Outburst Profiles and Triggering Mechanism of Transient Black Holes
Abstract
Most of the black holes (BHCs) reside in binary systems. They are observable when they accrete matter from the companion via Roche-Lobe overflow or via wind. Electromagnetic radiation emits from accretion disk (forms due to infalling matter which carries angular momentum) or outflowing jets in a wide energy band from radio to X-rays. Depending upon the activity they are mainly of two types: transient and persistent. Transient BHCs show outburst occasionally, whose duration varies from weeks to months. We see variation (even for same BH binary system) in nature (duration, peak intensity, etc.) of the outbursts and duration between two successive outbursts. The period between two outbursts is known as quiescence phase. The outburst profiles (daily average rates or fluxes) could be fitted with one or multiple profile(s) of the fast rise exponential decay (FRED). Chakrabarti et al. (2019) showed that viscosity plays a major role to play in this. When matter accretes from the companion, due to lack of viscosity it starts to accumulate at a distance, away from the central object, known to be as the pile-up radius ($X_p$). When a huge amount of matter is accumulated at this distance, the temperature increases. As a result, turbulence and instability increases, which in turn increase the viscosity much above the critical value. Due to this high viscosity, matter starts to move towards the BH from the pile-up radius and an outburst is triggered. This pile-up radius is a function of the viscosity itself. More distant is the pile-up radius, more is the accumulation of matter and more viscosity is needed to start the outburst. So, more will be the quiescence time also. In the case of different outbursts, this pile-up radii are different, resulting in different peak flux, outburst duration and quiescence period. The peak flux is proportional to the duration of the quiescence state prior to the outburst. We have taken archival RXTE/ASM (1.5-12 keV) and MAXI/GSC (2-10 keV) one day average light curve data for several outbursts of the BHCs GX 339-4, and 4U 1630-472. The outburst profiles are fitted with FRED model. This is done in ROOT software using C++ code. Fitting the outburst profiles, we have estimated the peak flux, duration of each outburst which depend on the distance of the pile-up radius. We also estimated the normalized flux per day in the period of both outburst and quiescence phases. We see that the peak flux depends on the duration of the quiescence state prior to it. From the normalized flux per day in quiescence, we can say that the matter supply from the companion takes place at a moreover constant rate.
- Publication:
-
43rd COSPAR Scientific Assembly. Held 28 January - 4 February
- Pub Date:
- January 2021
- Bibcode:
- 2021cosp...43E1707B