arXiv subjects

Hsiang-Kuang Chang

Publications and source records attributed to Hsiang-Kuang Chang.

At least 19 recordsLinked to original sources

arXiv2026

The Complete Catalog of Gamma-Ray Transients Observed by GRBAlpha & VZLUSAT-2 CubeSat Missions

We present the largest sample of gamma-ray transients observed by any CubeSat mission so far. Observations were acquired by a 1U CubeSat GRBAlpha, the smallest astrophysical space observatory, and a 3U CubeSat VZLUSAT-2. Both missions were technological pathfinders and carried a novel CsI scintillator-based detector read-out by silicon photomultipliers. They operated on Sun-synchronous low Earth orbits below 550 km for about four years; GRBAlpha between March 2021 and June 2025 while VZLUSAT-2 between January 2022 and November 2025. Despite being technological experiments, they observed over 300 gamma-ray transients including gamma-ray bursts (GRBs), solar flares, soft gamma repeaters and one outburst from an X-ray binary. Among these are the two brightest GRBs ever observed, GRB 221009A and GRB 230307A, without saturation and GRBs at redshifts up to z=4.2. GRBAlpha also contributed to the InterPlanetary Network. Regular monitoring of transients was demonstrated by a detection rate of two transients or one GRB a week and the shortest time between two subsequent detections of only 42 minutes. We show that a constellation of nanosatellites around the Earth would observe at least 60% of Fermi/GBM GRBs with 5$σ$ significance and over 90% at 3$σ$ level. GRBAlpha and VZLUSAT-2 prove that routine monitoring of the gamma-ray sky can also be done by low-cost and quickly developed nanosatellite missions.

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arXiv2026

Bridging X-ray Polarization with Timing & Spectroscopic Parameters of a galactic black hole: Swift J1727.8-1613

We report the discovery of a correlated energy-dependent time lag and degree of polarization for Swift J1727.8-1613 during its 2023 outburst. The energy-dependent time lag is measured around the type-C quasi-periodic oscillations (QPO) observed by IXPE on 2023-09-07, while the degree of polarization is obtained from energy-resolved polarimetric measurements. The Spearman correlation coefficient was found to be 0.8, with a null hypothesis probability of 4.2\%. Furthermore, the correlation value drops as the quality factor, or Q value, of the observed QPO frequencies decreases. The spectral properties of Swift J1727.8-1613 are analyzed using simultaneous Insight/HXMT data. Thereafter, we present model-independent theoretical arguments to show that processes other than inverse Comptonization also contributes to both the observed polarization and time lags. This correlation may therefore point to additional mechanisms contributing to the connection between the spectral, temporal, and polarimetric properties of black hole binaries in their hard state.

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arXiv2026

Extreme Variability Reveals How the Eddington Ratio Regulates Coronal Power in Active Galactic Nuclei

The bolometric luminosity ($L_{\rm bol}$) of active galactic nuclei (AGNs) is a key tracer of accretion physics, but its direct determination is often hindered by limited spectral coverage and contamination of the host galaxy. Bolometric corrections ($κ_λ = L_{\rm bol}/L_λ$) offer a practical means of estimating $L_{\rm bol}$, with the X-ray bolometric correction ($κ_{\rm 2-10}$) being crucial for exploring the coupling between the accretion disk and the X-ray corona. Here we present multi-epoch, multi-wavelength observations of five highly variable, changing-state AGNs that span more than three orders of magnitude in Eddington ratio ($-3.6\lesssim \log λ_{\rm Edd} \lesssim -0.5$). This unique data set reveals a remarkably tight relation between $κ_{\rm 2-10}$ and $λ_{\rm Edd}$, with an intrinsic scatter of only $\sim0.05$ dex. We find that while the sources show bolometric corrections following different tracks in luminosity space that depend on black hole mass, they all display the same $κ_{\rm 2-10}-λ_{\rm Edd}$ trend. This shows unambiguously that $λ_{\rm Edd}$ is the primary driver of X-ray bolometric corrections, and points to a tight underlying trend that can be used to obtain reliable estimates of bolometric output from X-ray luminosities. Our results highlight how time-domain, multi-wavelength observations of variable AGN offer unique insights into the accretion flow structure and its radiative output.

