NuSTAR OBSERVATIONS of MAGNETAR 1E 1048.1-5937

We report on simultaneous Nuclear Spectroscopic Telescope Array (NuSTAR) and XMM-Newton observations of the magnetar 1E 1048.1-5937, along with Rossi X-ray Timing Explorer (RXTE) data for the same source. The NuSTAR data provide a clear detection of this magnetar's persistent emission up to 20 keV. We detect a previously unreported small secondary peak in the average pulse profile in the 7-10 keV band, which grows to an amplitude comparable to that of the main peak in the 10-20 keV band. We show using RXTE data that this secondary peak is likely transient. We find that the pulsed fraction increases with energy from a value of ∼0.55 at ∼2 keV to a value of ∼0.75 near 8 keV but shows evidence of decreasing at higher energies. After filtering out multiple bright X-ray bursts during the observation, we find that the phase-averaged spectrum from combined NuSTAR and XMM data is well described by an absorbed double blackbody plus power-law model, with no evidence for the spectral turn-up near ∼10 keV as has been seen in some other magnetars. Our data allow us to rule out a spectral turn-up similar to those seen in magnetars 4U 0142+61 and 1E 2259+586 of ΔΓ ≳ 2, where ΔΓ is the difference between the soft-band and hard-band photon indexes. The lack of spectral turn-up is consistent with what has been observed from an active subset of magnetars given previously reported trends suggesting that the degree of spectral turn-up is correlated with spin-down rate and/or spin-inferred magnetic field.