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The enhanced X-ray Timing and Polarimetry mission - eXTP

ShuangNan Zhang, Andrea Santangelo, Marco Feroci, YuPeng Xu, FangJun Lu, Yong Chen, Hua Feng, Shu Zhang, Søren Brandt, Margarita Hernanz, Luca Baldini, Enrico Bozzo, Riccardo Campana, Alessandra De Rosa, YongWei Dong, Yuri Evangelista, Vladimir Karas, Norbert Meidinger, Aline Meuris, Kirpal Nandra, Teng Pan, Giovanni Pareschi, Piotr Orleanski, QiuShi Huang, Stephane Schanne, Giorgia Sironi, Daniele Spiga, Jiri Svoboda, Gianpiero Tagliaferri, Christoph Tenzer, Andrea Vacchi, Silvia Zane, Dave Walton, ZhanShan Wang, Berend Winter, Xin Wu, Jean J. M. in 't Zand, Mahdi Ahangarianabhari, Giovanni Ambrosi, Filippo Ambrosino, Marco Barbera, Stefano Basso, Jörg Bayer, Ronaldo Bellazzini, Pierluigi Bellutti, Bruna Bertucci, Giuseppe Bertuccio, Giacomo Borghi, XueLei Cao, Franck Cadoux, Riccardo Campana, Francesco Ceraudo, TianXiang Chen, YuPeng Chen, Jerome Chevenez, Marta Civitani, Wei Cui, WeiWei Cui, Thomas Dauser, Ettore Del Monte, Sergio Di Cosimo, Sebastian Diebold, Victor Doroshenko, Michal Dovciak, YuanYuan Du, Lorenzo Ducci, QingMei Fan, Yannick Favre, Fabio Fuschino, José Luis Gálvez, Min Gao, MingYu Ge, Olivier Gevin, Marco Grassi, QuanYing Gu, YuDong Gu, DaWei Han, Bin Hong, Wei Hu, Long Ji, ShuMei Jia, WeiChun Jiang, Thomas Kennedy, Ingo Kreykenbohm, Irfan Kuvvetli, Claudio Labanti, Luca Latronico, Gang Li, MaoShun Li, Xian Li, Wei Li, ZhengWei Li, Olivier Limousin, HongWei Liu, XiaoJing Liu, Bo Lu, Tao Luo, Daniele Macera, Piero Malcovati, Adrian Martindale, Malgorzata Michalska, Bin Meng, Massimo Minuti, Alfredo Morbidini, Fabio Muleri, Stephane Paltani, Emanuele Perinati, Antonino Picciotto, Claudio Piemonte, JinLu Qu, Alexandre Rachevski, Irina Rashevskaya, Jerome Rodriguez, Thomas Schanz, ZhengXiang Shen, LiZhi Sheng, JiangBo Song, LiMing Song, Carmelo Sgro, Liang Sun, Ying Tan, Phil Uttley, Juan Wang, LangPing Wang, YuSa Wang, Anna L. Watts, XiangYang Wen, Jörn Wilms, ShaoLin Xiong, JiaWei Yang, Sheng Yang, YanJi Yang, Nian Yu, WenDa Zhang, Gianluigi Zampa, Nicola Zampa, Andrzej A. Zdziarski, AiMei Zhang, ChengMo Zhang, Fan Zhang, Long Zhang, Tong Zhang, Yi Zhang, XiaoLi Zhang, ZiLiang Zhang, BaoSheng Zhao, ShiJie Zheng, YuPeng Zhou, Nicola Zorzi, J. Frans Zwart

arXiv:1812.04020v1astro-ph.IMastro-ph.HE

TL;DR

eXTP addresses fundamental-physics questions about matter under extreme density, gravity, and magnetism while also serving as a broad astrophysical observatory. The paper describes its payload technologies, expected instrument performance, and mission architecture, including capabilities for monitoring gravitational-wave counterparts. The mission combines focused timing and spectroscopy, large-area detection, and X-ray polarimetry with wide-field monitoring, with reported performance including LAD sensitivity and PFA polarimetric sensitivity benchmarks.

  • Problem

    eXTP is designed to investigate the equation of state of supra-nuclear-density matter, QED effects, and matter dynamics in strong-field gravity.

  • Method

    The paper presents the scientific payload's technologies and expected performance together with the mission's main elements and functions.

