Computed Light Curves Research Articles

Modeling photometric variations due to a global inhomogeneity on an obliquely rotating star: Application to light curves of white dwarfs

Abstract We develop a general framework to compute photometric variations induced by the oblique rotation of a star with an axisymmetric inhomogeneous surface. We apply the framework to compute light curves of white dwarfs adopting two simple models of their surface inhomogeneity. Depending on the surface model and the location of the observer, the resulting light curve exhibits a departure from a purely sinusoidal curve that is observed for a fraction of white dwarfs. As a specific example, we fit our model to the observed phase-folded light curve of a fast-spinning white dwarf ZTF J190132.9+145808.7 (with a rotation period of 419 s). We find that the size and obliquity angle of the spot responsible for the photometric variation are Δθs ≈ 60° and θ⋆ ≈ 60° or 90°, respectively, implying an interesting constraint on the surface distribution of the magnetic field on white dwarfs.

Fast Transit Computation Using Tabulated Stellar Intensities

Abstract Limb-darkening laws are convenient parameterizations of the stellar intensity center-to-limb variation, and their use is ubiquitous in eclipse and transit modeling. But they are not “laws” in any sense—they are simple approximations of the real intensity variations, and their limitations are becoming more and more apparent as stellar atmosphere models improve and higher precision data become available. When fitting eclipses and transit light curves, one would ideally like to use model intensities that are based on fundamental stellar parameters such as the mass, radius, and effective temperature of the star, rather than a limb-darkening law representation and its coefficients. This is especially true when attempting to detect higher-order effects such as planetary oblateness, rings, satellites, or atmospheres. However, using model intensities requires numerically integrating many small-area “tiles” on the model stellar surface(s), and this has traditionally been too computationally expensive for general use. Here we present a fast technique to compute light curves and the Rossiter–McLaughlin effect that uses tabulated stellar models intensities. This is a step in the development of tools that obviate the need for limb-darkening laws.

Monte Carlo simulations of fast Newtonian and mildly relativistic shock breakout from a stellar wind

ABSTRACT Strong explosion of a compact star surrounded by a thick stellar wind drives a fast (>0.1c) radiation mediated shock (RMS) that propagates in the wind, and ultimately breaks out gradually once photons start escaping from the shock transition layer. In exceptionally strong or aspherical explosions, the shock velocity may even be relativistic. The properties of the breakout signal depend on the dynamics and structure of the shock during the breakout phase. Here we present, for the first time, spectra and light curves of the breakout emission of fast Newtonian and mildly relativistic shocks, that were calculated using self-consistent Monte Carlo simulations of finite RMS with radiative losses. We find a strong dependence of the νFν peak on shock velocity, ranging from ∼1 keV for vs/c = 0.1 to ∼100 keV for vs/c = 0.5, with a shift to lower energies as losses increase. For all cases studied the spectrum below the peak exhibits a nearly flat component (Fν ∼ ν0) that extends down to the break frequency below which absorption becomes important. This implies much bright optical/ultraviolet emission than hitherto expected. The computed light curves show a gradual rise over tens to hundreds of seconds for representative conditions. The application to SN 2008D/XRT 080109 and the detectability limits are also discussed. We predict a detection rate of about one per year with eROSITA.

Fast Computational Convolution Methods for Extended Source Effects in Microlensing Light Curves

Abstract Extended source effects can be seen in gravitational lensing events when sources cross critical lines. Such events probe the stellar intensity profile and could be used to measure limb-darkening coefficients to test stellar model predictions. A database of accurately measured stellar profiles is needed to correctly subtract the stellar flux in planetary transient events. The amount of data that is being produced and that will be produced in current and future microlensing surveys, from both space and ground, requires algorithms that can quickly compute light curves for different source-lens configurations. Based on the convolution method, we describe a general formalism to compute those curves for single lenses. We develop approximations in terms of quadratures of elliptic integrals that we integrate by solving the associated first-order differential equations. We construct analytic solutions for a limb darkening and, for the first time, for a parabolic profile that are accurate at the ∼1%–3% and 0.5% level, respectively. These solutions can be computed orders of magnitude faster than other integration routines. They can be implemented in pipelines processing large data sets to extract stellar parameters in real time.

