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Cycle 1 Accepted IXPE GO Program Abstracts

Proposal Number: 1009
Proposal Type: REGULAR
PI Name: KARRI KOLJONEN
Title: IS MICROQUASAR GRS 1915+105 CURRENTLY IN A SUPER-EDDINGTON ACCRETION STATE?
Abstract: We propose an IXPE observation of GRS 1915+105 during its unusual obscured accretion state. Currently, GRS 1915+105 exhibits strong radio flaring and shares X-ray spectral properties similar to other X-ray binaries experiencing super-Eddington accretion rates (such as V404 Cyg, Cyg X-3, V4641 Sgr). These similarities strongly suggest super-Eddington accretion in GRS 1915+105, offering a rare opportunity to study this high accretion rate phase in detail. The potential detection of a high polarization degree would signify that the received emission is seen as scattered, in accordance with super-Eddington accretion. This discovery could hold profound implications for our understanding of black hole accretion processes and their impact on the surrounding environment.

Proposal Number: 1014
Proposal Type: THEORY
PI Name: HAOCHENG ZHANG
Title: PROBING PARTICLE ACCELERATION IN HIGH SYNCHROTRON PEAKED BLAZARS
Abstract: Understanding the particle acceleration in HSPs is among the key scientific goals of IXPE. We hereby propose a self-consistent theoretical study plan for the multi-wavelength signatures of HSPs. We will perform 3D particle-in-cell (PIC) and polarized radiation transfer simulations under the magnetic reconnection scenario in HSPs. We will address three main questions: Q1. What is the typical ratio between optical and X-ray polarization degrees for reconnection? Q2. Can reconnection explain the X-ray angle swing and sudden polarization degree changes as observed in Mrk 421? Q3. Do we expect correlated X-ray and TeV fast flares and X-ray polarization variations? These signatures can be compared with future IXPE and multi-wavelength observations, greatly enhance the scientific return of IXPE.

Proposal Number: 1031
Proposal Type: REGULAR
PI Name: FABRIZIO TAVECCHIO
Title: IXPE OBSERVATIONS OF EXTREME BLAZARS: PROBING THE MECHANISMS BEHIND HIGHLY EFFICIENT PARTICLE ACCELERATION IN JETS
Abstract: The study of particle acceleration in jets is at the focus of current efforts in high-energy astrophysics. Extreme BL Lacs (EHBL), play a unique role, because their features challenge theoretical efforts and provide insights into acceleration at the extreme. For EHBL, X-ray polarimetry gives access to the synchrotron emission from the most energetic, freshly accelerated electrons, allowing us to probe and characterize magnetic fields and source geometry in the vicinity of the acceleration sites. Only one EHBL, 1ES 0229+200, has been targeted by IXPE so far. In order to fully exploit the potential physical information encoded in the polarization channel, we propose to enlarge the sample of EHBL with measured X-ray polarimetric properties by observing with IXPE two bright EHBLs.

Proposal Number: 1039
Proposal Type: REGULAR
PI Name: PATRICK SLANE
Title: X-RAY POLARIMETRY OF SN1006 SOUTHWESTERN SHELL IN AN INHOMOGENEOUS INTERSTELLAR MEDIUM
Abstract: Whether supernova remnants (SNRs) can accelerate cosmic rays (CRs) up to PeV energies is an unsolved problem. To allow such a strong acceleration, the magnetic fields at the SNR shock need to be amplified over 100 times. CR-induced instability and turbulent dynamo are two popular mechanisms that drive turbulence and amply magnetic fields, but distinguishing their influence has been difficult. SN 1006 has been the most polarized SNR observed with IXPE, and its northeastern shell in a uniform medium has made a good case to reveal the CR-induced instability. We propose 1 Ms IXPE observation of SN 1006 southeastern shell that is impacting an HI cloud. The observation will allow us to understand whether and how efficiently the turbulent dynamo mechanism works in a young SNR.

