We report on new and archival X-ray and near-infrared ( near-IR ) observations of the anomalous X-ray pulsar 1E 1048.1 - 5937 performed between 2001-2007 with the Rossi X-ray Timing Explorer ( RXTE ) , the Chandra X-ray Observatory ( CXO ) , the Swift Gamma-ray Burst Explorer , the Hubble Space Telescope ( HST ) , and the Very Large Telescope .
During its \sim 2001-2004 active period , 1E 1048.1 - 5937 exhibited two large , long-term X-ray pulsed-flux flares as well as short bursts , and large ( > 10 \times ) torque changes .
Monitoring with RXTE revealed that the source entered a phase of timing stability in 2004 ; at the same time , a series of four simultaneous observations with CXO and HST in 2006 showed that its X-ray flux and spectrum and near-IR flux , all variable prior to 2005 , stabilized .
Specifically , we find the 2006 X-ray spectrum to be consistent with a two-component blackbody plus power law , with average kT = 0.52 keV and power-law index \Gamma = 2.8 at a mean flux level in the 2 - 10 keV range of \sim 6.5 \times 10 ^ { -12 } erg cm ^ { -2 } s ^ { -1 } .
The near-IR flux , when detected by HST ( H \sim 22.7 mag ) and VLT ( K _ { S } \sim 21.0 mag ) , was considerably fainter than previously measured .
Recently , in 2007 March , this newfound quiescence was interrupted by a sudden flux enhancement , X-ray spectral changes and a pulse morphology change , simultaneous with a large spin-up glitch and near-IR enhancement .
Our RXTE observations revealed a sudden pulsed flux increase by a factor of \sim 3 in the 2 - 10 keV band .
In observations with CXO and Swift , we found that the total X-ray flux increased much more than the pulsed flux , reaching a peak value of > 7 times the quiescent value ( 2 - 10 keV ) .
With these recent data , we find a strong anti-correlation between X-ray flux and pulsed fraction .
In addition , we find a correlation between X-ray spectral hardness and flux .
Simultaneously with the radiative and timing changes , we observed a significant X-ray pulse morphology change such that the profile went from nearly sinusoidal to having multiple peaks .
We compare these remarkable events with other AXP outbursts and discuss implications in the context of the magnetar model and other models of AXP emission .