Because of the lack of long-term pulsed emission in quiescence and the strong timing noise, it is impossible to directly measure the braking index n of a magnetar. Based on the estimated ages of their potentially associated supernova remnants (SNRs), we estimate the values of the mean braking indices of eight magnetars with SNRs, and find that they cluster in the range of 1-42. Five magnetars have smaller mean braking indices of 1 < n < 3, and we interpret them within a combination of magneto-dipole radiation and wind-aided braking. The larger mean braking indices of n > 3 for the other three magnetars are attributed to the decay of external braking torque, which might be caused by magnetic field decay. We estimate the possible wind luminosities for the magnetars with 1 < n < 3, and the dipolar magnetic field decay rates for the magnetars with n > 3, within the updated magneto-thermal evolution models. Although the constrained range of the magnetars' braking indices is tentative, as a result of the uncertainties in the SNR ages due to distance uncertainties and the unknown conditions of the expanding shells, our method provides an effective way to constrain the magnetars' braking indices if the measurements of the SNR ages are reliable, which can be improved by future observations.
Xinjiang Natural Science Foundation(2013211A053)
; Chinese National Science Foundation(11173041
; National Basic Research Programme of China grants 973 Programmes(2012CB821801)
; Strategic Priority Research Programme 'The Emergence of Cosmological Structures' of the Chinese Academy of Sciences(XDB09000000)
; West Light Foundation of Chinese Academy of Sciences(2172201302)
; 11173042
; 11273051
; 11373006
; 11133001)
1.Chinese Acad Sci, Xinjiang Astron Observ, Urumqi 830011, Xinjiang, Peoples R China 2.Chinese Acad Sci, Key Lab Radio Astron, Nanjing 210008, Jiangsu, Peoples R China 3.Nanjing Univ, Dept Astron, Nanjing 210046, Jiangsu, Peoples R China 4.Nanjing Univ, Key Lab Modern Astron & Astrophys, Nanjing 210046, Jiangsu, Peoples R China 5.Chinese Acad Sci, Natl Astron Observ, Beijing 100012, Peoples R China 6.Qian Xuesen Lab Space Technol, Beijing 100094, Peoples R China
