First detection of X-ray polarization from the accreting neutron star 4U 1820-303

This paper reports the first detection of polarization in the X-rays for atoll-source 4U 1820-303, obtained with the Imaging X-ray Polarimetry Explorer (IXPE) at 99.999% confidence level (CL). Simultaneous polarimetric measurements were also performed in the radio with the Australia Telescope Compact Array (ATCA). The IXPE observations of 4U 1820-303 were coordinated with Swift-XRT, NICER, and NuSTAR aiming to obtain an accurate X-ray spectral model covering a broad energy interval. The source shows a significant polarization above 4 keV, with a polarization degree of 2.0(0.5)% and a polarization angle of -55(7) deg in the 4-7 keV energy range, and a polarization degree of 10(2)% and a polarization angle of -67(7) deg in the 7-8 keV energy bin. This polarization also shows a clear energy trend with polarization degree increasing with energy and a hint for a position-angle change of about 90 deg at 96% CL around 4 keV. The spectro-polarimetric fit indicates that the accretion disk is polarized orthogonally to the hard spectral component, which is presumably produced in the boundary/spreading layer. We do not detect linear polarization from the radio counterpart, with a 99.97% upper limit of 50% at 7.25 GHz

Magneto-thermal evolution of neutron stars: a 3D approach

Abstract (IT): Le stelle di neutroni (NS) sono lo stadio evolutivo finale di stelle con massa sufficientemente elevata (8 M_sol < M < 25 M_sol), i resti del nucleo collassano a seguito dell'esplosione di una supernova che segna la morte del progenitore. Con una massa dell'ordine di quella solare e un raggio di solo 10-15 km, esse sono i corpi più compatti dell'universo, raggiungendo densità maggiori dei nuclei atomici. Inoltre, sono dotate di campi magnetici estremamente forti, i cui valori sono i più alti registrati ad oggi. Le NS nascono con alte temperature (T ~ 10^11 K), e si raffreddano invecchiando. La loro evoluzione termica é intrinsecamente legata a quella del loro campo magnetico. Facendo un confronto con le osservazioni, la conoscenza dell'evoluzione secolare magneto-termica permette una distinzione tra diversi scenari di raffreddamento, vincolando, quindi, l'equazione di stato (EOS) della materia ultra-densa. Inoltre, fornisce una mappa auto-consistente della temperatura superficiale, che rappresenta la chiave per derivare una stima affidabile del raggio della stella a partire dalle osservazioni in banda X. Questo porta alla determinazione della EOS per le NS. Lo scopo di questo studio è di creare un modello per l'evoluzione della temperatura e delle componenti del campo magnetico nella crosta delle NS attraverso simulazioni numeriche eseguite con PARODY, un codice 3D pseudo-spettrale che risolve l'equazione di induzione accoppiata con quella per la temperatura nella la crosta di una NS isolata con un campo magnetico molto intenso (B >~ 10^14 G). È necessario, dunque, un approccio interamente in 3D per trattare le configurazioni magnetiche non-asimmetriche ed esplorare le strutture su piccola scala che derivano direttamente dal termine di Hall, il cui ruolo é stato recentemente delucidato. Come fatto in precedenti studi sull'argomento, calcoliamo il campo magnetico e l'evoluzione della temperatura della crosta della NS, basandoci sul presupposto che l'effetto Meissner é in grado di espellere qualunque flusso magnetico dal nucleo superconduttivo di tipo I, in un tempo minore rispetto a quelli dell'evoluzione magnetica e termica. In questo lavoro presentiamo alcune delle prime simulazioni 3D magneto-termiche, considerando diverse configurazioni iniziali per il campo magnetico. Iniziamo dai casi assisimmetrici, per poter fare un miglior paragone con i precedenti studi, per poi considerare i modelli non-assisimmetrici, per cui é certamente necessario un approccio 3D per fare uno studio completo della topologia magnetica e della configurazione della temperatura nella crosta e sulla superficie. Abstract (ENG): Neutron stars (NSs) are the endpoints of the evolution of massive stars (8 M_sol < M < 25 M_sol), the remnants of the core collapse which follows the supernova explosion which marks the death of the progenitor. With a mass of the order of the solar mass and a radius of only 10-15 km, NSs are the most compact stellar objects in the present universe, reaching higher densities than those of atomic nuclei. Besides, they are endowed with extremely strong magnetic, the highest recorded to date. NSs are born very hot (T ~ 10^11 K), and cool down as they age. Their thermal evolution is inherently coupled to that of their magnetic field. Knowledge of the secular magneto-thermal evolution can discriminate between different cooling scenarios when compared to observations, thus constraining the equation of state (EOS) of ultra-dense matter. Moreover, it provides a self-consistent map of the surface temperature, which is key in deriving any reliable estimate of the star radius from X-ray observations. This, again, directly bears to the determination of the NS EOS. The aim of this study is to model the evolution of the temperature and the magnetic field in the NS crust by means of numerical simulations performed with PARODY, a 3D, pseudo-spectral code which solves the coupled induction and temperature equations in the crust of an highly-magnetised isolated NS (B >~ 10^14 G). A fully 3D approach is required in order to treat non-axysimmetric magnetic configurations and the explore small-scale structures which naturally arise as a consequence of the Hall term, the role of which has been recently elucidated. As in previous studies on the subject, we compute the magnetic field and temperature evolution in the NS crust, relying on the assumption that the Meissner effect is able to expel any magnetic flux from the type I, superconducting core on a timescale shorter than the typical timescales of magnetic and thermal evolution. We present in this work some of the first 3D magneto-thermal simulations, considering different initial configurations for the magnetic field. We start with axisymmmetric cases, to have a better comparison with previous investigations, and then turn to non-axisymmetric models, for which a 3D approach is indeed necessary for a complete study of the magnetic topology and the temperature configuration in the crust

