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Register a new userThe time delay of CLASS B1600+434 from VLA multi-frequency and polarization monitoring
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A. D. Biggs
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We present an analysis of archival multi-frequency Very Large Array monitoring data of the two-image gravitational lens system CLASS B1600+434, including the polarization properties at 8.5 GHz. From simulating radio light curves incorporating realistic external variability in image A, we find time delays consistent at 1 $sigma$ for all frequencies and in total flux density and polarization. The delay with the smallest uncertainty (total flux density at 8.5 GHz) is $42.3^{+2.0}_{-1.8}$ (random) $pm 0.5$ (systematic) d (equivalent to $42.3 pm 2.1$ d) whereas combining all delay estimates gives a slightly higher value of $43.6pm1.2$ d. Both values are lower than the previously published radio result and inconsistent with that found in the optical. $H_0$ determination is difficult due to the complicated lensing mass and the lack of constraints provided by only two images. However, analysis of archival Very Long Baseline Interferometry data reveals jets in this system for the first time, the orientations of which provide model constraints. In addition, extremely sensitive maps made from combining all the monitoring data reveal faint emission on one side of the lensing galaxy which we speculate might be the result of a naked-cusp lensing configuration. Finally, we find clear evidence for external variability in image A on time-scales of days to years, the frequency-dependence of which supports the previous conclusion that this is predominantly due to microlensing. External variability seems to be completely absent in image B and this does not appear to be a consequence of scatter-broadening in the interstellar medium of the lensing galaxy.
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We present Very Large Array (VLA) 8.5-GHz light curves of the two lens images of the Cosmic Lens All Sky Survey (CLASS) gravitational lens B1600+434. We find a nearly linear decrease of 18-19% in the flux densities of both lens images over a period of eight months (February-October) in 1998. Additionally, the brightest image A shows modulations up to 11% peak-to-peak on scales of days to weeks over a large part of the observing period. Image B varies significantly less on this time scale. We conclude that most of the short-term variability in image A is not intrinsic source variability, but is most likely caused by microlensing in the lens galaxy. The alternative, scintillation by the ionized Galactic ISM, is shown to be implausible based on its strong opposite frequency dependent behavior compared with results from multi-frequency WSRT monitoring observations (Koopmans & de Bruyn 1999). From these VLA light curves we determine a median time delay between the lens images of 47^{+5}_{-6} d (68%) or 47^{+12}_{-9} d (95%). We use two different methods to derive the time delay; both give the same result within the errors. We estimate an additional systematic error between -8 and +7 d. If the mass distribution of lens galaxy can be described by an isothermal model (Koopmans, de Bruyn & Jackson 1998), this time delay would give a value for the Hubble parameter, H_0=57^{+14}_{-11} (95% statistical) ^{+26}_{-15} (systematic) km/s/Mpc (Omega_m=1 and Omega_Lambda=0). Similarly, the Modified-Hubble-Profile mass model would give H_0=74^{+18}_{-15} (95% statistical) ^{+22}_{-22} (systematic) km/s/Mpc. For Omega_m=0.3 and Omega_Lambda=0.7, these values increase by 5.4%. ... (ABRIDGED)
We have analysed archival VLA 8.4-GHz monitoring data of the gravitational lens system JVAS B1030+074 with the goal of determining the time delay between the two lensed images via the polarization variability. In contrast to the previously published total intensity variations, we detect correlated variability in polarized flux density, percentage polarization and polarization position angle. The latter includes a fast ($<$5d) 90-degree rotation event. Our best estimate of the time delay is $146pm6$d (1$sigma$), considerably longer than that predicted by the lens model presented in the discovery paper. Additional model constraints will be needed before this system can be used to measure $H_0$, for example through a detection of the lensed sources VLBI jet in image B. No time delay is visible in total flux density and this is partially due to much greater scatter in the image B measurements. This must be due to a propagation effect as the radio waves pass through the ISM of the lensing galaxies or the Galaxy.
We present optical I-band light curves of the gravitationally lensed double QSO B1600+434 from observations obtained at the Nordic Optical Telescope (NOT) between April 1998 and November 1999. The photometry has been performed by simultaneous deconvolution of all the data frames, involving a numerical lens galaxy model. Four methods have been applied to determine the time delay between the two QSO components, giving a mean estimate of Delta_t = 51+/-4 days (95% confidence level). This is the fourth optical time delay ever measured. Adopting a Omega=0.3, Lambda=0 Universe and using the mass model of Maller et al. (2000), this time-delay estimate yields a Hubble parameter of H_0=52 (+14, -8) km s^-1 Mpc^-1 (95% confidence level) where the errors include time-delay as well as model uncertainties. There are time-dependent offsets between the two (appropriately shifted) light curves that indicate the presence of external variations due to microlensing.
We present Low-Frequency Array (LOFAR) telescope observations of the radio-loud gravitational lens systems MG 0751+2716 and CLASS B1600+434. These observations produce images at 300 milliarcseconds (mas) resolution at 150 MHz. In the case of MG 0751+2716, lens modelling is used to derive a size estimate of around 2 kpc for the low-frequency source, which is consistent with a previous 27.4 GHz study in the radio continuum with Karl G. Jansky Very Large Array (VLA). This consistency implies that the low-frequency radio source is cospatial with the core-jet structure that forms the radio structure at higher frequencies, and no significant lobe emission or further components associated with star formation are detected within the magnified region of the lens. CLASS B1600+434 is a two-image lens where one of the images passes through the edge-on spiral lensing galaxy, and the low radio frequency allows us to derive limits on propagation effects, namely scattering, in the lensing galaxy. The observed flux density ratio of the two lensed images is 1.19 +/- 0.04 at an observed frequency of 150 MHz. The widths of the two images give an upper limit of 0.035 kpc m^-20/3 on the integrated scattering column through the galaxy at a distance approximately 1 kpc above its plane, under the assumption that image A is not affected by scattering. This is relatively small compared to limits derived through very long baseline interferometry (VLBI) studies of differential scattering in lens systems. These observations demonstrate that LOFAR is an excellent instrument for studying gravitational lenses. We also report on the inability to calibrate three further lens observations: two from early observations that have less well determined station calibration, and a third observation impacted by phase transfer problems.
We have reanalysed the 1996/1997 VLA monitoring data of the gravitational lens system JVAS B0218+357 to produce improved total flux density and polarization variability curves at 15, 8.4 and 5 GHz. This has been done using improved calibration techniques, accurate subtraction of the emission from the Einstein ring and careful correction of various systematic effects, especially an offset in polarization position angle that is hour-angle dependent. The variations in total and polarized flux density give the best constraints and we determine a combined delay estimate of $11.3 pm 0.2$ d (1$sigma$). This is consistent with the $gamma$-ray value recently derived using the Fermi Gamma-ray Space Telescope and thus we find no evidence for a positional shift between the radio and $gamma$-ray emitting regions. Combined with the previously published lens model found using LensClean, the new delay gives a value for the Hubble constant of $H_0 = 72.9 pm 2.6$ km s$^{-1}$ Mpc$^{-1}$ (1$sigma$).
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