A Radiatively Quiet Glitch and Anti-glitch in the Magnetar 1E 2259+586

We report on the timing and spectral properties of the soft X-ray emission from the magnetar 1E 2259+586 from 2013 January, ∼8 months after the detection of an anti-glitch, until 2019 September, using the Neil Gehrels Swift and Neutron star Interior Composition ExploreR (NICER) observatories. During this time span, we detect two timing discontinuities. The first, occurring around 5 yr after the 2012 April anti-glitch, is a relatively large spin-up glitch with a fractional amplitude ${\rm{\Delta }}\nu /\nu =1.24(2)\times {10}^{-6}$. We find no evidence for flux enhancement or change in the spectral or pulse-profile shape around the time of this glitch. This is consistent with the picture that a significant number of magnetar spin-up glitches are radiatively quiet. Approximately 1.5 yr later in 2019 April, 1E 2259+586 exhibited an anti-glitch with spin-down of a fractional amplitude ${\rm{\Delta }}\nu /\nu =-5.8(1)\times {10}^{-7}$, similar to the fractional change detected in 2012. We do not, however, detect any change to the pulse-profile shape or increase in the rms pulsed flux of the source, nor do we see any possible bursts from its direction around the time of the anti-glitch, all of which occurred during the 2012 event. Hence, similar to spin-up glitches, anti-glitches can occur silently. This may suggest that these phenomena originate in the neutron star interior, and that their locale and triggering mechanism do not necessarily have to be connected to the magnetosphere. Last, our observations suggest that the occurrence rate of spin-up and spin-down glitches is about the same in 1E 2259+586, with the former having a larger net fractional change.