ASASSN-16oh: nova outburst with no-mass-ejection -- A new type of supersoft X-ray sources in old populations

Abstract

ASASSN-16oh is a peculiar transient supersoft X-ray source without mass-ejection signature in the field of the Small Magellanic Cloud. Maccarone et al. (2019) concluded that ASASSN-16oh is the first dwarf nova with supersoft X-ray originated from an equatorial accretion belt on a white dwarf (WD). Hillman et al. (2019) proposed a thermonuclear runaway model that both the X-rays and \(V\)/\(I\) photons are emitted from the hot WD. We calculated the same parameter models as Hillman et al.'s and found that they manipulated on/off the mass-accretion, and their best fit \(V\) light curves are 6 mag fainter, and decay about 10 times slower, than that of ASASSN-16oh. We propose a nova model induced by high rate mass-accretion during a dwarf nova outburst, i.e., the X-rays originate from a surface of the hydrogen-burning WD whereas the \(V/I\) photons from the irradiated disk. Our model explains the main observational properties of ASASSN-16oh. We also obtained thermonuclear runaway models with no-mass-ejection for a wide range of parameters of the WD mass and mass accretion rates including both natural and forced novae in low-metal environments of \(Z=0.001\) and \(Z=0.0001\). They are a new type of periodic supersoft X-ray sources with no-mass-ejection, and also a bright transient in \(V\)/\(I\) bands if they have a large disk. We suggest that such objects are a candidate of type Ia supernova progenitors because its mass is increasing at a very high efficiency \((\sim 100 \%)\).