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Calibration and performance studies of the balloon-borne hard X-ray polarimeter PoGO
KTH, School of Engineering Sciences (SCI), Physics, Particle and Astroparticle Physics.ORCID iD: 0000-0003-1999-2161
KTH, School of Engineering Sciences (SCI), Physics.
KTH, School of Engineering Sciences (SCI), Physics.
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2017 (English)In: Nuclear Instruments and Methods in Physics Research Section A: Accelerators, Spectrometers, Detectors and Associated Equipment, ISSN 0168-9002, E-ISSN 1872-9576, Vol. 859, p. 125-133Article in journal (Refereed) Published
Abstract [en]

Polarimetric observations of celestial sources in the hard X-ray band stand to provide new information on emission mechanisms and source geometries. PoGO+ is a Compton scattering polarimeter (20-150 keV) optimised for the observation of the Crab (pulsar and wind nebula) and Cygnus X-1 (black hole binary), from a stratospheric balloon-borne platform launched from the Esrange Space Centre in summer 2016. Prior to flight, the response of the polarimeter has been studied with polarised and unpolarised X-rays allowing a Geant4-based simulation model to be validated. The expected modulation factor for Crab observations is found to be M-Crab = (41.75 +/- 0.85)%, resulting in an expected Minimum Detectable Polarisation (MDP) of 7.3% for a 7 day flight. This will allow a measurement of the Crab polarisation parameters with at least 5 sigma statistical significance assuming a polarisation fraction similar to 20% - a significant improvement over the PoGOLite Pathfinder mission which flew in 2013 and from which the PoGO+ design is developed.

Place, publisher, year, edition, pages
ELSEVIER SCIENCE BV , 2017. Vol. 859, p. 125-133
Keywords [en]
X-ray, Polarisation, Compton scattering, Scientific ballooning, Crab, Cygnus X-1, Monte Carlo simulations
National Category
Physical Sciences
Identifiers
URN: urn:nbn:se:kth:diva-219572DOI: 10.1016/j.nima.2017.03.027ISI: 000402464700018Scopus ID: 2-s2.0-85018696451OAI: oai:DiVA.org:kth-219572DiVA, id: diva2:1163702
Note

QC 20171207

Available from: 2017-12-07 Created: 2017-12-07 Last updated: 2018-05-21Bibliographically approved
In thesis
1. Measurements of hard X-ray polarization from the Crab and Cygnus X-1
Open this publication in new window or tab >>Measurements of hard X-ray polarization from the Crab and Cygnus X-1
2018 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

Polarimetry provides insights into the emission mechanisms of astrophysical sources by elucidating their magnetic field and geometry. Hard X-rays are produced in \mbox{regions} with strong magnetic fields or strong gravitational effects, which makes them a probe of extreme environments. This thesis describes the design, \mbox{calibration} and data analysis from the balloon-borne hard X-ray polarimeters the PoGOLite Pathfinder and its upgrade PoGO+. These instruments have measured the polari-zation from the Crab nebula and pulsar, and of the black hole binary Cygnus X-1.

Paper I explores to what extent the statistical uncertainties on the polarization parameters are non-Gaussian when the number of photons is low, as tends to be the case for balloon-borne instruments.With this in mind, a Bayesian method is used for data analysis in the subsequent papers. Paper II describes the measurement of the polarization of the Crab system in the 20-120 keV energy range conducted by the PoGOLite Pathfinder. Although the result is modest in its statistical significance it paves the way for the design of the upgraded instrument PoGO+.

The PoGO+ mission was conceived to remedy the shortcomings of the PoGOLite Pathfinder design and observation strategy, as well as the pre-flight calibration, which the focus of Paper III. Significant improvements are made to the detector response model, optimization of data acquisition thresholds, online veto system and to the general calibration procedure. When combined with interspersed target and background measurements, systematic uncertainties are significantly smaller for PoGO+ than for the PoGOLite Pathfinder.

The main scientific results are presented in Papers IV and V for the Crab (20-160 keV) and Cygnus X-1 (20-180 keV), respectively. For the Crab, PoGO+ does not support a rapid increase in the polarization fraction claimed previously. Additionally, the hard X-ray emission must be produced close to the pulsar and possibly in the fine structures of the nebula. This is in agreement with X-ray images from other instruments. For Cygnus X-1, the polarization measurements constrain the geometry by rejecting the model where the hard X-rays are produced in a compact corona close to the black hole and support the extended corona model.

The thesis demonstrates how balloon-borne instruments can be improved over the course of several campaigns and can contribute to the testing of detector design, development of analysis methods and provide new scientific results for bright X-ray sources.

Place, publisher, year, edition, pages
KTH Royal Institute of Technology, 2018. p. 110
Series
TRITA-SCI-FOU ; 2018:14
National Category
Astronomy, Astrophysics and Cosmology
Research subject
Physics; Physics
Identifiers
urn:nbn:se:kth:diva-228216 (URN)978-91-7729-780-2 (ISBN)
Public defence
2018-05-31, FB42, Roslagstullsbacken 21, AlbaNova Universitetscentrum, Stockholm, 14:00 (English)
Opponent
Supervisors
Note

QC 20180521

Available from: 2018-05-21 Created: 2018-05-18 Last updated: 2018-05-21Bibliographically approved

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Chauvin, MaximeKiss, MózsiMikhalev, VictorPearce, Mark

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