22 research outputs found
Cold Solar Flares I. Microwave Domain
We identify a set of ~100 "cold" solar flares and perform a statistical
analysis of them in the microwave range. Cold flares are characterized by a
weak thermal response relative to nonthermal emission. This work is a follow up
of a previous statistical study of cold flares, which focused on hard X-ray
emission to quantify the flare nonthermal component. Here we focus on the
microwave emission. The thermal response is represented by the soft X-ray
emission measured by the GOES X-ray sensors. We obtain spectral parameters of
the flare gyrosynchrotron emission and investigate patterns of the temporal
evolution. The main results of the previous statistical study are confirmed: as
compared to a "mean" flare, the cold flares have shorter durations, higher
spectral peak frequencies, and harder spectral indices above the spectral peak.
Nonetheless, there are some cold flares with moderate and low peak frequencies.
In a majority of cold flares, we find evidence suggesting the presence of the
Razin effect in the microwave spectra, indicative of rather dense flaring
loops. We discuss the results in the context of electron acceleration
efficiency
A global model of the response of tropical and sub-tropical forest biodiversity to anthropogenic pressures
Habitat loss and degradation, driven largely by agricultural expansion and intensification, present the greatest immediate threat to biodiversity. Tropical forests harbour among the highest levels of terrestrial species diversity and are likely to experience rapid land-use change in the coming decades. Synthetic analyses of observed responses of species are useful for quantifying how land use affects biodiversity and for predicting outcomes under land-use scenarios. Previous applications of this approach have typically focused on individual taxonomic groups, analysing the average response of the whole community to changes in land use. Here, we incorporate quantitative remotely sensed data about habitats in, to our knowledge, the first worldwide synthetic analysis of how individual species in four major taxonomic groups—invertebrates, ‘herptiles’ (reptiles and amphibians), mammals and birds—respond to multiple human pressures in tropical and sub-tropical forests. We show significant independent impacts of land use, human vegetation offtake, forest cover and human population density on both occurrence and abundance of species, highlighting the value of analysing multiple explanatory variables simultaneously. Responses differ among the four groups considered, and—within birds and mammals—between habitat specialists and habitat generalists and between narrow-ranged and wide-ranged species
GRB 221009A, The BOAT
GRB 221009A has been referred to as the Brightest Of All Time (the BOAT). We
investigate the veracity of this statement by comparing it with a half century
of prompt gamma-ray burst observations. This burst is the brightest ever
detected by the measures of peak flux and fluence. Unexpectedly, GRB 221009A
has the highest isotropic-equivalent total energy ever identified, while the
peak luminosity is at the th percentile of the known distribution. We
explore how such a burst can be powered and discuss potential implications for
ultra-long and high-redshift gamma-ray bursts. By geometric extrapolation of
the total fluence and peak flux distributions GRB 221009A appears to be a once
in 10,000 year event. Thus, while it almost certainly not the BOAT over all of
cosmic history, it may be the brightest gamma-ray burst since human
civilization began.Comment: Resubmitted to ApJ
Cosmic gamma-ray bursts and soft gamma-repeaters — observations and modeling of extreme astrophysical phenomena (100th anniversary of the Ioffe Institute)
GRB 180128A: A Second Magnetar Giant Flare Candidate from the Sculptor Galaxy
International audienceMagnetars are slowly rotating neutron stars that possess the strongest magnetic fields () known in the cosmos. They display a range of transient high-energy electromagnetic activity. The brightest and most energetic of these events are the gamma-ray bursts (GRBs) known as magnetar giant flares (MGFs), with isotropic energy . There are only seven detections identified as MGFs to date: three unambiguous events occurred in our Galaxy and the Magellanic Clouds, and the other four MGF candidates are associated with nearby star-forming galaxies. As all seven identified MGFs are bright at Earth, additional weaker events remain unidentified in archival data. We conducted a search of the Fermi Gamma-ray Burst Monitor (GBM) database for candidate extragalactic MGFs and, when possible, collected localization data from the Interplanetary Network (IPN) satellites. Our search yielded one convincing event, GRB 180128A. IPN localizes this burst with NGC 253, commonly known as the Sculptor Galaxy. This event is the second MGF in modern astronomy to be associated with this galaxy and the first time two bursts are associated with a single galaxy outside our own. Here, we detail the archival search criteria that uncovered this event and its spectral and temporal properties, which are consistent with expectations for a MGF. We also discuss the theoretical implications and finer burst structures resolved from various binning methods. Our analysis provides observational evidence for an eighth identified MGF