Does Cyg X-1 have a small Standard Accretion Disc?

TL;DR

This study compares the accretion disc structures of Cyg X-1 and low mass X-ray binaries, revealing that Cyg X-1 has a very small or negligible Keplerian disc due to wind-fed accretion, unlike LMXBs.

Contribution

It provides the first detailed comparison of accretion disc delays in Cyg X-1 and LMXBs using TCAF spectral fits, highlighting the small disc in Cyg X-1.

Findings

Large delays in LMXBs indicate sizable Keplerian discs.

Negligible delay in Cyg X-1 suggests a very small disc.

Cyg X-1's sporadic spectral states are due to wind-fed accretion.

Abstract

We analyze several outbursts of a few transient sources using Proportional Counter Array (PCA) data (2.5-25 keV) as well as All Sky Monitor (ASM) data (1.5-12 keV) of Rossi X-ray Timing Explorer (RXTE) satellite. We find a time delay between the arrival times of the Keplerian disc component and the halo of the Two-Component Advective Flow (TCAF) when the spectral data is fitted with TCAF solution. We compare this time delay from the spectral fits with the TCAF solution of the transient low mass X-ray binaries (LMXBs) e.g., GX 339-4, H 1743-322 and MAXI J1836-194 with that of the high mass X-ray Binary (HMXB), Cyg X-1. We find that several days of time delays are observed in LMXBs while for Cyg X-1 the delay is negligible. We interpret the large delay to be due to the viscous delay of a large Keplerian component to reach the inner region as compared to nearly free-fall time taken by the low angular momentum halo component. The delay is of the order of a few days for the low mass X-ray binaries (LMXBs) where the feeding is primarily through the Roche-lobe. However, it is negligible in a wind-fed system like Cyg X-1 since a very small Keplerian disc is created here by slowly redistributing the low angular momentum of the wind. As a consequence, sporadic soft or intermediate spectral states are observed.