Abstract
Using the data from the Proportional Counter Array on board theRossi X-Ray Timing Explorersatellite, we study the orbital modulation for the spectrum and mass accretion rate of Cir X-1 during its two orbital periods. We use a model consisting of a blackbody, a multicolor disk blackbody, and a line component to fit the spectrum and find that the spectrum is obviously modulated by the orbital phase. It is shown that the disk accretion rate in Cir X-1 undergoes three states during the orbital period. At the periastron with orbital phase 0-0.1, the disk accretion rate is sup-Eddington, then from phase 0.1 to the apastron (phase 0.5) it decreases dramatically and becomes near-Eddington, and from the apastron to the next periastron (phase 1) the disk accretion rate approximates Eddington and tends to be steady. We argue that the evolution of the disk accretion rate is attributed to the high orbital eccentricity of this source. The mass accretion rate onto the neutron star is much less than that onto the inner disk, indicating significant outflows in this source.
Highlights
Cir X-1Circinus X-1 (Cir X-1)Circinus X-1 (Cir X-1), a peculiar X-ray binary with an orbital period of ~16.6 days [1] and a distance of 5.5 kpc [2], was suspected to be a black hole or a neutron star (NS) ago
Using the data from the Proportional Counter Array on board the Rossi X-Ray Timing Explorer satellite, we study the orbital modulation for the spectrum and mass accretion rate of Cir X-1Circinus X-1 (Cir X-1) during its two orbital periods
The neutron star (NS) surface magnetic field strength of atoll sources might be low, so that the magnetosphere possibly shrinks within the innermost stable circular orbit (ISCO) and the inner disk edge terminates at the ISCO, which is similar to the case of black hole X-ray binaries (BHXBs) [47]
Summary
Circinus X-1 (Cir X-1), a peculiar X-ray binary with an orbital period of ~16.6 days [1] and a distance of 5.5 kpc [2], was suspected to be a black hole or a neutron star (NS) ago. The high-resolution radio studies with the Australia Telescope showed clear evidence for jet-like structure within the nebula surrounding Cir X-1, implying that Cir X-1 is a runway binary from the supernova explosion [13]. From the X-ray spectra obtained with the High-Energy Transmission Grating Spectrometer on Chandra, a rich set of X-ray P Cygni lines were detected, which indicates the presence of a high-velocity outflow in this source, interpreted as an accretion-disk wind [18,28]. Using the observations from three satellites, i.e. RXTE, Swift, and Chandra, D’Aì et al [30] studied the broadband X-ray spectral evolution of Cir X-1 during its 2010 May-June outbursts and found that the X-ray spectrum can be satisfactorily described by a thermal Comptonization model
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