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 x 10^3 s < \Delta T_d < 1.10 x 10^4 s), a very rapid rise (< 5.5 x 10^3 s), and a huge peak luminosity of 2 x 10^40 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 (SMARTS), 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 extremely early turn-on of the super-soft X-ray source (SSS) phase (< 0.44 d), 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.
Highlights
Classical or recurrent novae are typically characterized by a rapid optical increase of 6 magnitudes or more, followed by a decline to quiescence over the 3–300 days (Warner 1995)
Further follow-up observations by Swift and groundbased optical observations confirmed that this source is a binary system consisting of a white dwarf (WD) and a Be star at the distance of the SMC (d = 60 kpc; Hilditch et al 2005; Li et al 2012)
The X-ray transient Monitor of All-sky X-ray Image (MAXI) J0158−744 is characterized by (1) a soft X-ray spectrum with most of the X-ray photons being detected below 4 keV (Figures 1 and 2), (2) a short duration, (3) a very rapid rise time (
Summary
Classical or recurrent novae are typically characterized by a rapid optical increase of 6 magnitudes or more, followed by a decline to quiescence over the 3–300 days (Warner 1995). The TNR blows off the outer layer of the accumulated mass and causes an optically thick wind expanding up to ∼100 R It produces bright blackbody emission (∼1038 erg s−1, comparable to the Eddington luminosity of a 1 M object) at optical bands. This optical nova phase lasts for ∼3–300 days (Warner 1995). At the time of the TNR, the early and short emission (a)
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