Tag Archives: gamma-rays

Interview with Rakhee Kushwah

Rakhee Kushwah

I am a talkative girl from India. I finished my Ph.D from Indian Space Research organisation, Bangalore. It is situated in South of India. I love to travel. My love for analysing human psychology means that I also like to meet new people.
I was always fascinated by science because it has a lot of practical applications in day to day life. Thus I chose to do science. I wanted to be in the field of research because I get bored if things are the same every day. Research is about finding something new. I like drawing conclusions based on practical experiences and so I like working with instruments in the lab.
Dislikes do always follow likes because life is all about pairs of opposites. I come from a country with a large population, where researchers often don’t get their due respect because they are just one among the many people living there! This is disheartening at times. I also do not like the uncertainty tagged with this career. Hopefully, it all works out fine at the end!

What is your field of research and/or what project are you involved in at the OKC?
I am working in the Particle and Astroparticle Physics group at KTH. My research is related to space instrumentation. I am currently involved in the activities of developing a new instrument to measure polarization from Gamma Ray Bursts (GRBs).

What are your research plans for your time in Sweden?
I am learning more about detectors which I have not worked with in my previous years of research. So it is very interesting. I am attending some lectures on spacecraft engineering from which I hope to learn more about satellite technologies.

Which of your skills are you most proud of? What new skills would you like to learn in the next year?
I am good with my hands when it comes to being in the laboratory. I am more organized and tidier than most researchers when planning and executing experiments.
I am being exposed to new software tools that are needed to understand different aspects of instrument building. I would love to learn these tools and use them for my experiments.

What advances or new results are you excited about or looking forward to?
I really got excited when the latest GW detection was declared and the associated analysis was presented by many scientists. I would love to learn the details of how such a sensitive detection was made. Hopefully our GRB polarimeter can contribute more to this field.

What is the biggest obstacle that is slowing down your research field right now?
I am not good at coding so it is taking time for me to develop instrument simulation skills. As I am new to Stockholm, the weather and dark evenings make me feel tired sooner in the day than in India.

If I offered you unlimited funding right now, to be spent on something scientifically relevant, what would you use it for?
I would love to make a small X-ray detector (with integrated read out) for undergraduate students to play with. The goal would be to make it at a minimal cost so that all the universities can have this in their labs. I think my knowledge of detectors will help me build such a unit.

What’s your favorite food? Why?
I love Indian food especially because it is prepared with a combination of a variety of spices. After coming here, I have learned to prepare pasta with Indian spices and I love it.

Why did you choose the OKC?
I wanted to be in instrumentation and found this position when I was looking for such an opportunity. The people in the group here have expertise in detection of X-ray polarisation from celestial bodies. I worked on a similar topic during my Ph.D so it was nice to join OKC for continuing in the field of my interest.

How do you relax after a hard day of work?
Watching a movie or preparing a different/new food while listening to my favourite music is what relaxes me most.

What do you hope to see accomplished scientifically in the next 50 years?
I would be happy to see a fully functional fusion reactor on Earth, providing abundant energy with a small input. And it would be nice to have tourist visits to our moon.

Rakhee is a postdoc in the SPHiNX group at KTH who joined the OKC in the fall of 2017.
Thanks Rakhee!

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!

Electromagnetic Counterparts to Black Hole Mergers?

The observations of the first gravitational wave by the Laser Interferometer Gravitational-Wave Observatory (LIGO) captured the attention of the world this February, confirming the existence of  gravitational waves as well as further confirming Einstein’s theory of general relativity.
The signature of merging blackholes resulted in a flurry of scientific articles reworking theories casting out models and creating new ones. Other observatories probed the sky looking for electromagnetic signatures across all wavelengths, but nothing was seen. Well almost nothing.

