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# Asymmetric mass ratios for bright double neutron-star mergers

## Abstract

The discovery of a radioactively powered kilonova associated with the binary neutron-star merger GW170817 remains the only confirmed electromagnetic counterpart to a gravitational-wave event1,2. Observations of the late-time electromagnetic emission, however, do not agree with the expectations from standard neutron-star merger models. Although the large measured ejecta mass3,4 could be explained by a progenitor system that is asymmetric in terms of the stellar component masses (that is, with a mass ratio q of 0.7 to 0.8)5, the known Galactic population of merging double neutron-star systems (that is, those that will coalesce within billions of years or less) has until now consisted only of nearly equal-mass (q > 0.9) binaries6. The pulsar PSR J1913+1102 is a double system in a five-hour, low-eccentricity (0.09) orbit, with an orbital separation of 1.8 solar radii7, and the two neutron stars are predicted to coalesce in $${470}_{-11}^{+12}$$ million years owing to gravitational-wave emission. Here we report that the masses of the pulsar and the companion neutron star, as measured by a dedicated pulsar timing campaign, are 1.62 ± 0.03 and 1.27 ± 0.03 solar masses, respectively. With a measured mass ratio of q = 0.78 ± 0.03, this is the most asymmetric merging system reported so far. On the basis of this detection, our population synthesis analysis implies that such asymmetric binaries represent between 2 and 30 per cent (90 per cent confidence) of the total population of merging binaries. The coalescence of a member of this population offers a possible explanation for the anomalous properties of GW170817, including the observed kilonova emission from that event.

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## Data availability

All data are available from the corresponding author on request.

## Code availability

The code used in this analysis is publicly available at: https://github.com/nanograv/tempo (pulsar timing analysis); https://github.com/NihanPol/2018-DNS-merger-rate (population synthesis); https://github.com/rferdman/pypsr (plotting tools).

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## Acknowledgements

We thank S. Nissanke, T. Tauris and B. Metzger for constructive discussions, as well as C. Belczynski and T. Tauris for useful comments. The Arecibo Observatory is operated by the University of Central Florida, Ana G. Mèndez-Universidad Metropolitana and Yang Enterprises under a cooperative agreement with the National Science Foundation (NSF; AST-1744119). R.D.F. and P.C.C.F. acknowledge the support of the PHAROS COST Action (CA16214). S.C., J.M.C., F. Crawford, M.A.M. and N.P. are members of the NANOGrav Physics Frontiers Center, which is supported by NSF award number PHY-1430284. S.C. and J.M.C. also acknowledge support from NSF award AAG-1815242. J.W.T.H. acknowledges funding from an NWO Vici grant (‘AstroFlash’). V.M.K. acknowledges support from an NSERC Discovery Grant and Herzberg Award, the Canada Research Chairs programme, the Canadian Institute for Advanced Research and FRQ-NT. E.P. is a Vanier Canada Graduate Scholar. I.H.S. acknowledges support for pulsar research at the University of British Columbia by an NSERC Discovery Grant and by the Canadian Institute for Advanced Research. J.v.L. acknowledges funding from an NWO Vici grant (‘ARGO’).

## Author information

Authors

### Contributions

R.D.F. contributed to timing analysis, wrote and ran mass probability contour code, led proposals for observing campaigns for this pulsar with the Arecibo telescope, composed the manuscript and produced Figs. 1, 3. P.C.C.F. led the timing analysis. B.P.P.P. and N.P. led the population synthesis analysis and produced Fig. 2. R.D.F., P.C.C.F., B.P.P.P., F. Camilo, S.C., J.M.C., F. Crawford, J.W.T.H., V.M.K., M.A.M., E.P., I.H.S. and J.v.L. contributed to the PALFA survey operations, as well as the discovery and follow-up observations of this pulsar. All authors contributed to discussion regarding the content of this manuscript.

### Corresponding author

Correspondence to R. D. Ferdman.

## Ethics declarations

### Competing interests

The authors declare no competing interests.

Peer review information Nature thanks Chris Belczynski and Thomas Tauris for their contribution to the peer review of this work.

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Ferdman, R.D., Freire, P.C.C., Perera, B.B.P. et al. Asymmetric mass ratios for bright double neutron-star mergers. Nature 583, 211–214 (2020). https://doi.org/10.1038/s41586-020-2439-x

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• ### Neutron star mergers and how to study them

• Eric Burns

Living Reviews in Relativity (2020)