arXiv subjects

E. Agol

Publications and source records attributed to E. Agol.

At least 37 records · Page 2Linked to original sources

arXiv2010

APOSTLE Observations of GJ 1214b: System Parameters and Evidence for Stellar Activity

We present three transits of GJ 1214b, observed as part of the Apache Point Observatory Survey of Transit Lightcurves of Exoplanets (APOSTLE). We used APOSTLE r-band lightcurves in conjunction with previously gathered data of GJ 1214b to re-derive system parameters. By using parameters such as transit duration and ingress/egress length we are able to reduce the degeneracies between parameters in the fitted transit model, which is a preferred condition for Markov Chain Monte Carlo techniques typically used to quantify uncertainties in measured parameters. The joint analysis of this multi-wavelength dataset confirms earlier estimates of system parameters including planetary orbital period, the planet-to-star radius ratio and stellar density. We fit the photometric spectralenergy distribution of GJ 1214 to derive stellar luminosity, which we then use to derive its absolute mass and radius. From these derived stellar properties and previously published radial velocity data we were able to refine estimates of the absolute parameters for the planet GJ 1214b. Transit times derived from our study show no evidence for strong transit timing variations. Some lightcurves we present show features that we believe are due to stellar activity. During the first night we observed a rise in the out-of-eclipse flux of GJ 1214 with a characteristic fast-rise exponential decay shape commonly associated with stellar flares. On the second night we observed a minor brightening during transit, which we believe might have been caused by the planet obscuring a star-spot on the stellar disk.

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arXiv2010

The climate of HD 189733b from fourteen transits and eclipses measured by Spitzer

We present observations of seven transits and seven eclipses of the transiting planet system HD 189733 taken with Spitzer IRAC at 8 microns. We use a new correction for the detector ramp variation with a double-exponential function. Our main findings are: (1) an upper limit on the variability of the day-side planet flux of 2.7% (68% confidence); (2) the most precise set of transit times measured for a transiting planet, with an average accuracy of 3 seconds; (3) a lack of transit-timing variations, excluding the presence of second planets in this system above 20% of the mass of Mars in low-order mean-motion resonance at 95% confidence; (4) a confirmation of the planet's phase variation, finding the night side is 64% as bright as the day side, as well as an upper limit on the night-side variability of 17% (68% confidence); (5) a better correction for stellar variability at 8 micron causing the phase function to peak 3.5 hrs before secondary eclipse, confirming that the advection and radiation timescales are comparable at the 8 micron photosphere; (6) variation in the depth of transit, which possibly implies variations in the surface brightness of the portion of the star occulted by the planet, posing a fundamental limit on non-simultaneous multi-wavelength transit absorption measurements of planet atmospheres; (7) a measurement of the infrared limb-darkening of the star, in agreement with stellar atmosphere models; (8) an offset in the times of secondary eclipse of 69 sec, which is mostly accounted for by a 31 sec light travel time delay and 33 sec delay due to the shift of ingress and egress by the planet hot spot; this confirms that the phase variation is due to an offset hot spot on the planet; (9) a retraction of the claimed eccentricity of this system due to the offset of secondary eclipse; and (10) high precision measurements of the parameters of this system.

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arXiv2009

Transit timing analysis of CoRoT-1b

CoRoT, the pioneer space-based transit search, steadily provides thousands of high-precision light curves with continuous time sampling over periods of up to 5 months. The transits of a planet perturbed by an additional object are not strictly periodic. By studying the transit timing variations (TTVs), additional objects can be detected in the system. A transit timing analysis of CoRoT-1b is carried out to constrain the existence of additional planets in the system. We used data obtained by an improved version of the CoRoT data pipeline (version 2.0). Individual transits were fitted to determine the mid-transit times, and we analyzed the derived $O-C$ diagram. N-body integrations were used to place limits on secondary planets. No periodic timing variations with a period shorter than the observational window (55 days) are found. The presence of an Earth-mass Trojan is not likely. A planet of mass greater than $\sim 1$ Earth mass can be ruled out by the present data if the object is in a 2:1 (exterior) mean motion resonance with CoRoT-1b. Considering initially circular orbits: (i) super-Earths (less than 10 Earth-masses) are excluded for periods less than about 3.5 days, (ii) Saturn-like planets can be ruled out for periods less than about 5 days, (iii) Jupiter-like planets should have a minimum orbital period of about 6.5 days.

