Hard X-ray Polarimeter Research Articles

The X-ray polarimeter of solar flares for the Interhelioprobe mission: experimental determination of the response of the physical model to polarized x-ray radiation

The work is devoted to the study of the characteristics of the PING-M hard x-ray polarimeter using its physical model (PING-P FM). The PING-M instrument is developed jointly by the Moscow Engineering Physical Iinstitute and the Ioffe Institute for the mission “Interhelioprobe”. The operation of the device is based on Compton scattering. The degree and direction of linear polarization are determined by measuring the asymmetry of the scattered radiation field. The device uses active scatterers that register the Compton recoil electron. A useful event is the case when two impulses in the detector-scatterer and in the detector-receiver of scattered radiation coincide. The physical model represents the detector part of the polarimeter. It contains three scatterers and six receivers of scattered radiation – a total of 18 pairs of detectors oriented at different azimuthal angles. As a result of the experiments, the dependences of counting rates in pairs of detectors on the positional angle of the polarization plane of the incident radiation were measured. The modulation depth of this dependence determines the sensitivity of the device to the polarization degree. The sensitivity of the device is estimated.

Observations of a GX 301–2 Apastron Flare with the X-Calibur Hard X-Ray Polarimeter Supported by NICER, the Swift XRT and BAT, and Fermi GBM

Abstract The accretion-powered X-ray pulsar GX 301−2 was observed with the balloon-borne X-Calibur hard X-ray polarimeter during late 2018 December, with contiguous observations by the Neutron star Interior Composition Explorer Mission (NICER) X-ray telescope, the Swift X-ray Telescope and Burst Alert Telescope, and the Fermi Gamma-ray Burst Monitor spanning several months. The observations detected the pulsar in a rare apastron flaring state coinciding with a significant spin up of the pulsar discovered with the Fermi Gamma-ray Burst Monitor. The X-Calibur, NICER, and Swift observations reveal a pulse profile strongly dominated by one main peak, and the NICER and Swift data show strong variation of the profile from pulse to pulse. The X-Calibur observations constrain for the first time the linear polarization of the 15–35 keV emission from a highly magnetized accreting neutron star, indicating a polarization degree of % (90% confidence limit) averaged over all pulse phases. We discuss the spin up and the X-ray spectral and polarimetric results in the context of theoretical predictions. We conclude with a discussion of the scientific potential of future observations of highly magnetized neutron stars with the more sensitive follow-up mission XL-Calibur.

A fully automated data center for the space-borne hard X-ray Compton polarimeter POLAR developed at PSI

Abstract Constant data inflow from the hard X-ray polarimeter POLAR onboard of the Chinese Space Laboratory TG-2 requires fully automated and safe guarded data processing. For this purpose, a dedicated data center was established at the Paul Scherrer Institute (PSI). This paper presents its design concept and structure, and describes the data flow and data products, data processing pipelines, quick-look utilities and web-based applications. Because the amount of POLAR telemetry data can reach up to 100 GB daily, specialized software and hardware with dedicated architecture are required to store and process this large amount of data. POLAR is a space-borne detector to precisely measure the linear polarization of hard X-rays emitted from solar flares and gamma-ray bursts in the energy range of 50–500 keV. It was launched into a low earth orbit on September 15th, 2016.

PoGO+ polarimetric constraint on the synchrotron jet emission of Cygnus X-1

Abstract We report a polarimetric constraint on the hard X-ray synchrotron jet emission from the Cygnus X-1 black hole binary system. The observational data were obtained using the PoGO+ hard X-ray polarimeter in 2016 July, when Cygnus X-1 was in the hard state. We have previously reported that emission from an extended corona with a low polarization fraction is dominating, and that the polarization angle is perpendicular to the disc surface. In the soft gamma-ray regime, a highly polarized synchrotron jet is reported with INTEGRAL observations. To constrain the polarization fraction and flux of such a jet component in the hard X-ray regime, we now extend analyses through vector calculations in the Stokes QU plane, where the dominant corona emission and the jet component are considered simultaneously. The presence of another emission component with different polarization angle could partly cancel out the net polarization. The 90 per cent upper limit of the polarization fraction for the additional synchrotron jet component is estimated as <10 per cent, <5 per cent, and <5 per cent for polarization angle perpendicular to the disc surface, parallel to the surface, and aligned with the emission reported by INTEGRAL data, respectively. From the 20–180 keV total flux of 2.6 × 10−8 erg s−1 cm−2, the upper limit of the polarized flux is estimated as < 3 × 10−9 erg s−1 cm−2.

