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

Abstract

We report on new and archival X-ray and near-IR observations of the anomalous X-ray pulsar 1E 1048.1À5937 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 $2001Y2004 active period, 1E 1048.1À5937 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 ¼ 0:52 keV and À ¼ 2:8, at a mean flux level of $6:5 ; 10 À12 erg cm À2 s À1 (2Y10 keV ). The near-IR counterpart in 2005Y2006 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 (2Y10 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 (2Y10 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.

Electrodynamics of Magnetars. III. Pair Creation Processes in an Ultrastrong Magnetic Field and Particle Heating in a Dynamic Magnetosphere

“…The variations in the spin-down torque of SGRs and AXPs are remarkable in their amplitude and longevity; the spin-down rate is observed to increase smoothly over a period of months, by up to a factor $8 (Kaspi et al 2001;Woods et al 2002Woods et al , 2007Tam et al 2008;Camilo et al 2007bCamilo et al , 2008. The strength of the torque variations correlates roughly with the level of activity that the magnetar has sustained over the preceding years, but this correlation is only indirect.…”

Mapping the Surface of the Magnetar 1e 1048.1–5937 in Outburst and Quiescence Through Phase-Resolved X-Ray Spectroscopy

“…In 2007 March, 1E 1048.1−5937 showed a pulsed flux increase, which was followed by a spin-up glitch (Dib et al 2009). Subsequent observations with imaging X-ray telescopes revealed that the total flux increased by a factor of ≈7, while the pulsed fraction decreased significantly from ≈70% to ≈30% (Tam et al 2008). The observed anti-correlation between the total flux and the pulsed fraction was attributed to a growing hot spot on the neutron star surface (Tam et al 2008;seeÖzel & Güver 2007).…”

“…Tam et al (2008) reported a 2-10 keV rms pulsed fraction of ≈70% for 1E 1048.1−5937, using several Chandra observations throughout 2006 when the source showed no activity. In 2007 March, 1E 1048.1−5937 showed a pulsed flux increase, which was followed by a spin-up glitch (Dib et al 2009).…”

“…Finally, it became active again in 2007, showing a large spin-up glitch, which is a sudden increase in its rotation frequency (Dib et al 2009). X-ray observations with imaging instruments revealed that, while the total flux was seven times greater compared to the quiescent levels, the pulsed fraction decreased from ≈70% to ≈30% (Tam et al 2008). This anti-correlation between the total flux and the pulsed fraction was also reported earlier for this source (see, e.g., Tiengo et al 2005).…”

“…We also calculated it in the 2-10 keV band, where we can directly compare our results with those of Tam et al (2008) We list the pulsed fractions in all energy bands in Table 2. We find that the rms pulsed fraction increases with photon energy in both observations.…”