10 research outputs found
Polarized radiation from an accretion shock in accreting millisecond pulsars using exact Compton scattering formalism
Pulse profiles of accreting millisecond pulsars can be used to determine
neutron star (NS) parameters, such as their masses and radii, and therefore
provide constraints on the equation of state of cold dense matter. Information
obtained by the Imaging X-ray Polarimetry Explorer (IXPE) can be used to
decipher pulsar inclination and magnetic obliquity, providing ever tighter
constraints on other parameters. In this paper, we develop a new emission model
for accretion-powered millisecond pulsars based on thermal Comptonization in an
accretion shock above the NS surface. The shock structure was approximated by
an isothermal plane-parallel slab and the Stokes parameters of the emergent
radiation were computed as a function of the zenith angle and energy for
different values of the electron temperature, the Thomson optical depth of the
slab, and the temperature of the seed blackbody photons. We show that our
Compton scattering model leads to a significantly lower polarization degree of
the emitted radiation compared to the previously used Thomson scattering model.
We computed a large grid of shock models, which can be combined with pulse
profile modeling techniques both with and without polarization included. In
this work, we used the relativistic rotating vector model for the oblate NS in
order to produce the observed Stokes parameters as a function of the pulsar
phase. Furthermore, we simulated the data to be produced by IXPE and obtained
constraints on model parameters using nested sampling. The developed methods
can also be used in the analysis of the data from future satellites, such as
the enhanced X-ray Timing and Polarimetry mission.Comment: Accepted to A&A on 11 August 202
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
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X-Ray Polarimetry of the Dipping Accreting Neutron Star 4U 1624â49
We present the first X-ray polarimetric study of the dipping accreting neutron star 4U 1624â49 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 2â8 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 4â6 keV band to 6% ± 2% in the 6â8 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 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 1624â49
International audienceWe present the first X-ray polarimetric study of the dipping accreting neutron star 4U 1624â49 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 2â8 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 4â6 keV band to 6% ± 2% in the 6â8 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 J1701â462
International audienceContext. After about 16 years since its first outburst, the transient neutron star low-mass X-ray binary XTE J1701â462 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 J1701â462 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 âŒ0.6% 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 J1701â462, 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
Discovery of a variable energy-dependent X-ray polarization in the accreting neutron star GX 5-1
International audienceWe report on the coordinated observations of the neutron star low-mass X-ray binary (NS-LMXB) GX 5â1 in X-rays (IXPE, NICER, NuSTAR, and INTEGRAL), optical (REM and LCO), near-infrared (REM), mid-infrared (VLT VISIR), and radio (ATCA). This Z-source was observed by IXPE twice in March-April 2023 (Obs. 1 and 2). In the radio band the source was detected, but only upper limits to the linear polarization were obtained at a 3Ï level of 6.1% at 5.5 GHz and 5.9% at 9 GHz in Obs. 1 and 12.5% at 5.5 GHz and 20% at 9 GHz in Obs. 2. The mid-IR, near-IR, and optical observations suggest the presence of a compact jet that peaks in the mid- or far-IR. The X-ray polarization degree was found to be 3.7%±0.4% (at 90% confidence level) during Obs. 1 when the source was in the horizontal branch of the Z-track and 1.8%±0.4% during Obs. 2 when the source was in the normal-flaring branch. These results confirm the variation in polarization degree as a function of the position of the source in the color-color diagram, as for previously observed Z-track sources (Cyg X-2 and XTE 1701â462). Evidence of a variation in the polarization angle of âŒ20° with energy is found in both observations, likely related to the different, nonorthogonal polarization angles of the disk and Comptonization components, which peak at different energies
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 2â8 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 âŒ44°; 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 âŒ7 and an electron temperature of âŒ3 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