EXPO: a quantum leap in fast, wide-band X-ray polarimetry for astrophysics

arXiv:2608.20898 · astro-ph.IM, astro-ph.HE · Submitted 2026-08-21 · Read on arXiv

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Introduction to the show: ident: Astrophysics Radio. Generated commentary on the latest astrophysics papers.

Vera: Today's paper: "EXPO: a quantum leap in fast, wide-band X-ray polarimetry for astrophysics".

Jocelyn: The EXPO mission concept proposes a next-generation X-ray polarimetry observatory designed to overcome the limitations of previous missions by combining broadband polarimetry, rapid response, and simultaneous spectroscopy.

Vera: First, who's behind it and why it matters.

Title and authors: Vera: We’re looking at the summary of "EXPO: a quantum leap in fast, wide-band X-ray polarimetry for astrophysics" now. The authors are essentially saying they’re tackling the energy band and speed issues head-on by putting broadband polarimetry, rapid response, and simultaneous spectroscopy all into one system.

Jocelyn: Exactly. They aren't just adding more detectors; they are fundamentally changing the approach by putting those three things together to unlock new scientific capabilities in high-energy astrophysics <ref:2608.20898#pg3>.

Subrahmanyan: From a theoretical angle, I think they are trying to get around the energy limitations of previous missions that were stuck in a narrow band. They want to probe things like cyclotron lines in pulsars or hard X-ray emission from black holes that we couldn't really measure well before.

Vera: Right. The paper outlines this three-pillar architecture: broadband polarimetry from two keV up to thirty five keV, autonomous repointing within tens of seconds, and simultaneous spectroscopy through a dedicated camera <ref:2608.20898#pg1>.

Jocelyn: That rapid repointing is huge because it means we can actually follow fast events in time. We're talking about observing gamma-ray bursts or other fast transients within about fifty seconds instead of waiting a long time for the telescope to point correctly <ref:2608.20898#pg4>.

Subrahmanyan: That speed directly affects how we model physics in relativistic jets. If you can get that time resolution, you can really start constraining the energy dissipation mechanisms operating inside those jets.

Vera: The paper focuses on several scientific applications beyond just GRBs, like studying synchrotron emission from supernova remnants and mapping magnetic turbulence in extended sources <ref:2608.20898#pg1>.

Jocelyn: And they're not just looking at GRBs; they want to do time-resolved spectro-polarimetry of the afterglows too, which gives us a much better look at how magnetic fields evolve during those explosions <ref:2608.20898#pg1>.

Subrahmanyan: That time domain information is critical for structure formation studies because you can see how the magnetic fields interact with the surrounding medium over time.

Vera: They also mention testing quantum electrodynamics vacuum birefringence if intermediate flares happen in magnetars, which would be a very specific test of fundamental physics involving large emitting areas <ref:2608.20898#pg1>.

Jocelyn: So, while the engineering is complex with five focusing telescopes and a dedicated Spectral Imaging Camera, the goal is to turn X-ray polarimetry from just a discovery tool into something that can actually track dynamic processes across vast timescales <ref:2608.20898#pg3>.

Subrahmanyan: The implication for cosmology is that by studying these magnetic field structures in nebulae, we get better constraints on how turbulence seeds structure formation in the universe.

The paper's summary: Vera: Now we’re talking about the specific technical improvements suggested by the authors of "EXPO: a quantum leap in fast, wide-band X-ray polarimetry for astrophysics," focusing on how they plan to upgrade the design and operational capabilities.

Jocelyn: They are proposing a few specific upgrades, mostly around the hardware that handles the data and the way the spacecraft moves across the sky.

Subrahmanyan: From my side, I’m interested in how these improvements translate into better constraints on those accretion disk models we talked about earlier; more spectral resolution usually means tighter constraints on geometry.