Gray Radiation Hydrodynamics with the FLASH Code for Astrophysical Applications

Abstract We present the newly incorporated gray radiation hydrodynamics capabilities of the FLASH code based on a radiation flux-limiter-aware hydrodynamics numerical implementation designed specifically for applications in astrophysical problems. The implemented numerical methods consist of changes in the unsplit hydrodynamics solver and adjustments in the flux-limited radiation diffusion unit. Our approach can handle problems in both the strong and weak radiation–matter coupling limits, as well as transitions between the two regimes. Appropriate extensions in the “Helmholtz” equation of state are implemented to treat two-temperature astrophysical plasmas involving the interaction between radiation and matter and the addition of a new opacity unit based on the OPAL opacity database, commonly used for astrophysical fluids. A set of radiation-hydrodynamics test problems is presented aiming to showcase the new capabilities of FLASH and to provide direct comparison to other similar software instruments available in the literature. To illustrate the capacity of FLASH to simulate phenomena occurring in stellar explosions, such as shock breakout, radiative precursors, and supernova ejecta heating due to the decays of radioactive 56Ni and 56Co, we also present 1D supernova simulations and compare the computed light curves to those of the SNEC code. The latest public release of FLASH with these enhanced capabilities is available for download and use by the broader astrophysics community.

Numerical simulations of the jetted tidal disruption event Swift J1644+57

In this work we focus on the technical details of the numerical simulations of the non-thermal transient Swift J1644+57, whose emission is probably produced by a two- component jet powered by a tidal disruption event. In this context we provide details of the coupling between the relativistic hydrodynamic simulations and the radiative transfer code. First, we consider the technical demands of one-dimensional simulations of a fast relativistic jet, and show to what extent (for the same physical parameters of the model) do the computed light curves depend on the numerical parameters of the different codes employed. In the second part we explain the difficulties of computing light curves from axisymmetric two dimensonal simulations and discuss a procedure that yields an acceptable tradeoff between the computational cost and the quality of the results.

Revisiting the rigidly rotating magnetosphere model for σ Ori E – II. Magnetic Doppler imaging, arbitrary field RRM, and light variability★

The initial success of the Rigidly Rotating Magnetosphere (RRM) model application to the B2Vp star sigma Ori E by Townsend, Owocki and Groote triggered a renewed era of observational monitoring of this archetypal object. We utilize high-resolution spectropolarimetry and the magnetic Doppler imaging (MDI) technique to simultaneously determine the magnetic configuration, which is predominately dipolar, with a polar strength Bd = 7.3–7.8 kG and a smaller non-axisymmetric quadrupolar contribution, as well as the surface distribution of abundance of He, Fe, C, and Si. We describe a revised RRM model that now accepts an arbitrary surface magnetic field configuration, with the field topology from the MDI models used as input. The resulting synthetic H alpha emission and broad-band photometric observations generally agree with observations, however, several features are poorly fit. To explore the possibility of a photospheric contribution to the observed photometric variability, the MDI abundance maps were used to compute a synthetic photospheric light curve to determine the effect of the surface inhomogeneities. Including the computed photospheric brightness modulation fails to improve the agreement between the observed and computed photometry. We conclude that the discrepancies cannot be explained as an effect of inhomogeneous surface abundance. Analysis of the UV light variability shows good agreement between observed variability and computed light curves, supporting the accuracy of the photospheric light variation calculation. We thus conclude that significant additional physics is necessary for the RRM model to acceptably reproduce observations of not only sigma Ori E, but also other similar stars with significant stellar wind-magnetic field interactions.

