This Letter reports on the photometric detection of transits of the Neptune-mass planet orbiting the nearby M-dwarf star GJ 436. It is by far the closest, smallest, and least massive transiting ...planet detected so far. Its mass is slightly larger than Neptune's at $M = 22.6 \pm 1.9~M_\oplus $. The shape and depth of the transit lightcurves show that it is crossing the host star disc near its limb (impact parameter $0.84 \pm 0.03$) and that the planet size is comparable to that of Uranus and Neptune, $R = 25\,200 \pm 2200$ km = $3.95 \pm 0.35~R_\oplus$. Its main constituant is therefore very likely to be water ice. If the current planet structure models are correct, an outer layer of H/He constituting up to ten percent in mass is probably needed on top of the ice to account for the observed radius.
Gravitationally lensed quasars can be used to map the mass distribution in lensing galaxies and to estimate the Hubble constant H sub(0) by measuring the time delays between the quasar images. Here ...we report the measurement of two independent time delays in the quadruply imaged quasar WFI J2033-4723 (z = 1.66). Our data consist of R-band images obtained with the Swiss 1.2 m EULER telescope located at la Silla and with the 1.3 m SMARTS telescope located at Cerro Tololo. The light curves have 218 independent epochs spanning 3 full years of monitoring between March 2004 and May 2007, with a mean temporal sampling of one observation every 4th day. We measure the time delays using three different techniques, and we obtain Delta t_ = 35.5 pm 1.4 days (3.8%) and Delta t_ = 62.6_ similar to {\rm days} similar to \ (_), where A is a composite of the close, merging image pair. After correcting for the time delays, we find R-band flux ratios of F A / F B = 2.88 pm 0.04, F A / F C = 3.38 pm 0.06, and F A 1 / F A 2 = 1.37 pm 0.05 with no evidence for microlensing variability over a time scale of three years. However, these flux ratios do not agree with those measured in the quasar emission lines, suggesting that longer term microlensing is present. Our estimate of H sub(0) agrees with the concordance value: non-parametric modeling of the lensing galaxy predicts H sub(0) = 67 super(+13) sub(-10) km s super(-1) Mpc super(-1), while the Single Isothermal Sphere model yields H sub(0) = 63 super(+7) sub(-3) km s super(-1) Mpc super(-1) (68% confidence level). More complex lens models using a composite de Vaucouleurs plus NFW galaxy mass profile show twisting of the mass isocontours in the lensing galaxy, as do the non- parametric models. As all models also require a significant external shear, this suggests that the lens is a member of the group of galaxies seen in field of view of WFI J2033-4723.
Aims. We measure the redshift of the lensing galaxy in eight gravitationally lensed quasars in view of determining the Hubble parameter H sub(0) from the time delay method. Methods. Deep VLT/FORS1 ...spectra of lensed quasars are spatially deconvolved in order to separate the spectrum of the lensing galaxies from the glare of the much brighter quasar images. A new observing strategy is devised. It involves observations in Multi-Object-Spectroscopy (MOS) which allows the simultaneous observation of the target and of several PSF and flux calibration stars. The advantage of this method over traditional long-slit observations is a much more reliable extraction and flux calibration of the spectra. Results. For the first time we measure the redshift of the lensing galaxy in three multiply-imaged quasars: SDSS J1138+0314 (z sub(lens) = 0.445), SDSS J1226-0006 (z sub(lens) = 0.517), SDSS J1335+0118 (z sub(lens) = 0.440), and we give a tentative estimate of the redshift of the lensing galaxy in Q 1355-2257 (z sub(lens) = 0.701). We confirm four previously measured redshifts: HE 0047-1756 (z sub(lens) = 0.407), HE 0230-2130 (z sub(lens) = 0.523), HE 0435-1223 (z sub(lens) = 0.454) and WFIJ2033-4723 (z sub(lens) = 0.661). In addition, we determine the red-shift of the second lensing galaxy in HE 0230-2130 (z sub(lens) = 0.526). The spectra of all lens galaxies are typical for early-type galaxies, except for the second lensing galaxy in HE 0230-2130 which displays prominent OII emission.
