Hadronic processes within collective stellar winds

Recently, we have proposed that the interaction between relativistic protons resulting from Fermi first order acceleration in the superbubble of a stellar OB association or in other nearby accelerator and ions residing in single stellar winds of massive stars could lead to TeV sources without strong counterparts at lower energies. Here we refine this analysis in several directions. We study collective wind configurations produced by a number of massive stars, and obtain densities and expansion velocities of the stellar wind gas that is to be target of hadronic interactions. We study the expected $\gamma$-ray emission from these regions, considering in an approximate way the effect of cosmic ray modulation. We compute secondary particle production (electrons from knock-on interactions and electrons and positrons from charged pion decay), and solve the loss equation with ionization, synchrotron, bremsstrahlung, inverse Compton, and expansion losses. We provide examples where configurations can produce sources for GLAST satellite, and the MAGIC/HESS/VERITAS telescopes in non-uniform ways, i.e., with or without the corresponding counterparts. We show that in all cases we studied no EGRET source is expected. Finally, we comment on HESS J1303-631 and on Cygnus OB 2 and Westerlund 1 as two associations where this scenario could be tested.Comment: Accepted for publication in A&

High energy gamma-ray emission from the starburst nucleus of NGC 253

The high density medium that characterizes the central regions of starburst galaxies and its power to accelerate particles up to relativistic energies make these objects good candidates as $\gamma$-rays sources. In this paper, a self-consistent model of the multifrequency emission of the starburst galaxy NGC 253, from radio to gamma-rays, is presented. The model is in agreement with all current measurements and provides predictions for the high energy behavior of the NGC 253 central region. Prospects for observations with the HESS array and GLAST satellite are especially discussed.Comment: 17 pages, 13 figures. To appear in A&A. Modelling parameters and results are unchanged from the previous version but we enhance here the detail with which they are described. 3 new figures and 1 new section have been added, including a figure showing the consistency of the model with the recently reported HESS observations (astro-ph/0507370) of NGC 25

High-energy gamma-rays from stellar associations

It is proposed that TeV gamma-rays and neutrinos can be produced by cosmic rays (CRs) through hadronic interactions in the innermost parts of the winds of massive O and B stars. Convection prevents low-energy particles from penetrating into the wind, leading to an absence of MeV-GeV counterparts. It is argued that groups of stars located close to the CR acceleration sites in OB stellar associations may be detectable by ground-based Cherenkov telescopes.Comment: 10 pages, 1 figure. Accepted for publication in Astrophysical Journal Letter

Pressure Impact on the Stability and Distortion of the Crystal Structure of CeScO3

[EN] The effects of high pressure on the crystal structure of orthorhombic (Pnma) perovskite-type cerium scandate were studied in situ under high pressure by means of synchrotron X-ray powder diffraction, using a diamond-anvil cell. We found that the perovskite-type crystal structure remains stable up to 40 GPa, the highest pressure reached in the experiments. The evolution of unit-cell parameters with pressure indicated an anisotropic compression. The room-temperature pressure¿volume equation of state (EOS) obtained from the experiments indicated the EOS parameters V0 = 262.5(3) Å3 , B0 = 165(7) GPa, and B0¿ = 6.3(5). From the evolution of microscopic structural parameters like bond distances and coordination polyhedra of cerium and scandium, the macroscopic behavior of CeScO3 under compression was explained and reasoned for its large pressure stability. The reported results are discussed in comparison with high-pressure results from otherThe authors are thankful for the financial support to this research from the Spanish Ministerio de Economia y Competitividad, the Spanish Research Agency, and the European Fund for Regional Development under Grant Nos. MAT2016-75S86-C4-1/2-P, MAT2013-46649-C4-1/2-P, and MAT2015-71070-REDC (MALTA Consolider). D.S.P. acknowledges the Spanish government for a Ramon y Cajal grant. The authors express gratitude to F. Aguado for fruitful discussions on the high-pressure behavior of perovskites. These experiments were performed at MSPD beamline at ALBA Synchrotron with the collaboration of ALBA staff.Errandonea, D.; Santamaria-Perez, D.; Martinez-Garcia, D.; Gomis, O.; Shukla, R.; Achary, SN.; Tyagi, AK.... (2017). Pressure Impact on the Stability and Distortion of the Crystal Structure of CeScO3. Inorganic Chemistry. 56(14):8363-8371. https://doi.org/10.1021/acs.inorgchem.7b01042S83638371561

Transition path to a dense efficient-packed post-delafossite phase. Crystal structure and evolution of the chemical bonding

