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Browsing UNAHUR by Subject "Ciencias Físicas"
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Publication A non-traditional fluid problem: transition between theoretical models from Stokes’ to turbulent flow(IOP Science, 2018) Salomone, Horacio D.; Olivieri, Néstor A.; Raviola, Lisandro A.; Véliz, Maximiliano EzequielIn the context of fluid mechanics courses, it is customary to consider the problem of a sphere falling under the action of gravity inside a viscous fluid. Under suitable assumptions, this phenomenon can be modelled using Stokes' law and is routinely reproduced in teaching laboratories to determine terminal velocities and fluid viscosities. In many cases, however, the measured physical quantities show important deviations with respect to the predictions deduced from the simple Stokes' model, and the causes of these apparent'anomalies'(for example, whether the flow is laminar or turbulent) are seldom discussed in the classroom. On the other hand, there are various variable-mass problems that students tackle during elementary mechanics courses and which are discussed in many textbooks. In this work, we combine both kinds of problems and analyse—both theoretically and experimentally—the evolution of a system composed of a sphere pulled by a chain of variable length inside a tube filled with water. We investigate the effects of different forces acting on the system such as weight, buoyancy, viscous friction and drag force. By means of a sequence of mathematical models of increasing complexity, we obtain a progressive fit that accounts for the experimental data. The contrast between the various models exposes the strengths and weaknessess of each one. The proposed experience can be useful for integrating concepts of elementary mechanics and fluids, and is suitable as laboratory practice, stressing the importance of the experimental validation of theoretical models and showing the model-building processes in a didactic framework.Publication Breaking the Degeneracy in Magnetic Cataclysmic Variable X-Ray Spectral Modeling Using X-Ray Light Curves(IOP Publishing, 2022) Belloni, Diogo; Luna, Gerardo Juan ManuelWe present an analysis of mock X-ray spectra and light curves of magnetic cataclysmic variables using an upgraded version of the 3D cyclops code. This 3D representation of the accretion flow allows us to properly model total and partial occultation of the postshock region by the white dwarf as well as the modulation of the X-ray light curves due to the phase-dependent extinction of the preshock region. We carried out detailed postshock region modeling in a four-dimensional parameter space by varying the white dwarf mass and magnetic field strength as well as the magnetosphere radius and the specific accretion rate. To calculate the postshock region temperature and density profiles, we assumed equipartition between ions and electrons; took into account the white dwarf gravitational potential, the finite size of the magnetosphere, and a dipole-like magnetic field geometry; and considered cooling by both bremsstrahlung and cyclotron radiative processes. By investigating the impact of the parameters on the resulting X-ray continuum spectra, we show that there is an inevitable degeneracy in the four-dimensional parameter space investigated here, which compromises X-ray continuum spectral fitting strategies and can lead to incorrect parameter estimates. However, the inclusion of X-ray light curves in different energy ranges can break this degeneracy, and it therefore remains, in principle, possible to use X-ray data to derive fundamental parameters of magnetic cataclysmic variables, which represents an essential step toward understanding their formation and evolution.Publication Chandra High Energy Transmission Gratings Spectra of V3890 Sgr(IOP Publishing, 2020) Orio, M.; Drake, J. J.; Ness, J. U.; Behar, E.; Darnley, M. J; Gallagher, J; Kuin, N. P. M; Luna, Gerardo Juan ManuelThe recurrent nova (RN) V3890 Sgr was observed during the