Magnetars are strongly magnetized, isolated neutron stars
with magnetic fields up to around 10
gauss, luminosities of approximately 10
-10
ergs per second and rotation periods of about 0.3-12.0 s. ...Very energetic giant flares from galactic magnetars (peak luminosities of 10
-10
ergs per second, lasting approximately 0.1 s) have been detected in hard X-rays and soft γ-rays
, and only one has been detected from outside our galaxy
. During such giant flares, quasi-periodic oscillations (QPOs) with low (less than 150 hertz) and high (greater than 500 hertz) frequencies have been observed
, but their statistical significance has been questioned
. High-frequency QPOs have been seen only during the tail phase of the flare
. Here we report the observation of two broad QPOs at approximately 2,132 hertz and 4,250 hertz in the main peak of a giant γ-ray flare
in the direction of the NGC 253 galaxy
, disappearing after 3.5 milliseconds. The flare was detected on 15 April 2020 by the Atmosphere-Space Interactions Monitor instrument
aboard the International Space Station, which was the only instrument that recorded the main burst phase (0.8-3.2 milliseconds) in the full energy range (50 × 10
to 40 × 10
electronvolts) without suffering from saturation effects such as deadtime and pile-up. Along with sudden spectral variations, these extremely high-frequency oscillations in the burst peak are a crucial component that will aid our understanding of magnetar giant flares.
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2.
First 10 Months of TGF Observations by ASIM Østgaard, N.; Neubert, T.; Reglero, V. ...
Journal of geophysical research. Atmospheres,
27 December 2019, Volume:
124, Issue:
24
Journal Article
Peer reviewed
Open access
The Atmosphere‐Space Interactions Monitor (ASIM) was launched to the International Space Station on 2 April 2018. The ASIM payload consists of two main instruments, the Modular X‐ray and Gamma‐ray ...Sensor (MXGS) for imaging and spectral analysis of Terrestrial Gamma‐ray Flashes (TGFs) and the Modular Multi‐spectral Imaging Array for detection, imaging, and spectral analysis of Transient Luminous Events and lightning. ASIM is the first space mission designed for simultaneous observations of Transient Luminous Events, TGFs, and optical lightning. During the first 10 months of operation (2 June 2018 to 1 April 2019) the MXGS has observed 217 TGFs. In this paper we report several unprecedented measurements and new scientific results obtained by ASIM during this period: (1) simultaneous TGF observations by Fermi Gamma‐ray Burst Monitor and ASIM MXGS revealing the very good detection capability of ASIM MXGS and showing substructures in the TGF, (2) TGFs and Elves produced during the same lightning flash and even simultaneously have been observed, (3) first imaging of TGFs giving a unique source location, (4) strong statistical support for TGFs being produced during the upward propagation of a leader just before a large current pulse heats up the channel and emits a strong optical pulse, and (5) the t50 duration of TGFs observed from space is shorter than previously reported.
Key Points
Simultaneous measurements of TGF by two spacecraft are presented
Simultaneous measurements of TGF and Elve are not rare coincidence
Imaging of TGF is presented
The sequence of TGF and main optical lightning pulse is resolved
TGFs observed from space have shorter duration than previously reported
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ABSTRACT
We report on detailed multiwavelength observations and analysis of the very bright and long GRB 210619B, detected by the Atmosphere-Space Interactions Monitor installed on the International ...Space Station and the Gamma-ray Burst Monitor (GBM) on-board the Fermi mission. Our main goal is to understand the radiation mechanisms and jet composition of GRB 210619B. With a measured redshift of z = 1.937, we find that GRB 210619B falls within the 10 most luminous bursts observed by Fermi so far. The energy-resolved prompt emission light curve of GRB 210619B exhibits an extremely bright hard emission pulse followed by softer/longer emission pulses. The low-energy photon index (αpt) values obtained using the time-resolved spectral analysis of the burst suggest a transition between the thermal (during harder pulse) to non-thermal (during softer pulse) outflow. We examine the correlation between spectral parameters and find that both peak energy and αpt exhibit the flux tracking pattern. The late time broad-band photometric data set can be explained within the framework of the external forward shock model with νm < νc < νx (where νm, νc, and νx are the synchrotron peak, cooling-break, and X-ray frequencies, respectively) spectral regime supporting a rarely observed hard electron energy index (p < 2). We find moderate values of host extinction of E(B − V) = 0.14 ± 0.01 mag for the small magellanic cloud extinction law. In addition, we also report late-time optical observations with the 10.4 m Gran Telescopio de Canarias placing deep upper limits for the host galaxy (z = 1.937), favouring a faint, dwarf host for the burst.
