ABSTRACT
The Fermi Large Area Telescope (LAT) has detected more than 5000 γ-ray sources in its first 8 yr of operation. More than 3000 of them are blazars. About 60 per cent of the Fermi-LAT blazars ...are classified as BL Lacertae objects (BL Lacs) or Flat Spectrum Radio Quasars (FSRQs), while the rest remain of uncertain type. The goal of this study was to classify those blazars of uncertain type, using a supervised machine learning method based on an artificial neural network, by comparing their properties to those of known γ-ray sources. Probabilities for each of 1329 uncertain blazars to be a BL Lac or FSRQ are obtained. Using 90 per cent precision metric, 801 can be classified as BL Lacs and 406 as FSRQs while 122 still remain unclassified. This approach is of interest because it gives a fast preliminary classification of uncertain blazars. We also explored how different selections of training and testing samples affect the classification and discuss the meaning of network outputs.
We use nine years of γ-ray data provided by the Fermi Large Area Telescope (LAT) to systematically study the light curves (LCs) of more than 2000 active galactic nuclei (AGN) included in recent ...Fermi-LAT catalogs. Ten different techniques are used, which are organized in an automatic periodicity-search pipeline, in order to search for evidence of periodic emission in γ rays. Understanding the processes behind this puzzling phenomenon will provide a better view about the astrophysical nature of these extragalactic sources. However, the observation of temporal patterns in γ-ray LCs of AGN is still challenging. Despite the fact that there have been efforts to characterize the temporal emission of some individual sources, a systematic search for periodicities by means of a full likelihood analysis applied to large samples of sources was missing. Our analysis finds 11 AGN, of which 9 are identified for the first time, showing periodicity at more than 4 in at least four algorithms. These findings will help in solving questions related to the astrophysical origin of this periodic behavior.
Converging lines of evidence suggest that the numerical abilities in Humans are rooted in the approximate number system (ANS): an innate, non-verbal mechanism that enables to estimate the numerosity ...of a set of items with little effort. Nevertheless, the high correlation between visual features and numerosity in the natural environment always constituted a relevant methodological problem that gathered growing concern throughout the years. This issue led some researchers to cast doubts on the existence of a system able to process numerical information independently from the influence of visual features. In the present study, we sought to shed light on the interplay between numerosity and visual features. To this aim, we implemented a non-symbolic estimation task which included a calibration phase. After performing a pre-calibration block, participants were presented with the calibration image for 20 s, and they were divided in three groups, according to the calibration stimulus they attended to: the three calibration stimuli contained the same number of items (30), but were characterized by a different amount of visual features. Results showed that performance was affected by numerosity and visual features in both phases of the experiment. However, calibration increased the weight of numerosity on performance while decreasing the weight of visual features. These results are hard to be reconciled with theories that attempt to explain human performance in non-symbolic number processing without taking into account both numerical and non-numerical aspects of the stimuli.
•In a non-symbolic estimation task, calibration significantly influenced performance.•Both visual features and numerosity of the calibration image affected performance.•Calibration increased impact of numerosity and decreased that of visual features.•Neither the ANS nor the SIS theories can fully explain the results.•Both numerical and non-numerical aspects of the stimuli need to be taken into account.
ABSTRACT
Machine learning is an automatic technique that is revolutionizing scientific research, with innovative applications and wide use in astrophysics. The aim of this study was to develop an ...optimized version of an Artificial Neural Network machine learning method for classifying blazar candidates of uncertain type detected by the Fermi Large Area Telescope γ-ray instrument. The final result of this study increased the classification performance by about 80 ${{\ \rm per\ cent}}$ with respect to previous method, leaving only 15 unclassified blazars out of 573 blazar candidates of uncertain type listed in the LAT 4-year Source Catalog.
When evaluating the properties of a set of elements in a natural environment, an increase in numerosity unavoidably corresponds to an increase in the physical properties of the set: Five apples ...differ from ten apples not only in numerosity, but also in their visual features, such as volume, density, and surface. Since nonsymbolic number processing is typically investigated through the presentation of arrays of elements, it is mandatory to keep track of the visual features characterizing the stimuli. A plethora of solutions have been proposed to address this complex methodological issue; yet, there is no agreed-upon standard for how to measure and control for visual features. Here we present the “customized ultraprecise standardization-oriented multipurpose” (CUSTOM) algorithm for generating nonsymbolic number stimuli. It is characterized by several core features: The absence of fixed parameters or rules—apart from geometrical constraints—lets the user freely manipulate the visual features of the stimuli; control over the visual features of the stimuli is extremely accurate; no modification is required in order to perform different types of manipulation; and users can re-create any set of stimuli described so far in previous experiments on numerical cognition, for a wide variety of tasks, including comparison, estimation, habituation, and match-to-sample. The CUSTOM algorithm could represent an asset in the field of numerical cognition, as a versatile instrument for effectively generating high-precision visual stimuli within an unbiased theoretical framework.