seventh day after the onset of its most recent outburst, with the Chandra ACIS-S camera and High Energy Transmission Gratings. A rich emission line spectrum was detected, due to transitions of Fe-L and K-shell ions ranging from neon to iron. The measured absorbed flux is ≈10−10 erg cm−2 s −1 in the 1.4–15 Å range (0.77–8.86 keV). The line profiles are asymmetric, blueshifted, and skewed toward the blue side, as if the ejecta moving toward us are less absorbed than the receding ejecta. The full width at half-maximum of most emission lines is 1000–1200 km s−1 , with some extended blue wings. The spectrum is thermal and consistent with a plasma in collisional ionization equilibrium with column density 1.3 × 1022 cm−2 and at least two components at temperatures of about 1 and 4 keV, possibly a forward and a reverse shock, or regions with differently mixed ejecta and a red giant wind. The spectrum is remarkably similar to the symbiotic RNe V745 Sco and RS Oph, but we cannot distinguish whether the shocks occurred at a distance of a few au from the red giant, or near the giant’s photosphere, in a high-density medium containing only a low mass. The ratios of the flux in lines of aluminum, magnesium, and neon relative to the flux in lines of silicon and iron probably indicate a carbon–oxygen white dwarf.Publication Evolution of the optical emission lines and the X-ray emission during the super-active stage of T CrB(2025) Stoyanov, K.A.; Luna, Gerardo Juan Manuel; Zamanov, R.K.; Ikiewicz, K.I.; Nikolov, Y.M.; Molyseev, M.; Minev, M.; Kurtenkov, A.Publication Expanding Bipolar X-Ray Structure after the 2006 Eruption of RS Oph(IOP Publishing, 2023) Luna, Gerardo Juan ManuelWe report on the detection and analysis of extended X-ray emission by the {\it Chandra} X-ray Observatory stemming from the 2006 eruption of the recurrent novae RS Oph. The extended emission was detected 1254 and 1927 days after the start of the 2006 eruption and is consistent with a bipolar flow oriented in the east-west direction of the sky with opening angles of approximately 70∘. The length of both lobes appeared to expand from 1.3 arcsec in 2009 to 2.0 arcsec in 2011, suggesting a projected expansion rate of 1.1±0.1 mas day−1 and an expansion velocity of 4600 km s−1 (D/2.4 kpc) in the plane of the sky. This expansion rate is consistent with previous estimates from optical and radio observations of material in a similar orientation. The X-ray emission does not show any evidence of cooling between 2009 and 2011, consistent with free expansion of the material. This discovery suggests that some mechanism collimates ejecta away from the equatorial plane, and that after that material passes through the red-giant wind, it expands freely into the cavity left by the 1985 eruption. We expect similar structures to arise from latest eruption and to expand into the cavity shaped by the 2006 eruption.Publication Increasing activity in T CrB suggests nova eruption is impending(IOP Publishing, 2020) Sokoloski, J. L.; Mukai, Koji; Kuin, Paul M.; Luna, Gerardo Juan ManuelEstimates of the accretion rate in symbiotic recurrent novae (RNe) often fall short of theoretical expectations by orders of magnitude. This apparent discrepancy can be resolved if the accumulation of mass by the white dwarf (WD) is highly sporadic, and most observations are performed during low states. Here we use a re-analysis of archival data from the Digital Access to a Sky Century @Harvard survey to argue that the most recent nova eruption in symbiotic RN T CrB, in 1946, occurred during—and was therefore triggered by—a transient accretion high state. Based on similarities in the optical light curve around 1946 and the time of the prior eruption, in 1866, we suggest that the WD in T CrB accumulates most of the fuel needed to ignite the thermonuclear runaways (TNRs) during accretion high states. A natural origin for such states is dwarf-nova like accretion-disk instabilities, which are expected in the presumably large disks in symbiotic binaries. The timing of the TNRs in