NICER / ISS Science Nugget
for October 15, 2020
Taking the Pulse of a Magnetar
On 10 October, 2020, the Burst Alert Telescope (BAT) aboard NASA's Swift observatory detected a short-duration burst in gamma rays from within the plane of our Milky Way Galaxy. Swift performed an automated maneuver to point its X-ray Telescope (XRT) in the direction of the BAT trigger and discovered a new magnetar candidate, designated SGR 1830-0645. Followup analysis of the XRT data indicated that the source had a likely pulsation period of about 10.4 seconds.
Within 5 hours of the initial burst, NICER carried out a Target of Opportunity observation and confirmed the pulse detection. With 20 times the XRT's collecting area, NICER provided a pulse profile (see figure) and spectral information resolved across pulse phase. The pulse period and X-ray spectrum reinforce the identification of SGR 1830-0645 with a magnetar, a highly magnetized neutron star with magnetic field strength trillions of times stronger than the Earth's. Magnetars can be characterized in terms of their pulse (spin) period as well as the rate at which the spin period increases with time as the star's rotation slows down. Pulse period and period time-derivative can be used to infer the magnetar's age as well as its magnetic field strength. NICER has continued to observe SGR 1830-0645 over the past week and has placed a limit on the spin-down rate so far; this implies an age > 4,800 years and a magnetic field strength of less than 6x10^14 Gauss. NICER observations are continuing; a measurement of the spin-down rate, and estimates of the age and field strength, are expected within a week or two.
NICER's initial results for SGR 1830-0645 were reported by Younes et al. in Astronomer's Telegram #14086.
Figure: The measured NICER pulse profile of SGR 1830-0645 shown over 2 of the 10.4 second periods. This pulse profile is determined by taking the entire NICER observation of the pulsar and folding it over the measured pulse period.