A “BOOSTED FIREBALL” MODEL FOR STRUCTURED RELATIVISTIC JETS

We present a model for relativistic jets which generates a particular angular distribution of Lorentz factor and energy per solid angle. We consider a fireball with specific internal energy E/M launched with bulk Lorentz factor \gamma_B. This "boosted fireball" model is motivated by the phenomenology of collapsar jets, but is applicable to a wide variety of relativistic flows. In its center-of-momentum frame the fireball expands isotropically, converting its internal energy into radially expanding flow with asymptotic Lorentz factor \eta_0 ~ E/M. In the lab frame the flow is beamed, expanding with Lorentz factor \Gamma = 2 \eta_0 \gamma_B in the direction of its initial bulk motion and with characteristic opening angle \theta_0 ~ 1/\gamma_B. The flow is jet-like with \Gamma \theta_0 ~ 2 \eta_0 such that jets with \Gamma > 1/\theta_0 are naturally produced. The choice \eta_0 ~ \gamma_B ~ 10 yields a jet with \Gamma ~ 200 on-axis and angular structure characterized by opening angle \theta_0 ~ 0.1 of relevance for cosmological GRBs, while \gamma_B >~ 1 may be relevant for low-luminosity GRBs. The model produces a family of outflows, of relevance for different relativistic phenomena with structures completely determined by \eta_0 and \gamma_B. We calculate the energy per unit solid angle for the model and use it to compute light curves for comparison with the widely used top-hat model. The jet break in the boosted fireball light curve is greatly subdued when compared to the top-hat model because the edge of the jet is smoother than for a top-hat. This may explain missing jet breaks in afterglow light curves.

GAMMA-RAY BURSTS IN CIRCUMSTELLAR SHELLS: A POSSIBLE EXPLANATION FOR FLARES

It is now generally accepted that long-duration gamma ray bursts (GRBs) are due to the collapse of massive rotating stars. The precise collapse process itself, however, is not yet fully understood. Strong winds, outbursts, and intense ionizing UV radiation from single stars or strongly interacting binaries are expected to destroy the molecular cloud cores that give birth to them and create highly complex circumburst environments for the explosion. Such environments might imprint features on GRB light curves that uniquely identify the nature of the progenitor and its collapse. We have performed numerical simulations of realistic environments for a variety of long-duration GRB progenitors with ZEUS-MP, and have developed an analytical method for calculating GRB light curves in these profiles. Though a full, three-dimensional, relativistic magnetohydrodynamical computational model is required to precisely describe the light curve from a GRB in complex environments, our method can provide a qualitative understanding of these phenomena. We find that, in the context of the standard afterglow model, massive shells around GRBs produce strong signatures in their light curves, and that this can distinguish them from those occurring in uniform media or steady winds. These features can constrain the mass of the shell and the properties of the wind before and after the ejection. Moreover, the interaction of the GRB with the circumburst shell is seen to produce features that are consistent with observed X-ray flares that are often attributed to delayed energy injection by the central engine. Our algorithm for computing light curves is also applicable to GRBs in a variety of environments such as those in high-redshift cosmological halos or protogalaxies, both of which will soon be targets of future surveys such as JANUS or Lobster.

