The Outburst Decay of the Low Magnetic Field Magnetar SGR 0418+5729

Abstract

We report on the long term X-ray monitoring of the outburst decay of the low magnetic field magnetar SGR 0418+5729 , using all the available X-ray data obtained with RXTE, Swift, Chandra, and XMM-Newton observations, from the discovery of the source in June 2009, up to August 2012. The timing analysis allowed us to obtain the first measurement of the period derivative of SGR 0418+5729: P = 4(1) × 10 −15 s s −1 , significant at ∼ 3.5σ confidence level. This leads to a surface dipolar magnetic field of B dip 6 × 10 12 Gauss. This measurement confirms SGR 0418+5729 as the lowest magnetic field magnetar. Following the flux and spectral evolution from the beginning of the outburst up to ∼1200 days, we observe a gradual cooling of the tiny hot spot responsible for the X-ray emission, from a temperature of ∼ 0.9 to 0.3 keV. Simultaneously, the X-ray flux decreased by about 3 orders of magnitude: from about 1.4 × 10 −11 to 1.2 × 10 −14 erg s −1 cm −2 . Deep radio, millimeter, optical and gamma-ray observations did not detect the source counterpart, implying stringent limits on its multiband emission, as well as constraints on the presence of a fossil disk. By modeling the magneto-thermal secular evolution of SGR 0418+5729 , we infer a realistic age of ∼550 kyr, and a dipolar magnetic field at birth of ∼ 10 14 G. The outburst characteristics suggest the presence of a thin twisted bundle with a small heated spot at its base. The bundle untwisted in the first few months following the outburst, while the hot spot decreases in temperature and size. We estimate the outburst rate of low magnetic field magnetars to be about one per year per galaxy, and we briefly discuss the consequences of such result in several other astrophysical contexts.

SGR 0418+5729,SwiftJ1822.3-1606, and 1E 2259+586 as massive, fast-rotating, highly magnetized white dwarfs

“…In the case of a white dwarf close to the critical mass, this new lower limit gives a very stringent constraint on the spindown rate of SGR 0418+5729,Ṗ = 4 × 10 −16 <Ṗ < 6 × 10 −15 , which we submit for observational verification. Indeed, after the submission of this work, Rea et al (2013) reported the confirmation of the spindown rate of SGR 0418+5729,Ṗ = 4 × 10 −15 , at 3.5 sigma confidence level. This measurement fully confirms the results of Malheiro et al (2012), see Eq.…”

“…As we show below, these predictions can be improved by considering realistic white dwarf parameters instead of fiducial values. It is important to mention at this point that, after the submission of this work, Rea et al (2013) presented the X-ray timing analysis of the long-term monitoring of SGR 0418+5729 with RXTE, Swift, Chandra, and XMM-Newton, which allowed the determination of the spindown rate of SGR 0418+5729,Ṗ = 4 × 10 −15 . These results confirm both our prediction given by Eq.…”

“…4.4 and given by Eq. (10), which are to be considered to be predictions of the white dwarf model because they were presented in advance of the observational results given in (Rea et al 2013). The situation has become even more striking considering the X-ray timing monitoring with Swift, RXTE, Suzaku, and XMM-Newton satellites of the recently discovered SGR Swift J1822.3-1606 (Rea et al 2012).…”

The Long-Term Post-Outburst Spin Down and Flux Relaxation of Magnetar Swift J1822.3–1606

“…The magnetic field measured from the long-term spin down of Swift J1822.3−1606 is 1.35 × 10 13 G. This is the second lowest spin-down inferred magnetar dipolar magnetic field and is about a factor of two higher than the lowest measured value, 6.1 × 10 12 G of SGR 0418+5729 (Rea et al 2013). However, since pulsar timing is only sensitive to the surface dipolar component of the field, the true magnetic field of the magnetar could be significantly higher than the value measured from pulsar timing if the toroidal component is much higher than the poloidal component or if the poloidal field has significant multipolar contributions.…”

The outburst decay of the low magnetic field magnetar SWIFT J1822.3−1606: phase-resolved analysis and evidence for a variable cyclotron feature

“…∼ 6 × 10 12 G; Rea et al 2013). Simulations of magnetic field evolution in neutron stars with different initial field strength and configuration have shown that, a relatively old magnetar such as SGR 0418+5729 despite its low surface dipolar field, might still harbor a sufficiently intense internal/crustal toroidal field to give rise to outbursts and short X-ray bursts (Turolla et al 2011).…”