arXiv subjects

Raymundo Baptista

Publications and source records attributed to Raymundo Baptista.

At least 19 recordsLinked to original sources

arXiv2026

The dwarf nova EX Draconis: a short review

EX Draconis (EX Dra) is a long period dwarf nova showing ~2 mag outburst which lasts for ~7 d and recur on a timescale of (20-30) d. Its deep eclipses allows one to trace the changes in surface brightness and radius of its accretion disk along the outburst cycle and to perform critical tests of the predictions of the thermal-viscous disk instability (DI) and the mass transfer outburst (MTO) models proposed to explain dwarf nova outbursts. The results of four critical tests are in clear contradiction with DI while in good agreement with MTO expectations. Furthermore, the observed variations in brightness and outer disk radius throughout EX Dra outbursts are well described by the response of a high-viscosity (alpha = 3-4) accretion disk to events in which the mass transfer rate increases by factors of ~30 for ~7 d, in line with MTO expectations. We further argue that the old expectation of accretion disk theory, alpha <= 1, seems unjustified and contradicts the values derived from dwarf nova outburst decline timescales if they are driven by MTO.

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arXiv2026

The accretion-driven eruption of the recurrent nova T Corona Borealis

T Corona Borealis (T CrB) is a symbiotic recurrent nova with an $\simeq 80$ yr recurrence interval, the eruptions of which occur on top of a $\simeq 15$ yr long high-brightness state. We show that the high-brightness state is best explained as the response of a high-viscosity ($α=3$) accretion disk to a unique event in which the mass transfer rate from the donor star increases by a factor $\simeq 100$, from $\dot{M}\mathrm{(quies)}= 2 \times 10^{-9} M_\odot$ yr$^{-1}$ up to $\dot{M}\mathrm{(out)}= 1.9 \times 10^{-7} M_\odot$ yr$^{-1}$; it can not be a thermal-viscous disk instability outburst neither a steady nuclear burning event. The constraint that the matter accreted onto the white dwarf in between eruptions equals the envelope mass $M_{ig}$ needed to trigger nova eruptions at the observed recurrence interval requires a white dwarf mass of $M_1= 1.29 M_\odot$, a donor star mass of $M_2= 0.7 M_\odot$, and an inclination of $i= 57.3^o$. As the high-brightness state responds for 95% of $M_{ig}$, the nova eruptions of T CrB are induced by accretion events. Without the 15 yr long enhanced mass transfer events, its nova recurrence interval would be significantly longer, $\simeq 5500$ yr. T CrB exhibits a conspicuous decrease in brightness during the 1-2 yr prior to the nova event. We argue that this pre-eruption dip occurs during the convection phase that precedes the nova eruption and is best explained by the slow, accelerated expansion of the accreted envelope (and inner disk radius) at an average velocity of $v_\mathrm{exp}= 0.02$ km s$^{-1}$ over a 2 yr timescale, likely as a consequence of excess heat being increasingly deposited at the accreted layer by thermonuclear reactions before the nova eruption stage.

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arXiv2025

Modeling the high-brightness state of the recurrent nova T CrB as an enhanced mass-transfer event

T~Coronae Borealis is the nearest symbiotic recurrent nova. Twice in the last two centuries, in 1866 and 1946, the accreted material ignited on the surface of the white dwarf via runaway thermonuclear fusion reactions and produced a nova eruption. Both eruptions occurred approximately midway through a transient state of high luminosity. A possible explanation of such a state is a dwarf-nova-like outburst, which may arise from a transient increase in the mass-transfer rate of the donor star. We simulate the response of an accretion disk to an event of enhanced mass-transfer that is ``interrupted'' by a pre-eruption dip associated to the convective phase leading to the thermonuclear runaway, and model the resulting optical light curve using the parameters of the T~CrB binary. Our model represents the first attempt to reproduce the transient high-accretion state. The observed brightening can be satisfactorily reproduced by models of an accretion disk with a viscosity parameter $α= 3$, an event of enhanced mass-transfer with a duration of $Δt = 15$\,yr, and quiescent and high-state mass-transfer rates of $2.0 \times 10^{-9} \, M_\odot$\,yr$^{-1}$ and $1.9 \times 10^{-7} \, M_\odot$\,yr$^{-1}$, respectively, while the pre-eruption dip can be reproduced by the small, accelerated expansion of the inner disk radius, at an average velocity of 0.02\,km\,s$^{-1}$. Our model is also capable of reproducing the observed changes in color of T~CrB throughout the transient event.

