18 research outputs found
Deep Learning Transient Detection with VERITAS
Ground-based -ray observatories, such as the VERITAS array of imaging
atmospheric Cherenkov telescopes, provide insight into very-high-energy (VHE,
) astrophysical transient events. Examples
include the evaporation of primordial black holes, gamma-ray bursts and flaring
blazars. Identifying such events with a serendipitous location and time of
occurrence is difficult. Thus, employing a robust search method becomes
crucial. An implementation of a transient detection method based on
deep-learning techniques for VERITAS will be presented. This data-driven
approach significantly reduces the dependency on the characterization of the
instrument response and the modelling of the expected transient signal. The
response of the instrument is affected by various factors, such as the
elevation of the source and the night sky background. The study of these
effects allows enhancing the deep learning method with additional parameters to
infer their influences on the data. This improves the performance and stability
for a wide range of observational conditions. We illustrate our method for an
historic flare of the blazar BL Lac that was detected by VERITAS in October
2016. We find a promising performance for the detection of such a flare in
timescales of minutes that compares well with the VERITAS standard analysis.Comment: Proceedings of the 37th International Cosmic Ray Conference (ICRC
2021
Optical Microlensing by Primordial Black Holes with IACTs
Primordial black holes (PBHs), hypothesized to be the result of density
fluctuations during the early universe, are candidates for dark matter. When
microlensing background stars, they cause a transient apparent enhancement of
the flux. Measuring these signals with optical telescopes is a powerful method
to constrain the PBH abundance in the range of to
. Especially for galactic stars, the finiteness of the
sources needs to be taken into account. For low PBH masses (in this work
) the average duration of the detectable event
decreases with the mass . For
we find . For this reason, fast sampling detectors may be
required as they could enable the detection of low mass PBHs. Current limits
are set with sampling speeds of 2 minutes to 24 hours in the optical regime.
Ground-based Imaging Atmospheric Cherenkov telescopes (IACTs) are optimized to
detect the ns long optical Cherenkov signals induced by atmospheric air
showers. As shown recently, the very-large mirror area of these instruments
provides very high signal to noise ratio for fast optical transients (s) such as asteroid occultations. We investigate whether optical
observations by IACTs can contribute to extending microlensing limits to the
unconstrained mass range . We discuss the
limiting factors to perform these searches for each telescope type. We
calculate the rate of expected detectable microlensing events in the relevant
mass range for the current and next-generation IACTs considering realistic
source parameters.Comment: 8 pages, 3 figures, Proceedings of the 37th International Cosmic Ray
Conference (ICRC 2021), Berlin, German
35th International Cosmic Ray Conference, ICRC 2017
We report on the detection of flaring activity from the prominent Fanaroff-Riley I radio galaxy NGC 1275 located in the Perseus cluster of galaxies in the very-high-energy gamma-ray band. The observations were performed with the MAGIC telescopes between 2016 and 2017 over several months. During this time period, the mean flux above 100 GeV was measured to be roughly ten times brighter than during previous observations. The night-by-night light curve above 100 GeV shows several peaks indicating flux-doubling time-scales of shorter than one day much smaller than the monthly time-scale previously detected by MAGIC. The brightest flux, observed around December 2016 and January 2017, reached 1.75 times the flux from the Crab Nebula, i.e. was about sixty times brighter than the average flux of the previous years. The spectral energy distribution measured up to > 1 TeV shows a curved shape unlike the previously measured simple power-law. The angle between the jet-axis and the line-of sight of NGC 1275 was formerly found to be 30-55 degree in the radio band excluding strong Doppler boosting of the emitted radiation towards the observer. The fast TeV flux variability and the spectral behaviour observed from a non-blazar object challenge current standard theoretical models and therefore, provide new and fascinating insights into the gamma-ray production and emission mechanism of active galactic nuclei. In the conference, we present the observational results and discuss possible physical processes responsible for the flaring events.</p
Demonstration of stellar intensity interferometry with the four VERITAS telescopes
High angular resolution observations at optical wavelengths provide valuable
insights in stellar astrophysics, directly measuring fundamental stellar