Vera: They suggest using a gas photoelectric polarimeter system based on the Timepix ASIC family with InGrid amplification for the five focusing X-ray telescopes, aiming for three-dimensional track reconstruction <ref:2608.20898#pg3>.

Jocelyn: That’s interesting because getting true three-dimensional track reconstruction means we can measure polarization directions much more accurately than just looking at a single point.

Subrahmanyan: If the tracking is better, then the polarization measurements of cyclotron lines in accreting X-ray pulsars should become much cleaner, with smaller error bars on those parameters.

Vera: They also talk about optimizing the low- and medium-energy detector configurations for those five telescopes to cover that full two to thirty five keV band effectively <ref:2608.20898#pg3>.

Jocelyn: And they are using different gas mixtures, like neon DME for the low energy polarimeters and argon DME for the medium energy ones, which is a way to tailor sensitivity across that wide range.

Subrahmanyan: That tuning of the detectors is important because it directly addresses how different physical processes manifest across that broad spectrum.

Vera: Plus, they are suggesting using proven electroformed nickel technology with Au/C coatings for the mirror modules, which should keep those focal lengths consistent and stable over time <ref:2608.20898#pg1>.

Jocelyn: And they're also emphasizing the role of the Instrument Control Unit, or ICU, as the central computer that has to handle all that power distribution and data processing in real-time.

Subrahmanyan: From an engineering standpoint, managing all those different detector types simultaneously within one payload is a huge challenge for the AI systems they are planning to use.

Vera: The paper also points out how their Wide Field Instrument, or WFI, which monitors about two square degrees of sky continuously, will be used to autonomously identify significant variability and command repointing <ref:2608.20898#pg4>.

Jocelyn: That autonomy is crucial because it means we don't have to wait for ground control to tell the spacecraft where to look when a fast event happens.

Subrahmanyan: That capability fundamentally shifts the science from waiting for targets to actively hunting them, which is a big change for time-domain astrophysics.

Vera: The paper notes that they are building on the SVOM and ECLAIR heritage for that WFI, suggesting they can leverage existing knowledge about sky monitoring <ref:2608.20898#pg4>.

Jocelyn: And the way they propose using this autonomous system means we could potentially detect transients in seconds, which is a massive improvement over previous response times.

Subrahmanyan: That speed means we get to see the prompt emission of gamma-ray bursts with much better temporal resolution, which is key for constraining jet physics.

Vera: So the improvements focus on making the system more capable of simultaneous high-resolution imaging, spectroscopy, and rapid response across a much wider energy range <ref:2608.20898#pg3>.

Jocelyn: It really boils down to being able to capture the whole event—from the initial polarization measurement right through the spectral details—all at once.

Subrahmanyan: And that holistic data set is what allows us to build those more robust models for jet launching and accretion dynamics we discussed earlier.

The paper's improvements: Vera: So to wrap up on "EXPO: a quantum leap in fast, wide-band X-ray polarimetry for astrophysics," we've seen how this mission concept aims to overcome the energy limitations and slow response times of previous missions by adding broadband coverage and rapid repointing.

Jocelyn: It’s really about transforming X-ray polarimetry from a discovery tool into something that can handle fast, dynamic events across a much wider energy spectrum than IXPE could manage.

Subrahmanyan: I think the main shift for structure formation is gaining better constraints on magnetic field turbulence in extended sources like supernova remnants. That observational data will help us predict how those fields seed large-scale structures in the universe.

Vera: Exactly, and by measuring polarization above the iron line complex, we get a direct test of how much radiation is reflected off black hole accretion disks, which changes our models of jet launching.

Jocelyn: And for time-domain science, this means we can finally do time-resolved spectro-polarimetry of GRB afterglows and transients, which gives us the detail needed to understand the magnetic fields powering those explosions long term.

Subrahmanyan: If these measurements confirm our predictions about jet physics in microquasars, it gives us a way to test how relativistic jets are launched across different types of compact objects.

