A complete X-ray spectral coverage of the 2010 May-June outbursts of Circinus X-1

Abstract

Circinus X-1 is a neutron-star-accreting X-ray binary in a wide (P${}_{\mathrm{orb}}$ = 16.6 d), eccentric orbit. After two years of relatively low X-ray luminosity, in May 2010 Circinus X-1 went into outburst, reaching 0.4 Crab flux. This outburst lasted for about two orbital cycles and was followed by another shorter and fainter outburst in June. We focus here on the broadband X-ray spectral evolution of the source as it spans about three order of magnitudes in flux. We attempt to relate luminosity, spectral shape, local absorption, and orbital phase. We use multiple Rossi-XTE/PCA (3.0--25 keV) and Swift/XRT (1.0--9.0 keV) observations and a 20 ks long Chandra/HETGS observation (1.0--9.0 keV), to comprehensively track the spectral evolution of the source during all the outbursting phases. These observations were taken every two/three days and cover about four orbital cycles. The PCA data mostly cover the major outburst, the XRT data monitor the declining phase of the major outburst and all the phases of the minor outburst, and Chandra data provide an essential snapshot of the end of this overall outbursting phase. The X-ray spectrum can be satisfactorily described by a thermal Comptonization model with variable neutral local absorption in all phases of the outburst. No other additive component is statistically required. The first outburst decays linearly, with an ankle in the light curve as the flux decreases below $\sim$\,5 $\times$ 10${}^{-10}$ erg cm${}^{-2}$ s${}^{-1}$. At the same time, the source shows a clear spectral state transition from an optically thick to an optically thin state. While the characteristics of the first, bright, outburst can be interpreted within the disk-instability scenario, the following, minor, outburst shows peculiarities that cannot be easily reconciled in this framework.

Accepted for publication in Astronomy and Astrophysics

References in corpus (8)

• Analysing the atolls: X-ray spectral transitions of accreting neutron stars
• Spectral distribution of Be/X-ray binaries in the Small Magellanic Cloud
• Detection of the radial velocity curve of the B5-A0 supergiant companion star of Cir X-1?
• A Double Outburst from IGR J00291+5934: Implications for Accretion Disk Instability Theory
• The large-scale jet-powered radio nebula of Circinus X-1
• Evidence for a parsec scale X-ray jet from the accreting neutron star Circinus X-1
• Mass transfer in eccentric binaries: the new Oil-on-Water SPH technique
• Mass transfer during low mass X-ray transient decays

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