Abstract
The Large Area Telescope (LAT) on board the Fermi Gamma-ray Space Telescope (Fermi) shows long-lasting high-energy emission in many gamma-ray bursts (GRBs), similar to X-ray afterglows ...observed by the Neil Gehrels Swift Observatory (Swift). Some LAT light curves (LCs) show a late-time flattening reminiscent of X-ray plateaus. We explore the presence of plateaus in LAT temporally extended emission analyzing GRBs from the second Fermi-LAT GRB Catalog from 2008 to 2016 May with known redshifts, and check whether they follow closure relations corresponding to four distinct astrophysical environments predicted by the external forward shock model. We find that three LCs can be fit by the same phenomenological model used to fit X-ray plateaus and show tentative evidence for the existence of plateaus in their high-energy extended emission. The most favorable scenario is a slow-cooling regime, whereas the preferred density profile for each GRBs varies from a constant-density interstellar medium to an
r
−2
wind environment. We also compare the end time of the plateaus in
γ
-rays and X-rays using a statistical comparison with 222 Swift GRBs with plateaus and known redshifts from 2005 January to 2019 August. Within this comparison, the case of GRB 090510 shows an indication of chromaticity at the end time of the plateau. Finally, we update the 3D fundamental plane relation among the rest-frame end time of the plateau, its correspondent luminosity, and the peak prompt luminosity for 222 GRBs observed by Swift. We find that these three LAT GRBs follow this relation.
We analyze the prompt emission of GRB 100724B and GRB 160509A, two of the brightest gamma-ray bursts (GRBs) observed by Fermi at MeV energies but surprisingly faint at 100 MeV energies. Time-resolved ...spectroscopy reveals a sharp high-energy cutoff at energies Ec ∼ 20-60 MeV for GRB 100724B and Ec ∼ 80-150 MeV for GRB 160509A. We first characterize phenomenologically the cutoff and its time evolution. We then fit the data to two models where the high-energy cutoff arises from intrinsic opacity to pair production within the source (τγγ): (i) a Band spectrum with τγγ from the internal-shocks-motivated model of Granot et al. (2008) and (ii) the photospheric model of Gill & Thompson (2014). Alternative explanations for the cutoff, such as an intrinsic cutoff in the emitting electron energy distribution, appear to be less natural. Both models provide a good fit to the data with very reasonable physical parameters, providing an estimate of bulk Lorentz factors in the range Γ ∼ 100-400, on the lower end of what is generally observed in Fermi GRBs. Surprisingly, their lower cutoff energies Ec compared to other Fermi/LAT GRBs arise not only predominantly from the lower Lorentz factors, but also at a comparable level from differences in variability time, luminosity, and high-energy photon index. Finally, particularly low Ec values may prevent detection by Fermi/LAT, thus introducing a bias in the Fermi/LAT GRB sample against GRBs with low Lorentz factors or variability times.
The physical origin of the >0.1 GeV emission detected from gamma-ray bursts (GRBs) by the Fermi satellite has not yet been completely understood. In this work, we consider the GeV light curves of 10 ...GRBs with measured redshift detected by the Fermi Large Area Telescope (LAT). These light curves are characterized by a long-lived (≳102 seconds) emission, whose luminosity decays in time as a power law. While the decay rate is similar for all GRBs (i.e. L
LAT ∝ t
−1.2), the normalization spans about two orders of magnitude in luminosity. However, after re-normalizing the luminosities to the prompt energetics E
prompt the light curves overlap. We consider the scenario in which the temporally extended LAT emission is dominated by synchrotron radiation from electrons accelerated at the forward external shock. According to this model, at high energies (i.e. above the typical synchrotron frequencies) a small dispersion of the E
prompt-normalized light curves is expected. The fact that the LAT temporally extended emission follows this behaviour reinforces its interpretation in terms of afterglow radiation from external shocks. Assuming this scenario, we argue that the parameters ϵe and ηγ (i.e. the fraction of shock-dissipated energy gained by the electrons, and the efficiency of the mechanism producing the prompt radiation, respectively) must be narrowly distributed.
