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Measurements of hard X-ray polarization from the Crab and Cygnus X-1
KTH, School of Engineering Sciences (SCI), Physics, Particle and Astroparticle Physics.ORCID iD: 0000-0002-5742-7553
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: urn:nbn:se:kth:diva-228216ISBN: 978-91-7729-780-2 (print)OAI: oai:DiVA.org:kth-228216DiVA, id: diva2:1208715
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
List of papers
1. Pitfalls of statistics-limited X-ray polarization analysis
Open this publication in new window or tab >>Pitfalls of statistics-limited X-ray polarization analysis
2018 (English)In: Astronomy and Astrophysics, ISSN 0004-6361, E-ISSN 1432-0746, Vol. 615, article id A54Article in journal (Refereed) Published
Abstract [en]

Context. One of the difficulties with performing polarization analysis is that the mean polarization fraction of sub-divided data sets is larger than the polarization fraction for the integrated measurement. The resulting bias is one of the properties of the generating distribution discussed in this work. The limitations of Gaussian approximations in standard analysis based on Stokes parameters for estimating polarization parameters and their uncertainties are explored by comparing with a Bayesian analysis. The effect of uncertainty on the modulation factor is also shown, since it can have a large impact on the performance of gamma-ray burst polarimeters. Results are related to the minimum detectable polarization (MDP), a common figure of merit, making them easily applicable to any X-ray polarimeter. Aims. The aim of this work is to quantify the systematic errors induced on polarization parameters and their uncertainties when using Gaussian approximations and to show when such effects are non-negligible. Methods. The probability density function is used to deduce the properties of reconstructed polarization parameters. The reconstructed polarization parameters are used as sufficient statistics for finding a simple form of the likelihood. Bayes theorem is used to derive the posterior and to include nuisance parameters. Results. The systematic errors originating from Gaussian approximations as a function of instrument sensitivity are quantified here. Different signal-to-background scenarios are considered making the analysis relevant for a large variety of observations. Additionally, the change of posterior shape and instrument performance MDP due to uncertainties on the polarimeteric response of the instrument is shown.

Place, publisher, year, edition, pages
EDP Sciences, 2018
Keywords
polarization; methods: data analysis; methods: statistical
National Category
Astronomy, Astrophysics and Cosmology
Identifiers
urn:nbn:se:kth:diva-228211 (URN)10.1051/0004-6361/201731971 (DOI)000438418300002000438418300002 ()
Funder
Swedish National Space Board
Note

QC 20180521

Available from: 2018-05-18 Created: 2018-05-18 Last updated: 2018-07-31Bibliographically approved
2. Observation of polarized hard X-ray emission from the Crab by the PoGOLite Pathfinder
Open this publication in new window or tab >>Observation of polarized hard X-ray emission from the Crab by the PoGOLite Pathfinder
Show others...
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
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
Keywords
instrumentation: polarimeters, techniques: polarimetric, stars: neutron, pulsars: individual: the Crab pulsar
National Category
Astronomy, Astrophysics and Cosmology
Identifiers
urn:nbn:se:kth:diva-182840 (URN)10.1093/mnrasl/slv177 (DOI)000368010000018 ()2-s2.0-84959175886 (Scopus ID)
Note

QC 20160224

Available from: 2016-02-24 Created: 2016-02-23 Last updated: 2018-05-21Bibliographically approved
3. Calibration and performance studies of the balloon-borne hard X-ray polarimeter PoGO
Open this publication in new window or tab >>Calibration and performance studies of the balloon-borne hard X-ray polarimeter PoGO
Show others...
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
Keywords
X-ray, Polarisation, Compton scattering, Scientific ballooning, Crab, Cygnus X-1, Monte Carlo simulations
National Category
Physical Sciences
Identifiers
urn:nbn:se:kth:diva-219572 (URN)10.1016/j.nima.2017.03.027 (DOI)000402464700018 ()2-s2.0-85018696451 (Scopus ID)
Note