Cadmium-Zinc-Telluride Imager On-Board Astrosat:A Multi-Faceted Hard X-Ray Instrument

The AstroSat satellite is designed to make multi-waveband observations of astronomical sources and the Cadmium Zinc Telluride Imager (CZTI) instrument of AstroSat covers the hard X-ray band. CZTI has a large area position sensitive hard X-ray detector equipped with a Coded Aperture Mask, thus enabling simultaneous background measurement. Ability to record simultaneous detection of ionizing interactions in multiple detector elements is a special feature of the instrument and this is exploited to provide polarization information in the 100 - 380 keV region. CZTI provides sensitive spectroscopic measurements in the 20 - 100 keV region, and acts as an all sky hard X-ray monitor and polarimeter above 100 keV. During the first year of operation, CZTI has recorded several gamma-ray bursts, measured the phase resolved hard X-ray polarization of the Crab pulsar, and the hard X-ray spectra of many bright Galactic X-ray binaries. The excellent timing capability of the instrument has been demonstrated with simultaneous observation of the Crab pulsar with radio telescopes like GMRT and Ooty radio telescope.

Calibration and performance studies of the balloon-borne hard X-ray polarimeter PoGO+

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 (20–150keV) 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 MCrab=(41.75±0.85)%, resulting in an expected Minimum Detectable Polarisation (MDP) of 7.3% for a 7day flight. This will allow a measurement of the Crab polarisation parameters with at least 5σ statistical significance assuming a polarisation fraction ∼20% – a significant improvement over the PoGOLite Pathfinder mission which flew in 2013 and from which the PoGO+ design is developed.

Optimising a balloon-borne polarimeter in the hard X-ray domain: From the PoGOLite Pathfinder to PoGO+

PoGOLite is a balloon-borne hard X-ray polarimeter dedicated to the study of point sources. Compton scattered events are registered using an array of plastic scintillator units to determine the polarisation of incident X-rays in the energy range 20–240 keV. In 2013, a near circumpolar balloon flight of 14 days duration was completed after launch from Esrange, Sweden, resulting in a measurement of the linear polarisation of the Crab emission. Building on the experience gained from this Pathfinder flight, the polarimeter is being modified to improve performance for a second flight in 2016. Such optimisations, based on Geant4 Monte Carlo simulations, take into account the source characteristics, the instrument response and the background environment which is dominated by atmospheric neutrons. This paper describes the optimisation of the polarimeter and details the associated increase in performance. The resulting design, PoGO+, is expected to improve the Minimum Detectable Polarisation (MDP) for the Crab from 19.8% to 11.1% for a 5 day flight. Assuming the same Crab polarisation fraction as measured during the 2013 flight, this improvement in MDP will allow a 5σ constrained result. It will also allow the study of the nebula emission only (Crab off-pulse) and Cygnus X-1 if in the hard state.

Solar X-ray polarimetry and spectrometry instrument PING-M for the Interhelioprobe mission

The PING-M experiment is designed to investigate solar X-ray activity. The instrument includes a hard X-ray polarimeter (PING-P), a hard X-ray spectrometer (HXRS) and a soft X-ray spectrometer (SXRS). PING-P has the energy range of 20–150keV and an effective area of about 2.5cm2. It uses three organic scintillation detectors as active scatterers, which work in coincidence with six absorber detectors, based on CsI(Tl) scintillator. This technique allows us to considerably improve the polarimeter sensitivity. HXRS has the energy range of 20–600keV and an effective area of about 15cm2. It is based on a fast inorganic scintillator (LaBr3(Ce) or CeBr3) with a relatively high energy resolution of 3.5–4.5% at 662keV. The SXRS energy range is 1.5–25keV, and its aperture is ø0.1mm, which provides the registration of solar flares in the range from C1 to X20 class of GOES scale. It is based on a SDD semiconductor detector with an energy resolution better than 200eV at 5.9keV line. The experiment will be performed onboard the Russian interplanetary mission Interhelioprobe which is planned for launch after 2025.The instrument will allow us to investigate angular and energy distributions of accelerated electrons, plasma heating processes, etc. Stereoscopic polarimetry and spectrometric observations will be possible if a similar instrument is installed onboard a near Earth satellite, or the second probe of the Interhelioprobe mission.

