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.
This is a preview of subscription content, access via your institution
Subscribe to Nature+
Get immediate online access to Nature and 55 other Nature journal
Subscribe to Journal
Get full journal access for 1 year
only $9.92 per issue
All prices are NET prices.
VAT will be added later in the checkout.
Tax calculation will be finalised during checkout.
Get time limited or full article access on ReadCube.
All prices are NET prices.
The model MSP dataset created with our code has been posted to Zenodo at: https://doi.org/10.5281/zenodo.6342560.
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.
Hooper, D. & Goodenough, L. Dark matter annihilation in the Galactic Center as seen by the Fermi Gamma Ray Space Telescope. Phys. Lett. B 697, 412–428 (2011).
Gordon, C. & Macías, O. Dark matter and pulsar model constraints from Galactic Center Fermi-LAT gamma-ray observations. Phys. Rev. D 88, 083521 (2013).
Abazajian, K. N., Canac, N., Horiuchi, S. & Kaplinghat, M. Astrophysical and dark matter interpretations of extended gamma-ray emission from the Galactic Center. Phys. Rev. D 90, 023526 (2014).
Daylan, T. et al. The characterization of the gamma-ray signal from the central Milky Way: a case for annihilating dark matter. Phys. Dark Universe 12, 1–23 (2016).
Calore, F., Cholis, I., McCabe, C. & Weniger, C. A tale of tails: dark matter interpretations of the Fermi GeV excess in light of background model systematics. Phys. Rev. D 91, 063003 (2015).
Hooper, D. & Slatyer, T. R. Two emission mechanisms in the Fermi bubbles: a possible signal of annihilating dark matter. Phys. Dark Universe 2, 118–138 (2013).
Ackermann, M. et al. Observations of M31 and M33 with the Fermi Large Area Telescope: a galactic center excess in Andromeda? Astrophys. J. 836, 208 (2017).
Abazajian, K. N. The consistency of Fermi-LAT observations of the Galactic Center with a millisecond pulsar population in the central stellar cluster. J. Cosmol. Astropart. Phys. 2011, 010 (2011).
Haggard, D., Heinke, C., Hooper, D. & Linden, T. Low mass X-ray binaries in the inner Galaxy: implications for millisecond pulsars and the GeV excess. J. Cosmol. Astropart. Phys. 2017, 056 (2017).
Ploeg, H. & Gordon, C. The effect of kick velocities on the spatial distribution of millisecond pulsars and implications for the Galactic center excess. J. Cosmol. Astropart. Phys. 2021, 020 (2021).
Macias, O. et al. Galactic bulge preferred over dark matter for the Galactic Centre gamma-ray excess. Nat. Astron. 2, 387–392 (2018).
Bartels, R., Storm, E., Weniger, C. & Calore, F. The Fermi-LAT GeV excess as a tracer of stellar mass in the Galactic bulge. Nat. Astron. 2, 819–828 (2018).
Macias, O. et al. Strong evidence that the Galactic bulge is shining in gamma rays. J. Cosmol. Astropart. Phys. 2019, 042 (2019).
Planck Collaboration Planck intermediate results. IX. Detection of the Galactic haze with Planck. Astron. Astrophys. 554, A139 (2013).
Chen, K. Gamma-ray emission from millisecond pulsars in globular clusters. Nature 352, 695–697 (1991).
Abdo, A. A. et al. A population of gamma-ray emitting globular clusters seen with the Fermi Large Area Telescope. Astron. Astrophys. 524, A75 (2010).
Wang, W., Jiang, Z. J. & Cheng, K. S. Contribution to diffuse gamma-rays in the Galactic Centre region from unresolved millisecond pulsars. Mon. Not. R. Astron. Soc. 358, 263–269 (2005).
Buschmann, M. et al. Foreground mismodeling and the point source explanation of the Fermi Galactic Center excess. Phys. Rev. D 102, 023023 (2020).
Hooper, D. & Mohlabeng, G. The gamma-ray luminosity function of millisecond pulsars and implications for the GeV excess. J. Cosmol. Astropart. Phys. 2016, 049 (2016).
