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A strangely light neutron star within a supernova remnant

Abstract

To constrain the equation of state of cold dense matter, astrophysical measurements are essential. These are mostly based on observations of neutron stars in the X-ray band, and, more recently, also on gravitational wave observations. Of particular interest are observations of unusually heavy or light neutron stars which extend the range of central densities probed by observations and thus permit the testing of nuclear-physics predictions over a wider parameter space. Here we report on the analysis of such a star, a central compact object within the supernova remnant HESS J1731-347. We estimate the mass and radius of the neutron star to be $$M=0.7{7}_{-0.17}^{+0.20}\,{M}_{\odot }$$ and $$R=10.{4}_{-0.78}^{+0.86}$$ km, respectively, based on modelling of the X-ray spectrum and a robust distance estimate from Gaia observations. Our estimate implies that this object is either the lightest neutron star known, or a ‘strange star’ with a more exotic equation of state. Adopting a standard neutron star matter hypothesis allows the corresponding equations of state to be constrained.

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Data availability

XMM-Newton and Suzaku data used in the publication are publicly available at the respective missions’ data centres and HEASARC archives. The data reduction was carried out using the software and instructions provided by the respective missions’ science operation centres. The tabulated EOSs considered in this work are available as part of the original publications23,24,25. The posterior samples for neutron star mass and radius obtained in this work are available via https://doi.org/10.5281/zenodo.6702216 (ref. 47).

Code availability

Model atmospheres are included as part of HEASOFT package at https://heasarc.gsfc.nasa.gov/docs/software/heasoft/. BXA software is also in the public domain and available at https://johannesbuchner.github.io/BXA/index.html. Code for calculation of theoretically expected pulsed fraction for arbitrary local spectra is available upon reasonable request from the authors.

References

1. Pavlov, G. G., Zavlin, V. E. & Sanwal, D. Thermal radiation from neutron stars: Chandra results. In Proc. 270th Heraeus Seminar on Neutron Stars, Pulsars and Supernova Remnants MPE Reports 278 (eds Becker, W. et al.) 273 (MPE, 2002).

2. Pavlov, G. G., Sanwal, D. & Teter, M. A. Central compact objects in supernova remnants. In Young Neutron Stars and Their Environments, IAU Symposium, Vol. 218 (eds Camilo, F. & Gaensler, B. M.) 239 (Astronomical Society of the Pacific, 2004).

3. De Luca, A. Central compact objects in supernova remnants. J. Phys. Conf. Ser. 932, 012006 (2017).

4. Halpern, J. P. & Gotthelf, E. V. Spin-down measurement of PSR J1852+0040 in Kesteven 79: central compact objects as anti-magnetars. Astrophys. J. 709, 436–446 (2010).

5. Gotthelf, E. V., Halpern, J. P. & Alford, J. The spin-down of PSR J0821-4300 and PSR J1210-5226: confirmation of central compact objects as anti-magnetars. Astrophys. J. 765, 58-74 (2013).

6. Lattimer, J. M. & Prakash, M. The equation of state of hot, dense matter and neutron stars. Phys. Rep. 621, 127–164 (2016).

7. Degenaar, N. & Suleimanov, V. F. Testing the equation of state with electromagnetic observations. Astrophys. Space Sci. Libr. 457, 185–253 (2018).

8. Zavlin, V. E., Pavlov, G. G. & Trumper, J. The neutron star in the supernova remnant PKS 1209-52. Astron. Astrophys. 331, 821–828 (1998).

9. Pavlov, G. G. & Luna, G. J. M. A dedicated Chandra ACIS observation of the central compact object in the Cassiopeia A supernova remnant. Astrophys. J. 703, 910–921 (2009).

10. Klochkov, D. et al. A non-pulsating neutron star in the supernova remnant HESS J1731-347/G353.6-0.7 with a carbon atmosphere. Astron. Astrophys. 556, A41 (2013).

11. Elshamouty, K. G., Heinke, C. O., Morsink, S. M., Bogdanov, S. & Stevens, A. L. The impact of surface temperature inhomogeneities on quiescent neutron star radius measurements. Astrophys. J. 826, 162 (2016).

12. Suleimanov, V. F., Klochkov, D., Poutanen, J. & Werner, K. Probing the possibility of hotspots on the central neutron star in HESS J1731-347. Astron. Astrophys. 600, A43 (2017).

13. Ho, W. C. G. & Heinke, C. O. A neutron star with a carbon atmosphere in the Cassiopeia A supernova remnant. Nature 462, 71–73 (2009).

14. Klochkov, D. et al. The neutron star in HESS J1731-347: central compact objects as laboratories to study the equation of state of superdense matter. Astron. Astrophys. 573, A53 (2015).

15. Doroshenko, V., Suleimanov, V. & Santangelo, A. CXOU J160103.1-513353: another central compact object with a carbon atmosphere? Astron. Astrophys. 618, A76 (2018).

16. Wu, Q. et al. What causes the absence of pulsations in central compact objects in supernova remnants? Res. Astron. Astrophys. 21, 294 (2021).

