A. D'Aì, E. Bozzo, A. Papitto, R. Iaria, T. Di Salvo, L. Burderi, A. Riggio, E. Egron, N. R. Robba
Circinus X-1 is a neutron star accreting X-ray binary in a wide, 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 it 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 aim at giving a description that relates luminosity, spectral shape, local absorption, and orbital phase. We use multiple Rossi-XTE/PCA (3.0-25 keV), Swift/XRT (1.0-9.0 keV) 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. PCA data mostly cover the major outburst, XRT data cover the declining phase of the major outburst and all the phases of the minor outburst, Chandra data give an essential snapshot at the end of this overall outbursting phase. The X-ray spectrum can satisfactorily be described by a thermal Comptonization model with variable neutral local absorption in all the phases of the outburst. No other additive component is statistically required. The first outburst has a clear linear decay, showing a knee as the source flux goes below 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 with this framework.
View original:
http://arxiv.org/abs/1204.2464
No comments:
Post a Comment