NICER detection of 376 Hz X-ray pulsations from IGR J17494-3030

Referred to by ATel #: 14126 Tweet In response to the recent report of renewed activity from the faint X-ray transient IGR J17494-3030 in INTEGRAL data (ATel #14119), NICER began monitoring observations of this source. We collected 4.1 ks of data between 2020 October 27 00:38 and 17:51 UTC, finding that the source is detected with an initial 1-10 keV count rate of about 16 c/s (including a background of < 1 c/s), with a slow decline to about 14 c/s.

We detect highly significant coherent pulsations with a barycentric frequency of 376.05 Hz (2.66 ms period). The pulse frequency is modulated by a circular orbit having a period of 4509 s (75 minutes), and a projected semi-major axis of 0.0164 lt-s. Hence, this system is an ultracompact binary accreting millisecond X-ray pulsar with a minimum companion mass of 0.015 Msun.

We find that the pulse profile has a 10% fractional sinusoidal amplitude in the 1-10 keV energy range, with a 2% second harmonic. We extracted the energy spectrum in the 0.5-10 keV range, finding that the continuum emission is well described by an absorbed power-law model. A preliminary fit provides a photon index of 1.84+/-0.04 and an absorption column density of (2.05+/-0.05)x1022 cm-2.

The 0.5-10 keV unabsorbed flux is 1.68x10-10 erg cm-2 s-1. The NICER team is continuing the monitoring and analysis of this source. NICER is a 0.2-12 keV X-ray telescope operating on the International Space Station. The NICER mission and portions of the NICER science team activities are funded by NASA.