17. Halpern, J. P. & Gotthelf, E. V. Spin-down measurement of PSR J1852+0040 in Kesteven 79: central compact objects as anti-magnetars. Astrophys. J. 709, 436–446 (2010).

18. Ho, W. C. G. et al. X-ray bounds on cooling, composition, and magnetic field of the Cassiopeia A neutron star and young central compact objects. Mon. Not. R. Astron. Soc. 506, 5015–5029 (2021).

19. Doroshenko, V. et al. Evidence for a binary origin of a central compact object. Mon. Not. R. Astron. Soc. 458, 2565–2572 (2016).

20. Bailer-Jones, C. A. L., Rybizki, J., Fouesneau, M., Demleitner, M. & Andrae, R. Estimating distances from parallaxes. V: geometric and photogeometric distances to 1.47 billion stars in Gaia Early Data Release 3. Astron. J. 161, 147 (2021).

21. Landstorfer, A., Doroshenko, V. & Pühlhofer, G. Dust scattering halo around the CCO in HESS J1731-347: a detailed analysis. Astron. Astrophys. 659, A82 (2022).

22. Lallement, R. et al. Gaia-2MASS 3D maps of galactic interstellar dust within 3 kpc. Astron. Astrophys. 625, A135 (2019).

23. Dietrich, T. et al. Multimessenger constraints on the neutron-star equation of state and the Hubble constant. Science 370, 1450–1453 (2020).

24. Pang, P. T. H. et al. Nuclear physics multimessenger astrophysics constraints on the neutron star equation of state: adding NICER’s PSR J0740+6620 measurement. Astrophys. J. 922, 14 (2021).

25. Nättilä, J. et al. Neutron star mass and radius measurements from atmospheric model fits to X-ray burst cooling tail spectra. Astron. Astrophys. 608, A31 (2017).

26. Hessels, J. W. T. et al. A radio pulsar spinning at 716 Hz. Science 311, 1901–1904 (2006).

27. Al-Mamun, M. et al. Combining electromagnetic and gravitational-wave constraints on neutron-star masses and radii. Phys. Rev. Lett. 126, 061101 (2021).

28. Adhikari, D. et al. Accurate determination of the neutron skin thickness of 208Pb through parity-violation in electron scattering. Phys. Rev. Lett. 126, 172502 (2021).

29. Suwa, Y., Yoshida, T., Shibata, M., Umeda, H. & Takahashi, K. On the minimum mass of neutron stars. Mon. Not. R. Astron. Soc. 481, 3305–3312 (2018).

30. Yakovlev, D. G. & Pethick, C. J. Neutron star cooling. Annu. Rev. Astron. Astrophys. 42, 169–210 (2004).

31. Ho, W. C. G. Evolution of a buried magnetic field in the central compact object neutron stars. Mon. Not. R. Astron. Soc. 414, 2567–2575 (2011).

32. Viganò, D. & Pons, J. A. Central compact objects and the hidden magnetic field scenario. Mon. Not. R. Astron. Soc. 425, 2487–2492 (2012).

33. Buchner, J. et al. X-ray spectral modelling of the AGN obscuring region in the CDFS: Bayesian model selection and catalogue. Astron. Astrophys. 564, A125 (2014).

34. Buchner, J. UltraNest – a robust, general purpose Bayesian inference engine. J. Open Source Softw. 6, 3001 (2021).

35. Freeman, P., Doe, S. & Siemiginowska, A. Sherpa: a mission-independent data analysis application. In Astronomical Data Analysis, Vol. 4477 of Society of Photo-Optical Instrumentation Engineers (SPIE) Conference Series (eds Starck, J.-L. & Murtagh, F. D.) 76–87 (SPIE, 2001).

36. Astropy Collaboration. et al. Astropy: a community Python package for astronomy. Astron. Astrophys. 558, A33 (2013).

37. Lindegren, L. et al. Gaia Early Data Release 3. The astrometric solution. Astron. Astrophys. 649, A2 (2021).

38. Groenewegen, M. A. T. The parallax zero-point offset from Gaia EDR3 data. Astron. Astrophys. 654, A20 (2021).

39. Huang, Y., Yuan, H., Beers, T. C. & Zhang, H. The parallax zero-point of Gaia Early Data Release 3 from LAMOST primary red clump stars. Astrophys. J. Lett. 910, L5 (2021).

40. Zinn, J. C. Validation of the Gaia Early Data Release 3 parallax zero-point model with asteroseismology. Astron. J. 161, 214 (2021).

41. Cash, W. Parameter estimation in astronomy through application of the likelihood ratio. Astrophys. J. 228, 939–947 (1979).

42. Arnaud, K. A. XSPEC: the first ten years. In Astronomical Data Analysis Software and Systems V, Vol. 101 of Astronomical Society of the Pacific Conference Series (eds Jacoby, G. H. & Barnes, J.) 17 (Astronomical Society of the Pacific, 1996).

43. Wilms, J., Allen, A. & McCray, R. On the absorption of X-rays in the interstellar medium. Astrophys. J. 542, 914–924 (2000).

