Accepted NuSTAR Programs through IXPE Cycle 3

Proposal number: 3091
PI last name: Gianolli
Title: A complete map of the X-ray polarization of NGC 4151: The polarization properties of the reflection component
Abstract: We propose to observe the Seyfert galaxy NGC 4151 in a low flux state with IXPE (900ks) and NuSTAR (75ks), with both observations
triggered by a Swift monitoring (33ks). NGC 4151 is the first radio-quiet Active Galactic Nucleus with an X-ray polarization measurement of the corona. However, the polarization properties of the reflection component, from close and distant matter, are unconstrained. During its extended low flux states, the reflection component dominates over the primary emission, offering the best opportunity for a detailed characterization of the polarization properties of this component. This would allow us to achieve the first comprehensive characterization of the X-ray polarization properties of both the primary continuum and reflection component within a single source.

Proposal number: 3111
PI last name: Ewing
Title: Searching for relativistic precession in accreting black holes with X-ray polarimetry-timing
Abstract: We propose a 300 ks IXPE ToO observation of a black hole X-ray binary exhibiting a Type-C quasi-periodic oscillation (QPO) in its X-ray flux, alongside a simultaneous 20 ks NuSTAR exposure and three 1 ks Swift/XRT pointings. We will use IXPE to search for modulations in the PD and PA on the QPO period, definitively testing whether or not such QPOs are driven by relativistic nodal precession of the inner accretion flow. We will use NuSTAR to constrain the physical source parameters, and the Swift/XRT pointings to confirm the presence of a QPO before triggering. The source may be known or unknown, as long as it displays a confirmed Type-C QPO and its 2--8 keV IXPE count rate (estimated from MAXI) is projected to be ≥30 c/s. We request a 10 day lead time (medium priority).

Proposal number: 3129
PI last name: Papitto
Title: Polarized X-rays from accreting millisecond pulsars: A pathway to the equation of state of neutron stars
Abstract: Modelling the waveform of the X-ray pulse of accreting ms pulsars (AMSP) to constrain the equation of state of neutron stars requires an extremely high number of counts to break the degeneracy between the many parameters that shape it. IXPE observations of an AMSP recently discovered polarized X-ray emission at a degree of ~2% but could not detect spin phase-resolved variability. Here, we propose a longer 900-ks-long ToO observation of the next outburst of an AMSP to constrain the binary inclination and the spot latitude to within a few degrees. This information will serve as a key prior in the fit of the waveform observed by instruments with a larger effective area (e.g. NuSTAR, XMM-Newton) and constrain the mass and the radius of these neutron stars with an accuracy of a few per cent.

Proposal number: 3140
PI last name: Papitto
Title: X-ray polarimetry of the highly accreting seyfert Ark 564
Abstract: We propose an IXPE and NuSTAR observation of the narrow-line Seyfert 1 galaxy Ark 564, one of the X-ray brightest high accretion rate AGN. IXPE observations of moderately accreting Seyferts to date point to radially extended slab or wedge-like coronae, yet the high-Eddington regime is still unexplored. A 2 Ms IXPE exposure of Ark 564 will allow polarization to be measured at a few percent level (MDP ~ 2.5%), sufficient to discriminate between competing models for the Comptonizing medium geometry. A simultaneous NuSTAR 100 ks exposure will provide independent constraints on the coronal temperature and the reflection component, ensuring a physically self-consistent interpretation. This campaign will deliver the first robust measurement of X-ray polarization in a highly accreting Seyfert.

Proposal number: 3207
PI last name: Chakraborty
Title: Probing the transient obscured black hole X-ray binaries with spectro-polarimetry
Abstract: We propose IXPE (150 ks) and NuSTAR (20 ks) observations of a transient obscured black hole binary during outburst. These systems, likely obscured by Compton-thick winds from super-Eddington accretion, show strong absorption and reflection features. Spectro-polarimetry will constrain the geometry of the obscurer. These observations will help us derive similarities between these galactic super-Eddington objects viewed at higher inclinations, to extragalactic ULXs viewed at lesser inclinations along the wind cone.

Proposal number: 3252
PI last name: Ravi
Title: Energy-dependent polarization rotation: Testing general relativistic disk models with IXPE
Abstract: We propose a 1.4 Ms triggered IXPE program, with contemporaneous NuSTAR and Swift coverage, to search for an energy-dependent rotation of the X-ray polarization angle in a black-hole X-ray binary. General relativistic models predict such swings in the disk-dominated soft state, with amplitudes of up to tens of degrees across 2 8 keV. Detecting this effect would provide a direct strong-field test of General Relativity and yield an independent probe of black-hole spin and inclination. By targeting a luminous soft-state transient during its decline and applying Bayesian analysis techniques, this program offers the strongest opportunity yet to reveal the predicted EVPA evolution.