[EN] A(I)B(III)O(2) delafossite-type oxides are important technological compounds characterized by the linear coordination of the monovalent A metal by oxygen atoms. Based on results of in situ synchrotron X-ray diffraction measurements and ab initio calculations, we herein report on the high-pressure behavior of AgGaO2, to the best of our knowledge the first compound showing step-wise transitions of Ag coordination from linear (2) to octahedral (6), through a leaning delafossite structure. These transformations take place at similar to 10.5 and similar to 16.5 GPa, respectively. Our structural analysis evidences that the initial rhombohedral delafossite structure first becomes dynamically unstable, and distorts continuously via a gliding motion of the [GaO2] octahedral layers within the ab plane, and subsequently transform into another rhombohedral phase 8% denser. This structural sequence is associated with a simultaneous decrease in the bond order of the Ag-O bonds and an increase in the ionicity of the crystal. These results may help to unveil the high-pressure phases of several delafossite compounds which were reported to undergo phase transitions under compression that could not be identified.We are thankful for the financial support received from the Spanish Ministerio de Ciencia e Innovacion and the Agencia Estatal de Investigacion under national projects PGC2018-094417-B-I00 (co-financed by EU FEDER funds), MAT2016-75586-C4-1-P/2-P, FIS2017-83295-P, PID2019-106383GB-C41/C42 and RED2018-102612-T (MALTA Consolider), and from Generalitat Valenciana under project PROMETEO/2018/123. D.S-P, A.O.R, and J.A.S acknowledge financial support of the Spanish MINECO for the RyC-2014-15643, RyC-2016-20301, and RyC-2015-17482 Ramon y Cajal Grants, respectively. Authors thank ALBA-CELLS synchrotron for providing beamtime (ALBA experiments 2012010170).Chuliá-Jordán, R.; Santamaria-Perez, D.; Pellicer-Porres, J.; Otero-De-La-Roza, A.; Martinez-Garcia, D.; García-Domene, B.; Gomis, O.... (2021). Transition path to a dense efficient-packed post-delafossite phase. Crystal structure and evolution of the chemical bonding. Journal of Alloys and Compounds. 867. https://doi.org/10.1016/j.jallcom.2021.15901215901286

Constraints on the steady and pulsed very high energy gamma-ray emission from observations of PSR B1951+32/CTB 80 with the MAGIC Telescope

We report on very high energy gamma-observations with the MAGIC Telescope of the pulsar PSR B1951+32 and its associated nebula, CTB 80. Our data constrain the cutoff energy of the pulsar to be less than 32 GeV, assuming the pulsed gamma-ray emission to be exponentially cut off. The upper limit on the flux of pulsed gamma-ray emission above 75 GeV is 4.3*10^-11 photons cm^-2 sec^-1, and the upper limit on the flux of steady emission above 140 GeV is 1.5*10^-11 photons cm^-2 sec^-1. We discuss our results in the framework of recent model predictions and other studies.Comment: 7 pages, 7 figures, replaced with published versio

Discovery of Very High Energy gamma-rays from 1ES 1011+496 at z=0.212

We report on the discovery of Very High Energy (VHE) gamma-ray emission from the BL Lacertae object 1ES1011+496. The observation was triggered by an optical outburst in March 2007 and the source was observed with the MAGIC telescope from March to May 2007. Observing for 18.7 hr we find an excess of 6.2 sigma with an integrated flux above 200 GeV of (1.58$\pm0.32) 10^{-11}$ photons cm$^{-2}$ s$^{-1}$. The VHE gamma-ray flux is >40% higher than in March-April 2006 (reported elsewhere), indicating that the VHE emission state may be related to the optical emission state. We have also determined the redshift of 1ES1011+496 based on an optical spectrum that reveals the absorption lines of the host galaxy. The redshift of z=0.212 makes 1ES1011+496 the most distant source observed to emit VHE gamma-rays up to date.Comment: 4 pages, 6 figures, minor changes to fit the ApJ versio

Discovery of Very High Energy γ\gamma-Rays from Markarian~180 Triggered by an Optical Outburst

The high-frequency-peaked BL Lacertae object Markarian~180 (Mrk~180) was observed to have an optical outburst in 2006 March, triggering a Target of Opportunity observation with the MAGIC telescope. The source was observed for 12.4 hr and very high energy $\gamma$-ray emission was detected with a significance of 5.5 $\sigma$. An integral flux above 200 GeV of $(2.3\pm0.7)\times10^{-11} {cm}^{-2} {s}^{-1}$ was measured, corresponding to 11% of the Crab Nebula flux. A rather soft spectrum with a photon index of $-3.3\pm0.7$ has been determined. No significant flux variation was found.Comment: Accepted by ApJ Letters, minor revision

First bounds on the very high energy gamma-ray emission from Arp 220

Using the Major Atmospheric Gamma Imaging Cherenkov Telescope (MAGIC), we have observed the nearest ultra-luminous infrared galaxy Arp 220 for about 15 hours. No significant signal was detected within the dedicated amount of observation time. The first upper limits to the very high energy $\gamma$-ray flux of Arp 220 are herein reported and compared with theoretical expectations.Comment: Accepted for publication in Ap