NICER Discovers the Ultracompact Orbit of the Accreting Millisecond Pulsar IGR J17062–6143

T. E. Strohmayer, Z. Arzoumanian, S. Bogdanov, P. M. Bult, D. Chakrabarty, T. Enoto, K. C. Gendreau, S. Guillot, A. K. Harding, W. C. G. Ho, J. Homan, G. K. Jaisawal, L. Keek, M. Kerr, S. Mahmoodifar, C. B. Markwardt, S. M. Ransom, P. S. Ray, R. Remillard, M. T. Wolff

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We present results of recent Neutron Star Interior Composition Explorer (NICER) observations of the accreting millisecond X-ray pulsar (AMXP) IGR J17062−6143 that show that it resides in a circular, ultracompact binary with a 38-minute orbital period. NICER observed the source for ≈26 ks over a 5.3-day span in 2017 August, and again for 14 and 11 ks in 2017 October and November, respectively. A power spectral analysis of the August exposure confirms the previous detection of pulsations at 163.656 Hz in Rossi X-ray Timing Explorer (RXTE) data, and reveals phase modulation due to orbital motion of the neutron star. A coherent search for the orbital solution using the Z2 method finds a best-fitting circular orbit with a period of 2278.21 s (37.97 minutes), a projected semimajor axis of 0.00390 lt-s, and a barycentric pulsar frequency of 163.6561105 Hz. This is currently the shortest known orbital period for an AMXP. The mass function is 9.12 × 10−8 M⊙, presently the smallest known for a stellar binary. The minimum donor mass ranges from ≈0.005 to 0.007 M⊙ for a neutron star mass from 1.2 to 2 M⊙. Assuming mass transfer is driven by gravitational radiation, we find donor mass and binary inclination bounds of 0.0175–0.0155 M⊙ and 19° 
Original languageEnglish
Article numberL13
JournalThe Astrophysical Journal Letters
Issue number2
Number of pages7
Publication statusPublished - 2018

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Strohmayer, T. E., Arzoumanian, Z., Bogdanov, S., Bult, P. M., Chakrabarty, D., Enoto, T., Gendreau, K. C., Guillot, S., Harding, A. K., Ho, W. C. G., Homan, J., Jaisawal, G. K., Keek, L., Kerr, M., Mahmoodifar, S., Markwardt, C. B., Ransom, S. M., Ray, P. S., Remillard, R., & Wolff, M. T. (2018). NICER Discovers the Ultracompact Orbit of the Accreting Millisecond Pulsar IGR J17062–6143. The Astrophysical Journal Letters, 858(2), [L13].