A MODEL FOR THERMAL PHASE VARIATIONS OF CIRCULAR AND ECCENTRIC EXOPLANETS

We present a semi-analytic model atmosphere for close-in exoplanets that captures the essential physics of phase curves: orbital and viewing geometry, advection, and re-radiation. We calibrate the model with the well-characterized transiting planet, HD 189733b, then compute light curves for seven of the most eccentric transiting planets. We present phase variations for a variety of different radiative times and wind speeds. In the limit of instant re-radiation, the light curve morphology is entirely dictated by the planet's eccentricity and argument of pericenter: the light curve maximum leads or trails the eclipse depending on whether the planet is receding from or approaching the star at superior conjunction, respectively. For a planet with non-zero radiative timescales, the phase peak occurs early for super- rotating winds, and late for sub-rotating winds. We find that for a circular orbit, the timing of the phase variation maximum with respect to superior conjunction indicates the direction of the dominant winds, but cannot break the degeneracy between wind speed and radiative time. For circular planets the phase minimum occurs half an orbit away from the phase maximum -despite the fact that the coolest longitudes are always near the dawn terminator- and therefore does not convey any additional information. In general, increasing the advective frequency or the radiative time has the effect of reducing the peak-to-trough amplitude of phase variations, but there are interesting exceptions to these trends. Lastly, eccentric planets with orbital periods significantly longer than their radiative time exhibit "ringing" whereby the hot spot generated at periastron rotates in and out of view. The existence of ringing makes it possible to directly measure the wind speed (the frequency of the ringing) and the radiative time constant (the damping of the ringing).

Radiative properties of highly magnetized isolated neutron star surfaces and approximate treatment of absorption features in their spectra

In the X-ray spectra of most X-ray dim isolated neutron stars (XDINSs) absorption features with equivalent widths (EWs) of 50 -- 200 eV are observed. We theoretically investigate different models to explain absorption features and compare their properties with the observations. We consider various theoretical models for the magnetized neutron star surface: naked condensed iron surfaces and partially ionized hydrogen model atmospheres, including semi-infinite and thin atmospheres above a condensed surface. The properties of the absorption features (especially equivalent widths) and the angular distributions of the emergent radiation are described for all models. A code for computing light curves and integral emergent spectra of magnetized neutron stars is developed. We assume a dipole surface magnetic field distribution with a possible toroidal component and corresponding temperature distribution. A model with two uniform hot spots at the magnetic poles can also be employed. Light curves and spectra of highly magnetized neutron stars with parameters typical for XDINSs are computed using different surface temperature distributions and various local surface models. Spectra of magnetized model atmospheres are approximated by diluted blackbody spectra with one or two Gaussian lines having parameters, which allow us to describe the model absorption features. To explain the prominent absorption features in the soft X-ray spectra of XDINSs a thin atmosphere above the condensed surface can be invoked, whereas a strong toroidal magnetic field component on the XDINS surfaces can be excluded.

Bursting SN 1996cr's bubble: hydrodynamic and X-ray modelling of its circumstellar medium

SN1996cr is one of the five closest SNe to explode in the past 30 years. Due to its fortuitous location in the Circinus Galaxy at ~ 3.7 Mpc, there is a wealth of recently acquired and serendipitous archival data available to piece together its evolution over the past decade, including a recent 485 ks Chandra HETG spectrum. In order to interpret this data, we have explored hydrodynamic simulations, followed by computations of simulated spectra and light curves under non-equilibrium ionization conditions, and directly compared them to the observations. Our simulated spectra manage to fit both the X-ray continuum and lines at 4 epochs satisfactorily, while our computed light curves are in good agreement with additional flux-monitoring data sets. These calculations allow us to infer the nature and structure of the circumstellar medium, the evolution of the SN shock wave, and the abundances of the ejecta and surrounding medium. The data imply that SN 1996cr exploded in a low-density medium before interacting with a dense shell of material about 0.03pc away from the progenitor star. We speculate that the shell could be due to the interaction of a blue supergiant or Wolf-Rayet wind with a previously existing red supergiant (RSG) wind. The shock wave has now exited the shell and is expanding in the medium exterior to it, possibly the undisturbed continuation of the dense RSG wind. The narrow lines that earned SN 1996cr its IIn designation possibly arise from dense, shocked clumps in the CSM. Although the possibility for an LBV progenitor for this Type IIn SN cannot be completely excluded, it is inconsistent with much of the data. These calculations allow us to probe the stellar mass loss in the very last phases ($ < 10^4$ years) of a massive star's life ($> 10^6$ years), and provide another means to deducing the progenitor of the SN.

