Galaxies

Review

10 pages, 4820 KiB

Open AccessEditor’s ChoiceReview

Probing Magnetic Fields in Young Supernova Remnants with IXPE

by Patrick Slane, Riccardo Ferrazzoli, Ping Zhou and Jacco Vink

Galaxies 2024, 12(5), 59; https://doi.org/10.3390/galaxies12050059 - 2 Oct 2024

Cited by 1 | Viewed by 863

Synchrotron emission from the shocked regions in supernova remnants provides, through its polarization, crucial details about the magnetic field strength and orientation in these regions. This, in turn, provides information on particle acceleration in these shocks. Due to the rapid losses of the [...] Read more.

Synchrotron emission from the shocked regions in supernova remnants provides, through its polarization, crucial details about the magnetic field strength and orientation in these regions. This, in turn, provides information on particle acceleration in these shocks. Due to the rapid losses of the highest-energy relativistic electrons, X-ray polarization measurements allow for investigations of the magnetic field to be carried outvery close to the sites of particle acceleration. Measurements of both the geometry of the field and the levels of turbulence implied by the observed polarization degree thus provide unique insights into the conditions leading to efficient particle acceleration in fast shocks. The Imaging X-ray Polarimetry Explorer (IXPE) has carried out observations of multiple young SNRs, including Cas A, Tycho, SN 1006, and RX J1713.7−3946. In each, significant X-ray polarization detections provide measurements of magnetic field properties that show some common behavior but also considerable differences between these SNRs. Here, we provide a summary of results from IXPE studies of young SNRs, providing comparisons between the observed polarization and the physical properties of the remnants and their environments. Full article

(This article belongs to the Special Issue X-Ray Polarimetry: A New Era Begins with the Imaging X-Ray Polarimetry Explorer)

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23 pages, 2586 KiB

Open AccessEditor’s ChoiceReview

IXPE View of BH XRBs during the First 2.5 Years of the Mission

by Michal Dovčiak, Jakub Podgorný, Jiří Svoboda, James F. Steiner, Philip Kaaret, Henric Krawczynski, Adam Ingram, Vadim Kravtsov, Lorenzo Marra, Fabio Muleri, Javier A. García, Guglielmo Mastroserio, Romana Mikušincová, Ajay Ratheesh and Nicole Rodriguez Cavero

Galaxies 2024, 12(5), 54; https://doi.org/10.3390/galaxies12050054 - 25 Sep 2024

Cited by 1 | Viewed by 907

Abstract

Accreting stellar-mass black holes represent unique laboratories for studying matter and radiation under the influence of extreme gravity. They are highly variable sources going through different accretion states, showing various components in their X-ray spectra from the thermal emission of the accretion disc [...] Read more.

Accreting stellar-mass black holes represent unique laboratories for studying matter and radiation under the influence of extreme gravity. They are highly variable sources going through different accretion states, showing various components in their X-ray spectra from the thermal emission of the accretion disc dominating in the soft state to the up-scattered Comptonisation component from an X-ray corona in the hard state. X-ray polarisation measurements are particularly sensitive to the geometry of the X-ray scatterings and can thus constrain the orientation and relative positions of the innermost components of these systems. The IXPE mission has observed about a dozen stellar-mass black holes with masses up to 20 solar masses in X-ray binaries with different orientations and in various accretion states. The low-inclination sources in soft states have shown a low fraction of polarisation. On the other hand, several sources in soft and hard states have revealed X-ray polarisation higher than expected, which poses significant challenges for theoretical interpretation, with 4U 1630–47 being one of the most puzzling sources. IXPE has measured the spin of three black holes via the measurement of their polarisation properties in the soft emission state. In each of the three cases, the new results agree with the constraints from the spectral observations. The polarisation observations of the black hole X-ray transient Swift J1727.8–1613 across its entire outburst has revealed that the soft-state polarisation is much weaker than the hard-state polarisation. Remarkably, the observations furthermore show that the polarisation of the bright hard state and that of the 100 times less luminous dim hard state are identical within the accuracy of the measurement. For sources with a radio jet, the electric field polarisation tends to align with the radio jet, indicating the equatorial geometry of the X-ray corona, e.g., in the case of Cyg X–1. In the unique case of Cyg X–3, where the polarisation is perpendicular to the radio jet, the IXPE observations reveal the presence and geometry of obscuring material hiding this object from our direct view. The polarisation measurements acquired by the IXPE mission during its first 2.5 years have provided unprecedented insights into the geometry and physical processes of accreting stellar-mass black holes, challenging existing theoretical models and offering new avenues for understanding these extreme systems. Full article

(This article belongs to the Special Issue X-Ray Polarimetry: A New Era Begins with the Imaging X-Ray Polarimetry Explorer)

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8 pages, 534 KiB

Open AccessReview

IXPE Observations of Magnetar Sources

by Roberto Turolla, Roberto Taverna, Silvia Zane and Jeremy Heyl

Galaxies 2024, 12(5), 53; https://doi.org/10.3390/galaxies12050053 - 18 Sep 2024

Cited by 1 | Viewed by 650

Abstract

Among the more than 60 sources observed in the first two years of operations, IXPE addressed four magnetars, neutron stars believed to host ultra-strong magnetic fields. We report here the main implication coming from IXPE measurements for the physics of magnetars. Polarimetric observations [...] Read more.

