NICER / ISS Science Nugget
for April 13, 2023

An old friend reappears

In October 2017, just three months after the start of NICER's science operations, NASA's Swift observatory discovered a new X-ray transient that, at the peak of a giant outburst lasting more than 150 days, briefly became one of the brightest sources of X-rays in the sky. NICER tracked the 2017 outburst of Swift J0243.6+6124, a binary system consisting of a neutron star that accretes matter from a massive companion star. The NICER campaign measured some of the system's key properties, such as the neutron star's spin period (9.8 seconds) and the binary orbital period (28 days). It also provided unprecedented evidence for a transition in the accretion flow close to the neutron star's surface, at a "critical luminosity" that was seen both during the brightening and then again during the fading of Swift J0243. The exceptional nature of this outburst, in which the luminosity of Swift J0243 grew a thousand-fold, placed this system in the rare category of "ultraluminous X-ray pulsars," of which only a handful are known - Swift J0243 was the first, and remains the only, example known in our home Galaxy.

On 8 April, 2023, JAXA's MAXI payload reported rapid brightening of X-ray emission from the vicinity of Swift J0243. Another highly variable X-ray emitter lies less than half a degree away, and MAXI could not distinguish which source was responsible for the occurrence. Swift captured an image of the region, and reported a 7-fold increase in the brightness of a source positionally consistent with Swift J0243. At the same time, NICER observed both Swift J0243 and its variable neighbor separately, finding not only that Swift J0243 had brightened, but also detecting its characteristic 9.8-second X-ray periodicity (see Figure), unambiguously confirming that Swift J0243 has reawakened in a new outburst. This result was reported on April 12 by Mason Ng (MIT) and collaborators in Astronomer's Telegram #15987. Early indications are that the current outburst is more modest than the one in 2017, but NICER continues to monitor its evolution.