  • Results

    The payload combines SFA, LAD, PFA, and wide-field monitoring, with LAD and PFA performance characterized by sensitivity benchmarks and detector measurements.

  • Takeaways & Limitations

    eXTP provides complementary spectral, timing, polarimetric, and wide-field X-ray capabilities for fundamental physics and astrophysical observations.

  • Takeaways & Limitations

    Polarimeter systematics can limit sensitivity regardless of source brightness or exposure, while optical contributions still require future testing.

Abstract

from arXiv · show

In this paper we present the enhanced X-ray Timing and Polarimetry mission - eXTP. eXTP is a space science mission designed to study fundamental physics under extreme conditions of density, gravity and magnetism. The mission aims at determining the equation of state of matter at supra-nuclear density, measuring effects of QED, and understanding the dynamics of matter in strong-field gravity. In addition to investigating fundamental physics, eXTP will be a very powerful observatory for astrophysics that will provide observations of unprecedented quality on a variety of galactic and extragalactic objects. In particular, its wide field monitoring capabilities will be highly instrumental to detect the electro-magnetic counterparts of gravitational wave sources. The paper provides a detailed description of: (1) the technological and technical aspects, and the expected performance of the instruments of the scientific payload; (2) the elements and functions of the mission, from the spacecraft to the ground segment.

1 Introduction

eXTP is a space mission for studying matter and physical processes under extreme density, gravity, and magnetic-field conditions. It combines fundamental-physics objectives with broad astrophysical observing and gravitational-wave counterpart monitoring.

  • eXTP targets the equation of state of supra-nuclear-density matter, QED effects near highly magnetized stars, and matter dynamics in strong-field gravity.
  • The mission investigates neutron stars, black holes, and magnetars as laboratories for fundamental physics.
  • eXTP will also observe a variety of galactic and extragalactic objects with high-quality X-ray measurements.
  • Its wide-field monitoring capabilities are intended to identify electromagnetic counterparts of gravitational-wave sources.
  • The project combines Chinese X-ray timing and polarimetry and European LOFT mission concepts after their consortiums merged in 2015.
  • This paper presents the payload technologies, instrument performance, and principal mission elements and functions.

2 The scientific payload

The eXTP scientific payload comprises four complementary instruments: focused SFA and PFA arrays, the large-area LAD, and the wide-field WFM.

  • The scientific payload includes the spectroscopic focusing array SFA, large area detector LAD, polarimetry focusing array PFA, and wide field monitor WFM.

2.1 Spectroscopic Focusing Array - SFA

The SFA combines nine Wolter-I grazing-incidence telescopes with silicon drift detectors for high-throughput spectral and timing observations from 0.5–10 keV. Its baseline optics, detector architecture, electronics, and filters are designed to meet demanding performance requirements, with effective-area performance assessed through simulations.

  • Optics: Nine identical Wolter-I telescopes provide SFA spectral and timing observations across 0.5–10 keV, with a 12-arcmin field of view.The SFA uses silicon drift detectors at the focal planes.
  • Optics: The thirteen-module SFA/PFA optical baseline expects at least 900 cm2 per telescope at 2 keV and at least 550 cm2 at 6 keV.The optics have a 12-arcmin diameter field of view and meet the approximately 820 cm2 requirement at 2 keV.
  • Optics: Nickel replication is the baseline mirror technology, while slumped glass optics remain a second technology option for the eXTP optics.The slumped-glass design uses lightweight thermally formed mirrors and requires about 200 shells for the target collecting area.
  • Optics: The best cylindrical glass mirror produced in the XTP study achieved an angular resolution of 36 arcsec half-power diameter.Other cylindrical glasses were produced with 50–60 arcsec resolution.
  • Detectors and Electronics: The SFA detector uses 19 hexagonal SDD cells to provide better than 180 eV energy resolution at 6 keV and time resolution below 10 µs over the mission lifetime.SDDs enable faster readout than CCDs, and the geometry focuses most source photons onto the inner seven cells.
  • Performance: The total SFA effective area, including detector efficiency and filters, and the expected background are evaluated using current simulations.The effective-area results are presented in Figure 8 and the background in Figure 9.

2.2 Large Area Detector - LAD

The LAD is a lightweight, highly redundant large-area X-ray detector using SDDs and capillary-plate collimators for high-throughput photon-by-photon spectral-timing observations. Its modular architecture and background-control measures support high sensitivity, negligible pile-up, and strong-field-gravity studies of faint sources.