symbiotic RNe could thus be set by the stability properties of their accretion disks. T CrB is in the midst of an accretion high state like the ones we posit led to the past two nova eruptions. Combined with the approach of the time at which a TNR would be expected based on the 80 yr interval between the prior two novae (2026 ± 3), the current accretion high state increases the likelihood of a TNR occurring in T CrB in the next few years.Publication INTEGRAL View on cataclysmic variables and symbiotic binaries(Elsevier, 2020) Lutovinov, Alexander; Suleimanov, Valery; Sazonov, Sergey; Martino, Domitilla de; Ducci, Lorenzo; Doroshenko, Victor; Falanga, MaurizioAccreting white dwarfs (WDs) constitute a significant fraction of the hard X-ray sources detected by the INTEGRAL observatory. Most of them are magnetic Cataclysmic Variables (CVs) of the intermediate polar (IP) and polar types, but the contribution of the Nova-likes systems and the systems with optically thin boundary layers, Dwarf Novae (DNs) and Symbiotic Binaries (or Symbiotic Stars, SySs) in quiescence is also not negligible. Here we present a short review of the results obtained from the observations of cataclysmic variables and symbiotic binaries by INTEGRAL. The highlight results include the significant increase of the known IP population, determination of the WD mass for a significant fraction of IPs, the establishment of the luminosity function of magnetic CVs, and uncovering origin of the Galactic ridge X-ray emission which appears to largely be associated with hard emission from magnetic CVsPublication K2 & TESS observations of symbiotic X-ray binaries : GX 1+4 and IGR J16194-2810(2023) Luna, Gerardo Juan ManuelPublication Modeling of a transient, high accretion state of the recurrent nova T CrB as a enhanced mass transfer event(2024) Schlindwein, W.; Luna, Gerardo Juan Manuel; Baptista, R.Publication NICER Monitoring of Supersoft X-Ray Sources(IOP Publishing, 2022) Luna, Gerardo Juan ManuelWe monitored four supersoft sources - two persistent ones, CAL 83 and MR Vel, and the recent novae YZ Ret (Nova Ret 2020), and V1674 Her (Nova Her 2021) - with NICER. The two persistent supersoft X-ray sources (SSS) were observed with unvaried X-ray flux level and spectrum, respectively, 13 and 20 yr after the last observations. Short-period modulations of the SSS appear where the spectrum of the luminous central source was fully visible (in CAL 83 and V1674 Her) and were absent in YZ Ret and MR Vel, in which the flux originated in photoionized or shocked plasma, while the white dwarf (WD) was not observable. We thus suggest that the pulsations occur on, or very close to, the WD surface. The pulsations of CAL 83 were almost unvaried after 15 yr, including an irregular drift of the ≃67 s period by 2.1 s. Simulations, including previous XMM-Newton data, indicate actual variations in period length within hours, rather than an artifact of the variable amplitude of the pulsations. Large amplitude pulsations with a period of 501.53 ± 0.30 s were always detected in V1674 Her, as long as the SSS was observable. This period seems to be due to rotation of a highly magnetized WD. We cannot confirm the maximum effective temperature of (≃145,000 K) previously inferred for this nova, and discuss the difficulty in interpreting its spectrum. The WD appears to present two surface zones, one of which does not emit SSS fluxPublication On the applicability of Benford law to exoplanetary and asteroid data(Elsevier, 2021) Melita, Mario Daniel; Miraglia, Jorge EstebanIn this article we investigate the masses, orbital periods, semimajor axis, eccentricities and radii of the existing exoplanets by comparing the first and second digit probabilities with Benford laws’s predictions. It is found that the masses, orbital periods and semimajor axis conform to Benfordós law quite well, but radii fail. It is also investigated the first digits occurrence corresponding to a given order of magnitude. We introduce a top function which can estimate all the probabilities of the