Soft Gamma-ray Research Articles

First results of a newly-built hard-X-ray/soft-Gamma spectrometer imaging system: on the aspect of plasma disruptions

Abstract A new Hard X-ray and soft gamma-ray spectrometer imaging System (HXS) has been built for the 2-Dimensional measurements of plasma emitted photons on the Experimental Advanced Superconducting Tokamak (EAST). The system uses a 2-D Cadmium Zinc Telluride (CZT) detector and an integrated electronics, which is as a whole shielded by a tungsten box with a pinhole and is tangential to the toroidal field. Three classes of typical energy spectra have been summarized in differently experimental scenarios over the past campaigns. After performing tomography calculations, the local emissivity contours have been obtained in different energy ranges which obviously show the asymmetry on the plasma cross section. The spatial perturbation structure is similar to the magnetohydrodynamics (MHD) modes with low mode numbers. In particular, the runaway island found by an infrared camera [R. Jaspers et al, Phys. Rev. Lett. 72, 4093 (1994)] is also measured by the HXS. There exists a reversal population on the energy spectra in both slide-away and strong neutral beam injection (NBI) shots. It is consistently observed that the count rate is increased in the low energy range before the plasma disruptions. Calculations in phase space indicate that the accelerated momentum flux can be deflected back to the low energy region by the large pitch-angle scattering. In the post-disruption phase the plasma current is not replaced by runaway electrons due to tearing modes or transiently bursting instabilities. This paper constructs the basics for the proper use of HXS for Hard X-ray and Soft Gamma-ray measurements in future investigations of plasma disruptions.

Data acquisition system for a newly-built hard x-ray/soft gamma-ray spectrometer imaging system on experimental advanced superconducting tokamak

A portable hard X-ray and soft gamma-ray spectrometer imaging system (HXS) has been constructed to gather physical information about fast electrons confined in the Experimental Advanced Superconducting Tokamak (EAST). The system is installed on the low field side of the mid-plane and provides a viewing field tangential to the toroidal field. The system utilizes a two-dimensional Cadmium Zinc Telluride (CdZnTe) semiconductor detector with 128 channels, and a data acquisition (DAQ) system has been designed for it. The DAQ system features a highly integrated signal processing system with the capability of high-speed processing and digital transmission of signals from 128 channels. In addition, a related DAQ software has been developed using a modular design approach, facilitating tasks such as data reception, storage, and preliminary processing. HXS, which has been applied during the recent EAST campaign, directly obtains the digital energy spectrum of incident photons. The DAQ system is described in detail in this paper. The hardware components and energy calibration have also been described. Experimental data have been successfully obtained and briefly discussed. More physical research will be reported in future publications.

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.

Fabrication of americium containing transmutation targets

An innovative, dust-free method for producing (U,Am)O2-x targets without the generation of Am-contaminated liquid waste has been successfully demonstrated in the Fuel Materials Laboratories at SCK CEN. The method comprises the fabrication of porous uranium oxide microspheres through internal gelation, using starch as a pore-former, and a single-step Am(NO3)3 aqueous solution infiltration process. Various samples containing targeted Am contents of 5, 10, 20, or 30 mol% were prepared. The concentration of the Am(III) stock solution was confirmed by ICP-MS and no other Am species than Am3+ were distinguished by UV–Vis spectrophotometry. The microstructural investigations revealed that the density of the porous host microspheres was approximately 65 %TD, with 30 vol% accessible porosity, suitable for an efficient infiltration. Microstructural analysis using SEM-EDS was performed on the sintered microspheres. Unexpectedly, EDS analysis was found to be incompatible due to detector interference from the 60 keV gamma-rays emitted by the sample. Therefore, examination of cross-sectioned (U,Am)O2-x microspheres was performed using wavelength dispersive spectrometry (WDS), which was unaffected by the soft gamma ray emission. A homogeneous mixing of Am and U was observed, without the formation of agglomerates with distinct doping levels. From the periphery to the center of the microspheres a radial gradient in Am content was observed, indicating a slight enrichment of Am on the surface of the microspheres. Overall, the nominal Am concentration was in very good agreement with the targeted content.

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.

