Alsatian Workshop on X-ray Polarimetry– Abstract booklet –

University of Strasbourg, Strasbourg, France

November, 13th - 15th, 2017

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Contents

AbstractsOral contributions 1A changing-look AGN to be checked by X-ray polarimetry (Beatriz Agıs-

Gonzalez ) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1Solar and stellar polarization: A synergy of optical and X-ray studies (Svet-

lana Berdyugina) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2Prospects and opportunities of future X-ray polarimetry of relativistic ac-

celerators: PWNe and SNRs (Niccolo Bucciantini) . . . . . . . . . . . 2X-ray polarization from black holes’ accretion disks: QED effects (Ilaria

Caiazzo) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3The Gas Pixel Detector yesterday, today and tomorrow (Enrico Costa) . . 3Discovering new fundamental physics with X-ray polarimetry (Francesca

Day) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4X-ray polarization by reflection from accretion disc in AGN (Michal Dovciak) 4Instrumentation and future missions (Sergio Fabiani) . . . . . . . . . . . . 5PoGO+ observations of Cygnus X-1 in the low/hard state (Mette Friis) . . 5Magnetar Bursts in the Era of X-ray Polarimeters (Ersin Gogus) . . . . . 5Generation of a monochromatic, polarized X-ray beam using HOPG and Si

crystals for testing polarimetry X-ray telescopes at PANTER (StefanF. Hartl) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6

X-ray polarimetry and fundamental physics (Jeremy Heyl) . . . . . . . . . 6Polarization of AGN X-ray Lines (Demosthenes Kazanas) . . . . . . . . . 6GRB polarisation measurements of POLAR (Merlin Kole) . . . . . . . . . 7Bounds on Axion-like Particles from X-ray observations of AGNs and

quasars (Sven Krippendorf ) . . . . . . . . . . . . . . . . . . . . . . . 7X-ray photo-polarimetry of HD 189733b (Frederic Marin) . . . . . . . . . 8Hot Coronae in local AGN: present status and future perspectives (Andrea

Marinucci) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8A Soft X-ray Polarimeter (Herman Marshall) . . . . . . . . . . . . . . . . 9Observing Cen A and Other Blazars with IXPE (Herman Marshall) . . . . 9Studying microquasars with X-ray polarimetry (Giorgio Matt) . . . . . . . 10Multi-wavelength polarimetry of isolated neutron stars (Roberto Mignani) 10PoGO+ measurement of Crab emissions in the hard X-ray band (Victor

Mikhalev) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11Hard X-ray and soft gamma ray polarimetry with CdTe/CZT spectro-

imager (Miguel Moita) . . . . . . . . . . . . . . . . . . . . . . . . . . 11

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Polarisation of high-energy emission in a pulsar striped wind (Jerome Petri) 12Mirion’s contribution to space projects (Pascal Quirin) . . . . . . . . . . . 12X-ray Polarization from Accreting Black Holes and Neutron Stars (Jeremy

Schnittman) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13Design and development of multilayer mirror based soft X-ray polarimeter

for Astronomical observations (Srikanth Panini Singam) . . . . . . . 13The high/soft state of Cygnus X-1 from the viewpoints of X-ray spectrum

and polarization (Hiromitsu Takahashi) . . . . . . . . . . . . . . . . . 14X-ray polarization as a tool to understand coronae in accreting sources

(Francesco Tamborra) . . . . . . . . . . . . . . . . . . . . . . . . . . 14On the spectrum and polarization of magnetar flare emission (Roberto Tav-

erna) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15Polarization of neutron star emission and future X-ray missions (Roberto

Turolla) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15X-ray imaging polarimetry (Jacco Vink) . . . . . . . . . . . . . . . . . . . 16An Overview of X-ray Polarimetry of Astronomical Sources (Martin C.

Weisskopf ) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 16SPHiNX: Compton scattering polarimeter for GRBs (Fei Xie) . . . . . . . 17TBA (Valentina Zharkova) . . . . . . . . . . . . . . . . . . . . . . . . . . . 17

AbstractsPoster contributions 19An extended nuclear torus revealed in NGC 1068 thanks to polarimetric

imaging at high angular resolution in the near-IR (Damien Gratadour) 19Polarimetric imaging of AGN at high angular resolution in near Infrared

- Observation and simulation of an extended nuclear torus (LucasGrosset) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 19

X-ray spectral properties of the blazar 3C 273 in quiescent states (NibeditaKalita) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 20

X-ray eclipsing events in active galactic nuclei (Elias Kammoun) . . . . . . 20X-ray spectroscopy of polar-scattered Seyfert 1 galaxies: The case of Fairall

51 (Romana Mikusincova) . . . . . . . . . . . . . . . . . . . . . . . . 21X-ray Polarization from Winds in Active Galactic Nuclei? (Francesco

