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Soft γ-ray repeaters exhibit bursting emission in hard X-rays and soft γ-rays. During the active phase, they emit random short (milliseconds to several seconds long), hard-X-ray bursts, with peak luminosities 1 of 10 36 to 10 43 erg per second. Occasionally, a giant flare with an energy of around 10 44 to 10 46 erg is emitted 2 . These phenomena are thought to arise from neutron stars with extremely high magnetic fields (10 14 to 10 15 gauss), called magnetars 1 , 3 , 4 . A portion of the second-long initial pulse of a giant flare in some respects mimics short γ-ray bursts 5 , 6 , which have recently been identified as resulting from the merger of two neutron stars accompanied by gravitational-wave emission 7 . Two γ-ray bursts, GRB 051103 and GRB 070201, have been associated with giant flares 2 , 8 – 11 . Here we report observations of the γ-ray burst GRB 200415A, which we localized to a 20-square-arcmin region of the starburst galaxy NGC 253, located about 3.5 million parsecs away. The burst had a sharp, millisecond-scale hard spectrum in the initial pulse, which was followed by steady fading and softening over 0.2 seconds. The energy released (roughly 1.3 × 10 46 erg) is similar to that of the superflare 5 , 12 , 13 from the Galactic soft γ-ray repeater SGR 1806−20 (roughly 2.3 × 10 46 erg). We argue that GRB 200415A is a giant flare from a magnetar in NGC 253. The γ-ray burst GRB 200415A is probably a giant flare emitted from a magnetar in the nearby starburst galaxy NGC 253.

Pulsar systems accelerate particles to immense energies. The detailed functioning of these engines is still poorly understood, but polarization measurements of high-energy radiation may allow us to locate where the particles are accelerated. We have detected polarized gamma rays from the vicinity of the Crab pulsar using data from the spectrometer on the International Gamma-Ray Astrophysics Laboratory satellite. Our results show polarization with an electric vector aligned with the spin axis of the neutron star, demonstrating that a substantial fraction of the high-energy electrons responsible for the polarized photons are produced in a highly ordered structure close to the pulsar.

High-energy, long gamma-ray bursts (GRBs) can be generated by the core collapse of massive stars at the end of their lives. When they happen in the close-by universe they can be exceptionally bright, as seen from the Earth in the case of the recent, giant, long-lasting GRB221009A. GRB221009A was produced by a collapsing star with a redshift of 0.152: this event was observed by many gamma-ray space experiments, which also detected an extraordinary long gamma-ray afterglow. The exceptionally large fluence of the prompt emission of about 0.013 erg cm−2 illuminated a large geographical region centered on India and including Europe and Asia. We report in this paper the observation of sudden electron flux changes correlated with GRB221009A and measured by the HEPP-L charged particle detector on board the China Seismo-Electromagnetic Satellite, which was orbiting over Europe at the time of the GRB event. The time structure of the observed electron flux closely matches the very distinctive time dependence of the photon flux associated with the main part of the emission at around 13:20 UTC on 2022 October 9. To test the origin of these signals, we set up a simplified simulation of one HEPP-L subdetector: the results of this analysis suggest that the signals observed are mostly due to electrons created within the aluminum collimator surrounding the silicon detector, providing real-time monitoring of the very intense photon fluxes. We discuss the implications of this observation for existing and forthcoming particle detectors on low Earth orbits.