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arXiv2026

Multimission Observations of GS 1354-64 during the 2025-2026 Outburst: First Results

The Galactic transient black hole GS 1354-64 recently showed a new outburst, which has been continuously monitored by {\it MAXI}, {\it NuSTAR} and {\it Insight-HXMT} missions. The ongoing $2025$--$2026$ outburst shows a slow rise with an unusually short period ($\sim 3$~days) of X-ray flare of peak flux $\sim 1.4$~Crab, followed by another relatively weak flare of intensity $\sim 0.8$ Crab. The source was observed to evolve through ``canonical'' spectral states in a hardness-intensity diagram (HID) during rising phase of the outburst, but the subsequent outburst profile during this study, the source did not follow the reverse trend of the HID. A rapid evolution of quasi-periodic oscillation (QPO) frequencies ($\sim 0.07-4$ Hz) is observed during hard and intermediate spectral states without any signature of QPOs in the soft state. The evolution of the observed low frequency QPOs shows a monotonically increasing (rising phase) signature as well as decreasing (decay phase) one, these are further studied with the propagating oscillatory shock model to understand the nature of the evolution of the shock wave responsible for the origin of the observed QPOs. The broadband energy spectra from {\it NuSTAR} ($3-70$ keV) and {\it Insight-HXMT} ($2-60$ keV) are well described by thermal (\textit{diskbb}) and reflection (\textit{relxill}) model components, indicating a strong signature of a relativistic reflection feature. Using ``canonical'' observations of a softer state, we found the source to be maximally rotating with $a_k \sim 0.998$ and inclination angle to be as $i \sim 63^\circ - 70^\circ$, which are consistent with earlier reports.

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arXiv2026

Instrument response generation using high-resolution 3D voxelization of GRBAlpha and VZLUSAT-2 satellites with MEGAlib

Pathfinder gamma-ray burst (GRB) detecting CubeSat missions such as GRBAlpha and VZLUSAT-2 have demonstrated the successful application of scintillator detectors with silicon photomultipliers in low Earth orbit (LEO). To produce more comprehensive scientific analysis of the data, the effective area of the detector needs to be characterised at different energies. A large part of this process requires a thorough understanding of the detectors response matrices based on the satellite mass model typically performed through Geant4 and MEGAlib simulations. We use a novel voxelization and binning methodology to turn complex 3D geometries into MEGAlib-compatible versions, and we validate these experiments by showing that the simulation results with Geant4 agree within an order of 10%. We use MEGAlib simulations from various angles around the spacecraft to generate instrumental response matrices for both simple and complex geometries, in the case of GRBAlpha and VZLUSAT-2, respectively.

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arXiv2026

Long-term performance of SiPMs in space environment measured by GRBAlpha, GRBBeta, and VZLUSAT-2 CubeSats

In this work, we report the successful application of silicon photomultipliers (SiPMs) in gamma-ray burst (GRB) detectors used in CubeSats operating in the low Earth orbit (LEO) radiation environment. It is known that SiPMs are susceptible to radiation damage, leading to an increase in the dark count rate. This results in an increase in the low-energy threshold in detectors combining SiPMs and scintillators. Despite this drawback, they became popular in gamma-ray detectors on CubeSats due to their low operating voltage, small size and fast response. Therefore, it is important to characterise their long-term performance in the space environment. Here, we describe the changes in the dark count rate and low-energy threshold of S13360-3050PE multi-pixel photon counters (MPPCs) by Hamamatsu Photonics K.K., using measurements from the GRBAlpha, GRBBeta, and VZLUSAT-2 CubeSats. In the case of GRBAlpha, the measurement of SiPM performance in space lasted over 4 years. GRBAlpha was a 1U CubeSat launched on 2021/03/22 to a 550 km altitude polar orbit carrying a CsI(Tl) scintillator GRB detector employing eight MPPCs and sensitive in the range of ~30-900 keV. GRBAlpha de-orbited on 2025/06/09. VZLUSAT-2 was a 3U CubeSat launched on 2022/01/13 to a 535 km altitude polar orbit and de-orbited on 2025/11/30. GRBBeta was launched on 2024/07/09 to a 580 km altitude, 62° inclination orbit. Both VZLUSAT-2 and GRBBeta carry detectors similar to the one on GRBAlpha. We have flight-proven the Hamamatsu MPPCs S13360-3050 PE and demonstrated that SiPMs, shielded by 2.5 mm of PbSb alloy, can be used in a LEO environment on a scientific mission lasting beyond 4 years. This shows the potential for SiPMs to be employed in future satellites.