The optical light generated simultaneously with x-rays and gamma rays during a gamma-ray burst (GRB) provides clues about the nature of the explosions that occur as massive stars collapse. We report ...on the bright optical flash and fading afterglow from powerful burst GRB 130427A. The optical and >100—megaelectron volt (MeV) gamma-ray flux show a close correlation during the first 7000 seconds, which is best explained by reverse shock emission cogenerated in the relativistic burst ejecta as ft collides with surrounding material. At later times, optical observations show the emergence of emission generated by a forward shock traversing the circumburst environment The link between optical afterglow and >100-MeV emission suggests that nearby early peaked afterglows will be the best candidates for studying gamma-ray emission at energies ranging from gigaelectron volts to teraelectron volts.
Abstract
We report on the X-ray dust-scattering features observed around the afterglow of the gamma-ray burst GRB 160623A. With an XMM–Newton observation carried out ∼2 d after the burst, we found ...evidence of at least six rings, with angular size expanding between ∼2 and 9 arcmin, as expected for X-ray scattering of the prompt gamma-ray burst (GRB) emission by dust clouds in our Galaxy. From the expansion rate of the rings, we measured the distances of the dust layers with extraordinary precision: 528.1 ± 1.2, 679.2 ± 1.9, 789.0 ± 2.8, 952 ± 5, 1539 ± 20 and 5079 ± 64 pc. A spectral analysis of the ring spectra, based on an appropriate dust-scattering model (BARE-GR-B) and the estimated burst fluence, allowed us to derive the column density of the individual dust layers, which are in the range 7 × 1020–1.5 × 1022 cm−2. The farthest dust layer (i.e. the one responsible for the smallest ring) is also the one with the lowest column density and it is possibly very extended, indicating a diffuse dust region. The properties derived for the six dust layers (distance, thickness and optical depth) are generally in good agreement with independent information on the reddening along this line of sight and on the distribution of molecular and atomic gas.
Aims.
The physical origin of the gamma-ray burst (GRB) prompt emission is still a subject of debate. Internal shock models have been widely explored, owing to their ability to explain most of the ...high-energy properties of this emission phase. While the
Band
function or other phenomenological functions are commonly used to fit GRB prompt emission spectra, we propose a new parametric function that is inspired by an internal shock physical model. We use this function as a proxy of the model to compare it easily to GRB observations.
Methods.
We built a parametric function that represents the spectral form of the synthetic bursts provided by our internal shock synchrotron model (
ISSM
). We simulated the response of the
Fermi
instruments to the synthetic bursts and fit the obtained count spectra to validate the
ISSM
function. Then, we applied this function to a sample of 74 bright GRBs detected by the
Fermi
GBM, and we computed the width of their spectral energy distributions around their peak energy. For comparison, we also fit the phenomenological functions that are commonly used in the literature. Finally, we performed a time-resolved analysis of the broadband spectrum of GRB 090926A, which was jointly detected by the
Fermi
GBM and LAT. This spectrum has a complex shape and exhibits a power-law component with an exponential cutoff at high energy, which is compatible with inverse Compton emission attenuated by gamma-ray internal absorption.
Results.
This work proposes a new parametric function for spectral fitting that is based on a physical model. The
ISSM
function reproduces 81% of the spectra in the GBM bright GRB sample, versus 59% for the
Band
function, for the same number of parameters. It gives also relatively good fits to the GRB 090926A spectra. The width of the MeV spectral component that is obtained from the fits of the
ISSM
function is slightly larger than the width from the
Band
fits, but it is smaller when observed over a wider energy range. Moreover, all of the 74 analyzed spectra are found to be significantly wider than the synthetic synchrotron spectra. We discuss possible solutions to reconcile the observations with the internal shock synchrotron model, such as an improved modeling of the shock microphysics or more accurate spectral measurements at MeV energies.
