Study of Gamma-Ray Bursts afterglow surprises scientists

Issued by University of Leicester Press Office on 30 April 2014

Key quotes:

“We believe that this detection means that most of the current theories of how electrons get accelerated in afterglows need re-examining.”

- Dr Klaas Wiersema, of the University of Leicester Department of Physics and Astronomy

“We are the first team to realise the importance of trying these technically difficult circular polarisation measurements at visible wavelengths – most people simply

assumed it wouldn't be worthwhile.” - Dr Klaas Wiersema, of the University of Leicester Department of Physics and Astronomy

Contact pressoffice@le.ac.uk to request images

SCIENCE CONTACT: Dr Wiersema, University of Leicester Department of Physics and Astronomy, email: kw113@le.ac.uk

Media contact: University of Leicester press office 0116 229 7467

Research from an international team of scientists led by the University of Leicester has discovered for the first time that one of the most powerful events in our universe – Gamma-Ray Bursts (GRB) – behave differently than previously thought.

The study, published in the prestigious scientific journal Nature, uses evidence from observation of a GRB to rule out most of the existing theoretical predictions concerning the afterglow of the explosions.

For Dr Klaas Wiersema, of the University of Leicester’s Department of Physics and Astronomy, it was handy that he was up in the middle of the night tending to his three-year-old son which is when he got the alert that a GRB had occurred.

He said: “When a suitable GRB is detected by a satellite, I get a text message on my phone, and then I have to very quickly tell the observatory in Chile exactly which observations I want them to take, and how.

“This is usually a rather stressed and frantic few hours of working, as fast as possible, on my laptop throughout our night-time – and I remember very well that my son, who was three at the time, was up a lot that night too, so I kept on running back and forth between my laptop, my phone to call the observatory in Chile, and my son’s cot!”

The effort was worth it - and has led to scientific findings that will change theoretical understandings of the afterglows of GRBs. The team from Leicester also included Dr Rhaana Starling, Professor Nial Tanvir, Professor Paul O’Brien and Professor Richard Willingale from the Department of Physics and Astronomy.

Dr Wiersema explains: “About once per day, a short, very bright flash of gamma-rays (the most energetic form of light) is detected by satellites. These flashes are called gamma-ray bursts (GRBs), and take place in galaxies far away, when a massive star collapses at the end of its life.

“These GRBs are followed by a so-called “afterglow”, slowly fading emission that can be seen at all wavelengths (including visible light), for a few days to weeks. We know that the afterglow emission is formed by a shockwave, moving at very high velocities, in which electrons are being accelerated to tremendous energies. These fast moving electrons then produce the afterglow light that we detect.

“However, how this acceleration process actually works is very hard to study on Earth in laboratories, or using computer simulations. What we do, is study the polarised light of the afterglow using large optical telescopes, and special filters, that work much like the filters in Polaroid sunglasses.”

So what is polarised light? Dr Wiersema says it is important to remember that light is a wave – when light is linearly polarised, it means that the wave vibrations lie in a plane; and when light is circularly polarised, it means that that this plane rotates on the sky.

He added: “Different theories for electron acceleration and light emission within the afterglow all predict different levels of linear polarisation, but theories all agreed that there should be no circular polarisation in visible light. This is where we come in: we decided to test this by carefully measuring both the linear and circular polarisation of one afterglow, of GRB 121024A, detected by the Swift satellite.

“Using the Very Large Telescope (VLT) in Chile, we measured both the linear and circular polarisation of an afterglow with high accuracy. Much to our surprise we clearly detected circular polarisation, while theories predicted we should not see any at all. We believe that the most likely explanation is that the exact way in which electrons are accelerated within the afterglow shockwave is different from what we always thought. It is a very nice example of observations ruling out most of the existing theoretical predictions – exactly why observes like me are in this game!

“We are the first team to realise the importance of trying these technically difficult circular polarisation measurements at visible wavelengths – most people simply assumed it wouldn't be worthwhile doing as theory predicted levels too low to be detectable. The detection of far stronger circular polarisation than expected makes it a particularly surprising result.

“We believe that this detection means that most of the current theories of how electrons get accelerated in afterglows need re-examining.”

Dr Wiersema said the research was also important because taking these high precision measurements of a rapidly fading afterglow is very difficult from a technical point of view. The research represents a great technological achievement, one the team would love to repeat for more sources.

“Extreme shocks like the ones in GRB afterglows are great natural laboratories to push our understanding of physics beyond the ranges that can be explored in laboratories,” said Dr Wiersema.