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arXiv2026

Evolution of Time-Lags of Swift J1727.8-1613 during the Rising Phase of Its Discovery Outburst

We investigate the accretion dynamics of the black hole X-ray binary Swift J1727.8-1613 during its $2023-2024$ discovery outburst that lasted for $\sim10$ months. Insight-HXMT monitored the rising phase of the outburst of Swift J1727.8-1613 roughly continuously from 2023 Aug 25 to 2023 Oct 05. Strong signatures of type-C Quasi-Periodic Oscillations (QPOs) are observed during this phase of the outburst. In our recent paper, nature of the QPOs are studied with the propagating oscillatory shock (POS) model. In this paper, we report on the observation of both positive (or hard) and negative (or soft) time-lags in the $4-10$ keV (LE), $10-30$ keV (ME), and $30 -150$ keV (HE) bands, computed with respect to the $2-4$ keV reference band. We detect a clear transition from hard to soft lags as the outburst evolves. We show the evolution of QPOs and associated time-lags between different X-ray energy bands, correlated with changes in the QPO frequency, spectral state, and the size of the Comptonizing region. Our analysis reveals strong anti-correlations between the time-lags and both QPO frequency and photon index, and a strong positive correlation with the shock location. These evolving lag characteristics and their correlations provide crucial insights into the changing accretion geometry and the interplay of radiative processes, further supporting dynamic models like the POS in explaining the coupled spectro-temporal evolution in black hole X-ray binaries.

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arXiv2026

GRBAlpha, VZLUSAT-2 and GRBBeta -- GRB observations with CubeSats

Results from GRBAlpha, VZLUSAT-2 and GRBBeta CubeSats and their on-board gamma-ray detectors for monitoring transients are summarised in this article. GRBAlpha was a 1U CubeSat launched in March 2021 to a 550 km altitude polar orbit carrying a CsI(Tl) scintillator gamma-ray burst (GRB) detector with a sensitive range of approximately 30-900 keV. It successfully operated for over four years until June 2025 when it de-orbited. VZLUSAT-2 was a 3U CubeSat launched in January 2022 to a 535 km altitude polar orbit and de-orbited in November 2025 after almost four years of smooth operation. It carried on board two GRB detectors very similar to the one used on GRBAlpha. Both missions have detected about 360 gamma-ray transients, including over 170 long and short gamma-ray bursts (GRBs), and including the most intense GRB ever recorded GRB 221009A and the second brightest GRB 230307A. The new family member, GRBBeta 2U CubeSat, integrated at Masaryk University, was launched in July 2024 to a 580 km altitude, 62 degree inclination orbit. It has been detecting GRBs since its launch without any trouble. Gamma-ray detectors on these nanosatellites are based on CsI(Tl) scintillator readout by silicon photomultipliers (SiPMs). These missions also provide a unique opportunity to study the radiation damage of SiPMs in the low Earth orbit environment and monitor the radiation belts. We have demonstrated that CubeSats can be used in missions lasting beyond three years and routinely detect GRBs.

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arXiv2026

Observational Relationship between Spectral Properties of Gamma-ray and X-ray Emissions from Pulsars

Correlations between gamma-ray and X-ray spectral properties of pulsars are investigated in order to provide observational hints on physics involved in pulsars' high-energy emissions. Using a sample of 43 pulsars detected in both X-ray and gamma-ray bands, we find that pulsars' gamma-ray luminosity, $L_γ$, clearly correlates with the luminosity of non-thermal X-ray emission, $L_{\rm p}$, and anti-correlates with non-thermal X-ray photon index. Other gamma-ray spectral parameters show weaker or negligible correlations. The found relation that $L_γ\propto L_{\rm p}^{0.49\pm 0.05}$ implies a certain connection between radiation mechanisms and energy distributions of radiating particles for these high-energy emissions. Pulsars with and without detected thermal emissions seem to show different dependencies in those correlations, suggesting the possible existence of two different kinds of pulsars. The ones without detected thermal emissions may represent a population of pulsars with low surface temperature. The origin and energetics of high-energy emitting electron-positron pairs for this group of pulsars probably do not depend on their surface thermal emissions, while that of the other group do. The low surface temperatures might be evidence for the working of some exotic processes of neutron-star cooling. Similar to $L_{\rm p}$, some tempting relationships are found among each gamma-ray spectral parameter, surface temperature and thermally radiating area radius. It again strengthens the connection between gamma-ray and X-ray emissions from pulsars.