Among the more than 60 sources observed in the first two years of operations, IXPE addressed four magnetars, neutron stars believed to host ultra-strong magnetic fields. We report here the main implication coming from IXPE measurements for the physics of magnetars. Polarimetric observations confirmed the expectations of high polarization degrees, up to ≈

80 %

, values which have not been detected in any other source so far, providing further proof (independent from the P-

\dot{P}

estimate) that magnetars host indeed ultra-magnetized neutron stars. Polarization measurements also indicate that softer X-rays likely come from surface regions where the overlying atmosphere underwent magnetic condensation. The agreement of the phase-dependent polarization angle with a simple rotating vector model strongly supports the presence of vacuum birefringence around the star. Full article

(This article belongs to the Special Issue X-Ray Polarimetry: A New Era Begins with the Imaging X-Ray Polarimetry Explorer)

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10 pages, 1034 KiB

Open AccessReview

X-ray Polarization of Blazars and Radio Galaxies Measured by the Imaging X-ray Polarimetry Explorer

by Alan P. Marscher, Laura Di Gesu, Svetlana G. Jorstad, Dawoon E. Kim, Ioannis Liodakis, Riccardo Middei and Fabrizio Tavecchio

Galaxies 2024, 12(4), 50; https://doi.org/10.3390/galaxies12040050 - 22 Aug 2024

Cited by 2 | Viewed by 961

Abstract

X-ray polarization, which now can be measured by the Imaging X-ray Polarimetry Explorer (IXPE), is a new probe of jets in the supermassive black hole systems of active galactic nuclei (AGNs). Here, we summarize IXPE observations of radio-loud AGNs that have been published [...] Read more.

X-ray polarization, which now can be measured by the Imaging X-ray Polarimetry Explorer (IXPE), is a new probe of jets in the supermassive black hole systems of active galactic nuclei (AGNs). Here, we summarize IXPE observations of radio-loud AGNs that have been published thus far. Blazars with synchrotron spectral energy distributions (SEDs) that peak at X-ray energies are routinely detected. The degree of X-ray polarization is considerably higher than at longer wavelengths. This is readily explained by energy stratification of the emission regions when electrons lose energy via radiation as they propagate away from the sites of particle acceleration as predicted in shock models. However, the 2–8 keV polarization electric vector is not always aligned with the jet direction as one would expect unless the shock is oblique. Magnetic reconnection may provide an alternative explanation. The rotation of the polarization vector in Mrk421 suggests the presence of a helical magnetic field in the jet. In blazars with lower-frequency peaks and the radio galaxy Centaurus A, the non-detection of X-ray polarization by IXPE constrains the X-ray emission mechanism. Full article

(This article belongs to the Special Issue X-Ray Polarimetry: A New Era Begins with the Imaging X-Ray Polarimetry Explorer)

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15 pages, 1308 KiB

Open AccessFeature PaperEditor’s ChoiceReview

Considerations on Possible Directions for a Wide Band Polarimetry X-ray Mission

by Paolo Soffitta, Enrico Costa, Nicolas De Angelis, Ettore Del Monte, Klaus Desch, Alessandro Di Marco, Giuseppe Di Persio, Sergio Fabiani, Riccardo Ferrazzoli, Markus Gruber, Takahashi Hiromitsu, Saba Imtiaz, Philip Kaaret, Jochen Kaminski, Dawoon E. Kim, Fabian Kislat, Henric Krawczynski, Fabio La Monaca, Carlo Lefevre, Hemanth Manikantan, Herman L. Marshall, Romana Mikusincova, Alfredo Morbidini, Fabio Muleri, Stephen L. O’Dell, Takashi Okajima, Mark Pearce, Vladislavs Plesanovs, Brian D. Ramsey, Ajay Ratheesh, Alda Rubini, Shravan Vengalil Menon and Martin C. Weisskopf Show full author list Hide full author list

Galaxies 2024, 12(4), 47; https://doi.org/10.3390/galaxies12040047 - 8 Aug 2024

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Abstract

The Imaging X-ray Polarimetry Explorer (IXPE) has confirmed that X-ray polarimetry is a valuable tool in astronomy, providing critical insights into the emission processes and the geometry of compact objects. IXPE was designed to be sensitive in the 2–8 keV energy range for [...] Read more.