first digit order-to-order.Publication Optical spectroscopy and X-ray observations of the D-type symbiotic star EF Aql(Wiley Blackwell, 2020) Stoyanov, K. A.; Ilkiewicz, K.; Mikołajewska, J.; Mukai, K.; Martí, J; Latev, G; Boeva, S.; Zamanov, R. K.; Luna, Gerardo Juan ManuelWe performed high-resolution optical spectroscopy and X-ray observations of the recently identified Mira-type symbiotic star EF Aql. Based on high-resolution optical spectroscopy obtained with the Southern African Large Telescope (SALT), we determine the temperature (∼55 000 K) and the luminosity (∼5.3 L⊙) of the hot component in the system. The heliocentric radial velocities of the emission lines in the spectra reveal possible stratification of the chemical elements.We also estimate the mass-loss rate of the Mira donor star. Our Swift observation did not detect EF Aql in X-rays. The upper limit of the X-ray observations is 10-12 erg cm-2 s-1, whichmeans that EF Aql is consistent with the faintestX-ray systems detected so far. Otherwise we detected it with the UltraViolet and Optical Telescope (UVOT) instrument with an average UVM2 magnitude of 14.05. During the exposure, EF Aql became approximately 0.2 UVM2 magnitudes fainter. The periodogram analysis of the V-band data reveals an improved period of 320.4 ± 0.3 d caused by the pulsations of the Mira-type donor star.Publication Physical properties of Centaur (60558) 174P/Echeclus from stellar occultations(2023) Pereira, C.L.; Braga-Rivas, F.; Sicardy, B.; Gómes-Junior, A.R.; Melita, M.Publication Taking a break : paused accretion in the symbiotic binary RT Cru(2023) Pujol, A.; Luna, Gerardo Juan Manuel; Mukai, K.Publication The contact binary system TYC 7275-1968-1 as seen by optical, UV and X-ray observations(2024) Lima, I.J.; Mattiucci, A.C.; Luna, Gerardo Juan Manuel; Oliveira, A.S.; Rodrigues, C.V.; Palivanas, N.; Nuñez, N.E.Publication The RS Oph outburst of 2021 monitored in X-rays with NICER(2023) Orio, Natalia; Gendreau, Keith; Giese, Morgan; Luna, Gerardo Juan ManuelPublication Transient and asymmetric dust structures in the TeV-bright nova RS Oph revealed by spectropolarimetry(2023) Nikolov, Y.; Luna, Gerardo Juan Manuel; Stoyanov, K.A.; Borisov, G.; Mukai, K.; Sokoloski, J.L.; Avramova-Boncheva, A.Publication V407 Lup, an intermediate polar nova(2024) Orio, Marina; Melicherčík, M.; Ciroi, S.; Luna, Gerardo Juan ManuelPublication X-ray spectra and light curves of cooling novae and a nova like(Wiley Blackwell, 2020) Sun, Bangzheng; Orio, Marina; Dobrotka, Andrej; Shugarov, Sergey; Zemko, Polina; Luna, Gerardo Juan ManuelWe present X-ray observations of novae V2491 Cyg and KT Eri about 9 yr post-outburst of the dwarf nova and post-nova candidate EY Cyg, and of a VY Scl variable. The first three objects were observed with XMM-Newton, KT Eri also with the Chandra ACIS-S camera, V794 Aql with the Chandra ACIS-S camera and High Energy Transmission Gratings. The two recent novae, similar in outburst amplitude and light curve, appear very different at quiescence. Assuming half of the gravitational energy is irradiated in X-rays, V2491 Cyg is accreting at ˙m = 1.4 × 10-9-10-8 M⊙yr-1, while for KT Eri, ˙m < 2 × 10-10 M⊙ yr. V2491 Cyg shows signatures of a magnetized WD, specifically of an intermediate polar. A periodicity of 39 min, detected in outburst, was still measured and is likely due to WD rotation. EY Cyg is accreting at ˙m ∼ 1.8 × 10-11 M⊙ yr-1, one magnitude lower than KT Eri, consistently with its U Gem outburst behaviour and its quiescent UV flux. The X-rays are modulated with the orbital period, despite the system's low inclination, probably due to the X-ray flux of the secondary. A period of 81 min is also detected, suggesting that it may also be an intermediate polar. V794 Aql had low X-ray luminosity during an optically high state, about the same level as in a recent optically low state. Thus, we find no clear correlation between optical and X-ray luminosity: The accretion rate seems unstable and variable. The very hard X-ray spectrum indicates a massive WD.