Bimodal brightness oscillations in models of young binary systems

We consider a model for the cyclic activity of young binary stars that accrete matter from the remnants of a protostellar cloud. If the orbit of such a binary system is inclined at a small angle to the line of sight, then the streams of matter and the density waves excited in the circumbinary disk can screen the primary component of the binary from the observer. To study these phenomena by the SPH (smoothed particle hydrodynamics) method, we have computed grids of hydrodynamic models for binary systems based on which we have constructed the light curves as a function of the orbital phase.The main emphasis is on investigating the properties of the brightness oscillations. Therefore, the model parameters were varied within the following ranges: the component mass ratio q = M2 : M1 = 0.2 - 0.5 and the eccentricity e = 0 - 0.7. The parameter that defined the binary viscosity was also varied. We adopted optical grain characteristics typical of circumstellar dust. Our computations have shown that bimodal oscillations are excited in binaries with eccentric orbits, provided that the binary components do not differ too much in mass. In this case, the ratios of the periods and amplitudes of the bimodal oscillations and their shape depend strongly on the inclination of the binary plane and its orientation relative to the observer. Our analysis shows that the computed light curves can be used in interpreting the cyclic activity of UX Ori stars.

THE DYNAMICS AND AFTERGLOW RADIATION OF GAMMA-RAY BURSTS. I. CONSTANT DENSITY MEDIUM

Direct multidimensional numerical simulation is the most reliable approach for calculating the fluid dynamics and observational signatures of relativistic jets in gamma-ray bursts (GRBs). We present a two-dimensional relativistic hydrodynamic simulation of a GRB outflow during the afterglow phase, which uses the fifth-order weighted essentially nonoscillatory scheme and adaptive mesh refinement. Initially, the jet has a Lorentz factor of 20. We have followed its evolution up to 150 years. Using the hydrodynamic data, we calculate synchrotron radiation based upon standard afterglow models and compare our results with previous analytic work. We find that the sideways expansion of a relativistic GRB jet is a very slow process and previous analytic works have overestimated its rate. In our computed light curves, a very sharp jet break is seen and the postbreak light curves are steeper than analytic predictions. We find that the jet break in GRB afterglow light curves is mainly caused by the missing flux when the edge of the jet is observed. The outflow becomes nonrelativistic at the end of the Blandford–McKee phase. But it is still highly nonspherical, and it takes a rather long time for it to become a spherical Sedov–von Neumann–Taylor blast wave. We find that the late-time afterglows become increasingly flatter over time. But we disagree with the common notion that there is a sudden flattening in light curves due to the transition into the Sedov–von Neumann–Taylor solution. We have also found that there is a bump in light curves at very late times (∼1000 days) due to radiation from the counter jet. We speculate that such a counter jet bump might have already been observed in GRB 980703.

Monte Carlo techniques for time-dependent radiative transfer in 3-D supernovae

Monte Carlo techniques based on indivisible energy packets are described for computing light curves and spectra for 3-D supernovae. The radiative transfer is time-dependent and includes all effects of O(v/c). Monte Carlo quantization is achieved by discretizing the initial distribution of 56Ni into radioactive pellets. Each pellet decays with the emission of a single energy packet comprising gamma-ray photons representing one line from either the 56Ni or the 56Co decay spectrum. Subsequently, these energy packets propagate through the homologously-expanding ejecta with appropriate changes in the nature of their contained energy as they undergo Compton scatterings and pure absorptions. The 3-D code is tested by applying it to a spherically-symmetric SN in which the transfer of optical radiation is treated with a grey absorption coefficient. This 1-D problem is separately solved using Castor's co-moving frame moment equations. Satisfactory agreement is obtained. The Monte Carlo code is a platform onto which more advanced treatments of the interactions of matter and radiation can be added. Some of these have already been developed and tested in previous papers and are summarized here.

