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De Diego J.A.,National Autonomous University of Mexico | De Diego J.A.,Institute of Astrophysics of Canarias
Astronomical Journal

Microvariations probe the physics and internal structure of quasars. Unpredictability and small flux variations make this phenomenon elusive and difficult to detect. Variance-based probes such as the C and F tests, or a combination of both, are popular methods to compare the light curves of the quasar and a comparison star. Recently, detection claims in some studies have depended on the agreement of the results of the C and F tests, or of two instances of the F-test, for rejecting the non-variation null hypothesis. However, the C-test is a non-reliable statistical procedure, the F-test is not robust, and the combination of tests with concurrent results is anything but a straightforward methodology. A priori power analysis calculations and post hoc analysis of Monte Carlo simulations show excellent agreement for the analysis of variance test to detect microvariations as well as the limitations of the F-test. Additionally, the combined tests yield correlated probabilities that make the assessment of statistical significance unworkable. However, it is possible to include data from several field stars to enhance the power in a single F-test, increasing the reliability of the statistical analysis. This would be the preferred methodology when several comparison stars are available. An example using two stars and the enhanced F-test is presented. These results show the importance of using adequate methodologies and avoiding inappropriate procedures that can jeopardize microvariability detections. Power analysis and Monte Carlo simulations are useful tools for research planning, as they can demonstrate the robustness and reliability of different research approaches. © 2014. The American Astronomical Society. All rights reserved. Source

Linares M.,Institute of Astrophysics of Canarias | Linares M.,University of La Laguna
Astrophysical Journal

Compact binary millisecond pulsars with main-sequence donors, often referred to as "redbacks," constitute the long-sought link between low-mass X-ray binaries and millisecond radio pulsars and offer a unique probe of the interaction between pulsar winds and accretion flows. We present a systematic study of eight nearby redbacks, using more than 100 observations obtained with Swift's X-ray Telescope. We distinguish between three main states: pulsar, disk, and outburst states. We find X-ray mode switching in the disk state of PSR J1023+0038 and XSS J12270-4859, similar to what was found in the other redback that showed evidence for accretion: rapid, recurrent changes in X-ray luminosity (0.5-10 keV, L X), between (6-9) × 1032 erg s-1 (disk-passive state) and (3-5) × 1033 erg s-1 (disk-active state). This strongly suggests that mode switching - which has not been observed in quiescent low-mass X-ray binaries - is universal among redback millisecond pulsars in the disk state. We briefly explore the implications for accretion disk truncation and find that the inferred magnetospheric radius in the disk state of PSR J1023+0038 and XSS J12270-4859 lies outside the light cylinder. Finally, we note that all three redbacks that have developed accretion disks have relatively high L X in the pulsar state (>1032 erg s-1). © 2014. The American Astronomical Society. All rights reserved.. Source

Martinez Pillet V.,Institute of Astrophysics of Canarias
Space Science Reviews

Various aspects of the magnetism of the quiet sun are reviewed. The suggestion that a small scale dynamo acting at granular scales generates what we call the quiet sun fields is studied in some detail. Although dynamo action has been proved numerically, it is argued that current simulations are still far from achieving the complexity that might be present on the Sun. We based this statement not so much on the low magnetic Reynolds numbers used in the simulations but, above all, in the smallness of the kinetic Reynolds numbers employed by them. It is argued that the low magnetic Prandtl number at the solar surface may pose unexpected problems for the identification of the observed internetwork fields with dynamo action at granular scales. Some form of turbulent dynamo at bigger (and deeper) scales is favored. The comparison between the internetwork fields observed by Hinode and the magnetism inferred from Hanle measurements are converging towards a similar description. They are both described as randomly oriented, largely transverse fields in the several hecto-Gauss range. These similarities are ever making more natural to assume that they are the same. However, and because of the large voids of magnetic flux observed in the spatial distribution of the internetwork fields, it is argued that they are not likely to be generated by dynamo action in the intergranular lanes. It is concluded that if a dynamo is acting at granular scales, the end product might have not been observed yet at current spatial resolutions and sensitivities with the Zeeman effect. Thus an effort to increase these resolutions and polarimetric sensitivities must be made. New ground- and space-based telescopes are needed. The opportunity offered by the Solar Orbiter mission to observe the Quiet Sun dynamics at the poles is seen as one of the most important tests for confirming the existence, or otherwise, of a granularly driven surface dynamo. © 2013 Springer Science+Business Media Dordrecht. Source

Casares J.,Institute of Astrophysics of Canarias | Casares J.,University of La Laguna
Astrophysical Journal

We compare Ha emission profiles of 12 dynamically confirmed black holes (BHs) and 2 neutron star X-ray transients (SXTs) in quiescence with those of a sample of 43 Cataclysmic Variables (CVs), also quiescent. The FWHM of the Ha line in SXTs is tightly correlated with the velocity semi-amplitude of the donor star K2 = 0.233(13) FWHM. This new correlation, when combined with orbital periods (i.e., through photometric light curves), allows for the possibility of estimating compact object mass functions from single integration, lowresolution spectroscopy. On the other hand, CVs above the period gap are found to follow a flatter correlation, likely consequence of their larger mass ratios. We also find that the FWHM traces the disk velocity at ≈42% RL1, independently of binary mass ratio. In addition, for a given FWHM, BHs tend to have lower EWs than CVs. This might be explained by the fact that CVs must be seen at higher inclinations to mimic the same projected disk velocities as BH SXTs. For the same reason, CVs with FWHM ≳1500 km s-1 are mostly eclipsing while none of our sample BHs are. Furthermore, we show that there is a vacant/unoccupied region for CVs in the FWHMEW plane defined by FWHM >2568√ (1 - (9 EW)2) (km s-1). Both the FWHMK2 correlation and the FWHM EW plane can be exploited, together with photometric light curves, to efficiently discover quiescent BHs in deep Ha surveys of the Galactic plane. © 2015. The American Astronomical Society. All rights reserved. Source

Agency: Cordis | Branch: H2020 | Program: RIA | Phase: ICT-10-2015 | Award Amount: 1.97M | Year: 2016

WHY: 2015 has been named by the United Nations as the International Year of Light (light2015.org). Light has had many obvious benefits for human mankind, but it also poses some relevant threats: the everyday-increasing excess of light thrown by humans to the sky seriously threatens to remove forever one of humanitys natural wonders, the view of our universe. More importantly, it has also an adverse impact on our environment and economy (energy wasted to the sky costs 2 billion US$ per year in the USA and 6,3 billion per year in Europe) and on the health of hundreds of species, including pathologies in human beings (e.g., stress, insomnia). Many professional and amateur scientists are already fighting against light pollution. However, it is necessary to increase social awareness about the importance of preserving the darkness of our cities and environment. WHAT: STARS4ALL will create an Light Pollution Initiative (LPI) incubation platform that will allow generating (and maintaining) customizable on-demand domain-focused LPIs (e.g., a light pollution working group in Brussels). The platform will be self-sustainable: it will integrate a crowdfunding tool to obtain funding for the LPIs; it will consider incentives that motivate citizens to participate in LPIs, as well as policies to handle those incentives; and it will provide innovations in data acquisition from sensors deployed by citizens and in games with a purpose. HOW: STARS4ALL will initially deploy 10 LPIs, which will be available by the end of the 1st semester of project execution, and will be operating and creating collective awareness during the rest of the project. At that moment we pave the way the creation of other LPIs by citizens, specially in other disciplines such as Energy Saving, Biodiversity, and Human Health, and will organize open competitions among them.

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