Temporal variability in flux and spectral shape is ubiquitous in the X-ray sky and carries crucial information about the nature and emission physics of the sources. The EPIC instrument on board the
...XMM-Newton
observatory is the most powerful tool for studying variability even in faint sources. Each day, it collects a large amount of information about hundreds of new serendipitous sources, but the resulting huge (and growing) dataset is largely unexplored in the time domain. The project called Exploring the X-ray transient and variable sky (EXTraS) systematically extracted all temporal domain information in the
XMM-Newton
archive. This included a search and characterisation of variability, both periodic and aperiodic, in hundreds of thousands of sources spanning more than eight orders of magnitude in timescale and six orders of magnitude in flux, and a search for fast transients that were missed by standard image analysis. All results, products, and software tools have been released to the community in a public archive. A science gateway has also been implemented to allow users to run the EXTraS analysis remotely on recent XMM datasets. We give details on the new algorithms that were designed and implemented to perform all steps of EPIC data analysis, including data preparation, source and background modelling, generation of time series and power spectra, and search for and characterisation of different types of variabilities. We describe our results and products and give information about their basic statistical properties and advice on their usage. We also describe available online resources. The EXTraS database of results and its ancillary products is a rich resource for any kind of investigation in almost all fields of astrophysics. Algorithms and lessons learnt from our project are also a very useful reference for any current and future experiment in the time domain.
•Silver nanoparticles pollution stress the microbial biomass in a forest soil.•Bacterial community is quantitatively and qualitatively influenced by silver NPs.•PCR-DGGE shows a selection of silver ...nanoparticles tolerant bacterial strains.
The extensive use of silver nanoparticles (SNPs) as antimicrobial in food, clothing and medicine, leads inevitably to a loss of such nanomaterial in soil and water. Little is known about the effects of soil contamination, in particular, on microbial cells, which play a fundamental ecological role.
In this work, the impact of SNPs on forest soil has been studied, investigating eco-physiological indicators of microbial biomass and microbial diversity with culture-dependent and independent techniques. Moreover, SNPs bioavailability and uptake were assessed. Soil samples were spiked with SNPs at two different concentrations (10 and 100μgg−1dw) and incubated with the relative controls for 30, 60 and 90 days. The overall parameters showed a significant influence of the SNPs on the soil microbial community, revealing a marked shift after 60 days of incubation.
At least a fraction of gravitational-wave (GW) progenitors are expected to emit an electromagnetic (EM) signal in the form of a short gamma-ray burst (sGRB). Discovering such a transient EM ...counterpart is challenging because the LIGO/VIRGO localization region is much larger (several hundreds of square degrees) than the field of view of X-ray, optical, and radio telescopes. The Fermi Large Area Telescope (LAT) has a wide field of view (∼2.4 sr) and detects ∼2-3 sGRBs per year above 100 MeV. It can detect them not only during the short prompt phase, but also during their long-lasting high-energy afterglow phase. If other wide-field, high-energy instruments such as Fermi-GBM, Swift-BAT, or INTEGRAL-ISGRI cannot detect or localize with enough precision an EM counterpart during the prompt phase, the LAT can potentially pinpoint it with arcmin accuracy during the afterglow phase. This routinely happens with gamma-ray bursts. Moreover, the LAT will cover the entire localization region within hours of any triggers during normal operations, allowing the γ-ray flux of any EM counterpart to be measured or constrained. We illustrate two new ad hoc methods to search for EM counterparts with the LAT and their application to the GW candidate LVT151012.
It is largely recognized that gamma-ray burst (GRB) jets involve ultrarelativistic motion. However, the value of the Lorentz factor G0 is still not clear and only lower limits are known for most ...bursts. We suggest here a new method to obtain upper limits on G0. The early high-energy synchrotron afterglow flux depends strongly on G0. Upper limits on GeV emission therefore provide upper limits on G0. Applying this method to 190 Fermi GRBs which have not been detected by the Fermi-LAT, we place upper limits on the high-energy afterglow flux, and in turn on G0. For bursts at a typical redshift z = 2, we find values of the order of 200 (and above) for a homogeneous density medium, and in the range 100-400 for a wind-like medium. These upper limits are consistent with (and are very close to) lower limits and direct estimates inferred using other methods, suggesting that the typical Lorentz factors of GRB jets are of the order of a few hundred.
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