NICER / ISS Science Nugget
for September 3, 2020
NICER Observations of Swift J1818.0-1607: a Missing Link between Magnetars and Rotation-Powered Pulsars
This week, a paper by Hu et al. was accepted for publication in The Astrophysical Journal, describing 100 days of NICER monitoring of the newly discovered magnetar Swift J1818.0Ð1607. A magnetar is a highly magnetized neutron star with magnetic field strength in excess of 10 billion times that of the Earth - the strongest known in the universe. These powerful fields play a role in releasing tremendous amounts of energy in the form of sudden X-ray flares. NICER's observations were triggered by the detection of such a flare from the Burst Alert Telescope (BAT) onboard NASA's Swift observatory. An X-ray-bright hot spot on the surface of Swift J1818.0 allowed NICER to measure the magnetar's rotation as it evolved, both gradually and suddenly (see Figure 1).
NICER found that the star's spin was noisy, with an overall spin-down rate that suggests this magnetar is young, with likely age less than 500 years. NICER detected sudden spin "glitches", both increasing and later decreasing the rotation rate, with no accompanying X-ray flaring. Over time, the surface hot spot was also observed to shrink in size, with the overall X-ray brightness falling to below the threshold where it can be attributed to the star's rotational kinetic energy loss. Traditionally, this is the distinction between magnetically-powered neutron stars (magnetars) and those powered by their rotation; Swift J1818 appears to bridge this divide, and thus provides a clue that these classes of objects may be related - e.g., by evolution in time from magnetars to canonical pulsars.
Figure: (Left) Evolution of the timing properties of Swift J1818, as measured by NICER. The upper panel shows spin frequency, with red lines indicating the constant spin-down timing solutions for three segments. Measured spin-down rate at each epoch is shown in the middle panel, and phase residuals in the bottom panel. Blue dashed vertical lines indicate the boundaries of four segments, with the first two being "glitch" timing discontinuities. Green dotted lines denote the times of short X-ray bursts observed by NICER. (Right) Pulse profile of Swift J1818 in multiple energy bands. The bottom panel shows the energy dependence of the pulsed fraction (i.e., the depth of modulation).