QC 20171207

Available from: 2017-12-07 Created: 2017-12-07 Last updated: 2018-05-21Bibliographically approved
4. Shedding new light on the Crab with polarized X-rays
Open this publication in new window or tab >>Shedding new light on the Crab with polarized X-rays
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2017 (English)In: Scientific Reports, ISSN 2045-2322, E-ISSN 2045-2322, Vol. 7, no 7816, p. 1-6Article in journal (Refereed) Published
Abstract [en]

Strong magnetic fields, synchrotron emission, and Compton scattering are omnipresent in compactcelestial X-ray sources. Emissions in the X-ray energy band are consequently expected to be linearlypolarized. X-ray polarimetry provides a unique diagnostic to study the location and fundamentalmechanisms behind emission processes. The polarization of emissions from a bright celestial X-raysource, the Crab, is reported here for the first time in the hard X-ray band (~20–160 keV). The Crab isa complex system consisting of a central pulsar, a diffuse pulsar wind nebula, as well as structures inthe inner nebula including a jet and torus. Measurements are made by a purpose-built and calibratedpolarimeter, PoGO+. The polarization vector is found to be aligned with the spin axis of the pulsar for apolarization fraction, PF = (20.9 ± 5.0)%. This is higher than that of the optical diffuse nebula, implyinga more compact emission site, though not as compact as, e.g., the synchrotron knot. Contrary tomeasurements at higher energies, no significant temporal evolution of phase-integrated polarisationparameters is observed. The polarization parameters for the pulsar itself are measured for the first timein the X-ray energy band and are consistent with observations at optical wavelengths.

Place, publisher, year, edition, pages
Nature Publishing Group, 2017
National Category
Astronomy, Astrophysics and Cosmology
Identifiers
urn:nbn:se:kth:diva-219424 (URN)10.1038/s41598-017-07390-7 (DOI)000407400500048 ()2-s2.0-85043356252 (Scopus ID)
Funder
Swedish National Space BoardKnut and Alice Wallenberg FoundationSwedish Research Council
Note

QC 20171212

Available from: 2017-12-05 Created: 2017-12-05 Last updated: 2018-06-19Bibliographically approved
5. Accretion geometry of the black-hole binary Cygnus X-1 from X-ray polarimetry
Open this publication in new window or tab >>Accretion geometry of the black-hole binary Cygnus X-1 from X-ray polarimetry
Show others...
2018 (English)In: Nature Astronomy, ISSN 2397-3366, Vol. 2, no 8, p. 652-655Article in journal, Letter (Refereed) Published
Abstract [en]

Black hole binary (BHB) systems comprise a stellar-mass black hole and a closely orbiting companion star. Matter is transferred from the companion to the black hole, forming an accretion disk, corona and jet structures. The resulting release of gravitational energy leads to the emission of X-rays1. The radiation is affected by special/general relativistic effects, and can serve as a probe for the properties of the black hole and surrounding environment, if the accretion geometry is properly identified. Two competing models describe the disk–corona geometry for the hard spectral state of BHBs, based on spectral and timing measurements2,3. Measuring the polarization of hard X-rays reflected from the disk allows the geometry to be determined. The extent of the corona differs between the two models, affecting the strength of the relativistic effects (such as enhancement of the polarization fraction and rotation of the polarization angle). Here, we report observational results on the linear polarization of hard X-ray emission (19–181 keV) from a BHB, Cygnus X-14, in the hard state. The low polarization fraction, <8.6% (upper limit at a 90% confidence level), and the alignment of the polarization angle with the jet axis show that the dominant emission is not influenced by strong gravity. When considered together with existing spectral and timing data, our result reveals that the accretion corona is either an extended structure, or is located far from the black hole in the hard state of Cygnus X-1.

Place, publisher, year, edition, pages
Nature Publishing Group, 2018
National Category
Astronomy, Astrophysics and Cosmology
Identifiers
urn:nbn:se:kth:diva-228215 (URN)10.1038/s41550-018-0489-x (DOI)2-s2.0-85051090128 (Scopus ID)
Note

QC 20180521

Available from: 2018-05-18 Created: 2018-05-18 Last updated: 2018-08-22Bibliographically approved

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