Abstract
Imaging X-ray Polarimetry Explorer (IXPE) is a Small Explorer mission that was launched at the end of 2021 to measure the polarization of X-ray emission from tens of astronomical sources. ...Its focal-plane detectors are based on the Gas Pixel Detector, which measures the polarization by imaging photoelectron tracks in a gas mixture and reconstructing their initial directions. The quality of the single track, and then the capability of correctly determining the original direction of the photoelectron, depends on many factors, e.g., whether the photoelectron is emitted at low or high inclination with respect to the collection plane or the occurrence of a large Coulomb scattering close to the generation point. The reconstruction algorithm used by IXPE to obtain the photoelectron emission direction also calculates several properties of the shape of the tracks that characterize the process. In this paper we compare several such properties and identify the best one to weight each track on the basis of the reconstruction accuracy. We demonstrate that significant improvement in sensitivity can be achieved with this approach and for this reason it will be the baseline for IXPE data analysis.
Abstract
We present an incremental version (4FGL-DR3, for Data Release 3) of the fourth Fermi Large Area Telescope (LAT) catalog of
γ
-ray sources. Based on the first 12 years of science data in the ...energy range from 50 MeV to 1 TeV, it contains 6658 sources. The analysis improves on that used for the 4FGL catalog over eight years of data: more sources are fit with curved spectra, we introduce a more robust spectral parameterization for pulsars, and we extend the spectral points to 1 TeV. The spectral parameters, spectral energy distributions, and associations are updated for all sources. Light curves are rebuilt for all sources with 1 yr intervals (not 2 month intervals). Among the 5064 original 4FGL sources, 16 were deleted, 112 are formally below the detection threshold over 12 yr (but are kept in the list), while 74 are newly associated, 10 have an improved association, and seven associations were withdrawn. Pulsars are split explicitly between young and millisecond pulsars. Pulsars and binaries newly detected in LAT sources, as well as more than 100 newly classified blazars, are reported. We add three extended sources and 1607 new point sources, mostly just above the detection threshold, among which eight are considered identified, and 699 have a plausible counterpart at other wavelengths. We discuss the degree-scale residuals to the global sky model and clusters of soft unassociated point sources close to the Galactic plane, which are possibly related to limitations of the interstellar emission model and missing extended sources.
With the specialization of VLSI ASICs for front-end signal processing electronics, the customization of the control back-end electronics (BEE) has become critical to fully deploy the ASIC ...performance. In the context of space operations, with typical constraints on power and reliability, the design and qualification of such integrated systems present significant challenges. In this paper, we review the design and performance of the BEE systems after two years of operations in low Earth orbit (LEO); these systems read out the custom ASICs inside the gas pixel detectors, which are located at the heart of the imaging X-ray polarimetry explorer (IXPE), a NASA-ASI small explorer mission designed to measure X-ray polarization in the 2–8 keV energy range.
We present our study on the reconstruction of photoelectron tracks in gas pixel detectors used for astrophysical X-ray polarimetry. Our work aims to maximize the performance of convolutional neural ...networks (CNNs) to predict the impact point of incoming X-rays from the image of the photoelectron track. A very high precision in the reconstruction of the impact point position is achieved thanks to the introduction of an artificial sharpening process of the images. We find that providing the CNN-predicted impact point as input to the state-of-the-art analytic analysis improves the modulation factor (\(\sim 1 \%\) at 3 keV and \(\sim 6 \%\) at 6 keV) and naturally mitigates a subtle effect appearing in polarization measurements of bright extended sources known as "polarization leakage".
IXPE is a Small Explorer mission that was launched at the end of 2021 to measure the polarization of X-ray emission from tens of astronomical sources. Its focal plane detectors are based on the Gas ...Pixel Detector, which measures the polarization by imaging photoelectron tracks in a gas mixture and reconstructing their initial directions. The quality of the single track, and then the capability of correctly determining the original direction of the photoelectron, depends on many factors, e.g., whether the photoelectron is emitted at low or high inclination with respect to the collection plane or the occurrence of a large Coulomb scattering close to the generation point. The reconstruction algorithm used by IXPE to obtain the photoelectron emission direction, also calculates several properties of the shape of the tracks which characterize the process. In this paper we compare several such properties and identify the best one to weight each track on the basis of the reconstruction accuracy. We demonstrate that significant improvement in sensitivity can be achieved with this approach and for this reason it will be the baseline for IXPE data analysis.
IXPE (Imaging X-ray Polarimetry Explorer) is a NASA Small Explorer mission -- in partnership with the Italian Space Agency (ASI) -- dedicated to X-ray polarimetry in the 2--8 keV energy band. The ...IXPE telescope comprises three grazing incidence mirror modules coupled to three detector units hosting each one a Gas Pixel Detector (GPD), a gas detector that allows measuring the polarization degree by using the photoelectric effect. A wide and accurate ground calibration was carried out on the IXPE Detector Units (DUs) at INAF-IAPS, in Italy, where a dedicated facility was set-up at this aim. In this paper, we present the results obtained from this calibration campaign to study the IXPE focal plane detector response to polarized radiation. In particular, we report on the modulation factor, which is the main parameter to estimate the sensitivity of a polarimeter.