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arXiv2009

The Sizes of the X-ray and Optical Emission Regions of RXJ1131-1231

We use gravitational microlensing of the four images of the z=0.658 quasar RXJ1131-1231 to measure the sizes of the optical and X-ray emission regions of the quasar. The (face-on) scale length of the optical disk at rest frame 400 nm is 1.3 10^15cm, while the half-light radius of the rest frame 0.3-17 keV X-ray emission is 2.3 10^14cm. The formal uncertainties are factors of 1.6 and 2.0, respectively. With the exception of the lower limit on the X-ray size, the results are very stable against any changes in the priors used in the analysis. Based on the Hbeta line-width, we estimate that the black hole mass is ~10^8 Msun, which corresponds to a gravitational radius of r_g~2 10^13 cm. Thus, the X-ray emission is emerging on scales of ~10r_g and the 400 nm emission on scales of ~70 r_g. A standard thin disk of this size should be significantly brighter than observed. Possible solutions are to have a flatter temperature profile or to scatter a large fraction of the optical flux on larger scales after it is emitted. While our calculations were not optimized to constrain the dark matter fraction in the lens galaxy, dark matter dominated models are favored. With well-sampled optical and X-ray light curves over a broad range of frequencies there will be no difficulty in extending our analysis to completely map the structure of the accretion disk as a function of wavelength.

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arXiv2009

Alien Maps of an Ocean-Bearing World

[Abridged] To simulate the kinds of observations that will eventually be obtained for exoplanets, the Deep Impact spacecraft obtained light curves of Earth at seven wavebands spanning 300-1000 nm as part of the EPOXI mission of opportunity. In this paper we analyze disc-integrated light curves, treating Earth as if it were an exoplanet, to determine if we can detect the presence of oceans and continents. We present two observations each spanning one day, taken at gibbous phases. The rotation of the planet leads to diurnal albedo variations of 15-30%, with the largest relative changes occuring at the reddest wavelengths. To characterize these variations in an unbiased manner we carry out a principal component analysis of the multi-band light curves; this analysis reveals that 98% of the diurnal color changes of Earth are due to only 2 dominant eigencolors. We use the time-variations of these two eigencolors to construct longitudinal maps of the Earth, treating it as a non-uniform Lambert sphere. We find that the spectral and spatial distributions of the eigencolors correspond to cloud-free continents and oceans; this despite the fact that our observations were taken on days with typical cloud cover. We also find that the near-infrared wavebands are particularly useful in distinguishing between land and water. Based on this experiment we conclude that it should be possible to infer the existence of water oceans on exoplanets with time-resolved broadband observations taken by a large space-based coronagraphic telescope.

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arXiv2008

Microlensing variability in the gravitationally lensed quasar QSO 2237+0305 = the Einstein Cross. II. Energy profile of the accretion disk

We present the continuation of our long-term spectroscopic monitoring of the gravitationally lensed quasar QSO 2237+0305. We investigate the chromatic variations observed in the UV/optical continuum of both quasar images A and B, and compare them with numerical simulations to infer the energy profile of the quasar accretion disk. Our procedure combines the microlensing ray-shooting technique with Bayesian analysis, and derives probability distributions for the source sizes as a function of wavelength. We find that the effective caustic crossing timescale is 4.0+/-1.0 months. Using a robust prior on the effective transverse velocity, we find that the source responsible for the UV/optical continuum has an energy profile well reproduced by a power-law R lambda^{zeta} with zeta=1.2+/-0.3, where R is the source size responsible for the emission at wavelength lambda. This is the first accurate, model-independent determination of the energy profile of a quasar accretion disk on such small scales.

