Pulsar wind nebulae are fascinating systems, and archetypal sources for high-energy astrophysics in general. Due to their vicinity, brightness, to the fact that they shine at multi-wavelengths, and ...especially to their long-living emission at gamma-rays, modelling their properties is particularly important for the correct interpretation of the visible Galaxy. A complication in this respect is the variety of properties and morphologies they show at different ages. Here we discuss the differences among the evolutionary phases of pulsar wind nebulae, how they have been modeled in the past and what progresses have been recently made. We approach the discussion from a phenomenological, theoretical (especially numerical) and observational point of view, with particular attention to the most recent results and open questions about the physics of such intriguing sources.
A black hole x-ray binary (XRB) system forms when gas is stripped from a normal star and accretes onto a black hole, which heats the gas sufficiently to emit x-rays. We report a polarimetric ...observation of the XRB Cygnus X-1 using the Imaging X-ray Polarimetry Explorer. The electric field position angle aligns with the outflowing jet, indicating that the jet is launched from the inner x-ray–emitting region. The polarization degree is 4.01 ± 0.20% at 2 to 8 kiloelectronvolts, implying that the accretion disk is viewed closer to edge-on than the binary orbit. These observations reveal that hot x-ray–emitting plasma is spatially extended in a plane perpendicular to, not parallel to, the jet axis.
x-ray polarization of Cygnus X-1
A black hole in a binary system can rip material off of its companion star, which heats up and forms an accretion disk. The disc emits light in the optical and x-ray bands, forming an x-ray binary (XRB) system. Some XRBs also launch a jet of fast-moving material that is visible at radio wavelengths. Krawczynski
et al
. observed the x-ray polarization of Cygnus X-1, a black hole XRB with a radio jet. By comparing the measured polarization properties with several competing XRB models, they eliminated some hypothesized geometries and determined that the x-ray–emitting region extends parallel to the accretion disc. —KTS
x-ray polarization measurements determine the geometric arrangement of hot material accreting onto a black hole.
Pulsar Wind Nebulae are among the most spectacular targets of X-ray astronomy. Their X-ray emission is the synchrotron radiation of the ultrarelativistic pulsar wind shocked in the ambient medium. ...Since the polarization position angle is perpendicular to the direction of the magnetic field at the site of emission, and the degree of polarization depends on the energy spectrum of emitting electrons, spatially resolved polarization measurements will probe the magnetic field topology and its connection with the PWN morphological elements, providing a unique opportunity to investigate physical processes not otherwise accessible. Future missions such as XIPE will be crucial in this respect. Keywords. X-Ray, Polarimetry, Supernova Remnants Le nebulose da pulsar sono fra gli oggetti piu spettacolari dell'astronomia nei raggi X. La loro emissione X non e altro che la radiazione di sincrotrone del vento ultrarelativistico della pulsar, rallentato e compresso dal mezzo circostante. Siccome l'angolo di polarizzazione della radiazione e perpendicolare alla direzione del campo magnetico nel punto di emissione, e il grado di polarizzazione dipende dalla distribuzione in energia degli elettroni emettenti, misure di polarizzazione risolte spazialmente possono dare informazioni sulla topologia del campo magnetico e la sua connessione con gli elementi morfologici della nebulosa, offrendo un'opportunita unica di studiare processi fisici altrimenti inaccessibili. Missioni future come il satellite XIPE saranno cruciali a tale riguardo. Parole chiave: Raggi-X, Polarimetria, Resti di Supernova
Pulsar wind nebulae are formed when outflows of relativistic electrons and positrons hit the surrounding supernova remnant or interstellar medium at a shock front. The Vela pulsar wind nebula is ...powered by a young pulsar (B0833-45, aged 11,000 years)
and located inside an extended structure called Vela X, which is itself inside the supernova remnant
. Previous X-ray observations revealed two prominent arcs that are bisected by a jet and counter jet
. Radio maps have shown high linear polarization of 60% in the outer regions of the nebula
. Here we report an X-ray observation of the inner part of the nebula, where polarization can exceed 60% at the leading edge-approaching the theoretical limit of what can be produced by synchrotron emission. We infer that, in contrast with the case of the supernova remnant, the electrons in the pulsar wind nebula are accelerated with little or no turbulence in a highly uniform magnetic field.
Particle acceleration mechanisms in supermassive black hole jets, such as shock acceleration, magnetic reconnection, and turbulence, are expected to have observable signatures in the multiwavelength ...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-synchrotron-peaked BL Lac objects (HSPs). We report the discovery of X-ray linear polarization (degree Πx = 15% ± 2% and electric vector position angle ψx = 35° ± 4°) from the jet of the HSP Mrk 421 in an average X-ray flux state. At the same time, the degree of polarization at optical, infrared, and millimeter wavelengths was found to be lower by at least a factor of 3. During the IXPE pointing, the X-ray flux of the source increased by a factor of 2.2, while the polarization behavior was consistent with no variability. The higher level of Πx compared to longer wavelengths, and the absence of significant polarization variability, suggest a shock is the most likely X-ray emission site in the jet of Mrk 421 during the observation. The multiwavelength polarization properties are consistent with an energy-stratified electron population, where the particles emitting at longer wavelengths are located farther from the acceleration site, where they experience a more disordered magnetic field.
