Suzaku Discovery of a Hard X-Ray Tail in the Persistent Spectra from the Magnetar 1E 1547.0-5408 during its 2009 Activity

The fastest-rotating magnetar 1E 1547.0-5408 was observed in broad-band X-rays with Suzaku for 33 ks on 2009 January 28-29, 7 days after the onset of its latest bursting activity. After removing burst events, the absorption-uncorrected 2-10 keV flux of the persistent emission was measured with the XIS as 5.7e-11 ergs cm-2 s-1, which is 1-2 orders of magnitude higher than was measured in 2006 and 2007 when the source was less active. The persistent emission was also detected significantly with the HXD in >10 keV up to at least ~110 keV, with an even higher flux of 1.3e-10 ergs cm-2 s-1 in 20-100 keV. The pulsation was detected at least up to 70 keV at a period of 2.072135+/-0.00005 s, with a deeper modulation than was measured in a fainter state. The phase-averaged 0.7-114 keV spectrum was reproduced by an absorbed blackbody emission with a temperature of 0.65+/-0.02 keV, plus a hard power-law with a photon index of ~1.5. At a distance of 9 kpc, the bolometric luminosity of the blackbody and the 2-100 keV luminosity of the hard power-law are estimated as (6.2+/-1.2)e+35 ergs s-1 and 1.9e+36 ergs s-1, respectively, while the blackbody radius becomes ~5 km. Although the source had not been detected significantly in hard X-rays during the past fainter states, a comparison of the present and past spectra in energies below 10 keV suggests that the hard component is more enhanced than the soft X-ray component during the persistent activity.

💡 Research Summary

The paper reports on a broadband X‑ray observation of the fastest‑rotating magnetar 1E 1547.0‑5408 performed with Suzaku on 2009 January 28–29, just seven days after the source entered a new bursting episode. After excising all burst intervals, the authors measured a persistent 2–10 keV flux of 5.7 × 10⁻¹¹ erg cm⁻² s⁻¹ with the XIS, which is one to two orders of magnitude higher than the flux recorded during the quiescent observations in 2006 and 2007. The hard X‑ray detector (HXD) detected significant emission above 10 keV, extending up to at least ∼110 keV, with a 20–100 keV flux of 1.3 × 10⁻¹⁰ erg cm⁻² s⁻¹.

Timing analysis revealed a coherent pulsation at a period of 2.072135 ± 0.00005 s, detectable up to ∼70 keV. The pulse fraction in the hard band is markedly larger than that measured during the source’s fainter state, indicating that the hard component is more strongly modulated.

The phase‑averaged spectrum from 0.7 to 114 keV is well described by an absorbed blackbody plus a hard power‑law. The blackbody temperature is 0.65 ± 0.02 keV, corresponding to an emitting radius of ≈5 km (assuming a distance of 9 kpc). The power‑law photon index is ≈1.5, significantly harder than the soft component. The bolometric luminosity of the blackbody is (6.2 ± 1.2) × 10³⁵ erg s⁻¹, while the 2–100 keV luminosity of the hard power‑law is ≈1.9 × 10³⁶ erg s⁻¹, i.e., the hard tail dominates the total X‑ray output.

Comparing these results with earlier, lower‑flux observations shows that while the soft X‑ray (≤10 keV) spectrum changes only modestly, the hard component is amplified far more strongly during the active phase. This behavior supports models in which a twisted magnetosphere, enhanced magnetospheric currents, and resonant cyclotron scattering (RCS) produce a hard X‑ray tail that grows disproportionately with magnetar activity. The increased twist raises the density of relativistic electrons, boosting up‑scattering of thermal photons and generating a hard power‑law with a relatively flat photon index. The larger pulse fraction at high energies is naturally explained by a more localized, anisotropic emission region in the magnetosphere.

The authors discuss the implications for magnetar emission mechanisms. The observed luminosities (∼10³⁶ erg s⁻¹) exceed the typical quiescent magnetar output (10³⁴–10³⁵ erg s⁻¹) and suggest a rapid release of magnetic energy stored in the internal field. The detection of a hard tail up to ∼110 keV demonstrates that magnetars can be significant contributors to the Galactic hard X‑ray background during outbursts.

In summary, the Suzaku observation provides the first clear detection of a hard X‑ray tail in the persistent emission of 1E 1547.0‑5408 during an active episode, quantifies its spectral and timing properties, and reinforces the view that magnetar hard X‑ray emission is tightly linked to magnetospheric reconfiguration and enhanced particle acceleration. This work thus offers a crucial observational benchmark for theoretical models of magnetar outbursts and their high‑energy phenomenology.