The Quiescent Counterpart of the Peculiar X-Ray Burster SAX J2224.9+5421

SAX J2224.9+5421 is an extraordinary neutron star low-mass X-ray binary. It was discovered when it was exhibiting a sime 10 s long thermonuclear X-ray burst, but it had faded to a 0.5-10 keV luminosity of L X lsim 8 × 1032(D/7.1 kpc)2 erg s-1 only sime 8 hr later. It is generally assumed that neutron stars are quiescent (i.e., not accreting) at such intensity, raising questions about the trigger conditions of the X-ray burst and the origin of the faint persistent emission. We report on a sime51 ks XMM-Newton observation aimed at finding clues explaining the unusual behavior of SAX J2224.9+5421. We identify a likely counterpart that is detected at L X sime 5 × 1031(D/7.1 kpc)2 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 ∞ sime 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 X sime 5 × 1032(D/7.1 kpc)2 erg s-1 and displays an X-ray spectrum that is best described by a power law with a photon index of Γ sime 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.