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

Recently, ultraluminous X-ray source (ULX) M82 X-2 has been identified to be an accreting neutron star, which has a P = 1.37 s spin period, and is spinning up at a rate. P = -2.0 x 10(-10) s s(-1). Interestingly, its isotropic X-ray luminosity L-iso = 1.8 x 10(40) erg s(-1) during outbursts is 100 times the Eddington limit for a 1.4M(circle dot) neutron star. In this Letter, based on the standard accretion model we attempt to constrain the dipolar magnetic field of the pulsar in ULX M82 X-2. Our calculations indicate that the accretion rate at the magnetospheric radius must be super-Eddington during outbursts. To support such a super-Eddington accretion, a relatively high multipole field (greater than or similar to 10(13) G) near the surface of the accretor is invoked to produce an accreting gas column. However, our constraint shows that the surface dipolar magnetic field of the pulsar should be in the range of 1.0-3.5 x 10(12) G. Therefore, our model supports that the neutron star in ULX M82 X-2 could be a low-magnetic-field magnetar (proposed by Tong) with a normal dipolar field (similar to 10(12) G) and relatively strong multipole field. For the large luminosity variations of this source, our scenario can also present a self-consistency interpretation.