Proposal number: 3258
PI last name: Nathan
Title: Targetting the return of the bright GRS 1915+105
Abstract: We propose three ToO observations to observe GRS 1915+105 if it leaves it's current absorbed state and returns to it's pre-2019 levels. These ToOs will target three different states of the source: the 'chi' state, analogous to a canonical hard XRB state; a disc dominated state, analogous to a canonical soft XRB state; and a state where the source is showing exotic variability. Together they will help uncover the mystery of the accretion system around GRS 1915+105. Even taking the observations individually, the high flux levels of the source means it's a perfect poster child to study XRB accretion via polarisation in these states. GRS 1915+105 has long been a keystone for studying XRBs, and any observations in its absorbed state will be a legacy of science done with IXPE.

Proposal number: 3260
PI last name: Loberger
Title: A legacy study of Cen X-3 with IXPE: Polarization as a probe of neutron star physics
Abstract: Cen X-3 is an X-ray binary system where a pulsar orbits a massive O star, producing some of the brightest and most complex X-ray emission known. While theory predicts strong polarization from such systems, past IXPE observations found unexpectedly low values, suggesting that additional effects such as scattering in the accretion disk or heating of the pulsar s atmosphere may dilute the signal. Recent XRISM data further revealed distinct absorption and scattering features tied to orbital phase, highlighting the importance of the surrounding environment. We propose 470 ks of IXPE observations spanning multiple orbits to disentangle these contributions, combining phase-resolved polarimetry with simultaneous broadband NuSTAR spectra.

Accepted NuSTAR Programs through IXPE Cycle 2

Proposal number: 2036
PI last name: Jorstad
Title: Mechanisms of X-Ray production in LSP and ISP blazars
Abstract: The investigators propose to observe an X-ray flaring blazar of the low-synchrotron-peak (LSP) or intermediate-synchrotron-peak (ISP) type with the Imaging X-ray Polarimetry Explorer (IXPE) using a 900 ks exposure to measure linear polarization parameters. We propose also 20 ks and 14 ks observations with the NuSTAR and Swift telescopes simultaneously with the IXPE long-look. We will organize polarization observations at lower frequencies. The polarization measured by IXPE, combined with the simultaneous 0.3-70 keV continuum spectrum along with mm-wave and optical polarization, will provide a definitive test of the synchrotron self-Compton and external Compton models for X-ray emission from an LSP blazar or for the hard X-ray emission from an ISP blazar.

Proposal number: 2037
PI last name: Tagliacozzo
Title: Constraining the coronal geometry in the Seyfert galaxy NGC 5506 with X-Ray polarimetry
Abstract: We propose to observe the Seyfert NGC 5506 (one of the brightest sources not yet observed by IXPE) for 1 Ms (MDP = 2%-3%). The well known shape of the high energy spectrum and the physical properties of the corona appear to be rather extreme (Ec > 350 keV, kT ~ 400 keV). However, spectroscopy alone was not enough to distinguish between coronal models. X-ray polarimetry can remove the degeneracies, since polarization properties depend on the degree of symmetry of the system. IXPE observed three RQ AGN so far, but only one provided a > 3sigma detection. These observations indicate a hint of alignment between the primary continuum PA and the radio jet, favouring radially extended coronae. The observation of NGC 5506 could provide a further step in the understanding of AGN coronal properties.

Proposal number:: 2121
PI last name: Di Marco
Title: Polarimetric study of the ultra-compact H-deficient system 4U 0614+091
Abstract: UCXBs are LMXBs with a short orbital period and H-deficient companions. X-ray polarimetric results for the UCXB 4U 1820-303 showed a peculiar behavior that, if confirmed to be common to other UCXBs, can be used as a signature for their identification. We propose to observe the brightest UCXB with a CO/ONeMg WD companion: 4U 0614+091. The reflection in this system is different compared to other LMXBs as a result of a different surrounding material. This new observation, combined with the spectral modeling of reflection, will give fundamental information to improve our understanding of accretion flow in H-deficient systems. Simultaneous observations at other wavelengths will allow for a comparison of the polarization in different bands, corresponding to different emission regions.

