Abstract
While X-ray spectroscopy, timing, and imaging have improved much since 1962 when the first astronomical nonsolar source was discovered, especially wi the launch of the 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, which in turn is a result of the fate of approved missions and because celestial X-ray sources appear less polarized than expected. Only one positive measurement has been available until now: the Orbiting Solar Observatory measured the polarization of the Crab Nebula in the 1970s. The advent of microelectronics techniques has allowed for designing a detector based on the photoelectric effect of gas in an energy range where the optics are efficient at focusing in X-rays. Here we describe the instrument, which is the major contribution of the Italian collaboration to the Small Explorer mission called IXPE, the Imaging X-ray Polarimetry Explorer, which will launch 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. We show the technological choices, their scientific motivation, and 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.
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
The X-ray pulsar 1A 0535+262 exhibited a giant outburst in 2020, offering us a unique opportunity for X-ray polarimetry of an accreting pulsar in the supercritical state. Measurement with ...PolarLight yielded a nondetection in 3–8 keV; the 99% upper limit of the polarization fraction (PF) is found to be 0.34 averaged over spin phases, or 0.51 based on the rotating vector model. No useful constraint can be placed with phase-resolved polarimetry. These upper limits are lower than a previous theoretical prediction of 0.6–0.8, but consistent with those found in other accreting pulsars, like Her X-1, Cen X-3, 4U 1626−67, and GRO J1008−57, which were in the subcritical state, or at least not confidently in the supercritical state, during the polarization measurements. Our results suggest that the relatively low PF seen in accreting pulsars cannot be attributed to the source not being in the supercritical state, but could be a general feature.
The gas pixel detector (GPD) is designed and developed for high-sensitivity astronomical X-ray polarimetry, which is a new window about to open in a few years. Due to the small mass, low power, and ...compact geometry of the GPD, we propose a CubeSat mission Polarimeter Light (PolarLight) to demonstrate and test the technology directly in space. There is no optics but a collimator to constrain the field of view to 2.3 degrees. Filled with pure dimethyl ether (DME) at 0.8 atm and sealed by a beryllium window of 100
μ
m thick, with a sensitive area of about 1.4 mm by 1.4 mm, PolarLight allows us to observe the brightest X-ray sources on the sky, with a count rate of, e.g., ∼0.2 counts s
− 1
from the Crab nebula. The PolarLight is 1U in size and mounted in a 6U CubeSat, which was launched into a low Earth Sun-synchronous orbit on October 29, 2018, and is currently under test. More launches with improved designs are planned in 2019. These tests will help increase the technology readiness for future missions such as the enhanced X-ray Timing and Polarimetry (eXTP), better understand the orbital background, and may help constrain the physics with observations of the brightest objects.
The Italian Space Agency plays a key role in the fulfillment of space missions, contributing to the scientific, technological and economic progress in Italy. The agency accomplishes space experiments ...by collaborating with scientific and industrial entities, supporting them in the realization of new projects able to achieve, over the last two decades, unprecedented results and obtention of fundamental information on the birth and evolution of the universe. The paper describes a selection of X-ray technologies developed by the synergy between the Italian Space Agency and its principal collaborators which contributed to the main scientific results achieved over the years, together with the latest advances addressed to the next astrophysics missions.
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.
Previous detections of individual astrophysical sources of neutrinos are limited to the Sun and the supernova 1987A, whereas the origins of the diffuse flux of high-energy cosmic neutrinos remain ...unidentified. On 22 September 2017, we detected a high-energy neutrino, IceCube-170922A, with an energy of ~290 tera-electron volts. Its arrival direction was consistent with the location of a known γ-ray blazar, TXS 0506+056, observed to be in a flaring state. An extensive multiwavelength campaign followed, ranging from radio frequencies to γ-rays. These observations characterize the variability and energetics of the blazar and include the detection of TXS 0506+056 in very-high-energy γ-rays. This observation of a neutrino in spatial coincidence with a γ-ray-emitting blazar during an active phase suggests that blazars may be a source of high-energy neutrinos.
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