Analytic Light Curves for Planetary Transit Searches

We present exact analytic formulae for the eclipse of a star described by quadratic or nonlinear limb darkening. In the limit that the planet radius is less than a tenth of the stellar radius, we show that the exact light curve can be well approximated by assuming the region of the star blocked by the planet has constant surface brightness. We apply these results to the Hubble Space Telescope observations of HD 209458, showing that the ratio of the planetary to stellar radii is 0.1207 ± 0.0003. These formulae give a fast and accurate means of computing light curves using limb-darkening coefficients from model atmospheres that should aid in the detection, simulation, and parameter fitting of planetary transits.

Burning regimes for thermonuclear supernovae and cosmological applications of SNe Ia

The method of multigroup radiation hydrodynamics is used to compute light curves for thermonuclear supernovae. Opacities are computed by taking into account spectral lines and expansion. UBVI fluxes are predicted. Our computed times of brightness rise to a maximum in B and V have been found to agree with observations better than those of other authors. The validity of our results is justified physically. The nuclear burning regime is shown to affect significantly the slope of the light curve in B and, to a slightly lesser extent, in V. If the prevailing burning regime during supernova explosions changed with age of the Universe, then the conclusion about a positive cosmological constant Λ drawn from observations of Type Ia supernovae may prove to be wrong.

Microlensing of an Elliptical Source by a Point Mass

We present an efficient method for computing light curves of an elliptical source that is microlensed by a point mass. The amplification of an extended source involves a two-dimensional integral over its surface brightness distribution. We show that for a general surface brightness profile with an elliptical symmetry, this integral can be reduced to one dimension. We derive analytical results for the entire light curve in the limit of low-mass (e.g., planetary) lenses and for the wings of all microlensing light curves in general. In both cases, the light curve carries information about deviations of the source from elliptical symmetry, e.g., deviations caused by spots. The method is used to find the amplification of a circular red giant photosphere and an inclined accretion disk. We demonstrate that microlensing of an emission line from a disk can be used to infer the disk velocity structure and surface brightness profile.

Two very similar late-type contact systems - BX and BB Pegasi

view Abstract Citations (15) References (9) Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS Two very similar late-type contact systems : BX and BB Pegasi. Leung, K. -C. ; Zhai, D. ; Zhang, Y. Abstract V and B observations of BX and BB Pegasi secured at Beijing Observatory were analyzed with the Wilson and Devinney method. An extensive search for reliable photometric mass ratios was performed. Both systems were found to be in contact with small degrees of overcontact, 10 percent and 12 percent, for BX and BB Peg, respectively. Their mass ratios were, respectively, 2.80 and 2.81. The eclipse at primary minimum was found to be total for both systems. They have the characteristics of W-type W UMa systems. The computed light curves fit the observations very well for BX Peg. The light curves of BB Peg are very complex: there is large asymmetry in the maxima, and significant discrepancy in the differential eclipse depths (Min I and Min II) between the V and B curves. As a result of these complications, the computed light curves do not fit the observations well. Adopted photometric parameters for BX Peg are similar to those obtained by Kaluzny, who used the Rucinski method. It is believed that adopted photometric parameters for BB Peg are more reliable than those obtained by Cerruti-Sola et al. Publication: The Astronomical Journal Pub Date: March 1985 DOI: 10.1086/113759 Bibcode: 1985AJ.....90..515L Keywords: Astronomical Photometry; Eclipsing Binary Stars; Electrophotometry; Late Stars; Light Curve; Mass Ratios; Astrophysics full text sources ADS | data products SIMBAD (4)

Interpretation of surface features of Europa obtained from occultations by Io

Light curves of occultations of Europa by Io have been used to generate a crude map of abledo features on Europa. The best values currently available for the impact parameters and magnitude ratios for each event have been imposed on our model. Residuals between the observed and computed light curves are interpreted as albedo features on Europa. In order to improve the fit between the observations and the model it became necessary to impose a general polar brightening. The effects of additional albedo features and alternate models are discussed.