Re-detection and a possible time variation of soft X-ray polarization from the Crab

Abstract

The Crab nebula is so far the only celestial object with a statistically significant detection in soft X-ray polarimetry1,2,3,4, a window that has not been explored in astronomy since the 1970s. However, soft X-ray polarimetry is expected to be a sensitive probe of magnetic fields in high-energy astrophysical objects, including rotation-powered pulsars5,6,7 and pulsar wind nebulae8. Here we report the re-detection of soft X-ray polarization after 40 years from the Crab nebula and pulsar with PolarLight9, a miniature polarimeter utilizing a novel technique10,11 onboard a CubeSat. The polarization fraction of the Crab in the on-pulse phases was observed to decrease after a glitch of the Crab pulsar on 23 July 2019, while that of the pure nebular emission remained constant within uncertainty. The phenomenon may have lasted about 100 days. If the association between the glitch and polarization change can be confirmed with future observations, it will place strong constraints on the physical mechanism of the high-energy emission12,13,14 and glitch15,16,17 of pulsars.

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Fig. 1: Polarization measurements of the Crab with PolarLight.
Fig. 2: Time dependent polarization of the Crab.
Fig. 3: Significance test for a polarization variation associated with the glitch.

Data availability

The datasets generated and analysed in this study are available from the corresponding author on reasonable request.

Change history

  • 30 June 2020

    An amendment to this paper has been published and can be accessed via a link at the top of the paper.

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Acknowledgements

We thank H.-K. Chang, J. Takata, M. Ge and J. Heyl for helpful discussions. H.F. acknowledges funding support from the National Natural Science Foundation of China under the grant numbers 11633003 and 11821303, and the National Key R&D Project (grants numbers 2018YFA0404502 and 2016YFA040080X).

Author information

Affiliations

Authors

Contributions

H.F. is the principal investigator of PolarLight and led the project. H.L. and X.L. conducted the daily operation of the CubeSat and had a major contribution to the data analysis. R.B. led the development of the GPD. E.C., P.S. and F.M. participated in the discussion, and E.C. made a special contribution to the initiation of the project. J.H. performed the simulation and modelling of the in-orbit background. Q.W., W.J., M.M., D.Y., L.B., S.C., H.N., A.J., J.Y., G.J., M.Z., P.A., A.B., L.L., C.S., G.S. and M.P. contributed to the development of the payload instrument. W.W. and R.X. participated in the interpretation of the results.

Corresponding author

Correspondence to Hua Feng.

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The authors declare no competing interests.

Additional information

Peer review information Nature Astronomy thanks Mozsi Kiss, Andrea Santangelo and the other, anonymous, reviewer(s) for their contribution to the peer review of this work.

Publisher’s note Springer Nature remains neutral with regard to jurisdictional claims in published maps and institutional affiliations.

Extended data

Extended Data Fig. 1 Energy spectra measured with PolarLight.

The solid curves are for the Crab and the dotted are for the background. The background spectra are obtained by observing source-free regions. The red spectra are constructed using all x-ray events passing from particle discrimination and the blue ones consist of events used for polarimetry (with one more criterion on the number of fired pixels). Errors of 1σ are shown on the two Crab spectra. We note that the background events shown in the plot are mainly due to charged particles but can not be distinguished by particle discrimination. A discussion on the time variation and modulation of the background can be found in Methods.

Extended Data Fig. 2 Quality factor as a function of energy.

Polarization quality factor of PolarLight when observing the Crab.

Extended Data Fig. 3 Pulse profile of the Crab pulsar.

Folded pulse profile of the Crab pulsar measured with PolarLight in the energy band of 3.0-4.5 keV. The on-pulse phase interval is indicated by the horizontal bar.

Extended Data Fig. 4 Posterior distributions.

Top: posterior distributions of PF and PA with data before the glitch (red) or data within 100 days after the glitch (blue). Bottom: posterior distribution of PF (marginalized over PA). Each measurement is not consistent with the other at a 3σ level, and this conclusion is valid if one chooses any date from 30 days to 100 days after the glitch.

Extended Data Fig. 5 Lightcurves for the Crab and background.

3.0–4.5 keV lightcurves measured with PolarLight when observing the Crab and background regions. The bars show typical errors. Each point is the count rate averaged in a continuous exposure, which varies and has a typical duration of 15 minutes. The gap in the Crab data from MJD 58620 to 58670 (early May to early July, 2019) is due to a small angular separation to the Sun, which precludes observations in this period.

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Feng, H., Li, H., Long, X. et al. Re-detection and a possible time variation of soft X-ray polarization from the Crab. Nat Astron 4, 511–516 (2020). https://doi.org/10.1038/s41550-020-1088-1

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