In this work, we present a new, high performance algorithm for background rejection in imaging atmospheric Cherenkov telescopes. We build on the already popular machine-learning techniques used in ...gamma-ray astronomy by the application of the latest techniques in machine learning, namely recurrent and convolutional neural networks, to the background rejection problem. Use of these machine-learning techniques addresses some of the key challenges encountered in the currently implemented algorithms and helps to significantly increase the background rejection performance between 100 GeV and 100 TeV energies. We apply these machine learning techniques to the H.E.S.S. telescope array, first testing their performance on simulated data and then applying the analysis to two well known gamma-ray sources. With real observational data we find significantly improved performance over the current standard methods, with a 20–25% reduction in the background rate when applying the recurrent neural network analysis. Importantly, we also find that the convolutional neural network results are strongly dependent on the sky brightness in the source region which has important implications for the future implementation of this method in Cherenkov telescope analyses.
Abstract
Stellar flares have been extensively studied in soft X-rays (SXRs) by basically every X-ray mission. Hard X-ray (HXR) emission from stellar superflares, however, have only been detected from ...a handful of objects over the past years. One very extreme event was the superflare from the young M-dwarf DG CVn binary star system, which triggered Swift/BAT as if it was a γ-ray burst. In this work, we estimate the expected γ-ray emission from DG CVn and the most extreme stellar flares by extrapolating from solar flares based on measured solar energetic particles (SEPs), as well as thermal and non-thermal emission properties. We find that ions are plausibly accelerated in stellar superflares to 100 GeV energies, and possibly up to TeV energies in the associated coronal mass ejections. The corresponding π0-decay γ-ray emission could be detectable from stellar superflares with ground-based γ-ray telescopes. On the other hand, the detection of γ-ray emission implies particle densities high enough that ions suffer significant losses due to inelastic proton–proton scattering. The next-generation Cherenkov Telescope Array (CTA) should be able to probe superflares from M dwarfs in the solar neighbourhood and constrain the energy in interacting cosmic rays and/or their maximum energy. The detection of γ-ray emission from stellar flares would open a new window for the study of stellar physics, the underlying physical processes in flares and their impact on habitability of planetary systems.
The binary system
η
Carinae is a unique laboratory that facilitates the study of particle acceleration to high energies under a wide range of conditions, including extremely high densities around ...periastron. To date, no consensus has emerged as to the origin of the gigaelectronvolt
γ
-ray emission in this important system. With a re-analysis of the full
Fermi
-LAT data set for
η
Carinae, we show that the spectrum is consistent with a pion decay origin. A single population leptonic model connecting X-ray to
γ
-ray emission can be ruled out. We revisit our physical model from 2015, based on two acceleration zones associated with the termination shocks in the winds of both stars. We conclude that inverse Compton emission from in-situ accelerated electrons dominates the hard X-ray emission detected with
NuSTAR
at all phases away from periastron and that pion-decay from shock accelerated protons is the source of
γ
-ray emission. Very close to periastron there is a pronounced dip in hard X-ray emission, concomitant with the repeated disappearance of the thermal X-ray emission, which we interpret as due to the suppression of significant electron acceleration in the system. Within our model, the residual emission seen by
NuSTAR
at this phase can be accounted for with secondary electrons produced in interactions of accelerated protons, which agrees with the variation in pion-decay
γ
-ray emission. Future observations with H.E.S.S., CTA, and
NuSTAR
should confirm or refute this scenario.
On the origin of γ-ray emission in η Carina Ohm, S; Zabalza, V; Hinton, J. A ...
Monthly notices of the Royal Astronomical Society. Letters,
05/2015, Letnik:
449, Številka:
1
Journal Article
Recenzirano
Odprti dostop
η Car is the only colliding-wind binary for which high-energy γ rays are detected. Although the physical conditions in the shock region change on time-scales of hours to days, the variability seen at ...GeV energies is weak and on significantly longer time-scales. The γ-ray spectrum exhibits two features that can be interpreted as emission from the shocks on either side of the contact discontinuity. Here, we report on the first time-dependent modelling of the non-thermal emission in η Car. We find that emission from primary electrons is likely not responsible for the γ-ray emission, but accelerated protons interacting with the dense wind material can explain the observations. In our model, efficient acceleration is required at both shocks, with the primary side acting as a hadron calorimeter, whilst on the companion side acceleration is limited by the flow time out of the system, resulting in changing acceleration conditions. The system therefore represents a unique laboratory for the exploration of hadronic particle acceleration in non-relativistic shocks.
