Soft Gamma-ray Repeaters Research Articles

A Comprehensive Timing Analysis of Individual Pulses in X-Ray Bursts from SGR J0501+4516

The pulses in X-ray-burst (XRB) light curves from soft gamma-ray repeaters (SGRs) are generally thought to arise from magnetic crustal fractures or magnetic reconnection, reflecting the evolution of the energy release process in magnetars. In this study, we conduct a comprehensive timing analysis of 27 XRBs from SGR J0501+4156 detected by the Gamma-ray Burst Monitor on board Fermi. Utilizing a improved pulse-finding algorithm, we identify a total of 95 pulses and fit them using multiple FRED functions to obtain pulse-shape parameters based on the Markov Chain Monte Carlo method. We calculate the minimum variability timescales (MVTs) of the XRBs based on the shortest pulse; the distribution of MVTs follows a log-Gaussian function with a mean of 7.22−2.11+2.93 ms (1σ). The distributions of rise time, decay time, waiting time, width, skewness, and peakedness all follow the log-Gaussian function, and multiple power-law dependencies are observed between them; for example, a power-law positive correlation between decay time and rise time with 4.7σ, and a power-law negative correlation between pulse width and peakedness with 6.8σ. Besides, there is a positive correlation with 3.7σ between the number of pulses and burst duration. Our findings favor a magnetospheric origin, and some similarities with gamma-ray bursts imply that they have similar radiation mechanisms, e.g., magnetic reconnection processes.

Fermi GBM Observations of the Galactic Magnetar SGR 1935+2154 during Its 2022 January Activity

The X-ray and radio burst-emitting soft gamma-ray repeater SGR 1935+2154 has been the most active magnetar during the past decade, with at least one active episode each year since 2019. We report on the 2022 January activity as observed by the Fermi Gamma-Ray Burst Monitor and present the temporal and time-integrated spectral properties of the observed 112 X-ray bursts. No radio bursts were reported during the activity. The mean burst duration is comparable to most of the previous activities but is shorter than that of 2020, during which the magnetar emitted radio bursts. The mean burst spectrum is softer than the earlier activities and a long-term softening trend since 2016 is evident and is accompanied by a rise in the mean burst fluence and blackbody emission radii. The power-law indices of the RBB2−kTBB correlation also show an evolution during 2019–2022, where the soft component started becoming steeper, whereas the hard component shows a less steep trend. The burst properties show no significant evolution within the activity, which lasted for ∼ 45 days, and none of the observed bursts showed properties reminiscent of those of the hard, long X-ray counterpart to the 2020 fast radio burst FRB 200428 from the source. We discuss the results comparatively with the 8 yr history of the source since its discovery in 2014.

Periodic activities of Fast Radio Burst repeaters from precessing magnetars with evolving obliquity

Abstract Fast radio bursts (FRBs) are cosmological radio transients with millisecond durations and extremely high brightness temperatures. One FRB repeater, FRB 180916.J0158+65 (FRB 180916B), was confirmed to appear 16.35-day periodic activities with 5-day activity window. Another FRB repeater, FRB 121102, and two soft gamma-ray repeaters (SGRs), SGR 1935+2154 and SGR 1806-20, also show possible periodic activities. These periodicities might originate from the precession process of young magnetars due to the anisotropic pressure from the inner magnetic fields as proposed in the literature. In this work, we analyze a self-consistent model for the rotation evolution of magnetars and obtain the evolutions of magnetar precession and obliquity. We find that if the FRB repeaters and the SGRs with (possible) periodic activities originate from the magnetar precession, their ages would be constrained to be hundreds to tens of thousands of years, which is consistent with the typical ages of magnetars. Assuming that the FRB emission is beaming in the magnetosphere as proposed in the literature, we calculate the evolution of the observable probability and the duty cycle of the active window period. We find that for a given magnetar the observable probability increases with the magnetar age in the early stage and decreases with the magnetar age in the later stage, meanwhile, there are one or two active windows in one precession period if the emission is not perfectly axisymmetric with respect to the deformation axis of a magnetar, which could be tested by the future observation for repeating FRB sources.

Asteroseismology of Compact Stars

Compact stars have been perceived as natural laboratories of matter at an extremely high density. The uncertainties of the equation of state (EOS) of matter can be constrained by observing compact stars. In this review, we investigate the EOSs, global structure, and elastic properties of compact stars. We focus in detail on how to constrain the above properties of compact stars via asteroseismology. Observations that include studies of quasi-periodic oscillations from giant flares of soft gamma-ray repeaters and gravitational waves provide information about the elastic properties and internal compositions of compact stars.

