Spectral investigations of the nature of the Scorpius X-1 like sources

Abstract

We present results of spectral investigations of the Sco X-1 like Z-track sources Sco X-1, GX 349+2, and GX 17+2 based on Rossi-XTE (RXTE) observations determining spectral evolution along the Z-track. Results are obtained for a spectral model describing an extended accretion disk corona. The results are compared with previous results for the Cyg X-2 like group: Cyg X-2, GX 340+0, and GX 5-1 and a general model for the Z-track sources proposed. On the normal branch, the Sco-like and Cyg-like sources are similar, the results indicating an increase of mass accretion rate Ṁ between soft and hard apex, not as in the standard view that this increases monotonically around the Z. In the Cyg-like sources, increasing Ṁ causes the neutron star temperature kT to increase from ∼1 to ∼2 keV. At the lower temperature, the radiation pressure is small, but at the higher kT, the emitted flux of the neutron star is several times super-Eddington and the high radiation pressure disrupts the inner disk and launches the relativistic jets observed on the upper normal and horizontal branches. In the Sco-like sources, the main physical difference is the high kT of more than 2 keV on all parts of the Z-track suggesting jets are always possible, even on the flaring branch. The flaring branch in the Cyg-like sources is associated with a release of energy on the neutron star consistent with unstable nuclear burning. The Sco-like sources are very different as flaring appears to be a combination of unstable burning and an increase of Ṁ which makes flaring much stronger. Analysis of 15 years of RXTE ASM data on all 6 classic Z-track sources shows the high rate and strength of flaring in the Sco-like sources suggesting that the continual release of energy heats the neutron star causing the high kT. Analysis of a Sco X-1 observation with unusually little flaring supports this. GX 17+2 appears to be transitional between the Cyg and Sco-like types. Our results do not support the suggestion that Cyg or Sco-like nature is determined by the luminosity. © ESO, 2012.