NuSTAR J095551+6940.8: a highly magnetised neutron star with super-Eddington mass accretion
Date Issued
2015
Author(s)
Abstract
The identification of the Ultraluminous X-ray source (ULX) X-2 in M82 as an
accreting pulsar has shed new light on the nature of a subset of ULXs, while
rising new questions on the nature of the super-Eddington accretion. Here, by
numerically solving the torque equation of the accreting pulsar within the
framework of the magnetically threaded-disk scenario, we show that three
classes of solutions, corresponding to different values of the magnetic field,
are mathematically allowed. We argue that the highest magnetic field one,
corresponding to B $\sim 10^{13}$ G, is favoured based on physical
considerations and the observed properties of the source. In particular, that
is the only solution which can account for the observed variations in $\dot{P}$
(over four time intervals) without requiring major changes in $\dot{M}$, which
would be at odds with the approximately constant X-ray emission of the source
during the same time. For this solution, we find that the source can only
accomodate a moderate amount of beaming, 0.5 $\lesssim b < 1$. Last, we show
that the upper limit on the luminosity, L$_X < 2.5 \times 10^{38}$ erg s$^{-1}$
from archival observations, is consistent with a highly-magnetized neutron star
being in the propeller phase at that time.
Volume
449
Issue
2
Start page
2144
Issn Identifier
0035-8711
Ads BibCode
2015MNRAS.449.2144D
Rights
open.access
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