We report the first Terrestrial Electron Beam detected by the Atmosphere‐Space Interactions Monitor. It happened on 16 September 2018. The Atmosphere‐Space Interactions Monitor Modular X and Gamma ...ray Sensor recorded a 2 ms long event, with a softer spectrum than typically recorded for Terrestrial Gamma ray Flashes (TGFs). The lightning discharge associated to this event was found in the World Wide Lightning Location Network data, close to the northern footpoint of the magnetic field line that intercepts the International Space Station location. Imaging from a GOES‐R geostationary satellite shows that the source TGF was produced close to an overshooting top of a thunderstorm. Monte‐Carlo simulations were performed to reproduce the observed light curve and energy spectrum. The event can be explained by the secondary electrons and positrons produced by the TGF (i.e., the Terrestrial Electron Beam), even if about 3.5% to 10% of the detected counts may be due to direct TGF photons. A source TGF with a Gaussian angular distribution with standard deviation between 20.6° and 29.8° was found to reproduce the measurement. Assuming an isotropic angular distribution within a cone, compatible half angles are between 30.6° and 41.9°, in agreement with previous studies. The number of required photons for the source TGF could be estimated for various assumption of the source (altitude of production and angular distribution) and is estimated between 1017.2 and 1018.9 photons, that is, compatible with the current consensus.
Plain Language Summary
Terrestrial Gamma Ray Flashes (TGFs) are the highest energy natural particle acceleration phenomena occurring on Earth. They are burst of energetic photons associated with thunderstorms and have a poorly understood production mechanism. When interacting with the atmosphere, TGFs produce secondary electrons and positrons of high energy. A fraction of them can reach space and forms a beam under the effect of Earth's magnetic field, so called Terrestrial Electron Beam (TEB). They can be detected over geographical location with no lightning activity. In the past, most of the TEBs have been detected by the Fermi space telescope and the Compton Gamma ray Observatory. In this article, we report the first detection of a TEB by the Atmosphere‐Space Interactions Monitor, docked on the International Space Station since April 2018. During this event, no lightning activity was detected below the instrument. The TEB's source lightning was actually found to be located 650 km away from detector, very close to an overshooting top of a thundercloud. The comparison of the observation with simulated data made it possible to constrain the geometry of the parent TGF. Our results point toward a relatively wide angular distribution and an intensity of 1017.2 to 1018.9 photons, in agreement with previous studies.
Key Points
Flying over an area with no nearby lightning activity, the ASIM‐MXGS instrument detected a 4 ms long event with a soft spectrum
Observations coupled with simulations suggest that more than 90% of the counts come from a TEB and the rest from the associated TGF
A source TGF with a broad angular distribution and 1017 to 1019 photons can explain the observation
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Gamma‐Ray Glows (GRGs) are high energy radiation originating from thunderclouds, in the MeV energy regime, with typical duration of seconds to minutes, and sources extended over several to tens of ...square kilometers. GRGs have been observed from detectors placed on ground, inside aircraft and on balloons. In this paper, we present a general purpose Monte‐Carlo model of GRG production and propagation. This model is first compared to a model from Zhou et al. (2016, https://doi.org/10.1016/j.astropartphys.2016.08.004) relying on another Monte‐Carlo framework, and small differences are observed. We then have built an extensive simulation library, made available to the community. This library is used to reproduce five previous gamma‐ray glow observations, from five airborne campaigns: balloons from Eack et al. (1996b, https://doi.org/10.1029/96gl02570), Eack et al. (2000, https://doi.org/10.1029/1999gl010849); and aircrafts from ADELE (Kelley et al., 2015, https://doi.org/10.1038/ncomms8845), ILDAS (Kochkin et al., 2017, https://doi.org/10.1002/2017jd027405) and ALOFT (Østgaard et al., 2019, https://doi.org/10.1029/2019jd030312). Our simulation results confirm that fluxes of cosmic‐ray secondary particles present in the background at a given altitude can be enhanced by several percent (MOS process), and up to several orders of magnitude (RREA process) due to the effect of thunderstorms' electric fields, and explain the five observations. While some GRG could be explained purely by the MOS process, E‐fields significantly larger than Eth are required to explain the strongest GRGs observed. Some of the observations also came with in‐situ electric field measurements, that were always lower than Eth, but may not have been obtained from regions where the glows are produced. This study supports the claim that kilometer‐scale E‐fields magnitudes of at least the level of Eth must be present inside some thunderstorms.
