To determine if the recently launched Imaging X-ray Polarimetry Explorer (IXPE) can follow the polarization variations induced by different particle acceleration mechanisms in blazars jets we simulate observations of a high synchrotron peak (HSP) blazar variable in polarization degree and angle according to the theoretical predictions of Tavecchio et al., 2020 (shock acceleration model), Bodo et al., 2021 (magnetic reconnection model) and Marscher \& Jorstad, 2021 (Turbulent Extreme Multi-Zone Model). We use the Monte Carlo tool ixpeobssim to create realistic IXPE data products. We create simulated light-curves of polarization degree and angle by time-slicing the simulated data into arbitrary short time bins. We use a $χ^2$ test to assess the performance of the observations in detecting the time variability of the polarization properties. A series of 10 ks long observations permits IXPE to follow the time variability of the polarization degree in the case of the shock acceleration model. In the case of the magnetic reconnection model, the nominal injected model provides the best fit of the simulated IXPE data for time bins of 5-10 ks, depending on the tested flux level. For the TEMZ model, shorter time slices of 0.5 ks are needed for obtaining a formally good fit of the simulated IXPE data with the injected model. On the other hand, we find that a fit with a constant model provides a $χ^2$ lower than the fit with the nominal injected model when using time slices of 20 ks, 60/70 ks, and 5 ks for the case of the shock acceleration, magnetic reconnection and TEMZ model, respectively. We conclude that, provided that the statistics of the observation allows slicing of the data in adequately short time bins, IXPE observations of an HSP blazar at a typical flux level can detect the time variability predicted by popular models for particle acceleration in jets.
Astronomical X-ray Polarimetry as a diagnostic for questions of fundamental Physics. What we learned from the Imaging X-Ray Polarimetry Explorer (IXPE)
Launched on December 9, 2021, the Imaging X-ray Polarimetry Explorer (IXPE) is the first imaging polarimeter ever flown, providing sensitivity in the 2--8 keV range, and during the 2-year initial phase of the mission will sample tens of X-ray sources among different source classes. While most of the measurements will be statistics-limited, for some of the brightest objects observed and long integration times, the systematic uncertainties in the detector response (primarily the effective area, the modulation factor and the absolute energy scale) will be important. In this contribution, we descr…
ixpeobssim is a simulation and analysis framework, based on the Python programming language and the associated scientific ecosystem, specifically developed for the Imaging X-ray Polarimetry Explorer (IXPE). Given a source model and the response functions of the telescopes, it is designed to produce realistic simulated observations, in the form of event lists in FITS format, containing a strict super-set of the information provided by standard IXPE level-2 files. The core ixpeobssim simulation capabilities are complemented by a full suite of post-processing applications, allowing for the implem…
In about four years, the National Aeronautics and Space Administration (NASA) will launch a small explorer mission named the Imaging X-ray Polarimetry Explorer (IXPE). IXPE is a satellite dedicated to the observation of X-ray polarization from bright astronomical sources in the 2-8 keV energy range. Using Gas Pixel Detectors (GPD), the mission will allow for the first time to acquire X-ray polarimetric imaging and spectroscopy of about a hundred of sources during its first two years of operation. Among them are the most powerful sources of light in the Universe: active galactic nuclei (AGN). I…
The NASA/ASI Imaging X-ray Polarimetry Explorer, which will be launched in 2021, will be the first instrument to perform spatially resolved X-ray polarimetry on several astronomical sources in the 2-8 keV energy band. These measurements are made possible owing to the use of a gas pixel detector (GPD) at the focus of three X-ray telescopes. The GPD allows simultaneous measurements of the interaction point, energy, arrival time, and polarization angle of detected X-ray photons. The increase in sensitivity, achieved 40 years ago, for imaging and spectroscopy with the Einstein satellite will thus …
We report on the Imaging X-ray Polarimetry Explorer (IXPE) observation of the closest and X-ray brightest Compton-thick active galactic nucleus (AGN), the Circinus galaxy. We find the source to be significantly polarized in the 2--6 keV band. From previous studies, the X-ray spectrum is known to be dominated by reflection components, both neutral (torus) and ionized (ionization cones). Our analysis indicates that the polarization degree is $28 \pm 7$ per cent (at 68 per cent confidence level) for the neutral reflector, with a polarization angle of $18° \pm 5°$, roughly perpendicular to the rad…
