An ultraluminous X-ray source powered by an accreting neutron star

Abstract

The majority of ultraluminous X-ray sources are point sources that are spatially offset from the nuclei of nearby galaxies and whose X-ray luminosities exceed the theoretical maximum for spherical infall (the Eddington limit) onto stellar-mass black holes1,2. Their X-ray luminosities in the 0.5–10 kiloelectronvolt energy band range from 1039 to 1041 ergs per second3. Because higher masses imply less extreme ratios of the luminosity to the isotropic Eddington limit, theoretical models have focused on black hole rather than neutron star systems1,2. The most challenging sources to explain are those at the luminous end of the range (more than 1040 ergs per second), which require black hole masses of 50–100 times the solar value or significant departures from the standard thin disk accretion that powers bright Galactic X-ray binaries, or both. Here we report broadband X-ray observations of the nuclear region of the galaxy M82 that reveal pulsations with an average period of 1.37 seconds and a 2.5-day sinusoidal modulation. The pulsations result from the rotation of a magnetized neutron star, and the modulation arises from its binary orbit. The pulsed flux alone corresponds to an X-ray luminosity in the 3–30 kiloelectronvolt range of 4.9 × 1039 ergs per second. The pulsating source is spatially coincident with a variable source4 that can reach an X-ray luminosity in the 0.3–10 kiloelectronvolt range of 1.8 × 1040 ergs per second1. This association implies a luminosity of about 100 times the Eddington limit for a 1.4-solar-mass object, or more than ten times brighter than any known accreting pulsar. This implies that neutron stars may not be rare in the ultraluminous X-ray population, and it challenges physical models for the accretion of matter onto magnetized compact objects.

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Figure 1: The X-ray light curve and pulsations from the region containing NuSTAR J095551+6940.8.
Figure 2: The spin-up behaviour of NuSTAR J095551+6940.8.
Figure 3: The counterpart of NuSTAR J095551+6940.8.

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Acknowledgements

This work was supported by NASA (grant no. NNG08FD60C), and made use of data from the Nuclear Spectroscopic Telescope Array (NuSTAR) mission, a project led by Caltech, managed by the Jet Propulsion Laboratory and funded by NASA. We thank the NuSTAR operations, software and calibration teams for support with execution and analysis of these observations. This work made use of data supplied by the UK Swift Science Data Centre at the University of Leicester. M.B. thanks the Centre National d’Études Spatiales (CNES) and the Centre National de la Recherche Scientifique (CNRS) for support. Line plots were done using Veusz software by J. Sanders.

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Contributions

M.B., reduction and timing analysis of the NuSTAR observations, interpretation of results, manuscript preparation; F.A.H., interpretation of results, manuscript preparation; D.J.W., NuSTAR and Chandra spectroscopy, point source analysis; B.W.G., NuSTAR image analysis; D.C., accretion torque analysis, interpretation; F.F., verification of timing analysis, interpretation; D.B., A.B., A.C.F., A.H., V.M.K., T.M., J.T., interpretation of results and manuscript review; S.B., F.C., W.W.C., C.J.H., D.S., S.P.T., N.W., W.W.Z., manuscript review.

Corresponding authors

Correspondence to M. Bachetti or F. A. Harrison.

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The authors declare no competing financial interests.

Extended data figures and tables

Extended Data Figure 1 X-ray sources identified by Chandra in the central region of M82.

a, Chandra image of the central region of M82 from the observation taken coincident with NuSTAR ObsID 006. The yellow circle shows the 70″ radius region used to extract NuSTAR fluxes. Within this region, 24 discrete X-ray point sources are identified, including X-1 and X-2. b, Expanded view of the crowded central region of a. Yellow crosses indicate the locations of identified point sources. We have used, where possible, the numbering from ref. 13 (sources up to no. 15). After this we assign our own numerical identification (note that sources 4 and 10 from ref. 13 are not detected in this observation).

Extended Data Figure 2 Swift imaging of the region containing M82 X-1 and M82 X-2.

a, b, In greyscale are images obtained via Swift automated processing over the 5–10 keV band for all of the observations during early February (a; 2014 February 04 through 2014 February 11) and mid-March (b; 2014 March 7 through 2014 March 11). The early February observations have 68.5 ks of exposure (mostly because of the increased cadence of observations due to the Swift monitoring of SN 2014J), while the mid-March snapshot has 1.8 ks of exposure. The images are 1.5 arcmin on a side and have been smoothed with a 2-pixel (4 arcsecond) Gaussian kernel (circle at lower left). The location of X-1 and X-2 are shown by the crosses in both panels. The late-time observation clearly shows a reduction in the flux from X-1 and that the flux is dominated by X-2.

Extended Data Table 1 List of NuSTAR observations used in this analysis
Extended Data Table 2 Best-fit orbital parameters
Extended Data Table 3 Best fit period and period derivatives for individual NuSTAR observations
Extended Data Table 4 3–10 keV flux contributions for bright sources near the nucleus of M82

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Bachetti, M., Harrison, F., Walton, D. et al. An ultraluminous X-ray source powered by an accreting neutron star. Nature 514, 202–204 (2014). https://doi.org/10.1038/nature13791

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