Gamma-Ray Bursts (GRBs) are the most energetic explosions in the universe.
We have already known the most distant GRBs at the redshift z=8.26
(spectroscopic)
and z=9.4 (photometric), so we can use GRBs as the cosmological probing tool.
However we could obtain little knowledge of physical conditions in the early
universe from these high-z GRBs while their redshifts were measured. We hope
to perform the high dispersion spectroscopy with the large area telescopes
during the afterglows are still bright. Then we can challenge to reveal
the reionization epoch and the origins, early metal enrichment, star formation
history as well as to detect GRBs from first stars (population-III stars).

Japanese GRB community has organized a working group of
a future small satellite mission, named "HiZ-GUNDAM"
(High-z Gamma-Ray Bursts for Unraveling the Dark Ages Mission),
to pioneer the observation of early universe with high-z GRBs.
This mission consists of two instruments, the X-ray imaging system
with coded aperture mask and the optical/near infrared telescope
with 30cm diameter. Once GRBs are detected and localized, HiZ-GUNDAM
rapidly perform the follow-up observation like the Swift satellite,
and determine the rough redshift with the photometric observation with
the optical/NIR telescope. The onboard measurement of redshift and
the redshift alert system will play an important role in leading
the follow-up observation for the early afterglows by large area telescope.

The X-ray imaging system continuously monitors a large field of view of
~1 steradian in the energy range of 1-20 keV with the effective area of
500-1,000 cm2. We are developing 1-dimensional silicon strip sensor with
the size of 19.2mm x 16mm (~3 cm2), and which has 64ch strip-type electrode
with the width of 0.3mm. We are also developing
the 64ch specific ASICs which is modified version of ones developed by
Prof. Takahashi and Prof. Ikeda group in ISAS/JAXA. According to the
current simulations by T-SPICE electric circuit simulator, they have
enough gain to readout the small charges from 1 keV X-ray photon,
and we can integrate appropriate trigger system. Using the coded aperture
mask with the same pitch of 0.3mm at the distance of 30cm from the detector
plane, we can determine the position of GRBs with ~10 arcmin accuracy in
geometrically and ~5 arcmin for bright GRBs. Since the high-z GRBs have
low brightness with long time duration because of the cosmological
time dilation,
we will enhance so-called imaging trigger system.

The optical/NIR telescope has the primary mirror with 30cm in diameter,
and we consider the offset Gregorian optics to avoid the stray light
from the sun and the earth. We will divide the optical ray of 0.5--1.7 um
into 4 bands with dichroic mirror, and perform the photometric observations
and select the candidates of high-z GRBs.
A reflection baffle (aperture shade) in front of the telescope gives us
the wide visibility larger than the field of view of X-ray imaging system.
Therefore we effectively perform the follow-up observations for GRBs detected
by the X-ray detector.

In this talk, we introduce the concept of HiZ-GUNDAM mission and
the developing/planning instrumentations.