parameters, and probing stellar atmospheres, circumstellar disks, elongation of
rapidly rotating stars, and pulsations of Cepheid variable stars. The angular
size of most stars are of order one milli-arcsecond or less, and to spatially
resolve stellar disks and features at this scale requires an optical
interferometer using an array of telescopes with baselines on the order of
hundreds of meters. We report on the successful implementation of a stellar
intensity interferometry system developed for the four VERITAS imaging
atmospheric-Cherenkov telescopes. The system was used to measure the angular
diameter of the two sub-mas stars Canis Majoris and Orionis
with a precision better than 5%. The system utilizes an off-line approach where
starlight intensity fluctuations recorded at each telescope are correlated
post-observation. The technique can be readily scaled onto tens to hundreds of
telescopes, providing a capability that has proven technically challenging to
current generation optical amplitude interferometry observatories. This work
demonstrates the feasibility of performing astrophysical measurements with
imaging atmospheric-Cherenkov telescope arrays as intensity interferometers and
the promise for integrating an intensity interferometry system within future
observatories such as the Cherenkov Telescope Array.Comment: Accepted for publication in Nature Astronomy (2020
An Archival Search for Neutron-Star Mergers in Gravitational Waves and Very-High-Energy Gamma Rays
The recent discovery of electromagnetic signals in coincidence with
neutron-star mergers has solidified the importance of multimessenger campaigns
in studying the most energetic astrophysical events. Pioneering multimessenger
observatories, such as LIGO/Virgo and IceCube, record many candidate signals
below the detection significance threshold. These sub-threshold event
candidates are promising targets for multimessenger studies, as the information
provided by them may, when combined with contemporaneous gamma-ray
observations, lead to significant detections. Here we describe a new method
that uses such candidates to search for transient events using archival
very-high-energy gamma-ray data from imaging atmospheric Cherenkov telescopes
(IACTs). We demonstrate the application of this method to sub-threshold binary
neutron star (BNS) merger candidates identified in Advanced LIGO's first
observing run. We identify eight hours of archival VERITAS observations
coincident with seven BNS merger candidates and search them for TeV emission.
No gamma-ray emission is detected; we calculate upper limits on the integral
flux and compare them to a short gamma-ray burst model. We anticipate this
search method to serve as a starting point for IACT searches with future
LIGO/Virgo data releases as well as in other sub-threshold studies for
multimessenger transients, such as IceCube neutrinos. Furthermore, it can be
deployed immediately with other current-generation IACTs, and has the potential
for real-time use that places minimal burden on experimental operations.
Lastly, this method may serve as a pilot for studies with the Cherenkov
Telescope Array, which has the potential to observe even larger fields of view
in its divergent pointing mode
Deep Learning Transient Detection with VERITAS
Ground-based -ray observatories, such as the VERITAS array of imaging atmospheric Cherenkov telescopes, provide insight into very-high-energy (VHE, ) astrophysical transient events. Examples include the evaporation of primordial black holes, gamma-ray bursts and flaring blazars. Identifying such events with a serendipitous location and time of occurrence is difficult. Thus, employing a robust search method becomes crucial. An implementation of a transient detection method based on deep-learning techniques for VERITAS will be presented. This data-driven approach significantly reduces the dependency on the characterization of the instrument response and the modelling of the expected transient signal. The response of the instrument is affected by various factors, such as the elevation of the source and the night sky background. The study of these effects allows enhancing the deep learning method with additional parameters to infer their influences on the data. This improves the performance and stability for a wide range of observational conditions. We illustrate our method for an historic flare of the blazar BL Lac that was detected by VERITAS in October 2016. We find a promising performance for the detection of such a flare in timescales of minutes that compares well with the VERITAS standard analysis
A Review of Models for the Sodium Boiling Phenomena in Sodium-Cooled Fast Reactor Subassemblies