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arXiv2024

Mass-transfer Outburts reborn: Modeling the light curve of the dwarf nova EX Draconis

EX Draconis is an eclipsing dwarf nova that shows outbursts with moderate amplitude ($\simeq 2$ mag) and a recurrence timescale of $\simeq 20$-30 d. Dwarf novae outbursts are explained in terms of either a thermal-viscous instability in the disc or an instability in the mass transfer rate of the donor star (MTIM). We developed simulations of the response of accretion discs to events of enhanced mass transfer, in the context of the MTIM, and applied them to model the light curve and variations in the radius of the EX Dra disc throughout the outburst. We obtain the first modeling of a dwarf nova outburst by using $χ^2$ to select, from a grid of simulations, the best-fit parameters to the observed EX Dra outbursts. The observed time evolution of the system brightness and the changes in the radius of the outer disc along the outburst cycle are satisfactorily reproduced by a model of the response of an accretion disc with a viscosity parameter $α= 4.0$ and a quiescent mass transfer rate $\dot{M}_2 (\textrm{quiescence}) = 4.0 \times 10^{16}$ g/s to an event of width $Δt = 6.0 \times 10^5$ s ($\sim 7$ d) where the mass-transfer rate increases to $\dot{M}_2 (\textrm{outburst}) = 1.5 \times 10^{18}$ g/s.

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arXiv2024

Cyclical period changes in cataclysmic variables: a statistical study

We report the results of a statistical study of cyclical period changes in cataclysmic variables (CVs). Assuming the third-body hypothesis as the cause of period changes, we estimate the third-body mass, $m_3$, and its separation from the binary, $a_3$, for 21 CVs showing cyclical period changes from well-sampled observed-minus-calculated diagrams covering more than a decade of observations. The inferred $a_3$ values are independent of the binary orbital period, $P_\mathrm{orb}$, whereas the $m_3$ values increase with $P_\mathrm{orb}$ by an order of magnitude from the shortest period (oldest) to the longest period (youngest) systems, implying significant mass loss from the third body with time. A model for the time evolution of the triple system is not able to simultaneously explain the observed behavior of the $m_3(P_\mathrm{orb})$ and $a_3(P_\mathrm{orb})$ distributions because the combined mass loss from the binary and the third body demands an increase in orbital separation by factors $\sim 140$ as the binary evolves toward shorter $P_\mathrm{orb}$'s, in clear disagreement with the observed distribution. We conclude that the third-body hypothesis is statistically inconsistent and cannot be used to explain cyclical period changes observed in CVs. On the other hand, the diagram of the amplitude of the period change versus the CV donor-star mass is consistent both with the alternative hypothesis that the observed cyclical period changes are a consequence of magnetic activity in the solar-type donor star, and with the standard evolutionary scenario for CVs.

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arXiv2021

Challenging the disk instability model: I -- The case of YZ LMi

Observations of YZ LMi show enhanced emission along the stream trajectory beyond impact at disk rim during outbursts as well as when the quiescent disk is large. We investigated whether these features can be explained in terms of either gas stream overflow or penetration within the frameworks of the disk-instability (DIM) and the mass-transfer instability (MTIM) models of outbursting disks. Gas stream overflow is not possible because the vertical scaleheight of the stream is significantly lower than that of the outer disk and because there is no combination of parameters which enables stream overflow on a larger disk while preventing it on a smaller disk. Stream penetration requires the gas stream to be denser than the outer disk regions. This requirement cannot be met by a low-viscosity DIM disk because its density is significantly larger than that of the gas stream over the whole range of mass transfer rates where the thermal-viscous instability occurs. On the other hand, the high-viscosity MTIM disk has much lower densities which decrease with increasing radius, easily allowing for gas stream penetration during outbursts (when mass transfer rate and stream density increase) as well as in large quiescent disks. The observed features are not consistent with DIM, but can be plausibly explained by MTIM. These results suggest that the outbursts of YZ LMi are the response of a high-viscosity disk to bursts of enhanced mass transfer rate. In this case, the outburst decline timescale of (2-3) d implies a viscosity parameter in the range alpha=3-4.

