| 1. |
- Demidem, Camilia, 1992-, et al.
(författare)
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Relativistic Collisionless Shocks in Inhomogeneous Magnetized Plasmas
- 2023
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Ingår i: Astrophysical Journal Letters. - : American Astronomical Society. - 2041-8205 .- 2041-8213. ; 947:1
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Tidskriftsartikel (refereegranskat)abstract
- Relativistic collisionless shocks are associated with efficient particle acceleration when propagating into weakly magnetized homogeneous media; as the magnetization increases, particle acceleration becomes suppressed. We demonstrate that this changes when the upstream carries kinetic-scale inhomogeneities, as is often the case in astrophysical environments. We use fully kinetic simulations to study relativistic perpendicular shocks in magnetized pair plasmas interacting with upstream density perturbations. For amplitudes of δρ/ρ ≳ 0.5, the upstream fluctuations are found to corrugate the shock front and generate large-scale turbulent shear motions in the downstream, which in turn are capable of accelerating particles. This can revive relativistic magnetized shocks as viable energization sites in astrophysical systems, such as jets and accretion disks. The generation of large-scale magnetic structures also has important implications for polarization signals from blazars.
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| 2. |
- Doroshenko, Victor, et al.
(författare)
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Complex variations in X-ray polarization in the X-ray pulsar LS V +44 17/RX J0440.9+4431
- 2023
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Ingår i: Astronomy and Astrophysics. - : EDP Sciences. - 0004-6361 .- 1432-0746. ; 677
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Tidskriftsartikel (refereegranskat)abstract
- We report on Imaging X-ray polarimetry explorer (IXPE) observations of the Be-transient X-ray pulsar LS V +44 17/RX J0440.9+4431 made at two luminosity levels during the giant outburst in January- February 2023. Considering the observed spectral variability and changes in the pulse profiles, the source was likely caught in supercritical and subcritical states with significantly different emission-region geometry, associated with the presence of accretion columns and hot spots, respectively. We focus here on the pulse-phase-resolved polarimetric analysis and find that the observed dependencies of the polarization degree and polarization angle (PA) on the pulse phase are indeed drastically different for the two observations. The observed differences, if interpreted within the framework of the rotating vector model (RVM), imply dramatic variations in the spin axis inclination, the position angle, and the magnetic colatitude by tens of degrees within the space of just a few days. We suggest that the apparent changes in the observed PA phase dependence are predominantly related to the presence of an unpulsed polarized component in addition to the polarized radiation associated with the pulsar itself. We then show that the observed PA phase dependence in both observations can be explained with a single set of RVM parameters defining the pulsar s geometry. We also suggest that the additional polarized component is likely produced by scattering of the pulsar radiation in the equatorial disk wind.
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| 3. |
- Gianolli, V. E., et al.
(författare)
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Uncovering the geometry of the hot X-ray corona in the Seyfert galaxy NGC 4151 with IXPE
- 2023
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Ingår i: Monthly notices of the Royal Astronomical Society. - : Oxford University Press (OUP). - 0035-8711 .- 1365-2966. ; 523:3, s. 4468-4476
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Tidskriftsartikel (refereegranskat)abstract
- We present an X-ray spectropolarimetric analysis of the bright Seyfert galaxy NGC 4151. The source has been observed with the Imaging X-ray Polarimetry Explorer (IXPE) for 700 ks, complemented with simultaneous XMM-Newton (50 ks) and NuSTAR (100 ks) pointings. A polarization degree Π = 4.9 ± 1.1 per cent and angle ψ = 86° ± 7° east of north (68 per cent confidence level) are measured in the 2-8 keV energy range. The spectropolarimetric analysis shows that the polarization could be entirely due to reflection. Given the low reflection flux in the IXPE band, this requires, however, a reflection with a very large (>38 per cent) polarization degree. Assuming more reasonable values, a polarization degree of the hot corona ranging from ∼4 to ∼8 per cent is found. The observed polarization degree excludes a 'spherical' lamppost geometry for the corona, suggesting instead a slab-like geometry, possibly a wedge, as determined via Monte Carlo simulations. This is further confirmed by the X-ray polarization angle, which coincides with the direction of the extended radio emission in this source, supposed to match the disc axis. NGC 4151 is the first active galactic nucleus with an X-ray polarization measure for the corona, illustrating the capabilities of X-ray polarimetry and IXPE in unveiling its geometry.
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| 4. |
- Ingram, A., et al.