Plain Language Summary
Gamma‐Ray Glows (GRGs) are high‐energy radiation that originates from thunderclouds. These radiations fall within the MeV energy range and last for seconds to minutes. The sources of GRGs are typically extended over a few to tens of square kilometers. In this study, we developed a general‐purpose model to understand the production of GRGs, including the cosmic ray fluxes and enhancement by thunderstorm's electric field, propagation, and instrumental response. Using this model, we were able to reproduce and constrain five previously reported airborne GRG observations, two from balloons, and three from aircraft. The results of our study showed that all of the observations could be explained by one of the two expected regimes: one involving purely particle acceleration (MOS: Modification of Spectrum) and the other involving particle multiplication (RREA: Relativistic Runaway Electron Avalanche). Our simulations suggest that the required large‐scale thunderstorm electric fields, which are compatible with our results, are generally larger than what was previously measured.
Key Points
A general‐purpose Monte‐Carlo model of gamma‐ray glow production is presented
Plausible Gamma‐ray Glow production conditions are provided for five previous airborne observations
Some cases could be explained by the Modification of Spectrum mechanism only while other require electric fields close to Relativistic Runaway Electron Avalanche process threshold, or above
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Terrestrial gamma ray flashes are natural bursts of X and gamma rays, correlated to thunderstorms, that are likely to be produced at an altitude of about 10 to 20 km. After the emission, the flux of ...gamma rays is filtered and altered by the atmosphere and a small part of it may be detected by a satellite on low Earth orbit (RHESSI or Fermi, for example). Thus, only a residual part of the initial burst can be measured and most of the flux is made of scattered primary photons and of secondary emitted electrons, positrons, and photons. Trying to get information on the initial flux from the measurement is a very complex inverse problem, which can only be tackled by the use of a numerical model solving the transport of these high‐energy particles. For this purpose, we developed a numerical Monte Carlo model which solves the transport in the atmosphere of both relativistic electrons/positrons and X/gamma rays. It makes it possible to track the photons, electrons, and positrons in the whole Earth environment (considering the atmosphere and the magnetic field) to get information on what affects the transport of the particles from the source region to the altitude of the satellite. We first present the MC‐PEPTITA model, and then we validate it by comparison with a benchmark GEANT4 simulation with similar settings. Then, we show the results of a simulation close to Fermi event number 091214 in order to discuss some important properties of the photons and electrons/positrons that are reaching satellite altitude.
Key Points
Detailed description of the model and comparison case with GEANT4
The magnetic mirroring ratio is 22% for electrons and 19% for positrons
Electron beams possibly 50 to 100% larger than previously expected
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Gamma Ray Glow Observations at 20‐km Altitude Østgaard, N.; Christian, H. J.; Grove, J. E. ...
Journal of geophysical research. Atmospheres,
16 July 2019, Volume:
124, Issue:
13
Journal Article
Peer reviewed
Open access
In the spring of 2017 an ER‐2 aircraft campaign was undertaken over continental United States to observe energetic radiation from thunderstorms and lightning. The payload consisted of a suite of ...instruments designed to detect optical signals, electric fields, and gamma rays from lightning. Starting from Georgia, USA, 16 flights were performed, for a total of about 70 flight hours at a cruise altitude of 20 km. Of these, 45 flight hours were over thunderstorm regions. An analysis of two gamma ray glow events that were observed over Colorado at 21:47 UT on 8 May 2017 is presented. We explore the charge structure of the cloud system, as well as possible mechanisms that can produce the gamma ray glows. The thundercloud system we passed during the gamma ray glow observation had strong convection in the core of the cloud system. Electric field measurements combined with radar and radio measurements suggest an inverted charge structure, with an upper negative charge layer and a lower positive charge layer. Based on modeling results, we were not able to unambiguously determine the production mechanism. Possible mechanisms are either an enhancement of cosmic background locally (above or below 20 km) by an electric field below the local threshold or an enhancement of the cosmic background inside the cloud but then with normal polarity and an electric field well above the Relativistic Runaway Electron Avalanche threshold.