The self-organized criticality behaviours of two new parameters in SGR J1935+2154

ABSTRACT 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.

Polarized X-Rays Detected from the Anomalous X-Ray Pulsar 1E 2259+586

Anomalous X-ray pulsars (AXPs) and soft gamma-ray repeaters (SGRs) are believed to be associated with magnetars, which have extremely strong magnetic fields. Recently, with the operation of the Imaging X-ray Polarimetry Explorer (IXPE), the polarization information of two AXPs and one SGR have been investigated. In this work, we report the observational results of the fourth magnetar, 1E 2259+586, with IXPE, and perform a joint analysis with observations from Neutron Star Interior Composition Explorer. We find that the emission from 1E 2259+586 is linearly polarized, with a polarization degree (5.3% ± 1.3%) and a polarization angle −77° ± 7° in the 2–8 keV energy range. Additionally, both the polarization degree and polarization angle exhibit variability with the pulse phase, and there is a hint of anticorrelation between the polarization degree and the flux, which is similar to AXP 1RXS J170849.0-400910. The phase-dependent polarization angle displays a sinusoidal profile and can be well fitted with the rotating vector model, indicating that the magnetic dipole field dominated the magnetic structure of the pulsar, and the variation in the polarization angle was modulated by the pulsar’s rotation.

The Minimum Variation Timescales of X-Ray Bursts from SGR J1935+2154

The minimum variation timescale (MVT) of soft gamma-ray repeaters can be an important probe to estimate the emission region in pulsar-like models, as well as the Lorentz factor and radius of the possible relativistic jet in gamma-ray burst (GRB)–like models, thus revealing their progenitors and physical mechanisms. In this work, we systematically study the MVTs of hundreds of X-ray bursts (XRBs) from SGR J1935+2154 observed by Insight-HXMT, GECAM, and Fermi/Gamma-ray Burst Monitor from 2014 July to 2022 January through the Bayesian block algorithm. We find that the MVTs peak at ∼2 ms, corresponding to a light-travel time size of about 600 km, which supports the magnetospheric origin in pulsar-like models. The shock radius and the Lorentz factor of the jet are also constrained in GRB-like models. Interestingly, the MVT of the XRB associated with FRB 200428 is ∼70 ms, which is longer than that of most bursts and implies its special radiation mechanism. In addition, the median of MVTs is 7 ms, shorter than the median MVTs of 40 ms and 480 ms for short GRBs or long GRBs, respectively. However, the MVT is independent of duration, similar to GRBs. Finally, we investigate the energy dependence of MVT and suggest that there is a marginal evidence for a power-law relationship like GRBs, but the rate of variation is at least about an order of magnitude smaller. These features may provide an approach to identify bursts with a magnetar origin.

The performance of SiPM-based gamma-ray detector (GRD) of GECAM-C

As a new member to the GECAM mission, the GECAM-C is a gamma-ray all-sky monitor onboard SATech-01 satellite. GECAM-C was launched on July 27th, 2022 and it is also called HEBS (High Energy Burst Searcher). GECAM-C is designed to detect gamma-ray transients from 6 keV to 6 MeV, such as Gamma-Ray Bursts (GRBs), high energy counterpart of Gravitational Waves (GWs), Fast Radio Bursts (FRBs), and Soft Gamma-ray Repeaters (SGRs). Together with GECAM-A and GECAM-B launched in December 2020, GECAM-C will greatly improve the monitoring coverage, localization, as well as temporal and spectral measurements of gamma-ray transients. GECAM-C employs 12 SiPM-based Gamma-Ray Detectors (GRDs) to detect gamma-ray transients. In this paper, we firstly give a brief description of the design of GECAM-C GRDs, and then focus on the on-ground tests and in-flight performance of GRDs. We also did the comparison study of the SiPM in-flight performance between GECAM-C and GECAM-B. The results show GECAM-C GRD works as expected and is ready to make scientific observations.

Neutron Star Phase Transition as the Origin for the Fast Radio Bursts and Soft Gamma-Ray Repeaters of SGR J1935 + 2154

Magnetars are believed to be neutron stars (NSs) with strong magnetic fields. X-ray flares and fast radio bursts (FRBs) have been observed from the magnetar (soft gamma-ray repeater, SGR J1935+2154). We propose that the phase transition of the NS can power the FRBs and SGRs. Based on the equation of state provided by the MIT bag model and the mean field approximation, we solve the Tolman–Oppenheimer–Volkoff equations to get the NS structure. With the spin-down of the NS, the hadronic shell gradually transfers to the quark shell. The gravitational potential energy released by one time of the phase transition can be achieved. The released energy, time interval between two successive phase transitions, and glitch are all consistent with the observations of the FRBs and the X-ray flares from SGR J1935 + 2154. We conclude that the phase transition of an NS is a plausible mechanism to power the SGRs as well as the repeating FRBs.

