Spectral Investigations of the nature of the Sco X-1 like sources
  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 observations using an
extended accretion disk corona model. 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 Mdot between soft and hard apex, not as in the standard view
that this increases around the Z. In the Cyg-like sources, increasing Mdot
causes the neutron star temperature kT to increase from ~1 to ~2 keV. At the
lower kT, 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 launching 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 that jets are always possible, even on the flaring
branch. The flaring branch in the Cyg-like sources is associated with 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 Mdot which makes flaring much stronger.
Analysis of 15 years or 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
continual release of energy heats the neutron star causing the high kT. 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 luminosity.
