A low-magnetic-field Soft Gamma Repeater

Soft gamma repeaters and anomalous x-ray pulsars form a rapidly increasing group of x-ray sources exhibiting sporadic emission of short bursts. They are believed to be magnetars, i.e. neutron stars powered by extreme magnetic fields, B~10^{14}-10^{15} Gauss. We report on a soft gamma repeater with low magnetic field, SGR 0418+5729, recently detected after it emitted bursts similar to those of magnetars. X-ray observations show that its dipolar magnetic field cannot be greater than 7.5x10^{12} Gauss, well in the range of ordinary radio pulsars, implying that a high surface dipolar magnetic field is not necessarily required for magnetar-like activity. The magnetar population may thus include objects with a wider range of B-field strengths, ages and evolutionary stages than observed so far.

💡 Research Summary

The paper reports the discovery and comprehensive analysis of SGR 0418+5729, a soft gamma repeater (SGR) that exhibits magnetar‑like bursting activity despite possessing a dipolar magnetic field far weaker than that of typical magnetars. The source was first identified in June 2009 when the Swift Burst Alert Telescope recorded a short, hard X‑ray burst with a duration of roughly 0.1 seconds, a hallmark of SGRs. Prompt follow‑up observations with Swift X‑Ray Telescope (XRT), Chandra, XMM‑Newton, and other facilities tracked the source over several years, allowing precise timing and spectral measurements.

Timing analysis revealed a spin period of 9.078 seconds. However, the period derivative ((\dot{P})) was not significantly detected; only an upper limit of (\dot{P}<6\times10^{-15}) s s⁻¹ could be established. Using the standard magnetic dipole formula (B\approx3.2\times10^{19}\sqrt{P\dot{P}}) G, this translates into an upper bound on the surface dipole field of (B_{\rm dip}<7.5\times10^{12}) G. This value lies squarely within the range of ordinary radio pulsars and is two orders of magnitude below the canonical magnetar field strength of (10^{14})–(10^{15}) G.

Spectral modeling of the X‑ray emission showed that the early post‑burst spectrum required both a thermal blackbody component (kT≈0.9 keV) and a non‑thermal power‑law tail (photon index Γ≈2.5). As the source faded over months, the power‑law component weakened while the thermal component dominated, indicating cooling of the neutron‑star surface and a gradual decline of magnetospheric activity. The X‑ray luminosity dropped from ∼10³⁵ erg s⁻¹ at outburst onset to ∼10³³ erg s⁻¹ after several months, a decay pattern reminiscent of other magnetars but at a lower absolute level.

The authors discuss several theoretical implications. First, a low dipole field does not preclude magnetar‑like bursts; internal magnetic fields (toroidal or higher‑order multipoles) could remain extremely strong (≥10¹⁴ G) and drive crustal fractures. Second, the presence of a strong, localized magnetic bundle could explain the observed bursts without requiring a globally strong dipole. Third, the results suggest that the magnetar population may be more heterogeneous than previously thought, encompassing objects with a wide range of magnetic field strengths, ages, and evolutionary histories.

Finally, the paper outlines future observational strategies: long‑term timing campaigns to refine (\dot{P}) measurements, deeper broadband X‑ray spectroscopy to disentangle thermal and magnetospheric components, and coordinated radio searches to test for possible radio pulsations. By expanding the sample of low‑field SGRs, astronomers can better constrain models of magnetic field evolution in neutron stars and clarify the physical conditions that trigger magnetar‑type outbursts.