A Comparison of the Variability of the Symbiotic X‐Ray Binaries GX 1+4, 4U 1954+31, and 4U 1700+24 from Swift BAT and RXTE ASM Observations

  • Corbet R
  • Sokoloski J
  • Mukai K
  • et al.
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Abstract

We present an analysis of the X-ray variability of three symbiotic X-ray binaries, GX 1+4, 4U 1700+24, and 4U 1954+31, using observations made with the Swift Burst Alert Telescope (BAT) and the Rossi X-Ray Timing Explorer (RXTE) All-Sky Monitor (ASM). Observations of 4U 1954+31 with the Swift BAT show modulation at a period near 5 hr. Models to explain this modulation are discussed, including the presence of an exceptionally slow X-ray pulsar in the system and accretion instabilities. We conclude that the most likely interpretation is that 4U 1954+31 contains one of the slowest known X-ray pulsars. Unlike 4U 1954+31, neither GX 1+4 nor 4U 1700+24 show any evidence for modulation on a timescale of hours. An analysis of the RXTE ASM light curves of GX 1+4, 4U 1700+24, and 4U 1954+31 does not show the presence of periodic modulation in any source, with the exception of a possible detection of the 5 hr period in 4U 1954+31, although there is considerable variability on long timescales for all three sources. There is no modulation in GX 1+4 on either the optical 1161 day orbital period or a previously reported 304 day X-ray period. For 4U 1700+24 we do not confirm the 404 day X-ray period previously proposed for this source from a shorter duration ASM light curve. We conclude that all three sources have substantial low-frequency noise in their power spectra that may give the appearance of periodic modulation if this noise is not properly accounted for, particularly if short-duration light curves are examined.

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Corbet, R. H. D., Sokoloski, J. L., Mukai, K., Markwardt, C. B., & Tueller, J. (2008). A Comparison of the Variability of the Symbiotic X‐Ray Binaries GX 1+4, 4U 1954+31, and 4U 1700+24 from Swift BAT and RXTE ASM Observations. The Astrophysical Journal, 675(2), 1424–1435. https://doi.org/10.1086/526337

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