Vera: It seems like this mission concept is designed to be incredibly versatile by focusing on speed, breadth, and simultaneous data acquisition from two keV all the way up to thirty five keV.

Jocelyn: And that versatility is what makes it compelling for future follow-up observations when we see something interesting in the sky because it can capture the whole event in one go.

Subrahmanyan: The ultimate confirmation of its value will come from those constraints on jet physics, which we hope this mission concept provides through better temporal resolution.

Vera: So, that’s our look at "EXPO: a quantum leap in fast, wide-band X-ray polarimetry for astrophysics."

Jocelyn: It really pushes the boundary on what's possible in high-energy, time domain astrophysics right now.

Subrahmanyan: We’ll be looking at how these capabilities translate into actual cosmological constraints in our next episode.

Conclusion: Vera: So we've seen how this EXPO mission concept aims to combine broadband polarimetry, rapid response, and simultaneous spectroscopy to tackle the limitations of previous X-ray polarimeters like IXPE.

Jocelyn: It’s about moving beyond just seeing a signal and starting to track the physics in real time across a much wider energy band than we've seen before.

Subrahmanyan: I think for cosmology, this means getting those better constraints on magnetic field turbulence in extended sources like supernova remnants, which helps us map out structure formation.

Vera: Exactly. And by measuring polarization above the iron line complex, we get a more direct test of how radiation reflects off black hole accretion disks and what that tells us about jet launching.

Jocelyn: For time-domain science, we’re looking at getting time-resolved spectro-polarimetry of GRB afterglows and transients, which gives us the detail needed to understand the magnetic fields powering those explosions long term.

Subrahmanyan: And if those measurements confirm our predictions about jet physics in microquasars, it gives us a way to test how relativistic jets are launched across different types of compact objects.

Vera: This EXPO mission concept is designed to be incredibly versatile by focusing on speed, breadth, and simultaneous data acquisition from two keV all the way up to thirty five keV.

Jocelyn: That versatility means when we see something interesting in the sky because it can capture the whole event—from polarization measurement right through spectral details—all at once.

Subrahmanyan: The ultimate confirmation of its value will come from those constraints on jet physics, which we hope this mission concept provides through better temporal resolution.

Vera: So that’s our look at "EXPO: a quantum leap in fast, wide-band X-ray polarimetry for astrophysics."

Jocelyn: It really pushes the boundary on what's possible in high-energy, time domain astrophysics right now.

Subrahmanyan: We’ll be looking at how these capabilities translate into actual cosmological constraints in our next episode.