We use numerical simulations to test a broad range of plausible observational strategies designed to measure the time delay between the images of gravitationally lensed quasars. Artificial quasar ...light curves are created along with Monte-Carlo simulations in order to determine the best temporal sampling to adopt when monitoring the photometric variations of systems with time delays between 5 and 120 days, i.e., always shorter than the visibility window across the year. Few and realistic assumptions are necessary on the quasar photometric variations (peak-to-peak amplitude and time-scale of the variations) and on the accuracy of the individual photometric points. The output of the simulations is the (statistical) relative error made on the time delay measurement, as a function of 1- the object visibility over the year; 2- the temporal sampling of the light curves; and 3- the time delay. Also investigated is the effect of long term microlensing variations which must be below the 5% level (either intrinsically or by subtraction) if the goal is to measure time delays with an accuracy of 1-2%. However, while microlensing increases the random error on the time delay, it does not significantly increase the systematic error, which is always a factor 5 to 10 smaller than the random error. Finally, it is shown that, when the time delay is comparable to the visibility window of the object, a logarithmic sampling can significantly improve the time delay determination. All results are presented in the form of compact plots to be used to optimize the observational strategy of future monitoring programs.
Aims. To provide the observational constraints required to use the gravitationally lensed quasar SDSS J0924+0219 for the determination of H sub(0) from the time delay method. We measure here the ...redshift of the lensing galaxy, we show the spectral variability of the source, and we resolve the lensed host galaxy of the source. Methods. We present our VLT/FORS1 deep spectroscopic observations of the lensed quasar SDSS J0924+0219, as well as archival HST/NICMOS and ACS images of the same object. The two-epoch spectra, obtained in the Multi Object Spectroscopy (MOS) mode, allow for very accurate flux calibration and spatial deconvolution. This strategy provides spectra for the lensing galaxy and for the quasar images A and B, free of any mutual light contamination. We deconvolve the HST images as well, which reveal a double Einstein ring. The mass distributions in the lens, reconstructed in several ways, are compared. Results. We determine the redshift of the lensing galaxy in SDSS J0924+0219: z sub(lens)= 0.394 plus or minus 0.001. Only slight spectral variability is seen in the continuum of quasar images A and B, while the C III, Mg II and Fe II emission lines display obvious changes. The flux ratio between the quasar images A and B is the same in the emission lines and in the continuum. One of the Einstein rings found using deconvolution corresponds to the lensed quasar host galaxy at z = 1.524 and a second bluer one, is the image either of a star-forming region in the host galaxy, or of another unrelated lower redshift object. A broad range of lens models give a satisfactory fit to the data. However, they predict very different time delays, making SDSS J0924+0219 an object of particular interest for photometric monitoring. In addition, the lens models reconstructed using exclusively the constraints from the Einstein rings, or using exclusively the astrometry of the quasar images, are not compatible. This suggests that multipole-like structures play an important role in SDSS J0924+0219.
Aims. We measure the redshift of the lensing galaxy in eight gravitationally lensed quasars in view of determining the Hubble parameter H-0 from the time delay method. Methods. Deep VLT/FORS1 spectra ...of lensed quasars are spatially deconvolved in order to separate the spectrum of the lensing galaxies from the glare of the much brighter quasar images. A new observing strategy is devised. It involves observations in Multi-Object-Spectroscopy (MOS) which allows the simultaneous observation of the target and of several PSF and flux calibration stars. The advantage of this method over traditional long-slit observations is a much more reliable extraction and flux calibration of the spectra. Results. For the first time we measure the redshift of the lensing galaxy in three multiply-imaged quasars: SDSS J1138+0314 (zlens = 0.445), SDSS J1226-0006 (z(lens) = 0.517), SDSS J1335+0118 (z(lens) = 0.440), and we give a tentative estimate of the redshift of the lensing galaxy in Q 1355- 2257 (z(lens) = 0.701). We confirm four previously measured redshifts: HE 0047-1756 (z(lens) = 0.407), HE 0230-2130 (z(lens) = 0.523), HE 0435-1223 (z(lens) = 0.454) and WFI J2033-4723 (z(lens) = 0.661). In addition, we determine the redshift of the second lensing galaxy in HE 0230-2130 (z(lens) = 0.526). The spectra of all lens galaxies are typical for early-type galaxies, except for the second lensing galaxy in HE 0230-2130 which displays prominent OII emission.
