skip to main content

Title: 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 ≅ 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 Lmore » {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.« less
Authors:
;  [1] ;  [2]
  1. Department of Astronomy, University of Michigan, 500 Church Street, Ann Arbor, MI 48109 (United States)
  2. Astronomical Institute Anton Pannekoek, University of Amsterdam, Postbus 94249, 1090 GE Amsterdam (Netherlands)
Publication Date:
OSTI Identifier:
22356832
Resource Type:
Journal Article
Resource Relation:
Journal Name: Astrophysical Journal; Journal Volume: 787; Journal Issue: 1; Other Information: Country of input: International Atomic Energy Agency (IAEA)
Country of Publication:
United States
Language:
English
Subject:
79 ASTROPHYSICS, COSMOLOGY AND ASTRONOMY; ACCRETION DISKS; COSMIC X-RAY BURSTS; COSMIC X-RAY SOURCES; DETECTION; EMISSION; HARD X RADIATION; KEV RANGE; LUMINOSITY; MAGNETIC FIELDS; MASS; NEUTRON STARS; NEUTRONS; SMALL ANGLE SCATTERING; SOFT X RADIATION; TRANSIENTS; X-RAY DIFFRACTION; X-RAY SPECTRA