We present a search for spatial extension in high-latitude ( ) sources in recent Fermi point source catalogs. The result is the Fermi High-Latitude Extended Sources Catalog, which provides source ...extensions (or upper limits thereof) and likelihood profiles for a suite of tested source morphologies. We find 24 extended sources, 19 of which were not previously characterized as extended. These include sources that are potentially associated with supernova remnants and star-forming regions. We also found extended γ-ray emission in the vicinity of the Cen A radio lobes and-at GeV energies for the first time-spatially coincident with the radio emission of the SNR CTA 1, as well as from the Crab Nebula. We also searched for halos around active galactic nuclei, which are predicted from electromagnetic cascades induced by the e+e− pairs that are deflected in intergalactic magnetic fields. These pairs are produced when γ-rays interact with background radiation fields. We do not find evidence for extension in individual sources or in stacked source samples. This enables us to place limits on the flux of the extended source components, which are then used to constrain the intergalactic magnetic field to be stronger than 3 × 10−16 G for a coherence length λ 10 kpc, even when conservative assumptions on the source duty cycle are made. This improves previous limits by several orders of magnitude.
We present optical, X-ray and gamma-ray observations of GRB 111209A, at a redshift of z = 0.677. We show that this event was active in its prompt phase for about 25000 seconds, making it the longest ...burst ever observed. This rare event could have been detected up to z ~ 1.4. Compared to other long GRBs, GRB 111209A is a clear outlier in the energy-fluence and duration plane. The high-energy prompt emission shows no sign of a strong black body component, as expected if the event was caused by a tidal disruption event or a supernova shock breakout. Given the extreme longevity of this event, and a lack of a supernova signature, we propose that GRB 111209A is a relatively rare stellar collapse of a low metallicity blue super giant star. Only this progenitor can supply mass to the central engine over a duration of thousands of seconds. Hence, GRB 111209A could have more in common with population III stellar explosions, rather than normal long gamma ray bursts.
ABSTRACT
The Fermi-LAT DR1 and DR2 4FGL catalogues feature more than 5000 gamma-ray sources of which about one fourth are not associated with already known objects, and approximately one third are ...associated with blazars of uncertain nature. We perform a three-category classification of the 4FGL DR1 and DR2 sources independently, using an ensemble of Artificial Neural Networks (ANNs) to characterize them based on the likelihood of being a Pulsar (PSR), a BL Lac type blazar (BLL) or a Flat Spectrum Radio Quasar (FSRQ). We identify candidate PSR, BLL, and FSRQ among the unassociated sources with approximate equipartition among the three categories and select 10 classification outliers as potentially interesting for follow-up studies.
Abstract
An incremental version of the fourth catalog of active galactic nuclei (AGNs) detected by the Fermi Large Area Telescope is presented. This version (4LAC-DR3) derives from the third data ...release of the 4FGL catalog based on 12 yr of
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> 50 MeV gamma-ray data, where the spectral parameters, spectral energy distributions (SEDs), yearly light curves, and associations have been updated for all sources. The new reported AGNs include 587 blazar candidates and four radio galaxies. We describe the properties of the new sample and outline changes affecting the previously published one. We also introduce two new parameters in this release, namely the peak energy of the SED high-energy component and the corresponding flux. These parameters allow an assessment of the Compton dominance, the ratio of the inverse-Compton to the synchrotron-peak luminosities, without relying on X-ray data.
The Fermi Large Area Telescope (LAT) has opened the way for comparative studies of cosmic rays (CRs) and high-energy objects in the Milky Way (MW) and in other, external, star-forming galaxies. Using ...2 yr of observations with the Fermi LAT, Local Group galaxy M31 was detected as a marginally extended gamma-ray source, while only an upper limit has been derived for the other nearby galaxy M33. We revisited the gamma-ray emission in the direction of M31 and M33 using more than 7 yr of LAT Pass 8 data in the energy range 0.1 - 100 GeV , presenting detailed morphological and spectral analyses. M33 remains undetected, and we computed an upper limit of 2.0 × 10 − 12 erg cm − 2 s − 1 on the 0.1 - 100 GeV energy flux (95% confidence level). This revised upper limit remains consistent with the observed correlation between gamma-ray luminosity and star formation rate tracers and implies an average CR density in M33 that is at most half of that of the MW. M31 is detected with a significance of nearly 10 . Its spectrum is consistent with a power law with photon index Γ = 2.4 0.1 stat + syst and a 0.1 - 100 GeV energy flux of ( 5.6 0.6 stat + syst ) × 10 − 12 erg cm − 2 s − 1 . M31 is detected to be extended with a 4 significance. The spatial distribution of the emission is consistent with a uniform-brightness disk with a radius of 0 4 and no offset from the center of the galaxy, but nonuniform intensity distributions cannot be excluded. The flux from M31 appears confined to the inner regions of the galaxy and does not fill the disk of the galaxy or extend far from it. The gamma-ray signal is not correlated with regions rich in gas or star formation activity, which suggests that the emission is not interstellar in origin, unless the energetic particles radiating in gamma rays do not originate in recent star formation. Alternative and nonexclusive interpretations are that the emission results from a population of millisecond pulsars dispersed in the bulge and disk of M31 by disrupted globular clusters or from the decay or annihilation of dark matter particles, similar to what has been proposed to account for the so-called Galactic center excess found in Fermi-LAT observations of the MW.
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