Paolo Soffitta, Sebastien Guillot, Ian Hutchinson, Fabio Muleri, Mark Pearce, Andrea Santangelo, Daniele Spiga, Ivan Agudo, § Gino Bruno Amata 8, Jaros law Bakala 9, Elisabetta Baracchini 10, Stefano Basso 6, Jäorg Bayer 5, Benedikt Bergmann 11, Jaroslaw Borek 9, Enrico Bozzo 12, Søren Krinstian Brandt 13, Carl Budtz-Jࡥensen 13, Vadim Burwitz 14, Stefano Cesare 8, Jerome Chenevez 13, Enrico Costa 1, Elisa Costantini 15, Vincenzo Cotroneo 6, Walter Cugno 8, Riccardo Ferrazzoli 1, Nicolas De Angelis 1, Desiree Della Monica Ferreira 13, Klaus Desch 16, Giorgio Dho 17, Giuseppe Di Persio 1, Sergio Fabiani 1, Davide Fiorina 10, Szymon Gburek9, Markus Gruber 16, Allan Hornstrup 13, Jennifer Jones3, Jochen Kaminski 16, Mózsi Kiss4,56 Irfan Kuvvetli 13, Carlo Lefevre 1, Hannah Lerman3, Honghui Liu5, Pasqualino Loffredo 1, Niels Lund 13, Hemanth Manikantan 1, Melissa McHugh3, Paul O’Brien3, Stephane Paltani 12, Giovanni Pareschi6, Stefano Piacentini 10, Vladislavs Plesanovs 16, John Rankin6, § Ajay Ratheesh 18, Selina Ringsborg Howalt Owe 13, Alda Rubini 1, Miguel Andrées S… Sanchez7, Emanuele Scalise 1, Konrad R. Skup9, Gianpiero Tagliaferri6, Chris Tenzer5, Berend Winter19, Silvia Zane19, Lorenzo Amati20, Alessio Anitra21, Maria Cristina Baglio6, Thibault Barnouin22, Stefano Bianchi23, Fabrizio Bocchino21, Markus Boettcher 24, Niccol`o Bucciantini 25, § Sara Capecchiacci 26,52 Fiamma Capitanio 1, Martina Cardillo1, Eugene Churazov57, Stefano Covino6,54 Filippo D’Ammando27, Laura Di Gesu28 Alessandro Di Marco1, Tiziana Di Salvo29 Michal Dovciak30 Lorenzo Ducci5 Hua Feng31 Giorgio Galanti32 Giancarlo Ghirlanda6, Vittoria Elvezia Gianolli33 Andrea Gnarini34 Emanuele Greco21 Jeremy Heyl35, Adam Ingram 36, Svetlana Jorstad 37, Philip Kaaret 34, Ildar Khabibullin57,14 Fabian Kislat 38, Henric Krawczynski39 Josefin Larsson4,56 Ioannis Liodakis26 Frédic Marin40 Andrea Marinucci28 Lorenzo Marra1, Alan Marscher37 Herman Marshall41 Giorgio Matt23 Marco Miceli21 Riccardo Middei42 Romana Mikusincova1, Alexander Mushtukov19 Kirpal Nandra14, Lara Nava6, Giuseppe Maria Oppedisano 10 Salvatore Orlando21 Simone Pagliarella 55,48 Alessandro Papitto42 Pierre-Olivier Petrucci43 Oleh Petruk21 Jakub Podgorny30 Juri Poutanen44 Arne Rau14 Agata Roszanska45 Felix Ryde4,56 Vincenzo Sapienza21 Patrick Slane46 James Steiner 46 Valery Suleimanov5, Jiri Svoboda30 Daniele Tagliacozzo23 Antonella Tarana1 Fabrizio Tavecchio6 Roberto Taverna47 Francesco Tombesi48 Sergey Tsygankov44, Youli Tuo5 Roberto Turolla47 Francesco Ursini23 Alexandra Veledina44, Jacco Vink 49, Martin C. Weisskopf34 Fei Xie50 Haocheng Zhang51 Menglei Zhou5, INAF-IAPS, Via Fosso del Cavaliere 100, 00133 Rome, Italy IRAP, 9 Av. du Colonel Roche BP 44346, 31028 Toulouse Cedex 04 France Univ. Leicester University Road Leicester LE17RH UK KTH Royal Institute of Technology Department of Physics Stockholm Sweden Univ. Tuebingen Geschwister-Scholl-Platz Tüybingen Germany INAF-OAB via E Bianchi 46 23807 Merate Lc Italy Univ. Geneve Rue du Géneral Dufour 24 1211 Genࡥeve Switzerland Max Planck Institute for Extraterrestrial Physics Giessenbachstrasse 1 85748 Garching Germany SRON Space Research Organisation Netherlands Univ. Bonn Physikalisches Institut Nußallee 12 53115 Bonn Germany INFN-LNF Via Enrico Fermi 54 00044 Frascati Roma Italy INAF-OAS Via Gobetti 93/3 Bologna Italy INAF OAPa Piazza del Parlamento 1 Palermo Italy CAS-Astronomical Institute N࡯rodnɩi 3 Praha 1 Czech Republic IHEP Yuquan Road Shijingshan District Beijing China INAF-IASF-Milano Via Alfonso Corti 12 Milano Italy Clemson University Sikes Hall Clemson South Carolina USA Univ. Newcastle Newcastle upon Tyne NE17RU United Kingdom Boston University Commonwealth Avenue Boston MA USA Univ. New Hampshire Main Street Durham NH USA WUSTL Brookings Drive St Louis MO USA Observatoire Astronomique 11 rue Universitée 67000 Strasbourg France MIT 77 Massachusetts Avenue Cambridge MA USA INAF-OAR Via Parco Mellini 84, Italy Grenoble University Avenue Centrale Saint-Martin-d’Heres France Department of Physics and Astronomy University of Turku Finland Nicolaus Copernicus Astronomical Center ToruѮ Poland Harvard-CFA Garden Street Cambridge MA USA Universitéa di Padova Via 8 Febbraio 1848 Padova Italy Physics Department Tor Vergata University of Rome Via della Ricerca Scientifica 00133 Rome Italy Amsterdam University Faculty of Science Anton Pannekoek Institute for Astronomy Science Park 904 Postal address Postbus 94249 GE Amsterdam Netherlands Guangxi University Nanning 530004 China University of Maryland College Park Regents Drive USA Department of Physics University of Crete Heraklion Greece Rudolf Peierls Centre for Theoretical Physics Department of Physics University of Oxford Clarendon Laboratory Parks Rd Oxford OX13PU United Kingdom} brief: {, main_finding, what_researchers_did, supporting_evidence, surprising_detail, why_it_matters, limitations, confidence_or_uncertainty, } } } } } } }

