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!
As avid readers of this blog, you no doubt remember PoGOLite – a balloon-borne hard X-ray polarisation mission which is part of the Swedish National Space Board national programme for balloon and sounding rocket research at the Esrange Space Centre. After a number of frustrating set-backs (broken balloons, bad weather, …), the Crab was successfully observed in July 2013 – providing the first measurement of the polarisation of emissions in the 20 – 120 keV energy band. Technical difficulties encountered during the flight meant that the polarisation parameters could only be determined with modest precision. The relatively high polarisation fraction observed suggests an ordered environment at the emission site, and the polarisation angle is consistent with the inferred projected direction of the pulsar spin axis reported in the optical regime. This alignment suggests that polarised hard X-ray emission originates close to the pulsar. The results also demonstrated that the instrument concept was sound and indicated how the design could be modified to achieve better performance for future flights. The improved polarimeter, called PoGO+, and the associated mission paraphernalia has been taking shape during the last couple of years at KTH and at our industrial partners, SSC and DST Control.
The PoGO+ launch campaign started at Esrange in mid-May 2016. Once the polarimeter was integrated with the attitude control system and the balloon systems, integrated system tests were conducted, including pointing tests and the final scientific calibration of the polarimeter. The launch window opened on July 1st (once the Crab was sufficient displaced from the sun). Weather conditions permitted launch attempts on 9th, 10th and 11th July with the final attempt resulting in a launch on July 12th 2016 at 05:17. Launch conditions were perfect with low winds and clear skies. We were pleased to hear from OKC colleague Tanja Nymark who spotted the 150 m diameter balloon flying at 40 km altitude over her holiday home in Vesterålen, close to the Lofoten Islands in northern Norway! Once at operational altitude, observations of the Crab and Cygnus X-1 started and continued without problem during the flight. Conditions were excellent and each source was observed daily, for the duration of the almost week long flight from Esrange to Victoria Island, Canada. Science data storage units were returned to Sweden soon after the flight ended and data reduction and analysis is now in progress.
In X-ray polarimetry, performance of is often expressed in terms of the MDP – Minimum Detectable Polarisation. If a measurement yields a polarisation fraction equal to the MDP, there is a 1% probability that the measured value arises from a statistical fluctuation of an unpolarised flux. For PoGOLite in 2013, the achieved MDP for the Crab was 28%. For PoGO+, the MDP is expected to be <10% which permits a 5 sigma determination of the polarisation parameters and should also allow pulsar and nebula contributions to be separated. In contrast to the 2013 flight, Cygnus X-1 was in the hard spectral state during the flight and there-by observable by PoGO+. In the hard state, X-rays from Compton up-scattering thermal X-rays may be reflected off the accretion disk and become polarised. The polarisation parameters observed can elucidate the inclination of the system – a characteristic which is challenging to acquire by other means.
PoGOLite launch activities have been terminated at Esrange. We have been waiting to launch since 1st July, but weather conditions have not been good enough. This is very unusual. The low pressure regions which have been oscillating back-and-forth over Esrange have lead to wind conditions which are incompatible with launching a million cubic metre volume balloon. Now, at the end of July, the stratospheric winds are no longer stable enough to support a flight Eastwards towards Canada and beyond.
Needless-to-say, we are all extremely disappointed (this is a massive understatement of course!). After last year’s failed launch, we were counting on finally observing the Crab in polarised X-rays. Disassembling, repairing, reassembling, and testing PoGOLite after last year’s failed launch has required a big effort from the team. I would like to thank them all for their dedication and many sacrifices which made this year’s flight attempt a reality.
One thing is already decided. We will try again next year!
Approximately one year ago, the PoGOLite team deployed to the Esrange Space Centre located outside of Kiruna in Northern Sweden. The result of that ill-fated flight attempt is well known to readers of this blog. Time flies (which is more than can be said for our balloon) and the last year has passed quickly. Now we find ourselves back at Esrange with some 6 weeks until PoGOLite is due to be airborne once more. It is a little bizarre to be commuting back-and-forth to 67 degrees N. Just as summer finally seems to be coming to the Stockholm region, there is still snow and chilly days up North.