Ploeg, H., Gordon, C., Crocker, R. & Macias, O. Comparing the Galactic bulge and Galactic disk millisecond pulsars. J. Cosmol. Astropart. Phys. 2020, 035 (2020).
Nataf, D. M. The controversial star-formation history and helium enrichment of the Milky Way Bulge. Publ. Astron. Soc. Aust. 33, e023 (2016).
Radhakrishnan, V. & Srinivasan, G. On the origin of the recently discovered ultra-rapid pulsar. Curr. Sci. 51, 1096–1099 (1982).
Ferrario, L. & Wickramasinghe, D. The birth properties of Galactic millisecond radio pulsars. Mon. Not. R. Astron. Soc. 375, 1009–1016 (2007).
Hurley, J. R., Tout, C. A., Wickramasinghe, D. T., Ferrario, L. & Kiel, P. D. Formation of binary millisecond pulsars by accretion-induced collapse of white dwarfs. Mon. Not. R. Astron. Soc. 402, 1437–1448 (2010).
Miyaji, S., Nomoto, K., Yokoi, K. & Sugimoto, D. Supernova triggered by electron captures. Publ. Astron. Soc. Jpn 32, 303–329 (1980).
Fryer, C., Benz, W., Herant, M. & Colgate, S. A. What can the accretion-induced collapse of white dwarfs really explain? Astrophys. J. 516, 892–899 (1999).
Kitaura, F. S., Janka, H. T. & Hillebrandt, W. Explosions of O–Ne–Mg cores, the Crab supernova, and subluminous type II-P supernovae. Astron. Astrophys. 450, 345–350 (2006).
Tauris, T. M., Sanyal, D., Yoon, S. C. & Langer, N. Evolution towards and beyond accretion-induced collapse of massive white dwarfs and formation of millisecond pulsars. Astron. Astrophys. 558, A39 (2013).
Lyne, A. G. & Lorimer, D. R. High birth velocities of radio pulsars. Nature 369, 127–129 (1994).
Freudenreich, H. T. A COBE model of the Galactic bar and disk. Astrophys. J. 492, 495–510 (1998).
Paczynski, B. Common envelope binaries. In Structure and Evolution of Close Binary Systems; Proc. of the IAU Symposium) Vol. 73 (eds Eggleton, P. et al.) 75 (1976).
de Kool, M. Statistics of cataclysmic variable formation. Astron. Astrophys. 261, 188–202 (1992).
Abdo, A. A. et al. The second Fermi Large Area Telescope catalog of gamma-ray pulsars. Astrophys. J. Suppl. 208, 17 (2013).
Sudoh, T., Linden, T. & Beacom, J. F. Millisecond pulsars modify the radio-star-formation-rate correlation in quiescent galaxies. Phys. Rev. D 103, 083017 (2021).
Song, D., Macias, O., Horiuchi, S., Crocker, R. M. & Nataf, D. M. Evidence for a high-energy tail in the gamma-ray spectra of globular clusters. Mon. Not. R. Astron. Soc. 507, 5161–5176 (2021).
Yuan, Q. & Ioka, K. Testing the millisecond pulsar scenario of the Galactic Center gamma-ray excess with very high energy gamma-rays. Astrophys. J. 802, 124 (2015).
Zhang, J. et al. Discriminating different scenarios to account for the cosmic e± excess by synchrotron and inverse Compton radiation. Phys. Rev. D 80, 023007 (2009).
Cherenkov Telescope Array Consortium. Science with the Cherenkov Telescope Array (World Scientific, 2019).
Maier, G. Performance of the Cherenkov Telescope Array. In 36th International Cosmic Ray Conference (ICRC2019) Vol. 36 733, eds. Desiati, P., Gaisser, T., and Karle, A. (2019).
Hurley, J. R., Pols, O. R. & Tout, C. A. Comprehensive analytic formulae for stellar evolution as a function of mass and metallicity. Mon. Not. R. Astron. Soc. 315, 543–569 (2000).
Hurley, J. R., Tout, C. A. & Pols, O. R. Evolution of binary stars and the effect of tides on binary populations. Mon. Not. R. Astron. Soc. 329, 897–928 (2002).