Proposal Number: 1043
Proposal Type: REGULAR
PI Name: IOANNIS LIODAKIS
Title: UNRAVELING PARTICLE ACCELERATION IN ASTROPHYSICAL JETS WITH THE ARCHETYPICAL BLAZAR MRK 501
Abstract: Multiwavelength polarization, and in particular the recently enabled X-ray polarization, is an important probe of the magnetic field geometry, acceleration physics, and high-energy emission processes of blazar jets. After 2 years of IXPE observations our understanding of blazars has increased tenfold, however, a lot of mysteries still remain. Here we propose observations of the archetypical blazar Mrk 501. The IXPE observations will be supplemented by our already planned multiwavelength campaign in radio, optical, X-rays, and TeV gamma-rays. This will allow us to produce the first legacy multi-year X-ray polarization light curve of a blazar jet, test particle acceleration models in different states, as well as detect rotations of the polarization angle and uncover their origin.

Proposal Number: 1045
Proposal Type: REGULAR
PI Name: LAURA DI GESU
Title: ACCELERATING PARTICLES TO THE EXTREME: IXPE OBSERVATIONS OF FLARES OF TEV BLAZARS
Abstract: The origin of major flaring activity in high-energy peaked BL Lac objects (HBL) is still an open question. The X-ray polarization offers a unique view of the particle acceleration process in astrophysical jets, yet, thus far, it has been exploited only for the case of HBL in the average state. We propose IXPE observations of HBLs during TeV flares. We will discriminate among particle acceleration processes during the flare, constrain the energy/particle distribution, and probe the connection between flaring activity and X-ray polarization angle rotation. These observations will provide us with unprecedented insight into the inner physics of jets in an extreme regime that is still completely unexplored by IXPE

Proposal Number: 1047
Proposal Type: REGULAR
PI Name: NICCOLO' BUCCIANTINI
Title: UNVEILING THE MAGNETIC GEOMETRY OF THE YOUNG PWN 3C58
Abstract: 3C58 (G130.7+3.1) is a young Pulsar Wind Nebula. It shows clear evidence of energy injection from the central pulsar PSR J0205+6449 in the form of a jet torus structure, observed edge-on. Polarization has been measured in Radio, with a typical average value of the PD for the entire nebula of 6%, with an average B-field roughly aligned with the nebular axis. Local highly polarised patches with PD up to 25% and sizes comparable to 1/10 of the nebula have also been reported. This is the only PWN, left out, that can be fully resolved by IXPE, and bright enough to allow possible detection of polarization. Given its peculiar viewing geometry, even lack of detection will enable us to set tight constraints on the level of turbulence.

Proposal Number: 1048
Proposal Type: THEORY
PI Name: HAOCHENG ZHANG
Title: PROBING MAGNETIC FIELD MORPHOLOGY AND PARTICLE ACCELERATION IN PULSAR WIND NEBULAE
Abstract: Understanding the magnetic field structure and particle acceleration mechanism in pulsar wind nebula is among the primary scientific goals of IXPE. We propose to study the magnetic reconnection scenario and compare our results with the IXPE observation of the flux and polarization maps of Crab and Vela PWNe. We aim to answer three questions: Q1. Can the reconnection model explain the IXPE flux and polarization maps of Crab and Vela PWNe? Q2. What is the polarization of the pulsar jet? Q3. What physical conditions of the Vela PWN are different from the Crab? The proposed study will significantly boost the scientific return of IXPE by constraining the magnetic field structure and particle acceleration mechanism in PWNe.

Proposal Number: 1050
Proposal Type: REGULAR
PI Name: FRANCESCO URSINI
Title: X-RAY POLARIMETRY OF BRIGHT ATOLL SOURCES
Abstract: IXPE has demonstrated that weakly magnetized neutron star low-mass X-ray binaries (NS-LMXBs) tend to be significantly polarized in the X-rays. IXPE is thus shedding new light on the geometry of the accretion flow in these sources. However, these observations also open new questions. Bright atoll sources are particularly puzzling, as the IXPE results range from non-detection in GS 1826-238 to 10% polarization in 4U 1820-303. We propose to observe with IXPE the remaining four X-ray brightest persistent atolls, doubling the current sample size. We request an exposure of 20 to 50 ks per source (150 ks in total), to reach a minimum detectable polarization of 1%. We also ask for joint 10 ks observations with NICER, to provide good spectral coverage.

