Search

Searching for X-ray and gamma-ray bursts, including Gamma-ray bursts (GRBs), Soft Gamma-ray Repeaters (SGRs), and high energy transients associated with Gravitational wave (GW) events or Fast radio bursts (FRBs), is of great importance in the multimessenger and multiwavelength era. Although a coherent search based on the likelihood ratio and Gaussian statistics has been established and utilized in many studies, this Gaussian-based method could be problematic for weak and short bursts which usually have very few counts. To deal with all bursts including weak ones, here we propose the coherent search in Poisson statistics. We studied the difference between Poisson-based and Gaussian-based search methods by Monte Carlo simulations, and find that the Poisson-based search method has advantages compared to the Gaussian case, especially for weak bursts. Our results show that, for very weak bursts with very low number of counts, the Poisson-based search can provide higher significance than the Gaussian-based search, and its likelihood ratio (for background fluctuation) still generally follows the χ2 distribution, making the significance estimation of searched bursts very convenient. Thus, we suggest that the coherent search based on Poisson-likelihood ratio is more appropriate in the search for generic transients including very weak ones. [ABSTRACT FROM AUTHOR]

Gravitational wave high-energy Electromagnetic Counterpart All-sky Monitor (GECAM) is a dedicated mission, launched in December 2020, for gamma-ray transients, including gamma-ray bursts (GRBs) and soft gamma repeater (SGR) bursts in the multimessenger and multiwavelength astronomy era. Since GECAM consists of two independent spacecrafts (or say instruments), and the framework of data analysis for multiple spacecrafts is distinctive from that for only one spacecraft, which is the case for most GRB missions, we developed a dedicated pipeline called Energetic Transients Joint Analysis System for Multi-INstrument (ETJASMIN) for GECAM mission. This pipeline has been naturally extended to incorporate data from other gamma-ray instruments, including the operating missions, such as Insight- HXMT/HE, Fermi /GBM, Swift /BAT, INTEGRAL/SPI-ACS, Konus-Wind, and GRID, as well as the forthcoming missions, such as SVOM/GRM and HEBS. In this paper, we present this pipeline with a focus on the data analysis procedures, methodology, and results in terms of the localization, verification (classification), spectral, and temporal analyses of gamma-ray transients. We show that this pipeline could provide more accurate, reliable, and comprehensive results than that of individual spacecraft, which is beneficial for gamma-ray transients observation. [ABSTRACT FROM AUTHOR]

The minimum variation time-scale (MVT) and spectral lag of hundreds of X-ray bursts (XRBs) from soft gamma-ray repeater (SGR) J1935+2154 were analysed in detail for the first time in our recent work, which are important probes for studying the physical mechanism and radiation region. In this work, we investigate their differential and cumulative distributions carefully and find that they follow power-law models. Besides, the distributions of fluctuations in both parameters follow the Tsallis q -Gaussian distributions and the q values are consistent for different scale intervals. Therefore, these results indicate that both parameters are scale-invariant, which provides new parameters for the study of self-organized criticality systems. Interestingly, we find that the q values for MVT and spectral lag are similar with duration and fluence, respectively. [ABSTRACT FROM AUTHOR]

The Doppler dipole signal dominates the cosmic microwave background (CMB) anisotropy maps obtained by the Wilkinson Microwave Anisotropy Probe ( WMAP) mission, and plays a key role throughout the data processing. Previously, we discovered a timing asynchronism of 25.6 ms between the timestamps of the spacecraft attitude and radiometer output in the original raw WMAP time-ordered data (TOD), which, if not corrected during data processing, would generate an artificial quadrupole component () in recovered CMB maps (). Recently, proves that there does exist a timing offset-induced error corresponding to about 25.6 ms in the WMAP calibrated TOD, by studying the fluctuation variance per pixel in the temperature map recovered from the TOD as a function of assumed timing offset. Here, we find evidence directly in the WMAP TOD for such an uncorrected timing error, which possibly occurred in calculating the Doppler dipole signal during the WMAP team's TOD data processing. The amplitude is highly significant and is consistent with previous work. We also show that the uncorrected timing offset can lead the WMAP CMB quadrupole to be substantially overestimated. [ABSTRACT FROM AUTHOR]

