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Ground calibration result of the Lobster Eye Imager for Astronomy

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Abstract

We report on results of the on-ground X-ray calibration of the Lobster Eye Imager for Astronomy (LEIA), an experimental space wide-field (18.6 \(\times \) 18.6 square degrees) X-ray telescope built from novel lobster eye micro-pore optics. LEIA was successfully launched on July 27, 2022 onboard the SATech-01 satellite. To achieve full characterisation of its performance before launch, a series of tests and calibrations have been carried out at different levels of devices, assemblies and the complete module. In this paper, we present the results of the end-to-end calibration campaign of the complete module carried out at the 100-m X-ray Test Facility at the Institute of High-energy Physics, Chinese Academy of Sciences (CAS). The Point Spread Function (PSF), effective area and energy response of the detectors were measured in a wide range of incident directions at several characteristic X-ray line energies. Specifically, the distributions of the PSF and effective areas are roughly uniform across the FoV, in large agreement with the prediction of lobster-eye optics. The mild variations and deviations from the prediction of idealized, perfect lobster-eye optics can be understood to be caused by the imperfect shapes and alignment of the micro-pores as well as the obscuration of incident photons by the supporting frames, which can be well reproduced by Monte Carlo simulations. The spatial resolution of LEIA defined by the full width at half maximum (FWHM) of the focal spot ranges from \(\textbf{4}\) to \(\textbf{8}\) arc minutes with a median of \(\mathbf{5.7}\) arcmin. The measured effective areas are in range of \(\mathbf{2-3}~\mathbf {cm^2}\) at \(\mathbf{\sim }\)1.25 keV across the entire FoV, and its dependence on photon energy is also in large agreement with simulations. The gains of the four complementary metal-oxide semiconductor (CMOS) sensors are in range of \(\mathbf{6.5-6.9}~\mathbf {eV/DN}\), and the energy resolutions in the range of \(\mathbf{\sim 120 - 140}\) eV at \(\mathbf{1.25}\) keV and \(\mathbf{\sim 170-190}\) eV at \(\mathbf{4.5}\) keV. These calibration results have been ingested into the first version of calibration database (CALDB) and applied to the analysis of the scientific data acquired by LEIA. This work paves the way for the calibration of the Wide-field X-Ray Telescope (WXT) flight model modules of the Einstein Probe (EP) mission.

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Data Availability

The data supports the findings of the study are not openly available due to the intellectual property belong to the partners and are available from the corresponding author upon reasonable request.

Notes

  1. The Einstein Probe is a time-domain X-ray mission of the Chinese Academy of Sciences (CAS) in collaboration with the European Space Agency (ESA), the Max-Planck Institute for Extraterrestrial Physics (MPE) and the France Space Agency (CNES).

  2. The focal surface of the lobster eye optics is a sphere with a radius of half the curvature of the optic.

  3. In this paper, the region to extract the photons of the focal spot is defined as a circle centered on the bary-center of the PSF with a radius of 1.25 mm (corresponding to 11.5 arcmin). Within this aperture the photons are dominated by those from the focal spot, with little contamination from the cruciform arms (e.g. at the energy of Mg K\(\alpha \) line the contribution of the cruciform arms to the total counts is found to be generally less than 5 per cent, which is smaller than the measurement error of the effective area).

  4. The Cu L\(\alpha \) line is excluded due to its asymmetric profile that may introduce uncertainties to the determination of the line energy.

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Acknowledgements

We thank all the members of the EP team, the EP consortium, and the SATech-01 team. The CAS team acknowledge contribution from ESA for calibration of the mirror assembly and tests of part of the devices. The Leicester and MPE teams acknowledge funding by ESA.

Funding

This work is supported by the Einstein Probe project, a mission in the Strategic Priority Program on Space Science of CAS (grant Nos. XDA15310000, XDA15052100), and the National Science Foundation of China (grant no. 12173055, 12173057, 12203071).

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Authors and Affiliations

  1. Key Laboratory of Space Astronomy and Technology, National Astronomical Observatories, Chinese Academy of Sciences, Datun Road 20A, Beijing, 100101, China

    Huaqing Cheng, Zhixing Ling, Chen Zhang, Yuan Liu, Yanfeng Dai, Zhenqing Jia, Haiwu Pan, Wenxin Wang, Donghua Zhao & Weimin Yuan

  2. School of Astronomy and Space Science, University of Chinese Academy of Sciences, Yuquan Road 19A, Beijing, 100049, China

    Zhixing Ling, Chen Zhang & Weimin Yuan

  3. Shanghai Institute of Technical Physics, Chinese Academy of Sciences, Yutian Road 500, Shanghai, 200083, China

    Xiaojin Sun, Shengli Sun, Yifan Chen, Zhiwei Cheng, Wei Fu, Yixiao Han, Junfei Li, Zhengda Li, Xiaohao Ma, Yulong Xue, Ailiang Yan & Qiang Zhang

  4. Institute of High Energy Physics, Chinese Academy of Sciences, Yuquan Road 19B, Beijing, 100049, China

    Yusa Wang, Xiongtao Yang & Zijian Zhao

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  1. Huaqing Cheng
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Contributions

H.-Q.C. and Z.-X.L. led the paper writing. C.Z. led the calibration experiment design and data analysis. W.-M.Y.contributed to the improvement of the manuscript. Z.-X.L., C.Z., W.-X.W., Y.-F.D, D.-H.Z., Z.-Q.J., X.-J.S., S.-L.S., Y.-F.C., Z.-W.C., W.F., Y.-X.H., J.-F.L., Z.-D.L., X.-H.M., Y.-L.X., A.-L.Y., Q.Z. and W.-M.Y. contributed to the design and development the instrument. H.-Q.C., Z.-X.L., C.Z.,D.-H.Z., Y.-F.D., Z.-Q.J., X.-J.S., Y.-F.C., Z.-W.C., Y.-X.H., J.-F.L., Z.-D.L., X.-H.M., Y.-L.X. participated in the calibration experiment. H.-Q.C., Z.-X.L., C.Z., Y.L., H.-W.P., W.-X.W., D.-H.Z. contributed to the analysis of calibration data. Y.-S.W. is responsible for the construction of the test facility. X.-T.Y. and Z.-J.Z. led the experimental setup and data acquisition. All authors read and approved the final manuscript.

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Correspondence to Zhixing Ling.

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Cheng, H., Ling, Z., Zhang, C. et al. Ground calibration result of the Lobster Eye Imager for Astronomy. Exp Astron 57, 10 (2024). https://doi.org/10.1007/s10686-024-09932-0

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