Santiago Pita - Academia.edu (original) (raw)

Papers by Santiago Pita

Research paper thumbnail of Is PKS 0625-354 another variable TeV active galactic nucleus?

Proceedings of 37th International Cosmic Ray Conference — PoS(ICRC2021)

Research paper thumbnail of Search for TeV emission from the base of the Fermi Bubbles with H.E.S.S

Proceedings of 37th International Cosmic Ray Conference — PoS(ICRC2021)

Research paper thumbnail of Deep observations of Kepler's SNR with H.E.S.S

Proceedings of 37th International Cosmic Ray Conference — PoS(ICRC2021)

Research paper thumbnail of Limits on primordial black hole evaporation from H.E.S.S. observations

Proceedings of 37th International Cosmic Ray Conference — PoS(ICRC2021)

Research paper thumbnail of Astronomy outreach in Namibia: H.E.S.S. and beyond

Proceedings of 37th International Cosmic Ray Conference — PoS(ICRC2021)

Research paper thumbnail of Testing cosmology and fundamental physics with the Cherenkov Telescope Array

Proceedings of 36th International Cosmic Ray Conference — PoS(ICRC2019)

Research paper thumbnail of Combined Dark Matter searches towards dwarf spheroidal galaxies with Fermi-LAT, HAWC, HESS, MAGIC and VERITAS

Proceedings of 36th International Cosmic Ray Conference — PoS(ICRC2019)

Research paper thumbnail of Studying cosmological γ-ray propagation with the Cherenkov Telescope Array

Proceedings of 35th International Cosmic Ray Conference — PoS(ICRC2017)

The measurement of γ-rays originating from active galactic nuclei offers the unique opportunity t... more The measurement of γ-rays originating from active galactic nuclei offers the unique opportunity to study the propagation of very-high-energy photons over cosmological distances. Most prominently, γ-rays interact with the extragalactic background light (EBL) to produce e + e − pairs, imprinting an attenuation signature on γ-ray spectra. The e + e − pairs can also induce electromagnetic cascades whose detectability in γ-rays depends on the intergalactic magnetic field (IGMF). Furthermore, physics beyond the Standard Model such as Lorentz invariance violation (LIV) or oscillations between photons and weakly interacting sub-eV particles (WISPs) could affect the propagation of γ-rays. The future Cherenkov Telescope Array (CTA), with its unprecedented γray source sensitivity, as well as enhanced energy and spatial resolution at very high energies, is perfectly suited to study cosmological effects on γ-ray propagation. Here, we present first results of a study designed to realistically assess the capabilities of CTA to probe the EBL, IGMF, LIV, and WISPs.

Research paper thumbnail of Looking for infrared counterparts of Fermi/LAT blazar candidates

Proceedings of 35th International Cosmic Ray Conference — PoS(ICRC2017)

Research paper thumbnail of Extragalactic source population studies at very high energies in the Cherenkov Telescope Array era

Proceedings of 35th International Cosmic Ray Conference — PoS(ICRC2017)

Research paper thumbnail of Gammapy: high level data analysis for extragalactic science cases with the Cherenkov Telescope Array

Proceedings of 35th International Cosmic Ray Conference — PoS(ICRC2017)

Research paper thumbnail of Spectroscopy of high-energy BL Lacertae objects with X-shooter on the VLT