X-Ray Emission from Isolated Neutron Stars Revisited: 3D Magnetothermal Simulations

X-ray emission from the surface of isolated neutron stars (NSs) has been now observed in a variety of sources. The ubiquitous presence of pulsations clearly indicates that thermal photons either come from a limited area, possibly heated by some external mechanism, or from the entire (cooling) surface but with an inhomogeneous temperature distribution. In an NS the thermal map is shaped by the magnetic \ufb01eld topology since heat \ufb02ows in the crust mostly along the magnetic \ufb01eld lines. Self-consistent surface thermal maps can hence be produced by simulating the coupled magnetic and thermal evolution of the star. We compute the evolution of the NS crust in three dimensions for different initial con\ufb01gurations of the magnetic \ufb01eld and use the ensuing thermal surface maps to derive the spectrum and the pulse pro\ufb01le as seen by an observer at in\ufb01nity, accounting for general-relativistic effects. In particular, we compare cases with a high degree of symmetry with inherently 3D ones, obtained by adding a quadrupole to the initial dipolar \ufb01eld. Axially symmetric \ufb01elds result in rather small pulsed fractions (<5%), while more complex con\ufb01gurations produce higher pulsed fractions, up to 3c25%. We \ufb01nd that the spectral properties of our axisymmetric model are close to those of the bright isolated NS RX J1856.5-3754 at an evolutionary time comparable with the inferred dynamical age of the source

Polarization Properties of the Weakly Magnetized Neutron Star X-Ray Binary GS 1826–238 in the High Soft State

The launch of the Imaging X-ray Polarimetry Explorer (IXPE) on 2021 December 9 has opened a new window in X-ray astronomy. We report here the results of the first IXPE observation of a weakly magnetized neutron star, GS 1826−238, performed on 2022 March 29–31 when the source was in a high soft state. An upper limit (99.73% confidence level) of 1.3% for the linear polarization degree is obtained over the IXPE 2–8 keV energy range. Coordinated INTEGRAL and NICER observations were carried out simultaneously with IXPE. The spectral parameters obtained from the fits to the broadband spectrum were used as inputs for Monte Carlo simulations considering different possible geometries of the X-ray emitting region. Comparing the IXPE upper limit with these simulations, we can put constraints on the geometry and inclination angle of GS 1826–238

