SWIFT and RXTE observations of Cir X-1 in an extended low-flux state

ATel #811; P. G. Jonker (SRON/CfA), J. H. Swank (NASA/GSFC), L. Kuiper (SRON), J. Homan (MIT), C. B. Markwardt (UMD/NASA/GSFC)
on 17 May 2006; 14:08 UT
Distributed as an Instant Email Notice Request For Observations
Credential Certification: P.G. Jonker (pjonker@cfa.harvard.edu)

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We report on our analysis of RXTE and two SWIFT observations of Cir X-1. RXTE/ASM observations show that Cir X-1 has been at or below the ASM detection level as of April 26 (MJD 53851.630). Hence the source has been undetected for more than 1 binary orbital period. An RXTE/PCA observation has been obtained on May 9 at 11:58 UTC 2005. A fit to the PCA X-ray spectrum gives an 0.5-10 keV flux of 2E-10 erg cm^-2 s^-1 (line emission is seen at 6.5 keV as well, although this could be related to the Galactic Ridge emission which contributes about 25% to the PCA flux). Two SWIFT observations have been performed on May 16, one at 01:22 UTC and one at 10:49 UTC. During the first observation lasting ~700 seconds the source count rate was ~0.59 counts/s, and the X-ray spectrum was well-fit by an absorbed power law with N_H ~(5+-4)E22 cm^2, and a power law index of 0.8+-0.8 (the errors are at the 90% confidence level). The 0.5-10 keV source flux was 1E-10 erg cm^-2 s^-1. The second SWIFT observation lasted ~720 seconds and the source count rate and flux had decreased to 0.27 counts/s and 3E-11 erg cm^-2 s^-1, respectively. The spectrum was well-fit by an absorbed power law with index 0.5+-0.5 and N_H (1+-1)E22 cm^2. Overall the source flux has decreased nearly a factor of 10 within a week, and whereas Cir X-1 is known to be highly variable the source is not usually this low. More SWIFT observations have been requested.