Tombesi) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 21

Author Index 23

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AbstractsOral contributions

A changing-look AGN to be checked by X-ray polarimetry

Beatriz Agıs-Gonzalez

Universit de Liege / KULeuven

We present a particular case where X-ray polarization can contribute to clarifychanging-look AGN scenarios. We carried out a detailed analysis of multi-epochobservations of the Seyfert galaxy ESO 362-G18 in X-ray, UV and optical ranges.In total, 45 X-ray observations and 4 optical data sets, 2 of them in polarized light,ranging from January 2003 till March 2016. From the X-ray data set, we foundESO 362-G18 to exhibit relativistic reflection, including a detected soft time lagbetween continuum and reflection components, whose results support the compactnature of the X-ray emitting regions. We unveiled a very rapidly spinning Kerr backhole and a very high inclination of ∼ 53◦ (in two ways, dependent and independentmodel; see Agıs-Gonzalez et al. 2014). Thanks to this derived model, we havealso detected two absorption events driven by clouds situated at torus scales, one ofthem followed almost completely from ingress to egress along 36 X-ray observationsperformed with Swift satellite.On the other hand, the four optical spectra revealESO 362-G18 as a changing-look Seyfert galaxy, since two spectra are of type 1.5and the remaining two show ESO 362-G18 as a type 1.9 AGN. Moreover, we havepolarized measurements of one of the type 1.5 data sets and also one of the type 1.9,allowing us to compare the polarization properties in both states of a changing-lookSeyfert galaxy. The relatively high inclination we derived is consistent with the ideathat our LOS is grazing the obscuring torus (which has a typical half-opening angleof the order of 45◦). If the torus is not homogeneous but clumpy, such high incli-nation may intercept from time to time some of the clumps of the obscuring torus,explaining the detected X-ray absorption events and possibly why ESO 362-G18exhibits changes of look from type 1.5 to type 1.9 in its optical spectra. Moreover,the X-ray luminosity keeps roughly constant along the available observations, whichdiscards a change in the accretion rate. According to this, we would expect a sig-nificant increase of the optical polarization degree in the 1.9 state. However, ourresults do not confirm that picture as the change in the polarization degree is notdetected with the change of state.From our multi-epoch analysis we reinforce ourtarget to be a potential candidate exhibiting occultation events, both in X-ray andoptical ranges. In the context of the X-ray polarization, we propose this particularcase to probe changes of absorption in the X-ray band as predicted by the models

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developed by Marin & Dovciak 2015, which found X-ray polarimetry to be a usefultool to probe X-ray eclipses due to Compton-thin and Compton-thick gas clouds.Checking if changes of polarization are tied to changes of look in X-ray bands wouldhelp us to clarify the general and polarization picture of ESO 362-G18.

Solar and stellar polarization: A synergy of optical andX-ray studies

Svetlana Berdyugina

Institut fuer Sonnenphysik - KIS.

Magnetic fields and scattering produce a wealth of polarization signatures in spectraof the Sun and other stars. A detailed analysis of observed polarization helps tofollow evolution of magnetic fields, reveal inhomogeneities in their atmospheres,and even detect exoplanets. I will review current efforts to achieve high precisionpolarization measurements on the Sun and other stars. It appears that furthersignificant advances in our understanding of stars and their activity require a synergyof optical polarization studies with those in other spectral bands, such as X-ray, UV,IR, etc.

Prospects and opportunities of future X-ray polarimetry ofrelativistic accelerators: PWNe and SNRs

Niccolo Bucciantini

Arcetri Astrophysical Observatory

SNRs and PWNe are among the most significant sources of non-thermal X-rays inthe sky, and the closest laboratories where relativistic plasma dynamics and particleacceleration can be investigated. Being strong synchrotron emitter, they are idealcandidates for X-ray polarimetry, and indeed the Crab nebula is up to presentthe only object where X-ray polarization has been detected with a high level ofsignificance. Future polarimetric measure will likely provide us crucial informationson the level of turbulence that is expected at the particle acceleration sites, togetherwith the spacial and temporal coherence of the magnetic field geometry, enabling usto set stronger constraints on our acceleration models. In PWNe it will also allowus to estimate the level of internal dissipation. I will review the current theoreticalwork on the polarization signatures in SNR/PWNe and I will briefly illustrate whatcan we hope to achieve with future missions like IXPE/XIPE.

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X-ray polarization from black holes’ accretion disks: QEDeffects

Ilaria Caiazzo

University of British Columbia

X-ray polarimetry will become a reality in the next few years. Several missionsthat include polarimeters are currently under development: eXTp, XIPE, IXPE,PolSTAR, Lamp, XPP... Observing the polarization of the X-ray emission fromblack holes accretion disks will provide us with information on the geometry ofthe emission region with unprecedented resolution. In order to truly understandfuture observations though, it’s important to take into account the effect of QEDvacuum birefringence. This effect, which was one of the first predictions of QED andwhich has only recently been observed, can modify dramatically the polarization oflight travelling through the magnetosphere of black holes. I will explain how X-raypolarimeters can probe the magnetic field structure of black holes. In particular,observations of the X-ray polarization from accreting black holes could verify therole of magnetic fields in astrophysical viscosity.

The Gas Pixel Detector yesterday, today and tomorrow

Enrico Costa

IAPS-INAF

The idea of polarimetry based on the photoelectric effect is almost as old as X-rayastronomy and is originally due to Vitali Ginzburg. The practical implementationwas much harder and required half a century. In this talk I will give a few examplesof the attempts performed through time. The first implementation of a fully bi-dimensional photoelectric polarimeter was presented in 2001. The next step wasthe development of an ASIC VLSI chip that could combine the functions of bottomof the detector, pixellated anode and front-end electronics. The system evolvedthrough two new versions with increased surface, smaller pixels and, eventually theinclusion of a self-triggering capability. The last version, in the focus of an X-raytelescope. can allow for space, time and energy resolved polarimetry. The ASICwas compatible with different gas filling. From a critical analysis of the scientificobjectives it was clear that the right band to disclose this new observational windowwas the classica 2-10 keV. both IXPE and XIPE missions have been proposed withthis approach. While waiting for an approved missions some developments havebeen carried on as well. A new ASIC VLSI chip with smaller pixels: less deadtimefor larger optics. Testing GPD with a mixture based on pressurized argon. Acombination of the two allows for sensitive polarimetry of solar flares In the focus ofmulti-layer optics allows for imaging polarimetry in 6-30 keV band and this couldbe an option for eXTP but is not the baseline.

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Discovering new fundamental physics with X-raypolarimetry

Francesca Day

University of Cambridge

I will discuss using X-ray polarimetry to constrain or discover new particle physics.X-ray telescopes are an exceptional tool for searching for new fundamental physics.In particular, X-ray observations have placed world leading bounds on the interac-tion between photons and axions. Axions are a hypothetical new ultra-light particlemotivated by string theory models. They can also act as dark matter and darkenergy, and provide a solution to the strong CP problem. In a background mag-netic field, axions and photons may interconvert. This leads to energy dependentmodulations in both the flux and polarisation of the spectra of point sources shiningthrough large magnetic fields. The next generation of polarising X-ray telescopeswill have an even greater discovery potential for the axion. I will present techniquesand projected bounds for searching for axions with X-ray polarimetry.