X-Ray Polarized View of the Accretion Geometry in the X-Ray Binary Circinus X-1

International audienceCir X-1 is a neutron star X-ray binary characterized by strong variations in flux during its eccentric ~16.6 day orbit. There are also strong variations in the spectral state, and it has historically shown both atoll and Z state properties. We observed the source with the Imaging X-ray Polarimetry Explorer during two orbital segments, 6 days apart, for a total of 263 ks. We find an X-ray polarization degree in these segments of 1.6% ± 0.3% and 1.4% ± 0.3% at polarization angles of 37° ± 5° and -12° ± 7°, respectively. Thus, we observed a rotation of the polarization angle by 49° ± 8° along the orbit. Because variations of accretion flow, and then of the hardness ratio, are expected during the orbit, we also studied the polarization binned in hardness ratio and found the polarization angle differing by 67° ± 11° between the lowest and highest values of the hardness ratio. We discuss possible interpretations of this result that could indicate a possible misalignment between the symmetry axes of the accretion disk and the Comptonizing region caused by the misalignment of the neutron star's angular momentum with respect to the orbital one

X-Ray Polarimetry of the Dipping Accreting Neutron Star 4U 1624–49

We present the first X-ray polarimetric study of the dipping accreting neutron star 4U 1624−49 with the Imaging X-ray Polarimetry Explorer. We report a detection of polarization in the nondip time intervals with a confidence level of 99.99%. We find an average polarization degree (PD) of 3.1% ± 0.7% and a polarization angle of 81° ± 6° east of north in the 2–8 keV band. We report an upper limit on the PD of 22% during the X-ray dips with 95% confidence. The PD increases with energy, reaching from 3.0% ± 0.9% in the 4–6 keV band to 6% ± 2% in the 6–8 keV band. This indicates the polarization likely arises from Comptonization. The high PD observed is unlikely to be produced by Comptonization in the boundary layer or spreading layer alone. It can be produced by the addition of an extended geometrically thin slab corona covering part of the accretion disk, as assumed in previous models of dippers, and/or a reflection component from the accretion disk

X-Ray Polarimetry of the Dipping Accreting Neutron Star 4U 1624–49

International audienceWe present the first X-ray polarimetric study of the dipping accreting neutron star 4U 1624‑49 with the Imaging X-ray Polarimetry Explorer. We report a detection of polarization in the nondip time intervals with a confidence level of 99.99%. We find an average polarization degree (PD) of 3.1% ± 0.7% and a polarization angle of 81° ± 6° east of north in the 2–8 keV band. We report an upper limit on the PD of 22% during the X-ray dips with 95% confidence. The PD increases with energy, reaching from 3.0% ± 0.9% in the 4–6 keV band to 6% ± 2% in the 6–8 keV band. This indicates the polarization likely arises from Comptonization. The high PD observed is unlikely to be produced by Comptonization in the boundary layer or spreading layer alone. It can be produced by the addition of an extended geometrically thin slab corona covering part of the accretion disk, as assumed in previous models of dippers, and/or a reflection component from the accretion disk

Discovery of strongly variable X-ray polarization in the neutron star low-mass X-ray binary transient XTE J1701−462

International audienceContext. After about 16 years since its first outburst, the transient neutron star low-mass X-ray binary XTE J1701−462 turned on again in September 2022, allowing for the first study of its X-ray polarimetric characteristics by a dedicated observing program with the Imaging X-ray Polarimeter Explorer (IXPE). Aims: Polarimetric studies of XTE J1701−462 have been expected to improve our understanding of accreting weakly magnetized neutron stars, in particular, the physics and the geometry of the hot inner regions close to the compact object. Methods: The IXPE data of two triggered observations were analyzed using time-resolved spectroscopic and polarimetric techniques, following the source along its Z-track of the color-color diagram. Results: During the first pointing on 2022 September 29, an average 2-8 keV polarization degree of (4.6 ± 0.4)% was measured, the highest value found up to now for this class of sources. Conversely, only a ∼0.6% average degree was obtained during the second pointing ten days later. Conclusions: The polarimetric signal appears to be strictly related to the higher energy blackbody component associated with the boundary layer (BL) emission and its reflection from the inner accretion disk, and it is as strong as 6.1% and 1.2% (> 95% significant) above 3-4 keV for the two measurements, respectively. The variable polarimetric signal is apparently related to the spectral characteristics of XTE J1701−462, which is the strongest when the source was in the horizontal branch of its Z-track and the weakest in the normal branch. These IXPE results provide new important observational constraints on the physical models and geometry of the Z-sources. Here, we discuss the possible reasons for the presence of strong and variable polarization among these sources

Highly Significant Detection of X-Ray Polarization from the Brightest Accreting Neutron Star Sco X-1

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