NICER Discovery of Millisecond X-Ray Pulsations and an Ultracompact Orbit in IGR J17494-3030

Mason Ng*, Paul S. Ray, Peter Bult, Deepto Chakrabarty, Gaurava K. Jaisawal, Christian Malacaria, Diego Altamirano, Zaven Arzoumanian, Keith C. Gendreau, Tolga Güver, Matthew Kerr, Tod E. Strohmayer, Zorawar Wadiasingh, Michael T. Wolff

*Corresponding author for this work

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    We report the detection of 376.05 Hz (2.66 ms) coherent X-ray pulsations in NICER observations of a transient outburst of the low-mass X-ray binary IGR J17494−3030 in 2020 October/November. The system is an accreting millisecond X-ray pulsar in a 75-minute ultracompact binary. The mass donor is most likely a 0.02 M finite-entropy white dwarf composed of He or C/O. The fractional rms pulsed amplitude is 7.4%, and the soft (1–3 keV) X-ray pulse profile contains a significant second harmonic. The pulsed amplitude and pulse phase lag (relative to our mean timing model) are energy dependent, each having a local maximum at 4 and 1.5 keV, respectively. We also recovered the X-ray pulsations in archival 2012 XMM-Newton observations, allowing us to measure a long-term pulsar spin-down rate of Hz s−1 and to infer a pulsar surface dipole magnetic field strength of 109 G. We show that the mass transfer in the binary is likely nonconservative, and we discuss various scenarios for mass loss from the system.
    Original languageEnglish
    Article numberL15
    JournalAstrophysical Journal Letters
    Issue number1
    Number of pages7
    Publication statusPublished - 2021


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