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Millisecond pulsars from accretion-induced collapse as the origin of the Galactic Centre gamma-ray excess signal

Abstract

Gamma-ray data from the Fermi Large Area Telescope reveal an unexplained, apparently diffuse, signal from the Galactic bulge1,2,3 that peaks near ~2 GeV with an approximately spherical4 intensity profile r−2.4 (refs. 3,5), where r is the radial distance to the Galactic centre, that extends to angular radial scales of at least ~10° and possibly to ~20° (refs. 6,7). The origin of this ‘Galactic Centre excess’ (GCE) has been debated, with proposed sources prominently including self-annihilating dark matter1,4 and a hitherto undetected population of millisecond pulsars (MSPs)8. However, the conventional channel for the generation of MSPs has been found to predict too many low-mass X-ray binary (LMXB) systems9 and, because of the expected large natal kicks, may not accommodate10 the close spatial correspondence11,12,13 between the GCE signal and stars in the bulge. Here we report a binary population synthesis (BPS) forward model that demonstrates that an MSP population arising from the accretion-induced collapse (AIC) of O–Ne white dwarfs in Galactic bulge binaries can naturally reproduce the morphology, spectral shape and intensity of the GCE signal while also obeying LMXB constraints. Synchrotron emission from MSP-launched cosmic ray electrons and positrons may simultaneously explain the mysterious microwave ‘haze’14 from the inner Galaxy.

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Fig. 1: Spin-down power liberated by magnetic dipole braking of Galactic bulge MSPs over cosmological history according to our BPS model.
Fig. 2: GCE spectrum at the Earth.
Fig. 3: Broad-band, non-thermal spectrum of the Galactic bulge region.

Data availability

The model MSP dataset created with our code has been posted to Zenodo at: https://doi.org/10.5281/zenodo.6342560.

Code availability

Our code is based on the BSE24 code available here: http://astronomy.swin.edu.au/~jhurley/bsedload.html. We have made some refinements and modifications to BSE to account for the spin evolution of AIC-formed neutron stars post formation. These code extensions are presented in a GitHub repository here: https://github.com/gautam-404/Binary-Evolution/tree/master.

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Acknowledgements

R.M.C. thanks M. Krumholz, I. Seitenzahl, G. Rowell, C. Boehm, M. Baring and C. O’Hare for enlightening conversations. N. Rodd and S. Horiuchi are thanked for a close reading of the paper in draft, and T. Slatyer for comments. R.M.C. acknowledges support from the Australian Government through the Australian Research Council, award DP190101258 (shared with M. Krumholz). A.J.R. has been supported by the Australian Research Council through grant number FT170100243. Parts of this research were undertaken with the assistance of resources and services from the National Computational Infrastructure, which is supported by the Australian Government through the National Computational Merit Allocation Scheme and the UNSW HPC Resource Allocation Scheme. The work of O.M. was supported by JSPS KAKENHI Grant Number JP20K14463 and by the World Premier International Research Center Initiative (WPI Initiative), MEXT, Japan.

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Authors and Affiliations

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Contributions

R.M.C. conceived the project in consultation with L.F. and A.J.R. A.G. performed the BPS under the supervision of L.F., A.J.R. and R.M.C. A.G. added new functionality to the existing BSE code to model neutron star period evolution under accretion torques. R.M.C. and H.P. analysed the BPS data to derive model γ-ray luminosities and spectra. C.G. consulted about data and statistical analysis. O.M. performed a number of novel γ-ray template analyses of the GCE in support of the project. An original draft of the paper was written by A.G. This was subsequently amended and extended by R.M.C. in consultation with all the other authors.

Corresponding author

Correspondence to Roland M. Crocker.

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Extended data

Extended Data Fig. 1 Cumulative AIC events and number of model bulge MSPs over cosmological time.

MSP periods P are as labelled in the legend; we define any NS with P < 40 ms as an MSP. The ±1σ error band on the red curve for all AIC events reflects the uncertainties stemming from the bulge stellar mass determination and binarity fraction. (For clarity, equivalent error bands on the other curves are not shown.).

Extended Data Fig. 2 The main evolutionary stages towards, and beyond, accretion induced collapse of a white dwarf.

This schematic is for the model binary whose history is described in the Methods section ‘A typical evolutionary pathway towards AIC’.

Extended Data Fig. 3 Cosmological time vs. donor star type at time of AIC for all AIC events in our simulated population.

Given our empirically-motivated parameter choices, this model binary population is as expected for a host stellar population of total zero age main sequence mass of 2 × 109 M . For more details on stellar types, the reader is referred to S.I. sec. 2.

Supplementary information

Supplementary Information

Supplementary Figs. 1–24 and Sections 1–14.

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Gautam, A., Crocker, R.M., Ferrario, L. et al. Millisecond pulsars from accretion-induced collapse as the origin of the Galactic Centre gamma-ray excess signal. Nat Astron 6, 703–707 (2022). https://doi.org/10.1038/s41550-022-01658-3

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