We present measurements of the polarization of X-rays in the 2-8 keV band from the pulsar in the ultracompact low mass X-ray binary 4U1626-67 using data from the Imaging X-ray Polarimetry Explorer (IXPE). The 7.66 s pulsations were clearly detected throughout the IXPE observations as well as in the NICER soft X-ray observations, which we use as the basis for our timing analysis and to constrain the spectral shape over 0.4-10 keV energy band. Chandra HETGS high-resolution X-ray spectra were also obtained near the times of the IXPE observations for firm spectral modeling. We find an upper limit …
X-ray polarimetry promises us an unprecedented look at the structure of magnetic fields and on the processes at the base of acceleration of particles up to ultrarelativistic energies in relativistic jets. Crucial pieces of information are expected from observations of blazars (that are characterized by the presence of a jet pointing close to the Earth), in particular of the subclass defined by a synchrotron emission extending to the X-ray band (so-called high synchrotron peak blazars, HSP). In this review, I give an account of some of the models and numerical simulations developed to predict t…
Particle acceleration mechanisms in supermassive black hole jets, such as shock acceleration, magnetic reconnection, and turbulence, are expected to have observable signatures in the multi-wavelength polarization properties of blazars. The recent launch of the Imaging X-ray Polarimetry Explorer (IXPE) enables us, for the first time, to use polarization in the X-ray band (2-8 keV) to probe the properties of the jet synchrotron emission in high-frequency-peaked BL Lac objects (HSPs). We report the discovery of X-ray linear polarization (degree $Π_{\rm x}=15\pm$2\% and electric-vector position an…
We present measurements of the polarization of X-rays in the $2-8 \thinspace \mathrm{keV}$ band from the nucleus of the radio galaxy Centaurus A (Cen A), using a 100ks observation from the Imaging X-ray Polarimetry Explorer (IXPE). Nearly simultaneous observations of Cen A were also taken with the Swift, NuSTAR, and INTEGRAL observatories. No statistically significant degree of polarization is detected with IXPE. These observations have a minimum detectable polarization at $99 \%$ percent confidence (MDP$_{99}$) of $6.5 \%$ using a weighted, spectral model-independent calculation in the $2-8 \…
Hard X-rays observed in Active Galactic Nuclei (AGNs) are thought to originate from the Comptonization of the optical/UV accretion disk photons in a hot corona. Polarization studies of these photons can help to constrain the corona geometry and the plasma properties. We have developed a ray-tracing code that simulates the Comptonization of accretion disk photons in coronae of arbitrary shape, and use it here to study the polarization of the X-ray emission from wedge and spherical coronae. We study the predicted polarization signatures for the fully relativistic and various approximate treatmen…
A Pipeline for the AI-Assisted Analysis of Imaging X-ray Polarimetry Explorer Observations of Black Hole X-ray Binaries and Limits on Their Polarization Variability
Washington University in St. Louis（华盛顿大学圣路易斯分校）；Washington University in St. Louis McDonnell Center for the Space Sciences and Center for Quantum Leaps（华盛顿大学圣路易斯分校麦克唐纳空间科学中心与量子跃迁中心）
Observations with RXTE (Rossi X-ray Timing Explorer) revealed the presence of High Frequency Quasi-Periodic Oscillations (HFQPOs) of the X-ray flux from several accreting stellar mass Black Holes. HFQPOs (and their counterparts at lower frequencies) may allow us to study general relativity in the strong gravity regime. However, the observational evidence today does not yet allow us to distinguish between different HFQPO models. In this paper we use a general relativistic ray-tracing code to investigate X-ray timing-spectroscopy and polarization properties of HFQPOs in the orbiting Hotspot mode…
Both leptonic and hadronic emission processes may contribute to blazar jet emission; which dominates in blazars's high energy emission component remains an open question. Some intermediate synchrotron peaked blazars transition from their low to high energy emission components in the X-ray band making them excellent laboratories to probe both components simultaneously, and good targets for the newly launched Imaging X-ray Polarimetry Explorer. We characterize the spectral energy distributions for three such blazars: CGRaBS~J0211+1051, TXS~0506+056, and S5~0716+714, predicting their X-ray polari…
X-ray polarimetry, sometimes alone, and sometimes coupled to spectral and temporal variability measurements and to imaging, allows a wealth of physical phenomena in astrophysics to be studied. X-ray polarimetry investigates the acceleration process, for example, including those typical of magnetic reconnection in solar flares, but also emission in the strong magnetic fields of neutron stars and white dwarfs. It detects scattering in asymmetric structures such as accretion disks and columns, and in the so-called molecular torus and ionization cones. In addition, it allows fundamental physics in…
