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

• Published in: Nature (London) Vol. 514; no. 7521; pp. 202 - 204
• Main Authors: Bachetti, M., Harrison, F. A., Walton, D. J., Grefenstette, B. W., Chakrabarty, D., Fürst, F., Barret, D., Beloborodov, A., Boggs, S. E., Christensen, F. E., Craig, W. W., Fabian, A. C., Hailey, C. J., Hornschemeier, A., Kaspi, V., Kulkarni, S. R., Maccarone, T., Miller, J. M., Rana, V., Stern, D., Tendulkar, S. P., Tomsick, J., Webb, N. A., Zhang, W. W.
• Format: Journal Article
• Language: English
• Published: London Nature Publishing Group UK 09.10.2014; Nature Publishing Group
• Subjects: 639/33/34/864; Accretion; Galactic center; Galaxies; Humanities and Social Sciences; letter; Magnetic fields; multidisciplinary; Neutron stars; Neutrons; Observations; Pulsars; Science; Stars; X-rays
• ISSN: 0028-0836; 1476-4687
• DOI: 10.1038/nature13791

X-ray pulsations with an average period of 1.37 seconds have been detected from a known ultraluminous X-ray source hitherto thought to be a black hole; the pulsations instead unequivocally identify the source as an accreting magnetized neutron star ten times brighter than any previously known. What drives ultraluminous X-ray sources? Ultraluminous X-ray sources (ULXs) are non-nuclear point sources that are widely believed to contain either intermediate mass black holes or smaller, stellar mass black holes accreting from a binary companion. The study of ULXs provides information about black hole formation and/or modes of high Eddington rate accretion. Two papers in this issue of Nature describe pulsating ULXs with unusual properties. Christian Motch et al . find that source P13 in the galaxy NGC 7793 is in a ∼64 day period binary system. By modelling the strong optical and UV modulations arising from X-ray heating of the B9Ia donor star, they constrain the black hole mass to be less than 15 solar masses. Matteo Bachetti et al . observe a source in the galaxy M82 that, the pulsation data imply, harbours a neutron star rather than a black hole, raising doubts over the assumption that black holes power the most luminous X-ray binaries. 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 holes 1 , 2 . Their X-ray luminosities in the 0.5–10 kiloelectronvolt energy band range from 10 39 to 10 41  ergs per second 3 . 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 systems 1 , 2 . The most challenging sources to explain are those at the luminous end of the range (more than 10 40  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 × 10 39  ergs per second. The pulsating source is spatially coincident with a variable source 4 that can reach an X-ray luminosity in the 0.3–10 kiloelectronvolt range of 1.8 × 10 40  ergs per second 1 . 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.