Burst Phase Distribution of SGR J1935+2154 Based on Insight-HXMT

On 2020 April 27, the soft gamma-ray repeater SGR J1935+2154 entered its intense outburst episode again. Insight-HXMT carried out about one month observation of the source. A total number of 75 bursts were detected during this activity episode by Insight-HXMT, and persistent emission data were also accumulated. We report on the spin period search result and the phase distribution of burst start times and burst photon arrival times of the Insight-HXMT high energy detectors and Fermi/Gamma-ray Burst Monitor (GBM). We find that the distribution of burst start times is uniform within its spin phase for both Insight-HXMT and Fermi/GBM observations, whereas the phase distribution of burst photons is related to the type of a burst’s energy spectrum. The bursts with the same spectrum have different distribution characteristics in the initial and decay episodes for the activity of magnetar SGR J1935+2154.

GECAM Localization of High-energy Transients and the Systematic Error

The Gravitational Wave High-energy Electromagnetic Counterpart All-sky Monitor (GECAM) is a pair of microsatellites (i.e., GECAM-A and GECAM-B) dedicated to monitoring gamma-ray transients including the high-energy electromagnetic counterparts of gravitational waves, such as gamma-ray bursts, soft gamma-ray repeaters, solar flares, and terrestrial gamma-ray flashes. Since launch in 2020 December, GECAM-B has detected hundreds of astronomical and terrestrial events. For these bursts, localization is the key for burst identification and classification as well as follow-up observations in multiple wavelengths. Here, we propose a Bayesian localization method with Poisson data with Gaussian background profile likelihood to localize GECAM bursts based on the distribution of burst counts in detectors with different orientations. We demonstrate that this method can work well for all kinds of bursts, especially extremely short ones. In addition, we propose a new method to estimate the systematic error of localization based on a confidence level test, which can overcome some problems of the existing method in the literature. We validate this method by Monte Carlo simulations, and then apply it to a burst sample with accurate location and find that the mean value of the systematic error of GECAM-B localization is ∼2.°5. By considering this systematic error, we can obtain a reliable localization probability map for GECAM bursts. Our methods can be applied to other gamma-ray monitors.

Formation, Possible Detection and Consequences of Highly Magnetized Compact Stars

Over the past several years, there has been enormous interest in massive neutron stars and white dwarfs due to either their direct or indirect evidence. The recent detection of gravitational wave event GW190814 has confirmed the existence of compact stars with masses as high as ∼2.5–2.67 M⊙ within the so-called mass gap, indicating the existence of highly massive neutron stars. One of the primary goals to invoke massive compact objects was to explain the recent detections of over a dozen Type Ia supernovae, whose peculiarity lies with their unusual light curve, in particular the high luminosity and low ejecta velocity. In a series of recent papers, our group has proposed that highly magnetised white dwarfs with super-Chandrasekhar masses can be promising candidates for the progenitors of these peculiar supernovae. The mass-radius relations of these magnetised stars are significantly different from those of their non-magnetised counterparts, which leads to a revised super-Chandrasekhar mass-limit. These compact stars have wider ranging implications, including those for soft gamma-ray repeaters, anomalous X-ray pulsars, white dwarf pulsars and gravitational radiation. Here we review the development of the subject over the last decade or so, describing the overall state of the art of the subject as it stands now. We mainly touch upon the possible formation channels of these intriguing stars as well as the effectiveness of direct detection methods. These magnetised stars can have many interesting consequences, including reconsideration of them as possible standard candles.

Burst search method based on likelihood ratio in Poisson statistics

ABSTRACTSearching 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.

An updated view and perspectives on high-energy gamma-ray emissionfrom SGR J1935+2154 and its environment

SGR J1935+2154 was discovered in 2016 and is currently one of the most burst-active Soft Gamma-ray Repeaters (SGR), having emitted many X-ray bursts in recent years. In one of our previous articles, we investigated the contribution to high-energy and very high-energy gamma-ray emission (VHE, E > 100 GeV) due to cosmic-ray acceleration of SNR G57.2+0.8 hosting SGR J1935+2154 using the GALPROP propagation code. However, follow-up observations of SGR 1935+2154 were made for 2 hours on April 28, 2020, using the High Energy Stereoscopic System (H.E.S.S.). The observations coincide with X-ray bursts detected by INTEGRAL and Fermi/Gamma-ray Burst Monitor (GBM). These are the first high-energy gamma-ray observations of an SGR in a flaring state, and upper limits on sustained and transient emission have been derived. Now that new H.E.S.S. observations have been made, it is interesting to update our model with respect to these new upper limits. We extend our previous results to a more general situation using the new version of GALPROP. We obtain a hadronic model that confirms the results discussed by H.E.S.S. . This leads to an optimistic prospect that cosmic ray gamma rays from SGR J1935+2154 can contribute to the overall gamma energy density distribution and in particular to the diffusion gamma rays from the Galactic center.

