Our project aims to identify the physical nature of gamma-ray burst (GRB) emission via measurement of the optical spectral shape of this emission during the prompt phase, usually lasting up to 70 sec. These measurements require a fast-moving optical telescope and instrumentation to respond autonomously to real-time GRB alerts. The Nazarbayev University Transient Telescope at Assy-Turgen Astrophysical Observatory (NUTTellA-TAO) has a 0.7 m aperture and can point anywhere above the local horizon in 8 seconds. We receive GRB Alerts via an internet socket connection to the Gamma Coordinates Network (GCN) at the telescope site. We measure the GRB prompt optical emission with the Burst Simultaneous Three-Channel Imager (BSTI), which incorporates 3 EMCCD cameras, at Sloan g', r', and i' bands, for simultaneous high time-resolution imaging as fast as a few hundred milliseconds per frame. NUTTellA-TAO is a fully automated telescope. In 2020 and 2023, we observed GRB afterglow starting from 58 and 41 seconds after the BAT trigger. It is the earliest afterglow optical observation with three filters simultaneously.
We report on the first ~two years of operation of the NUTTelA-TAO (Nazarbayev University Transient Telescope at Assy-Turgen Astrophysical Observatory). The instrument system is designed to respond autonomously, within seconds, to astronomical transient localization alerts, with simultaneous high-time resolution measurements in Sloan g’,r’, and i’ filters. Our primary objective is measurement of the shape of the gamma-ray burst (GRB) prompt optical spectrum by responding to rapid GRB localization alerts during bright y-ray emission. Measurements (detections or upper limits) have been made for 10 Swift-BAT alerts, with 2 very early afterglow detections, starting as early as 10 seconds after alert receipt. The NUTTelA Transient Catalog is available at ecl.nu.edu.kz/ntc. We describe the instrument performance and operational and technical challenges encountered in this experiment, which should inform other automated transient experiments. Overall, the system has performed well technically, but weather at the time of alerts, and possibly limitations of the Swift BAT alert system, have slowed our detection of prompt optical emission. We also describe future plans for improvement and expansion of our program
We aim to measure the optical spectral shape of gamma-ray bursts (GRBs) in the prompt phase, in order to identify the emission mechanism (after Grossan et al. 2019, JHEA 23, 14). This requires measuring GRB emission within ~10 s, measuring in at least 3 spectral channels simultaneously, at high time-resolution. The Nazarbayev University Transient Telescope at Assy-Turgen Astrophysical Observatory (NUTTelA-TAO) can point and track any celestial target above the horizon in < 8 s, with autonomous response to Swift and other real-time GRB alerts with ≤ 7' position errors. With three electron multiplied CCD (EMCCD) cameras, the Burst Simultaneous Three-Channel Imager (BSTI) can measure emission at Sloan g',r', and i' filters simultaneously, down to ~0.1 s time resolution. We report our instrument design and on-sky performance during commissioning. We detected early emission from GRBs 200925B and 201015A, though prompt emission was not obviously detected. We report early spectral shape measurements, along with other automatic responses to GRB alerts. The system is currently operating regularly, though with reduced time-resolution due to equipment failures, soon to be repaired. We anticipate completion of commissioning d performance; we discuss challenges and future plans for completion of commissioning and beyond.
In our Ultra-Fast Astronomy (UFA) program, we aim to improve measurements of variability of astronomical targets on millisecond and shorter time scales. In this work, we present initial on-sky measurements of the performance of silicon photomultiplier detectors (SiPMs) for UFA. We mounted two different SiPMs at the focal plane of the 0.7-m aperture Nazarbayev University Transient Telescope at the Assy-Turgen Astrophysical Observatory, with no filter in front of the detector. The 3 mm × 3 mm SiPM single-channel detectors have a field of view of 2.2716 ′ × 2.2716 ′ . During the nights of October 28–29, 2019, we measured sky background, bright stars, and an artificial source with a 100-Hz flashing frequency. We compared detected SiPM counts with Gaia satellite G-band flux values to show that our SiPMs have a linear response. With our two SiPMs (models S14520-3050VS and S14160-3050HS), we measured a dark current of ∼130 and ∼85 kilo counts per second (kcps), and a sky background of ∼201 and ∼203 kcps, respectively. We measured an intrinsic crosstalk of 10.34% and 10.52% and derived a 5σ sensitivity of 13.9 and 14 Gaia G-band magnitude for 200-ms exposures, for the two detectors, respectively. For a 10-μs window, and allowing a false alarm rate of once per 100 nights, we derived a sensitivity of 22 detected photons, or six Gaia G-band magnitudes. For nanosecond timescales, our detection is limited by crosstalk to 12 detected photons, which corresponds to a fluence of ∼155 photons per square meter.
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