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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transient sources and phenomena focusing on the detections with the nova-alert system, and some new transients yet unpublished or requiring attention.

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Summary Magnetotelluric (MT) observations have revealed sub-vertical electrical conductors that extend from shallow depths into the mid-crust at various geothermal zones, active volcanoes, and active faults worldwide. These deeply rooted sub-vertical conductors have typically been interpreted to represent entire zones of dedicated fluid transport through the crust. We estimate the high-resolution three-dimensional crustal resistivity structure below the Kuju Volcanoes, Japan, using dense observations from 146 broadband MT measurement sites and 47 telluric measurement sites. The resistivity structure highlights sub-vertical conductors that merge into a deep conductor to the north of the volcanoes, with deep low-frequency earthquakes occurring near the southeastern edge of this sub-vertical conductor at 10–30 km depth. This deep conductor branches into several sub-vertical conductors at 2–10 km depth, coinciding with a shallow zone where tectonic earthquakes rarely occur. The surface expressions of active geothermal areas and past volcanic eruptions are all located above the edges of the conductors at 2–6 km depth. Widespread conductive layers exist around the volcanoes above 2 km depth, and their distribution approximately corresponds to a low-gravity-anomaly zone. We discuss the nature of these sub-vertical conductors, the potential causes of their complex structure, and their relationship to local magmatic fluid transport. These sub-vertical conductors, a shallow clay-rich layer, developed fracture systems, and high-strength solidified magma may all contribute to magmatic fluid transport to the surface at the Kuju Volcanoes. In this study, we add the possibility that the edges of these sub-vertical conductors act as important magmatic fluid pathways.

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