2-mm-thick large-area CdTe double-sided strip detectors for high-resolution spectroscopic imaging of X-ray and gamma-ray with depth-of-interaction sensing

We developed a 2-mm-thick CdTe double-sided strip detector (CdTe-DSD) with a 250μm strip pitch, which has high spatial resolution with a uniform large imaging area of 10 cm2 and high energy resolution with high detection efficiency in tens to hundreds keV. The detector can be employed in a wide variety of fields for quantitative observations of hard X-ray and soft gamma-ray with spectroscopic imaging, for example, space observation, nuclear medicine, and non-destructive elemental analysis. This detector is thicker than the 0.75-mm-thick one previously developed by a factor of ∼2.7, thus providing better detection efficiency for hard X-rays and soft gamma rays. The increased thickness could potentially enhance bias-induced polarization if we do not apply sufficient bias and if we do not operate at a low temperature, but the polarization is not evident in our detector when a high voltage of 500 V is applied to the CdTe diode and the temperature is maintained at −20 °C during one-day experiments. The “Depth Of Interaction” (DOI) dependence due to the CdTe diode’s poor carrier-transport property is also more significant, resulting in much DOI information while complicated detector responses such as charge sharings or low-energy tails that exacerbate the loss in the energy resolution.In this paper, we developed 2-mm-thick CdTe-DSDs, studied their response, and evaluated their energy resolution, spatial resolution, and uniformity. We also constructed a theoretical model to understand the detector response theoretically, resulting in reconstructing the DOI with an accuracy of 100 μm while estimating the carrier-transport property. First, we evaluated the energy resolution using 22Na, 133Ba, and 57Co reconstructing the DOI effect from the energy correlation on both the anode and cathode sides. We obtained an energy resolution of 4.3 keV (FWHM) at 356 keV. Second, we formulated a theoretical detector-response model that reproduced the experimental data. Using the model, we determined the mobility-lifetime products of carriers μτe,h=(4±1)×10−3,(1.15±0.05)×10−4 [cm2/V] with the space charge density of nsp=−6.0×1010 [1/cm−3] and then reconstructed the DOI with an accuracy of 100 μm. Finally, we evaluated the spatial resolution and demonstrated to resolve patterns of 250-μm-width slits with an X-ray test chart. We found no positional variation in the detector response. We realized the detector that has high energy resolution and high 3D spatial resolution with a uniform large imaging area.

Centaurus A: Exploring the Nature of the Hard X-Ray/Soft Gamma-Ray Emission with INTEGRAL

The question of the origin of the hard X-ray/soft gamma-ray emission in Centaurus A (Cen A) persists despite decades of observations. Results from X-ray instruments suggest a jet origin since the implied electron temperature (kT e ) would cause runaway pair production in the corona. In contrast, instruments sensitive to soft gamma rays report electron temperatures indicating that a coronal origin may be possible. In this context, we analyzed archival INTEGRAL/IBIS-ISGRI and SPI data and observations from a 2022 Cen A monitoring program. Our analysis did not find any spectral variability. Thus we combined all observations for long-term average spectra, which were fit with a NuSTAR observation to study the 3.5 keV–2.2 MeV spectrum. Spectral fits using a CompTT model found kT e ∼ 550 keV, near runaway pair production. The spectrum was also well described by a log-parabola to model synchrotron self-Compton emission from the jet. Additionally, a spectral fit with the 12 yr catalog Fermi/LAT spectrum using a log-parabola can explain the data up to ∼3 GeV. Above ∼3 GeV, a power-law excess is present, which has been previously reported in LAT/H.E.S.S. analysis. However, including a coronal spectral component can also describe the data well. In this scenario, the hard X-rays/soft gamma rays are due the corona and the MeV to GeV emission is due to the jet.

Statistical X-ray analysis of gamma-ray-emitting radio galaxies

Abstract We systematically analyzed Swift/XRT X-ray data of gamma-ray-emitting radio galaxies (RGs) to study the origin of X-ray radiation. We studied the short-duration variability of spectra by using Swift/XRT data for 20 RGs. We found that gamma-ray-emitting RGs with a higher gamma-ray-to-X-ray flux ratio show a strong harder-when-brighter behavior. In addition, gamma-ray-emitting RGs with soft gamma-ray spectral index tend to have a hard X-ray spectral index and vice versa. Based on these properties, we imply a trend that gamma-ray-emitting RGs could be roughly classified into three types of X-ray emission: jet-dominated type, disk/corona-dominated type, and intermediate type. This classification is quite consistent with the optical and radio classification of RGs. In addition, we also discuss this result in terms of the Eddington luminosity ratio. These interpretations should be confirmed using larger samples of RGs in the future.