Proposal Number: 1051
Proposal Type: REGULAR
PI Name: LORENZO DUCCI
Title: PROBE THE GEOMETRY OF THE EMISSION REGION OF THE ACCRETION-POWERED PULSAR 4U 1907+09
Abstract: 4U 1907+09 is a binary system that hosts an accretion-powered pulsar whose X-ray flux and spectral properties make it ideal to obtain a solid detection of the polarization with IXPE. We propose an observation of 250 ks to measure with high significance the polarization parameters, conduct a phase-resolved spectro-polarimetric analysis to determine the basic geometry of the pulsar and of the binary system, and search for additional components of polarized emission.

Proposal Number: 1066
Proposal Type: REGULAR
PI Name: UNNATI KASHYAP
Title: IXPE POLARIZATION STUDY: MAPPING THE ACCRETION GEOMETRY OF NEUTRON STAR LOW MASS X-RAY BINARIES
Abstract: Despite intensive investigations of spectro-temporal features of neutron star low mass X-ray binaries across decades, their shape, geometry, and location of the "Comptonizing corona" are still not disentangled completely. The most debated classical models based on the choice of the spectral components, the Eastern and Western models, are largely degenerate spectroscopically. The different X-ray emission components in Z/atoll NS LMXBs are expected to be significantly polarized, and a spectro-polarimetric study using IXPE provides an excellent opportunity to probe their accretion geometry. Our proposed sample with the radio bright sources will also offer an ideal platform to investigate the jet's position angles with modest duration radio observations and, hence, jet-corona alignment.

Proposal Number: 1077
Proposal Type: REGULAR
PI Name: ALAN MARSCHER
Title: PROBING THE INNERMOST REGIONS OF THE JET OF BL LAC OBJECT MRK421
Abstract: The investigators propose to observe Mrk421 with IXPE over 800 ks to measure changes in the X-ray polarization of the relativistic jet. NuSTAR and Swift observations will be added to determine the X-ray spectrum. In previous IXPE observations of Mrk421 the X-ray polarization angle rotated while the degree of polarization remained essentially constant at several times the optical value, and the X-ray spectrum became harder at higher flux levels. This behavior can be explained if the X-ray emission occurs upstream of the optical emission, with the former containing a uniform helical magnetic field and the latter a mostly turbulent field. Simultaneous X-ray and optical measurements will further probe the magnetic field and particle acceleration in the most upstream regions of the jet.

Proposal Number: 1082
Proposal Type: REGULAR
PI Name: PHILIP KAARET
Title: X-RAY POLARIZATION OF A TEV BINARY
Abstract: Understanding particle acceleration in relativistic shocks occurs is a key goal in high energy astrophysics. It is important to study relativistic shock acceleration in a variety of systems in order to test models in different parameter regimes. IXPE observations have already provided new insights into particle acceleration in PWNe and blazars. We propose to observe a binary star system, containing a pulsar and a massive companion star, that is known to emit TeV gamma-ray indicating acceleration of particles to high energies. The observation proposed here will provide unique information for a novel class, TeV binaries, and have the potential to provide new insights into relativistic shock acceleration.

Proposal Number: 1087
Proposal Type: THEORY
PI Name: MITCHELL BEGELMAN
Title: X-RAY POLARIZATION FROM RELATIVISTIC BULK COMPTONIZATION AROUND ACCRETING BLACK HOLES
Abstract: High polarization levels from X-ray binary accretion disks are hard to explain with thermal Comptonization models. Bulk Comptonization, by a mildly relativistic jet or wind, can explain the observed polarization levels easily. Diffuse ambient radiation appears head-on in the flow frame, leading to beamed, highly polarized scattered radiation. We will study this mechanism by post-processing radiation GRMHD simulations of accretion disks modeling XRBs. Using our simulated polarization maps for different accretion rates and black hole spins, we will develop semi-analytic descriptions of the bulk Compton process. Our investigation will provide new tools for interpreting the data from IXPE, and enhance its potential for revealing the physics of accreting black holes on all scales.