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arXiv2019

Infrared photometry of the dwarf nova V2051 Ophiuchi: II -- The quiescent accretion disc and its spiral arms

We report the analysis of time-series of infrared $JHK_s$ photometry of the dwarf nova V2051 Oph in quiescence with eclipse mapping techniques to investigate structures and the spectrum of its accretion disc. The light curves after removal of the ellipsoidal variations caused by the mass-donor star show a double-wave modulation signalling the presence of two asymmetric light sources in the accretion disc. Eclipse maps reveal two spiral arms on top of the disc emission, one at $R_1= 0.28\pm 0.02 \,R_\mathrm{L1}$ and the other at $R_2= 0.42\pm 0.02 \,R_\mathrm{L1}$ (where $R_\mathrm{L1}$ is the distance from disc centre to the inner Lagrangian point), which are seen face-on at binary phases consistent with the maxima of the double-wave modulation. The wide open angle inferred for the spiral arms ($θ_s= 21^o \pm 4^o$) suggests the quiescent accretion disc of V2051 Oph has high viscosity. The accretion disc is hot and optically thin in its inner regions ($T_\mathrm{gas}\sim 10-12 \times 10^3\,K$ and surface densities $\sim 10^{-3}-10^{-2}\,g\,cm^{-2}$), and becomes cool and opaque in its outer regions.

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arXiv2018

VVV-WIT-07: another Boyajian's star or a Mamajek's object?

We report the discovery of VVV-WIT-07, an unique and intriguing variable source presenting a sequence of recurrent dips with a likely deep eclipse in July 2012. The object was found serendipitously in the near-IR data obtained by the VISTA Variables in the Vía Láctea (VVV) ESO Public Survey. Our analysis is based on VVV variability, multicolor, and proper motion (PM) data. Complementary data from the VVV eXtended survey (VVVX) as well as archive data and spectroscopic follow-up observations aided in the analysis and interpretation of VVV-WIT-07. A search for periodicity in the VVV Ks-band light curve of VVV-WIT-07 results in two tentative periods at P~322 days and P~170 days. Colors and PM are consistent either with a reddened MS star or a pre-MS star in the foreground disk. The near-IR spectra of VVV-WIT-07 appear featureless, having no prominent lines in emission or absorption. Features found in the light curve of VVV-WIT-07 are similar to those seen in J1407 (Mamajek's object), a pre-MS K5 dwarf with a ring system eclipsing the star or, alternatively, to KIC 8462852 (Boyajian's star), an F3 IV/V star showing irregular and aperiodic dips in its light curve. Alternative scenarios, none of which is fully consistent with the available data, are also briefly discussed, including a young stellar object, a T Tauri star surrounded by clumpy dust structure, a main sequence star eclipsed by a nearby extended object, a self-eclipsing R CrB variable star, and even a long-period, high-inclination X-ray binary.

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arXiv2018

Mapping the accretion disc of the short period eclipsing binary SDSS J0926+3624

We report the analysis of time-series of optical photometry of SDSS J0926+3624 collected with the Liverpool Robotic Telescope between 2012 February and March while the object was in quiescence. We combined our median eclipse timing with those in the literature to revise the ephemeris and confirm that the binary period is increasing at a rate $\dot{P}=(3.2 \pm 0.4)\times 10^{-13} \, s/s$. The light curves show no evidence of either the orbital hump produced by a bright spot at disc rim or of superhumps; the average out-of-eclipse brightness level is consistently lower than previously reported. The eclipse map from the average light curve shows a hot white dwarf surrounded by a faint, cool accretion disc plus enhanced emission along the gas stream trajectory beyond the impact point at the outer disc rim, suggesting the occurrence of gas stream overflow/penetration at that epoch. We estimate a disc mass input rate of $\dot{M}=(9 \pm 1)\times 10^{-12}\,M_\odot \,yr^{-1}$, more than an order of magnitude lower than that expected from binary evolution with conservative mass transfer.

