251 research outputs found

    Nonequilibrium phase transition to temporal oscillations in mean-field spin models

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    We propose a mean-field theory for nonequilibrium phase transitions to a periodically oscillating state in spin models. A nonequilibrium generalization of the Landau free energy is obtained from the join distribution of the magnetization and its smoothed stochastic time derivative. The order parameter of the transition is a Hamiltonian, whose nonzero value signals the onset of oscillations. The Hamiltonian and the nonequilibrium Landau free energy are determined explicitly from the stochastic spin dynamics. The oscillating phase is also characterized by a non-trivial overlap distribution reminiscent of a continuous replica symmetry breaking, in spite of the absence of disorder. An illustration is given on an explicit kinetic mean-field spin model.Comment: 5 pages, 1 figure, submitted to Physical Review Letter

    ComunicaciĂłn del peligro volcĂĄnico en erupciones de larga data: caso de Complejo VolcĂĄnico Nevados de ChillĂĄn

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    La Red Nacional de Vigilancia VolcĂĄnica (RNVV) del Servicio Nacional de GeologĂ­a y MinerĂ­a (SERNAGEOMIN), es el organismo tĂ©cnico del Estado de Chile, responsable de monitorear y evaluar el comportamiento de los 45 volcanes mĂĄs peligrosos dentro del territorio nacional. Para representar la actividad volcĂĄnica se utiliza un sistema de alertas tĂ©cnicas, mediante una escala de colores compuesta por verde (nivel base), amarilla (incremento en parĂĄmetros de monitoreo), naranja (posible incremento de actividad) y roja (erupciĂłn inminente o en curso), donde cada nivel estĂĄ asociado a recomendaciones y posibles manifestaciones esperables dentro del nivel de actividad. Los mecanismos formales para comunicar cambios en la actividad son los Reportes Especiales de Actividad VolcĂĄnica (REAV), emitidos cada vez que registra un evento instrumentalmente anĂłmalo y Reportes de Actividad VolcĂĄnica (RAV) que se emiten mensualmente en alerta verde, cada quince dĂ­as en caso de alerta amarilla o de forma diaria en caso de alerta naranja o roja (SERNAGEOMIN, 2022). Durante la Ășltima dĂ©cada, 15 de los 45 volcanes monitoreados han registrado un aumento significativo en su lĂ­nea base, implicando un cambio en su nivel de alerta. Estos niveles, usualmente amarillo, han tenido duraciones promedio en torno a un par de meses, sin embargo, sistemas volcĂĄnicos mĂĄs complejos, especialmente aquellos con conducto abierto o sistemas hidrotermales muy activos, pueden mantenerse sobre su lĂ­nea base por varios meses e incluso años de manera (semi) continua. Dentro de estos Ășltimos destacan los volcanes Villarrica, Copahue y Nevados de ChillĂĄn, los cuales han estado en alerta continua por periodos superiores a un año (Figura 1)

    Final results of the second prospective AIEOP protocol for pediatric intracranial ependymoma

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    BACKGROUND: This prospective study stratified patients by surgical resection (complete = NED vs incomplete = ED) and centrally reviewed histology (World Health Organization [WHO] grade II vs III). METHODS: WHO grade II/NED patients received focal radiotherapy (RT) up to 59.4 Gy with 1.8 Gy/day. Grade III/NED received 4 courses of VEC (vincristine, etoposide, cyclophosphamide) after RT. ED patients received 1-4 VEC courses, second-look surgery, and 59.4 Gy followed by an 8-Gy boost in 2 fractions on still measurable residue. NED children aged 1-3 years with grade II tumors could receive 6 VEC courses alone. RESULTS: From January 2002 to December 2014, one hundred sixty consecutive children entered the protocol (median age, 4.9 y; males, 100). Follow-up was a median of 67 months. An infratentorial origin was identified in 110 cases. After surgery, 110 patients were NED, and 84 had grade III disease. Multiple resections were performed in 46/160 children (28.8%). A boost was given to 24/40 ED patients achieving progression-free survival (PFS) and overall survival (OS) rates of 58.1% and 68.7%, respectively, in this poor prognosis subgroup. For the whole series, 5-year PFS and OS rates were 65.4% and 81.1%, with no toxic deaths. On multivariable analysis, NED status and grade II were favorable for OS, and for PFS grade II remained favorable. CONCLUSIONS: In a multicenter collaboration, this trial accrued the highest number of patients published so far, and results are comparable to the best single-institution series. The RT boost, when feasible, seemed effective in improving prognosis. Even after multiple procedures, complete resection confirmed its prognostic strength, along with tumor grade. Biological parameters emerging in this series will be the object of future correlatives and reports

