27 research outputs found
The time resolved Ep-L correlation in GRBs: characterization and implications
Our understanding of the GRB phenomenon is still affected by several open issues including the prompt emission physics, the problem related to the trigger of the GRB activity, the classification of different classes of GRBs and the degree of collimation of their emission. All of these issues have strong implications for a full comprehension of the nature of GRBs progenitors. In addition, their huge luminosity and their redshift distribution up to the early Universe make GRBs very promising as cosmological tools. The most investigated way for using them to measure cosmological parameters and the properties and evolution of dark energy is the correlation between the radiated energy and the photon energy at which the spectrum peaks Ep. This work aims at a further investigation of the time-resolved Ep - L correlation, based on large data sets from the BATSE/CGRO and Fermi/GBM experiments, and its application to cosmology. This is an innovative approach leading to a significant improvement in the estimates of cosmological parameters w/r to previous time integrated analysis. Indeed, in time-resolved analysis we can use the correlation between flux and Ep existing in individual GRBs allowing a cosmology independent and unbiased calibration of the slope and of the extra-statistical scatter of the correlation. The results of my thesis also provide important inputs for discriminating among different emission models of the prompt emission of GRBs and the comparison between the dispersion of the time-resolved correlation and that obtained through the averaged spectra provides an estimate of the dispersion of jet opening angles, with strong implications regarding the nature of GRB progenitors. Finally, my research activity includes also the fit of time-resolved spectra of some GRBs with both empirical models and a physical Comptonization model recently delevoped and simulations of the expected contribution by spectral measurements of GRBs through the Chinese satellite HXMT
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
Observation of the Gamma-Ray Binary HESS J0632+057 with the H.E.S.S., MAGIC, and VERITAS Telescopes
The results of gamma-ray observations of the binary system HESS J0632 + 057 collected during 450 hr over 15 yr, between 2004 and 2019, are presented. Data taken with the atmospheric Cherenkov telescopes H.E.S.S., MAGIC, and VERITAS at energies above 350 GeV were used together with observations at X-ray energies obtained with Swift-XRT, Chandra, XMM-Newton, NuSTAR, and Suzaku. Some of these observations were accompanied by measurements of the Hα emission line. A significant detection of the modulation of the very high-energy gamma-ray fluxes with a period of 316.7 4.4 days is reported, consistent with the period of 317.3 0.7 days obtained with a refined analysis of X-ray data. The analysis of data from four orbital cycles with dense observational coverage reveals short-timescale variability, with flux-decay timescales of less than 20 days at very high energies. Flux variations observed over a timescale of several years indicate orbit-to-orbit variability. The analysis confirms the previously reported correlation of X-ray and gamma-ray emission from the system at very high significance, but cannot find any correlation of optical Hα parameters with fluxes at X-ray or gamma-ray energies in simultaneous observations. The key finding is that the emission of HESS J0632 + 057 in the X-ray and gamma-ray energy bands is highly variable on different timescales. The ratio of gamma-ray to X-ray flux shows the equality or even dominance of the gamma-ray energy range. This wealth of new data is interpreted taking into account the insufficient knowledge of the ephemeris of the system, and discussed in the context of results reported on other gamma-ray binary systems
VERITAS Dark Matter search in dwarf spheroidal galaxies: an extended analysis
Dark matter (DM) is largely believed to be the dominant component of the matter content of the Universe. Astronomical measurements can be utilized to search for Standard Model (SM) annihilation or decay products of DM, complementing direct and collider-based searches. Among DM particle candidates, Weakly Interacting Massive Particles (WIMPs) are an attractive one. Their decay or annihilation could produce secondary particles including very-high-energy (VHE: GeV) gamma rays, which could be detected by imaging atmospheric Cherenkov telescopes (IACTs). One of the most favourable target classes for DM searches are dwarf spheroidal galaxies (dSphs), dark matter-dominated objects with a negligible predicted gamma-ray emission due to apparent absence of gas and on-going star formation. IACTs, whose point spread functions (PSFs, defined as 68\% containment radius) are typically at 1 TeV, have the necessary angular resolution to detect extended emission from some dSphs. Thus, an extended-source analysis may give an improvement to DM sensitivity, compared to a point-source analysis. In this work, we used observations made since 2007 to 2013 by VERITAS, an array of four imaging atmospheric Cherenkov telescopes sensitive to VHE gamma rays in the 100 GeV - 30 TeV energy range. We performed an unbinned maximum likelihood estimation incorporating the dSph angular profiles of four dSphs and tested its effectiveness against the traditional spectral analysis
Sustainable and cost-effective edge oxidized graphite/PEDOT:PSS nanocomposites with improved electrical conductivity
Poly(3,4-ethylenedioxythiophene): polystyrene sulfonate (PEDOT:PSS) is a soft and conjugated polymer whose conductive properties can be properly tuned through doping with various additives or solvents, preserving its excellent processability. In this work PEDOT:PSS was combined with a cost-effective graphite derivative named Edge Oxidized Graphite (EOG) for developing a nanocomposite with improved electrical conductivity, with respect to the pristine PEDOT:PSS, through an easy and environmentally friendly doping process. Firstly, the EOG powders, produced by a green oxidation process of graphite, were deeply characterized through Fourier transform infrared (FT-IR), Thermogravimetric (TGA), and Wide-angle X-ray scattering (WAXD) analysis, showing that this nanofiller has oxygenated functional groups on the sheet edges. The quality and the stability of the EOG dispersions within PEDOT:PSS were investigated at different carbon-filler concentrations, up to high loading of 25 %wt/V of EOG through rheological analyses, demonstrating pseudo-plastic behavior and excellent long-term stability of the inks due to the absence of inhomogeneities and aggregates over time; in fact, the same inks were tested under the same rheological conditions after 21 days, showing the same viscosity trend for all EOG concentrations (%wt/V). Transmission electron microscopy (TEM) and (Scanning Electron microscopy) SEM investigation of spin-coated samples onto glass substrates were performed to morphologically evaluate the nanocomposites and estimate the average size of the sheets, particularly the mean length of 1.2 μm and an approximated thickness of 26 nm of the EOG sheets dispersed into the polymer matrix (PEDOT:PSS) was determined, while WAXD analysis allowed to identify the average layer number of the EOG sheets, obtaining thus, a direct measurement of the EOG sheets aspect ratio equal to 45. Finally, sheet resistance tests showed that the increasing concentration of EOG leads to a significant improvement in the electrical conductivity of the nanocomposites, from 1.1 S/cm for pristine PEDOT:PSS to 21.9 S/cm for nanocomposites with the highest EOG content (25 %wt/V). This work demonstrates the successful development of nanocomposite based on PEDOT:PSS doped with carbon-based filler synthesized through a green and cost-effective process, promoting their use in the production of bio/electrochemical sensors or optoelectronic devices