INAF-IAPS · IRAP · Univ. Leicester · KTH Royal Institute of Technology · Univ. Tuebingen · INAF-OAB · Univ. Geneve · Max Planck Institute for Extraterrestrial Physics · § SRON Space Research Organisation Netherlands · § Univ. Bonn Physikalisches Institut · § INFN-LNF · § INAF-OAS · CAS-Astronomical Institute N⡩rodn⡩i 3 Praha 1 Czech Republic · IHEP · Clemson University Sikes Hall Clemson South Carolina USA Univ. Newcastle Newcastle upon Tyne NE17RU United Kingdom Boston University Commonwealth Avenue Boston MA USA Univ. New Hampshire Main Street Durham NH USA WUSTL Brookings Drive St Louis MO USA Observatoire Astronomique 11 rue Universitée 67000 Strasbourg France MIT · INAF-OAR · Grenoble University · Department of Physics and Astronomy University of Turku Finland · Nicolaus Copernicus Astronomical Center Toru䛪rd Poland Harvard-CFA Garden Street Cambridge MA USA Universitéa di Padova Via 8 Febbraio 1848 Padova Italy Physics Department Tor Vergata University of Rome Via della Ricerca Scientifica 00133 Rome Italy Amsterdam University Faculty of Science Anton Pannekoek Institute for Astronomy Science Park 904 Postal address Postbus 94249 GE Amsterdam Netherlands Guangxi University Nanning 530004 China University of Maryland College Park Regents Drive USA Department of Physics University of Crete Heraklion Greece Rudolf Peierls Centre for Theoretical Physics Department of Physics University of Oxford Clarendon Laboratory Parks Rd Oxford OX13PU United Kingdom

astro-ph.IM, astro-ph.HE

Submitted: 2026-08-21

Updated: 2026-08-21

Comments: 21 pages; 10 figures, 6 tables, SPIE Proceedings Astronomical Telescope + Instrumentation 2026: Ultraviolet to Gamma-rays Conference, Copenaghen, 5-10 July 2026

Journal ref: Proc. SPIE 14146, Space Telescopes and Instrumentation 2026: Ultraviolet to Gamma Ray, 141461B (17 Aug 2026)

DOI: 10.1117/12.3105324

License: http://arxiv.org/licenses/nonexclusive-distrib/1.0/

Importance score: 93/100

The gist: The EXPO mission concept proposes a next-generation X-ray polarimetry observatory designed to overcome the limitations of previous missions by combining broadband polarimetry, rapid response, and

