Predicting the X-ray signatures of the imminent T Coronae Borealis outburst through 3D hydrodynamic modeling

TL;DR

The paper tackles predicting the X-ray signatures of the imminent T CrB outburst by deriving CBM densities from inter-eruption radio data and performing comprehensive 3D HD simulations of the eruption. It models the circumbinary environment as a low-density spherical wind plus an equatorial density enhancement (EDE), explores ranges of explosion energy $E_{\mathrm{bw}}$ and ejecta mass $M_{\mathrm{ej}}$, and synthesizes X-ray light curves and spectra for XMM-Newton and XRISM, including non-equilibrium ionization and Doppler-shifted line profiles. Key findings show that T CrB's CBM is unusually tenuous, the blast is strongly collimated by the disk and EDE into a bipolar shock, and the X-ray emission evolves through three phases with distinct dominant components; early hard X-rays are disk-driven, mid-phase soft X-rays arise from ejecta, and late-time emission from shocked CBM, with blueshifted lines indicating LoS absorption. The results provide a predictive framework to interpret upcoming observations and to constrain the explosion physics (ejected mass and energy), informing broader studies of recurrent novae and Type Ia SN progenitors.

Abstract

T Coronae Borealis (T CrB) is a symbiotic recurrent nova with eruptions in 1866 and 1946. Mounting evidence suggests an imminent outburst, offering a rare opportunity to observe a nearby nova in detail. We constrain the circumbinary medium (CBM) by modeling inter-eruption radio observations and simulate the hydrodynamic evolution of the upcoming outburst to predict its X-ray signatures, focusing on the roles of the red giant companion, accretion disk, and equatorial density enhancement (EDE). We model thermal radio emission from a CBM composed of a spherical wind and a torus-like EDE to estimate its density. We then perform 3D hydrodynamic simulations of the nova outburst, varying explosion energy, ejecta mass, and CBM configuration. From these, we synthesize X-ray light curves and spectra as they would appear to XMM-Newton and XRISM. The CBM in T CrB is significantly less dense than in other symbiotic novae, with a mass-loss rate of $\dot{M} \approx 4 \times 10^{-9}$ M$_{\odot}$ yr$^{-1}$ for a 10 km s$^{-1}$ wind. Despite the low-density EDE, the blast is collimated along the poles by the accretion disk and EDE, producing a bipolar shock. The red giant partially shields the ejecta, forming a bow shock and hot wake. X-ray evolution proceeds through three phases: an early phase (first few hours) dominated by shocked disk material; an intermediate phase ($\sim 1$ week-1 month) driven by reverse-shocked ejecta; and a late phase dominated by shocked EDE. Soft X-rays trace shocked ejecta, hard X-rays arise from shocked ambient gas, and synthetic spectra show asymmetric, blueshifted lines due to absorption by expanding ejecta. The X-ray evolution resembles that of RS Oph and V745 Sco, with a peak luminosity of $L_\mathrm{X} \approx 10^{36}$ erg s$^{-1}$, but features a more prolonged soft X-ray phase, reflecting the lower CBM density and distinct ejecta-environment interaction.

Figures (15)

• Figure 1: Colour-coded cross-section of the gas density distribution (in cm$^{-3}$) illustrating the initial conditions of model M3-E3-D6-Z4.5-X4-W1 (Run 4 in Table \ref{['tab2']}). The cross-section emphasizes the EDE structure, visible as an enhanced density in the equatorial plane. The inset provides a zoomed view of the initial geometry of the T CrB system: the WD and the spherical blast wave are located at the origin (white sphere, left), while the RG companion is positioned along the $x$-axis at $x = 0.9$ au (orange sphere, right). The accretion disk (blue) is centered on the WD.
• Figure 2: Cumulative mass of the CBM enclosed within a given radius as a function of radial distance from the WD, for model M3-E3-D6-Z4.5-X4-W1 (Run 4 in Table \ref{['tab2']}). The plot shows contributions from the spherical RG wind (dashed line), the EDE (dotted line), and the accretion disk (solid line). The shaded region on the left marks the extent of the initial blast wave; the CBM is not described within this zone in the simulation. Vertical dot-dashed lines indicate the position of the RG companion along the $x$-axis.
• Figure 3: Evolution of the blast wave in reference model M3-E3-D6-Z4.5-X4-W1 (see Run 4 in Table \ref{['tab2']} for model parameters). Each panel displays the 3D density structure of the nova remnant at the indicated time (upper right corner of each panel), with a yardstick in the upper left corner denoting the spatial scale. The colored isosurface traces the distribution of the ejecta, with color representing local gas density (see color bar at right of each panel). The isosurface is partially clipped to expose the internal structure of the remnant. A semi-transparent gray isosurface outlines the forward shock. The RG companion is shown as an orange sphere, most visible at early times (top panels). The accretion disk, rendered in violet around the WD, appears only in the earliest frame (upper left panel). Green arrows indicate the velocity field of the outflowing plasma, with arrow color encoding the flow speed (color bar at lower right). The EDE is visible at times earlier than 40 days (upper panels and lower left panel), rendered in blue (color bar at lower left), and is partially clipped to reveal the remnant structure. The blast wave is visibly collimated along the polar directions, shaped by the combined influence of the EDE and the accretion disk.
• Figure 4: The 2D cross-sections in the $(x, z)$ plane display the logarithmic gas density distribution of the nova remnant at the indicated times for most models listed in Table \ref{['tab2']} (the model name is shown in the upper left corner of each panel). Runs 6 and 11 are omitted since their morphology is similar to Run 5. The bipolar structure of the blast wave is approximately aligned with the $z$-axis. A scale bar in the lower right corner of each panel indicates the physical length scale. The EDE itself is not visible in models M3-E3-D6-Z4.5-X4-W1 (Run 4) and M3-E3-D6-Z4.5-X4-W1-NOD (Run 14), as they adopt the most compact EDE configuration among all models considered (see Table \ref{['tab2']}), and the entire EDE has already been shocked by the time shown.
• Figure 5: Synthetic X-ray emission maps from model M3-E3-D6-Z4.5-X4-W1 (Run 4) in the $[0.5, 2]$ keV (left panels) and $[2, 10]$ keV (right panels) energy bands at the indicated times (top to bottom). Each image is normalized to its respective maximum intensity for visualization purposes. The scale bar in the lower-left corner of each panel denotes the physical length scale; at the distance of T CrB (890 pc), a length of 400 au corresponds to $0.45^{\prime\prime}$.