Soft gamma-ray spectral and time evolution of the GRB 221009A: Prompt and afterglow emission with INTEGRAL/IBIS-PICsIT

Aims. The gamma-ray burst (GRB) 221009A, with its extreme brightness, has provided the opportunity to explore GRB prompt and afterglow emission behavior on short timescales with high statistics. In conjunction with detection up to very-high-energy gamma rays, studies of this event shed light on the emission processes at work in the initial phases of GRBs’ emission. Methods. Using INTEGRAL/IBIS’s soft gamma-ray detector, PICsIT (200–2600 keV), we studied the temporal and spectral evolution during the prompt phase and the early afterglow period. Results. We found a “flux-tracking” behavior with the source spectrum “softer” when brighter. However, the relationship between the spectral index and the flux changes during the burst. The PICsIT light curve shows that afterglow emission begins to dominate at ∼T0 + 630 s and decays with a slope of 1.6 ± 0.2, consistent with the slopes reported at soft X-rays.

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.

Diffuse x-ray and gamma-ray limits on boson stars that interact with nuclei

Light bosonic dark matter can form gravitationally bound states known as boson stars. In this work, we explore a new signature of these objects interacting with the interstellar medium (ISM). We show how small effective couplings between the bosonic dark matter and the nucleon lead to a potential that accelerates ISM baryons as they transit the boson star, making the ISM within radiate at a high rate and energy. The low ISM density, however, implies the majority of Galactic boson stars will be too faint to be observable through this effect. By contrast, the diffuse photon flux, in hard x-rays and soft gamma-rays, produced by boson stars interacting with the ionized ISM phases can be sizable. We compute this diffuse flux and compare it to existing observations from HEAO-1, INTEGRAL and COMPTEL to infer limits on the fraction of these objects. This novel method places constraints on boson star dark matter while avoiding back-action effects from ambient baryons on the boson star configuration, unlike terrestrial searches where it has been noted that back-action can screen light bosonic fields. In addition, this study could be extended to other couplings and structures formed from light dark matter. For dark matter masses (10-14, 10-8) eV and boson star masses (10-10, 10-1) M ⊙, we find the constraints on the fraction can go down to f* ≲ 10-9 for dark matter in boson stars that is directly coupled to the Standard Model.

High-energy Photon Opacity in the Twisted Magnetospheres of Magnetars

Magnetars are neutron stars characterized by strong surface magnetic fields generally exceeding the quantum critical value of 44.1 TG. High-energy photons propagating in their magnetospheres can be attenuated by QED processes like photon splitting and magnetic pair creation. In this paper, we compute the opacities due to photon splitting and pair creation by photons emitted anywhere in the magnetosphere of a magnetar. Axisymmetric, twisted dipole field configurations embedded in the Schwarzschild metric are treated. The paper computes the maximum energies for photon transparency that permit propagation to infinity in curved spacetime. Special emphasis is given to cases where photons are generated along magnetic field loops and/or in polar regions; these cases directly relate to resonant inverse Compton scattering models for the hard X-ray emission from magnetars and Comptonized soft gamma-ray emission from giant flares. We find that increases in magnetospheric twists raise or lower photon opacities, depending on both the emission locale and the competition between field-line straightening and field strength enhancement. Consequently, given the implicit spectral transparency of hard X-ray bursts and persistent “tail” emission of magnetars, photon splitting considerations constrain their emission region locales and the twist angle of the magnetosphere; these constraints can be probed by future soft gamma-ray telescopes such as COSI and AMEGO. The inclusion of twists generally increases the opaque volume of pair creation by photons above its threshold, except when photons are emitted in polar regions and approximately parallel to the field.

