On the X-Ray Outbursts of Transient Anomalous X-Ray Pulsars and Soft Gamma-Ray Repeaters

Abstract

We show that the X-ray outburst light curves of four transient anomalous X-ray pulsars (AXPs) and soft gamma-ray repeaters (SGRs), namely, XTE J1810−197, SGR 0501+4516, SGR 1627−41, and CXOU J164710.2−455216, can be produced by the fallback disk model that was also applied to the outburst light curves of persistent AXPs and SGRs in our earlier work. The model solves the diffusion equation for the relaxation of a disk that has been pushed back by a soft gamma-ray burst. The sets of main disk parameters used for these transient sources are very similar to each other and to those employed in our earlier models of persistent AXPs and SGRs. There is a characteristic difference between the X-ray outburst light curves of transient and persistent sources. This can be explained by the differences in the disk surface density profiles of the transient and persistent sources in quiescence indicated by their quiescent X-ray luminosities. Our results imply that a viscous disk instability operating at a critical temperature in the range of ∼1300-2800 K is a common property of all fallback disks around AXPs and SGRs. The effect of the instability is more pronounced and starts earlier for the sources with lower quiescent luminosities, which leads to the observable differences in the X-ray enhancement light curves of transient and persistent sources. A single active disk model with the same basic disk parameters can account for the enhancement phases of both transient and persistent AXPs and SGRs. We also present a detailed parameter study to show the effects of disk parameters on the evolution of the X-ray luminosity of AXPs and SGRs in the X-ray enhancement phases.

VLA proper motion constraints on the origin, age, and potential magnetar future of PSR J1734$-$3333

“…An alternative scenario is that the spin evolution of the pulsar has been driven by accretion from a permanent fallback disc, for which models predict a pulsar age in a similar range to the one we obtain (Çalişkan et al 2013;Liu et al 2014). In this model magnetars are not powered by the decay of very powerful magnetic fields but by accretion (Çalışkan & Ertan 2012).…”

X-Ray Enhancement and Long-Term Evolution of Swift J1822.3–1606

“…Main disk parameters used in the present work are very similar to those employed in our earlier work to explain the X-ray enhancements and the evolution of AXP and SGRs (Ertan et al 2009, Ç alışkan & Ertan 2012, Ç alışkan et al 2013. The model sources with the dipole field strength in the 0.5 − 1.0 × 10 12 G range on the pole of the star and with the initial periods greater than ∼ 55 ms can reach the X-ray luminosity and the rotational properties of Swift J1822.3-1606 simultaneously.…”

“…We take the position r 0 of the pile-up to be outside the light cylinder, r 0 > r LC . X-ray enhancement light curves of the model sources are not sensitive to the exact positions of r 0 , r in , and the details of the Gaussian distribution, but sensitive to initial relative positions of these radii and the total mass included in the Gaussian distribution (Ertan et al 2006, Ç alışkan & Ertan 2012. In the simulations, we set r 0 = 4.1 × 10 10 cm, leaving Σ max and ∆r, which define the pile-up mass under the Gaussian, and r in as free parameters.…”

“…This effect does not modify the X-ray light curve in the early decay phase, when r h is not very close to r in , since the surface density gradients and local variations in the mass-flow rate occurring at larger radii are smoothed out on the way to the inner disk radius. A detailed investigation of this feature on the model light curves for different quiescent luminosities can be found in Ç alışkan & Ertan (2012).…”

Broad-band X-ray spectra of anomalous X-ray pulsars and soft γ-ray repeaters: pulsars in a weak-accretion regime?

“…In this case, the spin down of the neutron star is attributed to the interaction between the accretion disk and the magnetosphere (e.g. Ertan et al 2012). Owing to the low luminosities, the accretion rate of these systems is expected to be much lower than that of typical accreting pulsars in X-ray binaries.…”