Pasta properties of the neutron star within effective relativistic mean-field model

We study the properties of pasta structures and their influence on the neutron star observables employing the effective relativistic mean-field (E-RMF) model. The compressible liquid drop model is used to incorporate the finite size effects, considering the possibility of nonspherical structures in the inner crust. The unified equation of states are constructed for several E-RMF parameters to study various properties such as pasta mass and thickness in the neutron star's crust. The majority of the pasta properties are sensitive to the symmetry energy in the subsaturation density region. Using the results from Monte Carlo simulations, we estimate the shear modulus of the crust in the context of quasiperiodic oscillations from soft gamma-ray repeaters and calculate the frequency of fundamental torsional oscillation mode in the inner crust. Global properties of the neutron star such as mass-radius profile, the moment of inertia, crustal mass, crustal thickness, and fractional crustal moment of inertia are worked out. The results are consistent with various observational and theoretical constraints.

An Insight-HXMT Dedicated 33 day Observation of SGR J1935+2154. I. Burst Catalog

Magnetars are neutron stars with an extreme magnetic field and sometimes manifest as soft gamma-ray repeaters (SGRs). SGR J1935+2154 is one of the most prolific bursters and the first confirmed source of a fast radio burst (FRB; i.e., FRB 200428). Encouraged by the discovery of the first X-ray counterpart of FRBs, the Insight-Hard X-ray Modulation Telescope (Insight-HXMT) implemented a dedicated 33-day-long Target of Opportunity observation of SGR J1935+2154 since 2020 April 28. With the HE, ME, and LE telescopes, Insight-HXMT provides a thorough monitoring of burst activity evolution of SGR J1935+2154, in a very broad energy range (1–250 keV) with high temporal resolution and high sensitivity, resulting in a unique valuable data set for detailed studies of SGR J1935+2154. In this work, we conduct a comprehensive analysis of this observation, including detailed burst search, identification, and temporal analyses. After carefully removing false triggers, we find a total of 75 bursts from SGR J1935+2154, out of which 70 are single pulsed. The maximum burst rate is about 56 bursts day−1. Both the burst duration and the waiting time between two successive bursts follow lognormal distributions, consistent with previous studies. We also find that bursts with longer duration (some are multipulsed) tend to occur during the period with relatively high burst rate. There is no correlation between the waiting time and the fluence or duration of either the former or latter burst. It also seems that there is no correlation between burst duration and hardness ratio, in contrast to some previous reports. In addition, we do not find any X-ray burst associated with any reported radio bursts except for FRB 200428.

On-ground and on-orbit time calibrations of GECAM

ABSTRACT High time resolution and accuracy are of critical importance in the studies of timing analysis and time delay localization of gamma-ray bursts (GRBs), soft gamma-ray repeaters (SGRs) and pulsars. The Gravitational wave high-energy Electromagnetic Counterpart All-sky Monitor (GECAM) consisting of two micro-satellites, GECAM-A and GECAM-B, launched on 2020 December 10, is aimed at monitoring and locating X-ray and GRBs all over the sky. To achieve its scientific goals, GECAM is designed to have the highest time resolution (0.1 $\mu {\rm s}$) among all GRB detectors ever flown. Here, we make a comprehensive time calibration campaign including both on-ground and on-orbit tests to derive not only the relative time accuracy of GECAM satellites and detectors, but also the absolute time accuracy of GECAM-B. Using the on-ground calibration with a $\rm ^{22}Na$ radioactive source, we find that the relative time accuracy between GECAM-A and GECAM-B is about 0.15 $\mu {\rm s}$ (1σ). To measure the relative time accuracy between all detectors of a single GECAM satellite, cosmic-ray events detected on orbit are utilized since they could produce many secondary particles simultaneously record by multiple detectors. We find that the relative time accuracy among all detectors onboard GECAM-B is about 0.12 $\mu {\rm s}$ (1σ). Finally, we use the novel Li-CCF method to perform the absolute time calibration with Crab pulsar and SGR J1935+2154, both of which were jointly observed by GECAM-B and Fermi/GBM, and obtain that the time difference between GECAM-B and Fermi/GBM is 3.06 ± 6.04 $\mu {\rm s}$ (1σ).