Scheduled to launch in late 2021,the Imaging X-ray Polarimetry Explorer (IXPE) is a Small Explorer Mission designed to open up a new window of investigation --X-ray polarimetry. The IXPE observatory ...features 3 identical telescopes each consisting of a mirror module assembly with a polarization-sensitive imaging x-ray detector at its focus. An extending boom, deployed on orbit, provides the necessary 4 m focal length. The payload sits atop a 3-axis stabilized spacecraft which, among other things, provides power, attitude determination and control, commanding, and telemetry to the ground. During its 2-year baseline mission, IXPE will conduct precise polarimetry for samples of multiple categories of x-ray sources, with follow-on observations of selected targets. IXPE is a partnership between NASA and the Italian Space Agency (ASI).
While X-ray Spectroscopy, Timing and Imaging have improved verymuch since 1962, when the first astronomical non-solar source was discovered, especially with the launch of Newton/X-ray Multi-Mirror ...Mission, Rossi/X-ray Timing Explorer and Chandra/Advanced X-ray Astrophysics Facility, the progress of X-ray polarimetry has been meager. This is in part due to the lack of sensitive polarization detectors, in part due to the fate of approved missions and in part because the celestial X-ray sources appeared less polarized than expected. Only one positive measurement has been available until now. Indeed the eight Orbiting Solar Observatory measured the polarization of the Crab nebula in the 70s. The advent of techniques of microelectronics allowed for designing a detector based on the photoelectric effect in gas in an energy range where the optics are efficient in focusing X-rays. Herewe describe the Instrument, which is the major contribution of the Italian collaboration to the SmallExplorer mission called IXPE, the Imaging X-ray Polarimetry Explorer, which will be flown in late 2021. The instrument, is composed of three Detector Units, based on this technique, and a Detector Service Unit. Three Mirror Modules provided by Marshall Space Flight Center focus X-rays onto the detectors. In the following we will show the technological choices, their scientific motivation and the results from the calibration of the Instrument. IXPE will perform imaging, timing and energy resolved polarimetry in the 2-8 keV energy band opening this window of X-ray astronomy to tens of celestial sources of almost all classes.
The flux of positrons and electrons ($e^+ + e^-$) has been measured by the
$Fermi$ Large Area Telescope (LAT) in the energy range between 7 GeV and 2 TeV.
We discuss a number of interpretations of ...Pass 8 $Fermi$-LAT $e^+ + e^-$
spectrum, combining electron and positron emission from supernova remnants
(SNRs) and pulsar wind nebulae (PWNe), or produced by the collision of cosmic
rays with the interstellar medium. We find that the $Fermi$-LAT spectrum is
compatible with the sum of electrons from a smooth SNR population, positrons
from cataloged PWNe, and a secondary component. If we include in our analysis
constraints from AMS-02 positron spectrum, we obtain a slightly worse fit to
the $e^+ + e^-$ $Fermi$-LAT spectrum, depending on the propagation model. As an
additional scenario, we replace the smooth SNR component within 0.7 kpc with
the { individual sources} found in Green's catalog of Galactic SNRs. We find
that separate consideration of far and near sources helps to reproduce the $e^+
+ e^-$ $Fermi$-LAT spectrum. However, we show that the fit degrades when the
radio constraints on the positron emission from Vela SNR (which is the main
contributor at high energies) are taken into account. We find that a break in
the power-law injection spectrum at about 100 GeV can also reproduce the
measured $e^+ + e^-$ spectrum} and, among the cosmic-ray propagation models
that we consider, no reasonable break of the power-law dependence of the
diffusion coefficient can modify the electron flux enough to reproduce the
observed shape.
The flux of positrons and electrons (\(e^+ + e^-\)) has been measured by the \(Fermi\) Large Area Telescope (LAT) in the energy range between 7 GeV and 2 TeV. We discuss a number of interpretations ...of Pass 8 \(Fermi\)-LAT \(e^+ + e^-\) spectrum, combining electron and positron emission from supernova remnants (SNRs) and pulsar wind nebulae (PWNe), or produced by the collision of cosmic rays with the interstellar medium. We find that the \(Fermi\)-LAT spectrum is compatible with the sum of electrons from a smooth SNR population, positrons from cataloged PWNe, and a secondary component. If we include in our analysis constraints from AMS-02 positron spectrum, we obtain a slightly worse fit to the \(e^+ + e^-\) \(Fermi\)-LAT spectrum, depending on the propagation model. As an additional scenario, we replace the smooth SNR component within 0.7 kpc with the { individual sources} found in Green's catalog of Galactic SNRs. We find that separate consideration of far and near sources helps to reproduce the \(e^+ + e^-\) \(Fermi\)-LAT spectrum. However, we show that the fit degrades when the radio constraints on the positron emission from Vela SNR (which is the main contributor at high energies) are taken into account. We find that a break in the power-law injection spectrum at about 100 GeV can also reproduce the measured \(e^+ + e^-\) spectrum} and, among the cosmic-ray propagation models that we consider, no reasonable break of the power-law dependence of the diffusion coefficient can modify the electron flux enough to reproduce the observed shape.