Abstract
The first X-ray pulsar, Cen X-3, was discovered 50 yr ago. Radiation from such objects is expected to be highly polarized due to birefringence of plasma and vacuum associated with ...propagation of photons in the presence of the strong magnetic field. Here we present results of the observations of Cen X-3 performed with the Imaging X-ray Polarimetry Explorer. The source exhibited significant flux variability and was observed in two states different by a factor of ∼20 in flux. In the low-luminosity state, no significant polarization was found in either pulse phase-averaged (with a 3
σ
upper limit of 12%) or phase-resolved (the 3
σ
upper limits are 20%–30%) data. In the bright state, the polarization degree of 5.8% ± 0.3% and polarization angle of 49.°6 ± 1.°5 with a significance of about 20
σ
were measured from the spectropolarimetric analysis of the phase-averaged data. The phase-resolved analysis showed a significant anticorrelation between the flux and the polarization degree, as well as strong variations of the polarization angle. The fit with the rotating vector model indicates a position angle of the pulsar spin axis of about 49° and a magnetic obliquity of 17°. The detected relatively low polarization can be explained if the upper layers of the neutron star surface are overheated by the accreted matter and the conversion of the polarization modes occurs within the transition region between the upper hot layer and a cooler underlying atmosphere. A fraction of polarization signal can also be produced by reflection of radiation from the neutron star surface and the accretion curtain.
PulsarWind Nebulae (PWNe) are bubbles or relativistic plasma that form when the pulsar wind is confined by the SNR or the ISM. Recent observations have shown a richness of emission features that has ...driven a renewed interest in the theoretical modeling of these objects. In recent years a MHD paradigm has been developed, capable of reproducing almost all of the observed properties of PWNe, shedding new light on many old issues. Given that PWNe are perhaps the nearest systems where processes related to relativistic dynamics can be investigated with high accuracy, a reliable model of their behavior is paramount for a correct understanding of high energy astrophysics in general. I will review the present status of MHD models: what are the key ingredients, their successes, and open questions that still need further investigation.
Polarized x-rays from a magnetar Taverna, Roberto; Turolla, Roberto; Muleri, Fabio ...
Science (American Association for the Advancement of Science),
11/2022, Volume:
378, Issue:
6620
Journal Article
Peer reviewed
Open access
Magnetars are neutron stars with ultrastrong magnetic fields, which can be observed in x-rays. Polarization measurements could provide information on their magnetic fields and surface properties. We ...observed polarized x-rays from the magnetar 4U 0142+61 using the Imaging X-ray Polarimetry Explorer and found a linear polarization degree of 13.5 ± 0.8% averaged over the 2– to 8–kilo–electron volt band. The polarization changes with energy: The degree is 15.0 ± 1.0% at 2 to 4 kilo–electron volts, drops below the instrumental sensitivity ~4 to 5 kilo–electron volts, and rises to 35.2 ± 7.1% at 5.5 to 8 kilo–electron volts. The polarization angle also changes by 90° at ~4 to 5 kilo–electron volts. These results are consistent with a model in which thermal radiation from the magnetar surface is reprocessed by scattering off charged particles in the magnetosphere.
Polarization constrains magnetar emission
Magnetars are young neutron stars with high magnetic fields that are usually observed at x-ray wavelengths. The emission mechanism and geometry of the emitting region have been unclear. Taverna
et al
. measured the x-ray polarization of the magnetar 4U 0142+61. The polarization degree and angle change as a function of x-ray energy, indicating two different emission regions. The authors preferred a model in which most of the x-rays are emitted by an equatorial band on the surface of the neutron star, with some of the photons then being scattered to higher energies by collisions with electrons in the surrounding magnetic field. —KTS
Measurements of a magnetar’s x-ray polarization constrain models of the emission mechanism.
Abstract
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 4U 1626–67 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 used as the basis for our timing analysis and to constrain the spectral shape over the 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 found an upper limit on the pulse-averaged linear polarization of <4% (at 95% confidence). Similarly, there was no significant detection of polarized flux in pulse phase intervals when subdividing the bandpass by energy. However, spectropolarimetric modeling over the full bandpass in pulse phase intervals provided a marginal detection of polarization of the power-law spectral component at the 4.8% ± 2.3% level (90% confidence). We discuss the implications concerning the accretion geometry onto the pulsar, favoring two-component models of the pulsed emission.
X-ray polarimetry is a unique way to probe the geometrical configuration of highly magnetized accreting neutron stars (X-ray pulsars).GRO J1008−57 is the first transient X-ray pulsar observed at two ...different flux levels by the Imaging X-ray Polarimetry Explorer (IXPE) duringits outburst in November 2022. We find the polarization properties of GRO J1008−57 to be independent of its luminosity, with the polarization degree varying between nondetection and about 15% over the pulse phase. Fitting the phase-resolved spectro-polarimetric data with the rotatingvector model allowed us to estimate the pulsar inclination (130◦, which is in good agreement with the orbital inclination), the position angle (75◦)of the pulsar spin axis, and the magnetic obliquity (∼74◦). This makes GRO J1008−57 the first confidently identified nearly orthogonal rotator among X-ray pulsars. We discuss our results in the context of the neutron star atmosphere models and theories of the axis alignment of accreting pulsars.
PDF
