Asymmetric cross-correlation functions with delays in Sco X-1: evidence of possible jet triggering

Abstract

The formation and origin of jets in Z sources are not understood very well, although a strong X-ray-radio correlation has been noticed. We analyzed seventeen observations of Sco X-1 observed by the Rossi X-ray Timing Explorer (RXTE) where radio jet emissions were detected. In five observations, we report the detection of asymmetric cross-correlation functions (CCF) with delays of a few tens of seconds between soft and hard energy bands light curves in the horizontal branch associated with a flat-topped noise in the power density spectrum (PDS). Interestingly, these five observations were connected to a ballistic-type radio emission. We performed simulations to confirm and robust the cross-correlation coefficients and the observed lags. The CCF was highly symmetric in the remaining twelve observations, exhibiting NBO (Normal Branch Oscillations) or NBO+HBO (Horizontal Branch Oscillations) in the PDS. During these X-ray observations, the radio observations were found to be associated with an ultra-relativistic flow (URF) radio emission. The X-ray spectrum analysis of the two observations that showed core radio emission and abrupt variations in the PDS and CCF revealed that the bbody fractional flux varied by 10–20%, but the spectral parameters did not vary. We suggest that the ballistic jet might have triggered the instability in the inner region of the accretion disk, viz., the boundary layer (BL) plausibly along with the corona, causing the asymmetry with delays observed in the CCFs, and it also explains the absence of any oscillation features in the PDS, leaving behind a flat-topped noise. During symmetric CCF, the accretion flow was steady, hence, NBO / NBO+HBO was persistent. However, connecting the URF either to NBO or HBO is difficult since the majority of PDS exhibit an NBO alone rather than a NBO+HBO. We hypothesize that URF is most likely related to the phenomenon that causes NBO. Overall, we conclude that the asymmetric CCF shows that the inner part of the accretion disk is unstable due to the triggering of a ballistic jet and constrains the inner accretion region’s size to about 20-30 km, which possibly causes the accretion ejection.