The Peculiar Galactic Center Neutron Star X-Ray Binary XMM J174457-2850.3

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 sime2 hr and a radiated energy output of ~= 5 × 10⁴⁰ erg, which unambiguously demonstrates that the source harbors an accreting neutron star. It has a quiescent X-ray luminosity of L _X ~= 5 × 10³²(D/6.5 kpc)² erg s^-1 and exhibits occasional accretion outbursts during which it brightens to L _X ~= 10³⁵-10³⁶(D/6.5 kpc)² erg s^-1 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^-1. 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.