The brightest-of-all-time gamma-ray burst (GRB), GRB 221009A, is the first GRB observed to have emission line (up to 37 MeV) in its prompt emission spectra. It is naturally explained as |$e^-/e^+$|  annihilation line that was Doppler boosted in the relativistic jet of the GRB. In this work, we repeatedly apply the simple optical depth argument to different physical processes necessary to produce an observable |$e^-/e^+$|  annihilation line. This approach results in robust constraints on the physics of the line: We conclude that in GRB 221009A, the |$e^-/e^+$|  pairs were produced at a radius greater than |$4.3\times 10^{15}$|  cm from the central engine, and annihilated in a region between |$1.4\times 10^{16}$| and |$4.3\times 10^{16}$|  cm. From these constraints, we established a self-consistent picture of |$e^-/e^+$|  production, cooling, and annihilation. We also derived a criterion for pair production in the GRB prompt emission: |$E_{\rm {iso}} \gtrsim 3.3\times 10^{53} E_{\rm {peak},100} (1+z) R^2_{\rm {prod},16}~\text{erg}$|⁠. Using this criterion, we find tens of candidate GRBs that could have produced |$e^-/e^+$|  in prompt emissions to annihilate. GRB 221009A is with the highest likelihood according to this criterion. We also predict the presence of a thermal radiation, with a time-evolving blackbody temperature, sweeping through soft X-ray during the prompt emission phase. [ABSTRACT FROM AUTHOR]

One month after launching the Gravitational Wave High-energy Electromagnetic Counterpart All-sky Monitor, a bright thermonuclear X-ray burst from 4U 0614+09 was observed on 2021 January 24. We report the time-resolved spectroscopy of the burst and a hint of burst oscillation at 413 Hz with a fractional amplitude ∼2.0% (rms). This coincides with the burst oscillation previously discovered with Swift/Burst Alert Telescope (Strohmayer et al. 2008), and therefore supports the spin frequency of this source. This burst is a bright one in the normal bursts detected from 4U 0614+09, which leads to an upper limit of distance estimation of 3.1 kpc. The folded light curve during the burst oscillation shows a sinusoidal structure, which is consistent with previous observations. [ABSTRACT FROM AUTHOR]

The milestone discovery of GW170817-GRB 170817A-AT 2017gfo has shown that gravitational waves (GWs) could be produced during the merger of a neutron star–neutron star/black hole and that in electromagnetic (EM) waves, a gamma-ray burst (GRB) and a kilonova (KN) are generated in sequence after the merger. Observationally, however, EM properties before the merger phase are still unclear. Here we report a peculiar precursor in a KN-associated long-duration GRB 211211A, providing evidence of the EM before the merger. This precursor lasts ∼0.2 s, and the waiting time between the precursor and the main burst is ∼1 s, comparable to that between GW170817 and GRB 170817A. The spectrum of the precursor could be well fit with a nonthermal cutoff power-law model instead of a blackbody. In particular, a ∼22 Hz quasiperiodic oscillation candidate (∼3 σ) is detected in the precursor. These temporal and spectral properties indicate that this precursor is probably produced by a catastrophic flare accompanied with magnetoelastic or crustal oscillations of a magnetar in a binary compact merger. The strong magnetic field of the magnetar can also account for the prolonged duration of GRB 211211A. However, it poses a challenge to reconcile the rather short lifetime of a magnetar with the rather long spiraling time of a binary neutron star system only by the GW radiation before the merger. [ABSTRACT FROM AUTHOR]

Fast Radio Bursts (FRBs) are energetic phenomena that have significant implications for understanding fundamental physics and the Universe. Recent observations of FRB 121102, FRB 20220912A, and FRB 20201124A by the Five-hundred-meter Aperture Spherical Telescope showed high-burst rates and distinctive energy distribution and temporal properties. In this study, we examine these observations to investigate the scale invariance of the waiting times between bursts for intervals longer than approximately 1 s. Our analysis revealed a unified scaling law for these longer intervals, which is similar to the behaviour of solar flares. This discovery inspires us to suggest a dual analogy of the FRB scenario across the entire time intervals: with earthquake dynamics at subsecond scales and with solar flare dynamics beyond the one-second threshold. This threshold potentially aligns with the dynamic time-scale of neutron star crusts, offering insight of the occurrence of FRBs into the internal processes of neutron stars. [ABSTRACT FROM AUTHOR]

