  SAX J2224.9+5421 is an extraordinary neutron star low-mass X-ray binary.
Albeit discovered when it exhibited a ~10-s long thermonuclear X-ray burst, it
had faded to a 0.5-10 keV luminosity of Lx<8E32 (D/7.1 kpc)^2 erg/s only ~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 ~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 Lx~5E31 (D/7.1 kpc)^2 erg/s (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 ~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 Lx~5E32 (D/7.1
kpc)^2 erg/s and displays an X-ray spectrum that is best described by power law
with a photon index of ~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.
