http://arxiv.org/abs/1510.08597
Ultra luminous X-ray sources (ULXs) are usually believed to be black holes with mass about 10^{2–3}M_{sun}. However, the recent discovery of ULX NuSTAR J095551+6940.8 in M82 with the spin period P=1.37s and period derivation P_{dot}=-2*10^{-10} ss^{-1} provides a strong evidence that some ULXs are accreting neutron stars (NSs). To investigate such a particular accreting neutron star, we ascribe it as an evolved magnetar in the accretion binary system. By means of the model of accretion induced the NS magnetic evolution and standard spinup torque, we calculate the magnetic field decay and spin-up of M82 X-2, and show that its magnetic field is now 4.5*10^{12} G, which is evolved from a magnetar in a high mass Xray binary system (HMXB) with the initial values of magnetic field B~10^{14.5} G and spin period P~100 s by accreting ~10^{-3}M_{sun}, while the mass accretion rate for spin-up is set as 5.0*10^{18} gs^{-1}. The evolutionary track of magnetic field and spin period of M82 X-2 is simulated and plotted in the B-P diagram, with which we compare the observed pulsars, and find that several pulsars are consistent with the B-P track of M82 X-2. Since the birth rate of magnetar is about ten percent of the normal NSs, it is inferred that a couple of ULXs should also be the similar cases like M82 X-2. Furthermore, we argue that the existence of the local super-strong magnetic multipole structure of M82 X-2 destroys the spherical accretion condition of Eddington critical luminosity, which arises the ULX M82 X-2 to be different from the usual NS in HMXBs with the luminosity no more than the Eddington limit ……
Y. Pan, L. Song, C. Zhang, et. al.
Fri, 30 Oct 15
26/54
Comments: 4 pages, 1 figure, submitted to MNRAS
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