Understanding recent observations of isolated magnetars and accreting magnetars from the magnetospheric point of view

Understanding recent observations of isolated magnetars and accreting magnetars from the magnetospheric point of view

H. Tong

Understanding recent observations of isolated magnetars and accreting magnetars from the magnetospheric point of view


H. Tong

School of Physics and Electronic Engineering, Guangzhou University, 510006 Guangzhou, China Xinjiang Astronomical Observatory, Chinese Academy of Science, Urumqi, China

E-mail: htong_2005@163.com

(Received May XX, 2017)


The wind braking model and its applications of magnetars are discussed. The decreasing torque of magnetars during outbursts, anti-glitch, and anti-correlations between radiation and timing are understandable in the wind braking model. Recent timing observations of magnetars are also consistent with the previous modeling. A magnetism-powered wind nebula and a braking index smaller than three are the two predictions. Besides isolated magnetars, there may also be accreting magnetars in binary systems and magnetars accreting from fallback disks. Observationally, ultra-luminous X-ray pulsars may be accreting magnetars, while super-slow magnetars may be magnetars with fallback disks in the past. Many works are needed for both isolated magnetars and accreting magnetars.

KEYWORDS:  accretion, magnetar, neutron star, pulsar,


1 Introduction

After 50 years of their discovery, pulsars are still fascinating objects. They possess the strongest magnetic fields in the universe. Magnetars is a special kind of pulsars. They may have the strongest magnetic field in all pulsars, with magnetic field as high as [Mereghetti2008, Mereghetti2015]. Therefore, magnetars are new specimen to study the physics of normal pulsars and accretion-powered X-ray pulsars [Tong2015a, Tong2016a]. For isolated magnetars, their distribution on the period period-derivative diagram of pulsars is shown in figure LABEL:figure_PPdot. As can be seen this figure, both pulsars and magnetars are spinning down. One fundamental question of pulsar study is: what’s the braking mechanism of pulsars and magnetars? In both pulsar and magnetar studies, the magnetic dipole braking mechanism is usually assumed. However, this is only a crude approximation since it assumes a rotating dipole in vacuum. A physical braking mechanism must consider the presence of the pulsar magnetosphere. There are many models in this direction. Among them are the wind braking of pulsars [KouTong2015] and wind braking of magnetars [Tong2013].

Observationally, magnetars have varying spin-down torque [Archibald2015a], or decreasing torque during outburst [Scholz2017], or increasing torque [Younes2015]. Their spectra also contain a significant fraction of nonthermal emissions [Weng2015, Enoto2017]. Therefore, a physical picture is that: the magnetic energy of magnetars is first converted to a system of particles. These particles are responsible for both the X-ray emissions and the spin-down torque. Then, it is natural that correlations between radiation and timing are seen ubiquitously in magnetars. This means that the spin-down of magnetars may be dominated by the outflowing particles. This is dubbed as the “wind braking of magnetars” [Tong2013].

Recently, there are many interesting observations of magnetars. It is found that many of them can be understood safely in the wind braking model. Furthermore, the discovery of possible accreting magnetars enable people to study the magnetosphere of accreting magnetars. In the following, the recent observations of isolated magnetars and accreting magnetars are discussed. Understandings of these observations from the magnetospheric point of view are presented.

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