The design and flight performance of the PoGOLite Pathfinder balloon-borne hard X-ray polarimeter

In the 50 years since the advent of X-ray astronomy there have been many scientific advances due to the development of new experimental techniques for detecting and characterising X-rays. Observations of X-ray polarisation have, however, not undergone a similar development. This is a shortcoming since a plethora of open questions related to the nature of X-ray sources could be resolved through measurements of the linear polarisation of emitted X-rays. The PoGOLite Pathfinder is a balloon-borne hard X-ray polarimeter operating in the 25 - 240 keV energy band from a stabilised observation platform. Polarisation is determined using coincident energy deposits in a segmented array of plastic scintillators surrounded by a BGO anticoincidence system and a polyethylene neutron shield. The PoGOLite Pathfinder was launched from the SSC Esrange Space Centre in July 2013. A near-circumpolar flight was achieved with a duration of approximately two weeks. The flight performance of the Pathfinder design is discussed for the three Crab observations conducted. The signal-to-background ratio for the observations is shown to be 0.25$\pm$0.03 and the Minimum Detectable Polarisation (99% C.L.) is (28.4$\pm$2.2)%. A strategy for the continuation of the PoGOLite programme is outlined based on experience gained during the 2013 maiden flight.

Prospects of the Hard X-Ray Instrument POLAR to Measure Polarization of Solar Flares

In-depth studies of solar flares emissions and energy releases include analyses of polarization data. Polarization gives clear information about mechanisms and processes leading to electron acceleration and photon production. Despite of many past attempts, the key energy range of hard X-rays was only rarely explored and results were inconclusive. To large extend it was due to greater instrumental complications. Currently several novel polarimeters are either to be employed or under constructions for both balloon and satellite based observations. The novel hard X-ray polarimeter POLAR is an instrument developed by a collaboration between Switzerland, China and Poland. It is primarily designed for high accuracy polarization measurements from the prompt photon emissions of the gamma-ray bursts. The satellite orientation and instrument pointing direction make it also capable for precise measurements of polarization in solar flares. The instrument should fly in near future onboard of the Chinese Space Station TG2.

Design and Tests of the Hard X-ray Polarimeter X-Calibur

X-ray polarimetry promises to give qualitatively new information bout high-energy astrophysical sources, such as binary black hole systems, micro-quasars, active galactic nuclei, and gamma-ray bursts. We designed, built and tested ahard X-ray polarimeter, X-<em>Calibur</em>, to be used in the focal plane of the InFOCuS grazing incidence hard X-ray telescope.X-<em>Calibur</em> combines a low-Z Compton scatterer with a CZT detector assembly to measure the polarization of 20−60 keV X-rays making use of the fact that polarized photons Compton scatter preferentially perpendicular to the electric field orientation; in principal, a similar space-borne experiment could be operated in the 5−100 keV regime. X-<em>Calibur</em> achieves a high detection efficiency of order unity.

Optimization of the design of the hard X-ray polarimeter X-Calibur

We report on the optimization of the hard X-ray polarimeter X-Calibur for a high-altitude balloon-flight in the focal plane of the InFOCμS X-ray telescope from Fort Sumner (NM) in Fall 2013. X-Calibur combines a low-Z scintillator slab to Compton-scatter photons with a high-Z Cadmium Zinc Telluride (CZT) detector assembly to photo-absorb the scattered photons. The detector makes use of the fact that polarized photons Compton scatter preferentially perpendicular to the electric field orientation. X-Calibur achieves a high detection efficiency of order unity and reaches a sensitivity close to the best theoretically possible. In this paper, we discuss the optimization of the design of the instrument based on Monte Carlo simulations of polarized and unpolarized X-ray beams and of the most important background components. We calculate the sensitivity of the polarimeter for the upcoming balloon flight from Fort Sumner and for additional longer balloon flights with higher throughput mirrors. We conclude by emphasizing that Compton polarimeters on satellite borne missions can be used down to energies of a few keV.

