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At least 91 records · Page 5
All sourcesNASA NTRSDOE OSTI
NASA NTRS1983

Remarkable modification of light curves for shadowing effects on irregular surfaces - The case of the asteroid 37 Fides

Photometric observations of the asteroid 37 Fides carried out at four observatories are reported. The data were taken at phase angles ranging from approximately 2-23 deg. A composite of the light curves obtained revealed that, considering a period of 7.33 hr, the full cycle light curve exhibits one maximum and one minimum. An increase in brightness was observed following a primary minimum, a factor that indicated the cyclical appearance and disappearance of a large topographical feature. It is concluded that the 7.33 hr period is the rotational speed of the asteroid, in contrast with a previously held 14.66 hr rotational period. The method is concluded useful for identifying rotational periods of asteroids when only partial light curve data from different sources is available, and when the projections can be checked observationally.

Zappala, V.↗
NASA NTRS2012

The Prompt-afterglow Connection in Gamma-ray Bursts: a Comprehensive Statistical Analysis of Swift X-ray Light-curves

We present a comprehensive statistical analysis of Swift X-ray light-curves of Gamma- Ray Bursts (GRBs) collecting data from more than 650 GRBs discovered by Swift and other facilities. The unprecedented sample size allows us to constrain the rest-frame X-ray properties of GRBs from a statistical perspective, with particular reference to intrinsic time scales and the energetics of the different light-curve phases in a common rest-frame 0.3-30 keV energy band. Temporal variability episodes are also studied and their properties constrained. Two fundamental questions drive this effort: i) Does the X-ray emission retain any kind of "memory" of the prompt γ-ray phase? ii) Where is the dividing line between long and short GRB X-ray properties? We show that short GRBs decay faster, are less luminous and less energetic than long GRBs in the X-rays, but are interestingly characterized by similar intrinsic absorption. We furthermore reveal the existence of a number of statistically significant relations that link the X-ray to prompt γ-ray parameters in long GRBs; short GRBs are outliers of the majority of these 2-parameter relations. However and more importantly, we report on the existence of a universal 3-parameter scaling that links the X-ray and the γ-ray energy to the prompt spectral peak energy of both long and short GRBs: E(sub X,iso)∝ E(sup 1.00+/-0.06)(sub γ,iso) /E(sup 0.60+/-0.10)(sub pk).

Gamma-Ray Bursts↗
NASA NTRS2012

Light Curve Observations of Upper Stages in the Low Earth Orbit Environment

Active debris removal (ADR) is a potential means to remediate the orbital debris environment in low Earth orbit (LEO). Massive intact objects, including spent upper stages and retired payloads, with high collision probabilities have been suggested as potential targets for ADR. The challenges to remove such objects on a routine basis are truly monumental. A key piece of information needed for any ADR operations is the tumble motion of the targets. Rapid tumble motion (in excess of one degree per second) of a multiple-ton intact object could be a major problem for proximity and docking operations. Therefore, there is a need to characterize the general tumble motion of the potential ADR targets for future ADR planning. The NASA Orbital Debris Program Office has initiated an effort to identify the global tumble behavior of potential ADR targets in LEO. The activities include optical light curve observations, imaging radar data collection, and laboratory light curve simulations and modeling. This paper provides a preliminary summary of light curve data of more than 100 upper stages collected by two telescope facilities in Colorado and New Mexico between 2011 and 2012. Analyses of the data and implications for the tumble motions of the objects are also discussed in the paper.

Liou, J.-C.↗
NASA NTRS1995

The orbital light curve of PSR 1957 + 20

We make a detailed study of the optical light curve of PSR 1957 + 20. We show that the deep, smooth, and symmetrical modulation can be successfully modeled by a higl irradiated secondary. Two types of models are consistent with the data: (! a secodary close to filling its Rochhe lobe with 10%-20% of the incident flux converted to optical emission, and (2) a secondary considerably underfilling its Roche lobe, with a high degree of beaming of the neutron star flux. owever, the second model canbe rejected on the basis of current estimates of the extinction toward 1957 + 20. The proximity of the photosphere of the secondary to its Roche lobe facilitates mass loss by irradiation. Our ignorance of the (nonirradiated) luminosity of the secondary allows us to place only weak constraints on the inclination -50-80 deg. There is no evidence in the light curve for optical emission from any source in PSR 1957 + 20 other than the secondary itself.