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arXiv2026

Accretion Flow Properties of MAXI J1834-021 During Its Double-Outbursts In 2023

The Galactic transient black hole candidate MAXI J1834-021 exhibited `faint' outbursting activity for approximately $10$ months following its discovery on February 5, 2023. We study the evolution of both the temporal (hard and soft band photon count rates, hardness ratios, and QPO frequencies) and spectral properties of the source using NICER data between March 7 and October 4, 2023. The outburst profile and the nature of QPOs suggest that the source underwent a mini-outburst following the primary outburst. A monotonic evolution of low-frequency QPOs from higher to lower frequencies is observed during the primary outbursting phase. Both phenomenological (diskbb plus powerlaw) and physical (Two Component Advective Flow) model fitted spectral studies suggest that during the entire epoch, the source remained in harder spectral states, with a clear dominance of nonthermal emissions from the `hot' Compton cloud. Based on the evolution of the spectral and temporal properties, the 2023 outbursting activity of MAXI J1834-021 can be classified as a combination of double `failed' outbursts, as no softer spectral states were observed. The spectral analysis with the TCAF model also gives an estimate of the source mass as $12.3\pm0.2~M_\odot$.

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arXiv2025

Accretion Flow Properties of Swift J1727.8-1613 and Estimation of the BH Mass

The rising phase of the 2023-24 outburst of the recently discovered bright transient black hole candidate Swift J1727.8-1613 was monitored by {\it Insight}-HXMT. We study the evolution of hard ($4$-150$ keV) and soft ($2$-$4$ keV) band photon count rates, the hardness ratio (HR), QPO frequencies, and spectral features using daily observations from the HXMT/LE, ME, and HE instruments between August 25 and October 5, 2023. The QPO frequency is found to be strongly correlated with the soft-band X-ray count rates, and spectral photon indices. In contrast, a strong anti-correlation is observed between HR and QPO frequency, as well as between HR and photon index. Based on the evolution of the temporal and spectral properties, the rising phase of the outburst is subdivided into six parts. The evolution of the QPOs in parts 1-5 is fitted with the propagating oscillatory shock (POS) solution to understand the nature of the evolution from a physical perspective. An inward-propagating shock with weakening strength (except in part 4) is observed during the period of our study. The probable mass of the source is estimated to be $13.5 \pm 1.9~M_\odot$ using the QPO frequency ($ν$)-photon index ($Γ$) scaling method.

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arXiv2025

Distinct Fe-K Line Complexes in MAXI J1744-294 Revealed by XRISM High-Resolution Spectroscopy

The newly discovered Galactic transient MAXI J1744-294 went into its first X-ray outburst in 2025. We study the spectral properties of this source in the 2-10 keV energy band during this outburst using X-ray data from the XRISM satellite for both of its Resolve and Xtend instruments, taken on March 03, 2025. High-resolution spectroscopy has revealed, for the first time, complex iron line features in this source, corresponding to distinct components of Fe XXV emission and Fe XXVI absorption lines. Such a detailed structure has not been reported in other low-mass X-ray binaries to date, prior to the XRISM era. Our analysis shows that the line complexes arise from two highly ionized plasmas with ionization rate ~ 1000 erg-cm/s with distinct turbulent velocities: one broad (~2513 km/s) from hot gas at the inner accretion disk and one narrow (~153 km/s) scattered by nearby photoionized gas. These results offer new insight into the reprocessing of continuum in stratified media, either in the accretion disk or winds, or both, for XRBs in the soft state. The data are well described by models with spin, mass of the black hole, and accretion disk inclination 0.63-0.70, 5.7-10.1 Solar masses, and 19-24 degrees. The fitted spectral model parameters suggest that the source is in the soft spectral state. The source is situated in a crowded field near the Galactic center, resulting in a large hydrogen column density.