International audienceThe Euratom Horizon-2020 project European sodium fast reactor safety measures assessment and research tools (ESFR-SMART) aims to enhance the safety and performance of the European sodium-cooled fast reactor (ESFR) considering safety objectives envisaged for generation-IV reactors and the update of European and international safety frameworks, taking into account the Fukushima accident. Further, the project aims to support the development and validation of the computational tools for the situations to be considered at each defense-in-depth level in order to support the safety assessments using data produced in the project as well as selected legacy data. Within this activity, the focus is on the assessment and when needed further development of computer codes for the analysis of the sodium thermal-hydraulics phenomena in sodium cooled fast reactor (SFR) subassemblies under operational and accidental conditions including sodium boiling and transitional convection from forced to natural/mixed convection. In support of this activity, a review is performed for the sodium boiling models used in the codes participating in benchmark activities within the project. The objective of this paper is to summarize the result of the review, which encompasses both the phenomenological and mathematical models implemented in the codes. In particular, the review addresses the physical bases of sodium boiling models, the analytical models in connection with the numerical implementations in the codes, and the geometry representations ranging from a one-dimensional single channel and finite volume to full three-dimensional computational fluid dynamics (CFD) and porous media representations. Finally, the needs for further developments are discussed
Evaluation of Sodium Boiling Models Using KNS-37 Loss of Flow Experiments
International audienceAbstract The computational codes used in the evaluation of the European sodium fast reactor—safety measures assessment and research tools(ESFR-SMART) reactor performance and specifically their sodium boiling models are assessed using two KNS-37 loss of flow (LOF) experiments, i.e., L22 and L29 tests, where boiling onset and two-phase flow regime up to dry-out occurred. The well-equipped KNS-37 experimental facility provided very valuable information for understanding the physical phenomena occurring in a 37-pin subassembly under LOF conditions, as well as experimental data to be used for computational tools validation. NATOF-2D, SAS-SFR, TRACE, ASTEC-Na, CATHARE-2, CATHARE-3, and NEPTUNE_CFD codes have been used in this exercise in order to compare the various boiling models and conclude on the advantages and limitations of them based on the comparison against the experimental data. Beyond boiling onset, the various sodium two-phase flow approaches determine the ability of the code to correctly represent the rewetting and voiding phases as well as cladding dry-out onset. A simulation performed by a computational fluid dynamics (CFD) approach (NEPTUNE_CFD code) taking into account liquid–vapor interfaces by an interface-tracking method is also shown and compared with the others approaches. Conclusions on each code performance are presented where the improvements needed to solve the issues encountered are included. This paper provides a first step in the process to investigate the required evaluation of the sodium two-phase flow models able to assess the safety of new SFR core designs (e.g., low void cores) under accidental conditions such as unprotected loss of flow (ULOF) transients
VTSCat: The VERITAS Catalog of Gamma-Ray Observations
We present a catalog of results of gamma-ray observations made by VERITAS, published from 2008 to 2020. VERITAS is a ground based imaging atmospheric Cherenkov telescope observatory located at the Fred Lawrence Whipple Observatory (FLWO) in southern Arizona, sensitive to gamma-ray photons with energies in the range of 100 GeV - 30 TeV. Its observation targets include galactic sources such as binary star systems, pulsar wind nebulae, and supernova remnants, extragalactic sources like active galactic nuclei, star forming galaxies, and gamma-ray bursts, and some unidentified objects. The catalog includes in digital form all of the high-level science results published in 112 papers using VERITAS data and currently contains data on 57 sources. The catalog has been made accessible via GitHub and at NASA's HEASARC
A Search for TeV Gamma-Ray Emission from Pulsar Tails by VERITAS
We report on the search for very-high-energy gamma-ray emission from the regions around three nearby supersonic pulsars (PSR B0355+54, PSR J0357+3205, and PSR J1740+1000) that exhibit long X-ray tails. To date there is no clear detection of TeV emission from any pulsar tail that is prominent in X-ray or radio. We provide upper limits on the TeV flux, and luminosity, and also compare these limits with other pulsar wind nebulae detected in X-rays and the tail emission model predictions. We find that at least one of the three tails is likely to be detected in observations that are a factor of 2–3 more sensitive. The analysis presented here also has implications for deriving the properties of pulsar tails, for those pulsars whose tails could be detected in TeV