The quiescent counterpart of the peculiar X-ray burster SAX J2224.9+5421
- Department of Astronomy, University of Michigan, 500 Church Street, Ann Arbor, MI 48109 (United States)
SAX J2224.9+5421 is an extraordinary neutron star low-mass X-ray binary. It was discovered when it was exhibiting a ≅ 10 s long thermonuclear X-ray burst, but it had faded to a 0.5-10 keV luminosity of L {sub X} ≲ 8 × 10{sup 32}(D/7.1 kpc){sup 2} erg s{sup –1} only ≅ 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 ≅51 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 {sub X} ≅ 5 × 10{sup 31}(D/7.1 kpc){sup 2} erg s{sup –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 {sup ∞} ≅ 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 {sub X} ≅ 5 × 10{sup 32}(D/7.1 kpc){sup 2} erg s{sup –1} and displays an X-ray spectrum that is best described by a power law with a photon index of Γ ≅ 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.
- OSTI ID:
- 22356832
- Journal Information:
- Astrophysical Journal, Vol. 787, Issue 1; Other Information: Country of input: International Atomic Energy Agency (IAEA); ISSN 0004-637X
- Country of Publication:
- United States
- Language:
- English
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