The Fermi Gamma-ray Burst Monitor (GBM) claimed to see an event occurring 0.4 seconds after the LIGO event. It was a very week fluctuation in the data lasting only 1 second. SPI Anti-Coincidence Shield on board The INTErnational Gamma-Ray Astrophysics Laboratory (INTEGRAL), with a similar observing range as GBM, could not confirm the signal even though it should have seen the same event. This lead to an interesting conundrum. If GBM really saw gamma-rays from a black hole merger, many existing theories would have to be changed as it is very difficult to have a scenario where such merger can produce gamma-rays.

gamma-rays coming from merging black holes
A cartoon of gamma-rays coming from merging black holes and hitting Fermi GBM. The green histogram shows the result of our analysis. The signal is consistent with background.
Image credit: Composite NASA Goddard Space Flight Center/SXS, the Simulating eXtreme Spacetimes (SXS) project

Therefore, we teamed up with part of the GBM and INTEGRAL teams to do a thorough investigation of the event. Instead of relying on existing analysis tools, we went back to basic statistics and designed new analysis schemes for the data. We applied the tools to the data from the GBM event and could not find a signal above the normal background in the instrument. When we checked with the normal analysis tools, we could see it. Therefore, data from other weak events that were also seen by INTEGRAL were checked with both sets of tools. This was very strange and we needed to find a way to see which analysis was right.

Luckily, INTEGRAL and GBM see a lot of the same gamma-ray events. For events that both INTEGRAL and GBM agree are real events, we can predict what INTEGRAL would see using parameters from GBM data analysis. Using our method and the standard method, we found again that you get two different predictions for what INTEGRAL would see. Our method always predicted correctly the INTEGRAL signal, but the standard analysis over-predicted the strength in INTEGRAL: just like the presumed gravitational-wave counterpart! Thus, we could explain why GBM results were in conflict with INTEGRAL for the gravitational-wave counterpart and prove that the counterpart was merely a background fluctuation in the GBM data.

There will be a lot of exciting science to come from LIGO! GBM is very likely to see a coincident signal when two neutron stars collide as these events should generate copious gamma-rays. When this day comes, we must have the statistical tools to analyze the data ready so that we get the next event right. Once we know we have a signal, we can explore all the exotic theories.

– J. Michael Burgess ( jamesb@kth.se )
Article preprint: http://arxiv.org/abs/1606.00314
GBM instrument: http://gammaray.nsstc.nasa.gov/gbm/

Radioactive glow as smoking gun: cosmic explosions, heavy elements and gravitational waves

On June 3rd 2013 at 15:49 UT NASA’s Swift satellite detected an intense flash of γ -rays known as a short γ-ray burst. Follow-up observations by the Hubble Space Telescope revealed infrared emission that was present 9 days after the burst, but had faded away after 30 days. This infrared transient is likely the first ever observed example of a “macro-nova”, emission that is produced by the radioactive decay of very heavy nuclei that have been freshly synthesized in the merger of a compact binary system consisting of either two neutron stars or a neutron star with a black hole. If this interpretation is correct, the observation could have profound consequences for high-energy astrophysics, cosmic nucleosynthesis and detections of gravitational waves.

γ-ray bursts (GRBs) come in two flavors of different duration. Long bursts (longer than about 2 seconds) are produced in the death of a rare breed of massive stars, whereas short bursts (shorter than 2 seconds) are thought to result from compact-binary mergers. To date, we know 10 systems containing two neutron stars— extremely densely packed objects with masses around 1.4 times the mass of the Sun, but only about 12 kilometres in radius, and that consist predominantly of neutrons. As the stars orbit around each other they emit gravitational waves and therefore slowly spiral in towards one another until they finally merge. Such orbital decays have actually been observed2, and they agree remarkably well with the predictions from Einstein’s theory of general relativity.
Continue reading Radioactive glow as smoking gun: cosmic explosions, heavy elements and gravitational waves

Fermi/Swift GRB Symposium 2012: Polarisation and thermal emission in GRBs

The Fermi/Swift gamma-ray burst Symposium 2012 was held in Munich 7-11 May 2012.

Recent results on the prompt and afterglow emissions in gamma-ray burst were discussed at the Fermi/Swift gamma-ray burst Symposium 2012 which was held in Munich 7-11 May 2012.

Among the most important issues presented was the recent gamma-polarisation measurement with IKAROS-GAP. Significant degrees of polarisation in several bursts have now been detected. In particular, the change in polarisation angle was significantly detected. It was speculated that this is due to variation in emission patches in very narrowly collimated jets.

Another point which gained a lot of attention was on the existence of thermal components in GRB spectra. Previously the leading model for the prompt emission has been optically thin synchrotron emission. However, amounting observational evidence is showing that the photosphere in the relativistic flow is responsible for, at least a part of, the observed emission. Also recent progress in the theory and numerical simulations of relativistic jet was presented, and again thermal emission seems to be unavoidable.