Compton polarimeter as a focal plane detector for hard X-ray telescope: sensitivity estimation with Geant4 simulations

X-ray polarimetry can be an important tool for investigating various physical processes as well as their geometries at the celestial X-ray sources. However, X-ray polarimetry has not progressed much compared to the spectroscopy, timing and imaging mainly due to the extremely photon-hungry nature of X-ray polarimetry leading to severely limited sensitivity of X-ray polarimeters. The great improvement in sensitivity in spectroscopy and imaging was possible due to focusing X-ray optics which is effective only at the soft X-ray energy range. Similar improvement in sensitivity of polarisation measurement at soft X-ray range is expected in near future with the advent of GEM based photoelectric polarimeters. However, at energies >10 keV, even spectroscopic and imaging sensitivities of X-ray detector are limited due to lack of focusing optics. Thus hard X-ray polarimetry so far has been largely unexplored area. On the other hand, typically the polarisation degree is expected to increase at higher energies as the radiation from non-thermal processes is dominant fraction. So polarisation measurement in hard X-ray can yield significant insights into such processes. With the recent availability of hard X-ray optics (e.g. with upcoming NuSTAR, Astro-H missions) which can focus X-rays from 5 KeV to 80 KeV, sensitivity of X-ray detectors in hard X-ray range is expected to improve significantly. In this context we explore feasibility of a focal plane hard X-ray polarimeter based on Compton scattering having a thin plastic scatterer surrounded by cylindrical array scintillator detectors. We have carried out detailed Geant4 simulation to estimate the modulation factor for 100 % polarized beam as well as polarimetric efficiency of this configuration. We have also validated these results with a semi-analytical approach. Here we present the initial results of polarisation sensitivities of such focal plane Compton polarimeter coupled with the reflection efficiency of present era hard X-ray optics.

Design and tests of the hard X-ray polarimeter X-Calibur

X-ray polarimetry will give qualitatively new information about high-energy astrophysical sources. We designed, built and tested a hard X-ray polarimeter X-Calibur to be used in the focal plane of the InFOCμS grazing incidence hard X-ray telescope. X-Calibur combines a low-Z Compton scatterer with a CZT detector assembly to measure the polarization of 10–80keV X-rays making use of the fact that polarized photons Compton scatter preferentially perpendicular to the electric field orientation. X-Calibur achieves a high detection efficiency of order unity.

Development of a micro-satellite TSUBAME for X-ray polarimetry of GRBs

AbstractHard X-ray polarization is believed to be one of the most promising methods to investigate the physical processes just around the central engines by constraining the magnetic environment. For this purpose we are now developing a compact and highly sensitive hard X-ray polarimeter aboard a university class micro-satellite “TSUBAME”. We are now developing the flight model of the satellite aiming for the launch in late 2012 from Russia.

A method to localize gamma-ray bursts using POLAR

The hard X-ray polarimeter POLAR aims to measure the linear polarization of the 50–500keV photons arriving from the prompt emission of γ-ray bursts (GRBs). The position in the sky of the detected GRBs is needed to determine their level of polarization. We present here a method by which, despite of the polarimeter incapability of taking images, GRBs can be roughly localized using POLAR alone. For this purpose scalers are attached to the output of the 25 multi-anode photomultipliers (MAPMs) that collect the light from the POLAR scintillator target. Each scaler measures how many GRB photons produce at least one energy deposition above 50keV in the corresponding MAPM. Simulations show that the relative outputs of the 25 scalers depend on the GRB position. A database of very strong GRBs simulated at 10201 positions has been produced. When a GRB is detected, its location is calculated searching the minimum of the χ2 obtained in the comparison between the measured scaler pattern and the database. This GRB localization technique brings enough accuracy so that the error transmitted to the 100% modulation factor is kept below 10% for GRBs with fluence Ftot≥10−5ergcm−2. The POLAR localization capability will be useful for those cases where no other instruments are simultaneously observing the same field of view.

