2020ApJ...889..160A

Since its discovery, 1E 1048.1-5937 has been one of the most active magnetars, both in terms of radiative outbursts, and changes to its spin properties. Here we report on a continuing monitoring campaign with the Neil Gehrels Swift Observatory X-ray Telescope in which we observe two new outbursts from this source. The first outburst occurred in 2016 July, and the second in 2017 December, reaching peak 0.5-10 keV absorbed fluxes of 3.2_–0.3^+0.2×10^–11 erg s^–1 cm^–2 and 2.2_–0.2^+0.2×10^–11 erg s^–1 cm^–2, respectively, factors of ∼5 and ∼4 above the quiescent flux. Both new outbursts were accompanied by spin-up glitches with amplitudes of Δν=4.47(6)×10^–7 Hz and Δν=4.32(5)×10^–7 Hz, respectively. Following the 2016 July outburst, we observe, as for past outbursts, a period of delayed torque fluctuations, which reach a peak spin-down of 1.73 ± 0.01 times the quiescent rate, and which dominates the spin evolution compared to the spin-up glitches. We also report an observation near the peak of the first of these outbursts with NuSTAR in which hard X-ray emission is detected from the source. This emission is well characterized by an absorbed blackbody plus a broken power law, with a power-law index above 13.4 ± 0.6 keV of 0.5_–0.2^+0.3, similar to those observed in both persistent and transient magnetars. The hard X-ray results are broadly consistent with models of electron/positron cooling in twisted magnetic field bundles in the outer magnetosphere. However, the repeated outbursts and associated torque fluctuations in this source remain puzzling.