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Polarisation studies of the prompt gamma-ray emission from GRB 041219a using the spectrometer aboard INTEGRAL

Polarisation studies of the prompt gamma-ray emission from GRB 041219a using the spectrometer aboard INTEGRAL
Polarisation studies of the prompt gamma-ray emission from GRB 041219a using the spectrometer aboard INTEGRAL
The spectrometer aboard INTEGRAL, SPI, has the capability to detect the signature of polarised emission from a bright gamma-ray source. GRB 041219a is the most intense burst localised by INTEGRAL and is an ideal candidate for such a study. Polarisation can be measured using multiple events scattered into adjacent detectors because the Compton scatter angle depends on the polarisation of the incoming photon. A search for linear polarisation in the most intense pulse of duration 66 seconds and in the brightest 12 seconds of GRB 041219a was performed in the 100-350keV, 100-500keV and 100keV-1MeV energy ranges. The multiple event data from the spectrometer was analysed and compared with the predicted instrument response obtained from Monte-Carlo simulations using the GEANT 4 INTEGRAL mass model. The χ2 distribution between the real and simulated data as a function of the percentage polarisation and polarisation angle was calculated for all three energy ranges. The degree of linear polarisation in the brightest pulse of duration 66s was found to be 63±31% at an angle of 70±14 degrees in the 100-350keV energy range. The degree of polarisation was also constrained in the brightest 12s of the GRB and a polarisation fraction of 96±40% at an angle of 60±14 degrees was determined over the same energy range. However, despite extensive analysis and simulations, a systematic effect that could mimic the weak polarisation signal could not be definitively excluded. Our results over several energy ranges and time intervals are consistent with a polarisation signal of about 60% at a low level of significance (2 sigma). We conclude that the procedure described here demonstrates the effectiveness of using SPI as a polarimeter, and is a viable method of measuring polarisation levels in intense gamma-ray bursts.
gamma rays: bursts, gamma rays: observations polarisation
0004-6361
895-904
McGlynn, S.
8fc726f3-fceb-4627-a71c-10d9ca99a0c2
Clark, D.J.
4d37cdbe-d8f5-47c6-a135-7950843def36
Dean, A.J.
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Hanlon, L.
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McBreen, S.
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Willis, D.R.
7a2b80ac-8957-4fca-a7cc-feb8eec83dfe
Bird, A.J.
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Foley, S.
16e2ecef-f10d-44b6-b271-ccfa9c75c3a6
McBreen, B.
e3ebc062-f297-421e-9a24-4a10479f5e62
McGlynn, S.
8fc726f3-fceb-4627-a71c-10d9ca99a0c2
Clark, D.J.
4d37cdbe-d8f5-47c6-a135-7950843def36
Dean, A.J.
2f9093f2-855c-4769-b1aa-6dd621b5dcf1
Hanlon, L.
0364bd1e-826b-4bfb-800e-e42e4ee0fc08
McBreen, S.
a1945bfc-bed2-4056-a81c-9d56363bf429
Willis, D.R.
7a2b80ac-8957-4fca-a7cc-feb8eec83dfe
Bird, A.J.
045ee141-4720-46fd-a412-5aa848a91b32
Foley, S.
16e2ecef-f10d-44b6-b271-ccfa9c75c3a6
McBreen, B.
e3ebc062-f297-421e-9a24-4a10479f5e62

McGlynn, S., Clark, D.J., Dean, A.J., Hanlon, L., McBreen, S., Willis, D.R., Bird, A.J., Foley, S. and McBreen, B. (2007) Polarisation studies of the prompt gamma-ray emission from GRB 041219a using the spectrometer aboard INTEGRAL. Astronomy & Astrophysics, 466 (3), 895-904. (doi:10.1051/0004-6361:20066179).

Record type: Article

Abstract

The spectrometer aboard INTEGRAL, SPI, has the capability to detect the signature of polarised emission from a bright gamma-ray source. GRB 041219a is the most intense burst localised by INTEGRAL and is an ideal candidate for such a study. Polarisation can be measured using multiple events scattered into adjacent detectors because the Compton scatter angle depends on the polarisation of the incoming photon. A search for linear polarisation in the most intense pulse of duration 66 seconds and in the brightest 12 seconds of GRB 041219a was performed in the 100-350keV, 100-500keV and 100keV-1MeV energy ranges. The multiple event data from the spectrometer was analysed and compared with the predicted instrument response obtained from Monte-Carlo simulations using the GEANT 4 INTEGRAL mass model. The χ2 distribution between the real and simulated data as a function of the percentage polarisation and polarisation angle was calculated for all three energy ranges. The degree of linear polarisation in the brightest pulse of duration 66s was found to be 63±31% at an angle of 70±14 degrees in the 100-350keV energy range. The degree of polarisation was also constrained in the brightest 12s of the GRB and a polarisation fraction of 96±40% at an angle of 60±14 degrees was determined over the same energy range. However, despite extensive analysis and simulations, a systematic effect that could mimic the weak polarisation signal could not be definitively excluded. Our results over several energy ranges and time intervals are consistent with a polarisation signal of about 60% at a low level of significance (2 sigma). We conclude that the procedure described here demonstrates the effectiveness of using SPI as a polarimeter, and is a viable method of measuring polarisation levels in intense gamma-ray bursts.

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More information

Submitted date: 28 February 2007
Published date: May 2007
Additional Information: The discovery and measurement of polarisation in a gamma-ray burst is of fundamental astrophysical significance. The concept of exploiting the mass modelling technique and INTEGRAL instruments as a polarimeter was originated by AJD. The two research students involved were put as the first authors on this paper.
Keywords: gamma rays: bursts, gamma rays: observations polarisation

Identifiers

Local EPrints ID: 45359
URI: http://eprints.soton.ac.uk/id/eprint/45359
ISSN: 0004-6361
PURE UUID: b27fe7ae-30aa-4e7a-8ef0-bb58ff78fa2e
ORCID for A.J. Bird: ORCID iD orcid.org/0000-0002-6888-8937

Catalogue record

Date deposited: 26 Mar 2007
Last modified: 10 Nov 2021 02:42

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Contributors

Author: S. McGlynn
Author: D.J. Clark
Author: A.J. Dean
Author: L. Hanlon
Author: S. McBreen
Author: D.R. Willis
Author: A.J. Bird ORCID iD
Author: S. Foley
Author: B. McBreen

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