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

Abstract

Context. T Coronae Borealis (T CrB) is a symbiotic recurrent nova, with confirmed eruptions in 1866 and 1946. Mounting evidence suggests an imminent outburst, which would offer a rare opportunity to observe a nearby nova. Aims. We constrain the circumbinary medium (CBM) properties by modeling inter-eruption radio data, then simulate the hydrodynamic evolution of the upcoming T CrB outburst to predict its X-ray signatures, focusing on the impact of the red giant companion, accretion disk, and equatorial density enhancement (EDE) on the remnant and emission. Methods. We modeled the thermal radio signatures of a CBM that consisted of a spherical wind-like component and a torus-like EDE to quantify its density. We then ran three-dimensional hydrodynamic simulations of the nova outburst, varying the explosion energies, ejecta masses, and circumbinary configurations. From these, we synthesized X-ray light curves, spectra, and maps as observed by XMM-Newton and XRISM. Results. The CBM in T CrB is much less dense than in other symbiotic recurrent novae, with a mass-loss rate of Ṁ ≈ 4 × 10−9 M⊙ yr−1 for a 10 km s−1 wind. Despite the expected low-density CBM, the outburst’s blast will be strongly collimated along the poles by the combined influence of the accretion disk and EDE, producing a bipolar shock. The companion star partially shields the ejecta, thus forming a bow shock and a hot wake. The X-ray evolution proceeds through three phases: an early phase (the first few hours) dominated by shocked disk material; an intermediate phase (approximately one week to one month) driven by reverse-shocked ejecta; and a late phase dominated by shocked EDE. Soft X-rays trace shocked ejecta, while hard X-rays originate from shocked ambient gas. Synthetic spectra show asymmetric, blueshifted lines due to the absorption of redshifted emission by expanding ejecta. Conclusions. The predicted X-ray evolution of TCrB shows similarities to RS Oph and V745 Sco, as it reaches a comparable peak luminosity (LX ≈ 1036 erg s−1), but it has a more prolonged soft X-ray phase that reflects its less dense CBM and distinct ejecta-environment interaction.