X-ray polarization by reflection from accretion disc in AGN

Michal Dovciak

Astronomical Institute of the Czech Academy of Science

Theoretical computations showed that the reflection of X-ray radiation from theaccretion disc in AGN should result in significant (detectable) polarization signals.Originating from a primary power-law coronal emission situated above the disc sur-face, X-ray photons are partially reprocessed by Compton scattering in the discmaterial and show a polarization level that heavily depends on geometry of scatter-ing. In this contribution, we will examine the polarization in the lamp-post geometryscenario, where a compact patch of corona is positioned on the axis above the blackhole. The relativistic effects due to strong gravity close to the central black hole anddue to large velocities of accreting material will be shown together with the effectof initially polarised coronal emission.

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Instrumentation and future missions

Sergio Fabiani

INAF/IASF

The maturity of current detectors based on technologies form solid state to gasesrenewed the interest for X-ray polarimetry, raising the enthusiasm of a wide scientificcommunity to improve the performance of polarimeters as well as to produce moredetailed theoretical predictions. In this talk I will introduce the basic concepts aboutmeasuring the polarization of photons, especially in the X-rays and I will review thecurrent state of the art of polarimeters in a wide energy range from soft to hard X-rays. The panorama of next space missions and experiments to perform polarimetryfrom solar flares to far astrophysical sources will be discussed, as for example theNASA small Explorer mission IXPE, developed in collaboration with ASI, that willbe launched on late 2020 and will perform polarimetry in the 2 – 10 keV energy.

PoGO+ observations of Cygnus X-1 in the low/hard state

Mette Friis

KTH - Royal Institute of Technology

PoGO+ is a balloon-borne X-ray polarimeter which, in the summer of 2016, observedtwo celestial sources, the Crab pulsar/nebula system and the X-ray binary CygnusX-1, in the energy range 20 – 180 keV. I will present the polarisation measurement of,and discuss how the results affect current models for, Cygnus X-1. The X-ray binarycomprises a black hole, accreting matter from the stellar wind of its companion star.The binary itself is unresolved, with the only direct clue to orientation (with respectto us) coming from a radio jet. With polarisation measurements in hard X-rays,we have probed the geometry of the system in the near vicinity of the black hole.We use these results to test proposed models, providing strong constraints on theemission geometry of the system. Presented on behalf of the PoGO+ collaboration.

Magnetar Bursts in the Era of X-ray Polarimeters

Ersin Gogus

Faculty of Engineering and Natural Sciences

Energetic bursts from highly magnetized neutron stars (Soft Gamma Repeaters andAnomalous X-ray Pulsars) have unique characteristics. In recent years, we havewitnessed intriguing events; typical magnetar bursts have been seen from neutronstars with low inferred dipole magnetic field strengths, neutron star with extremelylong spin period, and a gamma ray pulsar with peculiar characteristics. Here wereview general properties of from the extended family of magnetars. We also discussprospects of detecting magnetar bursts with X-ray polarimeters.

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Generation of a monochromatic, polarized X-ray beamusing HOPG and Si crystals for testing polarimetry X-ray

telescopes at PANTER

Stefan F. Hartl

Max-Planck-Institute for Extraterrestrial Physics (MPE) in Garching (Munich)

The PANTER X-ray test facility of the Max-Planck-Institute for ExtraterrestrialPhysics (MPE) in Munich is designed for testing X-ray satellite optics. Upcomingpolarimetry missions are going to require the characterization of mirrors and detec-tors which makes it necessary to generate a polarized beam. The approach we havechosen is to use a Double Crystal Monochromator (DCM) at about 45 degrees to ob-tain linearly polarized monochromatic X-rays from an input Bremsstrahlung source.We have studied the use of Silicon and Highly Oriented Pyrolytic Graphite crystalsfor this purpose by making simulations with the ray-tracing programm SHADOWusing the physical dimensions of our existing DCM and test facility setup. The pre-liminary results show that we can expect to reach a degree of polarization greaterthan 99% thus enabling polarimetry X-ray telescope tests to be performed at PAN-TER.

X-ray polarimetry and fundamental physics

Jeremy Heyl

University of British Columbia

Compact objects provide a unique environment to probe physics at the extremefrom gravity to nuclear physics and plasma physics. I am going to focus on howstrongly neutron stars (magnetars) can help us probe quantum electrodynamics ina regime that hasn’t yet been tested. I will outline how to perform calculationsin non-perturbative QED and how observations of the polarization of X-rays fromcompact objects probes the non-linearity of the QED vacuum.

Polarization of AGN X-ray Lines

Demosthenes Kazanas

NASA/GSFC

We examine the possibility that Warm Absorber (WA) X-ray absorption line troughsmaybe polarized, in a way similar to those of BAL QSOs. Our models for AGN WAsemploy magnetically launched accretion disk winds with density profiles along theobserver’s line of sight close to n(r) ∼ 1/r and very steep dependence on the diskinclination angle. These have been employed to account for the properties of WAproperties in AGN as diverse as Seyfert galaxies and BAL QSOs. Because themodels are scale invariant the same winds account also the absorption properties ofthe galactic XRB GRO J1655-40 (Fukumura et al. 2017). The large wind scatteringdepth in certain objects allows for polarization of the WA troughs in a fashion similarto those of BAL QSOs. Such observations will further constrain the properties ofthese winds.

