Constraining the braking indices of magnetars 数字化校园建设解决方案 Constraining the braking indices of magnetars Zhi-Fu Gao Xinjiang Astronomical Observatory, Chinese Academy of Sciences，China 29 July. 2015. 沈阳职业技术学院信息中心

Contents Introduction Constraining the ages of magnetars Constraining the braking indices of magnetars Explaining magnetars’ braking indices Summary

Introduction It is commonly accepted that pulsars are produced in a core-collapse supernova explosions. Therefore, pulsars and their associated supernova remnants (SNRs ) should therefore have the same ages. Since the characteristic age of a is not available to estimate its true age , and the physically meaningful criterion to estimate is NS-SNR association. Their suggestions are in the same way applicable to magnetars with associated SNRs. A puzzle is that we can not measure magnetars’ braking indexes observationally due to strong timing noise and the lack of long-term persistent emission and strong timing noise.

Introduction A pulsar spins down with a power law The standard way to define the braking index n is

Braking indices of young radio pulsars

Observational braking indices of magnetars

Magnetars and their SNRs

Our assumptions Firstly, we assume that a NS's braking index keeps constant since its birth.This is a strong necessary assumption when estimating the age of a very young NS. In deed, one cannot derive an analytical expression, Eq. (8), to estimate the age of a NS, if we consider either K or n in Eq. (1) is a time-dependent quantity. The measured braking index is obviously influenced by the variation in the braking torque during the observational intervals.

Our Assumptions We assume that the age of a SNR is the age of a NS. Zhang \& Xie (2012) suggested that the physically meaningful criterion to estimate the true age of a NS is NS/SNR association. This suggestion is surely in the same way applicable to magnetars with SNRs. Finally, we assume that , the initial spin-period of a magnetar is far less than current period, because magnetars are supposed to be formed from rapidly rotating NSs ( Thompson \& Duncan 1996).

Our methods Note that t in Eq.(9) is the upper limit of the inferred age due to an approximation in Eq. (8), and n denotes the mean braking index of a magnetar since its formation due to the assumption of a constant n.

Our calculation results

Why magnetars appear `older’ or `younger’

Why n > 3? Due to the decay of crustal magnetic field Due to the decay of magnetospheric current Due to the decline in magnetic-cline angle

Why n < 3?

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