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arXiv2018

Infrared photometry of the dwarf nova V2051 Ophiuchi: I - The mass donor star and the distance

We report the analysis of time-series of infrared $JHK_s$ photometry of the dwarf nova V2051 Oph in quiescence. We modelled the ellipsoidal variations caused by the distorted mass-donor star to infer its $JHK_s$ fluxes. From its infrared colors we estimate a spectral type of $M(8.0\pm 1.5)$ and an equivalent blackbody temperature of $T_\mathrm{BB}=(2700\pm270)\,K$. We used the Barnes & Evans relation to infer a photometric parallax distance of $d_\mathrm{BE}=(102\pm16)$ pc to the binary. At this short distance, the corresponding accretion disc temperatures in outburst are too low to be explained by the disc-instability model for dwarf nova outbursts, underscoring a previous suggestion that the outbursts of this binary are powered by mass-transfer bursts.

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arXiv2016

SOAR observations of the high-viscosity accretion disc of the dwarf nova V4140 Sagitarii in quiescence and in outburst

We report the analysis of 22 B-band light curves of the dwarf nova V4140 Sgr obtained with SOI/SOAR during two nights along the decline of a superoutburst in 2006 Sep 12-24 and in quiescence over 50 days following the superoutburst. Three-dimensional eclipse mapping of the outburst light curves indicates that the accretion disc is elliptical (eccentricity e=0.13) and that superhump maximum occurs when the mass donor star is aligned with the bulge of the elliptical disc. The accretion disc is geometrically thin both in outburst and in quiescence; it fills the primary Roche lobe in outburst and shrinks to about half this size in quiescence. The stability of the eclipse shape, width and depth along quiescence and the derived disc surface brightness distribution indicate that the quiescent accretion disc is in a high-viscosity, steady-state. Flickering mapping of the quiescent data reveal that the low-frequency flickering arises from an azimuthally-extended stream-disc impact region at disc rim and from the innermost disc region, whereas the high-frequency flickering originates in the accretion disc. Assuming the disc-related flickering to be caused by fluctuations in the energy dissipation rate induced by magneto-hydrodynamic turbulence (Gertseema & Achterberg 1992), we find that the quiescent disc viscosity parameter is large (alpha ~ 0.2-0.4) at all radii. The high-viscosity quiescent disc and the inferred low disc temperatures in superoutburst are inconsistent with expectations of the disc-instability model, and lead to the conclusion that the outbursts of V4140 Sgr are powered by mass transfer bursts from its donor star.

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arXiv2015

Eclipse Mapping: Astrotomography of Accretion Discs

The Eclipse Mapping Method is an indirect imaging technique that transforms the shape of the eclipse light curve into a map of the surface brightness distribution of the occulted regions. Three decades of application of this technique to the investigation of the structure, the spectrum and the time evolution of accretion discs around white dwarfs in cataclysmic variables have enriched our understanding of these accretion devices with a wealth of details such as (but not limited to) moving heating/cooling waves during outbursts in dwarf novae, tidally-induced spiral shocks of emitting gas with sub-Keplerian velocities, elliptical precessing discs associated to superhumps, and measurements of the radial run of the disc viscosity through the mapping of the disc flickering sources. This chapter reviews the principles of the method, discusses its performance, limitations, useful error propagation procedures, as well as highlights a selection of applications aimed at showing the possible scientific problems that have been and may be addresses with it.

