arXiv subjects

C. Alvarez

Publications and source records attributed to C. Alvarez.

125 records · Page 7Linked to original sources

arXiv2013

Sensitivity of the High Altitude Water Cherenkov Detector to Sources of Multi-TeV Gamma Rays

The High Altitude Water Cherenkov (HAWC) observatory is an array of large water Cherenkov detectors sensitive to gamma rays and hadronic cosmic rays in the energy band between 100 GeV and 100 TeV. The observatory will be used to measure high-energy protons and cosmic rays via detection of the energetic secondary particles reaching the ground when one of these particles interacts in the atmosphere above the detector. HAWC is under construction at a site 4100 meters above sea level on the northern slope of the volcano Sierra Negra, which is located in central Mexico at 19 degrees N latitude. It is scheduled for completion in 2014. In this paper we estimate the sensitivity of the HAWC instrument to point-like and extended sources of gamma rays. The source fluxes are modeled using both unbroken power laws and power laws with exponential cutoffs. HAWC, in one year, is sensitive to point sources with integral power-law spectra as low as 5x10^-13 cm^-2 sec^-1 above 2 TeV (approximately 50 mCrab) over 5 sr of the sky. This is a conservative estimate based on simple event parameters and is expected to improve as the data analysis techniques are refined. We discuss known TeV sources and the scientific contributions that HAWC can make to our understanding of particle acceleration in these sources.

astro-ph.HE↗

arXiv2009

Operating Water Cherenkov Detectors in high altitude sites for the Large Aperture GRB Observatory

Water Cherenkov Detectors (WCD) are efficient detectors for detecting GRBs in the 10 GeV - 1 TeV energy range using the single particle technique, given their sensitivity to low energy secondary photons produced by high energy photons when cascading in the atmosphere. The Large Aperture GRB Observatory (LAGO) operates arrays of WCD in high altitude sites (above 4500 m a.s.l.) in Bolivia, Mexico and Venezuela, with planned extension to Peru. Details on the operation and stability of these WCD in remote sites with high background rates of particles will be detailed, and compared to simulations. Specific issues due to operation at high altitude, atmospheric effects and solar activity, as well as possible hardware enhancements will also be presented.

astro-ph.HE↗

arXiv2009

The Large Aperture GRB Observatory

The Large Aperture GRB Observatory (LAGO) is aiming at the detection of the high energy (around 100 GeV) component of Gamma Ray Bursts, using the single particle technique in arrays of Water Cherenkov Detectors (WCD) in high mountain sites (Chacaltaya, Bolivia, 5300 m a.s.l., Pico Espejo, Venezuela, 4750 m a.s.l., Sierra Negra, Mexico, 4650 m a.s.l). WCD at high altitude offer a unique possibility of detecting low gamma fluxes in the 10 GeV - 1 TeV range. The status of the Observatory and data collected from 2007 to date will be presented.

astro-ph.HE↗

arXiv2009

Water Cherenkov Detectors response to a Gamma Ray Burst in the Large Aperture GRB Observatory

In order to characterise the behaviour of Water Cherenkov Detectors (WCD) under a sudden increase of 1 GeV - 1 TeV background photons from a Gamma Ray Burst (GRB), simulations were conducted and compared to data acquired by the WCD of the Large Aperture GRB Observatory (LAGO). The LAGO operates arrays of WCD at high altitude to detect GRBs using the single particle technique. The LAGO sensitivity to GRBs is derived from the reported simulations of the gamma initiated particle showers in the atmosphere and the WCD response to secondaries.

astro-ph.HE↗

arXiv2007

Discovery of a wide companion near the deuterium burning mass limit in the Upper Scorpius association

We present the discovery of a companion near the deuterium burning mass limit located at a very wide distance, at an angular separation of 4.6+/-0.1 arcsec (projected distance of ~ 670 AU) from UScoCTIO108, a brown dwarf of the very young Upper Scorpius association. Optical and near-infrared photometry and spectroscopy confirm the cool nature of both objects, with spectral types of M7 and M9.5, respectively, and that they are bona fide members of the association, showing low gravity and features of youth. Their masses, estimated from the comparison of their bolometric luminosities and theoretical models for the age range of the association, are 60+/-20 and 14^{+2}_{-8} MJup, respectively. The existence of this object around a brown dwarf at this wide orbit suggests that the companion is unlikely to have formed in a disk based on current planet formation models. Because this system is rather weakly bound, they did not probably form through dynamical ejection of stellar embryos.

astro-ph↗

arXiv2005

A search for hot massive extrasolar planets around nearby young stars with the adaptive optics system NACO