The past year has by no means been a quiet one for the PoGOLite team. During the Autumn, the result of an investigation into the cause of the flight failure was published. The investigation showed that a sudden change of wind speed and wind direction just as the balloon was released was primarily to blame. After licking our wounds, we started the painstaking task of completely disassembling our X-ray polarimeter, repairing damage, reassembling and testing. Much of this work was completed around the start of 2012 and since then we’ve been based in a drafty hangar at Linköping airport for tests together with the attitude control system (which keeps the polarimeter accurately aligned with observation targets), developed with DST Control AB. The team’s obsession with checking weather forecasts in order to pinpoint clear nights for tests of our star trackers lead to colleagues commenting that some of us had truly made the transition from our particle physics pasts.
On July 1st, the PoGOLite launch window will open. We stand to benefit from a 15 day long flight with corresponding multiple observations of our scientific targets – The Crab and Cygnus X-1. This is very exciting, but we’ll also be somewhat nervous… such a large balloon has never made such a circumpolar traverse before. The balloon will be carried in a Westerly direction by stratospheric winds, flying over Norway, Greenland, Canada and Russia before returning to Scandinavia where the flight will be terminated and our telescope returned to ground by parachute.
More news from PoGOLite as the launch date approaches. In the meantime, if you want to know more why not come to Merlin Kole’s licentiate seminar on May 31st: “PoGoLite: 2011 flight results and 2012 pre-flight predictions“? Although we were aloft for a short time during the 2011 flight, it was still possible to squeeze some very useful information from the data we collected and Merlin will reveal all.
While we’re on the subject of thesis presentations, I would also like to alert you to two PAMELA Ph.D. thesis presentations in the near future. Juan Wu will present her thesis, “Measurements of cosmic ray antiprotons with PAMELA and studies of propagation models“, on Friday June 1st with Fiorenza Donato from Turn University as opponent. Juan has worked closely with Antje Putze, giving this work a true OKC flavour. Laura Rossetto presents her thesis, “PAMELA measurements of high energy cosmic ray positrons“, on May 11th with Tatsuya Nakada as opponent. The particle physicists amongst you may know him as a past Spokesperson for LHCb – you may not know that he is also leading a balloon-borne positron mission called PEBS (cosmic-ray positron measurements up to the TeV scale – something for OKC, maybe?). These will be the final two PAMELA Ph.D. students at KTH. The end of an era! Of course, we’re all waiting with bated-breath for first results from PAMELA’s larger sibling, AMS. I am particularly curious to see their positron results. Now that the PAMELA rise in the positron fraction has been confirmed by an inventive use of the Fermi Gamma-ray Space Telescope, that AMS also sees the rise will not come as much of a surprise. What will be interesting, however, is the maximum energy at which AMS can reliably resolve positrons from the large background of cosmic-ray protons. The Twittersphere was recently awash with congratulatory messages as AMS passed 17 billion triggers – so far, so good. Rumour has it that something ‘interesting’ will be shown at the summer conferences. Stay tuned…
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.
The PoGOLite flight did not turn out as we had hoped. A few hours after the spectacular launch at 2 AM on Thursday 7th July it became clear that the balloon’s altitude was lower than expected. It was soon after determined that the balloon was in fact leaking and that the altitude had started to steadily decrease. Since the balloon was approaching a mountainous region it was decided to terminate the flight – a 5 day flight to Canada was no longer an option. We were, of course, extremely disappointed and frustrated. During our few hours aloft we managed to commission the polarimeter and start pointing exercises with the attitude control system. The first guide stars we selected appeared nicely centred in our field-of-view and we were eagerly awaiting the Crab rising in a couple of hours time.
Before shutting down the instrument we did manage to point at Cygnus X-1 and take some data. The pointing worked beautifully, but we were already too low to allow meaningful X-ray observations. The gondola was finally cut from the balloon around 0730 on Thursday morning in the vicinity of Kebnekaise and landed by parachute near to Nikkaluokta. Initial GPS information indicated that it had splashed down in a lake, but fortunately this was not the case. We managed to locate the gondola the day after and were relieved to find it relatively intact.
So, what happens now? The first step is to understand the status of the polarimeter, attitude control system, star trackers and electronic control systems. Today we have made an initial appraisal and first results are promising. It will take more time to form a proper understanding of the situation however.
