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 rateṀ between soft and hard apex, not as in the standard view that this increases monotonically around the Z. In the Cyg-like sources, increasingṀ 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 ofṀ 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.

KeplerK2 observations of Sco X-1: orbital modulations and correlations withFermiGBM and MAXI

“…This is consistent with findings of other groups for the energy ranges considered (e.g. Church et al 2012; component to the neutron star and model the spectrum as a black body with a cut-off power-law tail, subject to absorption. The differences we see between the states can readily be associated with the color changes in the Z diagram.…”

“…An obvious interpretation is that the upper limit in the optical comes from a maximum luminosity associated with the nuclear burning event on the neutron star, much like in the case of X-ray bursters. This is consistent with the model of Church et al (2012) for the FB where unstable nuclear burning is providing part of the luminosity of flares.…”

“…For more typically assumed H/R ∼ 0.05, the viscous time-scale becomes tens of days (comparable to the decay time-scales of outbursts of transient LMXBs; King & Ritter 1998). Church et al (2012) have argued that the Comptonization region responsible for the hard X-rays is not a compact inner region, but rather an extended thin, hot layer above the disc. This is supported by observations of progressive covering of the hard X-rays during X-ray dips in dipping systems, permitting an estimate of the spatial extent of the hard X-ray region.…”

“…Church et al 2012;Titarchuk, Seifina, & Shrader 2014, and references therein). The key innovation is that we have shown that state-dependent hard X-ray spectra can be constructed using optically defined states only.…”

Sco X-1 revisited withKepler, MAXI and HERMES: outflows, time-lags and echoes unveiled

“…Indeed Hasinger & van der Klis (1989) have reported "very low frequency" noise observed on similar timescales. Given that the high optical luminosity state is somewhat associated to the FB for Sco X-1, and given that according to the Church et al (2012) model this is where nuclear burning occurs on the NS surface, it is possible that the additional noise component in the high optical luminosity state observed here is in fact associated to the Bildsten (1993) mechanism.…”