Callanan, Paul J.↗
NASA NTRS2012

A Correlation Between Intrinsic Brightness and Average Decay Rate of Swift UVOT GRB Optical/UV Light Curves

We examine a sample of 48 Swift/UVOT long Gamma-ray Burst light curves and find a correlation between the logarithmic luminosity at 200s and average decay rate determined from 200s onwards, with a Spearman rank coefficient of -0.58 at a significance of 99.998% (4.2 sigma ). We discuss the causes of the log L200s - alpha (greater than) 200s correlation, finding it to be an intrinsic property of long GRBs, and not resulting from the selection criteria. We find two ways to produce the correlation. One possibility is that there is some property of the central engine, outflow or external medium that affects the rate of energy release so that the bright afterglows release their energy more quickly and decay faster than the fainter afterglows. Alternatively, the correlation may be produced by variation of the observers viewing angle, with observers at large viewing angles observing fainter and slower decaying light curves.

Oates, S. R.↗
NASA NTRS2016

Gamma-Ray Pulsar Light Curves as Probes of Magnetospheric Structure

The large number of gamma-ray pulsars discovered by the Fermi Gamma-Ray Space Telescope since its launch in 2008 dwarfs the handful that were previously known. The variety of observed light curves makes possible a tomography of both the ensemble-averaged field structure and the high-energy emission regions of a pulsar magnetosphere. Fitting the gamma-ray pulsar light curves with model magnetospheres and emission models has revealed that most of the high-energy emission, and the particles acceleration, takes place near or beyond the light cylinder, near the current sheet. As pulsar magnetosphere models become more sophisticated, it is possible to probe magnetic field structure and emission that are self-consistently determined. Light curve modeling will continue to be a powerful tool for constraining the pulsar magnetosphere physics.

Harding, A. K.↗
NASA NTRS1992

Light curves of rotating, oscillating neutron stars

A technique has been developed for computing the light curve produced by a rotating, oscillating neutron star that emits radiation from circular polar cap regions, as are thought to exist in pulsars, X-ray binaries, and perhaps X-ray bursters. Several examples of light curves produced by single, low-order (l = 1, 2) oscillation modes are given. A Gaussian beaming function is used to simulate typical radio pulsar beam widths in order to investigate a neutron star oscillation model for subpulse drift in pulsars. X-ray bursts and X-ray pulsars have also been simulated to assess the possibility of detecting such oscillations in these sources with XTE and AXAF.

Strohmayer, T. E.↗
NASA NTRS1985

A new method for determining temperature and emission measure during solar flares from light curves of soft X-ray line fluxes

A method for using the shapes of the light curves of soft X-ray line fluxes observed during the decay phase of solar flares to infer the temperature and emission measure with time is described. Light curves of the November 5, 1980 flare and several additional flares that exhibit breaks are presented, and possible causes for these breaks are discussed. Numerical models that incorporate the effects of temperature changes on the emissivity function are examined, and the model calculations are compared with the line fluxes observed during the November 5, 1980 flare. The model results are used to derive other properties of the flare.

Bornmann, P. L.↗
NASA NTRS1982

Infrared photometry of the X-ray binary 2A 1822-371 - A model for the ultraviolet, optical, and infrared light curve

The X-ray source 2A 1822-371 may be prototypical of a class of accreting close binary systems in which the X-ray emission is diffused by a cloud of ionized material which is associated with the accretion disk. The present investigation provides a report on infrared photometry of 2A 1822-371 in the J (1.25 micrometer), H (1.65 micrometer), and K (2.2 micrometer) bands. A complete 5.57 hr light curve has been obtained at J which makes it possible to extend the investigations of the energy dependence of the ultraviolet and optical curve begun by Mason and Cordova (1982). The work conducted by White and Holt (1982) is extended to the long wavelength light curve by incorporating the influence of emission from the inner and outer rim of the disk and from the companion star. Including such effects, and assuming that reprocessed X rays are the major source of light at wavelengths greater than 1100 A, it is found possible to reproduce the morphology and energy dependence which is observed in the light curve of 2A 1822-371 from the far-ultraviolet to the near-infrared.

