Probing the potential high-energy messengers of the anticipated T Coronae Borealis outburst

Abstract

T Coronae Borealis (T CrB) is a nearby recurrent nova expected to erupt in the near future, offering a unique opportunity to study particle acceleration and high-energy emission from novae in real time. We investigate the production of gamma-rays and neutrinos following the T CrB outburst by combining three-dimensional hydrodynamical simulations with a detailed diffusive shock acceleration model. Our simulations account for the complex circumbinary medium, including the red giant wind, equatorial density enhancement, and accretion disk. We compute spatially resolved spectra of accelerated protons and electrons at the forward shock, accounting for downstream velocity gradients and variations in shock properties. Using a multi-zone approach, we synthesize hadronic gamma- ray emission from proton-proton interactions, leptonic gamma-rays from inverse-Compton scattering, and the associated neutrino emission. We present predicted gamma-ray spectra, light curves, and images from our numerical models of T CrB, and assess their detectability with current gamma-ray and neutrino observatories. We find that the early high-energy emission is dominated by the ejecta, with the accretion disk significantly boosting the gamma-ray flux and particle normalization during the first hours after the outburst. By incorporating velocity gradients in the post-shock flow, we demonstrate that maximum particle energies can reach the PeV scale in high-energy explosion scenarios. We show that while GeV gamma-rays are prominent messengers, neutrino detection is feasible primarily in models with high explosion energy and high ambient density.

Figures (18)

• Figure 1: Histograms (rows 1 and 3) and corresponding spatial distributions (rows 2 and 4) for parameters determining the shape of the proton spectrum, across the shock surface for RUN04, $t = 153$ days, $\hat{\xi}_\mathrm{cr}=0.1$, $\mu\neq 0$. The vertical axes on histograms show the number of pixels. The color scales for the surface plots have the same units as the horizontal axes in histograms. Top two rows: the shock speed $V$ (left), the disordered magnetic field $\delta B$ (center), the maximum energy of protons $E_\mathrm{m1}$ and $E_\mathrm{m2}$ (right). $E_\mathrm{m3}$ is not shown here because it is a few orders of magnitude higher than $E_\mathrm{m1}$ and $E_\mathrm{m2}$. The surface distribution shows $\min(E_\mathrm{m1},E_\mathrm{m2})$. Bottom two rows: the gradient of the flow speed $\mu$ (left), the diffusion distance $x_\mathrm{p}(p_\mathrm{max})/R$ for protons with momentum $p_\mathrm{max}$ (center), the injection energy $E_\mathrm{in}$ (right). The distribution of the flow gradient is characterized by $\mu = 0.2 \pm 27.9$. The blue line on the histogram for $\mu$ represents a Gaussian with the mean $0.2$ and standard deviation $27.9$. The shock radius varies, being smaller in the orbital plane and larger out of it, but the spread in shock radius across all possible directions is rather small: $R = 252 \pm 25{\,\rm a.u.}$.
• Figure 2: The average proton energy distributions for several time moments in the RUN04 model. The solid lines correspond to the model that accounts for variations in $\mu$ across the shock surface, and the short-dashed lines are for $\mu = 0$. The gray long-dashed lines show the electron spectra in the case of variable $\mu$ and $K_\mathrm{ep}=1$, enabling a direct comparison with protons. The dashed and solid lines are not aligned at low energies because the spectra have the same acceleration efficiency $\hat{\xi}_\mathrm{cr}=0.1$. At lower energies, all these spectra are $E^{-2}$ power laws, in agreement with Eq. (\ref{['tcb:eqN']}).
• Figure 3: The $\gamma$-ray spectra for the same time moments as in Fig. \ref{['tcorbor:fig-proton-specta']}. The solid and short-dashed lines are for hadronic $\gamma$-rays. The solid lines correspond to the model that accounts for variations in $\mu$ across the shock surface, and the short-dashed lines are for $\mu = 0$. The long-dashed lines represent the IC $\gamma$-ray spectra for the case $\mu\neq 0$ and $K_\mathrm{ep}=0.01$. Spectra of neutrinos are similar to hadronic $\gamma$-ray spectra with the high-energy cutoff at about 0.8 times smaller energy. The shaded regions correspond to the differential sensitivity of Fermi LAT (pink, 10 yrs, taken from https://www.slac.stanford.edu/exp/glast/groups/canda/lat_Performance.htm), H.E.S.S. (blue, 50 hours, 2015arXiv150902902H), LHAASO (gray, 1 yr, 2019arXiv190502773C), ASTRI (red, 50 hours, 2022icrc.confE.884L), CTAO North Alpha (green, 50 hours, 2022icrc.confE.884L). The sensitivity of VERITAS is quite similar to that of H.E.S.S.
• Figure 4: The light curves for $\gamma$-rays in three photon energy ranges estimated from the RUN04 model of T CrB. The solid lines correspond to the model that accounts for variations in $\mu$ across the shock surface, and dashed lines correspond to the model with $\mu = 0$. The thin horizontal line marks the sensitivity limit of different instruments (see Fig. \ref{['tcorbor:fig-pp-specta']}). Its intersection with the light curve provides a rough estimate of a nova's visibility duration.
• Figure 5: Contribution of different components to the $\gamma$-ray light curves in two photon energy ranges for the RUN04 model of T CrB with $\mu\neq 0$. Contributions shown are from the shocked ejecta (dotted line), the shocked disk material (dashed line), and the shocked CBM plasma (dot-dashed line). Each solid line is the sum of these components; they are the same as solid lines in Fig. \ref{['tcorbor:fig-pp-gamma-ray-light-curve']}.