(författare)
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The X-ray polarization of the Seyfert 1 galaxy IC 4329A
- 2023
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Ingår i: Monthly notices of the Royal Astronomical Society. - : Oxford University Press (OUP). - 0035-8711 .- 1365-2966. ; 525:4, s. 5437-5449
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Tidskriftsartikel (refereegranskat)abstract
- We present an X-ray spectro-polarimetric analysis of the bright Seyfert galaxy IC 4329A. The Imaging X-ray Polarimetry Explorer (IXPE) observed the source for ∼500 ks, supported by XMM-Newton (∼60 ks) and NuSTAR (∼80 ks) exposures. We detect polarization in the 2-8 keV band with 2.97σ confidence. We report a polarization degree of 3.3 ± 1.1 per cent and a polarization angle of 78° ± 10° (errors are 1σ confidence). The X-ray polarization is consistent with being aligned with the radio jet, albeit partially due to large uncertainties on the radio position angle. We jointly fit the spectra from the three observatories to constrain the presence of a relativistic reflection component. From this, we obtain constraints on the inclination angle to the inner disc (<39° at 99 per cent confidence) and the disc inner radius (<11 gravitational radii at 99 per cent confidence), although we note that modelling systematics in practice add to the quoted statistical error. Our spectropolarimetric modelling indicates that the 2-8 keV polarization is consistent with being dominated by emission directly observed from the X-ray corona, but the polarization of the reflection component is completely unconstrained. Our constraints on viewer inclination and polarization degree tentatively favour more asymmetric, possibly out-flowing, coronal geometries that produce more highly polarized emission, but the coronal geometry is unconstrained at the 3σ level.
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| 5. |
- Ingram, Adam, et al.
(författare)
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Tracking the X-Ray Polarization of the Black Hole Transient Swift J1727.8-1613 during a State Transition
- 2024
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Ingår i: Astrophysical Journal. - : American Astronomical Society. - 0004-637X .- 1538-4357. ; 968:2
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Tidskriftsartikel (refereegranskat)abstract
- We report on an observational campaign on the bright black hole (BH) X-ray binary Swift J1727.8-1613 centered around five observations by the Imaging X-ray Polarimetry Explorer. These observations track for the first time the evolution of the X-ray polarization of a BH X-ray binary across a hard to soft state transition. The 2-8 keV polarization degree decreased from ∼4% to ∼3% across the five observations, but the polarization angle remained oriented in the north-south direction throughout. Based on observations with the Australia Telescope Compact Array, we find that the intrinsic 7.25 GHz radio polarization aligns with the X-ray polarization. Assuming the radio polarization aligns with the jet direction (which can be tested in the future with higher-spatial-resolution images of the jet), our results imply that the X-ray corona is extended in the disk plane, rather than along the jet axis, for the entire hard intermediate state. This in turn implies that the long (≳10 ms) soft lags that we measure with the Neutron star Interior Composition ExploreR are dominated by processes other than pure light-crossing delays. Moreover, we find that the evolution of the soft lag amplitude with spectral state does not follow the trend seen for other sources, implying that Swift J1727.8-1613 is a member of a hitherto undersampled subpopulation.
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| 6. |
- Kaaret, Philip, et al.
(författare)
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X-Ray Polarization of the Eastern Lobe of SS 433
- 2024
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Ingår i: Astrophysical Journal Letters. - : American Astronomical Society. - 2041-8205 .- 2041-8213. ; 961:1
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Tidskriftsartikel (refereegranskat)abstract
- How astrophysical systems translate the kinetic energy of bulk motion into the acceleration of particles to very high energies is a pressing question. SS 433 is a microquasar that emits TeV γ-rays indicating the presence of high-energy particles. A region of hard X-ray emission in the eastern lobe of SS 433 was recently identified as an acceleration site. We observed this region with the Imaging X-ray Polarimetry Explorer and measured a polarization degree in the range 38%-77%. The high polarization degree indicates the magnetic field has a well-ordered component if the X-rays are due to synchrotron emission. The polarization angle is in the range −12° to +10° (east of north), which indicates that the magnetic field is parallel to the jet. Magnetic fields parallel to the bulk flow have also been found in supernova remnants and the jets of powerful radio galaxies. This may be caused by interaction of the flow with the ambient medium.
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| 7. |
- La Monaca, Fabio, et al.
