Evolution of accretion disc-corona in the TDE Candidate AT 2019avd

TL;DR

The paper investigates how accretion disc and corona evolve in the TDE AT 2019avd by applying a disc-corona model where a fraction $f$ of gravitational energy heats a hot corona and upscatters disc photons via inverse Compton scattering. NICER spectra from late-time Phases 3–4 reveal that the accretion rate declines as $\dot{m} \propto (t-t_0)^{-\xi}$ with $\xi \approx 1.86$, while $f$ anti-correlates with $\dot{m}$ as $f \propto \dot{m}^{-0.30}$, indicating a hotter, more tenuous corona dominates at low $\dot{m}$. The coronal properties $(\Gamma, T_e, \tau)$ evolve systematically with $\dot{m}$, and the disc–corona framework successfully explains the observed spectral hardening, NH evolution, and the limited optical/UV emission from the disc. The findings imply stronger magnetic turbulence in the TDE disc compared to typical AGNs and offer a diagnostic for magnetized corona formation in transient accretion events.

Abstract

X-ray observations of the tidal disruption event (TDE) candidate AT 2019avd show drastic variabilities in flux and spectral shape over hundreds of days, providing clues on the accretion disc-corona evolution. We utilize a disc-corona model, in which a fraction of the gravitational energy released in the disc is transported into the hot corona above/below. Some soft photons emitted from the disc are upscattered to X-ray photons by the hot electrons in the optically thin corona. By fitting the NICER observations of AT 2019avd during epochs when the spectra exhibit significant hardening, we derive the evolution of the mass accretion rate, $\dot{m}$, and the coronal energy fraction, $f$. Our results show that $f$ decreases with increasing $\dot{m}$, which is qualitatively consistent with that observed in active galactic nuclei (AGNs), while the slope of this source, $f\propto \dot{m}^{-0.30}$, is much shallower than that of AGNs. We also find that the non-thermal X-ray spectrum in this source is significantly softer than those typically seen in AGNs and black-hole X-ray binaries. We argue that these quantitative differences can be a powerful diagnostic of the underlying magnetic turbulence, which may imply a stronger magnetic field within the TDE accretion disc than that in typical AGNs. It is also found that the evolution of the fitted neutral hydrogen column density follows a similar pattern to that of the accretion rate evolution, which may reflect the accumulation of absorbing material originating from the inflowing streams of stellar debris and/or other related sources.

Figures (6)

• Figure 1: Radial profiles of the disc-corona system. The left column of panels shows the effect of varying the coronal fraction $f$ at a fixed accretion rate of $\dot{m}=0.1$. The right column shows the effect of varying the accretion rate $\dot{m}$ at a fixed coronal fraction of $f=0.2$. From top to bottom, the panels show: the effective temperature of the accretion disc, the electron temperature, the optical depth, and the electron number density of the corona. A black hole mass of $M_{\rm BH} = 10^6 M_\odot$ is adopted.
• Figure 2: The spectra of the disc-corona systems for different accretion rates $\dot{m}$ and coronal fractions $f$. The dashed lines indicate the modified disc emission, while the dash-dotted and dotted lines represent inverse Compton scattering and thermal bremsstrahlung components of the corona, respectively. Grey shaded regions highlight the 0.3--2 keV X-ray bands. A black hole mass of $M_{\rm BH} = 10^6 M_\odot$ is adopted.
• Figure 3: Representative unfolded X-ray spectra of AT 2019avd observed with NICER, along with best-fitting model components and residuals. The left and right panels correspond to observations on Day 188 (Phase 3) and Day 244 (Phase 4), respectively. Residuals (bottom panels) are expressed in units of photon counts.
• Figure 4: Time evolution of the intrinsic unabsorbed spectra of AT 2019avd in the 0.1--2 keV band, with emission from IC 505 subtracted. Each solid line represents the best-fitting spectrum at a given epoch and the shaded regions indicate the uncertainty range derived from the confidence intervals of the fitted model parameters.
• Figure 5: Temporal evolution of the best-fitting parameters for AT 2019avd. From top to bottom, the panels show: the accretion rate $\dot{m}$, coronal energy fraction $f$, and hydrogen column density $N_\text{H}$. The golden dashed line marks Day 205, corresponding to the onset of the rapid evolution phase. Cyan vertical dashed lines denote the start and end of Phase 4 (Days 224 and 249, respectively). In the top panel, purple and magenta dashed lines represent power-law fits to the declining segments of $\dot{m}$. In the bottom panel, the black dashed line indicates the lower bound of the fitted hydrogen column density.