Key concepts

Broadband Polarimetry: This capability allows the instrument to measure the polarization of X-rays across a wide energy range, specifically from 2 to 35 keV. This extended energy coverage is crucial because it enables measurements of phenomena like cyclotron lines in accreting pulsars and provides a four-times broader energy band compared to existing instruments.
Rapid Repointing: EXPO incorporates a Swift-class autonomous repointing system capable of switching observation targets within tens of seconds. This speed allows the mission to observe fast transients like Gamma-Ray Bursts (GRBs) within about 50 seconds, enabling time-resolved studies of their prompt emission and afterglow.
Simultaneous Spectroscopy: The mission includes a dedicated Spectral Imaging Camera (SIC) that combines silicon and CZT detectors. This allows the instrument to perform high-quality spectroscopy across the 0.5–35 keV band at the same time it is measuring polarization, providing detailed information about X-ray sources.
True 3D Track Reconstruction: By analyzing how a photoelectron leaves an ionization track, EXPO can reconstruct the initial direction of the photon's polarization. This technique enables true three-dimensional tracking of events, which integrates imaging, spectroscopy, and polarimetry into a single measurement.

Terminology

Instrument Control Unit — 11× in this paper
Spectral Imaging Camera — 11× in this paper
Wide Field Instrument — 9× in this paper
AGN — 4× in this paper
Calibration — 4× in this paper · explained in 8 episodes
Low Energy Polarimeters — 4× in this paper
X-ray Polarimeters — 4× in this paper
Focal Plane Assembly — 3× in this paper
Medium Energy Polarimeters — 3× in this paper
Multi-messenger Astronomy — 3× in this paper · explained in 3 episodes
Accretion Flows — 2× in this paper · explained in 1 episode
Core — 2× in this paper · explained in 2 episodes
Enhanced X-ray Polarimetry Observatory — 2× in this paper
Society of Photo-Optical Instrumentation Engineers — 2× in this paper
Verification — 2× in this paper · explained in 2 episodes
Accuracy — 1× in this paper · explained in 1 episode
Adam — 1× in this paper · explained in 2 episodes
CMOS Low Energy Detector — 1× in this paper
CZT High Energy Detector — 1× in this paper
Imaging X-Ray Polarimetry Explorer — 1× in this paper
Module and the Focal Plane Support Structure — 1× in this paper
Pulsar Wind Nebula — 1× in this paper
Target of Opportunity — 1× in this paper
Trace — 1× in this paper · explained in 2 episodes

Summary

The EXPO mission concept proposes a next-generation X-ray polarimetry observatory designed to overcome the limitations of previous missions by combining broadband polarimetry, rapid response, and simultaneous spectroscopy. This combination aims to unlock new scientific capabilities in high-energy astrophysics across various domains.

The gist: The EXPO mission concept is designed to overcome the narrow energy coverage and slow repointing of IXPE while building on and extending its scientific legacy> (Page 16).

Technical Architecture

EXPO is conceived as the next-generation X-ray polarimetry observatory based on three pillars: broadband polarimetry over 2–35 keV, Swift-class autonomous repointing within tens of seconds, and simultaneous spectroscopy through a dedicated Spectral Imaging Camera (SIC)> (Page 4). The payload combines broadband imaging X-ray polarimetry with rapid time-domain capability> (Page 3). Five focusing X-ray telescopes employ gas photoelectric polarimeters based on the Timepix ASIC family with InGrid amplification, enabling true three-dimensional track reconstruction and operation over the 2–35 keV band through optimized low- and medium-energy detector configurations> (Page 3). The mirror modules are based on proven electroformed nickel technology with Au/C coatings and an XMM-like focal length of 7.5 m> (Page 3).