Mason, K. O.↗
NASA NTRS2016

Marginalizing Instrument Systematics in HST WFC3 Transit Light Curves

Hubble Space Telescope (HST) Wide Field Camera 3 (WFC3) infrared observations at 1.1-1.7 microns probe primarily the H2O absorption band at 1.4 microns, and have provided low-resolution transmission spectra for a wide range of exoplanets. We present the application of marginalization based on Gibson to analyze exoplanet transit light curves obtained from HST WFC3 to better determine important transit parameters such as "ramp" probability (R (sub p)) divided by "ramp" total (R (sub asterisk)), which are important for accurate detections of H2O. We approximate the evidence, often referred to as the marginal likelihood, for a grid of systematic models using the Akaike Information Criterion. We then calculate the evidence-based weight assigned to each systematic model and use the information from all tested models to calculate the final marginalized transit parameters for both the band-integrated and spectroscopic light curves to construct the transmission spectrum. We find that a majority of the highest weight models contain a correction for a linear trend in time as well as corrections related to HST orbital phase. We additionally test the dependence on the shift in spectral wavelength position over the course of the observations and find that spectroscopic wavelength shifts delta (sub lambda) times lambda) best describe the associated systematic in the spectroscopic light curves for most targets while fast scan rate observations of bright targets require an additional level of processing to produce a robust transmission spectrum. The use of marginalization allows for transparent interpretation and understanding of the instrument and the impact of each systematic evaluated statistically for each data set, expanding the ability to make true and comprehensive comparisons between exoplanet atmospheres.

Wakeford, H. R.↗
NASA NTRS2004

Recovering the rotational light curve of Phoebe

We present rotational light-curve data for Saturn's satellite Phoebe taken over the observing period prior to the Cassini mission's encounter with that moon.

planetsasteroids satellite Phoebe↗
NASA NTRS1993

X-ray light curves of active galactic nuclei are phase incoherent

We compute the Fourier phase spectra for the light curves of five low-luminosity active galactic nuclei observed by EXOSAT. There is no statistically significant phase coherence in any of them. This statement is equivalent, subject to a technical caveat, to a demonstration that their fluctuation statistics are Gaussian. Models in which the X-ray output is controlled wholly by a unitary process undergoing a nonlinear limit cycle are therefore ruled out, while models with either a large number of randomly excited independent oscillation modes or nonlinearly interacting spatially dependent oscillations are favored. We also demonstrate how the degree of phase coherence in light curve fluctuations influences the application of causality bounds on internal length scales.

Krolik, Julian↗
NASA NTRS2022

Probing the High-energy Gamma-Ray Emission Mechanism in the Vela Pulsar via Phase-resolved Spectral and Energy-dependent Light-curve Modeling

Recent kinetic simulations sparked a debate regarding the emission mechanism responsible for pulsed GeV γ-ray emission from pulsars. Some models invoke curvature radiation, while other models assume synchrotron radiation in the current sheet. We interpret the curved spectrum of the Vela pulsar as seen by H.E.S.S. II (up to ∼100 GeV) and the Fermi Large Area Telescope to be the result of curvature radiation due to primary particles in the pulsar magnetosphere and current sheet. We present phase-resolved spectra and energy-dependent light curves using an extended slot gap and current-sheet model, invoking a step function for the accelerating electric field as motivated by kinetic simulations. We include a refined calculation of the curvature radius of particle trajectories in the lab frame, impacting the particle transport, predicted light curves, and spectra. Our model reproduces the decrease of the flux of the first peak relative to the second one, evolution of the bridge emission, near-constant phase positions of peaks, and narrowing of pulses with increasing energy. We can explain the first of these trends because we find that the curvature radii of the particle trajectories in regions where the second γ-ray light-curve peak originates are systematically larger than those associated with the first peak, implying that the spectral cutoff of the second peak is correspondingly larger. However, an unknown azimuthal dependence of the E field, as well as uncertainty in the precise spatial origin of the GeV emission, precludes a simplistic discrimination of emission mechanisms.

Monica Barnard↗
NASA NTRS1989

The late behavior of supernova 1987A. I - The light curve. II - Gamma-ray transparency of the ejecta

Observations of the late (t = 20-1500 days) bolometric light curve and the gamma-lines and X-rays from supernova 1987A are compared to theoretical models. It is found that 0.073 + or - 0.015 solar masses of freshly synthesized Ni-56 must be present to fit the bolometric light curve. The results place limits on the luminosity and presumed period of the newly formed pulsar/neutron star. In the second half of the paper, the problem of computing the luminosities in gamma-ray lines and in X-rays from supernova 1987A is addressed. High-energy observations suggest the development of large-scale clumping and bubbling of radioactive material in the ejecta. A model is proposed with a hydrogen envelope mass of about 7 solar masses, homologous scale expansion velocities of about 3000 km/s, and an approximately uniform mass distribution.

