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GRB which occurs in the field of view (FOV) of MAXI will be detected automatically by the nova-alert system on the ground. Alert will be issued instantly to world-wide astronomers. The instantaneous field of view of MAXI GSC is 2% of the whole sky. The expected number of GRB in the FOV is 3.5 per year. The one of the GRB afterglow is 2.5 per year.
GRBs are hyper-novae explosions of super massive stars in distant galaxies located at the redshift >~ 1. The peak X-ray luminosity reaches 100 million of the whole galaxy. They are the largest explosions in the universe. Riken had an important role in the study of the GRB origin with HETE-2 satellite. Now we continue the GRB study with MAXI.
There is a phenomenon of sudden appearance of an X-ray star, called "X-ray nova". It becomes the brightest star in the sky in one day. The intensity decreases exponentially in about 100 days. The recurrent X-ray nova has not been observed so far. The first X-ray nova is A0620-00 discovered in 1980's. Since then 29 X-ray novae were found, in other word one in every year. They are binaries with a blackhole with 100 solar mass and a late-type star. The 90% of the known blackhole stars appeared as X-ray novae. The gas of the companion star falls to the blackhole and forms an accretion disk around it. At first, the disk is stable and continues to rotate. When the amount of the disk
reaches a value, the gas falls onto the blackhole at once. The gas is heated up to 100 million K by the strong gravity of the blackhole and emits X-ray. It appears as an X-ray nova.
MAXI is 10 times more sensitive and covers three times in distance. The volume of the galactic disk is 10 times more. Thus we expect 10 times more X-ray novae will be found with MAXI. It is one in every MONTH.
X-ray objects are highly variable. There is a flare phenomenon that X-ray star becomes 10 – 100 times
brighter than the normal and get back to the normal brightness in about a month.
There are several reasons of flares.
The neutron star of the binary goes through the periastron, accretes much gas, and emits X-ray.
We need to watch them in several years to discover those with long orbital periods.
SGR (Soft gamma-ray repeater) and AXP (Anomalous X-ray pulsar) are neutron stars with extremely
strong magnetic fields. Those stars explode in gamma-ray with the magnetic energy.
There was a giant burst from SGR 1806-20 in December 2004. The X-ray flux reached 10 million
times more than that of the strongest persistent X-ray source Sco X-1. Two new SGRs were discovered this year.
Flare stars which are smaller ones than the sun have a stronger magnetic fields (black spots)
and often have X-ray flares.
AGN (Active galactic nuclei) has a super massive blackhole in the center. The gas accretes and emits X-ray. It appears in a bright spot in the center of a galaxy in Seyfert galaxies, or it is brighter than the whole galaxy and appears as a point source in quasars.
X-ray objects have some types of variability : orbital period of the binary, no periodicity at all, and quasi-periodicity. MAXI can observe continuously the bright 100 objects. The time scale of the variability of the blackhole is proportional to the mass of the blackhole (0.1 second to years). That of a neutron star is related to the spin period as 1000 rotations per second. The time variability contains important information on the central objects. There are only a few observations of the galactic and extra-galactic objects in the longer period than a week. MAXI searches in the discovery space in time domain.
The star like the sun mainly emits optical lights, and the X-ray and radio emissions are very weak. On the other hand, the AGNs and blackhole binaries emits equally strong radio, infrared, optical, X-ray, gamma-ray and TEV gamma-ray. To study the behavior of these objects we need not only optical telescope, but also those of multi wave-lengths. Blazars and glactic jet objects emits jets. We use X-ray to detect mass accretion, and radio to detect the jet emission. The optical and radio observation can be done from the ground, while the X-ray, which contains useful information, can only be done from space. Thus astronomers of mulch wave-lengths world-wide hope the X-ray monitor, MAXI.
The X-ray catalog was first produced by the HEAO-1 satellite in 1978 followed by the ROSAT in the lower energy band in 1991. But nowadays most of the galaxies are dimmer than the cataloged values. X-ray objects are variable. In 10 years, many objects has become dimmer and some would be brighter. MAXI has the same energy band as HEAO-1 and will create a new X-ray catalog in the 21st century. High sensitivity of MAXI can create a HEAO-1 catalog in every month. It also creates a daily sky map for bright sources.
The solar system is surrounded by a hot bubble of an old supernova remnant. The temperature of the gas is low, and it emits strong oxygen and neon emission lines. The mapping of the hot gas was done with the ROSAT in 1991 but only with X-ray band intensity. MAXI can make a map with oxygen and neon line only, which traces the distribution of the hot gas more precisely.