Key Points
Gamma ray glows were observed for the first time at 20‐km altitude above thunderclouds
The thunderclouds below the aircraft had an anomalous charge structure
Possible production mechanisms for gamma ray glow are tested by Monte Carlo modeling
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Terrestrial Gamma‐ray Flashes (TGFs) are short emissions of high energy photons associated with thunderstorms. It has been known since the discovery of TGFs that they are associated with lightning, ...and several case studies have shown that the TGFs are produced at the initial phase of the lightning flash. However, it has not been tested whether this is true in general. By using the largest TGF sample up to date, combined with ground‐based radio lightning detection data, we perform a statistical study to test this. One of the TGF missions is the Atmosphere‐Space Interactions Monitor (ASIM) consisting of the innovative combination of X‐ and gamma‐ray detectors, optical photometers and cameras. This allows us to investigate the temporal relation between gamma‐rays produced by TGFs and the optical signal produced by lightning discharges. Based on stacking analysis of the TGF sample and ground‐based measurements of associated lightning activity, together with the high temporal resolution of the optical signal from the ASIM photometers, it is shown that TGFs are produced in the beginning of the lightning flashes. In addition, for a significant fraction of the TGFs, the lightning activity detected in radio is enhanced in an interval between 150 and 750 ms following the TGFs, and is co‐located with the lightning associated with the TGFs. The enhanced lightning activity is not evident in a randomly selected sample of flashes. This indicates that the activity between 150 and 750 ms is a characteristic property of a significant fraction of flashes that start with a TGF.
Key Points
VLF radio and optical measurements show that upward TGFs are typically produced in the beginning of a lightning flash
Stacking analysis confirms an excess of lightning activity 150–750 ms after the TGFs
When a TGF is simultaneous to a lightning stroke, the enhanced activity after is usually co‐located with the first lightning stroke
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BFBNIB, FZAB, GIS, IJS, KILJ, NLZOH, NUK, OILJ, SAZU, SBCE, SBMB, UL, UM, UPUK
Terrestrial gamma‐ray flashes are bursts of X/gamma photons, correlated to thunderstorms. By interacting with the atmosphere, the photons produce a substantial number of electrons and positrons. Some ...of these reach a sufficiently high altitude that their interactions with the atmosphere become negligible, and they are then guided by geomagnetic field lines, forming a Terrestrial Electron Beam. On 9 December 2009, the Gamma‐Ray Burst Monitor (GBM) instrument on board the Fermi Space Telescope made a particularly interesting measurement of such an event. To study this type of event in detail, we perform Monte‐Carlo simulations and focus on the resulting time histograms. In agreement with previous work, we show that the histogram measured by Fermi GBM is reproducible from a simulation. We then show that the time histogram resulting from this simulation is only weakly dependent on the production altitude, duration, beaming angle, and spectral shape of the associated terrestrial gamma‐ray flash. Finally, we show that the time histogram can be decomposed into three populations of leptons, coming from the opposite hemisphere, and mirroring back to the satellite with or without interacting with the atmosphere, and that these populations can be clearly distinguished by their pitch angles.
Key Points
Fermi 091214 event time histogram accurately reproduced
The synthetic time histogram is reproducible by a wide range of possible TGFs
The lepton time histogram can be split into three populations by looking at the pitch angles
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We present a complete and systematic search for terrestrial gamma‐ray flashes (TGFs), detected by AGILE, that are associated with radio sferics detected by the World Wide Lightning Location Network ...(WWLLN) in the period February 2009 to September 2018. The search algorithms and characteristics of these new TGFs will be presented and discussed. The number of WWLLN identified (WI) TGFs shows that previous TGF selection criteria needs to be reviewed as they do not identify all the WI TGFs in the data set. In this analysis we confirm with an independent data set that WI TGFs tend to have shorter time duration than TGFs without a WWLLN match. TGFs occurs more often on coastal and ocean regions compared to the distribution of lightning activity. Several multipulse TGFs were identified and their WWLLN match are always associated with the last gamma‐ray pulse. We also present the first Terrestrial Electron Beam detected by AGILE. This data set together with the TGF sample identified by selection criteria (companion paper Maiorana et al., 2020) constitute the 3rd AGILE TGF catalog.
Key Points
A sample of 592 AGILE detected TGFs associated to lightning sferics is presented
TGF duration, geographic dependence of the TGF/lightning ratio, and multipulse TGFs are discussed
The first Terrestrial Electron Beam detected by AGILE is presented
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