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GRB polarisation measurements of POLAR

Merlin Kole

University of Geneva

The polarisation of the gamma-ray component of Gamma-Ray Bursts is expectedto hold a wealth of information on the nature of these transient high-energy events.Several GRB polarisation measurements have been performed, including some bya dedicated instrument, however both the precision of these measurements and thenumber of measurements has not been enough to exclude any of the GRB emissionmodels. POLAR is a new space-borne gamma-ray polarimeter launched on Septem-ber 15th 2016 as part of the Chinese spacelab TG-2. The instrument is optimizedfor dedicated polarisation measurements of the of the 50 – 500 keV component ofthe prompt emission of GRBs. The instrument is foreseen to produce a catalogueof GRB polarisation measurements with an unprecedented precision during its twoyears lifetime. In the detector the polarisation degree is measured using the degreeof non-uniformity of the Compton scattering angles for photons interacting in thedetector. POLAR uses a f inely segmented plastic scintillator array consisting of1600 bars to measure the scattering angles of the incoming photons. Due to this finesegmentation POLAR can measure the photon interaction locations, and thereforethe scattering angles, giving it a large sensitivity to polarisation. The instrumentfurthermore has a relatively large effective area and a field of view of 1/2 of thesky thereby optimising it for studying transient sources such as GRBs. The instru-ment commenced data taking on the 22nd of September after which it operatedalmost continuously. In the first months of operation POLAR has detected about10 of GRBs per month, approximately 11 of which are candidates for detailed polar-ization measurements. The results from the instrument calibration measurements,performed using both radioactive sources and synchrotron facilities will be presentedalong with the the first preliminary polarisation studies and the used methodology.

Bounds on Axion-like Particles from X-ray observations ofAGNs and quasars

Sven Krippendorf

LMU Munich – University of Oxford

Axion-like particles are a well-motivated extension of the Standard Model. Theyarise generically in string theory and they can be (part of) dark matter. Axion-like particles can induce localised oscillatory modulations in the spectra of photonsources passing through astrophysical magnetic fields, in particular through mag-netic fields in galaxy clusters. By the absence of modulations in AGN/Quasar spec-tra (in particular from observations of the central AGN of NGC1275 in the PerseusCluster) we are able to place world-leading bounds on light axion-like particles. Iwill present how we obtain these bounds from archived Chandra observations, how

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they compare to direct (upcoming) experimental searches (e.g. IAXO) and how fu-ture observations (for instance with Athena) could improve these bounds. Besides,I will discuss how X-ray polarisation can help to constrain axion-like particles.

X-ray photo-polarimetry of HD 189733b

Frederic Marin

Observatoire astronomique de Strasbourg

Thousands of exoplanets have been detected, but only one exoplanetary transit waspotentially observed in X-rays from HD 189733A. What makes the detection ofexoplanets so difficult in this band? Using Monte-Carlo radiative transfer simula-tions, we compute the amount of X-ray coronal flux reprocessed by the extendedevaporating-atmosphere of HD 189733b. In the 0.25 – 2 keV energy band, the max-imum depth of the XMM-Newton light curve is about 1.6 percent at 47 min apartfrom the transit center on the geometrically thick and optically thin corona, andlittle sensitive to the metal abundance (assuming that adding metals in the atmo-sphere would not dramatically change the density-temperature profile). Regardinga direct detection of HD 189733b in X-rays, we find that the amount of flux repro-cessed by the exoplanetary atmosphere from egress to transit decays from 3 to 5orders of magnitude fainter than the flux of the host star. Additionally, the degreeof diluted linear polarization emerging from HD 189733A is lower than 0.003%, andlargest near planetary greatest elongations. This implies that neither the modula-tion of the X-ray reprocessed flux with the orbital phase nor the scattered-inducedcontinuum polarization can be observed with current X-ray facilities.

Hot Coronae in local AGN: present status and futureperspectives

Andrea Marinucci

University Roma Tre

The nuclear X-ray emission in radio-quiet AGN is commonly believed to be due toinverse Compton scattering of soft UV photons in a hot corona. The radiation isexpected to be polarized, the polarization degree depending mainly on the geometryand optical depth of the corona. NuSTAR observations are providing for the firsttime high quality measurements of the coronal physical parameters - temperatureand optical depth. We hereby review the NuSTAR results on the coronal physicalparameters (temperature and optical depth) and discuss their implications for futureX-ray polarimetric studies.

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A Soft X-ray Polarimeter

Herman Marshall

MIT Kavli Institute

The Rocket Experiment Demonstration of a Soft X-ray Polarimeter (REDSoX Po-larimeter) is a sounding rocket instrument that can make the first measurementof the linear X-ray polarization of an extragalactic source in the 0.2 – 0.6 keVband as low as 10%. We employ multilayer-coated mirrors as Bragg reflectors atthe Brewster angle. By matching the dispersion of a spectrometer using replicatedoptics from MSFC and critical angle transmission gratings from MIT to three later-ally graded multilayer mirrors (LGMLs), we achieve polarization modulation factorsover 90%. We present a novel arrangement of gratings, designed optimally for thepurpose of polarimetry with a converging beam. The entrance aperture is dividedinto six equal sectors; pairs of blazed gratings from opposite sectors are oriented todisperse to the same LGML. The LGML position angles are 120 degrees to eachother. CCD detectors then measure the intensities of the dispersed spectra afterreflection and polarizing by the LGMLs, giving the three Stokes parameters neededto determine the source polarization. The REDSoX polarimeter has been proposedto NASA and, if funded, would observe blazars and isolated neutron stars whileIXPE is operational.

Observing Cen A and Other Blazars with IXPE

Herman Marshall

MIT Kavli Institute

Cen A is the closest active galaxy and brightest in the 2 – 10 keV band. Withthe Imaging X-ray Polarization Explorer (IXPE), we will be able to measure thepolarization of the core as a function of energy from 2 to 7 keV. I will show re-sults from previous X-ray missions and radio VLBI that give us a good basis forpredicting results of IXPE observations. With the imaging capability of IXPE, wecan separately measure the polarization along the jet in order to determine if theX-ray emission is predominantly synchrotron emission and how the magnetic fieldis oriented in the regions where X-rays are produced. We will also observe otherblazars such as 3C 273 and Mrk 421.

