Extraordinary Luminous Soft X-Ray Transient MAXI J0158-744 as an Ignition of a Nova on a Very Massive O-Ne White Dwarf
Sakamoto, T.; Mihara, T.; Sugizaki, M.; Gehrels, N.; Kawai, N.; Matsuoka, M.; Kennea, J. A.; Osborne, J. P.; Page, K. L.; Curran, P. A.; Kuin, N. P. M.; Negoro, H.; Walter, F. M.; Tsunemi, H.; Ueda, Y.; Morii, M.; Nakahira, S.; Nakajima, M.; Serino, M.; Tomida, H.; Tsuboi, Y.; Ueno, S.; Yoshida, A.; Pritchard, T.; Kimura, M.; Kotani, T.; Roming, P. W. A.; Hiroi, K.; Usui, R.; Suwa, F.; Kohama, M.; Sugimori, K.
Japan, United States, United Kingdom, Australia
Abstract
We present the observation of an extraordinary luminous soft X-ray transient, MAXI J0158-744, by the Monitor of All-sky X-ray Image (MAXI) on 2011 November 11. This transient is characterized by a soft X-ray spectrum, a short duration (1.3 × 103 s < ΔTd < 1.10 × 104 s), a rapid rise (<5.5 × 103 s), and a huge peak luminosity of 2 × 1040 erg s-1 in 0.7-7.0 keV band. With Swift observations and optical spectroscopy from the Small and Moderate Aperture Research Telescope System, we confirmed that the transient is a nova explosion, on a white dwarf in a binary with a Be star, located near the Small Magellanic Cloud. An early turn-on of the super-soft X-ray source (SSS) phase (<0.44 days), the short SSS phase duration of about one month, and a 0.92 keV neon emission line found in the third MAXI scan, 1296 s after the first detection, suggest that the explosion involves a small amount of ejecta and is produced on an unusually massive O-Ne white dwarf close to, or possibly over, the Chandrasekhar limit. We propose that the huge luminosity detected with MAXI was due to the fireball phase, a direct manifestation of the ignition of the thermonuclear runaway process in a nova explosion.