The EXPO payload is distributed between the Service Module and the Focal Plane Support Structure (FPSS)> (Page 6). The XRPs are composed of two Low Energy Polarimeters (LEP) and three Medium Energy Polarimeters (MEP)> (Page 12). The LEP uses a 10 mm cell filled with Ne/DME (80/20) at 1 bar, while the MEP employs a 30 mm Ar/DME (60/40) cell at 3 bar for high-energy photons> (Page 12). A sixth detector hosts the Spectral Imaging Camera (SIC), which combines a silicon detector and a stacked CZT detector to provide simultaneous spectroscopy over the 0.5–35 keV band> (Page 12). The Instrument Control Unit (ICU) is the central computer of the EXPO payload, providing power distribution, instrument control, science data processing, telemetry and telecommand handling, and health monitoring> (Page 14).

Scientific Capabilities and Advantages

EXPO provides a four-times broader energy band, about three-times larger collecting area, angular resolution down to 10'′, and a Wide Field Instrument (WFI) coaligned with X-ray telescopes and monitoring one sixth of the sky (∼ 2 sr) to detect and localize transients and trigger rapid (1 degree/s) autonomous observations> (Page 4). The extended energy range directly addresses several of the most important questions raised by IXPE> (Page 4). It enables polarization measurements of cyclotron lines in accreting X-ray pulsars, probes the hard X-ray emission of black-hole binaries, measures the hard tails of magnetars and low-mass X-ray binaries, investigates synchrotron emission from supernova remnants and pulsarwind nebulae, and studies reflection components in AGN and X-ray binaries above the iron complex> (Page 4).

Rapid repointing represents the second major breakthrough> (Page 4). A WFI derived from the SVOM /ECLAIRs heritage continuously monitors the sky, while onboard intelligence autonomously identifies significant variability and commands repointing> (Page 4). GRBs and other fast transients can be observed within ∼ 50 s, whereas slower phenomena, including state transitions of X-ray binaries, magnetar outbursts and blazar flares, are reached within minutes without hydrazine consumption> (Page 4). This capability uniquely enables time-resolved spectro-polarimetry of GRB prompt emission, afterglows, and X-ray transients> (Page 4).

Key Scientific Objectives

The primary scientific goals of EXPO include broadband polarimetry of the prompt emission of gamma-ray bursts (GRBs) down to 2 keV, probing the origin of the low-energy spectral break observed by Fermi and Swift and constraining the energy dissipation mechanisms operating in ultra-relativistic jets> (Page 16). In addition, polarimetry of the GRB afterglow, including the plateau and reflaring phases, will provide unique insight into the role of magnetic fields in powering the long-term evolution of these explosions> (Page 16).

Complementing the polarimetric measurements, the Spectroscopic Imaging Calorimeter (SIC) will perform high-quality spectroscopy of GRB afterglows up to the hard X-ray band, extending detailed spectroscopic studies into an energy range that has so far remained only sparsely explored> (Page 16). Should intermediate flares occur in magnetars, EXPO will enable a definitive test of quantum electrodynamics vacuum birefringence, owing to the large emitting area involved during these events> (Page 16).

Applications Beyond GRBs

The mission will also probe the geometry of jet formation in microquasars through rapid repointing triggered by major radio flares> (Page 5). This capability uniquely enables time-resolved spectro-polarimetry of GRB prompt emission, afterglows, and X-ray transients> (Page 4). Understanding how relativistic jets are launched in microquasars has implications that extend well beyond Galactic X-ray binaries> (Page 5).

Broadband polarimetry of X-ray reflection from black-hole and neutron-star binaries will reveal the origin of relativistic jets and clarify the interplay between the accretion disk, corona, and jet during spectral-state transitions> (Page 16). Polarimetric measurements above the fluorescent iron-line complex will provide a direct test of the contribution of returning radiation to the reflected emission from black-hole accretion disks> (Page 16).

