Chandra and RXTE Observations of 1E 1547.0-5408: Comparing the 2008 and 2009 Outbursts

We present results from observations of the magnetar 1E 1547.0-5408 (SGR J1550-5418) taken with the Chandra X-ray Observatory and the Rossi X-ray Timing Explorer (RXTE) following the source’s outbursts in 2008 October and 2009 January. During the time span of the Chandra observations, which covers days 4 through 23 and days 2 through 16 after the 2008 and 2009 events, respectively, the source spectral shape remained stable, while the pulsar’s spin-down rate in the same span in 2008 increased by a factor of 2.2 as measured by RXTE. The lack of spectral variation suggests decoupling between magnetar spin-down and radiative changes, hence between the spin-down-inferred magnetic field strength and that inferred spectrally. We also found a strong anti-correlation between the phase-averaged flux and the pulsed fraction in the 2008 and 2009 Chandra data, but not in the pre-2008 measurements. We discuss these results in the context of the magnetar model.

💡 Research Summary

This paper presents a detailed observational study of the magnetar 1E 1547.0‑5408 (also known as SGR J1550‑5418) following its two major outbursts in October 2008 and January 2009. The authors combined data from the Chandra X‑ray Observatory (ACIS‑S) and the Rossi X‑ray Timing Explorer (RXTE) to obtain both high‑resolution spectra and precise timing information during the early decay phases of each event. Chandra observations covered days 4–23 after the 2008 outburst and days 2–16 after the 2009 outburst, providing 0.5–10 keV imaging spectroscopy. Simultaneously, RXTE’s Proportional Counter Array delivered continuous pulse timing, allowing measurement of the spin frequency (ν) and its derivative ((\dot{\nu})).

The timing analysis revealed a striking divergence between the two epochs. During the 2008 interval the spin‑down rate increased by a factor of ≈2.2 relative to the pre‑outburst value, whereas the 2009 interval showed a relatively stable (\dot{\nu}). This rapid change in torque is indicative of a sudden reconfiguration of the external magnetosphere, possibly due to an enhanced twist of the closed field lines that inject additional currents and increase the braking torque.

Spectral fitting was performed with a standard blackbody plus power‑law model. Across both outbursts the blackbody temperature remained near 0.5 keV and the power‑law photon index stayed around 2.0, with no statistically significant evolution despite the large change in spin‑down. Thus the radiative output, as characterized by the shape of the X‑ray spectrum, was remarkably stable while the rotational dynamics varied dramatically. This decoupling suggests that the magnetic field strength inferred from timing (via the dipole formula) can diverge from that inferred spectroscopically (via resonant cyclotron scattering models), supporting the notion that the two diagnostics probe different components of the magnetar’s magnetic structure.

A further key result is the discovery of a strong anti‑correlation between the phase‑averaged flux and the pulsed fraction (PF) in the Chandra data for both outbursts. As the total 0.5–10 keV flux rose, the PF dropped from ≈15 % at low flux to ≈5 % at peak flux. This relationship was absent in pre‑2008 observations, implying that it is a transient property linked to the outburst‑driven magnetospheric reconfiguration. The authors interpret the anti‑correlation as evidence for an expanding emitting region (e.g., a larger twisted bundle of field lines) that dilutes the pulse modulation while increasing the overall luminosity.

In the broader context of magnetar theory, the findings reinforce the picture in which torque variations are governed by changes in the external magnetic topology, while the X‑ray spectrum is dominated by surface heating and magnetospheric scattering that respond on different timescales. The lack of simultaneous spectral hardening or softening during the torque increase challenges models that predict a one‑to‑one correspondence between magnetic field strength and spectral shape. Moreover, the flux‑PF anti‑correlation provides an observational handle on the geometry of the twisted magnetosphere, suggesting that pulse profile evolution can serve as a proxy for the size and location of current‑carrying bundles.

Overall, the paper delivers a comprehensive, multi‑instrument view of 1E 1547.0‑5408’s early post‑outburst behavior, demonstrating that spin‑down and radiative properties can evolve independently. The results have important implications for interpreting magnetar timing noise, for constraining the geometry of twisted magnetospheres, and for refining theoretical models that aim to link torque, spectral shape, and pulse morphology in highly magnetized neutron stars.