Magnetar giant flare originating from GRB 200415A: transient GeV emission, time-resolved Ep – L iso correlation and implications

Giant flares (GFs) are unusual bursts from soft gamma-ray repeaters (SGRs) that release an enormous amount of energy in a fraction of a second. The afterglow emission of these SGR-GFs or GF candidates is a highly beneficial means of discerning their composition, relativistic speed and emission mechanisms. GRB 200415A is a recent GF candidate observed in a direction coincident with the nearby Sculptor galaxy at 3.5 Mpc. In this work, we searched for transient gamma-ray emission in past observations by Fermi-LAT in the direction of GRB 200415A. These observations confirm that GRB 200415A is observed as a transient GeV source only once. A pure pair-plasma fireball cannot provide the required energy for the interpretation of GeV afterglow emission and a baryonic poor outflow is additionally needed to explain the afterglow emission. A baryonic rich outflow is also viable, as it can explain the variability and observed quasi-thermal spectrum of the prompt emission if dissipation is happening below the photosphere via internal shocks. Using the peak energy (Ep ) of the time-resolved prompt emission spectra and their fluxes (Fp ), we found a correlation between Ep and Fp or isotropic luminosity L iso for GRB 200415A. This supports the intrinsic nature of Ep -L iso correlation found in SGRs-GFs, hence favoring a baryonic poor outflow. Our results also indicate a different mechanism at work during the initial spike, and that the evolution of the prompt emission spectral properties in this outflow would be intrinsically due to the injection process.

Similar Scale-invariant Behaviors between Soft Gamma-Ray Repeaters and an Extreme Epoch from FRB 121102

Abstract The recent discovery of a Galactic fast radio burst (FRB) associated with a hard X-ray burst from the soft gamma-ray repeater (SGR) J1935+2154 has established the magnetar origin of at least some FRBs. In this work, we study the statistical properties of soft gamma-ray/hard X-ray bursts from SGRs 1806–20 and J1935+2154 and of radio bursts from the repeating FRB 121102. For SGRs, we show that the probability density functions for the differences of fluences, fluxes, and durations at different times have fat tails with a q-Gaussian form. The q values in the q-Gaussian distributions are approximately steady and independent of the temporal interval scale adopted, implying a scale-invariant structure of SGRs. These features indicate that SGR bursts may be governed by a self-organizing criticality (SOC) process, confirming previous findings. Very recently, 1652 independent bursts from FRB 121102 have been detected by the Five-hundred-meter Aperture Spherical radio Telescope (FAST). Here we also investigate the scale-invariant structure of FRB 121102 based on the latest observations of FAST, and show that FRB 121102 and SGRs share similar statistical properties. Given the bimodal energy distribution of FRB 121102 bursts, we separately explore the scale-invariant behaviors of low- and high-energy bursts of FRB 121102. We find that the q values of low- and high-energy bursts are different, which further strengthens the evidence of the bimodality of the energy distribution. Scale invariance in both the high-energy component of FRB 121102 and SGRs can be well explained within the same physical framework of fractal-diffusive SOC systems.

Statistical properties of fast radio bursts elucidate their origins: magnetars are favored over gamma-ray bursts

Fast radio bursts (FRBs) are extremely strong radio flares lasting several milliseconds, most of which come from unidentified objects at a cosmological distance. They can be apparently repeating or not. In this paper, we analyzed 18 repeaters and 12 non-repeating FRBs observed in the frequency bands of 400–800 MHz from Canadian Hydrogen Intensity Mapping Experiment (CHIME). We investigated the distributions of FRB isotropic-equivalent radio luminosity, considering the K correction. Statistically, the luminosity distribution can be better fitted by Gaussian form than by power-law. Based on the above results, together with the observed FRB event rate, pulse duration, and radio luminosity, FRB origin models are evaluated and constrained such that the gamma-ray bursts (GRBs) may be excluded for the non-repeaters while magnetars or neutron stars (NSs) emitting the supergiant pulses are preferred for the repeaters. We also found the necessity of a small FRB emission beaming solid angle (about 0.1 sr) from magnetars that should be considered, and/or the FRB association with soft gamma-ray repeaters (SGRs) may lie at a low probability of about 10%. Finally, we discussed the uncertainty of FRB luminosity caused by the estimation of the distance that is inferred by the simple relation between the redshift and dispersion measure (DM).