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arXiv2008

Microlensing to probe the quasar structure: spectrophotometry of Q2237+0305 and of J1131-1231

We present the main results of the first long-term spectrophotometric monitoring of the ``Einstein cross'' Q2237+0305 and of the single-epoch spectra of the lensed quasar J1131-1231. From October 2004 to December 2006, we find that two prominent microlensing events affect images A & B in Q2237+0305 while images C & D remain grossly unaffected by microlensing on a time scale of a few months. Microlensing in A & B goes with chromatic variations of the quasar continuum. We observe stronger micro-amplification in the blue than in the red part of the spectrum, as expected for continuum emission arising from a standard accretion disk. Microlensing induced variations of the CIII] emission are observed both in the integrated line intensity and profile. Finally, we also find that images C & D are about 0.1-0.3 mag redder than images A & B. The spectra of images A-B-C in J1131-1231 reveal that, in April 2003, microlensing was at work in images A and C. We find that microlensing de-amplifies the continuum emission and the Broad Line Region (BLR) in these images. Contrary to the case of Q2237+0305, we do not find evidence for chromatic microlensing of the continuum emission. On the other hand, we observe that the Balmer and MgII broad line profiles are deformed by microlensing. These deformations imply an anti-correlation between the width of the emission line and the size of the corresponding emitting region. Finally, the differential microlensing of the FeII emission suggests that the bulk of FeII is emitted in the outer parts of the BLR while another fraction of FeII is produced in a compact region.

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arXiv2008

Microlensing variability in the gravitationally lensed quasar Q2237+0305 = the Einstein Cross

We present the results of the first long-term (2.2 years) spectroscopic monitoring of a gravitationally lensed quasar, namely the Einstein Cross Q2237+0305. We spatially deconvolve deep VLT/FORS1 spectra to accurately separate the spectrum of the lensing galaxy from the spectra of the quasar images. Accurate cross-calibration of the observations at 31 epochs from October 2004 to December 2006 is carried out using foreground stars observed simultaneously with the quasar. The quasar spectra are further decomposed into a continuum component and several broad emission lines. We find prominent microlensing events in the quasar images A and B, while images C and D are almost quiescent on a timescale of a few months. The strongest variations are observed in the continuum, and their amplitude is larger in the blue than in the red, consistent with microlensing of an accretion disk. Variations in the intensity and profile of the broad emission lines are also reported, most prominently in the wings of the CIII] and in the center of the CIV emission lines. During a strong microlensing episode observed in quasar image A, the broad component of the CIII] is more magnified than the narrow component. In addition, the emission lines with higher ionization potentials are more magnified than the lines with lower ionization potentials, consistent with the stratification of the broad line region (BLR) infered from reverberation mapping observations.

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arXiv2008

Extending the Model of KH 15D: Estimating the Effects of Forward Scattering and Curvature of the Occulting Ring Edge

The periodic eclipses of the pre-main-sequence binary, KH 15D, have been explained by a circumbinary dust ring inclined to the orbital plane, which causes occultations of the stars as they pass behind the ring edge. We compute the extinction and forward scattering of light by the edge of the dust ring to explain (1) the gradual slope directly preceding total eclipse, (2) the gradual decline at the end of ingress, and (3) the slight rise in flux at mid-eclipse. The size of the forward scattering halo indicates that the dust grains have a radius of a ~ 6 (D/3 AU) microns, where D is the distance of the edge of the ring from the system barycenter. This dust size estimate agrees well with estimates of the dust grain size from polarimetry, adding to the evidence that the ring lies at several AU. Finally, the ratio of the fluxes inside and outside eclipse independently indicates that the ring lies at a few astronomical units.