Time-dependent energy spectra of galactic cosmic rays (GCRs) carry crucial information regarding their origin and propagation throughout the interstellar environment. When observed at the Earth, after traversing the interplanetary medium, such spectra are heavily affected by the solar wind and the embedded solar magnetic field permeating the inner sectors of the heliosphere. The activity of the Sun changes significantly over an 11 yr solar cycle—and so does the effect on cosmic particles; this translates into a phenomenon called solar modulation. Moreover, GCR spectra during different epochs of solar activity provide invaluable information for a complete understanding of the plethora of mechanisms taking place in various layers of the Sun’s atmosphere and how they evolve over time. The High-Energy Particle Detector (HEPD-01) has been continuously collecting data since 2018 August, during the quiet phase between solar cycles 24 and 25; the activity of the Sun is slowly but steadily rising and is expected to peak around 2025/2026. In this paper, we present the first spectra for ∼50–250 MeV galactic protons measured by the HEPD-01 instrument—placed on board the CSES-01 satellite—from 2018 August to 2022 March over a one-Carrington-rotation time basis. Such data are compared to the ones from other spaceborne experiments, present (e.g., EPHIN, Parker Solar Probe) and past (PAMELA), and to a state-of-the-art three-dimensional model describing the GCRs propagation through the heliosphere.

Galactic cosmic-ray (GCR) intensities exhibit recurrent variations caused by their passage through plasma interaction regions corotating with the Sun, with the ∼27 day periodicity being the most prominent one. Data collected by the High-Energy Particle Detector (HEPD-01) on board the China Seismo-Electromagnetic Satellite in Low-Earth Orbit have been used to derive daily proton fluxes from 2018 to 2019 August, in the energy range between ∼55 and ∼200 MeV. Daily fluxes from HEPD-01 have been analyzed along with proton fluxes measured during the same period by ERNE and EPHIN, on board the SOHO spacecraft, and by AMS-02, on board the International Space Station. Using a time-frequency analysis, we confirm a slight energy dependence for the power of the ∼27 day variation as a function of time, with the periodicity maximum occurring earlier for HEPD-01 than for high-energy data from AMS-02. Additionally, as already obtained in previous studies, the rigidity dependence of the amplitude of the aforementioned GCR variation cannot be described by the same power law at both low and high energies, as a consequence of different physical mechanisms playing roles at different rigidity ranges. HEPD-01 GCR measurements cover the energy range from tens to a few hundreds of MeV, which is not accessible to existing detectors (EPHIN and ERNE covering from a few MeV up to tens or a hundred MeV, respectively, and AMS-02 in the GeV–TeV energy range), providing important information for understanding GCR periodicities.

The intricate behavior of particle acceleration and transport mechanisms complicates the overall efforts in formulating a comprehensive understanding of solar energetic particle (SEP) events; these efforts include observations of low-energy particles (from tens of keV to hundreds of MeV) by space-borne instruments and measurements by the ground-based neutron monitors of the secondary particles generated in the Earth atmosphere by SEPs in the GeV range. Numerous space-borne missions provided good data on the nature/characteristics of these solar particles in past solar cycles, but more recently—concurrently with the rise toward the maximum of solar cycle 25—the High-Energy Particle Detector (HEPD-01) proved to be well suited for the study of solar physics and space weather. Its nominal 30–300 MeV energy range for protons can enlarge the detection capabilities of solar particles at low Earth orbit, closer to the injection limit of many SEP events. In this work, we characterize three SEP events within the first six months of 2022 through spectral and velocity dispersion analysis, assessing the response of HEPD-01 to >M1 events.

The High-Energy Particle Detector (HEPD-01) on board the China Seismo-Electromagnetic Satellite, located on a Sun-synchronous orbit at 500 km of altitude with an inclination of 97°, features a dedicated logic counting low-energy event rates, which proved sensitive to intense Gamma-Ray Burst (GRB). The present work reports a comprehensive analysis of signals induced by GRBs in the event-rate data collected between 2018 August and 2022 June. After accurately modeling the background rate as observed in different passages of the satellite over the same geographical area, we detected significant deviations to be compared with observations of GRB candidates from other observatories. The analysis revealed 12 statistically significant excesses, that have been associated with GRB 181222B, GRB 190114C, GRB 190129B, GRB 190305A, GRB 190928A, GRB 200412B, GRB 200422A, GRB 200826B, GRB 201009A, GRB 210702A, GRB 211211A, and GRB 220624A. We report light curves for 0.3–50 MeV photons, comparing them with findings from other space telescopes. The catalog of observations is published, complete of GRB observation time, duration, integrated counts, and fluence.