  • Instrument overview: The LAD records photon arrival times and energies over 2–30 keV, extending to 80 keV in expanded mode for out-of-field-of-view bursts.A mechanical collimator limits the field of view to approximately 1° to reduce source confusion and X-ray background.
  • Instrument overview: Large-area SDDs and capillary-plate collimators make the detector highly efficient while reducing weight, volume, and power requirements by about an order of magnitude.The detector elements are only a few millimeters thick and weigh a few hundred grams.
  • LAD architecture: Each LAD module contains 16 SDDs, 16 collimator tiles, front-end electronics, back-end electronics, power supplies, shielding, and thermal components.The MBEE controls detectors and electronics, processes and time-stamps events, generates housekeeping data, and transmits events to the PBEE.
  • LAD architecture: Forty modules arranged across two panels provide 640 detectors, while the segmented readout uses 144 × 10^3 electronics channels to keep individual-channel rates low.This segmentation makes pile-up and dead-time effects negligible even for very bright sources.
  • Detectors and electronics: The SDDs drift photon-generated electrons to edge anodes, where diffusion encodes the impact position and supports low-noise readout over a large collecting area.The detector uses a 370 V/cm drift field, approximately 5 mm/µs drift velocity, and a maximum drift time of approximately 7 µs.
  • Performance: The LAD achieves approximately 0.5 mCrab/s 3σ sensitivity for persistent sources, with background-subtraction systematics dominating for exposures longer than 10^4 s.If background variations are modeled to 0.3% of the mean level on few-kilosecond timescales, simulations indicate 20% black-hole-spin accuracy for AGN samples and enable AGN reverberation mapping.

2.3 Polarimetry Focusing Array – PFA

The PFA combines four imaging polarimetry telescopes with gas pixel detectors to measure X-ray polarization through photoelectron tracks and modulation. Its sensitivity depends on effective area, modulation factor, background, and systematic errors, with peak sensitivity near 3 keV.

  • PFA overview: The PFA comprises four identical telescopes optimized for 2–8 keV X-ray imaging polarimetry.It provides spatial, energy, and temporal resolution in synergy with the SFA and LAD, with imaging better than an arcminute.
  • Detectors and electronics: The gas pixel detector converts X-rays into photoelectrons whose emission angles encode polarization, while a GEM amplifies the resulting electron signal.The detector uses a 50 µm beryllium entrance window, DME gas at 0.8 atm, and a drift field of about 2 kV cm−1.
  • Polarization measurement: Polarized X-rays produce a cosine-squared modulation curve, whose amplitude scales linearly with the incident beam’s polarization degree.For a fully polarized source, the modulation amplitude defines the modulation factor µ.
  • Detector performance: 58.2% ± 0.7% is the measured modulation degree for fully polarized 6.14 keV X-rays.The measurement used 45° Bragg diffraction with a LiF crystal.
  • Performance: The four GPDs have an estimated 2–8 keV background of about 6 × 10−3 counts s−1, compared with approximately 1,200 counts s−1 from the Crab nebula.The background is therefore negligible for most bright Galactic targets.
  • Performance: The PFA sensitivity peaks around 3 keV, although its effective area peaks at about 2 keV.The sensitivity peak reflects the increase of modulation factor with energy.
  • Systematics: GPD systematic errors can generally be controlled below 1%, but the optics’ contribution still requires future testing.Calibration can allow systematic effects to be determined and subtracted.
  • Performance: For a Crab-like source, a 1 ks exposure gives an MDP of 1.7% when background is negligible.The quoted MDP corresponds to 99% confidence; a precise measurement at 3σ or more requires additional observing time.

2.4 Wide Field Monitor – WFM

The WFM combines six coded-mask cameras with position-sensitive SDDs to monitor a wide sky area while providing source positions, energy measurements, and precise timing. Its large field of view and burst-alert system support rapid detection and localization of transient sources.

