Evolution of cross-correlation and time lag of 4U 1735-44 along the branches

TL;DR

This study investigates the cross-correlation and time lag behavior between soft and hard X-rays in the atoll source 4U 1735-44, revealing anti-correlated soft and hard X-ray lags primarily on the banana branch, with implications for understanding accretion processes.

Contribution

It provides the first detailed analysis of cross-correlation and time lag evolution along different spectral states of 4U 1735-44, comparing with Z sources and black hole candidates.

Findings

Anti-correlated soft and hard X-ray lags of about hecto-seconds were observed.

Anti-correlations are detected mainly on the upper banana branch.

Lag timescales are similar to those in Z sources, but differ from black hole candidates.

Abstract

We analyze the cross-correlation function between the soft and hard X-rays of atoll source 4U 1735-44 with RXTE data, and find the anti-correlated soft and hard time lags of about hecto-second. On the island state, the observations do not show any obvious correlations, and most observations of banana branch show positive correlation. However, the anti-correlations are detected on the upper banana branch. These results are different from those of Z sources (Cyg X-2, GX 5-1), where the anti-correlation is detected in the low luminosity states, then the lag timescales of both this atoll and Z sources are found to be similar, at the magnitude of several tens to hundreds of seconds. As a comparison, it is noted that the anti-correlated lags of thousand-second have been reported from the several black hole candidates in their intermediate states. Finally, we compare the correspondent results of atoll source 4U 1735-44 with those observed in black hole candidates and Z sources, and the possible origins of the anti-correlated time lags are discussed.