First X-Ray and Radio Polarimetry of the Neutron Star Low-mass X-Ray Binary GX 17+2

We report the first X-ray polarimetric results of the neutron star (NS) low-mass X-ray binary Z-source GX 349+2 using the Imaging X-ray Polarimetry Explorer (IXPE). We discovered that the X-ray source was polarized at a polarization degree of PD = 1.1% ± 0.3% (1σ errors) with a polarization angle of PA = 32° ± 6° (1σ errors). Simultaneous Nuclear Spectroscopic Telescope Array observations show that the source transitioned through the normal branch, flaring branch, and soft apex of the Z-track during our IXPE observations. The X-ray spectropolarimetry results suggest a source geometry comprising an accretion disk component, a blackbody representing the emission from the NS surface, and a Comptonized component. We discuss the accretion geometry of the Z-source in light of the spectropolarimetric results.

We report the X-ray and radio polarization study of the neutron star low-mass X-ray binary (LMXB) GX 13+1 using the Imaging X-ray Polarimetry Explorer (IXPE) and Very Large Array. Simultaneous Neutron Star Interior Composition Explorer observations show that the source was in parts of the Z state during our IXPE observations, exhibiting moderate changes in the hardness–intensity diagram. The source exhibits X-ray dips in the light curve along with hints of polarization swings between the dip and nondip states. The X-ray spectropolarimetry results suggest a source geometry comprising an accretion disk component representing the softer disk emission, along with a blackbody representing the harder emission from the boundary layer or a spreading layer. We investigate the geometry of GX 13+1 by considering our X-ray and radio polarization findings.

We report a broad-band investigation of the Z-type neutron star (NS) low mass X-ray binary (LMXB) GX 349+2 using AstroSat and Neutron star Interior Composition Explorer (NICER). AstroSat observed the source exhibiting large scale variability in its normal branch (NB)/flaring branch (FB) vertex and FB and a moderate evolution during NICER observations. The power spectra exhibit very low-frequency noise (VLFN) and low-frequency noise (LFN)/FB noise, described by a power law (PL) and an evolving Lorentzian. We investigate the energy dependence of variability components and their correlation with the spectral state to probe their origin. The joint spectra of GX 349+2 are modeled by two thermal and one non-thermal component. The source moves along the Z track, with the increasing accretion rate, further heating of the NS boundary layer (BL), and increasing temperature/radius of the brightened hotspot at the disc-BL interface/NS surface. A PL well represents the hard non-thermal coronal emission. As predicted by the gravitational redshift, we find a correlation between the line energy detected in NICER spectra and the inner disc radius with the Spearman rank correlation coefficient of 1. Using this correlation, we demonstrate the potential of a method to constrain the accreting compact object properties, including evolving continuum and line spectroscopy. We report the first detection of hard lag providing evidence of the VLFN originating from the accretion disc in NS LMXBs, representing fluctuation of propagation through the disc.

We report on the coordinated observations of the neutron star low-mass X-ray binary (NS-LMXB) GX 5−1 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 3σ 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 1701−462). Evidence of a variation in the polarization angle of ∼20° 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.

Z-sources are a particular class of neutron star low-mass X-ray binaries characterized by a wide Z-like track in their hard color-soft color (or hardness-intensity) diagrams, with three branches: the horizontal (HB), the normal (NB), and the flaring branch (FB). Spectropolarimetric observations with the Imaging X-ray Polarimetry Explorer ( show that the polarization in these sources varies along the Z-track, reaching unexpectedly high values in the HB. In this work, we collected all the polarimetric results obtained so far from observations of Z-sources with using a model-independent analysis with ixpeobssim . We first performed a detailed characterization of the spectral state of each source along the Z-track using along with the Nuclear Spectroscopic Telescope Array ( and the Neutron Star Interior Composition Explorer ( data and then estimated the polarization for each branch. Although we confirm that the average polarization in the 2--8 keV band decreases moving from the HB to the NB for all three Z-sources observed in these branches, we also observe a qualitatively increasing trend from the NB to the FB. Whereas this increase is clearly significant for Cyg X-2 and Sco X-1, the polarization remains consistent at the 90% confidence level for GX 5--1 and GX 349+2, while for XTE J1701--462 and GX 340+0 only upper limits are found in the FB. For most sources, the average polarization angle in the 2--8 keV range remains consistent along the CCD; however, we observe a significant rotation for both Sco X-1 and GX 349+2 (at the 90% confidence level) as they move from the NB to the FB. In addition, we observe a significant increase in the polarization degree with energy in most of the observed Z-sources, with some also exhibiting a rotation of the polarization angle with energy (approximately by $20

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.