GRS 1915+105 has been in a bright flux state for more than 2 decades, but in 2018 a significant drop in flux was observed, partly due to changes in the central engine along with increased X-ray absorption. The aim of this work is to explore how X-ray spectro-polarimetry can be used to derive the basic geometrical properties of the absorbing and reflecting matter. In particular, the expected polarisation of the radiation reflected off the disc and the putative outflow is calculated. We use \textit{NuSTAR} data collected after the flux drop to derive the parameters of the system from hard X-ray …
Measurements of the angular momentum (spin) of astrophysical black holes are extremely important, as they provide information on the black hole formation and evolution. We present simulated observations of a X-ray binary system with the Imaging X-ray Polarimetry Explorer (IXPE), with the aim to study the robustness of black hole spin and geometry measurements using X-ray polarimetry. As a representative example, we used the parameters of GRS 1915+105 in its former unobscured, soft state. In order to simulate the polarization properties, we modeled the source emission with a multicolor blackbod…
We investigate with relativistic MHD simulations the dissipation physics of BL Lac jets, by studying the synchrotron polarization signatures of particles accelerated by the kink instability in a magnetically-dominated plasma column. The nonlinear stage of the kink instability generates current sheets, where particles can be efficiently accelerated via magnetic reconnection. We identify current sheets as regions where s = J d/B is above some predefined threshold (where B is the field strength, J the current density and d the grid scale), and assume that the particle injection efficiency scales …
Polarimetric measurements, especially if extended at high energy, are expected to provide important insights into the mechanisms underlying the acceleration of relativistic particles in jets. In a previous work we have shown that the polarization of the synchrotron X-ray emission produced by highly energetic electrons accelerated by a mildly relativistic shock carries essential imprints of the geometry and the structure of the magnetic fields in the downstream region. Here we present the extension of our analysis to the non-stationary case, especially suitable to model the highly variable emis…
Over the next year, a new era of observations of compact objects in X-ray polarization will commence. Among the key targets for the upcoming Imaging X-ray Polarimetry Explorer mission, will be the magnetars 4U 0142+61 and 1RXS J170849.0-400910. Here we present the first detailed predictions of the expected polarization from these sources that incorporate realistic models of emission physics at the surface (gaseous or condensed), the temperature distribution on the surface, general relativity, quantum electrodynamics and scattering in the magnetosphere, and also account for the broadband spectr…
The dynamics, energetics and evolution of pulsar wind nebulae (PWNe) and supernova remnants (SNRs), are strongly affected by their magnetic field strength and distribution. They are usually strong, extended, sources of non-thermal X-ray radiation, producing intrinsically polarised radiation. The energetic wind around pulsars produces a highly-magnetised, structured flow, often displaying a jet and a torus and different features (i.e. wisps, knots). This magnetic-dominant wind evolves as it moves away from the pulsar magnetosphere to the surrounding large-scale nebula, becoming kinetic-dominant…
The recently measured 3.5 keV line in a number of galaxy clusters, the Andromeda galaxy (M31) and the Milky Way (MW) center can be well accounted for by a scenario in which dark matter decays to axion-like particles (ALPs) and subsequently convert to 3.5 keV photons in magnetic fields of galaxy clusters or galaxies. We propose to test this hypothesis by performing X-ray polarization measurements. Since ALPs can only couple to photons with polarization orientation parallel to magnetic field, we can confirm or reject this model by measuring the polarization of 3.5 keV line and comparing it to th…
X-ray polarization of Z-type neutron star low-mass X-ray binaries -- I. Model-independent, time-resolved X-ray polarimetry
Università degli Studi Roma Tre（罗马第三大学）；INAF – Istituto di Astrofisica e Planetologia Spaziali（意大利国家天体物理研究所）；INAF – Osservatorio Astronomico di Cagliari（意大利国家天体物理研究所卡利亚里天文台）；INAF – Osservatorio di Astrofisica e Scienza dello Spazio（意大利国家天体物理研究所空间科学天文台）
IXPE (Imaging X-ray Polarimetry Explorer) is a Small Explorer mission by NASA and ASI, launched on December 9$^{th}$ 2021, dedicated to investigating X-ray polarimetry allowing angular-, time- and energy-resolved observations in the 2--8 keV energy band. IXPE is in Science Observation phase since January 2022; it comprises of three identical telescopes with grazing-incidence mirrors, each one having in the focal plane a Gas Pixel Detector (GPD). In this paper, we present a possible guideline to obtain an optimal background selection in polarimetric analysis, and a rejection strategy to remove …
The dissipation of energy flux in blazar jets plays a key role in the acceleration of relativistic particles. Two possibilities are commonly considered for the dissipation processes, magnetic reconnection -- possibly triggered by instabilities in magnetically-dominated jets -- , or shocks -- for weakly magnetized flows. We consider the polarimetric features expected for the two scenarios analyzing the results of state-of-the-art simulations. For the magnetic reconnection scenario we conclude, using results from global relativistic MHD simulations, that the emission likely occurs in turbulent r…
