Sensitivity of stacked imaging detectors to hard X-ray polarization

The development of multi-layer optics which allow to focus photons up to 100 keV and more promises an enormous jump in sensitivity in the hard X-ray energy band. This technology is already planned to be exploited by future missions dedicated to spectroscopy and imaging at energies >10 keV, e.g. Astro-H and NuSTAR. Nevertheless, our understanding of the hard X-ray sky would greatly benefit from carrying out contemporaneous polarimetric measurements, because the study of hard spectral tails and of polarized emission often are two complementary diagnostics of the same non-thermal and acceleration processes. At energies above a few tens of keV, the preferred technique to detect polarization involves the determination of photon directions after a Compton scattering. Many authors have asserted that stacked detectors with imaging capabilities can be exploited for this purpose. If it is possible to discriminate those events which initially interact in the first detector by Compton scattering and are subsequently absorbed by the second layer, the direction of scattering is singled out from the hit pixels in the two detectors. In this paper we give the first detailed discussion of the sensitivity of such a generic design to the X-ray polarization. The efficiency and the modulation factor are calculated analytically from the geometry of the instruments and then compared with the performance as derived by means of Geant4 Monte Carlo simulations.Comment: 14 pages, 11 figures. Accepted for publication in Ap

Measurement of the position resolution of the Gas Pixel Detector

The Gas Pixel Detector was designed and built as a focal plane instrument for X-ray polarimetry of celestial sources, the last unexplored subtopics of X-ray astronomy. It promises to perform detailed and sensitive measurements resolving extended sources and detecting polarization in faint sources in crowded fields at the focus of telescopes of good angular resolution. Its polarimetric and spectral capability were already studied in earlier works. Here we investigate for the first time, with both laboratory measurements and Monte Carlo simulations, its imaging properties to confirm its unique capability to carry out imaging spectral-polarimetry in future X-ray missions.Comment: Submitted to Nuclear Instruments and Methods in Physics Research Section A; 15 figures, 3 table

Correlation methods for the analysis of X-ray polarimetric signals

X-ray polarimetric measurements are based on studying the distribution of the directions of scattered photons or photoelectrons and on the search of a sinusoidal modulation with a period of {\pi}. We developed two tools for investigating these angular distributions based on the correlations between counts in phase bins separated by fixed phase distances. In one case we use the correlation between data separated by half of the bin number (one period) which is expected to give a linear pattern. In the other case, the scatter plot obtained by shifting by 1/8 of the bin number (1/4 of period) transforms the sinusoid in a circular pattern whose radius is equal to the amplitude of the modulation. For unpolarized radiation these plots are reduced to a random point distribution centred at the mean count level. This new methods provide direct visual and simple statistical tools for evaluating the quality of polarization measurements and for estimating the polarization parameters. Furthermore they are useful for investigating distortions due to systematic effects

Modelling the X-ray polarimetric signatures of complex geometry: the case study of the "changing look" AGN NGC 1365

"Changing look" Active Galactic Nuclei (AGN) are a subset of Seyfert galaxies characterized by rapid transitions between Compton-thin and Compton-thick regimes. In their Compton-thin state, the central engine is less obscured, hence spectroscopy or timing observations can probe their innermost structures. However, it is not clear if the observed emission features and the Compton hump are associated with relativistic reflection onto the accretion disc, or complex absorption by distant, absorbing gas clouds passing by the observer's line-of-sight. Here, we investigate these two scenarios under the scope of X-ray polarimetry, providing the first polarisation predictions for an archetypal "changing look" AGN: NGC 1365. We explore the resulting polarisation emerging from lamp-post emission and scattering off an accretion disc in the immediate vicinity of a supermassive black hole. The computed polarisation signatures are compared to the results of an absorption-dominated model, where high column density gas partially covers the central source. While the shape of the polarisation spectrum is similar, the two models differ in net polarisation percentage, with the relativistic reflection scenario producing significantly stronger polarisation. Additionally, the variation of the polarisation position angle is distinctly different between both scenarios: the reflection-dominated model produces smooth rotations of the polarisation angle with photon energy whereas circumnuclear absorption causes an orthogonal switch of the polarisation angle between the soft and the hard X-ray bands. By comparing the predicted polarisation of NGC 1365 to the detectability levels of X-ray polarimetry mission concepts proposed in the past, we demonstrate that with a large, soft X-ray observatory or a medium-sized mission equipped with a hard (6 - 35 keV) polarimeter, the correct interpretation would be unambiguous.Comment: 6 pages, 4 figures, accepted for publication in the Monthly Notices of the Royal Astronomical Societ