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arXiv2025

Detection of X-ray Polarization in the Hard State of IGR J17091-3624: Spectro-Polarimetric Study with IXPE and NuSTAR Data

The class-transition Galactic X-ray binary IGR~J17091--3624 was simultaneously monitored by the \textit{IXPE} and \textit{NuSTAR} satellites. We present a detailed spectro-polarimetric study of the source using data from both satellites covering the period from March~7--10, 2025. A polarimetric analysis in the $2$--$8$~keV band using a model-independent method reveals a significant detection of polarization degree (PD) of $(11.3\pm2.35)\%$ at a polarization angle (PA) of $82^\circ.7\pm5^\circ.96$ (significant at $>4σ$). The model-dependent polarization analysis using the \texttt{polconst} and \texttt{polpow} models yields consistent values of PD and PA. In both methods, an energy-dependent increasing trend of PD is observed. In the $6$--$8$~keV band, a maximum PD of $(29.9\pm8.46)\%$ (significant at $>3σ$) is detected at a PA of $88^\circ.0\pm8^\circ.15$ ($>3σ$). The joint spectral analysis using \textit{IXPE} and \textit{NuSTAR} data in the $2$--$70$~keV band was performed with four different sets of phenomenological and physical models. Our results indicate a strong dominance of non-thermal photons originating from a `hot' Compton cloud, suggesting that the source was in a hard spectral state. Spectral fitting with the physical {\fontfamily{qcr}\selectfont kerrbb} and {\fontfamily{qcr}\selectfont TCAF} models provides an estimate of the black hole mass $M_{\rm BH} = 14.8^{+4.7}_{-3.4}~M_\odot$ and dimensionless spin parameter $a^* \sim 0.54$. The requirement of a higher hydrogen column density in the spectral fit of the second \textit{NuSTAR} observation is attributed to the obscuration of non-thermal photons during the dip phase, likely caused by the presence of wind accreted from the companion star.

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arXiv2025

Investigating the Anti-Correlation between Photon Index and Flux of the Crab using RXTE and NuSTAR

We present a systematic study of the recently reported anti-correlation between X-ray flux and photon index ($Γ$) in the Crab Nebula, using archival RXTE/PCA (3 - 50 keV), RXTE/HEXTE (20 - 100 keV), and NuSTAR (3 - 78 keV) observations. Spectra were extracted in soft (3 - 10 keV) and hard bands (10 - 50 keV, 10 - 78 keV, 20 - 100 keV) and fitted with an absorbed power-law model. Across all instruments and energy ranges, we confirm the existence of a persistent negative correlation -- harder spectra at higher flux levels. The correlation is stronger in the hard bands compared to the soft bands. This is consistent with synchrotron emission modulated by magnetic field variations in the pulsar wind nebula.

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arXiv2025

Evolution of QPO During Discovery Outburst of MAXI J1834-021: Estimation of Intrinsic Parameters from Spectro-Temporal Study

The Galactic transient black hole candidate (BHC) MAXI~J1834-021 was detected for the first time by MAXI/GSC on February 05, 2023 and it was active for next $\sim 10$~months. A monotonic evolution of low-frequency QPOs from higher to lower frequencies is observed in the middle-phase of the outburst. We study this evolution of the QPO with the propagating oscillatory shock (POS) model, and it suggests the presence of a receding shock. The POS model fit also estimates the mass of the source to be $12.1\pm0.3~M_\odot$. We also study the boradband ($0.5$-$70$~keV) nature of the source using archival data of NICER and NuSTAR on March 10, 2023 with the both phenomenological (combined disk blackbody plus powerlaw) and physical (\textit{nthComp, kerrbb, TCAF}) models. The mass of the BHC estimated by the \texttt{kerbb} and \texttt{TCAF} models is found to be consistent with POS model fits as well as reported in our recent work. Combining all these methods, we predict mass of the source as $12.3^{+1.1}_{-2.0}~M_\odot$. The \texttt{kerbb} model fit also estimates the spin, distance, and inclination of the source to be $0.13^{+0.03}_{-0.02}$, $9.2^{+0.4}_{-0.9}$~kpc, and $80^\circ.0$$^{+2.7}_{-6.0}$, respectively. The combined spectral study suggests harder spectral state of the source with a higher dominance of the sub-Keplerian halo accretion rate over the Keplerian disk rate. The consistency of the observed frequency of the QPO with that of obtained from the \texttt{TCAF} model fitted shock parameters, confirms shock oscillation as the origin of the QPO.