(författare)
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Highly Significant Detection of X-Ray Polarization from the Brightest Accreting Neutron Star Sco X-1
- 2024
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Ingår i: Astrophysical Journal Letters. - : American Astronomical Society. - 2041-8205 .- 2041-8213. ; 960:2
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Tidskriftsartikel (refereegranskat)abstract
- The Imaging X-ray Polarimetry Explorer measured with high significance the X-ray polarization of the brightest Z-source, Sco X-1, resulting in the nominal 2–8 keV energy band in a polarization degree of 1.0% ± 0.2% and a polarization angle of 8° ± 6° at a 90% confidence level. This observation was strictly simultaneous with observations performed by NICER, NuSTAR, and Insight-HXMT, which allowed for a precise characterization of its broadband spectrum from soft to hard X-rays. The source has been observed mainly in its soft state, with short periods of flaring. We also observed low-frequency quasiperiodic oscillations. From a spectropolarimetric analysis, we associate a polarization to the accretion disk at <3.2% at 90% confidence level, compatible with expectations for an electron scattering dominated optically thick atmosphere at the Sco X-1 inclination of ∼44°; for the higher-energy Comptonized component, we obtain a polarization of 1.3% ± 0.4%, in agreement with expectations for a slab of Thomson optical depth of ∼7 and an electron temperature of ∼3 keV. A polarization rotation with respect to previous observations by OSO-8 and PolarLight, and also with respect to the radio-jet position angle, is observed. This result may indicate a variation of the polarization with the source state that can be related to relativistic precession or a change in the corona geometry with the accretion flow.
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| 8. |
- Loktev, Vladislav, et al.
(författare)
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ARTPOL: Analytical ray-tracing method for spectro-polarimetric properties of accretion disks around Kerr black holes
- 2024
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Ingår i: Astronomy and Astrophysics. - : EDP Sciences. - 0004-6361 .- 1432-0746. ; 685
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Tidskriftsartikel (refereegranskat)abstract
- Spectro-polarimetric signatures of accretion disks in X-ray binaries and active galactic nuclei contain information on the masses and spins of their central black holes, as well as the geometry of matter in proximity to the compact objects. This information can be extracted by means of X-ray polarimetry. In this work, we present a fast analytical ray-tracing technique for polarized light (artpol) that helps us to obtain the spinning black hole parameters from the observed properties. This technique can replace the otherwise time-consuming numerical ray-tracing calculations for any optically thick or geometrically thin accretion flow. For the purposes of illustration, we considered a standard optically thick, geometrically thin accretion disk in the equatorial plane of the Kerr black hole. We show that artpol proves accurate for dimensionless spin parameter a ≤ 0.94 with a speed that is over four orders of magnitude faster than direct ray-tracing calculations. This approach opens up broader prospects for direct fittings of the spectro-polarimetric data from the Imaging X-ray Polarimetry Explorer.
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| 9. |
- Marra, L., et al.
(författare)
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IXPE observation confirms a high spin in the accreting black hole 4U 1957+115
- 2024
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Ingår i: Astronomy and Astrophysics. - 0004-6361 .- 1432-0746. ; 684
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Tidskriftsartikel (refereegranskat)abstract
- We present the results of the first X-ray polarimetric observation of the low-mass X-ray binary 4U 1957+115, performed with the Imaging X-ray Polarimetry Explorer in May 2023. The binary system has been in a high-soft spectral state since its discovery and is thought to host a black hole. The ∼571 ks observation reveals a linear polarisation degree of 1.9%±0.6% and a polarisation angle of −41.°8±7.°9 in the 2–8 keV energy range. Spectral modelling is consistent with the dominant contribution coming from the standard accretion disc, while polarimetric data suggest a significant role of returning radiation: photons that are bent by strong gravity effects and forced to return to the disc surface, where they can be reflected before eventually reaching the observer. In this setting, we find that models with a black hole spin lower than 0.96 and an inclination lower than 50° are disfavoured.
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| 10. |
- Podgorný, J., et al.
(författare)
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Recovery of the X-ray polarisation of Swift J1727.8-1613 after the soft-to-hard spectral transition
- 2024
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Ingår i: Astronomy and Astrophysics. - : EDP Sciences. - 0004-6361 .- 1432-0746. ; 686
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Tidskriftsartikel (refereegranskat)abstract
- We report on the detection of X-ray polarisation in the black-hole X-ray binary Swift J1727.8-1613 during its dim hard spectral state by the Imaging X-ray Polarimetry Explorer (IXPE). This is the first detection of X-ray polarisation at the transition from the soft to the hard state in an X-ray binary. We find an averaged 2- 8 keV polarisation degree of (3.3 ± 0.4)% and a corresponding polarisation angle of 3 ±4, which matches the polarisation detected during the rising stage of the outburst, in September- October 2023, within 1σuncertainty. The observational campaign complements previous studies of this source and enables comparison of the X-ray polarisation properties of a single transient across the X-ray hardness-intensity diagram. The complete recovery of the X-ray polarisation properties, including the energy dependence, came after a dramatic drop in the X-ray polarisation during the soft state. The new IXPE observations in the dim hard state at the reverse transition indicate that the accretion properties, including the geometry of the corona, appear to be strikingly similar to the bright hard state during the outburst rise despite the X-ray luminosities differing by two orders of magnitude.
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