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Circinus X-1 -- Return to high radio flaring state

ATEL # 985; G. D. Nicolson (Hartebeesthoek Radio Astronomy Observatory)
on 31 Jan 2007; 7:46 UT
Password Certification: G. D. Nicolson (

Subjects: Radio, X-ray, Binaries, Neutron Stars

Circinus X-1 has returned to a high radio flaring state last seen during 1975-1985. Observations with the 26-m telescope at the Hartebeesthoek Radio Astronomy Observatory detected strong flares at 3.5 cm wavelength, reaching levels between 180 and 1030 mJy for seven flares observed between 2006 Aug 03 and 2006 Nov 26. This coincides with a marked change in the X-ray flares observed with with RXTE All Sky Monitor over the corresponding 16.5 day cycles. This new behaviour is characterised by strong short flares (>100 ASM counts) with a rapid turn on (1.5 hr) and was preceded by an extended low state. Flares of this form have not been observed with RXTE over the past decade and are similar to those reported by Kaluzienski et al (1979), IAUC # 3320 , which were also accompanied by strong radio flares. The start of the X-ray flares coincides with the onset of the radio flares which in both cases last for several days, followed by a decay of about one week.

A new ephemeris derived from the onset times of 21 well observed flares spanning the past 28 years predicts the start of the radio flares as:

t(N) = MJD 43076.32 +(16.57794 - 0.0000401*N)*N, where N is the number of 16.6 day cycles since MJD 43076.32. The rms scatter in the residuals is 0.069d and 95% error bounds for the period and period decrement are 16.5770d, -0.0000386 d per cycle and 16.57888, -0.0000416d per cycle.

Transient features in the X-ray data, such as the short X-ray dips observed by RXTE from 1996-1998, are consistent with this ephemeris, but the best defined short dips occur at phase 0.019 (0.26d late), with an rms error of 0.073d.

Radio flaring over the last three cycles has weakened and the X-ray flares have also changed, both weakening and peaking at later phases in the 1.5 day cycle. While there is correlation between the the X-ray and radio data, there are occasions when the RXTE data remains low throughout the cycle, despite radio flares of several hundred mJy. The next radio flare is predicted to start on 2007 February 01 at 10:41 UT.

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