Probing magnetars magnetosphere through X-ray polarization measurements

The study of magnetars is of particular relevance since these objects are the only laboratories where the physics in ultra-strong magnetic fields can be directly tested. Until now, spectroscopic and timing measurements at X-ray energies in soft gamma-repeaters (SGRs) and anomalous X-ray pulsar (AXPs) have been the main source of information about the physical properties of a magnetar and of its magnetosphere. Spectral fitting in the ~ 0.5-10 keV range allowed to validate the "twisted magnetosphere" model, probing the structure of the external field and estimating the density and velocity of the magnetospheric currents. Spectroscopy alone, however, may fail in disambiguating the two key parameters governing magnetospheric scattering (the charge velocity and the twist angle) and is quite insensitive to the source geometry. X-ray polarimetry, on the other hand, can provide a quantum leap in the field by adding two extra observables, the linear polarization degree and the polarization angle. Using the bright AXP 1RXS J170849.0-400910 as a template, we show that phase-resolved polarimetric measurements can unambiguously determine the model parameters, even with a small X-ray polarimetry mission carrying modern photoelectric detectors and existing X-ray optics. We also show that polarimetric measurements can pinpoint vacuum polarization effects and thus provide an indirect evidence for ultra-strong magnetic fields.Comment: 12 pages, 8 figures, accepted for publication in MNRA

Characterization of scatterers for an active focal plane Compton polarimeter

In this work we present an active Compton scattering polarimeter as a focal plane instrument able to extend the X-ray polarimetry towards hard X-rays. Other authors have already studied various instrument design by means of Monte Carlo simulations, in this work we will show for the first time the experimental measurements of "tagging efficiency" aimed to evaluate the polarimeter sensitivity as a function of energy. We performed a characterization of different scattering materials by measuring the tagging efficiency that was used as an input to the Monte Carlo simulation. Then we calculated the sensitivity to polarization of a design based on the laboratory set-up. Despite the geometry tested is not optimized for a realistic focal plane instrument, we demonstrated the feasibility of polarimetry with a low energy threshold of 20 keV. Moreover we evaluated a Minimum Detectable Polarization of 10% for a 10 mCrab source in 100 ks between 20 and 80 keV in the focal plane of one multilayer optics module of NuSTAR. The configuration used consisted of a doped p-terphenyl scatterer 3 cm long and 0.7 cm of diameter coupled with a 0.2 cm thick LaBr3 absorber.Comment: Accepted in press on Astroparticle Physics, 201

A versatile facility for the calibration of X-ray polarimeters with polarized and unpolarized controlled beams

We devised and built a versatile facility for the calibration of the next generation X-ray polarimeters with unpolarized and polarized radiation. The former is produced at 5.9 keV by means of a Fe55 radioactive source or by X-ray tubes, while the latter is obtained by Bragg diffraction at nearly 45 degrees. Crystals tuned with the emission lines of X-ray tubes with molybdenum, rhodium, calcium and titanium anodes are employed for the efficient production of highly polarized photons at 2.29, 2.69, 3.69 and 4.51 keV respectively. Moreover the continuum emission is exploited for the production of polarized photons at 1.65 keV and 2.04 keV and at energies corresponding to the higher orders of diffraction. The photons are collimated by means of interchangeable capillary plates and diaphragms, allowing a trade-off between collimation and high fluxes. The direction of the beam is accurately arranged by means of high precision motorized stages, controlled via computer so that long and automatic measurements can be done. Selecting the direction of polarization and the incidence point we can map the response of imaging devices to both polarized and unpolarized radiation. Changing the inclination of the beam we can study the systematic effects due to the focusing of grazing incidence optics and the feasibility of instruments with large field of view.Comment: 12 pages, 11 figure