Given the excellent performance we experienced during our few hours aloft, we are keen to fly again as soon as possible!
After a weather briefing this morning, it was decided to start the PoGOLite countdown. We currently foresee a launch around 2230-2300. Preflight tests are on-going and all is OK. We will move to the launch position in an hour or so.
POGOLite is almost ready for launch! As you can see from the photograph, the polarimeter, which once filled our lab at AlbaNova, is now dwarfed by the protective gondola and solar cell arrays. The picture was taken just before Midsummer, during a launch rehearsal. This provided us with a realistic environment to tune-up our pre-flight checklists and confirm that we can operate the polarimeter, pointing system and our satellite communication systems together with the other balloon systems.
There are a few items remaining on our ‘to do‘ list and then we’re ready to launch. How long we need to wait will depend on the weather. There are very strict requirements placed on wind speeds at the ground and at altitudes of a few hundred metres. The countdown itself takes about 24 hours once a positive weather prognosis is received. The polyethylene balloon is unpacked at the last moment. Being about as thick as a standard sandwich bag, once unpacked it so fragile that it must either be used or thrown away.
For the latest news updates, you might like to take a look at our recently overhauled web-page.
In a little over two weeks, just after Midsummer, the launch window for PoGOLite will open. In my last post, I talked about the conclusion of PoGOLite tests at Linköping airport. Since then, PoGOLite has been moved up to the Esrange Space Centre thereby marking the start of the launch campaign. Esrange is located some 40 km east of Kiruna and provides unique opportunities to launch large helium-filled balloons into the stratosphere. We are hoping to make a circumpolar navigation of the North Pole, overflying Norway, Iceland, Greenland, Canada and Russia before landing back in Sweden some 20 days later. Such a long flight will allow repeated observations of our primary science targets, the Crab and Cygnus X-1 and plenty of time to study backgrounds – which is essential for a counting rate anisotropy measurement such as polarisation. Obtaining permission to fly over Russia is a complicated business and we are still on tenterhooks waiting for the green light. If this is not forthcoming, we’ll have to cut the balloon down over Western Canada and strike out into the wilderness to recover the payload and, in particular, the valuable cargo of hard disks.
There is a lot of activity at Esrange right now with several large balloons being prepared for launch. First up are three NASA-sponsored payloads. The first two, AESOP and LEE, have flown many times before in their quest to monitor the effect of solar modulation on low energy cosmic rays. The relatively high latitude of Esrange means that cosmic rays are not screened by the Earth’s magnetic field. LEE (Low Energy Electrons), has already been launched and landed in Western Canada after 5 days. AESOP (Anti-Electron Sub Orbital Payload) will be launched as soon as weather conditions allow. The final payload in the queue is called Highwind and uses a Fabry-Perot interferometer to probe the upper atmosphere. Interestingly, LEE and AESOP have connections to my ‘other’ experiment PAMELA. You may recall that the low energy part of the well-known PAMELA positron fraction did not agree with the majority of other published balloon-borne data since these measurements were taken during a different solar configuration (polarity and activity). Measurements from AESOP collected around the time PAMELA was launched agreed with the PAMELA observations, however.
PoGOLite rounds off the summer launch programme for large balloons, with the late date chosen to maximise the angle between the Sun and the Crab. So, how large is large? Well, the balloon which will lift PoGOLite to an altitude of 40 km has a volume of a little over 1 million cubic metres – about twice the volume of the Globen Arena. Such a large balloon is needed since PoGOLite is, in reality, not so light and weighs in at about 2 metric Tonnes.
An exciting few weeks lie ahead at Esrange. On-ground tests of the polarimeter continue, with a polarised radioactive source replacing photons from the Crab. The attitude control system which keeps the polarimeter aligned to targets of interest is also being put through its paces. As PoGOLite drifts Westward, Esrange will eventually drop below the horizon and the only way to contact the balloon is through Iridium satellite links. This requires that the payload can operate autonomously, rather like a satellite. Not always so easy to test realistically on the ground!
For more details about PoGOLite, you may wish to take a look at a paper which was presented a couple of weeks ago at the 20th ESA Symposium on European Rocket & Balloon Programmes (arXiv:1106.1322).