The Imaging X-ray Polarimetry Explorer (IXPE) has confirmed that X-ray polarimetry is a valuable tool in astronomy, providing critical insights into the emission processes and the geometry of compact objects. IXPE was designed to be sensitive in the 2–8 keV energy range for three primary reasons: (1) celestial X-ray sources are bright within this range, (2) the optics are effective, and (3) most sources across various classes were expected to exhibit some level of polarization. Indeed, IXPE is a great success, and its discoveries are necessitating the revision of many theoretical models for numerous sources. However, one of IXPE’s main limitations is its relatively narrow energy band, coupled with rapidly declining efficiency. In this paper, we will demonstrate the benefits of devising a mission focused on a broader energy band (0.1–79 keV). This approach leverages current technologies that align well with theoretical expectations and builds on the successes of IXPE. Full article

(This article belongs to the Special Issue X-Ray Polarimetry: A New Era Begins with the Imaging X-Ray Polarimetry Explorer)

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14 pages, 1795 KiB

Open AccessEditor’s ChoiceReview

X-ray Polarimetry of X-ray Pulsars

by Juri Poutanen, Sergey S. Tsygankov and Sofia V. Forsblom

Galaxies 2024, 12(4), 46; https://doi.org/10.3390/galaxies12040046 - 7 Aug 2024

Cited by 2 | Viewed by 847

Abstract

Radiation from X-ray pulsars (XRPs) was expected to be strongly linearly polarized owing to a large difference in their ordinary and extraordinary mode opacities. The launch of IXPE allowed us to check this prediction. IXPE observed a dozen X-ray pulsars, discovering pulse-phase dependent [...] Read more.

Radiation from X-ray pulsars (XRPs) was expected to be strongly linearly polarized owing to a large difference in their ordinary and extraordinary mode opacities. The launch of IXPE allowed us to check this prediction. IXPE observed a dozen X-ray pulsars, discovering pulse-phase dependent variation of the polarization degree (PD) and polarization angle (PA). Although the PD showed rather erratic profiles resembling flux pulse dependence, the PA in most cases showed smooth variations consistent with the rotating vector model (RVM), which can be interpreted as a combined effect of vacuum birefringence and dipole magnetic field structure at a polarization-limiting (adiabatic) radius. Application of the RVM allowed us to determine XRP geometry and to confirm the free precession of the NS in Her X-1. Deviations from RVM in two bright transients led to the discovery of an unpulsed polarized emission likely produced by scattering off the accretion disk wind. Full article

(This article belongs to the Special Issue X-Ray Polarimetry: A New Era Begins with the Imaging X-Ray Polarimetry Explorer)

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8 pages, 2851 KiB

Open AccessEditor’s ChoiceReview

The IXPE Science of Pulsars and Their Nebulae

by Niccolò Bucciantini, Roger W. Romani, Fei Xie and Josephine Wong

Galaxies 2024, 12(4), 45; https://doi.org/10.3390/galaxies12040045 - 7 Aug 2024

Cited by 1 | Viewed by 756

Abstract

Pulsars (PSRs) and Pulsar Wind Nebulae (PWNe) form some of the most interesting high-energy astrophysical systems. Their prominent synchrotron emission makes them ideal candidates for polarimetry. Here, after briefly summarizing the scientific rationale underpinning the importance of their polarimetric studies, we review the [...] Read more.

Pulsars (PSRs) and Pulsar Wind Nebulae (PWNe) form some of the most interesting high-energy astrophysical systems. Their prominent synchrotron emission makes them ideal candidates for polarimetry. Here, after briefly summarizing the scientific rationale underpinning the importance of their polarimetric studies, we review the current status and achievements obtained by the IXPE mission. For the first time, we have space- and phase-resolved X-ray data that allow us to set constraints on the geometry and level of turbulence of the magnetic field which, in turn, can help us to better understand which acceleration and radiation model(s) might be at work in these systems. Interestingly, PWNe show a large variety in terms of polarization degree that might indicate key physical differences, still to be further investigated. Full article

(This article belongs to the Special Issue X-Ray Polarimetry: A New Era Begins with the Imaging X-Ray Polarimetry Explorer)

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8 pages, 1762 KiB

Open AccessFeature PaperEditor’s ChoiceReview

The IXPE View of Neutron Star Low-Mass X-ray Binaries

by Francesco Ursini, Andrea Gnarini, Fiamma Capitanio, Anna Bobrikova, Massimo Cocchi, Alessandro Di Marco, Sergio Fabiani, Ruben Farinelli, Fabio La Monaca, John Rankin, Mary Lynne Saade and Juri Poutanen

Galaxies 2024, 12(4), 43; https://doi.org/10.3390/galaxies12040043 - 4 Aug 2024

Cited by 3 | Viewed by 994

Abstract

Low-mass X-ray binaries hosting weakly magnetized neutron stars (NS-LMXBs) are among the brightest sources in the X-ray sky. Since 2021, the Imaging X-ray Polarimetry Explorer (IXPE) has provided new measurements of the X-ray polarization of these sources. IXPE observations have revealed that most [...] Read more.