The study of quasi-periodic oscillations (QPOs) and power density spectra (PDSs) continuum properties can help shed light on the still elusive emission physics of magnetars and as a window into the interiors of neutron stars using asteroseismology. In this work, we employ a PDS method to search for the QPOs in the hundreds of X-ray bursts from SGR J1935+2154 observed by Insight - HXMT, GECAM , and Fermi/ GBM from 2014 July to 2022 January. Although no definitive QPO signal (significance >3 σ) is detected in individual bursts or the averaged periodogram of the bursts grouped by duration, we identify several bursts exhibiting features at ∼40 Hz, attributed to the the proximity intervals between adjacent pulses and consistent with that reported in the X-ray burst associated with FRB 200428. We investigate the PDS continuum properties and find that the distribution of the PDS slope in the simple power-law model peaks at ∼2.5, which is consistent with other magnetars but higher than 5/3 commonly seen in gamma-ray bursts. Besides, the distribution of the break frequency in the broken power-law model peaks at ∼60 Hz. Finally, we report that the power-law index of PDS has an anti-correlation and power-law dependence on the burst duration as well as the minimum variation time-scale. [ABSTRACT FROM AUTHOR]

As the main detector of the Gravitational wave high-energy Electromagnetic Counterpart All-sky Monitor mission (GECAM), the calibration of the energy response and detection efficiency of the gamma-ray detector (GRD) is the main content of the ground-based calibration. This article mainly focuses on the calibration of the energy response and detection efficiency in the 8–160 keV with a refined measurement around the absorption edges of the lanthanum bromide crystal (⁠|$\rm {LaBr_3}$|⁠). The GRD performances for different crystal types, data acquisition modes, working modes, and incident positions are also analysed in detail. We show that the calibration campaign is comprehensive, the detector performance meets the flight requirements, and the calibration results generally agree with simulations as expected. The detector's model was corrected by the ground-based calibration, which led to the establishment of the calibration data base. [ABSTRACT FROM AUTHOR]

Spectral lag of the low-energy photons with respect to the high-energy ones is a common astrophysical phenomenon (such as gamma-ray bursts and the Crab Pulsar) and may serve as a key probe to the underlying radiation mechanism. However, spectral lag in keV range of the magnetar bursts has not been systematically studied yet. In this work, we perform a detailed spectral lag analysis with the Li et al.'s Cross-Correlation Function (Li-CCF) method for SGR J1935+2154 bursts observed by Insight - Hard X-ray Modulation Telescope (HXMT), Gravitational Wave High-energy Electromagnetic Counterpart All-sky Monitor (GECAM), and Fermi /Gamma-ray Burst Monitor (GBM) from 2014 July to 2022 January. We discover that the spectral lags of about 61 per cent (non-zero significance >1σ) bursts from SGR J1935+2154 are linearly dependent on the photon energy (E) with t lag(E) = α(E /keV) + C , which may be explained by a linear change of the temperature of the blackbody-emitting plasma with time. The distribution of the slope (α) approximately follows a Gaussian function with mean and standard deviation of 0.02 ms keV−1 (i.e. high-energy photons arrive earlier) and 0.02 ms keV−1, respectively. We also find that the distribution can be well fitted with three Gaussians with mean values of ∼−10.009, 0.013, and 0.039 ms keV−1, which may correspond to different origins of the bursts. These spectral lag features may have important implications on the magnetar bursts. [ABSTRACT FROM AUTHOR]