Kiel, P. D. & Hurley, J. R. Populating the galaxy with low-mass X-ray binaries. Mon. Not. R. Astron. Soc. 369, 1152–1166 (2006).
Wickramasinghe, D. T., Tout, C. A. & Ferrario, L. The most magnetic stars. Mon. Not. R. Astron. Soc. 437, 675–681 (2014).
Briggs, G. P., Ferrario, L., Tout, C. A., Wickramasinghe, D. T. & Hurley, J. R. Merging binary stars and the magnetic white dwarfs. Mon. Not. R. Astron. Soc. 447, 1713–1723 (2015).
Hurley, J. R., Tout, C. A. & Pols, O. R. BSE: Binary Star Evolution, Astrophysics Source Code Library, ascl:1303.014, (2013).
Ruiter, A. J. et al. On the formation of neutron stars via accretion-induced collapse in binaries. Mon. Not. R. Astron. Soc. 484, 698–711 (2019).
Kroupa, P., Tout, C. A. & Gilmore, G. The distribution of low-mass stars in the Galactic disc. Mon. Not. R. Astron. Soc. 262, 545–587 (1993).
Ferrario, L. Constraints on the pairing properties of main-sequence stars from observations of white dwarfs in binary systems. Mon. Not. R. Astron. Soc. 426, 2500–2506 (2012).
Duchêne, G. & Kraus, A. Stellar multiplicity. Annu. Rev. Astron. Astrophys. 51, 269–310 (2013).
Eggleton, P. P. The evolution of low mass stars. Mon. Not. R. Astron. Soc. 151, 351–364 (1971).
Pols, O. R., Tout, C. A., Eggleton, P. P. & Han, Z. Approximate input physics for stellar modelling. Mon. Not. R. Astron. Soc. 274, 964–974 (1995).
Pols, O. R., Schröder, K.-P., Hurley, J. R., Tout, C. A. & Eggleton, P. P. Stellar evolution models for Z = 0.0001 to 0.03. Mon. Not. R. Astron. Soc. 298, 525–536 (1998).
Moe, M. & Di Stefano, R. The close binary properties of massive stars in the Milky Way and low-metallicity Magellanic Clouds. Astrophys. J. 778, 95 (2013).
Mazeh, T., Simon, M., Prato, L., Markus, B. & Zucker, S. The mass ratio distribution in main-sequence spectroscopic binaries measured by infrared spectroscopy. Astrophys. J. 599, 1344–1356 (2003).
Willems, B. & Kolb, U. Population synthesis of wide binary millisecond pulsars. Mon. Not. R. Astron. Soc. 337, 1004–1016 (2002).
Moe, M. & Di Stefano, R. Mind your Ps and Qs: the interrelation between period (P) and mass-ratio (Q) distributions of binary stars. Astrophys. J. Suppl. 230, 15 (2017).
Provencal, J. L. et al. Whole Earth Telescope observations of the helium interacting binary PG 1346+082 (CR Bootis). Astrophys. J. 480, 383–394 (1997).
Ivanova, N., Heinke, C. O., Rasio, F. A., Belczynski, K. & Fregeau, J. M. Formation and evolution of compact binaries in globular clusters. II. Binaries with neutron stars. Mon. Not. R. Astron. Soc. 386, 553–576 (2008).
Lyutikov, M. & Toonen, S. Fast-rising blue optical transients and AT2018cow following electron-capture collapse of merged white dwarfs. Mon. Not. R. Astron. Soc. 487, 5618–5629 (2019).
Portail, M., Wegg, C., Gerhard, O. & Martinez-Valpuesta, I. Made-to-measure models of the Galactic box/peanut bulge: stellar and total mass in the bulge region. Mon. Not. R. Astron. Soc. 448, 713–731 (2015).
Maraston, C. Evolutionary synthesis of stellar populations: a modular tool. Mon. Not. R. Astron. Soc. 300, 872–892 (1998).
Gonthier, P. L. et al. Population syntheses of millisecond pulsars from the Galactic disk and bulge. Astrophys. J. 863, 199 (2018).
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.
The authors declare no competing interests.
Peer review information
Nature Astronomy thanks the anonymous reviewers 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.
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.
About this article
Cite this article
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
This article is cited by
Nature Astronomy (2022)