2014ApJ...792..109D

The recent discovery of a millisecond radio pulsar experiencing an accretion outburst similar to those seen in low mass X-ray binaries, has opened up a new opportunity to investigate the evolutionary link between these two different neutron star manifestations. The remarkable X-ray variability and hard X-ray spectrum of this object can potentially serve as a template to search for other X-ray binary/radio pulsar transitional objects. Here we demonstrate that the transient X-ray source XMM J174457-2850.3 near the Galactic center displays similar X-ray properties. We report on the detection of an energetic thermonuclear burst with an estimated duration of ≃2 hr and a radiated energy output of ≃ 5x10⁴⁰ erg, which unambiguously demonstrates that the source harbors an accreting neutron star. It has a quiescent X-ray luminosity of L_X≃ 5x10³²(D/6.5 kpc)² erg/s and exhibits occasional accretion outbursts during which it brightens to L_X≃ 10³⁵-10³⁶(D/6.5 kpc)² erg/s for a few weeks (2-10 keV). However, the source often lingers in between outburst and quiescence at L_X≃ 10³³-10³⁴(D/6.5 kpc)² erg/s. This peculiar X-ray flux behavior and its relatively hard X-ray spectrum, a power law with an index of Γ ≃ 1.4, could possibly be explained in terms of the interaction between the accretion flow and the magnetic field of the neutron star.