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Observation of polarized hard X-ray emission from the Crab by the PoGOLite Pathfinder
KTH, School of Engineering Sciences (SCI), Physics, Particle and Astroparticle Physics.ORCID iD: 0000-0003-1999-2161
KTH, School of Engineering Sciences (SCI), Physics, Particle and Astroparticle Physics.
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2016 (English)In: Monthly notices of the Royal Astronomical Society, ISSN 0035-8711, E-ISSN 1365-2966, Vol. 456, no 1, p. L84-L88Article in journal (Refereed) Published
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Abstract [en]

We have measured the linear polarization of hard X-ray emission from the Crab in a previously unexplored energy interval, 20-120 keV. The introduction of two new observational parameters, the polarization fraction and angle stands to disentangle geometrical and physical effects, thereby providing information on the pulsar wind geometry and magnetic field environment. Measurements are conducted using the PoGOLite Pathfinder - a balloon-borne polarimeter. Polarization is determined by measuring the azimuthal Compton scattering angle of incident X-rays in an array of plastic scintillators housed in an anticoincidence well. The polarimetric response has been characterized prior to flight using both polarized and unpolarized calibration sources. We address possible systematic effects through observations of a background field. The measured polarization fraction for the integrated Crab light curve is 18.4(-10.6)(+9.8) per cent, corresponding to an upper limit (99 per cent credibility) of 42.4 per cent, for a polarization angle of (149.2 +/- 16.0)degrees.

Place, publisher, year, edition, pages
Oxford University Press, 2016. Vol. 456, no 1, p. L84-L88
Keywords [en]
instrumentation: polarimeters, techniques: polarimetric, stars: neutron, pulsars: individual: the Crab pulsar
National Category
Astronomy, Astrophysics and Cosmology
Identifiers
URN: urn:nbn:se:kth:diva-182840DOI: 10.1093/mnrasl/slv177ISI: 000368010000018Scopus ID: 2-s2.0-84959175886OAI: oai:DiVA.org:kth-182840DiVA, id: diva2:906617
Note

QC 20160224

Available from: 2016-02-24 Created: 2016-02-23 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)
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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, VictorRyde, FelixPearce, Mark

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