γ-ray emission from the Westerlund 1 region Ohm, S; Hinton, J. A; White, R
Monthly notices of the Royal Astronomical Society,
2013, Letnik:
434, Številka:
3
Journal Article
Recenzirano
Odprti dostop
Westerlund 1 (Wd 1) is the most massive stellar cluster in the Galaxy and is associated with an extended region of TeV emission. Here, we report the results of a search for GeV γ-ray emission in this ...region. The analysis is based on ∼4.5 yr of Fermi-Large Area Telescope data and reveals significantly extended emission which we model as a Gaussian, resulting in a best-fitting sigma of σ
S
= (0
475 ± 0
05) and an offset from Wd 1 of ∼1°. A partial overlap of the GeV emission with the TeV signal as reported by the High Energy Stereoscopic System (HESS) is found. We investigate the spectral and morphological characteristics of the γ-ray emission and discuss its origin in the context of two distinct scenarios. Acceleration of electrons in a pulsar wind nebula provides a reasonably natural interpretation of the GeV emission, but leaves the TeV emission unexplained. A scenario in which protons are accelerated in or near Wd 1 in supernova explosion(s) and are diffusing away and interacting with molecular material seems consistent with the observed GeV and TeV emission, but requires a very high energy input in protons, ∼1051 erg, and rather slow diffusion. Observations of Wd 1 with a future γ-ray detector such as the Cherenkov Telescope Array (CTA) provide a very promising route to fully resolve the origin of the TeV and GeV emission in Wd 1 and provide a deeper understanding of the high-energy astrophysics of massive stellar clusters.
The Large Array Survey Telescope—Science Goals Ben-Ami, S.; Ofek, E. O.; Polishook, D. ...
Publications of the Astronomical Society of the Pacific,
08/2023, Letnik:
135, Številka:
1050
Journal Article
The recently detected γ-ray emission from starburst galaxies is most commonly considered to be diffuse emission arising from strong interactions of accelerated cosmic rays. Mannheim, Elsässer & ...Tibolla, however, have argued that a population of individual pulsar-wind nebulae (PWNe) could be responsible for the detected TeV emission. Here, we show that the starburst environment plays a critical role in the TeV emission from starburst PWN, and perform the first detailed calculations for this scenario. Our approach is based on the measured star formation rates in the starburst nuclei of NGC 253 and M82, assumed pulsar birth periods and a simple model for the injection of non-thermal particles. The two-zone model applied here takes into account the high far-infrared radiation field and different densities and magnetic fields in the PWN and the starburst regions, as well as particle escape. We confirm that PWN can make a significant contribution to the TeV fluxes, provided that the injection spectrum of particles is rather hard and that the average pulsar birth period is rather short (∼35 ms). The PWN contribution should lead to a distinct spectral feature which can be probed by future instruments such as Cherenkov Telescope Array.
We present a detailed analysis of the interstellar medium towards the tera electron volt (TeV) ...-ray sources HESS J1640-465 and HESS J1641-463 using results from the Mopra Southern Galactic Plane ...CO Survey and from a Mopra 7 mm-wavelength study. The ...-ray sources are positionally coincident with two supernova remnants (SNRs) G338.3-0.0 and G338.5+0.1, respectively. A bright complex of H ii regions connect the two SNRs and TeV objects. Observations in the CO(1-0) transition lines reveal substantial amounts of diffuse gas positionally coincident with the ...-ray sources at multiple velocities along the line of sight, while 7 mm observations in CS, SiO, HC sub( 3)N and CH sub( 3)OH transition lines reveal regions of dense, shocked gas. Archival H i data from the Southern Galactic Plane Survey was used to account for the diffuse atomic gas. Physical parameters of the gas towards the TeV sources were calculated from the data. We find that for a hadronic origin for the ...-ray emission, the cosmic ray enhancement rates are ~10 super( 3) and 10 super( 2) times the local solar value for HESS J1640-465 and HESS J1641-463, respectively. (ProQuest: ... denotes formulae/symbols omitted.)
Escape from Vela X Hinton, J. A; Funk, S; Parsons, R. D ...
Astrophysical journal. Letters,
12/2011, Letnik:
743, Številka:
1
Journal Article
Recenzirano
Odprti dostop
While the Vela pulsar and its associated nebula are often considered as the archetype of a system powered by a ~104 year old isolated neutron star, many features of the spectral energy distribution ...of this pulsar wind nebula (PWN) are both puzzling and unusual. Here we develop a model that for the first time relates the main structures in the system, the extended radio nebula (ERN) and the X-ray cocoon through continuous injection of particles with a fixed spectral shape. We argue that diffusive escape of particles from the ERN can explain the steep Fermi-LAT spectrum. In this scenario Vela X should produce a distinct feature in the locally measured cosmic ray (CR) electron spectrum at very high energies. This prediction can be tested in the future using the Cherenkov Telescope Array. If particles are indeed released early in the evolution of PWNe and can avoid severe adiabatic losses, PWN provides a natural explanation for the rising positron fraction in the local CR spectrum.
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