The gas pixel detector as a solar X-ray polarimeter and imager

The Sun is the nearest astrophysical source with a very intense emission in the X-ray band. The study of energetic events, such as solar flares, can help us to understand the behaviour of the magnetic field of our star. There are in the literature numerous studies published about polarization predictions, for a wide range of solar flares models involving the emission from thermal and/or non-thermal processes, but observations in the X-ray band have never been exhaustive. The gas pixel detector (GPD) was designed to achieve X-ray polarimetric measurements as well as X-ray images for far astrophysical sources. Here we present the possibility to employ this instrument for the observation of our Sun in the X-ray band.Comment: J. Adv. Space Res. (2011

Expectations and perspectives of X-ray photoelectric polarimetry

La polarimetria è l'ultima branca dell'Astronomia nella banda dei raggi X ancora inesplorata. Nonostante un vasto interesse testimoniato da un'estesa letteratura, il suo sviluppo è stato impedito dalla mancanza di dispositivi la cui sensibilità giustificasse l'inserimento di polarimetri X a bordo delle missioni spaziali moderne. Ciononostante nuovi strumenti basati sull'effetto fotoelettrico, capaci di risolvere le tracce dei fotoelettroni in un gas, offrono oggi la possibilità di colmare il divario tra aspettative teoriche e gli scarsi risultati ad oggi raggiunti: in questo ambito, uno dei progetti più avanzati è il Gas Pixel Detector (GPD), sviluppato in Italia dall'INFN di Pisa e dall'INAF/IASF di Roma. Questa tesi ha contribuito dell'inserimento del GPD a bordo delle prossime missioni in differenti aspetti. Innanzitutto è stata misurata la risposta dello strumento a radiazione polarizzata di alcuni keV, ovvero nell'intervallo energetico di massima sensibilità, grazie ad una sorgente di calibrazione basata sulla diffrazione di Bragg a circa 45 gradi. Quest'ultima, insieme a sorgenti radioattive e a tubi a raggi X, è stata montata su un sistema meccanico, progettato e costruito in modo tale che lo strumento in esame potesse essere spostato, inclinato e ruotato rispetto al fascio incidente. Questo rende disponibile una struttura per la completa caratterizzazione del GPD (e di altri strumenti): prime e preliminari misure di calibrazione sono state effettuate grazie ad essa e sono di seguito presentate. Infine ho calcolato la sensibilità del GPD nel caso in cui esso sia posto nel piano focale di telescopi progettati per le prossime missioni spaziali, come PolariX e IXO, e ho discusso ulteriori applicazioni del GPD e dei polarimetri fotoelettrici in generale. In particolare, sono presentati il possibile utilizzo di questi dispositivi come strumenti a grande campo di vista o e quello come rivelatori sensibili fino ad energie di decine di keV.Polarimetry is the last unexplored branch of X-ray Astronomy. Despite a wide interest proved by an extended literature, its development has suffered the lack of instruments which could assure a sufficient sensitivity to justify the inclusion of X-ray polarimeters on-board modern missions. Nevertheless new devices based on photoelectric absorption, which are able to image the track of photoelectrons in a gas mixture, offer today the possibility to fill the gap between theoretical expectations and the current meagre results: in this regard, one of the most advanced project is the GPD (Gas Pixel Detector), developed in Italy by INFN of Pisa and INAF/IASF of Rome. This work of thesis contributed to the use of the GPD on-board future space missions from different points of view. The response of the instrument to polarized radiation at a few keV, namely in the energy range of maximum sensitivity, was measured thanks to a calibration source based on Bragg diffraction at nearly 45 degrees. This source, together with radioactive unpolarized sources and X-ray tubes, were also interfaced with a mechanical assembly which was designed and built allowing for the movement, the inclination and the rotation of the instrument with respect to the incident beam. This makes available a facility for the complete characterization of the GPD (and other instruments) and the first and preliminary systematic measurements of calibration are presented. I also derive the scientific performances of the GPD when the instrument is placed in the focal plane of realistic X-ray telescopes planned for future missions, such as PolariX or IXO, and discuss some further applications of the GPD and of the photoelectric polarimeters in general. In particular the possible use of these devices as large field of view instruments or at energies up to tens of keV are presented