Astronomy & Astrophysics

Context. BL Lac objects detected in γ-rays and, particularly, those detected at very high energie... more Context. BL Lac objects detected in γ-rays and, particularly, those detected at very high energies (E > 100 GeV) by Cherenkov telescopes are extreme sources with most having redshifts lower than 0.2. Their study gives insights on the acceleration mechanisms in play in such systems and is also a valuable tool for putting constraints on the density of extragalactic background light, especially if several objects are detected at different redshifts. As their spectra are dominated by the non-thermal emission of the jet and the spectral features are weak and narrow in the optical domain, measuring their redshift is challenging. However, such a measure is fundamental as it allows a firm determination of the distance and luminosity of the source, and, therefore, a consistent model of its emission. Aims. Measurement of the redshift of BL Lac objects detected in γ-rays and determination of global properties of their host galaxies is the aim of this study. Methods. We observed a sample of eight BL Lac objects with the X-shooter spectrograph installed at the ESO Very Large Telescope (VLT) to take advantage of its unprecedented wavelength coverage and its resolution, which is about five times higher than generally used in such studies. We extracted UVB to NIR spectra that we then corrected for telluric absorption and calibrated in flux. We systematically searched for spectral features. When possible, we determined the contribution of the host galaxy to the overall emission. Results. Of the eight BL Lac sources, we measured the redshift of five of them and determined lower limits for two through the detection of intervening systems. All seven of these objects have redshifts greater than 0.2. For the remaining one, we estimated, using an indirect method, that its redshift is greater than 0.175. In two cases, we refuted redshift values reported in other publications. Through careful modelling, we determined the magnitude of the host galaxies. In two cases, the detection of emission lines allowed to provide hints on the overall properties of the gas in the host galaxies. Even though we warn that we are dealing with a very small sample, we remark that the redshift determination efficiency of our campaign is higher than for previous campaigns. We argue that it is mainly the result of the comparatively higher resolution of X-shooter.

Research paper thumbnail of Searching for VHE gamma-ray emission associated with IceCube astrophysical neutrinos using FACT, H.E.S.S., MAGIC, and VERITAS

Proceedings of 35th International Cosmic Ray Conference — PoS(ICRC2017)

on behalf of the VERITAS Collaboration † D. Dorner on behalf of the FACT Collaboration ‡ J. Dumm ... more on behalf of the VERITAS Collaboration † D. Dorner on behalf of the FACT Collaboration ‡ J. Dumm on behalf of the IceCube Collaboration § K. Satalecka on behalf of the MAGIC Collaboration ¶ F. Schüssler on behalf of the H.E.S.S. Collaboration * Speaker.

Research paper thumbnail of HAP-Fr, a pipeline of data analysis for the HESS-II experiment

Proceedings of The 34th International Cosmic Ray Conference — PoS(ICRC2015)

Research paper thumbnail of Gammapy - A prototype for the CTA science tools

Proceedings of 35th International Cosmic Ray Conference — PoS(ICRC2017)

Gammapy is a Python package for high-level gamma-ray data analysis built on Numpy, Scipy and Astr... more Gammapy is a Python package for high-level gamma-ray data analysis built on Numpy, Scipy and Astropy. It enables us to analyze gamma-ray data and to create sky images, spectra and lightcurves, from event lists and instrument response information, and to determine the position, morphology and spectra of gamma-ray sources. So far Gammapy has mostly been used to analyze data from H.E.S.S. and Fermi-LAT, and is now being used for the simulation and analysis of observations from the Cherenkov Telescope Array (CTA). We have proposed Gammapy as a prototype for the CTA science tools. This contribution gives an overview of the Gammapy package and project and shows an analysis application example with simulated CTA data.

Research paper thumbnail of Research and characterisation of blazar candidates among the Fermi/LAT 3FGL catalogue using multivariate classifications

