-funded GREAT (Gravitational Radiation and Electromagnetic Astrophysical Transients) research environment they plan to carry out end-to-end simulations of the electromagnetic signals from scenarios involving mergers of compact objects accompanied by emission of gravitational radiation. Based on these simulations, they will optimize search strategies and perform searches for electromagnetic counterparts of GW events in leading time-domain astronomical surveys.

KTH Theoretical Particle Physics Joins the OKC

Welcome to the newest members of the Oskar Klein Centre : the KTH Theoretical Particle Physics Group!

The group conducts research in physics beyond the Standard Model, including neutrino physics, new dark matter particles, primordial black holes, and LHC phenomenology.

Group members include Tommy Ohlsson, Mattias Blennow, Florian Kühnel, Sofiane Boucenna, Stella Riad, Stefan Clementz, and Håkan Snellman.

The group is a part of the Department of Physics at the KTH Royal Institute of Technology. Their offices are on the 4th floor of AlbaNova.

A group of theoretical particle physicists standing on one of the walking bridges in the AlbaNova building.
The KTH Theoretical Particle Physics group.

Cosmology, astrophysics, astroparticle physics