    Avances y desafĂ­os en la evaluaciĂłn de peligros volcĂĄnicos en la Red Nacional de Vigilancia VolcĂĄnica de Chile

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    El territorio continental de Chile alcanza un largo de 4.270 km, en cuya extensiĂłn posee un arco volcĂĄnico cuasi-continuo que alberga cerca de 90 volcanes con actividad registrada en el periodo Holoceno (SERNAGEOMIN, 2019). Asimismo, en los Andes chilenos existe una marcada segmentaciĂłn longitudinal en el espesor de la corteza, variando desde 70 km en los Andes Centrales hasta espesores menores a 40 km en los Andes PatagĂłnicos (Tassara y Yañez, 2003). Estas caracterĂ­sticas propician la variabilidad en los procesos de generaciĂłn de magmas y, por ende, favorecen la heterogeneidad en estilos eruptivos, magnitudes y peligros volcĂĄnicos. En los Ășltimos 50 años en Chile, han ocurrido cerca de 20 erupciones de Índice de Explosividad VolcĂĄnica (IEV) superior a 2, incluyendo las erupciones plinianas del volcĂĄn Hudson del año 1991 y del volcĂĄn ChaitĂ©n entre los años 2008 y 2009, que tuvieron IEV superiores a 4. Adicional a esto, numerosas explosiones y/o emisiones menores de IEV 0-1 se han registrado en volcanes con actividad frecuente, tales como el Villarrica, Llaima, LĂĄscar, PlanchĂłn-Peteroa, Copahue y Nevados de ChillĂĄn. Sumado a la alta frecuencia eruptiva, un 20% de la poblaciĂłn de Chile estĂĄ expuesta al impacto directo de procesos volcĂĄnicos, la cual se concentra mayoritariamente en la zona centro-sur del paĂ­s (desde Santiago hasta Puerto Montt) (Lara et al., 2011). Sin embargo, la afectaciĂłn por peligros volcĂĄnicos aumenta ampliamente al considerar que elementos de importancia polĂ­tico-administrativa (ej. rutas de conectividad regional, nacional e internacional), econĂłmica y social se encuentran en zonas de alta susceptibilidad. Este escenario de actividad volcĂĄnica y exposiciĂłn, representa un desafĂ­o para la evaluaciĂłn de peligros volcĂĄnicos en el corto y largo plazo, por lo que en este trabajo se desarrolla una revisiĂłn enfocada en la evoluciĂłn de la cartografĂ­a de peligros volcĂĄnicos en Chile, sus variaciones metodolĂłgicas, la diversificaciĂłn de productos y los desafĂ­os futuros en el contexto nacional