Astronomy & Astrophysics

Context. In the recently published 3FGL catalogue, the Fermi/LAT collaboration reports the detect... more Context. In the recently published 3FGL catalogue, the Fermi/LAT collaboration reports the detection of γ-ray emission from 3034 sources obtained after four years of observations. The nature of 1010 of those sources is unknown, whereas 2023 have wellidentified counterparts in other wavelengths. Most of the associated sources are labelled as blazars (1717/2023), but the BL Lac or FSRQ nature of 573 of these blazars is still undetermined. Aims. The aim of this study was twofold. First, to significantly increase the number of blazar candidates from a search among the large number of Fermi/LAT 3FGL unassociated sources (case A). Second, to determine the BL Lac or FSRQ nature of the blazar candidates, including those determined as such in this work and the blazar candidates of uncertain type (BCU) that are already present in the 3FGL catalogue (case B). Methods. For this purpose, multivariate classifiers-boosted decision trees and multilayer perceptron neural networks-were trained using samples of labelled sources with no caution flag from the 3FGL catalogue and carefully chosen discriminant parameters. The decisions of the classifiers were combined in order to obtain a high level of source identification along with well controlled numbers of expected false associations. Specifically for case A, dedicated classifications were generated for high (|b| > 10 •) and low (|b| ≤ 10 •) galactic latitude sources; in addition, the application of classifiers to samples of sources with caution flag was considered separately, and specific performance metrics were estimated. Results. We obtained a sample of 595 blazar candidates (high and low galactic latitude) among the unassociated sources of the 3FGL catalogue. We also obtained a sample of 509 BL Lacs and 295 FSRQs from the blazar candidates cited above and the BCUs of the 3FGL catalogue. The number of expected false associations is given for different samples of candidates. It is, in particular, notably low (∼9/425) for the sample of high-latitude blazar candidates from case A.

Research paper thumbnail of Research and characterisation of blazar candidates in the Fermi/LAT 3FGL catalogue using multivariate classifications

Research paper thumbnail of Redshift measurement of Fermi blazars for the Cherenkov telescope array

Research paper thumbnail of Reactions de transfert d'un nucleon induites par un faisceau secondaire de 1 1be : etude de la structure des noyaux exotiques 1 1be et 1 0li

Research paper thumbnail of PKS 2155-304 in July 2006: H.E.S.S. results and simultaneous multi-wavelength observations

AIP Conference Proceedings, 2008

The high‐frequency‐peaked BL Lac PKS 2155‐304 is one of the brightest and best‐studied VHE γ‐ray ... more The high‐frequency‐peaked BL Lac PKS 2155‐304 is one of the brightest and best‐studied VHE γ‐ray sources in the southern hemisphere. The High Energy Stereoscopic System (HESS) has monitored PKS 2155‐304 in 2006 and a multi‐wavelength campaign ...

Research paper thumbnail of Is PKS 0625-354 another variable TeV active galactic nucleus?

Proceedings of 37th International Cosmic Ray Conference — PoS(ICRC2021)

Research paper thumbnail of Search for TeV emission from the base of the Fermi Bubbles with H.E.S.S

Proceedings of 37th International Cosmic Ray Conference — PoS(ICRC2021)

Research paper thumbnail of Deep observations of Kepler's SNR with H.E.S.S

Proceedings of 37th International Cosmic Ray Conference — PoS(ICRC2021)

Research paper thumbnail of Limits on primordial black hole evaporation from H.E.S.S. observations

Proceedings of 37th International Cosmic Ray Conference — PoS(ICRC2021)

Research paper thumbnail of Astronomy outreach in Namibia: H.E.S.S. and beyond

Proceedings of 37th International Cosmic Ray Conference — PoS(ICRC2021)

Research paper thumbnail of Testing cosmology and fundamental physics with the Cherenkov Telescope Array

Proceedings of 36th International Cosmic Ray Conference — PoS(ICRC2019)

Research paper thumbnail of Combined Dark Matter searches towards dwarf spheroidal galaxies with Fermi-LAT, HAWC, HESS, MAGIC and VERITAS

Proceedings of 36th International Cosmic Ray Conference — PoS(ICRC2019)

Research paper thumbnail of Studying cosmological γ-ray propagation with the Cherenkov Telescope Array

Proceedings of 35th International Cosmic Ray Conference — PoS(ICRC2017)

