The quiescent counterpart of the peculiar X-ray burster SAX J2224.9+5421

Journal Article · Tue May 20 04:00:00 EDT 2014 · Astrophysical Journal

DOI:https://doi.org/10.1088/0004-637X/787/1/67· OSTI ID:22356832

Degenaar, N.; Miller, J. M. ^[1]

---

1. 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, Journal Name: Astrophysical Journal Journal Issue: 1 Vol. 787; ISSN ASJOAB; ISSN 0004-637X
• Country of Publication: United States
• Language: English

Related Subjects

79 ASTRONOMY AND ASTROPHYSICS
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