Spectral Evolution of Circinus X-1 Along its Orbit

Abstract

We report on the spectral analysis of Circinus X-1 observed by the ASCA satellite in March 1998 along one orbital period. The luminosity of the source (in the 0.1-100 keV band) ranges from $2.5\times 10^{38}$ erg s${}^{-1}$ at the periastron (orbital phase 0.01) to $1.5\times 10^{38}$ erg s${}^{-1}$ at orbital phase 0.3. From the spectral analysis and the lightcurve we argue that Cir X-1 shows three states along the orbital evolution. The first state is at the orbital phase interval 0.97-0.3: the luminosity becames super-Eddington and a strong flaring activity is present. In this state a shock could form in the inner region of the system due to the super-Eddington accretion rate, producing an outflow of ionized matter whose observational signature could be the prominent absorption edge at $\sim 8.7$ keV observed in the energy spectrum at these phases. In the second state, corresponding to the orbital phase interval between 0.3 and 0.7, the accretion rate is sub-Eddington and we observe a weaker outflow, with smaller hydrogen column: the absorption edge is now at $\sim 8.3$ keV with an optical depth a factor of 2.5 to 6 smaller. The third state corresponds to the orbital phase interval 0.78-0.97. In this state the best fit model to the spectrum requires the presence of a partial covering component, indicating that the emission from the compact object is partially absorbed by neutral matter, probably the atmosphere of the companion star and/or the accreting matter from the companion.

18 pages, 3 figures. Accepted by ApJ

References in corpus (4)

• The Discovery of a State Dependent Hard Tail in the X-ray Spectrum of the Luminous Z-source GX 17+2
• Detection of a Hard Tail in the X-ray Spectrum of the Z Source GX 349+2
• The Discovery of Broad P Cygni X-ray Lines from Circinus X-1 with the Chandra High Energy Transmission Grating Spectrometer
• The Composite Broad Band Spectrum of Cir X-1 at the Periastron: a Peculiar Z-source

Cited by in corpus (11)

• Variability of the X-ray P Cygni Line Profiles from Circinus X-1 Near Zero Phase
• The return of the bursts: Thermonuclear flashes from Circinus X-1
• Long-term X-ray Variability of Circinus X-1
• On the Soft Excess in the X-ray Spectrum of Cir X-1: Revisitation of the Distance to Cir X-1
• Study of the Cir X--1 Broad Band Spectrum at Orbital Phases Close to the Apoastron
• A search for very high-energy flares from the microquasars GRS 1915+105, Circinus X-1, and V4641 Sgr using contemporaneous H.E.S.S. and RXTE observations
• HST observations rule out the association between Cir X-1 and SNR G321.9-0.3
• A complete X-ray spectral coverage of the 2010 May-June outbursts of Circinus X-1
• Evolution of Hard X-Ray Spectra Along the Branches in Cir X-1
• Sudden End of X-Ray Outbursts around Periastron of Circinus X-1 Observed with MAXI
• The broad-band spectrum of Cyg X-2 with INTEGRAL