NICER / ISS Science Nugget
for January 14, 2021

NICER Tracks Giant Outburst of X-ray Binary 1A 0535+262 to Completion

NICER began observations of the high-mass X-ray binary (HMXB) 1A 0535+262 in early November 2020, following a notification of a new outburst from NASA's Swift observatory. A0535 is a neutron star in a 110-day orbit with a "Be"-type star; the highly eccentric orbit makes it prone to regular outbursts that vary in brightness and duration. The neutron star spins once around every 103 seconds. NICER has continuously tracked the source as it evolved through its brightest outburst since at least 2009 and likely in the 45 years since its discovery. The only gap in NICER's coverage was during the ISS Circuit 4B power anomaly, when the outburst was in decline. Now, after two months, it appears the outburst is over (see figure). NICER is continuing to track A0535 into quiescence, to track the evolving spin period of the neutron star as a function of mass-transfer rate, and to search for a possible transition to the "propeller" phase. During the outburst, a high rate of mass transfer from the companion star overcomes the neutron star's magnetic field, resulting in accretion onto the latter's surface and bright X-ray emission. The propeller effect takes over when the mass transfer rate drops and accretion may be inhibited by the neutron star's strong magnetic field.