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.
@article{arxiv.2001.06450,
title = {Two new outbursts and transient hard X-rays from 1E 1048.1$-$5937},
author = {R. F. Archibald and P. Scholz and V. M. Kaspi and S. P. Tendulkar and A. P. Beardmore},
journal= {arXiv preprint arXiv:2001.06450},
year = {2020}
}