44. Posselt, B., Pavlov, G. G., Suleimanov, V. & Kargaltsev, O. New constraints on the cooling of the central compact object in Cas A. Astrophys. J. 779, 186 (2013).

45. Smith, R. K., Valencic, L. A. & Corrales, L. The impact of accurate extinction measurements for X-ray spectral models. Astrophys. J. 818, 143 (2016).

46. Mathis, J. S., Rumpl, W. & Nordsieck, K. H. The size distribution of interstellar grains. Astrophys. J. 217, 425–433 (1977).

47. Doroshenko, V., Suleimanov, V. F., Pühlhofer, G. & Santangelo, A. MCMC samples for X-ray spectra fits summarised in the paper ‘A strangely light neutron star’. Zenodo https://doi.org/10.5281/zenodo.6702216 (2022).

48. Suleimanov, V. F., Klochkov, D., Pavlov, G. G. & Werner, K. Carbon neutron star atmospheres. Astrophys. J. Suppl. 210, 13 (2014).

49. Schwope, A. et al. Phase-resolved X-ray spectroscopy of PSR B0656+14 with SRG/eROSITA and XMM-Newton. Astron. Astrophys. 661, A41 (2022).

50. Greenstein, G. & Hartke, G. J. Pulselike character of uchlackbody radiation from neutron stars. Astrophys. J. 271, 283–293 (1983).

51. Miller, M. C. et al. The radius of PSR J0740+6620 from NICER and XMM-Newton data. Astrophys. J. Lett. 918, L28 (2021).

52. Buccheri, R. et al. Search for pulsed γ-ray emission from radio pulsars in the COS-B data. Astr. Astrophys. 128, 245–251 (1983).

53. Huppenkothen, D. et al. Stingray: a modern Python library for spectral timing. Astrophys. J. 881, 39 (2019).

54. Vaughan, B. A. et al. Searches for millisecond pulsations in low-mass X-ray binaries. II. Astrophys. J. 435, 362 (1994).

55. Miller, M. C. et al. PSR J0030+0451 mass and radius from NICER data and implications for the properties of neutron star matter. Astro. Phys. J. Lett. 87, L24 (2019).

Acknowledgements

This research made use of observations obtained with XMM-Newton, a European Space Agency (ESA) science mission with instruments and contributions directly funded by ESA Member States and NASA. For analysing X-ray spectra, we use the analysis software BXA33, which connects the nested sampling algorithm UltraNest34 with the fitting environment CIAO/Sherpa35. This research also made use of the astropy package36. The work was supported by the German Research Foundation (DFG) grant WE 1312/53-1 (VFS).

Author information

Authors

Contributions

V.D. carried out data analysis and modelling and drafted the initial version of the manuscript. V.S. developed atmosphere models used in the work and calculated upper limits on theoretically allowed pulsation amplitudes. G.P. and A.S. contributed to the interpretation of the results. G.P. was also principal investigator for some of the XMM observations used in this work. All authors contributed to the writing of the manuscript.

Corresponding author

Correspondence to Victor Doroshenko.

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Nature Astronomy thanks Adriana Pires and the other, anonymous, reviewer(s) for their contribution to the peer review of this work.

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Extended data

Extended Data Fig. 1 Comparison of the fit results for NS mass and radius from this work with those reported by 14.

The dotted line shows results from the latter work for a fixed distance of 3.2 kpc (1σ credible interval). The labeled contours show results obtained in this work: (1) - same model and energy range as 14, (2) - same as 1 but with data below 1 keV included, (3) - same as 2 but with the wabs model component substituted with tbabs, (3a) - same as (3) but also accounting for the scattering component, (4) same as (3a) but with distance fixed to 2.5 kpc, and (5) - same as 4 but with distance priors set to the Gaia estimate as described in the text.

Extended Data Fig. 2 Theoretically expected pulsed fraction limits as a function of angles defining the viewing geometry.

The expected pulsed fraction is calculated given the best-fit spectral parameters for each model as described in section ‘More complex temperature distributions’ of the Methods, and contours represent limits on possible angle values when upper limits on the observed pulsed fraction obtained in this work for various frequency ranges and reported in the Extended Data Table 2 are considered. The region to the lower left of the respective contours represents the range of angles allowed for a given model and corresponding upper limit on pulsed fraction. The shaded region corresponds thus to the weakest of the upper limits on the observed pulsed fraction (that is 9.7%), and thus represents the most conservative estimate.

Extended Data Fig. 3 Corner plots corresponding to the final fit with single temperature carbon atmosphere model including full distance priors and EOS constrain priors.

Unweighted samples from the BXA modeling described in the text for all relevant parameters are used to produce the plots, using the corner module for 1σ credibility intervals. The panels corresponding to the (well constrained) cross-normalization constants also included in the fit are omitted for clarity.

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Doroshenko, V., Suleimanov, V., Pühlhofer, G. et al. A strangely light neutron star within a supernova remnant. Nat Astron 6, 1444–1451 (2022). https://doi.org/10.1038/s41550-022-01800-1

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