MRI-drivenαΩ dynamos in protoneutron stars

Context.Magnetars are highly magnetized neutron stars that can produce a wide diversity of X-ray and soft gamma-ray emissions that are powered by magnetic dissipation. Their magnetic dipole is constrained in the range of 1014–1015G by the measurement of their spin-down. In addition to fast rotation, these strong fields are also invoked to explain extreme stellar explosions, such as hypernovae, which are associated with long gamma-ray bursts and superluminous supernovae. A promising mechanism for explaining magnetar formation is the amplification of the magnetic field by the magnetorotational instability (MRI) in fast-rotating protoneutron stars (PNS). This scenario is supported by recent global incompressible models, which showed that a dipole field with magnetar-like intensity can be generated from small-scale turbulence. However, the impact of important physical ingredients, such as buoyancy and density stratification, on the efficiency of the MRI in generating a dipole field is still unknown.Aims.We assess the impact of the density and entropy profiles on the MRI dynamo in a global model of a fast-rotating PNS. The model focuses on the outer stratified region of the PNS that is stable to convection.Methods.Using the pseudo-spectral code MagIC, we performed 3D Boussinesq and anelastic magnetohydrodynamics simulations in spherical geometry with explicit diffusivities and with differential rotation forced at the outer boundary. The thermodynamic background of the anelastic models was retrieved from the data of 1D core-collapse supernova simulations from the Garching group. We performed a parameter study in which we investigated the influence of different approximations and the effect of the thermal diffusion through the Prandtl number.Results.We obtain a self-sustained turbulent MRI-driven dynamo. This confirms most of our previous incompressible results when they are rescaled for density. The MRI generates a strong turbulent magnetic field and a nondominant equatorial dipole, which represents about 4.3% of the averaged magnetic field strength. Interestingly, an axisymmetric magnetic field at large scales is observed to oscillate with time, which can be described as a mean-fieldαΩ dynamo. By comparing these results with models without buoyancy or density stratification, we find that the key ingredient explaining the appearance of this mean-field behavior is the density gradient. Buoyancy due to the entropy gradient damps turbulence in the equatorial plane, but it has a relatively weak influence in the low Prandtl number regime overall, as expected from neutrino diffusion. However, the buoyancy starts to strongly impact the MRI dynamo for Prandtl numbers close to unity.Conclusions.Our results support the hypothesis that the MRI is able to generate magnetar-like large-scale magnetic fields. The results furthermore predict the presence of aαΩ dynamo in the protoneutron star, which could be important to model in-situ magnetic field amplification in global models of core-collapse supernovae or binary neutron star mergers.

GRB 211227A as a Peculiar Long Gamma-Ray Burst from a Compact Star Merger

Long-duration gamma-ray bursts (GRBs) associated with supernovae (SNe) are believed to originate from massive star core-collapse events, whereas short-duration GRBs that are related to compact star mergers are expected to be accompanied by kilonovae. GRB 211227A, which lasted about 84 s, had an initial short/hard spike followed by a series of soft gamma-ray extended emission at redshift z = 0.228. We performed follow-up observations of the optical emission using BOOTES, LCOGT, and the Lijiang 2.4 m telescope, but we detected no associated supernova signature, even down to very stringent limits at such a low redshift. We observed the host galaxy within a large error circle and roughly estimated the physical offset of GRB 211227A as 20.47 ± 14.47 kpc from the galaxy center. These properties are similar to those of GRB 060614, and suggest that the progenitor of GRB 211227A is not favored to be associated with the death of massive stars. Hence, we propose that GRB 211227A originates from a compact star merger. Calculating pseudo-kilonova emission for this case by adopting the typical parameters, we find that any associated pseudo-kilonova is too faint to be detected. If this is the case, it explains naturally the characteristics of the prompt emission, the lack of SN and kilonova emission, and the large physical offset from the galaxy center.

Gamma-rays from reaccelerated cosmic rays in high-velocity clouds colliding with the Galactic disc

ABSTRACT High-velocity clouds moving towards the disc will reach the Galactic plane and will inevitably collide with the disc. In these collisions, a system of two shocks is produced, one propagating through the disc and the other develops within the cloud. The shocks produced within the clouds in these interactions have velocities of hundreds of kilometres per second. When these shocks are radiative they may be inefficient in accelerating fresh particles; however, they can reaccelerate and compress Galactic cosmic rays from the background. In this work, we investigate the interactions of Galactic cosmic rays within a shocked high-velocity cloud, when the shock is induced by the collision with the disc. This study is focused in the case of radiative shocks. We aim to establish under which conditions these interactions lead to significant non-thermal emission, especially gamma-rays. We model the interaction of cosmic ray protons and electrons reaccelerated and further energized by compression in shocks within the clouds, under very general assumptions. We also consider secondary electron–positron pairs produced by the cosmic ray protons when colliding with the material of the cloud. We conclude that nearby clouds reaccelerating Galactic cosmic rays in local shocks can produce high-energy radiation that might be detectable with existing and future gamma-ray detectors. The emission produced by electrons and secondary pairs is important at radio wavelengths, and in some cases it may be relevant at hard X-rays. Concerning higher energies, the leptonic contribution to the spectral energy distribution is significant at soft gamma-rays.

