CubeSat for ultraviolet time-domain astronomy

A wide-field ultraviolet observatory for time-domain astronomy utilizing 6U CubeSat is presented. Ultraviolet waveband is one of the unexplored fields in astronomy. Potential targets are short duration transient sources in UVband: early-phase emission from gravitational wave sources, supernovae shock-breakouts, tidal disruption events around super massive blackholes, etc. The telescope was designed for covering the large error circle of GW detectors, FoV~100 deg2. Thanks to the high quantum efficiency of “delta-doping” detector, the detection limit achieves 20 mag (AB) for 1800 s exposure in NUV band, which is sufficient to detect UV emission from a binary neutron star merger within 200 Mpc from the earth. The satellite has a high-performance on-board computer for onorbit analysis to detect transient sources and measure the magnitude and the accurate position of the target. The obtained information is required to be transferred to the ground within 30 min from the detection to start multimessenger follow-up observations utilizing ground-based observatories and astronomical satellites. In this presentation we show the mission overview and conceptual design of the satellite system. INTRODUCTION The universe is active. In this decade many astronomers have focused on short duration phenomena with a time scale shorter than ~1 day. This new field is referred to as "time-domain astronomy" and is believed to be a promising discovery space in astrophysics. The progresses of astronomy have been invoked by enlargement of aperture diameter of telescopes, because a bigger telescope can search deeper, and deeper observations can provide not only the precise measurements but also the information of the ancient universe. Therefore, a small satellite with limited payload regrettably does not have any advantages in classical astronomy. In contrast, time-domain astronomy has another evaluation axis, namely "time". In this field, a telescope is expected to detect transient phenomena earlier than ever. From this perspective, a recent high-performance small satellite can be an attractive platform on the other hand. The new field, time-domain astronomy, has been opened up thanks to the high-speed/real-time communication (Internet), robotics technology and high-performance computing technology, and those technologies have almost eliminated human eyes and judgements from the observation work flow. Consequently, the response time of the robotic telescopes has been drastically improved. In HETE-II era (2001~2005), the typical delay in starting follow-up observation was about 60 min after gamma-ray detection on the satellite. That delay arose from the satellite communication lag and limited computational resources on satellite. Currently, we can start observation only 10 s after the Swift satellite detects gamma-rays. Recent dramatic progresses in small satellite can provide fast detection alerts from space that can be an alternative to Swift satellite with 1/100 cost. Most of transient sources are believed related to explosive phenomena. When a huge amount of energy is released in a short period, the stellar material around the central engine is heated up to ~105 K and then radiates in UV light and soft X-rays. The heated material then rapidly cooled down as expands and radiates. As the apparent temperature decreases the observed color evolves redder. UV and soft X-rays are therefore ideal for detecting the prompt emission faster than the other wavelength and for investigating the very early phase behavior of transients. In terms of detector technique, X-rays and UV are entirely different. Soft X-ray photons can be treated as particles therefore the sensor must detect every single photon individually (A photon-counting detector is required). Moreover, X-ray photons requires very special optics, such as Walter-I. On the other hand, UV photons behave similar to optical photons, which can be accumulated with usual optics and detected with Sibased semiconductor imaging devices. These properties of UV can suppress the developing cost drastically, and enables the early realization accordingly. In this paper we propose an ultra-wide field UV monitor for time-domain astronomy. In the next section