  • WFM architecture: Six cameras arranged as three orthogonal pairs cover 4.1 sr, approximately 33% of the sky, across 2–50 keV.Each camera pair has an effective field of view of approximately 70° × 70°, extending to approximately 90° × 90° at zero response.
  • WFM architecture: The WFM measures photon energy, two-dimensional position, and event time using SDD charge distributions processed onboard by FPGA-based electronics.The X-position is derived from the charge-cloud center, while the Y-position is proportional to its width.
  • WFM architecture: The camera uses four SDDs, four front-end electronics units, beryllium windows, a back-end electronics assembly, collimator, and coded mask, with redundant camera-pair design.Thermal control includes a sunshield, detector operating temperatures from −30 °C to −3 °C, and blankets that stabilize mask temperature and camera alignment.
  • WFM architecture: 4.24 arcmin and 4.5 deg are the angular resolutions in the fine- and coarse-mask directions, respectively.The on-axis resolution follows Arctan(p/d), where p is mask pitch and d is the detector–mask distance; orthogonal cameras improve two-coordinate positioning.
  • Performance: The WFM has a 1.75π sr field of view at 0% response and 1.33π sr at 20% of peak response, larger than comparable facilities.This broad coverage is intended to support detection of electromagnetic counterparts of gravitational-wave sources.
  • Performance: Approximately 100 GRBs per year are expected, with burst positions and trigger times delivered to end users within 30 s.Image-trigger and count-rate-trigger algorithms identify burst candidates, while a VHF transmitter relays short alerts to ground stations.

3 The mission profile

The mission profile integrates modular spacecraft and payload architecture with an equatorial low-Earth orbit selected to limit radiation damage. China leads spacecraft, launch, and ground operations, while European partners provide the LAD and WFM and contribute to other payload elements.

  • Launch and deployment: The satellite is launched stowed within a 4600 mm diameter and 8835 mm height envelope, then deployed to an in-orbit size of 4737 mm × 8781 mm × 11500 mm.The LM-7 launch vehicle is planned to place the spacecraft directly into orbit from the Wenchang Satellite Launch Center.
  • Spacecraft and configuration: The mission uses pointing observations as its main mode and scans small regions near known sources for calibration or point-spread-function verification.Its design maximizes focal length and effective area within the LM-7 fairing constraints and separates the spacecraft into functional modules.
  • Spacecraft and configuration: The spacecraft modules are the mirror assembly, focal-plane module, LAD assembly, and WFM, with a 5.25 m focal length supported by the telescope tube.The focal-plane module contains 13 cameras, while the mirror modules are mounted on an optical bench to maintain co-alignment.
  • Orbit: The selected orbit is an equatorial LEO at 550 km altitude with inclination below 2.5°, reducing radiation exposure and South Atlantic Anomaly effects.The orbit is also intended to support passive thermal control and acceptable instrument energy-resolution losses over the mission.
  • Ground segment: The ground segment comprises TT&C, the operational ground segment, and the science ground segment, including mission operations, telemetry stations, proposal support, and science data centers.Sanya and Malindi receive telemetry, while the science segment coordinates proposals, target-of-opportunity requests, and multi-wavelength or multi-messenger programs.

4 Observing strategy

eXTP is planned as an observatory open to the scientific community, with observing time allocated through annual announcements and peer review. Target-of-opportunity observations provide a route for responding to transient or changing sources.

  • Observing strategy: Observing time will be allocated through annual Announcements of Opportunities and scientific peer review.The strategy aims to maximize scientific return while providing returns to the institutions and countries supporting the mission.
  • Observing strategy: The mission will operate as an observatory open to the scientific community, with target-of-opportunity requests considered by the eXTP PI after consultation with the OTAC.Depending on the request, either a one-year proprietary-data rule or broader community access may apply.

5 Conclusions

The paper presents eXTP’s payload technologies, expected instrument performance, and mission architecture for studying matter, gravity, and magnetism under extreme conditions. Its combined timing, spectroscopy, polarimetry, and wide-field monitoring capabilities support both fundamental physics and broad astrophysical investigations.

  • Conclusions: eXTP targets the equation of state of supra-nuclear matter, QED effects near highly magnetized stars, and matter dynamics in strong-field gravity.The relevant environments include neutron stars, regions close to black holes, and magnetars.
  • Conclusions: The payload combines the SFA, LAD, PFA, and WFM to enable spectral-timing-polarimetry studies of many variable-universe sources.The PFA is described as five to six times more sensitive than the polarimeter on NASA’s IXPE mission.
  • Conclusions: The WFM is expected to be highly instrumental for detecting electromagnetic counterparts of transient multi-messenger sources.The mission is also presented as a powerful observatory for galactic and extragalactic astrophysics.
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