Proposal Number: 1102
Proposal Type: REGULAR
PI Name: RYOTA TOMARU
Title: THE ORIGIN OF ACCRETION DISK WINDS IN LOW MASS X-RAY BINARIES.
Abstract: Accretion disk winds in low-mass X-ray binaries (stellar mass black holes and neutron stars) are smaller scale analogues of the winds in AGN, which feedback energy from the accretion flow to the host galaxy. Still, they can be observed at higher signal-to-noise. We propose joint IXPE-NICER observation with the performance verification XRISM observations of GX 13+1 and V395 Car to understand the geometry and velocity structure of the winds. The high spectral resolution of XRISM will measure the wind velocity structures, while IXPE gives independent constraints on the geometry by X-ray polarization. The NICER helps constrain the intrinsic continuum. Together these will enable the physical wind launch mechanism to be distinguished.

Proposal Number: 1103
Proposal Type: REGULAR
PI Name: MARIA CRISTINA BAGLIO
Title: PROBING THE EMISSION MECHANISMS OF TRANSITIONAL MILLISECOND PULSARS USING X-RAY POLARIMETRIC OBSERVATIONS
Abstract: The discovery of three transitional millisecond pulsars (tMSPs) that swing between accretion-powered and rotation-powered states has challenged our understanding of neutron-star evolution in low mass X-ray binaries. PSR J1023+0038 (J1023) is currently the only tMSP known to be in a unique dim X-ray state. Recently, a scenario involving a permanently-active rotation-powered pulsar, an accretion disk and discrete mass ejection episodes on top of a compact jet has been proposed to explain its puzzling phenomenology. We propose 750 ks of IXPE observations to search for polarised X-ray emission from J1023 for the first time. These observations aim to conclusively probe the aforementioned scenario, providing crucial insights into the emission mechanisms of tMSPs in their dim X-ray state.

Proposal Number: 1116
Proposal Type: REGULAR
PI Name: ALESSANDRO PAPITTO
Title: POLARIZED X-RAYS FROM ACCRETING MILLISECOND PULSARS: A PATHWAY TO THE EQUATION OF STATE OF NEUTRON STARS
Abstract: Modelling of the waveform of the X-ray pulsations of accreting ms pulsars (AMSP) is one of the most promising ways to constrain the equation of state of neutron stars. However, it requires an extremely high number of counts to break the degeneracy between the many parameters that shape the X-ray pulse profiles. Here, we propose a 900 ks-long ToO observation of the next X-ray outburst of an accreting MSP to measure for the first time the polarization of its X-ray emission and constrain the binary inclination and the spot latitude to within a few degrees. This information will serve as a key prior to fit the waveform observed by instruments with a larger effective area (e.g. NICER, XMM-Newton) and constrain the mass and the radius of these neutron stars with an accuracy of a few per cent.

Proposal Number: 1124
Proposal Type: REGULAR
PI Name: GUGLIELMO MASTROSERIO
Title: COMBINING X-RAY POLARIZATION AND REVERBERATION LAGS TO CONSTRAIN THE ACCRETION DISK GEOMETRY IN GX 339-4
Abstract: X-ray polarization has proven to be effective in constraining the orientation of the innermost regions of black hole binaries (BHBs). However, polarimetric measurements can tell us little about the extension of the corona and how it interacts with the accretion disk during the outbursts of BHBs. On the other hand, X-ray reverberation lags, produced by the different light crossing time between the Comptonized photons and the reprocessed ones, is sensitive to the size of the corona and its separation from the accretion disk. We aim to combine these two analyses to study the evolution of GX 339-4 coronal geometry. We propose to observe the source in 3 states: hard, intermediate, and soft state with IXPE + NICER to measure the polarization evolution and reverberation lags during the outburst.

Proposal Number: 1140
Proposal Type: REGULAR
PI Name: MAXIME PARRA
Title: A NOVEL VIEW OF DISK WINDS IN TRANSIENT 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 one of 6 canonical high-inclined wind-emitting BHLMXBs in the soft state, plus 10ks of NICER exposure for complementary spectral coverage.

Proposal Number: 1158
Proposal Type: REGULAR
PI Name: VLADISLAV LOKTEV
Title: EXPLORING POLARIZATION AND GEOMETRY IN THE X-RAY PULSAR 4U 1538-52
Abstract: X-ray pulsar (XRP) observations using IXPE have been substantial for astrophysical insights. For most of them rotation of theneutron star (NS) aligns with orbital inclinations. 4U 1538-52, with constrained orbital features, is poised for deep IXPE study, testing the spin alignment. With a 360 ks observation, we'll gather substantial data on the system's geometry. Using the rotating vector model, we aim to constrain the pulsar's geometric layout. This study also enhances datasets on NS magnetic obliquities and could detect super-orbital variability tied to the precession of the NS.