    A population of luminous accreting black holes with hidden mergers

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    Major galaxy mergers are thought to play an important part in fuelling the growth of supermassive black holes. However, observational support for this hypothesis is mixed, with some studies showing a correlation between merging galaxies and luminous quasars and others showing no such association. Recent observations have shown that a black hole is likely to become heavily obscured behind merger-driven gas and dust, even in the early stages of the merger, when the galaxies are well separated (5 to 40 kiloparsecs). Merger simulations further suggest that such obscuration and black-hole accretion peaks in the final merger stage, when the two galactic nuclei are closely separated (less than 3 kiloparsecs). Resolving this final stage requires a combination of high-spatial-resolution infrared imaging and high-sensitivity hard-X-ray observations to detect highly obscured sources. However, large numbers of obscured luminous accreting supermassive black holes have been recently detected nearby (distances below 250 megaparsecs) in X-ray observations. Here we report high-resolution infrared observations of hard-X-ray-selected black holes and the discovery of obscured nuclear mergers, the parent populations of supermassive-black-hole mergers. We find that obscured luminous black holes (bolometric luminosity higher than 2x10^44 ergs per second) show a significant (P<0.001) excess of late-stage nuclear mergers (17.6 per cent) compared to a sample of inactive galaxies with matching stellar masses and star formation rates (1.1 per cent), in agreement with theoretical predictions. Using hydrodynamic simulations, we confirm that the excess of nuclear mergers is indeed strongest for gas-rich major-merger hosts of obscured luminous black holes in this final stage.Comment: To appear in the 8 November 2018 issue of Nature. This is the authors' version of the wor

    Effects of gravitational-wave recoil on the dynamics and growth of supermassive black holes

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    Simulations of binary black hole mergers indicate that asymmetrical gravitational wave (GW) emission can cause black holes to recoil at speeds up to thousands of km/s. These GW recoil events can dramatically affect the coevolution of recoiling supermassive black holes (SMBHs) and their host galaxies. However, theoretical studies of SMBH-galaxy evolution almost always assume a stationary central black hole. In light of the numerical results on GW recoil velocities, we relax that assumption here and consider the consequences of recoil for SMBH evolution. We follow the trajectories of SMBHs ejected in a smooth background potential that includes both a stellar bulge and a multi-component gaseous disk. In addition, we calculate the accretion rate onto the SMBH as a function of time using a hybrid prescription of viscous (alpha-disk) and Bondi accretion. We find that recoil kicks between 100 km/s and the escape speed cause SMBHs to wander through the galaxy and halo for about 1 Myr - 1 Gyr before settling back to the galactic center. However, the mass accreted during this time is roughly constant at about 10% of the initial mass, independent of the recoil velocity. This indicates that while large recoils may disrupt active galactic nuclei feedback processes, recoil itself is an effective means of regulating SMBH growth. Recoiling SMBHs may be observable as spatially or kinematically offset quasars, but finding such systems could be challenging, because the largest offsets correspond to the shortest quasar lifetimes.Comment: 16 pages, 15 figures, minor revisions made, accepted to MNRA

    Measurement of the top quark-pair production cross section with ATLAS in pp collisions at \sqrt{s}=7\TeV

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    A measurement of the production cross-section for top quark pairs(\ttbar) in pppp collisions at \sqrt{s}=7 \TeV is presented using data recorded with the ATLAS detector at the Large Hadron Collider. Events are selected in two different topologies: single lepton (electron ee or muon Ό\mu) with large missing transverse energy and at least four jets, and dilepton (eeee, ΌΌ\mu\mu or eΌe\mu) with large missing transverse energy and at least two jets. In a data sample of 2.9 pb-1, 37 candidate events are observed in the single-lepton topology and 9 events in the dilepton topology. The corresponding expected backgrounds from non-\ttbar Standard Model processes are estimated using data-driven methods and determined to be 12.2±3.912.2 \pm 3.9 events and 2.5±0.62.5 \pm 0.6 events, respectively. The kinematic properties of the selected events are consistent with SM \ttbar production. The inclusive top quark pair production cross-section is measured to be \sigmattbar=145 \pm 31 ^{+42}_{-27} pb where the first uncertainty is statistical and the second systematic. The measurement agrees with perturbative QCD calculations.Comment: 30 pages plus author list (50 pages total), 9 figures, 11 tables, CERN-PH number and final journal adde
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