We report on a survey devoted for the search of exo-planets around young and nearby stars carried out with NACO at the VLT. The detection limit for 28 among the best available targets vs. the angular separation from the star is presented. The non-detection of any planetary mass companion in our survey is used to derive, for the first time, the frequency of the upper limit of the projected separation planet-stars. In particular, we find that in 50 % of cases, no 5 MJ planet (or more massive) has been detected at projected separations larger than 14 AU and no 10 MJ planet (or more massive) has been detected at projected separations larger than 8.5 AU. In 100 % of cases, these values increase to 36 AU and 65 AU respectively. The excellent sensitivity reached by our study leads to a much lower upper limit of the projected planet-star separation compared with previous studies. For example, for the Beta Pictoris group, (~ 12 Myr), we did not detect any 10 MJ planet at distances larger than 15 AU. A previous study carried out with 4 m class telescopes put an upper limit for 10 MJ planets at ~ 60 AU. For our closest target (V 2306 Oph - d = 4.3 pc) it is shown that it would be possible to detect a 10 MJ planet at a minimum projected separation from the star of 1 AU and a 5 MJ planet at a minimum projected separation of 3.7 AU. Our results are discussed with respect to mechanisms explaining planet formation and migration and to forthcoming observational strategies and future planet finder observations from the ground.

astro-ph↗

arXiv2005

The sub-arcsecond dusty environment of Eta Carinae

The core of the nebula surrounding Eta Carinae has been observed with the VLT Adaptive Optics system NACO and with the interferometer VLTI/MIDI to constrain spatially and spectrally the warm dusty environment and the central object. In particular, narrow-band images at 3.74 and 4.05 micron reveal the butterfly shaped dusty environment close to the central star with unprecedented spatial resolution. A void whose radius corresponds to the expected sublimation radius has been discovered around the central source. Fringes have been obtained in the Mid-IR which reveal a correlated flux of about 100Jy situated 0.3" south-east of the photocenter of the nebula at 8.7 micron, which corresponds with the location of the star as seen in other wavelengths. This correlated flux is partly attributed to the central object, and these observations provide an upper limit for the SED of the central source from 2.2 to 13.5 micron. Moreover, we have been able to spectrally disperse the signal from the nebula itself at PA=318 degree, i.e. in the direction of the bipolar nebula 310 degree) within the MIDI field of view of 3". A large amount of corundum (Al2O3) is discovered, peaking at 0.6-1.2" south-east from the star, whereas the dust content of the Weigelt blobs is dominated b silicates. We discuss the mechanisms of dust formation which are closely related to the geometry of this Butterfly nebulae.

astro-ph↗

arXiv2004

Near-IR speckle imaging of massive young stellar objects

We present near-IR speckle images of 21 massive Young Stellar Objects (YSOs) associated with outflows. The aim of this study is to search for sub-arcsecond reflection nebulae associated with the outflow cavity. We find that 6 of the massive YSOs show a conical nebula which can be interpreted in terms of reflected light from the dusty walls of the outflow cavity. In all cases, the small scale structures seen in our images are compared with outflow indicators found in the literature. No clear correlation is found between the presence of the reflection nebulosity and any property such as degree of embeddedness. We also note that 3 of the sources show close companions, one of them belonging also to the sample with conical nebula.

astro-ph↗

arXiv2004

Constraints in the circumstellar density distribution of massive Young Stellar Objects

We use a Monte Carlo code to generate synthetic near-IR reflection nebulae that resemble those (normally associated with a bipolar outflow cavity) seen towards massive young stellar objects (YSOs). The 2D axi-symmetric calculations use an analytic expression for a flattened infalling rotating envelope with a bipolar cavity representing an outflow. We are interested in which aspects of the circumstellar density distribution can be constrained by servations of these reflection nebulae. We therefore keep the line of sight optical depth constant in the model grid, as this is often constrained independently by observations. It is found that envelopes with density distributions corresponding to mass infall rates of ~ 1E-4 M_sun/yr (for an envelope radius of 4700 AU) seen at an inclination angle of ~ 45 deg approximately reproduce the morphology and extension of the sub-arcsecond nebulae observed in massive YSOs. Based on the flux ratio between the approaching and receding lobe of the nebula, we can constrain the system inclination angle. The cavity opening angle is well constrained from the nebula opening angle. Our simulations indicate that to constrain the outflow cavity shape and the degree of flattening in the envelope, near-IR imaging with higher resolution and dynamic range than speckle imaging in 4m-class telescopes is needed. The radiative transfer code is also used to simulate the near-IR sub-arcsecond nebula seen in Mon R2 IRS3. We find indications of a shallower opacity law in this massive YSO than in the interstellar medium, or possibly a sharp drop in the envelope density distribution at distances of ~ 1000 AU from the illuminating source.