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Studying microquasars with X-ray polarimetry

Giorgio Matt

University Roma Tre

While timing and spectroscopy of microquasars are well established techniques, X-ray polarimetry is lagging behind, despite its widely recognized importance in pro-viding vital information on the physics and geometry of these sources, includingstrong gravity effects. Happily, this will change very soon thanks to the approval byNASA of IXPE (the Imaging X-ray Polarimetry Explorer), the next mission in theSMEX program, to be launched in 2020, not forgetting XIPE, at the time of writ-ing this abstract still a contender in the ESA M4 program, and eXTP, a proposedChinese-led X-ray mission with polarimetric capabilities. In this contribution, themaWhile timing and spectroscopy of microquasars are well established techniques,X-ray polarimetry is lagging behind, despite its widely recognized importance inproviding vital information on the physics and geometry of these sources, includingstrong gravity effects. Happily, this will change very soon thanks to the approval byNASA of IXPE (the Imaging X-ray Polarimetry Explorer), the next mission in theSMEX program, to be launched in 2020, not forgetting XIPE, at the time of writ-ing this abstract still a contender in the ESA M4 program, and eXTP, a proposedChinese-led X-ray mission with polarimetric capabilities. In this contribution, themain scientific results expected by IXPE and other missions on microquasars willbe discussed, with particular emphasis on the possibility to measure the black holespin via energy-dependent polarization observations.in scientific results expected byIXPE and other missions on microquasars will be discussed, with particular emphasison the possibility to measure the black hole spin via energy-dependent polarizationobservations.

Multi-wavelength polarimetry of isolated neutron stars

Roberto Mignani

INAF/IASF-Milan

Polarisation studies of neutron stars have been traditionally performed in the radiodomain and in the optical. Only for the Crab pulsar measurements of the polarisa-tions exist in X rays and soft gamma-rays. The new fleet of planned missions withX and gamma-ray polarimetry capabilities, such as XIPE and e-ASTROGAM, pro-posed to ESA in the M4 and M5 calls, will open a new window in multi-wavelengthpolarimetry studies of neutron stars. In this talk, I review the current status ofnon-radio polarimetry observations of neutron stars and outline perspectives withfuture missions.

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PoGO+ measurement of Crab emissions in the hard X-rayband

Victor Mikhalev

KTH - Royal Institute of Technology

The polarization of emissions from the Crab, is reported in the hard X-ray band( 20 – 160 keV) (arXiv:1706.09203). The Crab is a complex system consisting of acentral pulsar, a diffuse pulsar wind nebula, as well as structures in the inner nebulaincluding a jet and torus. Measurements are made by PoGO+ – a purpose-builtand calibrated balloon borne polarimeter flown from the north of Sweden in thesummer of 2016. Source and background observations are interspersed allowing fora rigorous background subtraction eliminating systematic effects. The polarizationvector is found to be aligned with the spin axis of the pulsar for a polarizationfraction, PF = (20.9 ± 5.0)%. Contrary to measurements at higher energies, no sig-nificant temporal evolution of phase-integrated polarisation parameters is observed.Through phase selections the off-pulse (OP) and pulsar (P1, P2) can be studiedseparately. The polarization properties of nebula dominated OP are compatiblewith the phase-integrated results. The polarization parameters for the pulsar it-self (P2) are measured and are consistent with observations at optical wavelengths.Additionally, the instrument design and analysis methodology is detailed.

Hard X-ray and soft gamma ray polarimetry withCdTe/CZT spectro-imager

Miguel Moita

INAF/IASF-Bologna

CdTe/CZT spectroscopic 2D/3D imagers when operated in the Compton regimecan work as high performance scattering polarimeters, mainly for high-energy as-trophysics applications. Indeed, polarimetry in high-energy astrophysics has beenlittle explored. To date, X- and γ-ray source emissions have been studied almostexclusively through spectral, imaging and timing analysis. Polarization measure-ments provide two additional observational parameters: the polarization angle andthe level of linear polarization. These additional parameters should allow a betterdiscrimination between different emission models characterizing the same celestialobject. Therefore, scattering polarimetry will play a strategic role in new instru-mentations for future high-energy astronomy missions. We describe the results weobtained, both experimentally and by MC simulations, in the development of differ-ent configuration CdTe/CZT detection plane prototypes for high-energy polarime-try. 2D and 3D CZT/CdTe spectroscopic imagers with coincidence readout logiccan efficiently handle scattering events to perform simultaneously polarisation, spec-troscopy, imaging, and timing measurements. We give an overview on achievablepolarimetric performance by CdTe/CZT spectroscopic imagers and how these per-formance are affected by detector configuration parameters. Furthermore we wouldaddress the perspective of scattering polarimetry opened by the implementation of

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new high energy focussing optics as broad band Laue lens in next generation of hardX and soft gamma ray astronomy instrumentation. The unprecedented sensitivityachievable by these telescope will open definitely the window of polarimetry also inthe field of hard X- and soft gamma-rays.

Polarisation of high-energy emission in a pulsar stripedwind

Jerome Petri

Observatoire astronomique de Strasbourg

Although discovered 50 years ago, pulsars remain enigmatic compact objects butexquisite places where to test our current theories in strong magnetic and gravita-tional fields. Within the last few years, the physics of pulsars has greatly benefitedfrom terrestrial and space based observations. These magnetized neutron stars har-bour strong gravitational fields and host strong electromagnetic fields giving rise tonon-linear corrections to Maxwell equations described by quantum electrodynamics(QED). Magnetic field strengths around the critical value of B = 4.4×109 T aresuspected to sustain their electromagnetic activity like pair creation. It is thereforeimportant to include general relativistic (GR) effects as well as QED corrections.This is achieved through a 3+1 formalism for Maxwell equations with GRQED.These equations are then solved with standard finite volume methods for hyperbolicconservation laws including GR and QED into the magnetospheric picture. As anapplication, we compare spin-down luminosity corrections associated to QED fromthose associated to GR. Future phase-resolved X-ray and gamma-ray spectroscopyand polarization will strongly discriminate between competing high-energy modelsand would be valuable indicators for the magnetic field topology at the emissionsites: magnetosphere or wind.

