SAX J2224.9+5421 is an extraordinary neutron star low-mass X-ray binary .
Albeit discovered when it exhibited a \simeq 10-s long thermonuclear X-ray burst , it had faded to a 0.5–10 keV luminosity of L _ { \mathrm { X } } \lesssim 8 \times 10 ^ { 32 } ~ { } ( D / \mathrm { 7.1 ~ { } kpc } ) ^ { 2 } ~ { } \mathrm { % erg~ { } s } ^ { -1 } only \simeq 8 hr later .
It is generally assumed that neutron stars are quiescent ( i.e. , not accreting ) at such an intensity , raising questions about the trigger conditions of the X-ray burst and the origin of the faint persistent emission .
We report on a \simeq 51 ks XMM-Newton observation aimed to find clues explaining the unusual behavior of SAX J2224.9+5421 .
We identify a likely counterpart that is detected at L _ { \mathrm { X } } \simeq 5 \times 10 ^ { 31 } ~ { } ( D / \mathrm { 7.1 ~ { } kpc } ) ^ { 2 } ~ { } \mathrm { erg~ { } s } ^ { -1 } ( 0.5–10 keV ) and has a soft X-ray spectrum that can be described by a neutron star atmosphere model with a temperature of kT ^ { \infty } \simeq 50 eV .
This would suggest that SAX J2224.9+5421 is a transient source that was in quiescence during our XMM-Newton observation and experienced a very faint ( ceasing ) accretion outburst at the time of the X-ray burst detection .
We consider one other potential counterpart that is detected at L _ { \mathrm { X } } \simeq 5 \times 10 ^ { 32 } ~ { } ( D / \mathrm { 7.1 ~ { } kpc } ) ^ { 2 } ~ { } \mathrm { erg~ { } s } ^ { -1 } and displays an X-ray spectrum that is best described by power law with a photon index of \Gamma \simeq 1.7 .
Similarly hard X-ray spectra are seen for a few quiescent neutron stars and may be indicative of a relatively strong magnetic field or the occurrence of low-level accretion .