This white paper highlights compact object and fundamental physics science opportunities afforded by high-throughput broadband (0.1-60 keV) X-ray polarization observations. X-ray polarimetry gives new observables with geometric information about stellar remnants which are many orders of magnitude too small for direct imaging. The X-ray polarimetric data also reveal details about the emission mechanisms and the structure of the magnetic fields in and around the most extreme objects in the Universe. Whereas the Imaging X-ray Polarimetry Explorer (IXPE) to be launched in 2021 will obtain first re…
We briefly review a fruitful approach to compute a variety of radiation signatures of General Relativity (GR) originating from accretion disks in strong gravity. A set of transfer functions can be pre-computed and then employed to accelerate the ray-traycing computations, generate a series of model spectra, and to fit the model predictions to actual data sets in X-rays. We have been developing this method to examine spectra and light curves and to model the expected polarimetric properties; in particular, to analyse the properties of the "corona--disk-line" geometry with GR effects taken into …
Various quantum gravity approaches that extend beyond the standard model predict Lorentz Invariance and Charge-Parity-Time Violation at energies approaching the Planck scale. These models frequently predict a wavelength dependent speed of light, which would result in time delays between promptly emitted photons at different energies, as well as a wavelength-dependent rotation of the plane of linear polarization for photons resulting from vacuum birefringence. Here, we describe a pilot program with an automated system of small telescopes that can simultaneously conduct high cadence optical phot…
We present modelling of X-ray polarisation spectra emerging from the two competing scenarios that are proposed to explain the broad Fe Kα line in the Seyfert 1 galaxy MCG-6-30-15. The polarisation signature of complex absorption is studied for a partial covering scenario using a clumpy wind and compared to a reflection model based on the lamp-post geometry. The shape of the polarisation percentage and angle as a function of photon energy are found to be distinctly different between the reflection and the absorption case. Relativistic reflection produces significantly stronger polarisation in t…
Using observations of X-ray pulsar Her X-1 by the Imaging X-ray Polarimetry Explorer, we report on a highly significant ($>17σ$) detection of the polarization signal from an accreting neutron star. The observed degree of the linear polarization of $\sim$10\% is found to be far below theoretical expectations for this object, and stays low throughout the spin cycle of the pulsar. Both the polarization degree and the angle exhibit variability with pulse phase, which allowed us to measure the pulsar spin position angle 57(2) deg and the magnetic obliquity 12(4) deg, which is an essential step towa…
Harvard-Smithsonian Center for Astrophysics（哈佛-史密森尼天体物理中心）；Universitá di Pisa（比萨大学）；INFN-Sezione di Pisa（意大利国家核物理研究所比萨分部）
We present a method for enhancing the sensitivity of X-ray telescopic observations with imaging polarimeters, with a focus on the gas pixel detectors (GPDs) to be flown on the Imaging X-ray Polarimetry Explorer (IXPE). Our analysis determines photoelectron directions, X-ray absorption points and X-ray energies for 1-9 keV event tracks, with estimates for both the statistical and model (reconstruction) uncertainties. We use a weighted maximum likelihood combination of predictions from a deep ensemble of ResNet convolutional neural networks, trained on Monte Carlo event simulations. We define a …
Stanford University（斯坦福大学）；Department of Physics, Stanford University（斯坦福大学物理系）；Kavli Institute for Particle Astrophysics and Cosmology, Stanford University（斯坦福大学卡弗里粒子天体物理学与宇宙学研究所）
We argue that measurements of X-ray polarization using the recently launched Imaging X-ray Polarimetry Explorer will answer many open questions about magnetars in particular the physical state of their surfaces, whether vacuum birefringence exists, and the nature of the hard X-ray emission from these objects. We outline the capabilities of the instrument, specific models and the results of simulations for the magnetar 4U~0142+61.
Emission from an accretion disc around compact objects, such as neutron stars and black holes, is expected to be significantly polarized. The polarization can be used to put constraints on geometrical and physical parameters of the compact sources -- their radii, masses and spins -- as well as to determine the orbital parameters. The radiation escaping from the innermost parts of the disc is strongly affected by the gravitational field of the compact object and relativistic velocities of the matter. The straightforward calculation of the observed polarization signatures involves computationall…