The X/gamma-ray polarimetry of the Crab pulsar/nebula is believed to hold crucial information on their emission models. In the past, several missions have shown evidence of polarized emission from the Crab. The significance of these measurements remains, however, limited. New measurements are therefore required. POLAR is a wide Field of View Compton-scattering polarimeter (sensitive in 50–500 keV) onboard the Chinese spacelab Tiangong-2 that took data from 2016 September to 2017 April. Although not designed to perform polarization measurements of pulsars, we present here a novel method, which can be applied to POLAR as well as that of other wide Field of View polarimeters. The novel polarimetric joint-fitting method for the Crab pulsar observations with POLAR , allows us to obtain constraining measurements of the pulsar component. The best-fitting values and corresponding 1 σ deviations for the averaged phase interval: (PD =  |$14\substack{+15 \\-10}$|  per cent, PA =  |$108\substack{+33 \\-54} ^{\circ }$|⁠), for Peak 1: (PD =  |$17\substack{+18 \\-12}$|  per cent, PA =  |$174\substack{+39 \\-36} ^{\circ }$|⁠) and for Peak 2: (PD =  |$16\substack{+16 \\-11}$|  per cent, PA =  |$78\substack{+39 \\-30} ^{\circ }$|⁠). Furthermore, the 3 σ upper limits on the polarization degree are for the averaged phase interval (55 per cent), Peak 1 (66 per cent) and Peak 2 (57 per cent). Finally, to illustrate the capabilities of this method in the future, we simulated 2 yr observation to the Crab pulsar with POLAR - 2. The results show that POLAR - 2 is able to confirm the emission to be polarized with 5 σ and 4 σ confidence level if the Crab pulsar is polarized at |$20{{\,\rm per\,cent}}$| and |$10{{\,\rm per\,cent}}$|⁠ , respectively. [ABSTRACT FROM AUTHOR]

The giant flares of soft gamma-ray repeaters (SGRs) have long been proposed to contribute to at least a subsample of the observed short gamma-ray bursts (GRBs). In this paper, we perform a comprehensive analysis of the high-energy data of the recent bright short GRB 200415A, which was located close to the Sculptor galaxy. Our results suggest that a magnetar giant flare provides the most natural explanation for most observational properties of GRB 200415A, including its location, temporal and spectral features, energy, statistical correlations, and high-energy emissions. On the other hand, the compact star merger GRB model is found to have difficulty reproducing such an event in a nearby distance. Future detections and follow-up observations of similar events are essential to firmly establish the connection between SGR giant flares and a subsample of nearby short GRBs. [ABSTRACT FROM AUTHOR]

As a new member of the GECAM mission, GECAM-C (also named High Energy Burst Searcher, HEBS), launched onboard the SATech-01 satellite on July 27th, 2022, is capable of monitoring gamma-ray transients from ∼ 6 keV to ∼ 6 MeV. GECAM-C is equipped with 12 gamma-ray detectors (GRDs) as its main detectors. To test the performance of GECAM-C GRDs and verify the energy response simulated by Monte Carlo, comprehensive on-ground calibration experiments have been performed using X-ray beam and radioactive sources, including Channel–Energy relation, energy resolution, detection efficiency, Silicon Photomultipliers (SiPM) voltage-gain relation, and non-uniformity of positional response. This paper presents the calibration campaigns and the results of data analysis for GECAM-C GRDs. [ABSTRACT FROM AUTHOR]

The Gamma-ray Burst polarimeter - POLAR is a dedicated instrument for polarization measurements of prompt emissions of the Gamma-ray Bursts (GRBs). POLAR detected 55 GRBs jointly with other satellites in about 6 months after launch onboard the Chinese Space Laboratory "Tiangong-2 (TG-2)" in September 2016. We simulated GRBs from various directions to get the trigger-counting distribution responses of the 1600 pixels of the 25 detector modules of POLAR. The observed trigger-counting distribution of each GRB was compared with the simulated distributions to find the most likely location of the GRB. We studied the localization capabilities of POLAR using the data of 30 GRBs which have reference locations from other GRB detection instruments, as well as the systematic error on GRB localization of POLAR using a Bayesian method. The results show that POLAR is able to localize GRBs with a systematic error of about 3. 8 ∘ . For typical GRB in 10 keV to 1000 keV with a fluence from 1 0 − 5 erg/cm 2 to 1 0 − 4 erg/cm 2 , the statistical error of localization is between 10° to 2.8°. Further simulation studies were performed to understand the systematic error on the polarization measurements caused by the location errors. [ABSTRACT FROM AUTHOR]