Mirion’s contribution to space projects

Pascal Quirin

Mirion Technologies (Canberra) France

Mirion Technologies (Canberra) is developing special gamma ray detectors based onhigh purity Germanium material for the sensor to address demanding space appli-cations. The goal is to combine state of the art technologies like ultra-high vacuumencapsulation or dedicated passivation or contacting technology to offer ultimatereliability with minimized payload to fulfill drastic book of specifications. Our pre-sentation will show Mirion’s contribution to many spaces missions including SPIIntegral, Mars Odyssey and Selene. We will also highlight new detector technologiesnow available for space projects.

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X-ray Polarization from Accreting Black Holes and NeutronStars

Jeremy Schnittman

NASA / Goddard Space Flight Center

Accreting neutron stars and stellar-mass black holes are among the brightest X-raysources in the sky. Thus, they are excellent candidates for X-ray polarimetry, a tech-nique that requires very large number of photons for a sensitive measurement. Foraccreting black holes in the thermal state, polarization provides important informa-tion about the black hole’s spin magnitude and orientation relative to the observer.For black holes in the “low-hard” or “steep power-law” states, polarization providesa unique probe of the geometry of the hot electron corona. Accreting (low magneticfield) neutron stars also provide the opportunity to measure the geometry of theboundary layer and possibly even the neutron star radius.

Design and development of multilayer mirror based softX-ray polarimeter for Astronomical observations

Srikanth Panini Singam

Indian Institute of Astrophysics

X-ray polarimetric study of astrophysical sources is one of the last investigativewindows in X-ray astronomy that remains poorly explored. These studies promiseto reveal unique information about physical processes in a variety of astrophysicalobjects in our universe. The difficulty in measuring X-ray polarization has beenmainly due to limitations in efficiency of experiments designed to extract polar-ization information. Different techniques are beingused for X-ray polarimetry atdifferent energies. Photo electron tracking technique for energy range 2 – 10 keVand Compton/ Thompson scattering techniques for energies 5 – 100 keV are beingproposed for future x-ray polarimetric missions. Bragg reflection polarimetry at subkeV X-rays is possible using multilayer mirrors. We present a simple design of asoft X-ray polarimeter using multilayer mirrors for broadband polarization sensitiv-ity. The instrumentconsists of an X-ray concentrator, a set of multilayer mirrorsplaced at 45◦ from the optical axis and a detector at the Nasmyth focus. The in-strument rotating about its optical axis during observations, can measure the linearpolarization of 0.2 – 0.7 keV X-rays from astronomical sources. The use of a softX-ray concentrator with geometrical area ∼ 630 cm2 provides sufficient sensitivityto address key scientific questions. Five different multilayer mirrors placed on arotating wheel provide the option to measure polarization in any of the five narrowbands spanning the 0.2 – 0.7 keV range. Design, estimated performance and currentinstrumental developmental status will be presented.

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The high/soft state of Cygnus X-1 from the viewpoints ofX-ray spectrum and polarization

Hiromitsu Takahashi

Hiroshima University

Cygnus X-1 is a persistently bright black-hole binary in our Galaxy. In the high/softstate of black-hole binaries, the energy spectrum is dominated by a soft thermalemission from the accretion disk. If the inner part of the disk reaches the innermoststable circular orbit, it is suitable to study the black spin. Suzaku satellite observedthe softest ever spectrum of Cygnus X-1 in 2013. We analyzed the energy spectrumwith several emission models and concluded that the so-called high/soft state ofCygnus X-1 does not become the typical one observed in other transient sources(Kawano et al. 2017 PASJ). We discuss the results by comparing them with thepolarization information of Cygnus X-1 previously reported by OSO-8 satellite.

X-ray polarization as a tool to understand coronae inaccreting sources

Francesco Tamborra

Astronomical Institute of the Czech Academy of Sciences

Up-scattering of low-energy photons by Inverse Compton on a hot gas of electrons(i.e. Comptonization) is an important mechanism in Astrophysics. In particular, inaccreting sources such as X-Ray Binaries (XRBs) and Active Galactic Nuclei (AGN)this mechanism is believed to be responsible for their hard X-ray emission: soft pho-tons produced by the accretion disc are Comptonized by a corona of hot electronswhose physical parameters and especially the geometry are not well known. Spec-troscopical analysis alone is not able to discriminate between different geometriesand parameters of the corona. In the context of this scenario we developed MoCA:a Monte Carlo code for Comptonization in Astrophysics which includes polarizationand we are going to show how X-ray polarimetry can be the tool to understandcoronae in accreting sources.

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On the spectrum and polarization of magnetar flareemission

Roberto Taverna

Physics and Astronomy Department - University of Padova

Bursts and flares are among the distinctive observational manifestations of magne-tars, isolated neutron stars endowed with an ultra-strong magnetic field (B ≈ 1014

– 1015 G). It is believed that these events arise in a hot electron-positron plasma,injected in the magnetosphere, due to a magnetic field instability, which remainstrapped within the closed magnetic field lines (the “trapped-fireball” model). Wehave developed a simple radiative transfer model to simulate magnetar flare emis-sion in the case of a steady trapped fireball. After dividing the fireball surface ina number of plane-parallel slabs, the local spectral and polarization properties areobtained integrating the radiative transfer equations for the two normal modes. Weassume that magnetic Thomson scattering is the dominant source of opacity, andneglect contributions from second-order radiative processes, although the presenceof double-Compton scattering is accounted for in establishing local thermal equi-librium in the fireball atmospheric layers. The observed spectral and polarizationproperties as measured by a distant observer are obtained summing the contribu-tions from the patches which are visible for a given viewing geometry by means ofa ray-tracing code. The spectra we obtained in the 1 – 100 keV energy range arethermal and can be described in terms of the superposition of two blackbodies. Theblackbody temperature and the ratio of the emitting areas are in broad agreementwith the observations available so far. The predicted linear polarization degree isin general greater than 80% over the entire energy range. Such a large degree ofpolarization should be easily detectable by new-generation X-ray polarimeters, likeIXPE, XIPE and eXTP, allowing to confirm the model predictions.