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arXiv2010

Near-Infrared SOAR Photometric Observations of Post Common Envelope Binaries

{From a number of today known Post Common Envelopes Binaries (PCEB) only a handful has yet been observed at near-infrared (NIR) wavelengths and an even smaller number has modeled NIR light curves. At shorter wavelengths one has access to the cooler and larger components of these systems and has the chance to detect emission from its faint and heavily irradiated atmospheres. } {By modeling NIR light curves of PCEBs we intent to constrain their system parameters and study the properties of the system components.} {Here we present simultaneous NIR $JHK_s$ light curves of two PCEBs obtained with the $4m$ SOAR telescope.} {%For this work we have selected 3 systems from a previously selected sample of 8 PCEBs. KV Vel and TW Crv are long period (P$_{\rm orb} =$ 8.6h and 7.9h, respectively) PCEBs with large irradiation effects. The results of light curve fitting provided solutions with inclination $i = (47\pm5)^\circ$, mass ratio $q = 0.3\pm0.1$ and radius of the secondary $R_2/a = 0.24^{+0.05}_{-0.03}$ (where $a$ is the orbital separation) for KV Vel, and $i = (42\pm9)\degr$, $q = 0.28\pm0.04$ and $R_2/a = 0.22\pm0.01$ for TW Crv, respectively. For KV Vel, we obtain an average value for the albedo of the secondary star of $α= 0.43$, consistent in the $J$, $H$ and $K_s$-bands. For TW Crv, on the other hand, we obtain values of $α_{J} = (0.4\pm0.1)$ and $α_{H} = (0.3\pm0.1)$ for the $J$- and $H$-bands, respectively.

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arXiv2010

Spectral Mapping of the Intermediate Polar DQ Herculis

We report an eclipse mapping study of the intermediate polar DQ Her based on time-resolved optical spectroscopy (~3800-5000A) covering 4 eclipses. Eclipse maps of the HeII 4686 line indicate that an azimuthally-and vertically-extended bright spot at disk rim is important source of reprocessing of x-rays from the magnetic poles. The disk spectrum is flat with no Balmer or Helium lines in the inner regions, and shows double-peaked emission lines in the intermediate and outer disk regions while the slope of the continuum becomes progressively redder with increasing radius. The inferred disk temperatures are in the range T~13500-5000K and can be reasonably well described by a steady-state disk with mass accretion rate of dM/dt=(2.7+/-1.0)x10^-9 Msun/yr. A comparison of the radial intensity distribution for the Balmer lines reveals a linear correlation between the slope of the distribution and the transition energy. The spectrum of the uneclipsed light is dominated by Balmer and HeI lines in emission with narrow absorption cores. The observed narrow and redshifted CaII 3934 absorption line in the total light spectra plus the inverse P-Cygni profiles of the Balmer and HeII 4686 emission lines in spectra of the asymmetric component indicate radial inflow of gas in the innermost disk regions and are best explained in terms of magnetically-controlled accretion inside the white dwarf magnetosphere. We infer projected radial inflow velocities of ~200-500km/s, significantly lower than both the rotational and the free-fall velocities for the corresponding range of radii. A combined net emission HeII plus Hbeta low-velocity eclipse map reveals a twisted dipole emitting pattern near disk center. This is interpreted as being the projection of accretion curtains onto the orbital plane at two specific spin phases, as a consequence of the selection in velocity provided by the spectral eclipse mapping.

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arXiv2007

A two-armed pattern in flickering maps of the nova-like variable UU Aquarii

We report the analysis of a uniform sample of 31 light curves of the nova-like variable UU Aqr with eclipse mapping techniques. The data were combined to derive eclipse maps of the average steady-light component, the long-term brightness changes, and low- and high-frequency flickering components. The long-term variability responsible for the 'low' and 'high' brightness states is explained in terms of the response of a viscous disk to changes of 20-50 per cent in the mass transfer rate from the donor star. Low- and high-frequency flickering maps are dominated by emission from two asymmetric arcs reminiscent of those seen in the outbursting dwarf nova IP Peg, and are similarly interpreted as manifestation of a tidally-induced spiral shock wave in the outer regions of a large accretion disk. The asymmetric arcs are also seen in the map of the steady-light aside of the broad brightness distribution of a roughly steady-state disk. The arcs account for 25 per cent of the steady-light flux and are a long-lasting feature in the accretion disk of UU Aqr. We infer an opening angle of 10+/-3 degrees for the spiral arcs. The results suggest that the flickering in UU Aqr is caused by turbulence generated after the collision of disk gas with the density-enhanced spiral wave in the accretion disk.