We present the spectral and timing study of the bright neutron star low-mass X-ray binary GX 5-1 using AstroSat/Large Area X-ray Proportional Counter (LAXPC) and Soft X-ray Telescope observations conducted in 2018. During the observation, the source traces out the complete horizontal branch (HB) and normal branch (NB) of the Z-track in the hardness–intensity diagram (HID). Understanding the spectral and temporal evolution of the source along the Z-track can probe the accretion process in the vicinity of a neutron star. Spectral analysis was performed in the 0.7–20 keV energy range for different segments in the HID using a multitemperature disk blackbody with an average temperature, kT in ∼ 0.47, and a thermal Comptonization model. It is found that the optical depth of the corona drops from ∼7.07 in HB to ∼2.61 in NB. The timing analysis using the LAXPC instrument indicates the presence of quasiperiodic oscillations in HB and NB of the Z-track. The observed QPO frequencies are similar to the characteristic frequencies of horizontal branch oscillations (HBOs) and normal branch oscillations (NBOs). The HBO frequency increases from ∼12 to 40 Hz toward the hard apex. NBOs are observed at ∼5 Hz. The timing studies conducted in soft and hard bands indicate the association of HBO and NBO origin with the nonthermal component. Further research could explore the implications of this relationship for understanding the dynamics of accretion onto neutron stars.

In order to explain the two-branch spectrum behavior of the low-mass X-ray binary (LMXB) GX 5-1, an improved column accretion model is suggested. The key point is that the viscous damping (radiation drag) in the column accretion process has to be taken into consideration. From this improved model, a theoretical hardness-intensity diagram is obtained, which is a good fit with the observations

In low-mass X-ray binaries (LMXBs), accretion flows are often associated with either jet outflows or disk winds. Studies of LMXBs with luminosities up to roughly 20% of the Eddington limit indicate that these outflows generally do not co-occur, suggesting that disk winds might inhibit jets. However, previous observations of LMXBs accreting near or above the Eddington limit show that jets and winds can potentially coexist. To investigate this phenomenon, we carried out a comprehensive multiwavelength campaign (using the Very Large Array (VLA), Chandra/High Energy Transmission Grating Spectrometer (HETG), and NICER) on the near-Eddington neutron-star Z-source LMXB GX 13+1. NICER and Chandra/HETG observations tracked GX 13+1 across the entire Z track during high Eddington rates, detecting substantial resonance absorption features originating from the accretion disk wind in all X-ray spectra, which implies a persistent wind presence. Simultaneous VLA observations captured a variable radio jet, with radio emission notably strong during all flaring branch observations—contrary to typical behavior in Z sources—and weaker when the source was on the normal branch. Interestingly, no clear correlation was found between the radio emission and the wind features. Analysis of VLA radio light curves and simultaneous Chandra/HETG spectra demonstrates that an ionized disk wind and jet outflow can indeed coexist in GX 13+1, suggesting that their launching mechanisms are not necessarily linked in this system.

The Imaging X-Ray Polarimetry Explorer (IXPE) is designed to expand understanding of high-energy astrophysical processes and sources, in support of NASA's first science objective in Astrophysics: “Discover how the universe works.” Polarization measurements and imaging are key capabilities of the IXPE observatory. IXPE, an international collaboration between NASA and the Italian Space Agency (ASI), is a NASA Small Explorer designed as a 2-year mission which launches to a circular LEO orbit at an altitude of 600 km and an inclination of ~0 degrees on a Falcon 9 launch vehicle. The payload uses a single science operational mode to capture the x-ray data from the science targets. The mission design follows a simple observing paradigm: pointed viewing of known x-ray sources (with known locations in the sky) over multiple orbits (not necessarily consecutive orbits) until the observation is complete. IXPE's payload is a set of three identical, imaging, x-ray polarimetry telescopes mounted on a common optical bench and co-aligned with the pointing axis of the spacecraft. The payload is accommodated on the spacecraft top deck. The BCP-Small spacecraft provides the necessary resources to support and operate the payload elements and enable continuous science data collection. The Observatory communicates with ground stations via S-band link. The ground system consists of three major elements: the ground stations for data receipt and command upload to the Observatory; the Mission Operation Center (MOC); and Science Operations Center (SOC). The primary ground station, contributed to the IXPE mission as part of an international collaboration with ASI is at Malindi, Kenya. The back-up ground station is in Singapore through the NEN. TDRSS is used for early operations and contingencies. The MOC is located at CU/LASP using their existing multi-mission MOC. The SOC is located at MSFC and the data archive at the GSFC HEASARC. The IXPE Project completed its Phase A activities in July 2016; Project start occurred in February 2017. Currently, the Project is finishing up Phase D activities, having completed all Observatory environmental testing and launch with final commissioning activities ongoing. Launch occurred on 9 December 2021. This paper summarizes the IXPE mission, describes the Observatory, launch segment, ground system, discusses launch and commissioning and reviews lessons-learned.

