Discovery of 105 Hz coherent pulsations in the ultracompact binary IGR J16597-3704

A. Sanna, A. Bahramian, E. Bozzo, C. Heinke, D. Altamirano, R. Wijnands, N. Degenaar, T. Maccarone, A. Riggio, T. Di Salvo, R. Iaria, M. Burgay, A. Possenti, C. Ferrigno, A. Papitto, G. R. Sivakoff, N. D'Amico, L. Burderi

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We report the discovery of X-ray pulsations at 105.2 Hz (9.5 ms) from the transient X-ray binary IGR J16597-3704 using NuSTAR and Swift. The source was discovered by INTEGRAL in the globular cluster NGC 6256 at a distance of 9.1 kpc. The X-ray pulsations show a clear Doppler modulation that implies an orbital period of ∼46 min and a projected semi-major axis of ∼5 lt-ms, which makes IGR J16597-3704 an ultracompact X-ray binary system. We estimated a minimum companion mass of 6.5 × 10-10 M, assuming a neutron star mass of 1.4 M, and an inclination angle of <75° (suggested by the absence of eclipses or dips in its light curve). The broad-band energy spectrum of the source is well described by a disk blackbody component (kT ∼ 1.4 keV) plus a comptonised power-law with photon index ∼2.3 and an electron temperature of ∼30 keV. Radio pulsations from the source were unsuccessfully searched for with the Parkes Observatory.

Original languageEnglish
Article numberL2
JournalAstronomy and Astrophysics
StatePublished - Feb 1 2018


  • Accretion, accretion disks
  • Binaries: general
  • Stars: neutron
  • X-rays: binaries


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