In this paper we report the detection of five strong gamma-ray bursts (GRBs) by the High-Energy Particle Detector (HEPD-01) mounted on board the China Seismo-Electromagnetic Satellite, operational since 2018 on a Sun-synchronous polar orbit at a ∼507 km altitude and 97° inclination. HEPD-01 was designed to detect high-energy electrons in the energy range 3–100 MeV, protons in the range 30–300 MeV, and light nuclei in the range 30–300 MeV n−1. Nonetheless, Monte Carlo simulations have shown HEPD-01 is sensitive to gamma-ray photons in the energy range 300 keV–50 MeV, even if with a moderate effective area above ∼5 MeV. A dedicated time correlation analysis between GRBs reported in literature and signals from a set of HEPD-01 trigger configuration masks has confirmed the anticipated detector sensitivity to high-energy photons. A comparison between the simultaneous time profiles of HEPD-01 electron fluxes and photons from GRB190114C, GRB190305A, GRB190928A, GRB200826B, and GRB211211A has shown a remarkable similarity, in spite of the different energy ranges. The high-energy response, with peak sensitivity at about 2 MeV, and moderate effective area of the detector in the actual flight configuration explain why these five GRBs, characterized by a fluence above ∼3 × 10−5 erg cm−2 in the energy interval 300 keV–50 MeV, have been detected.

The nature of a substantial percentage (about one fifth) of hard X-ray sources discovered with the BAT instrument onboard the Neil Gehrels Swift Observatory (hereafter Swift) is unknown because of the lack of an identified longer-wavelength counterpart. Without such follow-up, an X-ray catalogue is of limited astrophysical value: we therefore embarked, since 2009, on a long-term project to uncover the optical properties of sources identified by Swift by using a large suite of ground-based telescopes and instruments.In this work, we continue our programme of characterization of unidentified or poorly studied hard X-ray sources by presenting the results of an optical spectroscopic campaign aimed at pinpointing and classifying the optical counterparts of 35 hard X-ray sources taken from the 70-month BAT catalogue. This sample was selected out of the available information about the chosen objects: either they are completely unidentified sources, or their association with a longer-wavelength counterpart is still ambiguous.With the use of optical spectra taken at six different telescopes we were able to identify the main spectral characteristics (continuum type, redshift, and emission or absorption lines) of the observed objects, and determined their nature.We identify and characterize a total of 41 optical candidate counterparts corresponding to 35 hard X-ray sources given that, because of positional uncertainties, multiple lower energy counterparts can sometimes be associated with higher energy detections. We discuss which ones are the actual (or at least most likely) counterparts based on our observational results.In particular, 31 sources in our sample are active galactic nuclei: 16 are classified as Type 1 (with broad and narrow emission lines) and 13 are classified as Type 2 (with narrow emission lines only); two more are BL Lac-type objects. We also identify one LINER, one starburst, and 3 elliptical galaxies. The remaining 5 objects are galactic sources: we identify 4 of them as cataclysmic variables, whereas one is a low mass X-ray binary.

We present an observational multiwavelength campaign during 2018–19 for PBC J2333.9–2343, a giant radio galaxy with a bright central core associated to a blazar nucleus, whose structure could be due to a significant jet reorientation. We report flux increases by a factor of two or more on timescales shorter than a month, resembling flaring events. The cross correlation between the NIR and optical bands shows quasi-simultaneous variations arising from the jet. The optical variability properties of PBC J2333.9–2343 are more comparable to a sample of blazars than to non-blazar AGN. The SED of the nucleus shows two peaks, with a derived jet angle of 3 degrees, also typical of a blazar. Therefore, we confirm the presence of a blazar-like core in the center of this galaxy.