Polarization of neutron star emission and future X-raymissions

Roberto Turolla

University of Padova

Radiation emitted in the vicinity of an isolated neutron star is expected to be intrin-sically polarized because the high magnetic field (B ∼ 1012-1015 G) strongly affectsthe plasma opacity. The polarization fraction and polarization angle measured byan instrument, however, do not necessary coincide with the intrinsic ones, due to theeffects of both quantum electrodynamics in the highly magnetized vacuum aroundthe star (the vacuum polarization) and rotation of the Stokes parameters in theplane perpendicular to the line of sight induced by the non-uniform magnetic field.I will review theoretical estimates for the polarization observables in the case ofthermal surface emission from neutron stars and of the (soft) X-ray emission frommagnetars, where magnetospheric reprocessing of radiation by resonant comptonscattering is important. The potentials of X-ray polarimetry to probe the physical

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conditions in neutron star sources and to test, for the first time, vacuum polarizationare discussed in connection with the recently proposed polarimetric missions: IXPE,recently selected by NASA, XIPE, under evaluation by ESA for the M4 competition,and eXTP.

X-ray imaging polarimetry

Jacco Vink

University of Amsterdam

With the launch of IXPE and potentially later, XIPE and eXTP, a new era in X-ray polarisation will begin. It will be the first time that X-ray polarisation can bemeasured with positional information: the start of X-ray imaging polarimetry.

Traditionally X-ray polarisation detection techniques depend on fitting a mod-ulation curve using a chi-squared technique, using the fact that the distribution ofphoton electron angles will be modulated as cos(2α+φ0). However, this requiresfirst choosing an extraction region and then testing whether the overall X-ray signalis polarised. However, there is method that directly creates Q and U maps andderivatives from the photon list. These maps can be easily (adaptively) rebinned,making it much more clear what the position-dependent signal looks like, and notrequiring a priori knowledge about the region that needs to be extracted: the mapswill show which regions are significantly polarised. Moreover, the method allows foran energy dependent-correction factor on a photon by photon basis, which can alsobe propagated to the error on the maps. The resulting maps then contain the bestestimates for the actual X-ray U and Q.

Finally, note that this method is also suitable for making Q and U spectra ortime series, as this involves binning of time and energy, rather than position.

An Overview of X-ray Polarimetry of Astronomical Sources

Martin C. Weisskopf

NASA/MSFC

We review the history of astronomical X-ray polarimetry, beginning with earlysounding-rocket experiments by Robert Novick at Columbia University and his team,of which I was a member. After describing various techniques for measuring X-ray polarization, we discuss the polarimeter aboard Orbiting Solar Observatory 8(OSO-8) and its scientific results. Next, we consider the X-ray polarimeter aboardthe ill-fated original Spectrum-X mission, which provided important lessons on po-larimeter design, systematic effects, and programmatics of a shared focal plane. Weconclude with a description of the Imaging X-ray Polarimetry Explorer (IXPE) andits prospective scientific return. IXPE, a partnership between NASA and ASI, hasbeen selected as a NASA Astrophysics Small Explorers Mission and will launch in2021.

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SPHiNX: Compton scattering polarimeter for GRBs

Fei Xie

KTH - Royal Institute of Technology

Polarisation measurements of gamma-ray bursts probe how and where the energyis released in GRBs. Understanding the emission mechanism holds the key to usingGRBs as probes of the early universe and of extreme physical processes occurringin the universe, including the newly discovered gravitational waves. SPHiNX islarge field-of-view (∼ 120◦) Compton scattering polarimeter covering the energyrange ∼ 30 – 600 keV. The polarimeter is proposed for the second Swedish Innosatmission with launch foreseen around 2021. The polarimeter design and expectedperformance will be discussed.

TBA

Valentina Zharkova

Northumbria University

TBA

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AbstractsPoster contributions

An extended nuclear torus revealed in NGC 1068 thanks topolarimetric imaging at high angular resolution in the

near-IR

Damien Gratadour

LESIA - Observatoire de Paris

In this paper we show strong evidence for an extended nuclear torus at the center ofNGC 1068 thanks to new adaptive optics assisted polarimetric observations in thenear-infrared with SPHERE on the VLT. The orientation of the polarization vectorsclearly evidences the presence of a structured hourglass-shaped bicone and a compactelongated nuclear structure perpendicular to the bicone axis. The linearly polarizedemission in the bicone is dominated by a centro-symmetric pattern, but the centralcompact region shows a clear deviation from the latter with linear polarizationaligned along a direction perpendicular to the bicone axis. We believe this extendedpatch of linear polarization to be the first direct evidence for an extended torus atthe core of the archetypal Seyfert 2 galaxy.

