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)
Subjects: X-ray, Request for Observations, Binary, Neutron Star
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.