Proposal Number: 1160
Proposal Type: REGULAR
PI Name: SUDIP CHAKRABORTY
Title: DECIPHERING THE COMPTONIZATION PROPERTIES OF LOW-LUMINOSITY AGN THROUGH X-RAY POLARIMETRY
Abstract: Through their place in the universality of Comptonization processes in tau-kTe space, Low-Luminosity Active Galactic Nuclei (LLAGN) are key to the puzzle of non-thermal emission from accreting black holes. However, to decipher the exact nature of the Comptonizing 'corona', its geometry and emission mechanism have to be understood beyond the limits of spectro-timing techniques. To break the degeneracies related to coronae, we propose upto 1.4 Ms IXPE polarimetry observations in the relatively pristine 2-8 keV band for upto 2 carefully selected Compton-thin LLAGN (NGC 2110 600ks, NGC 3227 800ks) using IXPE. The detected polarization degree and angle will help answer our two central questions: the exact geometry of the corona, and the contrast in coronal properties between brighter and LLAGN.

Proposal Number: 1161
Proposal Type: REGULAR
PI Name: ZORAWAR WADIASINGH
Title: PROBING THE MAGNETIC FIELD GEOMETRY OF PULSAR WIND SHOCKS WITH THE BRIGHTEST "SPIDER" BINARY
Abstract: Shocked pulsar winds are already known to produce high levels of synchrotron polarization in pulsar wind nebulae. Yet, magnetic structures along intrabinary shocks in pulsar binary systems sample much more intense magnetic fields and are particularly challenging to understand from theory or simulations. We propose 1.4 Ms observations of the brightest X-ray "redback" binary J1723-2837 for an orbital phase-resolved polarization study. This exposure time is guided by new models of potential time-dependent polarization across the orbit of these systems. The proposed observation would be the first X-ray polarization study of a redback and stimulate theoretical and observational scrutiny of other similar systems.

Proposal Number: 1162
Proposal Type: REGULAR
PI Name: VADIM KRAVTSOV
Title: SUPERORBITAL VARIABILITY OF CYG X-1 IN X-RAY POLARIZATION
Abstract: We propose to observe Cyg X-1 5 times with 60 ksec exposure per pointing, at different phases of its superorbital period of 294 days. The detection of superorbital variability in the X-ray polarization, on top of the already detected variations in the radio and X-ray flux as well as in the optical polarization, can verify the presence of the accretion disc tilt, may shed light on the details of matter exchange in the binary and test the suggestion on the larger inclination of the inner disc at the time of the first IXPE observation. The absence of periodic long-term changes in the X-ray polarization will require a revision of the mechanisms of superorbital variability.

Proposal Number: 1163
Proposal Type: THEORY
PI Name: JEREMY SCHNITTMAN
Title: X-RAY SPECTROPOLARIMETRY FROM MAGNETO-HYDRODYNAMIC SIMULATIONS OF ACCRETING BLACK HOLES
Abstract: We propose to carry out a theory/computation project to improve and combine the capabilities of three existing simulation codes. These codes (1) model the accretion flow of gas onto a black hole, using fully relativistic magnetohydrodynamics; (2) employ a Monte Carlo radiation transport scheme to calculate the X-ray polarization signature of photons scattering through an accretion disk and corona; and (3) detailed photo-ionization balance to predict the ionization state and X-ray line emission and absorption properties of a gas disk being irradiated by an external source.

Proposal Number: 1166
Proposal Type: REGULAR
PI Name: JAMES STEINER
Title: EVOLVING SLIM-DISK POLARIZATION WITH LMC X-3
Abstract: We propose a monitoring program for LMC X-3 to capture this persistent and soft source which ranges over a month-long timescale from a few percent Eddington to near its Eddington limit. Polarimetric data over this luminosity range allows assessment of slim-disk effects in the inner-disk geometry. At low luminosities, the thermal state of black holes in X-ray binaries is simple and stable, with a fixed inner-disk radius and exhibiting low rms-variability. At high luminosities >~ 30% of the Eddington limit, slim-disk effects manifest as the disk scale-height grows. X-ray polarimetry can uniquely map the inner-disk geometry vs luminosity, and can distinguish the role of returning radiation (enhanced for thicker disks), which may be important in governing polarization strength.