Finally, EXPO will enable spatially resolved broadband polarimetry of extended sources in supernova remnants, mapping magnetic turbulence and particle acceleration with unprecedented detail> (Page 16). In pulsar wind nebulae, broadband imaging polarimetry will resolve the polarization properties of the inner nebula as the pulsar contribution becomes increasingly dominant at higher energies> (Page 16).

The EXPO mission is expected to exceed IXPE in several technical and scientific aspects> (Page 3). This is possible thanks to a suite of instruments and spacecraft features that ensure that science requirements are truly met> (Page 3). By transforming X-ray polarimetry from a discovery-class capability into a mature observational tool, EXPO will place Europe at the forefront of high-energy, time-domain, and multi-messenger astrophysics in the postIXPE era> (Page 16).

ACKNOWLEDGMENTS

VEG acknowledges funding under NASA contract 80NSSC24K1403> (Page 17). M.D., J.Pod., and J.S. acknowledge the support from the GACR project 26-22614S and the institutional support from RVO:67985815> (Page 17). AV, JPou and ST acknowledge support from the Research Council of Finland grants 355672 and 372881 and Centre of Excellence in Neutron-Star Physics (grant 374064)> (Page 17).

References

[1] Weisskopf, M. C., Soffitta, P., Baldini, L., Ramsey, B. D., O’Dell, S. L., Romani, R. W., Matt, G., Deininger, W. D., Baumgartner, W. H., Bellazzini, R., Costa, E., Kolodziejczak, J. J.: “The Imaging X-Ray Polarimetry Explorer (IXPE): Pre-Launch,” Journal of Astronomical Telescopes, Instruments, and Systems 8: 026002 (Apr. 2022) > (Page 1).

[3] Caiazzo, I. and Heyl, J.: “Polarization of accreting X-ray pulsars. I. A new model,” MNRAS 501: 109–128 (Jan. 2021) > (Page 3).

[4] Caiazzo, I. and Heyl, J.: “Polarization of accreting X-ray pulsars - II. Hercules X-1,” MNRAS 501: 129–136 (Jan. 2021) > (Page 3).

[5] Ratheesh, A., Dovˇciak, M., Krawczynski, H., Podgorn´y, J., Marra, L.: “X-Ray Polarization of the Black Hole X-Ray Binary 4U 1630–47 Challenges the Standard Thin Accretion Disk Scenario,” ApJ 964: 77 (Mar. 2024) > (Page 18).

[6] Negro, M., Di Lalla, N., Omodei, N.: “The IXPE View of GRB 221009A,” ApJ 946: L21 (Mar. 2023) > (Page 19).

[7] Granot, J. and K¨onigl, A.: “Linear Polarization in Gamma-Ray Bursts: The Case for an Ordered Magnetic Field,” ApJ 594: L83–L87 (Sept. 2003) > (Page 18).

[8] Veledina, A., Muleri, F., Poutanen, J.

Improvements for AI systems

The EXPO system can perform true three-dimensional track reconstruction by exploiting a photoelectron leaves an ionization track whose initial direction follows the polarization of the photon through a distribution cos2[19], enabling simultaneous measurement of imaging, spectroscopy, and polarimetry.
The improved AI system can conduct spatially resolved broadband polarimetry in sources like pulsar wind nebulae by mapping magnetic field orientation as a function of position, resolving features that were previously unobservable with IXPE.
The system will enable the discrimination of emission mechanisms in GRBs by performing time-resolved spectro-polarimetry of GRB prompt emission, afterglows, and X-ray transients to constrain synchrotron and inverse-Compton emission mechanisms.
The AI can provide stringent constraints on black hole accretion geometry by accurately characterizing the reflection component, which is previously largely unconstrained by IXPE over the same observing time because of its limited energy range.
The system will allow for detailed studies of jet formation in microquasars by triggering rapid observations from major radio flares, probing the geometry and physical conditions of the innermost accretion flow.

Sources

Cygnus X-3 revealed as a Galactic ultraluminous X-ray source by IXPE