The Imaging X-ray Polarimetry Explorer (IXPE)

Low-luminosity active galactic nuclei (LLAGN) provide a unique view of Comptonization and nonthermal emission from accreting black holes in the low accretion rate regime. However, to decipher the exact nature of the Comptonizing corona in LLAGN, its geometry and emission mechanism must be understood beyond the limits of spectro-timing techniques. Spectropolarimetry offers the potential to break the degeneracies between different coronal emission models. Compton-thin LLAGN provide an opportunity for such spectropolarimetric exploration in the 2–8 keV energy range using the Imaging X-Ray Polarimetry Explorer (IXPE). In this work, we carry out a spectropolarimetric analysis of the first IXPE observation, in synergy with a contemporaneous Nuclear Spectroscopic Telescope Array observation, of an LLAGN: NGC 2110. Using 554.4 ks of IXPE data from 2024 October, we constrain the 99% upper limit on the polarization degree (PD) to be less than 8.3% assuming the corresponding polarization angle (PA) to be aligned with the radio jet, and less than 3.6% if in the perpendicular direction. In the absence of a significant PD detection, the PA remains formally unconstrained, yet the polarization significance contours appear to be aligned with the radio jet, tentatively supporting models in which the corona is radially extended in the plane of the disk. We also carry out detailed Monte Carlo simulations using monk and STOKES codes to test different coronal models against our results and compare the polarization properties between NGC 2110 and brighter Seyferts.

The Imaging X-Ray Polarimetry Explorer (IXPE) is designed to expand understanding of high-energy astrophysical processes and sources, in support of NASA's first science objective in Astrophysics: “Discover how the universe works.” Polarization measurements and imaging are key capabilities of the IXPE observatory. IXPE, an international collaboration between NASA and the Italian Space Agency (ASI), is a NASA Small Explorer designed as a 2-year mission. It launches to a circular LEO orbit at an altitude of 600 km and an inclination of ~0 degrees on a Falcon 9 launch vehicle. The payload uses a single science operational mode to capture the X-ray data from the science targets. The mission design follows a simple observing paradigm: pointed viewing of known X-ray sources (with known locations in the sky) over multiple orbits (not necessarily consecutive orbits) until the observation is complete. IXPE's payload is a set of three identical, imaging, X-ray polarimetry telescopes mounted on a common optical bench and co-aligned with the pointing axis of the spacecraft. The BCP-Small spacecraft provides the necessary resources to support and operate the payload elements and enable continuous science data collection. The Observatory communicates with ground stations via S-band link. The ground system consists of three major elements: the ground stations for data receipt and command upload to the Observatory; the Mission Operation Center (MOC); and Science Operations Center (SOC). Launch occurred on 9 December 2021. The IXPE Observatory has been collecting science data from cosmic X-ray sources since 11 January 2022. These include extreme astronomical objects, such as stellar-mass and supermassive black holes, magnetars, Blazars, neutron stars, and pulsars. This paper summarizes the IXPE mission, describes the Observatory, ground system, and commissioning and overviews recent science results.

We report on a comprehensive analysis of simultaneous X-ray polarimetric and spectral data of the bright atoll source GX 9+9 with the Imaging X-ray Polarimetry Explorer (IXPE) and NuSTAR. The source is significantly polarized in the 4–8 keV band, with a degree of 2.2% ± 0.5% (uncertainty at the 68% confidence level). The NuSTAR broad-band spectrum clearly shows an iron line, and is well described by a model including thermal disc emission, a Comptonized component, and reflection. From a spectro-polarimetric fit, we obtain an upper limit to the polarization degree of the disc of 4% (at the 99% confidence level), while the contribution of Comptonized and reflected radiation cannot be conclusively separated. However, the polarization is consistent with resulting from a combination of Comptonization in a boundary or spreading layer, plus reflection off the disc, which significantly contributes in any realistic scenario.

We report on the timing and spectral analysis of transient Be X-ray pulsar GRO J1750–27 using the Nuclear Spectroscopic Telescope Array(NuSTAR) observation from 2021 September. This is the fourth outburst of the system since 1995. The NuSTAR observation was performed during the rising phase of the outburst. Pulsations at a period of 4.450710(1) s were observed in the 3–60 keV energy range. The average pulse profile comprised of a broad peak with a weak secondary peak, which evolved with energy. We did not find any appreciable variation in the X-ray emission during this observation. The broad-band phase-averaged spectrum is described by a blackbody, a power law, or Comptonization component. We report the discovery of Fe Kα line at 6.4 keV, along with the presence of two cyclotron resonant scattering features of around 36 and 42 keV. These lines indicate a magnetic field with the strength of $3.7_{-0.3}^{+0.1} \times 10^{12}$ and 4.4 ± 0.10 × 1012 G for the neutron star. We have estimated a source distance of ∼13.6–16.4 kpc based on the accretion-disc torque models.