Swift observations of Circinus X-1

ATel #2650; A. Papitto (INAF-OAC), A. D'Ai' (Univ. Palermo), E. Bozzo (ISDC), R. Iaria, T. Di Salvo (Univ. Palermo,)
on 28 May 2010; 20:54 UT
Distributed as an Instant Email Notice Request For Observations
Credential Certification: Luciano Burderi (burderi@mporzio.astro.it)

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After the recent detection of an X-ray brightening (ATel #2608), Swift has observed the LMXB, Cir X-1, starting on 2010 May 27.827 (UTC; MJD 55343.826), for a total exposure time of 4.5ks. Considering the ephemeris calculated by Clarkson et al. 2004, the Swift observations we report here took place in the proximity of the predicted epoch of the X-ray dip (MJD 55346.7+/-2.8). During this interval the Swift XRT count-rate was stable within 20% from the average.

A 0.5-10 keV spectrum is extracted from Swift XRT data taken in Windowed Timing mode, considering only grade 0 events. The spectrum is dominated by a thermal component with energy kT_1=1.48+/-0.02 keV. A soft excess is indeed clearly visible below 1.5 keV. We successfully model it with a second thermal component (kT_2=0.074+/-0.001 keV), coming from a much wider region than the hotter blackbody. The best fitting absorption column density is nH=(2.14+/-0.05)E22 cm2. The 2-10 keV unabsorbed flux is 9.4+/-0.1 E-10 erg cm-2 s-1 (0.04 Crab). Considering also the values given by Linares et al. 2010 (ATel #2643), this measurement is consistent with the known variations of the X-ray flux along the 16.6 d orbital cycle, even if a source fading on a longer timescale cannot be excluded.

No type I X-ray burst is detected during the Swift coverage (see Linares et al. 2010, ATel #2643). In the proximity of the Swift observation we report here, RXTE caught other three bursts with respect to those reported in ATel #2643. The onset of these bursts happened at MJD 55343.354, 55343.757 and 55344.205, respectively. None was simultaneous to the Swift pointing, and no manifest periodic signal is present in the (non barycentred) light curves.

Swift will keep monitoring the source activity in the following days, to enlighten the source spectral variations along the binary orbital cycle. We thank the Swift team for promptly scheduling this Target of Opportunity Observation.