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ATEL # 2671; A.K.H. Kong (NTHU, Taiwan), Y.-J. Yang (Amsterdam)
on 12 Jun 2010; 22:05 UT
Password Certification: Albert Kong (email@example.com)
Subjects: X-ray, Binaries, Neutron Stars, Transients
Referred to by ATEL #: 2674
Following the detection of a brightening and several X-ray bursts of Cir X-1 (ATel #2608,#2643,#2650,#2651,#2653; Linares et al. arXiv:1006.1379), the source has returned to low flux level since June based on observations taken with RXTE/ASM and MAXI/GSC. We examined the Swift/XRT imaging mode data taken on 2010 June 6 and June 10. On June 6 (at orbital phase 0.8; defined by Stewart et al. 1993), the 0.3-10 keV XRT count rate of Cir X-1 is about 0.07 c/s while it brightened to 0.8 c/s on June 10 (at phase 0.03). By comparing with the single-orbit 1.5-20 keV MAXI/GSC data, the source intensity varied only by a factor of 2 between the two periods. We therefore conclude that the Swift observation on June 10 was taken during a short flaring. It is worth noting that radio flares are common during zero phase (Stewart et al. 1993). The energy spectrum during the flaring can be best described by an absorbed two-blackbody model with N_H=(4.3+/-1.4)e22 cm^-2, kT_1=0.14+/-0.04 keV, and kT_2=1.36+/-0.17 keV. The 2-10 keV unabsorbed flux is 8.3e-11 erg/s/cm^2.
The Swift/XRT image taken on June 6 clearly shows an extended X-ray emission around Cir X-1 (see Figure 1). In particular, two extended structures are located along the southeast and northwest direction, at a distance of 30 arcsec from Cir X-1. Both features are consistent with the X-ray jets detected by previous Chandra HETGS and HRC-I observations (Heinz et al. 2007; Soleri et al. 2009). The X-ray knot along the northeast direction reported in Soleri et al. (2009) is also evident. We further examined a recent 100ks Chandra ACIS-S image taken on 2009 May 1; the jet emission is clearly detected and is roughly consistent with the Swift data (see Figure 1). For the data taken at much higher flux level (May 29 and June 10), while the extended X-ray emission is evident, the jet features are not clear presumably due to the bright central source. It is also possible that the jets quench during X-ray high state.
On 2010 June 12 (at phase 0.16), the XRT count rate of Cir X-1 increased dramatically to 14 c/s. This suggests that Cir X-1 may be re-brightening. We encourage multi-wavelength observations to monitor the evolution of the jet emission.
Swift and Chandra images of Cir X-1