Tag Archives: PoGOLite

From PoGOLite to PoGO+

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.

The PoGO+ flight path
The PoGO+ flight trajectory between the Esrange Space Centre, Sweden, and Victoria Island, Canada. The flight lasted approximately 1 week with observations conducted from an altitude of 40 km. Courtesy: SSC.

 

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.

The PoGO+ campaign team
The PoGO+ campaign team – from left to right: Jan-Erik Strömberg (DST Control), Nagomi Uchida (Hiroshima Uni.), Christian Lockowandt (SSC), H-G. Florén (Stockholm Uni.), Mark Pearce (KTH), Victor Mikhalev (KTH), Hiromitsu Takahashi (Hiroshima Uni.), Maxime Chauvin (KTH), Mette Friis (KTH), Takafumi Kawano (Hiroshima Uni.), Mózsi Kiss (KTH), Thedi Stana (KTH).

 

Links:

PoGOLite mission design (Experimental Astronomy)

PoGOLite calibration (Astroparticle Physics)

PoGOLite results on the Crab (MNRAS Letters)

PoGO+ design (Astroparticle Physics)

PoGO+ on Rymdkanalen

 

-Mark Pearce (pearce@kth.se)

PoGOLite landed!

After a pioneering circumpolar journey lasting almost 14 days, the PoGOLite flight ended on the Siberian tundra. The gondola was cut from the balloon in the early hours of 26th July and touched down by parachute approximately 1 hour later. The gondola landed near, but luckily not in, a lake (this seems to be a recurring theme for us…). The landing site was close to the Siberian city of Norilsk which houses a large nickel and copper mine, as well as good infrastructure for a helicopter-based recovery of the gondola.

PoGOLite flight path 2013
The path followed by PoGOLite during the ~14 day long flight.

Photographs provided by the Russian recovery team show that the gondola is in good shape. Recovery operations are still on-going with the ultimate aim of returning the gondola to Stockholm once customs issues are solved – hopefully during the next couple of weeks. While it was hoped that PoGOLite would make a full circumpolar transit and return to Scandinavia, the stratospheric winds pushed the gondola too far to the North. Continue reading PoGOLite landed!

PoGOLino successfully launched

Last Wednesday at 17:20 a balloon was successfully launched from Esrange in Northern Sweden to an altitude of 30.5 km. This balloon was carrying, besides test equipment from SSC, a 14 kg spin-off instrument from PoGOLite called PoGOLino.

Unlike the future planned PoGO and the currently flight-ready PoGOLite instrument, PoGOLino is not built for measuring the polarisation of X-rays emitted by point sources such as the Crab. Rather, it is built to measure the atmospheric radiation which will be the main source of background for the PoGOLite measurements – more specifically it is designed to measure the neutron flux in different energy bands. It does this using LiCAF
scintillators which contain 6Li to capture neutrons, the subsequent decay of the lithium produces a characteristic signal we can measure using a photomultiplier.

The reason we built this experiment is that in the high radiation environment between 30 km and 40 km altitude in which PoGOLite will fly, neutrons are expected to form the main
background. This is due to their ability to move through the PoGOLite anticoincidence shields undetected and scatter in the plastic scintillator array, producing a signal which is
indistinguishable from that expected from a Compton scattering X-ray. Especially at the high latitudes where PoGOLite is going to fly this summer, this neutron background is relatively large.
The exact magnitude is however unknown due to a lack of measurements. On top of that the neutron background is directly correlated to solar activity since the neutrons are secondary particles originating from cosmic rays. A direct measurement of the background now will therefore be very useful for the analysis of the PoGOLite data after its flight this summer. The PoGOLino data will also be used to verify existing software models used to simulate neutron production in the atmosphere.

For the PoGOLite Collaboration, a successful flight seems to indicate that after a cancelled launch in 2010, a balloon leak in 2011 and a summer with weather which made launching impossible in 2012, our luck seems finally to be changing!

Merlin Kole, PhD student – merlin@kth.se

Bad news for PoGOLite once again.

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.
PoGOLite during tests at Esrange
PoGOLite during tests at Esrange in June
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!

 

 

A second chance for PoGOLite and some news from PAMELA

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.

PoGOLite preparations
Preparing to roll out the X-ray polarimeter for another night of observations at Linköping airport. The long tube is a star tracker - the subject of the test. Mark at the wheel, Mozsi on the hook and MSc student Anders lending a helping hand.

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.

Gondola
Hanging the gondola at Esrange. Can we have a larger OKC sticker, please?

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…

 

Interview with a new Oskar Klein Fellow

Elena Moretti is the first of the about 300 applicants who was selected to become an Oskar Klein Fellow this year. She comes from a little country-side town, called Cartura, on the south of Padua in Italy, where she graduated in physics in 2006. She got her PhD in Trieste where she worked with the AGILE and Fermi experiments on GRBs. She developed a method that was used to calculate the flux upper limits on the GRB emission that was used in both experiments. In 2010 she moved to Stockholm working as a postdoc at the KTH. We ask her to tell us more about herself and the work she will be doing at the Oskar Klein Centre.

Congratulations Elena! You have been offered an Oskar Klein Fellowship. How does it feel?
It feels good! It gives me the opportunity to develop my newborn interest in the polarimetry field. Wen I came here 2 years ago I was working only in the high energy astrophysics field with the 2 gamma-ray experiments Fermi and AGILE. After one year a new interest was tickling me: PoGOLite. I started to work on it as a “side job” on my spare time….well I guess that would change soon.
Continue reading Interview with a new Oskar Klein Fellow

PoGOLite flight cut short

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.

Happy faces.
The first data download from the polarimeter. Happy faces!

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.

Landing...
PoGOLite after flight termination.

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!

 

 

 

 

 

 

 

PoGOLite launched!

At 01:57 on Thursday 7th July 2011, PoGOLite was successfully launched from Esrange. It was a beautiful sight. About an hour after the launch we are now at an altitude of 20000 metres and climbing steadily. We have activated the polarimeter and all is OK. So far, so good… We are now working through our in-flight check-lists and will soon start to position the gondola for first observations of the Crab.

PoGOLite is launched!