Swift-XRT detected a new outburst of Cir X-1

ATel #9547; A. Sanna (University of Cagliari); S. E. Motta (University of Oxford)
on 25 Sep 2016; 16:17 UT
Distributed as an Instant Email Notice Transients
Credential Certification: Sara Elisa Motta (sara.motta@physics.ox.ac.uk)

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Based on the MAXI transient alert on 2016-09-16 16:56:22 UT, we requested a Swift-XRT observation. Swift performed a pointed ~1ks observation on 2016-09-18T13:50:57 in the direction of the neutron star binary system Cir X-1, detecting X-ray activity at a source position RA=230.1695 deg, DEC=-57.16703 deg (Enhanced position with 2.2'' radius uncertainty at 90% confidence level) compatible within errors with Cir X-1.The unabsorbed X-ray flux in the range 0.3-10 keV of (3+/-1)E-11 erg/cm2/s revealed week activity of the source. A follow-up Swift-XRT observation performed 2016-09-21T21:21:58, with exposure 1.7 ks confirmed the X-ray activity of Cir X-1, returning an unabsorbed X-ray flux in the range 0.3-10 keV of (5+/-1)E-11 erg/cm2/s. The latest Swift-XRT ~2ks observations (2016-09-24T09:58:58) showed a dramatic increase of the source activity, with a mean count rate of 180 cts/s in the 0.3-10 keV. The energy spectrum extracted from the last Swift/XRT pointing in the energy range 1-10 keV is well fitted by an absorbed power-law plus black body model with nH=3.56(6)E22 cm-2, power-law photon index 3.1(1) and black body temperature of 0.98(2) keV. We estimate an unabsorbed flux in the range 0.3-10 keV of 7E-8 erg/cm2/s. Multi-wavelength observations to this source in outburst are strongly encouraged. We are grateful to Neil Gehrels and the Swift team for rapidly planning the requested observations.