NICER / ISS Science Nugget
for March 26, 2020

A new magnetar, Swift J1818.0-1607, offers clues to neutron star evolution

At 21:16:47 UT on March 12, a gamma-ray burst from the Galactic plane was detected with the Burst Alert Telescope (BAT) onboard the Neil Gehrels Swift Observatory, which was originally designed to observe gamma-ray bursts from cosmological distances. Spectral and timing behaviors of this event were different from those of cosmological gamma-ray bursts, and more similar to short bursts emitted from magnetars, which are a subclass of neutron stars exhibiting the strongest known magnetic fields.

Four hours after the first alert from the Swift team (GCN circular #27373), NICER started follow-up observations of this target. In the first data downlinked from ISS, the NICER team discovered that this source pulsates with a period of 1.36 sec (Figure 1). This period is similar to but shorter than the rotation periods of most magnetars, which are in the range 2-11 seconds. This pulse detection, additional evidence of a new magnetar, was reported to the astronomical community (Astronomer's Telegram #13551).

This new magnetar, Swift J1818.0-1607, is becoming important for astronomers to study the diversity of neutron stars because its characteristics draw from both magnetars and rotation-powered pulsars, which had been historically recognized as different subclasses of neutron stars. Subsequent NICER monitoring between March 13 and 24 revealed that the neutron star's rotation is gradually slowing down (Astronomer's Telegram #13588).

Using the stellar rotation period and its time-derivative, astronomers can derive approximate values for the star's magnetic field strength, age, and rotational energy loss (spin-down luminosity). The magnetic field of Swift J1818.0-1607 is typical of known magnetars (6.1x10¹⁰ Tesla), whereas the estimated age was revealed to be very young (of order 300 years!). The spin-down luminosity is the highest among magnetars, and rather close to those of young rotation-powered pulsars. Thus, Swift J1818.0-1607 appears to lie between the two categories (Figure 2).

This conclusion was further supported by multi-wavelength follow-up. NICER's discovery of the 1.36-sec pulsation encouraged monitoring campaigns not only by X-ray satellites, but also by many ground-based radio telescopes, which have reported detections of radio pulses from Swift J1818.0-1607. Most magnetars are radio-quiet, while rotation-powered pulsars usually show radio pulses. The radio detection again suggests a combination of properties of magnetars and rotation-powered pulsars.

The NICER team continues its monitoring campaign and is coordinating simultaneous coverage with radio observatories. During the NICER observations, several short bursts were observed. Some of them may be associated with a timing anomaly in the pulsar spin evolution, namely a pulsar glitch (Figure 3). During the bursting activity, the star's magnetosphere might undergo a rearrangement of its magnetic field, in turn provoking a change in torque on the star and thus on its rotation (Astronomer's Telegram #13588).

The NICER team will keep a close eye on this mysterious new Galactic source.