Irradiation-Driven Formation of Supersoft X-ray Sources Following Classical Novae

TL;DR

The paper addresses the puzzle that short-period supersoft X-ray sources (SSSs) require high mass-transfer rates from low-mass donors, which traditional binary evolution struggles to supply. Using long-term MESA simulations, the authors show that irradiation from a classical nova eruption, followed by accretion luminosity, heats and expands the companion, driving sustained mass transfer into the hydrogen-burning regime. This irradiation-driven channel yields SSS phases that can persist for well over $10^4$ years across a broad parameter space, aligning with observed short-period SSSs. The work also links irradiation-driven mass transfer to short-period recurrent novae, offering a unified framework for CN, SSS, and RN phenomena with clear observational predictions for future tests.

Abstract

Supersoft X-ray sources (SSSs) are characterized by persistent thermonuclear burning on the surfaces of white dwarfs (WDs).The standard model requires high mass transfer rates of $\sim 10^{-7}\, {\rm M_{\odot}}\,yr^{-1}$ from massive companions, presenting a theoretical impediment to the observed short-period SSSs, whose orbital periods imply low-mass donors theoretically incapable of sustaining such accretion.To resolve this paradox,we propose and demonstrate through detailed simulations that irradiative feedback following a classical nova (CN) eruption provides a natural formation channel.Through detailed binary evolution simulations with MESA, we reveal that sustained WD irradiation initially from the outburst and subsequently from accretion luminosity triggers significant and stable expansion of the low mass companion.This,in turn,drives mass-transfer rates into the stable hydrogen-burning regime and sustains it beyond $10^4$ years after the initiation of hydrogen burning.This mechanism robustly explains the observed population of short-period SSSs. Moreover,when irradiation-driven mass transfer rate drops below the stable accretion rate,it may lead to the rapid accumulation of sufficient material on shorter time scales to trigger a recurrent nova outburst instead of SSS, thereby also offering an explanation for the origin of short-period recurrent novae.

Figures (4)

• Figure 1: Irradiation-induced changes in mass transfer rate and companion star radius following nova outburst in our model. The WD mass, mass of the companion star, the orbital period, and irradiation efficiency are 1.0 ${\rm M}_{\odot}$, 0.5 ${\rm M}_{\odot}$, 0.154 days, and 0.1, respectively. Upper Panel: The black solid line represents the modeled luminosity during the nova outburst for the WD mass of 1.0 ${\rm M}_{\odot}$. The black dashed line represents the mass transfer rate modulated by irradiation of the companion star. The red dashed line shows the accretion luminosity resulting from material accreted onto the WD in the aftermath of the outburst. The horizontal black dotted line indicates the stable accretion rate for a WD of 1.0 ${\rm M}_{\odot}$. Lower Panel: The black solid and dashed lines represent the stellar and the Roche lobe radii of the companion star, respectively.
• Figure 2: Regions in the initial orbital period-secondary mass plane (${\rm log} P$, $M_{2}$) for WD binaries that produce SSSs for initial WD masses of 0.60, 0.80, 1.0, 1.2, and 1.36 $\rm M_{\odot}$. Here, the irradiation efficiency is 1. The red symbols represent observed short period SSSs in this model, with the examples RX J0439.8–6809 Schmidtke1996bvanTeeseling1997, 1E 0035.4–7230 Schmidtke1996a, and RX J0537.7–7034 Greiner2000 corresponding to their potential locations and inferred companion mass ranges.
• Figure 3: Possible properties of hydrogen-burning shells onto the accreting WDs in the plane of the WD mass, $M_\mathrm { WD}$, and the mass transfer rate, $\dot{M}$. The green dashed and solid lines represent $\dot{M}_\mathrm { stable}$ and $\dot{M}_\mathrm { cr}$, respectively. The black dashed and solid lines represent the irradiation efficient $\eta = 1$ and $\eta = 0.1$, respectively.
• Figure 4: Similar to Figure \ref{['fig:2']} but for different irradiation efficiently, and the initial WD masses was set be 0.9$\rm M_{\odot}$.