Proposal Number: 1169
Proposal Type: REGULAR
PI Name: VITTORIA GIANOLLI
Title: FOLLOW-UP OF THE FIRST X-RAY POLARIZATION MEASUREMENT OF THE AGN CORONA: A NEW OBSERVATION FOR NGC 4151
Abstract: We propose to observe the Seyfert galaxy NGC4151 with IXPE for 750ks. NGC4151 is the first radio-quiet Active Galactic Nucleus with an X-ray polarization measure for the corona, showing a polarization degree of 4.9 pm 1.1 % and angle of 86 pm 7 degree(east of north). With its changing-look nature and luminosity, NGC4151 stands out as the only candidate for observing potential variations in the flux/absorption state of the sources, offering the unique opportunity to investigate possible shifts in the polarization properties of the system. To assess each spectral component (primary emission, reflection, absorption) contribution, simultaneous XMM-Newton and NuSTAR observations will be arranged. NGC4151 is also a target for XRISM program, offering the chance for contemporaneous observations.

Proposal Number: 1174
Proposal Type: THEORY
PI Name: PAOLO COPPI
Title: UNDERSTANDING IXPE HSP BLAZAR OBSERVATIONS: AN MHD-PIC STUDY OF THE POLARIZATION SIGNATURES OF JET RECOLLIMATION SHOCKS
Abstract: IXPE observations of HSP blazars, where the synchrotron component peaks in X-rays and is produced by the most energetic electrons, finally let us probe the geometry and dynamics of the magnetic fields in the vicinity of their acceleration site(s). To move beyond the toy model stage and unravel the implications of the IXPE observations, which show significant X-ray polarization (10-15%) aligned with the jet in contrast to the much weaker (2-3%) polarization at radio/optical energies, we will use state-of-the art relativistic MHD and PIC codes augmented by an improved tracer particle/radiation transfer module to explore the synchrotron polarization signatures of a recollimation shock, a leading candidate to explain the relative stability of HSP emission on several day-month timescales.

Proposal Number: 1182
Proposal Type: REGULAR
PI Name: STEFANO CIPRINI
Title: POLARIZATION EVOLUTION IN HIGH-ENERGY PERIODIC BLAZAR PG 1553+113
Abstract: The gamma-ray HE BL Lac object PG 1553+113 is a special target. X-ray, multifrequency polarization probes magnetic field topology, particle composition, HE acceleration/emission processes in blazar jets, and detailed spectral-temporal analysis is here enabled by this archetypal GeV-TeV blazar. Peculiarly, a binary supermassive black-hole hypothesis has been introduced, related to the reliability of a modulating gamma-ray quasi-periodic oscillation (QPO) discovered by the Fermi LAT. Two 150 ksec IXPE observations are proposed in the two visibility windows of 2024, with coordinated multifrequency observing campaign. Our twofold approach aim both to study relativistic jet, and to detail origin and processes of the X-ray emission, that also may support or weaken the gamma-ray QPO scenario.

Proposal Number: 1184
Proposal Type: REGULAR
PI Name: JOSEPHINE WONG
Title: PROBING X-RAY POLARIZATION OF KES 75 (G29.7-0.3)'S WIND NEBULA POWERED BY MAGNETAR-LIKE PSR J1846-0258
Abstract: SNR Kes 75 hosts a dynamic wind nebula powered by a young PSR J1846-0258 that evolves energetically like a rotation-powered pulsar but occasionally undergoes magnetar-like bursts. Only a radio PWN PD = 10% measurement exists. With a 1Ms observation, we have MDP99 = 4.85% in the quiescent state and can expect a detection of the integrated PWN. Using a more sensitive technique to separate the pulsar and the nebula components, we can detect polarization in both with careful binning. Augmented with a new 2021 eVLA observation, a 1Ms IXPE observation of Kes 75's PWN complex can have great scientific return, helping us to investigate the connection between magnetars and rotation-powered pulsars and build our growing database of PWN X-ray polarization measurements to constrain emission models.

