NICER / ISS Science Nugget for April 12, 2018

NICER discovers a new pulsar in IGR J17379-3747

Following a 2018 March 19 alert from the ISS payload MAXI of a new outburst from the neutron star low-mass X-ray binary IGR J17379-3747, NICER has observed the source daily since 2018 March 29. From that date onward, the mean count rates detected each day through April 1 were 12.9, 11.0, 8.7, and 4.7 ct/s (0.5–12 keV), respectively. NICER detected a clear pulsation (> 7 sigma significance for a single trial) at a frequency of 468.05 Hz, making this NICER's first discovery of a previously unknown pulsar.

Furthermore, NICER looked at variations in the detected pulse frequency with time to determine that this pulsar is in a binary with an orbital period of 1.88 hrs and a minimum companion mass of 0.055 M_sun (assuming a 1.4 M_sun neutron star). Correcting for the binary orbit, the pulse profile is shown in the attached figure. The NICER pulsation detection conclusively identifies IGR J17379-3747 as an accreting millisecond X-ray pulsar.

NICER monitoring of the source continues. NICER reported these results in its 6th Astronomer's Telegram: ATEL #11507.