The measurement of γ-rays originating from active galactic nuclei offers the unique opportunity t... more The measurement of γ-rays originating from active galactic nuclei offers the unique opportunity to study the propagation of very-high-energy photons over cosmological distances. Most prominently, γ-rays interact with the extragalactic background light (EBL) to produce e + e − pairs, imprinting an attenuation signature on γ-ray spectra. The e + e − pairs can also induce electromagnetic cascades whose detectability in γ-rays depends on the intergalactic magnetic field (IGMF). Furthermore, physics beyond the Standard Model such as Lorentz invariance violation (LIV) or oscillations between photons and weakly interacting sub-eV particles (WISPs) could affect the propagation of γ-rays. The future Cherenkov Telescope Array (CTA), with its unprecedented γray source sensitivity, as well as enhanced energy and spatial resolution at very high energies, is perfectly suited to study cosmological effects on γ-ray propagation. Here, we present first results of a study designed to realistically assess the capabilities of CTA to probe the EBL, IGMF, LIV, and WISPs.

Research paper thumbnail of Looking for infrared counterparts of Fermi/LAT blazar candidates

Proceedings of 35th International Cosmic Ray Conference — PoS(ICRC2017)

Research paper thumbnail of Extragalactic source population studies at very high energies in the Cherenkov Telescope Array era

Proceedings of 35th International Cosmic Ray Conference — PoS(ICRC2017)

Research paper thumbnail of Gammapy: high level data analysis for extragalactic science cases with the Cherenkov Telescope Array

Proceedings of 35th International Cosmic Ray Conference — PoS(ICRC2017)

Research paper thumbnail of Spectroscopy of high-energy BL Lacertae objects with X-shooter on the VLT

Astronomy & Astrophysics

Context. BL Lac objects detected in γ-rays and, particularly, those detected at very high energie... more Context. BL Lac objects detected in γ-rays and, particularly, those detected at very high energies (E > 100 GeV) by Cherenkov telescopes are extreme sources with most having redshifts lower than 0.2. Their study gives insights on the acceleration mechanisms in play in such systems and is also a valuable tool for putting constraints on the density of extragalactic background light, especially if several objects are detected at different redshifts. As their spectra are dominated by the non-thermal emission of the jet and the spectral features are weak and narrow in the optical domain, measuring their redshift is challenging. However, such a measure is fundamental as it allows a firm determination of the distance and luminosity of the source, and, therefore, a consistent model of its emission. Aims. Measurement of the redshift of BL Lac objects detected in γ-rays and determination of global properties of their host galaxies is the aim of this study. Methods. We observed a sample of eight BL Lac objects with the X-shooter spectrograph installed at the ESO Very Large Telescope (VLT) to take advantage of its unprecedented wavelength coverage and its resolution, which is about five times higher than generally used in such studies. We extracted UVB to NIR spectra that we then corrected for telluric absorption and calibrated in flux. We systematically searched for spectral features. When possible, we determined the contribution of the host galaxy to the overall emission. Results. Of the eight BL Lac sources, we measured the redshift of five of them and determined lower limits for two through the detection of intervening systems. All seven of these objects have redshifts greater than 0.2. For the remaining one, we estimated, using an indirect method, that its redshift is greater than 0.175. In two cases, we refuted redshift values reported in other publications. Through careful modelling, we determined the magnitude of the host galaxies. In two cases, the detection of emission lines allowed to provide hints on the overall properties of the gas in the host galaxies. Even though we warn that we are dealing with a very small sample, we remark that the redshift determination efficiency of our campaign is higher than for previous campaigns. We argue that it is mainly the result of the comparatively higher resolution of X-shooter.

Research paper thumbnail of Searching for VHE gamma-ray emission associated with IceCube astrophysical neutrinos using FACT, H.E.S.S., MAGIC, and VERITAS

Proceedings of 35th International Cosmic Ray Conference — PoS(ICRC2017)

on behalf of the VERITAS Collaboration † D. Dorner on behalf of the FACT Collaboration ‡ J. Dumm ... more on behalf of the VERITAS Collaboration † D. Dorner on behalf of the FACT Collaboration ‡ J. Dumm on behalf of the IceCube Collaboration § K. Satalecka on behalf of the MAGIC Collaboration ¶ F. Schüssler on behalf of the H.E.S.S. Collaboration * Speaker.