Proposal Number: 1186
Proposal Type: REGULAR
PI Name: ANUVAB BANERJEE
Title: POLARIMETRIC SIGNATURES OF ACCELERATION MECHANISMS IN HIGH-SYNCHROTRON PEAKED BLAZARS
Abstract: Extreme High Synchrotron Peaked BL Lac sources (EHSPs) harbor non-thermal particle populations within their relativistic jets. These particles can be accelerated via two physical mechanisms: diffusive shock acceleration (DSA) or magnetic reconnection. In the X-ray and optical bands, these scenarios are characterized by different levels of polarization, because the particles emitting in these two bands experience different levels of orderedness of the magnetic field. Here, we request two 107 ks pointings with IXPE to constrain, together with optical polarimetry, the dominant acceleration mechanism in the EHSP source H 1426+428.

Proposal Number: 1191
Proposal Type: REGULAR
PI Name: YASH BHARGAVA
Title: POLARIMETRIC OBSERVATION OF GX 340+0 ALONG THE Z-TRACK
Abstract: Z sources are some of the most enigmatic persistent sources in the X-ray sky. We propose observations of the Z source GX 340+0 with IXPE and NICER for total exposures of 300 ks and 12 ks, respectively. The main goal of this program is to constrain the accretion geometry, in particular, that of the thermal component and the corona, by measuring the energy dependent polarization. By comparing our results with spectro-polarimetric modeling of other Z sources, we aim to develop a more robust picture of the complex accretion flows in these systems. GX 340+0 traces all the spectral states along its Z-track on a time scale of a few days; with our proposed observations we will likely sample a full Z track with IXPE for the first time.

Proposal Number: 1194
Proposal Type: THEORY
PI Name: MATTHEW BARING
Title: COUPLING X-RAY POLARIZATION TO TURBULENCE IN SHOCKED BLAZAR JETS
Abstract: Our understanding of blazar jets has improved significantly with the X-ray polarimetry observations of IXPE. The relatively modest polarization degrees lead to the broad inference of significant turbulence in the X-ray emission region. To progress beyond these generalities, a detailed understanding of the turbulence-polarization connection in the environs of jet shocks is needed. This program addresses such by developing a Monte Carlo simulation of lepton propagation in a power spectrum of MHD turbulence, accompanied by predictions of polarized synchrotron radiation. The spectra and Stokes parameters for IXPE blazars will be employed to constrain the variance and inertial range of the shock-connected turbulence, and also the (time varying) projection of the mean field on the sky plane.

Proposal Number: 1197
Proposal Type: REGULAR
PI Name: FABIAN KISLAT
Title: JOINT OBSERVATIONS OF CYG X-1 AND CYG X-3 WITH IXPE AND XL-CALIBUR
Abstract: Cyg X-1 and Cyg X-3 are among the best studied X-ray sources in the sky. This makes them ideal laboratories for studying accretion onto compact objects. Combined IXPE and XL-Calibur polarimetric observations that cover the 2 80keV energy range have the potential to distinguish corona geometries in Cyg X-1 and to confirm the nature of the obscuring funnel in Cyg X-3. We request joint IXPE and NICER observations of Cyg X-1 and Cyg X-3 to coincide with the XL-Calibur flight. The distinguishing feature of these new observations will be the availability of near-contemporaneous 2 8keV and 15 80keV polarization information. By combining the broad-band polarimetric information and spectral information from NICER, we will make use of the unique opportunity provided by the XL-Calibur flight.

Proposal Number: 1215
Proposal Type: REGULAR
PI Name: ANUVAB BANERJEE
Title: PROBING THE TORUS OF NGC 4945 WITH IXPE
Abstract: The reflection spectrum in active galactic nuclei (AGN) holds critical insight into the torus, the fundamental component of the unification model. The polarization degree and angle of the reflection spectrum are closely tied to the geometry of the torus, including its opening and inclination angles. We propose to observe NGC 4945 for 1 Ms with IXPE. NGC 4945 is among the brightest Compton-thick AGN, with a 2-8 keV spectrum entirely dominated by the reflection component. NGC 4945 also stands out among similar AGN because previous spectral modeling has indirectly shown that the torus is thin (covering factor of 0.15) and is observed nearly edge-on. Such configuration leads to rather large polarization fractions (~15%). The IXPE observations will unveil the geometry of the torus in NGC 4945.

