NICER follow-up observation and a candidate timing anomaly from Swift J1818.0-1607

We report on continuing NICER observations of the magnetar candidate Swift J1818.0-1607 (GCN circular #27373) designed to track the evolving X-ray timing and spectral behavior. Previously, coherent X-ray pulsations at F0 = 0.73 Hz were discovered using NICER (ATel #13551), followed by the discovery of pulsed radio emission (ATel #13553) allowing the frequency derivative to be measured as F1 = -4.39(1) x 10^-11 s^-2 with F0=0.7334110(2) at MJD 58922.31 (ATel #13559).
NICER has observed Swift J1818.0-1607 with a total exposure time of 21 ks between 2020 March 13 and 25. We corrected the photon arrival times to the barycenter of the Solar System using the Swift/XRT position reported in GCN circular #27373 and the JPL DE405 ephemeris. We confirmed a frequency derivative of F1 = -4.38(3) x 10^-11 s^-2 before March 20 (ATel #13559). We found that the phase residuals deviated from the best-fit model after March 20, possibly caused by a timing anomaly event. Fitting this with a glitch model, we obtain \Delta F0 = 2.8(2) x 10^-6 Hz (or \Delta F0/F0 = 3.8(3) x 10^-6) at MJD 58928.55 (March 20). The frequency derivative also changed to F1 = -3.70(8) x 10^-11 s^-2 (or \Delta F1/F1 = -0.15(2)).

The long-term average frequency derivative is F1 = -3.74(1) x 10^-11 s^-2. The characteristic spin-down age of Swift J1818.0-1607 can then be derived as 310 years by assuming a braking index of 3 and rapid spin at birth. This value is slightly larger than that reported in ATel #13559 but Swift J1818.0-1607 remains one of the youngest magnetars known. The inferred dipolar magnetic field is as high as 3 x 10^14 G and the spin-down luminosity is estimated as 1.1 x 10^36 erg s^-1. This spin-down luminosity is the largest among magnetars (ATel #13569) and close to that of the high magnetic field rotation-powered pulsar PSR J1119+6127 (dE/dt=-2.3 x 10^36 erg s^-1), which showed magnetar-like activity in 2016 (Gogus et al. 2016, Archibald et al. 2018).

We searched for short X-ray bursts by creating a light curve with a sampling rate of 256 Hz. Four candidate bursts are found to occur at MJD (TDB) 58921.2003463, 58927.7803163, 58929.0729996, and 58929.3775590 with detection significance higher than 5-sigma. The duration of these bursts is in the range of 20-50 ms.

The X-ray spectrum of Swift J1818.0-1607 obtained with NICER can be described with a strongly absorbed (NH = 6.4x10^22 cm^-2) blackbody with a temperature of ~1.1 keV within the 2-7 keV band. The unabsorbed X-ray flux in 2-7 keV is ~2.6 x 10^-11 erg s^-1 cm^-2. During the NICER observations between March 13 and 24, we did not observe a statistically significant variation in spectral parameters that would correlate to the observed timing anomaly or a flux decrease that would have indicated an overall cooling trend.

NICER is a 0.2-12 keV X-ray telescope operating on the International Space Station. The NICER mission and portions of the NICER science team activities are funded by NASA.