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arXiv2025

Characterization of more than three years of in-orbit radiation damage of SiPMs on GRBAlpha and VZLUSAT-2 CubeSats

Silicon photomultipliers (SiPMs) are prone to radiation damage which causes an increase of dark count rate. This leads to an increase in low-energy threshold in a gamma-ray detector combining SiPM and a scintillator. Despite this drawback, they are becoming preferred for scintillator-based gamma-ray detectors on CubeSats due to their low operation voltage, small size, linear response to low light intensity and fast response. This increasing popularity of SiPMs among new spaceborne missions makes it important to characterize their long-term performance in the space environment. In this work, we report the change of the dark count rate and low-energy threshold of S13360-3050 PE multi-pixel photon counters (MPPCs) by Hamamatsu, using measurements acquired by the GRBAlpha and VZLUSAT-2 CubeSats at low Earth orbit (LEO) spanning over three years. Such a long measurement of the performance of MPPCs in space has not been published before. GRBAlpha is a 1U CubeSat launched on March 22, 2021, to a 550 km altitude sun-synchronous polar orbit (SSO) carrying on board a gamma-ray detector based on CsI(Tl) scintillator readout by eight MPPCs and regularly detecting gamma-ray transients such as gamma-ray bursts and solar flares in the energy range of ~30-900 keV. VZLUSAT-2 is a 3U CubeSat launched on January 13, 2022 also to a 535 km altitude SSO carrying on board, among other payloads, two gamma-ray detectors similar to the one on GRBAlpha. We have flight-proven the Hamamatsu MPPCs S13360-3050 PE and demonstrated that MPPCs, shielded by 2.5 mm of PbSb alloy, can be used in LEO environment on a scientific mission lasting beyond three years. This manifests the potential of MPPCs being employed in future satellites.

astro-ph.IM↗

arXiv2025

Anti-correlation between Flux and Photon Index of Hard X-ray Emission from The Crab

Using \textit{Swift} Burst Alert Telescope (BAT) event-mode data during Gamma Ray Burst (GRB) occurrences, we conducted spectral analysis for the Crab system. From 38 good observations, which spans over a period of 18 years from 2006 to 2023, we found that the Crab's X-ray flux does not only flicker, but also significantly anti-correlates to its spectral power-law photon index. Since emission contribution of the Crab pulsar in this energy range is small, this anti-correlation is mainly about the emission of the Crab nebula. We suggest that this anti-correlation is an observational supporting evidence for the long-standing notion that the nebula emission is due to synchrotron radiation of shocked pulsar winds in the nebula.

astro-ph.HE↗

arXiv2024

Energy calibration of GTM on ground

The Gamma-ray Transients Monitor (GTM) on board the Formosat-8B (FS-8B) satellite is designed to detect and localize Gamma-Ray Bursts (GRBs). By utilizing 2+2 CITIROC chips to manipulate 4+4 detectors, which are composed of GAGG(Ce) scintillators coupled with Silicon Photomultipliers (SiPMs) and oriented in various directions to achieve all-sky coverage, the GRB saturation fluences of GTM in the 50 keV to 1 MeV range for Short GRBs (SGRBs) and Long GRBs (LGRBs) were estimated to be about $3.1 \times 10^{-4}$ and $5.0 \times 10^{-3}\ {\rm erg/cm^2}$, respectively, based on simulations. To precisely interpret the GTM readout signal in terms of energy, several measurements for isotope and gain calibration were conducted. Despite encountering issues with crosstalk and SiPM saturation effect in the data, the energy spectrum can still be recovered by appropriately discarding channel noise and mapping with the correct ADC-to-energy relation. This paper summarizes the energy resolution of GTM and the linear variations in the relationship between photon energy and readout signal. At 662 keV, the energy resolution is about 16 %. Also, it demonstrates that greater gain is achieved by increasing voltage or decreasing temperature.

astro-ph.IM↗