Proposal Number: 1219
Proposal Type: REGULAR
PI Name: MICHAEL NOWAK
Title: POLARIZATION IN THE BRIGHT STATE OF 4U 1957+11
Abstract: IXPE has provided us with the new tool of X-ray polarization to begin exploring the physics and geometry of black hole X-ray states. It has sampled a small set of sources and states, but the states are highly dynamic, and the intrinsic properties of the systems are likely varied (e.g., differences in black hole spin, system inclination, and the accretion feeding mechanism: Roche lobe overflow vs. focused wind accretion). We propose a triggered IXPE/NICER observation of the persistently bright low mass X-ray binary 4U1957+11, which is hypothesized to harbor a near-maximally spinning black hole viewed close to edge-on to its accretion disk. It has been observed by IXPE in a nearly pure disk state. We will use MAXI monitoring to target a bright soft state with a significant hard tail.

Proposal Number: 1220
Proposal Type: REGULAR
PI Name: JIRI SVOBODA
Title: X-RAY POLARIZATION ACROSS THE BLACK-HOLE ACCRETION STATES
Abstract: We aim to trace the X-ray polarization properties across different accretion states of a black-hole X-ray binary in a single outburst. The best candidate to accomplish this goal is a newly discovered transient Swift J1727.8-1613, which is currently one of the brightest X-ray sources in the sky. IXPE has already observed it at the beginning of its outburst and detected a high average 2-8keV polarization fraction of 4% during the hard state. The source is poised to imminently undergo a spectral transition to the soft state. With this proposal, we aim to catch this source in the soft state and in the return transition to the low/hard state, which will allow us to study changes in the polarization properties and reveal how different X-ray emission regions evolve during the accretion states.

Proposal Number: 1222
Proposal Type: THEORY
PI Name: HENRIC KRAWCZYNSKI
Title: MODELING TRUNCATED DISK-HOT INNER FLOW GEOMETRY FOR THE HARD STATE BLACK HOLE X-RAY BINARIES
Abstract: One of the science goals of IXPE is to reveal the geometry and location of the coronal gas of Black Hole X-ray Binaries (BHXRBs). The IXPE observations of the BHXRB Cyg X-1 revealed a corona extended perpendicular to the radio jet. We propose here to use general relativistic ray tracing calculations to simulate a library of models with a thin but truncated accretion disk surrounding a hot inner flow. We will use the library to fit all existing and upcoming IXPE observations of BHXRBs in the hard state. We will furthermore make the fitting model public so that the X-ray community can use it to fit IXPE and multi-mission data sets.

Proposal Number: 1224
Proposal Type: REGULAR
PI Name: MANEL ERRANDO
Title: IXPE OBSERVATIONS OF THE NEXT OUTBURST OF THE NAKED-EYE RECURRENT NOVA T CORONA BOREALIS
Abstract: T Corona Borealis, a recurrent nova, is set to experience a thermonuclear ignition event between Feb 2024 and August 2026. During its last nova event in 1946, it reached a visible magnitude of 3.0, becoming visible to the naked eye. The X-ray flux is expected to peak above 1E-9 erg cm-2 s-1 in the IXPE band. We propose an observation campaign of T CrB with IXPE to cover the intitial and late stages of the nova event evolution. This strategy aims to provide X-ray polarization measurements with MDP_99 of ~3%. It will enable us to capture the initial shock interactions of the nova ejecta with the red giant donor, along with the transition to the characteristic thermal emission of the late super-soft state.

Proposal Number: 1236
Proposal Type: LARGE
PI Name: EILEEN MEYER
Title: A UNIQUE X-RAY POLARIMETRY TARGET: THE WESTERN HOTSPOT OF PICTOR A
Abstract: Jetted AGN are a primary target class for the groundbreaking IXPE mission, and prime-mission phase observations of blazers have revealed a number of exciting findings related to the compact parsec-scale emission zone near the base of the jet, aka the bright core of the AGN. However, no studies to date have focused on extended jets or terminal hotspots, where the physical conditions are very different, and many open questions remain, including the identity of the X-ray emission mechanism and the mode of in-situ particle acceleration. We propose IXPE observations of the only feasible target for an extended jet/hotspot, which is Pictor A.