Research paper thumbnail of HAP-Fr, a pipeline of data analysis for the HESS-II experiment

Proceedings of The 34th International Cosmic Ray Conference — PoS(ICRC2015)

Research paper thumbnail of Gammapy - A prototype for the CTA science tools

Proceedings of 35th International Cosmic Ray Conference — PoS(ICRC2017)

Gammapy is a Python package for high-level gamma-ray data analysis built on Numpy, Scipy and Astr... more Gammapy is a Python package for high-level gamma-ray data analysis built on Numpy, Scipy and Astropy. It enables us to analyze gamma-ray data and to create sky images, spectra and lightcurves, from event lists and instrument response information, and to determine the position, morphology and spectra of gamma-ray sources. So far Gammapy has mostly been used to analyze data from H.E.S.S. and Fermi-LAT, and is now being used for the simulation and analysis of observations from the Cherenkov Telescope Array (CTA). We have proposed Gammapy as a prototype for the CTA science tools. This contribution gives an overview of the Gammapy package and project and shows an analysis application example with simulated CTA data.

Research paper thumbnail of Research and characterisation of blazar candidates among the Fermi/LAT 3FGL catalogue using multivariate classifications

Astronomy & Astrophysics

Context. In the recently published 3FGL catalogue, the Fermi/LAT collaboration reports the detect... more Context. In the recently published 3FGL catalogue, the Fermi/LAT collaboration reports the detection of γ-ray emission from 3034 sources obtained after four years of observations. The nature of 1010 of those sources is unknown, whereas 2023 have wellidentified counterparts in other wavelengths. Most of the associated sources are labelled as blazars (1717/2023), but the BL Lac or FSRQ nature of 573 of these blazars is still undetermined. Aims. The aim of this study was twofold. First, to significantly increase the number of blazar candidates from a search among the large number of Fermi/LAT 3FGL unassociated sources (case A). Second, to determine the BL Lac or FSRQ nature of the blazar candidates, including those determined as such in this work and the blazar candidates of uncertain type (BCU) that are already present in the 3FGL catalogue (case B). Methods. For this purpose, multivariate classifiers-boosted decision trees and multilayer perceptron neural networks-were trained using samples of labelled sources with no caution flag from the 3FGL catalogue and carefully chosen discriminant parameters. The decisions of the classifiers were combined in order to obtain a high level of source identification along with well controlled numbers of expected false associations. Specifically for case A, dedicated classifications were generated for high (|b| > 10 •) and low (|b| ≤ 10 •) galactic latitude sources; in addition, the application of classifiers to samples of sources with caution flag was considered separately, and specific performance metrics were estimated. Results. We obtained a sample of 595 blazar candidates (high and low galactic latitude) among the unassociated sources of the 3FGL catalogue. We also obtained a sample of 509 BL Lacs and 295 FSRQs from the blazar candidates cited above and the BCUs of the 3FGL catalogue. The number of expected false associations is given for different samples of candidates. It is, in particular, notably low (∼9/425) for the sample of high-latitude blazar candidates from case A.

Research paper thumbnail of Research and characterisation of blazar candidates in the Fermi/LAT 3FGL catalogue using multivariate classifications

Research paper thumbnail of Redshift measurement of Fermi blazars for the Cherenkov telescope array

Research paper thumbnail of Reactions de transfert d'un nucleon induites par un faisceau secondaire de 1 1be : etude de la structure des noyaux exotiques 1 1be et 1 0li

Research paper thumbnail of PKS 2155-304 in July 2006: H.E.S.S. results and simultaneous multi-wavelength observations

AIP Conference Proceedings, 2008

The high‐frequency‐peaked BL Lac PKS 2155‐304 is one of the brightest and best‐studied VHE γ‐ray ... more The high‐frequency‐peaked BL Lac PKS 2155‐304 is one of the brightest and best‐studied VHE γ‐ray sources in the southern hemisphere. The High Energy Stereoscopic System (HESS) has monitored PKS 2155‐304 in 2006 and a multi‐wavelength campaign ...