The magnetic field evolution of ULX NuSTAR J095551+6940.8 in M82 – a legacy of accreting magnetar

Abstract

Ultra-luminous X-ray sources are usually believed to be black holes with mass about 10 2−3 M ⊙ . However, the recent discovery of NuSTAR J095551+6940.8 in M82 by Bachetti et al. shows that it holds the spin period P = 1.37 s and period derivativeṖ ≈ −2 × 10 −10 s s −1 , which provides a strong evidence that some ultra-luminous X-ray sources could be neutron stars. We obtain that the source may be an evolved magnetar according to our simulation by employing the model of accretion induced the polar magnetic field decay and standard spin-up torque of an accreting neutron star. The results show that NuSTAR J095551+6940.8 is still in the spin-up process, and the polar magnetic field decays to about 4.5 × 10 12 G after accreting ∼ 10 −2.5 M ⊙ , while the strong magnetic field exists in the out-polar region, which could be responsible for the observed low field magnetar. The ultra luminosity of the source can be explained by the beaming effect and two kinds of accretion-radial random accretion and disk accretion. Since the birth rate of magnetars is about ten percent of the normal neutron stars, we guess that several ultra-luminous X-ray sources should share the similar properties to that of NuSTAR J095551+6940.8.

Constraining the dipolar magnetic field of M82 X-2 by the accretion model

“…Based on the accretion induced the polar magnetic field decay model, the simulation given by Pan et al (2016) show that the polar magnetic field decays to 4.5×10 13 G when the pulsar in M82 X-2 accreted ∼ 0.005 M⊙, while the strong magnetic field still remain in the out-polar region. According to the observed parameters of M82 X-2, and adopting a typical accretion column factor l0/d0 = 30, the dipole magnetic field in the pulsar pole and the beaming factor at peak luminosity are constrained to be Bp = 1.0 − 3.5 × 10 12 G, and b = 0.03 − 0.05, respectively.…”

How to make a mature accreting magnetar

“…In our analysis we did not include possible influence of accretion on the field decay. If this effect is taken into account (see, for example, Pan et al 2016) then the field might be even lower than in our estimates. I.e., smaller values of Q might be appropriated to fit properties of the systems discussed in this paper.…”

Study on the magnetic field strength of NGC 300 ULX1

“…It is suspected that the extraordinary characteristics of PULXs can be understanding through their magnetic field. One interpretation is that PULXs are with fields in the magnetar-level 10 14 G. Such a strong magnetic field can reduce the electron scattering cross-section and promotes super-Eddington luminosities (Dall'Osso et al 2015;Eksi et al 2015;Tong 2015;Pan et al 2016;Israel et al 2017). The other explanation is that the magnetic fields of PULXs are in the 10 12 -10 13 G range, which matches with the field range of normal pulsars (Bachetti et al 2014;Fürst et al 2017;Carpano et al 2018;King & Lasota 2020).…”