Arnett, W. David↗
NASA NTRS2020

SN2020bvc: A Broad-line Type Ic Supernova with a Double-peaked Optical Light Curve and a Luminous X-Ray and Radio Counterpart

We present optical, radio, and X-ray observations of SN 2020bvc (=ASASSN-20bs, ZTF 20aalxlis), a nearby (z = 0.0252; d = 114Mpc) broad-line (BL) Type Ic supernova (SN) and the first double-peaked Ic-BL discovered without a gamma-ray burst (GRB) trigger. Our observations show that SN 2020bvc shares several properties in common with the Ic-BL SN 2006aj, which was associated with the low-luminosity gamma-ray burst (LLGRB) 060218. First, the 10 GHz radio luminosity (L(radio) ≈ 10^(37) erg/s) is brighter than ordinary core-collapse SNe but fainter than LLGRB SNe such as SN 1998bw (associated with LLGRB 980425). We model our VLA observations (spanning 13–43 days) as synchrotron emission from a mildly relativistic (v ≳ 0.3c) forward shock. Second, with Swift and Chandra, we detect X-ray emission (L(X) ≈ 10^(41) erg/s) that is not naturally explained as inverse Compton emission or part of the same synchrotron spectrum as the radio emission. Third, high-cadence (6 × per night) data from the Zwicky Transient Facility (ZTF) show a double-peaked optical light curve, the first peak from shock cooling of extended low-mass material (mass Me < 10^(-2) Mꙩ at radius Re > 10^(12) cm) and the second peak from the radioactive decay of 56Ni. SN 2020bvc is the first double-peaked Ic-BL SN discovered without a GRB trigger, so it is noteworthy that it shows X-ray and radio emission similar to LLGRB SNe. For four of the five other nearby (z ≲ 0.05) Ic-BL SNe with ZTF high-cadence data, we rule out a first peak like that seen in SN 2006aj and SN 2020bvc, i.e., that lasts ≈1 day and reaches a peak luminosity M ≈ −18. Follow-up X-ray and radio observations of Ic-BL SNe with well-sampled early optical light curves will establish whether double-peaked optical light curves are indeed predictive of LLGRB-like X-ray and radio emission.

Anna Y. Q. Ho↗
NASA NTRS1995

A Simple Model for the Light Curve Generated by a Shoemaker-Levy 9 Impact

The impact of a typical Shoemaker-Levy 9 fragment produced three light peaks as seen from Earth. The first peak is related to the entry of the fragment into the Jovian atmosphere. The second peak occurs when the exploding fireball rises above Jupiter's limb into direct view from Earth. The third peak, much the brightest, occurs when the ejecta plume falls back on the atmosphere. By contrast, Galileo, which had a direct view of the impacts, saw two peaks, one at entry, and one at plumefall. Here we present a simple, highly idealized model of a ballistic plume, which we then use to fit the observed light curve of the R impact as recorded at Mauna Kea and Mount Palomar. From the light curve we find that the nominal R fragment had diameter 450-500 m and mass approx. 2-3 x 10(exp 13) g. The uncertainty in the mass is probably about a factor of 3, with a smaller event more likely than a larger one.

Zahnle, Kevin↗
NASA NTRS1976

An elementary theory of eclipsing depths of the light curve and its application to Beta Lyrae

An elementary theory of the ratio of depths of secondary and primary eclipses of a light curve has been proposed for studying the nature of component stars. It has been applied to light curves of Beta Lyrae in the visual, blue, and far-ultraviolet regions with the purpose of investigating the energy sources for the luminosity of the disk surrounding the secondary component and determining the dominant radiative process in the disk. No trace of the spectrum of primary radiation has been found in the disk. Therefore, it is suggested that LTE is the main radiative process in the disk, which radiates at a temperature of approximately 12,000 K in the portion that undergoes eclipse. A small source corresponding to 14,500 K has also been tentatively detected and may represent a hot spot caused by hydrodynamic flow of matter from the primary component to the disk.

Huang, S.-S.↗