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

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  • 13 July 2007
Abstract
(Abridged) Monitoring of the anomalous X-ray pulsar 1E 1048.1-5937 in 2005-2006 with the RXTE, CXO, and HST has revealed that the source entered a phase of X-ray and near-IR radiative quiescence, simultaneous with timing stability. During its ~2001-2004 active period, the source exhibited two large, long-term X-ray pulsed-flux flares as well as short bursts, and large (>10x) torque changes. A series of four simultaneous observations with CXO and HST approximately equispaced in 2006 showed that its X-ray flux and spectrum and near-IR flux, both variable prior to 2005, stabilized. The near-IR flux (m_{F110W} > 24.8 mag, m_{F160W} ~ 22.70 mag) is considerably fainter in 2006 than previously measured. Recently, in 2007 March, this newfound quiescence was interrupted by a sudden flux enhancement, spectral changes and a pulse morphology change, simultaneous with a large spin-up glitch and near-IR enhancement. Specifically, our RXTE observations revealed a sudden pulsed flux increase by a factor of ~3 in the 2-10 keV band. In Target of Opportunity 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). The total and pulsed flux are slowly decaying with time but the source continues, in 2007 July, to be much brighter than in quiescence. 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.

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