Low-mass X-ray binaries hosting weakly magnetized neutron stars (NS-LMXBs) are among the brightest sources in the X-ray sky. Since 2021, the Imaging X-ray Polarimetry Explorer (IXPE) has provided new measurements of the X-ray polarization of these sources. IXPE observations have revealed that most NS-LMXBs are significantly polarized in the X-rays, providing unprecedented insight into the geometry of their accretion flow. In this review paper, we summarize the first results obtained by IXPE on NS-LMXBs, the emerging trends within each class of sources (atoll/Z), and possible physical interpretations. Full article

(This article belongs to the Special Issue X-Ray Polarimetry: A New Era Begins with the Imaging X-Ray Polarimetry Explorer)

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9 pages, 2148 KiB

Open AccessReview

On the Making of IXPE

by Martin C. Weisskopf and Brian Ramsey

Galaxies 2024, 12(4), 38; https://doi.org/10.3390/galaxies12040038 - 16 Jul 2024

Viewed by 799

Abstract

Drs. Weisskopf and Ramsey were the original Principal and Deputy Principal Investigators of the Imaging X-ray Polarimetry Explorer (IXPE). They outline the path to the development of IXPE and discuss the technical and programmatic history that led up to the mission, a partnership [...] Read more.

Drs. Weisskopf and Ramsey were the original Principal and Deputy Principal Investigators of the Imaging X-ray Polarimetry Explorer (IXPE). They outline the path to the development of IXPE and discuss the technical and programmatic history that led up to the mission, a partnership between the Italian Space Agency and NASA, and the first fully dedicated to imaging X-ray polarimetry in the 2–8 keV band. An admittedly biased, as seen through the eyes of the original and Deputy Principal Investigators, technical overview of the development of the historical and critical scientific instrumentation is provided. The outstanding, and often paradigm-shifting results are presented in the papers following this one. Full article

(This article belongs to the Special Issue X-Ray Polarimetry: A New Era Begins with the Imaging X-Ray Polarimetry Explorer)

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11 pages, 548 KiB

Open AccessReview

An Examination of the Very First Polarimetric X-ray Observations of Radio-Quiet Active Galactic Nuclei

by Frédéric Marin, Vittoria E. Gianolli, Adam Ingram, Dawoon E. Kim, Andrea Marinucci, Daniele Tagliacozzo and Francesco Ursini

Galaxies 2024, 12(4), 35; https://doi.org/10.3390/galaxies12040035 - 2 Jul 2024

Cited by 1 | Viewed by 1063

Abstract

Active galactic nuclei (AGNs), either radio-quiet or radio-loud, had never been observed in X-ray polarized light until the advent of the Imaging X-ray Polarimetry Explorer (IXPE) in the end of 2021. This satellite opened a new observational window for studying supermassive black holes [...] Read more.

Active galactic nuclei (AGNs), either radio-quiet or radio-loud, had never been observed in X-ray polarized light until the advent of the Imaging X-ray Polarimetry Explorer (IXPE) in the end of 2021. This satellite opened a new observational window for studying supermassive black holes and their complex environment. In this regard, radio-quiet AGNs are probably better targets than radio-loud objects to probe accretion processes due to the lack of synchrotron emission from jets that can dilute the polarized signal from the central engine. Their relatively clean environment not only allows to detect and measure the X-ray polarization originating from the hot corona responsible for X-ray emission, but also to assess the geometry of the media immediately surrounding the supermassive black hole. Such geometrical measurements work just as well for characterizing the corona morphology in pole-on AGNs as it does for determining the three-dimensional shape of the circumnuclear cold obscurer (the so-called torus) in edge-on AGNs. In this review paper, we will return to each of the observations made by IXPE so far in the field of radio-quiet AGNs and highlight the fundamental contribution of X-ray polarimetry to our understanding of how light is emitted and how matter is shaped around supermassive black holes. Full article

(This article belongs to the Special Issue X-Ray Polarimetry: A New Era Begins with the Imaging X-Ray Polarimetry Explorer)

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