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arXiv2004

Eclipse mapping of the flickering sources in the dwarf nova V2051 Ophiuchi

We report on the eclipse mapping analysis of an ensemble of light curves of the dwarf nova V2051 Oph with the aim to study the spatial distribution of its steady-light and flickering sources. The data are combined to derive the orbital dependency of the steady-light and the flickering components at two different brightness levels, named the 'faint' and 'bright' states. The differences in brightness are caused by long-term variations in the mass transfer rate from the secondary star. Eclipse maps of the steady-light show enhanced emission along the ballistic stream trajectory, in a clear evidence of gas stream overflow. We identify two different and independent sources of flickering in V2051 Oph. Low-frequency flickering arises in the overflowing gas stream and is associated to the mass transfer process. It maximum emission occurs at the position of closest approach of the gas stream to the white dwarf, and its spatial distribution changes in response to variations in mass transfer rate. High-frequency flickering originates in the accretion disk, showing a radial distribution similar to that of the steady-light maps and no evidence of emission from the hot spot, gas stream or white dwarf. This disk flickering component has a relative amplitude of about 3 per cent of the steady disk light, independent of disk radius and brightness state. If the disk flickering is caused by fluctuations in the energy dissipation rate induced by MHD turbulence, its relative amplitude lead to a viscosity parameter alpha= 0.1-0.2 at all radii for the quiescent disk. This value seems uncomfortably high to be accommodated by the disk instability model [abridged].

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arXiv2004

What can we learn from Accretion Disc Eclipse Mapping experiments?

The accretion disc eclipse mapping method is an astrotomographic inversion technique that makes use of the information contained in eclipse light curves to probe the structure, the spectrum and the time evolution of accretion discs in cataclysmic variables. This paper presents examples of eclipse mapping results that have been key to improve our understanding of accretion physics.

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arXiv2002

Multicolor flickering studies of X1822-371

We report on the analysis of high-speed multicolor photometry of the eclipsing X-ray binary X1822-371. We used new eclipse timings to derive a revised optical ephemeris. A quadratic fit to the eclipse timings is not statistically significant but suggests that the orbital period is increasing on a timescale of P/|Pdot|= (4.2 +/- 1.4) 10^6 yr. We find no systematic delay or advance of the optical timings with respect to the X-ray timings. Average UBVRI light curves show the deep eclipse of the disc by the secondary star superimposed on the broader and shallower occultation of the inner disc regions by the outer disc (dip), and an orbital hump centred at phase +0.25 which is mostly seen in the U and B bands. The starting phase of the dip occurs earlier for shorter wavelengths, while the egress occurs at the same phase in all bands. This suggests that the thickening of the outer, occulting disc rim is gradual with azimuth at ingress but decreases sharply at egress. We fit synthetic photometry to the extracted colors of the inner and outer disc regions to estimate their effective temperatures. We find Teff= (9+/-5) 10^7 K and Teff= (6+/-2) 10^4 K, respectively, for the inner and outer disc regions. The orbital dependency of the flickering activity is derived from the mean scatter of the individual light curves with respect to the average UBVRI light curves. The flickering curves show a broad eclipse at the dipping phases, the depth of which decreases with increasing wavelength. The blue, eclipsed flickering component is associated with the inner disc regions and can be fitted by a blackbody spectrum of Teff= (2.1+/-0.8) 10^8 K, whereas the uneclipsed flickering component probably arises from the outermost disc regions and is well described by a blackbody of Teff= (9.6+/-0.7) 10^3 K.

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