23 giant flares from 13 active stars (eight RS CVn systems, one Algol system, three dMe stars and one YSO) were detected during the first two years of our all-sky X-ray monitoring with the gas propotional counters (GSC) of the Monitor of All-sky X-ray Image (MAXI). The observed parameters of all of these MAXI/GSC flares are found to be at the upper ends for stellar flares with the luminosity of 10 31−34 ergs s −1 in the 2-20 keV band, the emission measure of 10 54−57 cm −3 , the e-folding time of 1 hour to 1.5 days, and the total radiative energy released during the flare of 10 34−39 ergs. Notably, the peak X-ray luminosity of 5 +4 −2 × 10 33 ergs s −1 in the 2-20 keV band was detected in one of the flares on II Peg, which is one of the, or potentially the, largest ever observed in stellar flares. X-ray flares were detected from GT Mus, V841 Cen, SZ Psc, and TWA-7 for the first time in this survey. Whereas most of our detected sources are multiple-star systems, two of them are single stars (YZ CMi and TWA-7). Among the stellar sources within 100 pc distance, the MAXI/GSC sources have larger rotation velocities than the 2 other sources. This suggests that the rapid rotation velocity may play a key role in generating large flares. Combining the X-ray flare data of nearby stars and the sun, taken from literature and ou r own data, we discovered a universal correlation of τ ∝ L 0.2 X for the flare duration τ and the intrinsic X-ray luminosity L X in the 0.1-100 keV band, which holds for 5 and 12 orders of magnitude in τ and L X , respectively. The MAXI/GSC sample is located at the highest ends on the correlation.

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 × 10 3 s < ∆T d < 1.10 × 10 4 s), a very rapid rise (< 5.5 × 10 3 s), and a huge peak luminosity of 2 × 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.

Various transient phenomena on a timescale ranging from seconds to days appear at unexpected sky positions in X-rays. MAXI, Monitor of All-sky X-ray Image, on the International Space Station has been monitoring about 95% of the sky a day and has detected transient objects since 2009 August. Here, we describe quasi-real-time data processing systems of MAXI and a subsequent nova-alert system to find transient objects, and present the capabilities for the nova-alert system to detect transient events with excess fluxes from ≳80 mCrab in a single scan transit to ≳8 mCrab for 4 d, and to send prompt alert information to the world in less than 30 s after the onboard detection of a burst, making the best use of the International Space Station (ISS) real-time network. We also report on highlights of scientific results obtained with the system until the end of the first extended mission phase, 2015 March. Including 15 X-ray novae solely or independently discovered, we have reported on 177 transient phenomena, such as X-ray bursts, outbursts, and state transitions of X-ray binaries and X-ray flares from active stars and blazars, via the Astronomer's Telegram, and on 63 burst phenomena of other types via the Gamma-ray Coordinates Network. We summarize the results of these transient sources and phenomena focusing on the detections with the nova-alert system, and some new transients yet unpublished or requiring attention.

The Monitor of All-sky X-ray Image (MAXI) Gas Slit Camera (GSC) detects gamma-ray bursts (GRBs), including bursts with soft spectra, such as X-ray flashes (XRFs). MAXI/GSC is sensitive to the energy range from 2 to 30 keV. This energy range is lower than other currently operating instruments which are capable of detecting GRBs. Since the beginning of the MAXI operation on 2009 August 15, GSC observed 35 GRBs up to the middle of 2013. One third of them were also observed by other satellites. The rest of them show a trend to have soft spectra and low fluxes. Because of the contribution of those XRFs, the MAXI GRB rate is about three times higher than those expected from the BATSE log N–log P distribution. When we compare it to the observational results of the Wide-field X-ray Monitor on the High Energy Transient Explorer 2, which covers the the same energy range as that of MAXI/GSC, we find the possibility that many of the MAXI bursts are XRFs with Epeak lower than 20 keV. We discuss the source of soft GRBs observed only by MAXI. The MAXI log N–log S distribution suggests that the MAXI XRFs are distributed over a closer distance than hard GRBs. Since the distributions of the hardness of galactic stellar flares and X-ray bursts overlap with those of MAXI GRBs, we discuss the possibility of confusion of such galactic transients with the MAXI GRB samples.

We present a large X-ray flare from a nearby weak-lined T Tauri star TWA-7 detected with the Gas Slit Camera (GSC) on the Monitor of All-sky X-ray Image (MAXI). The GSC captured X-ray flaring from TWA-7 with a flux of 3 $\times$ 10 $^{-9}\ $ erg cm $^{-2}\ $ s $^{-1}$ in 2–20 keV band during the scan transit starting at 2010-09-07 18:24:30 (UT). The estimated X-ray luminosity at the scan in the energy band is 3 $\times$ 10 $^{32}\ $ erg s $^{-1}$ , indicating that the event is among the largest X-ray flares from T Tauri stars. Since MAXI GSC monitors a target only during a scan transit of about a minute per 92 min orbital cycle, the luminosity at the flare peak might have been higher than that detected. At the scan transit, we observed a high X-ray-to-bolometric luminosity ratio, log $\ L_{\rm X}/L_{\rm bol}$$=$$-$ 0.1 $^{+0.2}_{-0.3}$ ; i.e., the X-ray luminosity is comparable to the bolometric luminosity. Since TWA-7 has neither an accreting disk nor a binary companion, the observed event implies that none of those are essential to generate such big flares in T Tauri stars.