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arXiv2008

2MASS J01542930+0053266 : A New Eclipsing M-dwarf Binary System

We report on 2MASS J01542930+0053266, a faint eclipsing system composed of two M dwarfs. The variability of this system was originally discovered during a pilot study of the 2MASS Calibration Point Source Working Database. Additional photometry from the Sloan Digital Sky Survey yields an 8-passband lightcurve, from which we derive an orbital period of 2.6390157 +/- 0.0000016 days. Spectroscopic followup confirms our photometric classification of the system, which is likely composed of M0 and M1 dwarfs. Radial velocity measurements allow us to derive the masses (M_1 = 0.66 +/- 0.03 M_sun; M_2 = 0.62 +/- 0.03 M_sun) and radii (R_1 = 0.64 +/- 0.08 R_sun; R_2 = 0.61 +/- 0.09 R_sun) of the components, which are consistent with empirical mass-radius relationships for low-mass stars in binary systems. We perform Monte Carlo simulations of the lightcurves which allow us to uncover complicated degeneracies between the system parameters. Both stars show evidence of H-alpha emission, something not common in early-type M dwarfs. This suggests that binarity may influence the magnetic activity properties of low-mass stars; activity in the binary may persist long after the dynamos in their isolated counterparts have decayed, yielding a new potential foreground of flaring activity for next generation variability surveys.

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arXiv2008

Microlensing variability in the gravitationally lensed quasar Q2237+0305 = the Einstein Cross, I. Spectrophotometric monitoring with the VLT

We present the results of the first long-term (2.2 years) spectroscopic monitoring of a gravitationally lensed quasar, namely the Einstein Cross Q2237+0305. The goal of this paper is to present the observational facts to be compared in follow-up papers with theoretical models to constrain the inner structure of the source quasar. We spatially deconvolve deep VLT/FORS1 spectra to accurately separate the spectrum of the lensing galaxy from the spectra of the quasar images. Accurate cross-calibration of the 58 observations at 31-epoch from October 2004 to December 2006 is carried out with non-variable foreground stars observed simultaneously with the quasar. The quasar spectra are further decomposed into a continuum component and several broad emission lines to infer the variations of these spectral components. We find prominent microlensing events in the quasar images A and B, while images C and D are almost quiescent on a timescale of a few months. The strongest variations are observed in the continuum of image A. Their amplitude is larger in the blue (0.7 mag) than in the red (0.5 mag), consistent with microlensing of an accretion disk. Variations in the intensity and profile of the broad emission lines are also reported, most prominently in the wings of the CIII] and center of the CIV emission lines. During a strong microlensing episode observed in June 2006 in quasar image A, the broad component of the CIII] is more highly magnified than the narrow component. In addition, the emission lines with higher ionization potentials are more magnified than the lines with lower ionization potentials, consistent with the results obtained with reverberation mapping. Finally, we find that the V-band differential extinction by the lens, between the quasar images, is in the range 0.1-0.3 mag.

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arXiv2007

Hot Nights on Extrasolar Planets: Mid-IR Phase Variations of Hot Jupiters

We present results from Spitzer Space Telescope observations of the mid-infrared phase variations of three short-period extrasolar planetary systems: HD 209458, HD 179949 and 51 Peg. We gathered IRAC images in multiple wavebands at eight phases of each planet's orbit. We find the uncertainty in relative photometry from one epoch to the next to be significantly larger than the photon counting error at 3.6 micron and 4.5 micron. We are able to place 2-sigma upper limits of only 2% on the phase variations at these wavelengths. At 8 micron the epoch-to-epoch systematic uncertainty is comparable to the photon counting noise and we detect a phase function for HD 179949 which is in phase with the planet's orbit and with a relative peak-to-trough amplitude of 0.00141(33). Assuming that HD 179949b has a radius R_J < R_p < 1.2R_J, it must recirculate less than 21% of incident stellar energy to its night side at the 1-sigma level (where 50% signifies full recirculation). If the planet has a small Bond albedo, it must have a mass less than 2.4 M_J (1-sigma). We do not detect phase variations for the other two systems but we do place the following 2-sigma upper limits: 0.0007 for 51 Peg, and 0.0015 for HD 209458. Due to its edge-on configuration, the upper limit for HD 209458 translates, with appropriate assumptions about Bond albedo, into a lower limit on the recirculation occuring in the planet's atmosphere. HD 209458b must recirculate at least 32% of incident stellar energy to its night side, at the 1-sigma level, which is consistent with other constraints on recirculation from the depth of secondary eclipse depth at 8 micron and the low optical albedo. These data indicate that different Hot Jupiter planets may experience different recirculation efficiencies.