HX-POL—A Balloon-Borne Hard X-Ray Polarimeter

We report on the design and estimated performance of a balloon-borne hard X-ray polarimeter called HX-POL. The experiment uses a combination of Si and Cadmium Zinc Telluride detectors to measure the polarization of 50 keV-400 keV X-rays from cosmic sources through the dependence of the angular distribution of Compton scattered photons on the polarization direction. On a one-day balloon flight, HX-POL would allow us to measure the polarization of bright Crab-like sources for polarization degrees well below 10%. On a longer (15-30 day) flight from Australia or Antarctica, HX-POL would be be able to measure the polarization of bright galactic X-ray sources down to polarization degrees of a few percent. Hard X-ray polarization measurements provide unique venues for the study of particle acceleration processes by compact objects and relativistic outflows. In this paper, we discuss the overall instrument design and performance. Furthermore, we present results from laboratory tests of the Si and CZT detectors.

POLAR: Design of a novel X-ray polarimeter based on plastic scintillators and MAPMTs

POLAR is a space-borne hard X-ray polarimeter whose design has been optimized to measure the level of linear polarization of gamma-ray bursts (GRB) in the energy range of 50–500 keV. In POLAR, the GRB photons undergo Compton scattering in a target constituted by 1600 plastic scintillator bars. The light output from the whole target is read by 25 multi-anode photomultipliers (MAPMTs). The azimuthal distribution of the scattered photons inside the target provides the information on the GRB polarization. To be able to measure polarization of photons with energy as low as 50 keV, an energy threshold for each single channel of maximum 5 keV is required. This introduces strong constraints in the photon collection efficiency. To maximize it, detailed studies of the scintillator bar surfaces and the available wrapping materials have been performed using both Monte Carlo simulations and laboratory measurements. At present, a POLAR demonstration model (2 of the 25 units of the final design) is being tested in the laboratory. The engineering-qualification model will be ready in 2010.

Expected performance of a hard X-ray polarimeter (POLAR) by Monte Carlo simulation

Polarization measurements of the prompt emission in gamma-ray bursts (GRBs) can provide diagnostic information for understanding the nature of the central engine. POLAR is a compact polarimeter dedicated to the polarization measurement of GRBs between 50 and 300keV and is scheduled to be launched aboard the Chinese Space Laboratory around the year 2012. A preliminary Monte Carlo simulation has been accomplished to model the expected performance of POLAR while a prototype of POLAR is being constructed. The modulation factor, efficiency and effective area, background rates and minimum detectable polarization (MDP) were calculated for different detector configurations and trigger strategies. With the optimized detector configuration and trigger strategy and the total weight constraint of less than 30kg, the primary science goal to determine whether most GRBs are strongly polarized can be achieved, and about 9GRBs/yr can be detected with an MDP<10% for the conservative detector configuration.

Performance of Prototype Hard X-ray Polarimeter Utilizing Compton Scattering

In X-ray astronomy, various kinds of information on stellar objects have been obtained from energy spectrum, time variabilities, and images. Although the observation of polarization is also important, it has not been carried out for 25 years, because of the difficulty of developing a polarimeter with a high sensitivity. Thus, we have been developing a Compton-scattering-type hard X-ray polarimeter sensitive to the energy range from 40 to 300 keV, employing segmented plastic and CsI(Tl) scintillators mounted on a flat-panel multianode photomutiplier (MAPMT). Constructing the prototype polarimeter and using polarized hard X rays of 80 keV, we carried out basic experiments to investigate the performance of our polarimeter. Moreover, we carried out computer simulations and confirmed that the results obtained by computer simulation are in good agreement with those obtained by experiment. Finally, it was found that the prototype polarimeter can obtain a modulation factor and a detection efficiency of ∼40 and ∼20%, respectively.