astro-ph↗

arXiv2004

First Results on In-Beam gamma Spectroscopy of Neutron-Rich Na and Mg Isotopes at REX-ISOLDE

After the successful commissioning of the radioactive beam experiment at ISOLDE (REX-ISOLDE) - an accelerator for exotic nuclei produced by ISOLDE - first physics experiments using these beams were performed. Initial experiments focused on the region of deformation in the vicinity of the neutron-rich Na and Mg isotopes. Preliminary results show the high potential and physics opportunities offered by the exotic isotope accelerator REX in conjunction with the modern Germanium gamma spectrometer MINIBALL.

nucl-ex↗

arXiv1996

Testing the Equivalence Principle by Lamb shift Energies

The Einstein Equivalence Principle has as one of its implications that the non-gravitational laws of physics are those of special relativity in any local freely-falling frame. We consider possible tests of this hypothesis for systems whose energies are due to radiative corrections, i.e. which arise purely as a consequence of quantum field theoretic loop effects. Specifically, we evaluate the Lamb shift transition (as given by the energy splitting between the $2S_{1/2}$ and $2P_{1/2}$ atomic states) within the context of violations of local position invariance and local Lorentz invariance, as described by the $T H εμ$ formalism. We compute the associated red shift and time dilation parameters, and discuss how (high-precision) measurements of these quantities could provide new information on the validity of the equivalence principle.

gr-qc↗

arXiv1996

The Equivalence Principle and Anomalous Magnetic Moment Experiments

We investigate the possibility of testing of the Einstein Equivalence Principle (EEP) using measurements of anomalous magnetic moments of elementary particles. We compute the one loop correction for the $g-2$ anomaly within the class of non metric theories of gravity described by the \tmu formalism. We find several novel mechanisms for breaking the EEP whose origin is due purely to radiative corrections. We discuss the possibilities of setting new empirical constraints on these effects.

gr-qc↗

arXiv1996

Testing the Equivalence Principle using Atomic Vacuum Energy Shifts

We consider possible tests of the Einstein Equivalence Principle for quantum-mechanical vacuum energies by evaluating the Lamb shift transition in a class of non-metric theories of gravity described by the \tmu formalism. We compute to lowest order the associated red shift and time dilation parameters, and discuss how (high-precision) measurements of these quantities could provide new information on the validity of the equivalence principle.

gr-qc↗

arXiv1996

The Equivalence Principle in the Non-baryonic Regime

We consider the empirical validity of the equivalence principle for non-baryonic matter. Working in the context of the THεμformalism, we evaluate the constraints experiments place on parameters associated with violation of the equivalence principle (EVPs) over as wide a sector of the standard model as possible. Specific examples include new parameter constraints which arise from torsion balance experiments, gravitational red shift, variation of the fine structure constant, time-dilation measurements, and matter/antimatter experiments. We find several new bounds on EVPs in the leptonic and kaon sectors.

gr-qc↗

arXiv1996

Testing the Equivalence Principle in the Quantum Regime

We consider possible tests of the Einstein Equivalence Principle for physical systems in which quantum-mechanical vacuum energies cannot be neglected. Specific tests include a search for the manifestation of non-metric effects in Lamb-shift transitions of Hydrogenic atoms and in anomalous magnetic moments of massive leptons. We discuss how current experiments already set bounds on the violation of the equivalence principle in this sector and how new (high-precision) measurements of these quantities could provide further information to this end.

gr-qc↗

arXiv1995

The Equivalence Principle and g-2 Experiments

We consider the possibility of using measurements of anomalous magnetic moments of elementary particles as a possible test of the Einstein Equivalence Principle (EEP). For the class non-metric theories of gravity described by the \tmu formalism we find several novel mechanisms for breaking the EEP, and discuss the possibilities of setting new empirical constraints on such effects.

gr-qc↗

arXiv1993

Local Position Invariance and Vacuum Energy Shifts

We discuss tests of the Einstein Equivalence Principle due to energies which are purely quantum mechanical in origin. In particular, we consider using Lamb Shift energies to test for possible quantum violations of Local Position Invariance. (to appear in the Proceedings of the 5th Canadian Conference on General Relativity and Relativistic Astrophysics, University of Waterloo, May 13-- 15, 1993) (acknowledgements changed to correct an omission in the originally submitted version)

gr-qc↗