X-Ray and Near-IR Variability of the Anomalous X-Ray Pulsar 1E 1048.1-5937: From Quiescence Back to Activity

We report on new and archival X-ray and near-IR observations of the anomalous X-ray pulsar 1E 1048.1-5937 performed between 2001 and 2007 with the Rossi X-Ray Telescope Explorer (RXTE), the Chandra X-Ray Observatory, the Swift Gamma-Ray Burst Explorer, the Hubble Space Telescope (HST), and the Very Large Telescope. Monitoring with RXTE revealed that following its ~2001-2004 active period, 1E 1048.1-5937 entered a phase of timing stability; at the same time, simultaneous observations with Chandra and HST in 2006 showed that its X-ray and near-IR radiative properties, all variable prior to 2005, stabilized. Specifically, the 2006 X-ray spectrum is consistent with a two-component blackbody plus power law, with an average kt = 0.52 keV and Γ = 2.8, at a mean flux level of ~6.5 × 10^-12 erg cm^-2 s^-1 (2-10 keV). The near-IR counterpart in 2005-2006 was detected at H ~ 22.7 mag and K_s ~ 21.0 mag, considerably fainter than previously measured. In 2007 March, this newfound quiescence was interrupted by sudden X-ray flux, spectral, and pulse morphology changes, simultaneous with a large glitch and near-IR enhancement. Our RXTE observations revealed a factor of ~3 increase in pulsed flux (2-10 keV), while observations with Chandra and Swift saw the total X-ray flux increase much more than the pulsed flux, reaching a peak value of >7 times the quiescent value (2-10 keV). We find a strong anticorrelation between X-ray flux and pulsed fraction, and a correlation between X-ray spectral hardness and flux. Simultaneously with the radiative and timing changes, we observed the X-ray pulse profile change significantly from nearly sinusoidal to having multiple peaks. We compare these remarkable events with other magnetar outbursts and discuss implications in the context of AXP emission models.