We expand our Bayesian Monte Carlo method for analyzing the light curves of gravitationally lensed quasars to simultaneously estimate time delays and the sizes of quasar continuum emission regions ...including their mutual uncertainties. We apply the method to HE1104-1805 and QJ0158-4325, two doubly imaged quasars with microlensing and intrinsic variability on comparable timescales. For HE1104-1805 the resulting time delay of image days and accretion disk size estimate of image at 0.2 mum in the rest frame and for inclination i are consistent with earlier estimates but suggest that existing methods for estimating time delays in the presence of microlensing underestimate the uncertainties. We are unable to measure a time delay for QJ0158-4325, but the accretion disk size is image at 0.3 mum in the rest frame.
Aims.To provide the observational constraints required to use the gravitationally lensed quasar SDSS J0924+0219 for the determination of HSUB0/SUB from the time delay method. We measure here the ...redshift of the lensing galaxy, we show the spectral variability of the source, and we resolve the lensed host galaxy of the source. Methods.We present our VLT/FORS1 deep spectroscopic observations of the lensed quasar SDSS J0924+0219, as well as archival HST/NICMOS and ACS images of the same object. The two-epoch spectra, obtained in the Multi Object Spectroscopy (MOS) mode, allow for very accurate flux calibration and spatial deconvolution. This strategy provides spectra for the lensing galaxy and for the quasar images A and B, free of any mutual light contamination. We deconvolve the HST images as well, which reveal a double Einstein ring. The mass distributions in the lens, reconstructed in several ways, are compared. Results.We determine the redshift of the lensing galaxy in SDSS J0924+0219: z_lens = 0.394±0.001. Only slight spectral variability is seen in the continuum of quasar images A and B, while the C III , Mg II and Fe II emission lines display obvious changes. The flux ratio between the quasar images A and B is the same in the emission lines and in the continuum. One of the Einstein rings found using deconvolution corresponds to the lensed quasar host galaxy at z=1.524 and a second bluer one, is the image either of a star-forming region in the host galaxy, or of another unrelated lower redshift object. A broad range of lens models give a satisfactory fit to the data. However, they predict very different time delays, making SDSS J0924+0219 an object of particular interest for photometric monitoring. In addition, the lens models reconstructed using exclusively the constraints from the Einstein rings, or using exclusively the astrometry of the quasar images, are not compatible. This suggests that multipole-like structures play an important role in SDSS J0924+0219.
Aims. Our aim is to measure the time delay between the two gravitationally lensed images of the z_{\rm qso} = 1.547 quasar SDSS J1650+4251, in order to estimate the Hubble constant H_{\rm0}. Methods. ...Our measurement is based on R-band light curves with 57 epochs obtained at Maidanak Observatory, in Uzbekistan, from May 2004 to September 2005. The photometry is performed using simultaneous deconvolution of the data, which provides the individual light curves of the otherwise blended quasar images. The time delay is determined from the light curves using two very different numerical techniques, i.e., polynomial fitting and direct cross-correlation. The time delay is converted into H_{\rm0} following analytical modeling of the potential well. Results. Our best estimate of the time delay is \Delta t = 49.5 \pm 1.9 days, i.e., we reach a 3.8% accuracy. The R-band flux ratio between the quasar images, corrected for the time delay and for slow microlensing, is F_{\rm A}/F_{\rm B} = 6.2 \pm 5%. Conclusions. The accuracy reached on the time delay allows us to discriminate well between families of lens models. As for most other multiply imaged quasars, only models of the lensing galaxy that have a de Vaucouleurs mass profile plus external shear give a Hubble constant compatible with the current most popular value (H_{\rm0} = 72 \pm 8 km s super(-1) Mpc super(-1)). A more realistic singular isothermal sphere model plus external shear gives H_{\rm0} = 51.7 super(+4.0) sub(-3.0) km s super(-1) Mpc super(-1).
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