Dr Wiersema’s post at the University of Leicester is funded by the Science and Technology Facilities Council (STFC). Key collaborators are Prof. Stefano Covino (INAF / Observatory Brera, Italy) and Prof. Kenji Toma (Osaka University, Japan)

The observations were obtained in October 2012

Support for operation of Swift at the University of Leicester is funded by the UK Space Agency (UKSA)

Ends

Notes to editors:

Dr Wiersema is available via email: kw113@le.ac.uk

The full listing of authors is as follows:

K. Wiersema1, S. Covino2, K. Toma3,4,5, A. J. van der Horst6, K. Varela7, M. Min6, J. Greiner7, R. L. C. Starling1, N. R. Tanvir1, R. A. M. J. Wijers6, S. Campana2, P. A. Curran8, Y. Fan9, J. P. U. Fynbo10, J. Gorosabel11,12,13, A. Gomboc14, D. Götz15, J. Hjorth10, Z. P. Jin9, S. Kobayashi16, C. Kouveliotou17, C. Mundell16, P. T. O'Brien1, E. Pian18,19, A. Rowlinson6, D. M. Russell20,21,22, R. Salvaterra23, S. di Serego Alighieri24, G. Tagliaferri2, S. D. Vergani2, J. Elliott7, C. Fariña25, O. E. Hartoog6, R. Karjalainen25, S. Klose26, F. Knust7, A. J. Levan27, P. Schady7, V. Sudilovsky7 & R. Willingale1

1 Department of Physics & Astronomy, University of Leicester, Leicester, LE1 7RH, United Kingdom

2 INAF/Brera Astronomical Observatory, via Bianchi 46, I-23807, Merate (LC), Italy

3 Department of Earth and Space Science, Osaka University, Toyonaka 560-0043, Japan

4 Astronomical Institute, Tohoku University, Sendai 980-8578, Japan

5 Frontier Research Institute for Interdisciplinary Sciences, Tohoku University, Sendai 980-8578, Japan

6 Astronomical Institute ‘Anton Pannekoek’, University of Amsterdam, PO Box 94248, 1090 SJ Amsterdam, the Netherlands

7 Max-Planck-Institut für extraterrestrische Physik, Giessenbachstrasse 1, D-85748 Garching, Germany

8 International Centre for Radio Astronomy Research, Curtin University, GPO Box U1987, Perth, WA 6845, Australia

9 Key Laboratory of Dark Matter and Space Astronomy, Purple Mountain Observatory, Chinese Academy of Science, Nanjing, 210008, China

10 Dark Cosmology Centre, Niels Bohr Institute, University of Copenhagen, Juliane Maries Vej 30, DK 2100 Copenhagen , Denmark

11 Instituto de Astrofísica de Andalucía (IAA-CSIC), Glorieta de la Astronomía s/n, E-18008, Granada, Spain

12 Unidad Asociada Grupo Ciencia Planetarias UPV/EHU-IAA/CSIC, Departamento de Física Aplicada I, E.T.S. Ingeniería, Universidad del País Vasco UPV/EHU, Alameda de Urquijo s/n, E 48013 Bilbao, Spain

13 Ikerbasque, Basque Foundation for Science, Alameda de Urquijo 36-5, E-48008 Bilbao, Spain

14 Faculty of Mathematics and Physics, University of Ljubljana, Jadranska 19, 1000 Ljubljana, Slovenia

15 AIM (UMR 7158 CEA/DSM-CNRS-Université Paris Diderot) Irfu/Service d'Astrophysique, Saclay, F-91191 Gif-sur-Yvette Cedex, France

16 Astrophysics Research Institute, Liverpool John Moores University, Liverpool Science Park,IC2 Building, 146 Brownlow Hill, Liverpool L3 5RF, United Kingdom

17 Space Science Office, ZP12, NASA/Marshall Space Flight Center, Huntsville, AL 35812, USA

18 Scuola Normale Superiore, 7, I-56126, Pisa, Italy

19 INAF/IASF Bologna, via Gobetti 101, I-40129, Bologna, Italy

20 Instituto de Astrofísica de Canarias (IAC), E-38200 La Laguna, Tenerife, Spain

21 Departamento de Astrofísica, Universidad de La Laguna, E-38206 La Laguna, Tenerife, Spain

22 New York University Abu Dhabi, P.O. Box 129188, Abu Dhabi, United Arab Emirates

23 INAF/IASF Milano, via E. Bassini 15, 20133 Milano, Italy

24 INAF-Osservatorio Astrofisico di Arcetri, L.go E. Fermi 5, I-50125 Firenze, Italy

25 Isaac Newton Group of Telescopes, Apartado de Correos 321, E-38700 Santa Cruz de la Palma, Canary Islands, Spain

26 Thüringer Landessternwarte Tautenburg, Sternwarte 5, 07778, Tautenburg, Germany

27 Department of Physics, University of Warwick, Coventry, CV4 7AL, United Kingdom

The following funding acknowledgements from the authors appear at the end of the paper:

KW acknowledges support from STFC. KT acknowledges support from JSPS Research Fellowships for Young Scientists No. 231446. AJvdH, RAMJW and AR acknowledge support by the European Research Council via Advanced Investigator Grant no. 247295. RLCS is supported by a Royal Society Fellowship. YZF is supported by the 973 Programme of China, under grant 2013CB837000. DMR acknowledges support from a Marie Curie Intra European Fellowship within the 7th European Community Framework Programme under contract no. IEF 274805. This work was supported by Australian Research Council grant DP120102393.