Featured on www.nature.com/
Conference website

Gamma ray spectrum of GRB110721A
Gamma-ray spectrum of GRB110721A which will soon be presented in a Fermi publication led by Magnus Axelsson. In addition to the dominant broad component, there is a bump at lower energies which is likely emission from the photosphere. The spectrum cannot be explained by synchrotron emission and thus disproves the long-held view that synchrotron emission alone can explain GRB spectra.

The Fermi symposium 2011: AGNs, pulsars and gamma ray bursts

The results presented at the III Fermi symposium in Rome reflected, in particular, what a magnificent instrument the Fermi LAT is for observing active galactic nuclei and pulsars. The 2 source catalogue 2FGL was presented and will soon be released with 1888 sources. Much attention was given to the blazar 3C454.3 which has been monitored since the launch and has undergone a series of very bright outbursts. The multiwavelength analysis by Stefan Larsson revealed a far more complex behaviour than expected in the simple picture we had of AGN jets before the launch of Fermi. The discovery of spectral breaks at GeV energies was nicely interpreted by the former Stockholm astronomer Juri Poutanen and collaborators as a result of gamma-ray absorption via photon-photon pair production on He II Lyman recombination continuum and lines within the broad-line region.

The Aula Magna at La Sapienza, Rome

It was also made clear that all models we have for description of the high energy emission around pulsars are, more or less, wrong. Fermi has told us for certain that the emission is from high altitudes in the outer magnetosphere; Fermi has killed the polar cap model and the classical TPC, while the other models are in need of modifications. Continue reading The Fermi symposium 2011: AGNs, pulsars and gamma ray bursts

Dissipative photospheres in gamma-ray bursts

The initial stage of a gamma-ray burst. The core of the star has collapsed deep inside the star. A black hole has formed within the star, and within a few seconds launches a jet of matter away. (Larger image) (Credit: NASA / SkyWorks Digital)
The origin of the emission during the prompt phase in gamma-ray bursts is still a mystery. One suggestion is that the photosphere of the relativistic jet plays an important role. Indeed, recently the Fermi gamma-ray space telescope has made interesting observations of the gamma-ray spectra of several GRBs which show a clear signature of a photospheric emission component.

Another recent development in the field is the realisation that energy dissipation naturally should occur close to the jet photosphere. Such theoretical predictions for kinetic outflow as well as for Poynting flux dominated outflows are confirmed by numerical jet simulations.

In a recent paper, that has just been accepted for publication in the Monthly Notices of the Royal Astronomical Society, we present observational evidence for the onset of such subphotospheric dissipation. This is clearly seen during the prompt phase in the exceptionally bright burst GRB090902B. Initially the main spectral emission component is close to a Planck function, expected for a photosphere. Later this component broadens into a spectral shape that is typical for GRBs. This illustrates that the photosphere emission can have a variety shapes. This is indeed what is expected if the dissipation pattern in the jet changes and gives rise to subphotospheric heating. This we show through numerical simulation of the dissipation processes and argue that the change in spectral behaviour as being due to a decrease in the outflow Lorentz factor. This leads to a substantial part of the kinetic energy being dissipated at optical depth of approximately 10. This causes the change in spectral shape since the photons do not have time to thermalise into a Planck function.

These observations show that the photosphere emission indeed is important in GRBs and can even be a common feature.
The spectral shape of the photospheric emission can have a variety of shapes and not only a Planck function shape.
The identification of the photosphere as cause of the main emission in GRBs provides us a way to study the physics of the relativistic jet. This allows us to learn more about these enigmatic events that are the largest explosions in the Universe.

If you want to read the original paper, check this out: http://arxiv.org/abs/1103.0708

Collaboration between OKC and Dublin Institute of Advanced Studies

Last week, Jan Conrad and I spent a couple of days in Dublin, Ireland, invited by Felix Aharonian and his colleagues at DIAS. We had interesting dicussions around many topics, the most interesting being the possibility to obtain a lower energy threshold for gamma-rays at imaging air Cherenkov telescopes, for instance by using large mirrors at high altitude. This would be of importance for the dark matter search (that we are experts on at OKC) and for GRBs and AGNs (where Dublin has very good expertise and we also have activity in OKC). We decided that this topic would be interesting enough for both parties that we should try to arrange a small workshop together to investigate the scientific aspects of such a detector. Continue reading Collaboration between OKC and Dublin Institute of Advanced Studies