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Open Access. This article is distributed under the terms of the Creative Commons Attribution 4.0 International License (http://creativecommons.org/licenses/by/4.0/), which permits unrestricted use, distribution, and reproduction in any medium, provided you give appropriate credit to the original author(s) and the source, provide a link to the Creative Commons license, and indicate if changes were made. The Ultra-Fast Flash Observatory (UFFO) Burst Alert and Trigger Telescope (UBAT) has been designed and built for the localization of transient X-ray sources such as Gamma Ray Bursts (GRBs). As one of main instruments in the UFFO payload onboard the Lomonosov satellite (hereafter UFFO/Lomonosov), the UBAT’s roles are to monitor the X-ray sky, to rapidly locate and track transient sources, and to trigger the slewing of a UV/optical telescope, namely Slewing Mirror Telescope (SMT). The SMT, a pioneering application of rapid slewing mirror technology has a line of sight parallel to the UBAT, allowing us to measure the early UV/optical GRB counterpart and study the extremely early moments of GRB evolution. To detect X-rays, the UBAT utilizes a 191.1 cm scintillation detector composed of Yttrium Oxyorthosilicate (YSO) crystals, Multi-Anode Photomultiplier Tubes (MAPMTs), and associated electronics. To estimate a direction vector of a GRB source in its field of view, it employs the well-known coded aperture mask technique. All functions are written for implementation on a field programmable gate array to enable fast triggering and to run the device’s imaging algorithms. The UFFO/Lomonosov satellite was launched on April 28, 2016, and is now collecting GRB observation data. In this study, we describe the UBAT’s design, fabrication, integration, and performance as a GRB X-ray trigger and localization telescope, both on the ground and in space. The Korean work is supported by the National Research Foundation grants funded by MSIP of Korea (Creative Research Initiatives program for RCMST, No. 2015R1A2A1A01006870, and No. 2015R1A2A1A15055344). SJ acknowledges the support of the Korea Basic Science Research Program through NRF-2015R1D1A4A01020961. MBK acknowledges support from the NRF-2015-Global Ph.D. Fellowship Program in Korea. The Russian work was partially supported by ROSCOSMOS grants and by RFFI grants No. 13-02-12175 and No. 15-35-21038 and also acknowledges support from the Development Program of Lomonosov Moscow State University. AJCT acknowledges support from the Spanish MINECO Projects AYA 2009-14000-C03-01/ESP and AYA 201571718R (including EU/FEDER funds). The Taiwan authors thank Taiwan's National Science Council Vanguard Program (100-2119-M-002-025) as well as the Ministry of Science and Technology (MOST) for its funding (104-2811-M-002-160). We also acknowledge the support of the National Space Organization (NSPO) of Taiwan.

Open Access. This article is distributed under the terms of the Creative Commons Attribution 4.0 International License (http://creativecommons.org/licenses/by/4.0/), which permits unrestricted use, distribution, and reproduction in any medium, provided you give appropriate credit to the original author(s) and the source, provide a link to the Creative Commons license, and indicate if changes were made. The payload of the UFFO (Ultra-Fast Flash Observatory)-pathfinder now onboard the Lomonosov spacecraft (hereafter UFFO/Lomonosov) is a dedicated instrument for the observation of GRBs. Its primary aim is to capture the rise phase of the optical light curve, one of the least known aspects of GRBs. Fast response measurements of the optical emission of GRB will be made by a Slewing Mirror Telescope (SMT), a key instrument of the payload, which will open a new frontier in transient studies by probing the early optical rise of GRBs with a response time in seconds for the first time. The SMT employs a rapidly slewing mirror to redirect the optical axis of the telescope to a GRB position prior determined by the UFFO Burst Alert Telescope (UBAT), the other onboard instrument, for the observation and imaging of X-rays. UFFO/Lomonosov was launched successfully from Vostochny, Russia on April 28, 2016, and will begin GRB observations after completion of functional checks of the Lomonosov spacecraft. The concept of early GRB photon measurements with UFFO was reported in 2012. In this article, we will report in detail the first mission, UFFO/Lomonosov, for the rapid response to GRB observations. © The Author(s) 2017. The Korean work is supported by the National Research Foundation grants funded by MSIP of Korea (Creative Research Initiatives program for RCMST, No. 2015R1A2A1A01006870, and No. 2015R1A2A1A15055344). The Russian work was partially supported by ROSCOSMOS grants and by RFFI grants No. 13-02-12175 and No. 15-35-21038 and acknowledges support from the Development Program of Lomonosov Moscow State University. AJCT acknowledges support from the Spanish MINECO Projects AYA 2009-14000-C03-01/ESP and AYA 2015-71718R (including EU/FEDER funds). The Taiwanese authors thank Taiwan's National Science Council Vanguard Program (100-2119-M-002-025) and Ministry of Science and Technology (MOST) funding (104-2811-M-002-160). SJ acknowledges the support from the Korea Basic Science Research Program through NRF-2014R1A6A3A03057484 and NRF-2015R1D1A4A01020961. MBK acknowledges the support from NRF-2015-GPF.

The UFFO/Lomonosov has been successfully launched into Sun synchronous orbit and is operational through tests and calibrations since its launch on Apr. 28, 2016. As a pathfinder of UFFO, it will be the first space instrument to use a fast slewing mirror which reduce the trigger latency of optical telescope, less than a second, to explore early time domain of GRB evolution.In this article, we will report in detail the first mission, UFFO/Lomonosov, for the rapid response to observe early photons from GRBs in orbit.