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arXiv2007

Discovery of Probable Relativistic Fe Emission and Absorption in the Cloverleaf Quasar H 1413+117

We present results from Chandra and XMM-Newton observations of the low-ionization broad absorption line (LoBAL) quasar H 1413+117. Our spatial and spectral analysis of a recent deep Chandra observation confirms a microlensing event in a previous Chandra observation performed about 5 years earlier. We present constraints on the structure of the accretion flow in H 1413+117 based on the time-scale of this microlensing event. Our analysis of the combined spectrum of all the images indicates the presence of two emission peaks at rest-frame energies of 5.35 keV and 6.32 keV detected at the > 98% and > 99% confidence levels, respectively. The double peaked Fe emission line is fit well with an accretion-disk line model, however, the best-fitting model parameters are neither well constrained nor unique. Additional observations are required to constrain the model parameters better and to confirm the relativistic interpretation of the double peaked Fe Kalpha line. Another possible interpretation of the Fe emission is fluorescent Fe emission from the back-side of the wind. The spectra of images C and D show significant high-energy broad absorption features that extend up to rest-frame energies of 9 keV and 15 keV respectively. We propose that a likely cause of these differences is significant variability of the outflow on time-scales that are shorter than the time-delays between the images. The Chandra observation of H 1413+117 has made possible for the first time the detection of the inner regions of the accretion disk and/or wind and the high ionization component of the outflowing wind of a LoBAL quasar.

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arXiv2004

Chandra Observations of the Cloverleaf Quasar H 1413+117: A Unique Laboratory for Microlensing Studies of a Lobal Quasar

We present new results uncovered by a re-analysis of a Chandra observation of the gravitationally lensed, low-ionization broad absorption line (LoBAL) quasar H 1413+117. Previous analyses of the same Chandra observation led to the detection of a strong, redshifted Fe Kalpha line from the combined spectrum of all images. We show that the redshifted Fe Kalpha line is only significant in the brighter image A. The X-ray flux fraction of image A is larger by a factor of 1.55 +/- 0.17 than the optical R-band flux fraction, indicating that image A is significantly enhanced in the X-ray band. We also find that the Fe Kalpha line and the continuum are enhanced by different factors. A microlensing event could explain both the energy-dependent magnification and the significant detection of Fe Kalpha line emission in the spectrum of image A only. In the context of this interpretation we provide constraints on the spatial extent of the inferred scattered continuum and reprocessed Fe Kalpha line emission regions in a LoBAL quasar.

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arXiv2003

Chandra Observations of QSO 2237+0305

We present the observations of the gravitationally lensed system QSO 2237+0305 performed with the Chandra X-ray Observatory on 2000 Sept. 6, and on 2001 Dec. 8 for 30.3 ks and 9.5 ks, respectively. Imaging analysis resolves the four X-ray images of the Einstein Cross. A possible fifth image is detected; however, this detection less certain. Fits to the combined spectrum of all images of the Einstein Cross assuming a simple power law with Galactic and intervening absorption at the lensing galaxy yield a photon index of 1.90(+0.05,-0.05). For the first observation, this spectral model yields a 0.4-8.0 keV X-ray flux of 4.6e-13 erg cm-2 s-1 and a 0.4-8.0 keV lensed luminosity of 1.0e46 erg s-1. The source exhibits variability both over long and short time scales. The X-ray flux has dropped by 20% between the two observations, and the K-S test showed that image A is variable at the 97% confidence level within the first observation. Furthermore, a possible time-delay of 2.7(+0.5,-0.9) hours between images A and B with image A leading is detected in the first observation. The X-ray flux ratios of the images are consistent with the optical flux ratios which are affected by microlensing suggesting that the X-ray emission is also microlensed. A comparison between our measured column densities and those inferred from extinction measurements suggests a higher dust-to-gas ratio in the lensing galaxy than the average value of our Galaxy. Finally, we report the detection at the 99.99% confidence level of a broad emission feature near the redshifted energy of the Fe Kαline in only the spectrum of image A. The rest frame energy, width, and equivalent width of this feature are E = 5.7(+0.2,-0.3) keV, sigma = 0.87(+0.30,-0.15) keV, and EW = 1200(+300,-200) eV, respectively.