Polarimetric imaging of AGN at high angular resolution innear Infrared - Observation and simulation of an extended

nuclear torus

Lucas Grosset

LESIA - Observatoire de Paris

The unified model of Active Galactic Nuclei (AGN) interpret the observational dif-ferences between AGN types as being driven by the inclination of an equatorialdusty torus, with all the AGN observed being intrinsically the same objects. Be-cause of the high optical depth of the torus dust at short wavelengths, blocking theemission from the central source, is it challenging to detect this fundamental pieceof the model. We took advantage of the SPHERE high contrast capacities and highangular resolution to obtain polaro-images of NGC 1068, the archetypal Seyfert 2galaxy in the near infrared (NIR). The polarisation position angle map clearly tracesthe expected centro-symmetric pattern and diverging from this pattern, a centralregion of constant polarisation position angle at the expected torus location. In

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order to analyse this feature, we developed a radiative transfer code, MontAGN,optimised for NIR wavelengths. With simulations, we were able to reproduce mostof the observed features on the polaro-images of SPHERE thanks to a model withan optically thick torus, extending into a fainter outer part. Simulated maps areconsistent with the observed centro-symmetric pattern and a central region of con-stant polarisation orientation. These constrain the equatorial optical depth at 20 inK band at maximum, and to an electrons density in the ionisation cone consistentwith previous observations.

X-ray spectral properties of the blazar 3C 273 in quiescentstates

Nibedita Kalita

ARIES, Nainital, India

We present the results of X-ray spectral analysis and multi-waveband correlationstudy of the blazar 3C 273 with 15 years of XMM-Newton observations. An ab-sorbed simple power law can well describe all the spectra in the energy range 2.5– 10 keV. A “harder-when-brighter” trend for the spectrum is found in these long-term multi-epoch monitorning of the source for the first time. Our study findsan anti-correlation between X-ray spectral slope and logarithm of the ratio of thesimultaneous X-ray to optical/UV fluxes in the low/quiescent state; however, therelation we have observed is not applicable during a flaring state of the source. Thisresult is statistically 95% significant while checked with Bootstarp method. TheX-ray emission in the source originates from inverse Compton scattering of opti-cal/UV photons, most likely in a thermal corona residing above the accretion disk.The significant variation in the hard X-ray continuum can be explained in terms ofchanging optical depth of the Comptonizing medium, along the lines of wind-shockmodel proposed by Courvoisier and Camenzind (1989).

X-ray eclipsing events in active galactic nuclei

Elias Kammoun

SISSA - Trieste

X-ray eclipses of the innermost regions by distant clouds (associated to the broadline region and/or the molecular torus) have been detected in several local brightactive galactic nuclei (AGN). Combining the high-quality spectra, below 10 keV,provided by XMM-Newton and the availability of relativistic X-ray spectral modelsof such occultation events, this allows us to perform a detailed modelling of theseevents in order to obtain accurate information on the nature and the geometry ofthe system, by performing “tomography” experiments on the accretion disc. Wewill revisit in this poster X-ray eclipsing events, from archival data, that we modelusing the KYN model. We will present also theoretical predictions on the variousparameters inferred by studying such events in simulated ATHENA spectra. Weintend to enhance this work to a next level by modelling the X-ray polarization thatis induced by X-ray eclipses in AGN.

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X-ray spectroscopy of polar-scattered Seyfert 1 galaxies:The case of Fairall 51

Romana Mikusincova

Institute of Theoretical Physics of the Charles University in Prague / AstronomicalInstitute of the Czech Academy of Sciences

We present a spectral re-analysis of Fairall 51, optically a Seyfert 1 galaxy, which,however, shows a very strong polarization. As this feature is not characteristic forSeyfert 1 objects, this galaxy may represent a borderline object between Seyfert 1and Seyfert 2 galaxies and provide with valuable information about the structureof Active Galactic Nuclei. We present results from a study of two archival sets ofobservations performed in September 2005 and March 2006 by the European satelliteXMM-Newton and four observations performed in September 2013 by the Japaneseobservatory Suzaku. It follows from the spectral analysis that there are at least twoor three ionized absorbers in the studied AGN. Based on the spectral variability,we estimated the location of the variable absorber to be 3 to 60 light days from thecentral engine, which indicates its origin in the Broad Line Region.

X-ray Polarization from Winds in Active Galactic Nuclei?

Francesco Tombesi

University of Rome “Tor Vergata” / NASA/GSFC, University of Maryland

Powerful winds driven by active galactic nuclei (AGN) are often invoked to play afundamental role in the evolution of both supermassive black holes (SMBHs) andtheir host galaxies. Moreover, disk winds are found to be ubiquitous in accretingcompact objects. X-ray observations are fundamental to detect the most powerfulwinds, with velocities up to mildly relativistic values. Such highly ionized, highcolumn density winds are expected to be observed close to the accretion disk andtheir possible polarization signature has not been sufficiently investigated. X-raypolarization may provide information regarding the wind geometry, location, density,and ionization. It may also provide information on the acceleration, especially ifthe winds are driven by large-scale, ordered magnetic field lines. Polarization frommassive disk winds may also potentially influence the signal expected from accretiondisks.

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Author Index

Agıs-GonzalezBeatriz, 1

BerdyuginaSvetlana, 2

BucciantiniNiccolo, 2

CaiazzoIlaria, 3

CostaEnrico, 3

DayFrancesca, 4

DovciakMichal, 4

FabianiSergio, 5

FriisMette, 5

GogusErsin, 5

GratadourDamien, 19

GrossetLucas, 19

HartlStefan F., 6

HeylJeremy, 6

KalitaNibedita, 20

KammounElias, 20

KazanasDemosthenes, 6

KoleMerlin, 7

KrippendorfSven, 7

MarinFrdric, 8

MarinucciAndrea, 8

MarshallHerman, 9

MattGiorgio, 10

MignaniRoberto, 10

MikhalevVictor, 11

MikusincovaRomana, 21

MoitaMiguel, 11

PetriJerome, 12

QuirinPascal, 12

SchnittmanJeremy, 13

SingamSrikanth Panini, 13

TakahashiHiromitsu, 14

TamborraFrancesco, 14

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TavernaRoberto, 15

TombesiFrancesco, 21

TurollaRoberto, 15

VinkJacco, 16

WeisskopfMartin C., 16

XieFei, 17

ZharkovaValentina, 17

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