Proposal number: 2142
PI last name: Soffitta
Title: Probing the accretion ejection-geometry of the hyper-Eddington black-hole binary SS 433 with X-Ray spectro-polarimetry
Abstract: SS 433 is the only micro-quasar with hadronic persistent jets and, at variance with other binary systems, is characterized by a non-transient supercritical accretion of gas. SS433 is X-ray faint due to heavy absorption. Recent observations of highly obscured sources have shown reflection-dominated X-ray spectra thought to be the walls of wind outflow originating from the accretion disk to form a reflection cone. IXPE observation of SS 433 (500 ks) would for the first time unambiguously constrain its reflector geometry and nature (wind or thick disk). This would be the first X-ray polarization observation of the central engine of this exotic X-ray binary and it will help to probe the rare high Eddington accretion regime in galactic accreting sources and show its X-ray Ultra Luminous nature.

Proposal number: 2151
PI last name: Steiner
Title: A test of disk-jet misalignment and black hole spin measurement for the X-Ray binary H1743-322
Abstract: The BH microquasar H1743-322 is uniquely poised to deliver key insights into questions on putative misalignment between the binary orbital and BH spin axes. H1743 is a propitious source with recurrent transient activity, a high expected PD (owing both to its high inclination and its predicted low spin), and is one of the strongest and most prolific sources of QPOs. The leading model of QPO production requires significant misalignment, but this prediction hasn't been readily testable. An IXPE campaign of 320ks divided between two observations of H1743, one in a hard state and the other in a soft/thermal state, will enable a direct constraint on potential misalignment, testing the QPO mechanism. Moreover, this campaign will also produce an orthogonal constraint on H1743's spin.

Proposal number: 2154
PI last name: Anitra
Title: Exploring X-Ray polarization of the high-inclined low-mass X-Ray pulsar 4U 1822-37
Abstract: Recent advancements in polarimetric capabilities with IXPE have opened new opportunities to understand the geometry of X-ray pulsars (XRPs) in low-mass X-ray binaries (LMXBs). Among XRPs, 4U 1822 37 stands out for its peculiar geometrical configuration and remains one of the most puzzling sources despite numerous studies. The detected cyclotron resonance line, the high inclination angle, and the theorized presence of an extended optically thin corona make this source an ideal target for IXPE observation. We request the first IXPE observation of 4U 1822 37 with a total exposure of 500 ks to ensure a minimum detectable polarization of 2%. To achieve comprehensive spectral and temporal analysis, we also request joint NuSTAR (80 ks) and NICER (50 ks) observations.

Proposal number: 2156
PI last name: Bhargava
Title: Identifying the geometry of weakly magnetized low mass x-ray binary Aql X-1 using joint IXPE, NICER and NuSTAR
Abstract: Aquila X-1 is one of the most recurrent transient sources in the X-ray sky. It shows clear hysteresis during its spectral evolution. To investigate the longstanding question of the geometry of spectral components, we propose joint observations of the source during its next outburst using IXPE, NICER, and NuSTAR for a total exposure of 400 ks, 10 ks, and 40 ks respectively. The proposed observations will provide first-ever estimates of X-ray polarization in this source during the soft and hard states. The spectro-polarimetric study will provide crucial insights into the geometry of the corona and its orientation relative to the thermal component. Comparing the X-ray polarization estimates with radio polarization will yield strong constraints on the origin of non-thermal emission in Aql X-1.

Proposal number: 2184
PI last name: Ratheesh
Title: Solving the mystery behind the X-Ray dimming of GRS 1915+105 using X-Ray polarimetry
Abstract: GRS 1915+105 is currently in a noval obscured state, adding to the exotic variability it has exhibited in the past. The X-ray spectrum is heavily absorbed locally and dominated by reflection features. The primary source of this obscuration is believed to be a Compton-thick accretion disk wind. There is an ongoing debate about whether the source is undergoing super-Eddington accretion or if the accretion rate has decreased and the source transited into low hard state. X-ray Polarimetry, as an excellent tool for estimating the geometry of reflecting matter, could help resolve this by determining the opening angle of the wind and derive the intrinsic luminosity. Further, IXPE observations would also help to understand the nature of aperiodic X-ray flares in the source.

Proposal number: 2191
PI last name: Parra
Title: A novel view of disk winds in outbursting black hole low-mass X-Ray binaries with X-Ray spectro-polarimetry
Abstract: The first IXPE observations of wind emitting BHLMXBs showed that the wind is likely to play a pivotal role in the significant polarization of these sources. While the scattering of the incident flux in the outflowing material leads to a polarization signal, different launching mechanisms are expected to leave different imprints on the polarization measurements, and the results are also extremely dependent on the wind structure. New IXPE data of more high-inclined sources will be tremendously helpful to understand and disentangle the dependence of the wind and inclination angle on the polarization properties. We request an IXPE ToO observation of a high-inclined BHLMXB with wind detections, in the soft state, plus 15ks of NICER exposure, and 20ks of NuSTAR exposure, for spectral coverage.