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arXiv2001

Constraints on the mass-profile of the lens galaxy G2237+0305

Published parametric models of the Einstein Cross gravitational lens demonstrate that the image geometry can be reproduced by families of models. In particular, the slope of the mass-profile for the lens galaxy is unconstrained. However, recent models predict a dependence of image flux ratios on the slope of the mass profile. We use this dependence to constrain the mass profile by calculating the likelihood of the slope using published mid-IR flux ratios (including microlensing variability). We find that the galaxy is likely to be flatter than isothermal, and therefore that the mass-to-light ratio is decreasing in the inner kpc.

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arXiv2001

The size of a quasar's mid-IR emission region inferred from microlensed images of Q2237+0305

We use published mid-IR and V-band flux ratios for images A and B of Q2237+0305 to demonstrate that the size of the mid-IR emission region has a scale comparable to or larger than the microlens Einstein Radius (ER, ~10^17 cm for solar mass stars). Q2237+0305 has been monitored extensively in the R and V-bands for ~15 years. The variability record shows significant microlensing variability of the optical emission region, and has been used by several studies to demonstrate that the optical emission region is much smaller than the ER for solar-mass objects. For the majority of the monitoring history, the optical flux ratios have differed significantly from those predicted by macro-models. In contrast, recent observations in mid-IR show flux ratios similar to those measured in the radio, and to predictions of some lens models, implying that the mid-IR flux is emitted from a region that is at least 2 orders of magnitude larger than the optical emission region. We have calculated the likeli-hood of the observed mid-IR flux ratio as a function of mid-IR source size given the observed V-band flux ratio. The expected flux ratio for a source having dimensions of ~1 ER is a sensitive function of the macro model adopted. However we find that the probability of source size given the observed flux ratios is primarily sensitive to the ratio of the macro-model magnifications. The majority of published macro models for Q2237+0305 yield a flux ratio for images B and A of 0.8 - 1.1. By combining probabilities from the ratios A/B and C/D we infer that the diameter of a circular IR emission region is >1ER with >95% confidence. For microlensing by low-mass stars, this source size limit rules out non-thermal processes such as synchrotron as mechanisms for mid-IR emission.

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arXiv2001

Caught in the Act; Chandra Observations of Microlensing of the Radio-Loud Quasar MG J0414+0534

We present results from monitoring of the distant (z = 2.64), gravitationally lensed quasar MG J0414+0534 with the Chandra X-ray Observatory. An Fe Ka line at 6.49 +/- 0.09 keV (rest-frame) with an equivalent width of ~ 190eV consistent with fluorescence from a cold medium is detected at the 99 percent confidence level in the spectrum of the brightest image A. During the last two observations of our monitoring program we detected a five-fold increase of the equivalent width of a narrow Fe Ka line in the spectrum of image B but not in the brighter image A whereas image C is too faint to resolve the line. The continuum emission component of image B did not follow the sudden enhancement of the iron line in the last two observations. We propose that the sudden increase in the iron line strength from ~ 190eV to 900eV only in image B can be explained with a caustic crossing due to microlensing that selectively enhances a strip of the line emission region of the accretion disk. The non-enhancement of the continuum emission in the spectrum of image B suggests that the continuum emission region is concentrated closer to the center of the accretion disk than the iron line emission region and the magnification caustic has not reached close enough to the former region to amplify it. A model of a caustic crossing event predicts discontinuities in the light-curve of the magnification and provides an upper limit of ~ 5 x 10^(-4) pc on the outer radius of the Fe Ka emission region. The non-detection of any relativistic or Doppler shifts of the iron line in the spectrum of image B implies that the magnification caustic for the last two observations was located at a radius greater than ~ 100 gravitational radii.

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