Polarimetry With a Multilayer CdTe Prototype for Soft Gamma-Ray Astrophysics

The development of a new generation of soft gamma-ray telescopes should allow polarimetric measurements of astrophysical sources to be associated with the results from spectroscopy and imaging due to the promising developments in polarimetry achieved over the last decade. In this perspective, the advanced surveyor of transient events and nuclear astrophysics mission which includes a narrow field telescope (NFT) that comprises a wide band Laue lens (60–600 keV) associated with a high performance thick and stacked CdZnTe focal plane operating as spectrometer, imager, and scattering polarimeter has been submitted for the ESA Voyage 2050 long term scientific planning preparation. In this context, and with the aim of optimizing the focal plane configuration of an instrument such as NFT, several studies are underway. In particular, with this work we study some performance issues such as polarimeters of segmented semiconductor detectors at room temperature, such as CdZnTe and cadmium telluride (CdTe), by means of a prototype based on two planar CdTe spectroimagers operating in coincidence. The two CdTe detectors are 2-mm-thick crystals with the anode segmented in $8\times8$ pixels with 2-mm pitch. This prototype configuration allows the Compton polarimetric performance of a spectroimager to be assessed by changing the distance between the two detection layers while operating in the 100–600-keV energy range, thereby allowing the analysis of Compton polarimetry 3-D dependence. The results obtained provide relevant input to the design of large sensitive volume multilayer and 3-D sensitive focal planes.

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.

A Month of Monitoring the New Magnetar Swift J1555.2−5402 during an X-Ray Outburst

Abstract The soft gamma-ray repeater Swift J1555.2−5402 was discovered by means of a short burst detected with Swift BAT on 2021 June 3. Then, 1.6 hr after the burst, the Neutron star Interior Composition Explorer (NICER) started daily monitoring of this target for a month. The absorbed 2–10 keV flux stayed nearly constant at around 4 × 10−11 erg s−1 cm−2 during the monitoring, showing only a slight gradual decline. An absorbed blackbody with a temperature of 1.1 keV approximates the soft X-ray spectrum. A 3.86 s periodicity is detected, and the period derivative is measured to be 3.05(7) × 10−11 s s−1. The soft X-ray pulse shows a single sinusoidal shape with an rms pulsed fraction that increases as a function of energy from 15% at 1.5 keV to 39% at 7 keV. The equatorial surface magnetic field, characteristic age, and spin-down luminosity are derived under the dipole field approximation to be 3.5 × 1014 G, 2.0 kyr, and 2.1 × 1034 erg s−1, respectively. We detect 5 and 45 bursts with Swift/BAT and NICER, respectively. Based on these properties, this new source is classified as a magnetar. A hard X-ray power-law component that extends up to at least 40 keV is detected with the Nuclear Spectroscopic Telescope Array (NuSTAR). The 10–60 keV flux is ∼9 × 10−12 erg s−1 cm−2 with a photon index of ∼1.2. The pulsed fraction has a sharp cutoff at around 10 keV with an upper limit (≲10%) in the hard-tail band. No radio pulsations are detected during the DSN or VERA observations. The 7σ upper limits of the flux density are 0.043 and 0.026 mJy at the S and X bands, respectively.

Searching for TeV Gamma-Ray Emission from SGR 1935+2154 during Its 2020 X-Ray and Radio Bursting Phase

Abstract Magnetar hyperflares are the most plausible explanation for fast radio bursts (FRBs)—enigmatic powerful radio pulses with durations of several milliseconds and high brightness temperatures. The first observational evidence for this scenario was obtained in 2020 April when an FRB was detected from the direction of the Galactic magnetar and soft gamma-ray repeater SGR 1935+2154. The FRB was preceded by two gamma-ray outburst alerts by the BAT instrument aboard the Swift satellite, which triggered follow-up observations by the High Energy Stereoscopic System (H.E.S.S.). H.E.S.S. observed SGR 1935+2154 for 2 hr on 2020 April 28. The observations are coincident with X-ray bursts from the magnetar detected by INTEGRAL and Fermi-GBM, thus providing the first very high energy gamma-ray observations of a magnetar in a flaring state. High-quality data acquired during these follow-up observations allow us to perform a search for short-time transients. No significant signal at energies E > 0.6 TeV is found, and upper limits on the persistent and transient emission are derived. We here present the analysis of these observations and discuss the obtained results and prospects of the H.E.S.S. follow-up program for soft gamma-ray repeaters.