The X-ray rise and fall of the Recurrent Symbiotic System T CrB

TL;DR

This study analyzes X-ray observations of the symbiotic recurrent nova T CrB over 16 years, revealing a large accretion disk, evolving boundary layer temperatures linked to accretion rate changes, and evidence of jet-like ejections, with T CrB's evolution resembling other accreting systems.

Contribution

It provides a detailed model of T CrB's accretion disk, boundary layer evolution, and jet activity, offering new insights into its long-term X-ray behavior and similarities to other accreting systems.

Findings

The accretion disk has a radius of about 1 AU and significantly contributes to X-ray reflection.

Boundary layer temperature decreased from 14.8 keV to 2.8 keV, then stabilized around 8 keV.

Evidence of jet-like ejections with gas velocities of 110-200 km/s.

Abstract

We present the analysis of publicly available NuSTAR, Suzaku and XMM-Newton observations of the symbiotic recurrent nova T CrB covering the 2006.77-2022.66 yr period. The X-ray spectra are analysed by adopting a model that includes a reflection component produced by the presence of a disk that mimics the accretion disk and the immediate surrounding medium. Our best-fit model requires this disk to have a radius of 1 AU, effective thickness of 0.1 AU, averaged column density 10$^{25}$ cm$^{-2}$ and orientation of 50$^{\circ}$ with respect to the line of sight. This disk is about a factor of two larger than recent estimations for the accretion disk and its presence contributes significantly via reflection to the total X-ray flux detected from T CrB, which naturally produces the emission of the 6.4 keV Fe line. Our analysis suggests that the temperature of the boundary layer evolved from 14.8 keV in the steady-state phase (before 2016), to 2.8 keV in the 2017.24 epoch, to finally stabilise to about $\sim$8 keV in the subsequent epochs. These variations in the plasma temperature of the boundary layer are attributed to the evolution of the mass accretion rate ($\dot{M}_\mathrm{acc}$), which is estimated to have an averaged value of $\dot{M}_\mathrm{acc}$=2.6$\times10^{-8}$ M$_\odot$ yr$^{-1}$ for the current active phase. The presence of emission lines in the XMM-Newton RGS spectrum of 2017.24 prevents from adopting a black body emission model to fit the soft X-ray range. Instead, we use plasma emission models that suggest the presence of adiabatically-shocked gas produced by gas velocities of 110-200 km s$^{-1}$, very likely tracing jet-like ejections similar to what is found in other symbiotic systems. The analysis of X-ray and optical data together show that T CrB has a similar evolution as black hole binaries, accreting neutron stars and AGN in the hardness-intensity diagram.