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Register a new userFree-free absorption in the gravitational lens JVAS B0218+357
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We address the issue of anomalous image flux ratios seen in the double-image gravitational lens JVAS B0218+357. From the multi-frequency observations presented in a recent study (Mittal et al. 2006) and several previous observations made by other authors, the anomaly is well-established in that the image flux-density ratio (A/B) decreases from 3.9 to 2.0 over the observed frequency range from 15 GHz to 1.65 GHz. In Mittal et al. (2006), the authors investigated whether an interplay between a frequency-dependent structure of the background radio-source and a gradient in the relative image-magnification can explain away the anomaly. Insufficient shifts in the image centroids with frequency led them to discard the above effect as the cause of the anomaly. In this paper, we first take this analysis further by evaluating the combined effect of the background source extension and magnification gradients in the lens plane in more detail. This is done by making a direct use of the observed VLBI flux-distributions for each image to estimate the image flux-density ratios at different frequencies from a lens-model. As a result of this investigation, this mechanism does not account for the anomaly. Following this, we analyze the effects of mechanisms which are non-gravitational in nature on the image flux ratios in B0218+357. These are free-free absorption and scattering, and are assumed to occur under the hypothesis of a molecular cloud residing in the lens galaxy along the line-of-sight to image A. We show that free-free absorption due to an H II region covering the entire structure of image A at 1.65 GHz can explain the image flux ratio anomaly. We also discuss whether H II regions with physical parameters as derived from our analysis are consistent with those observed in Galactic and extragalactic H II regions.
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We present the results of phase-referenced VLBA+Effelsberg observations at five frequencies of the gravitational lens B0218+357 to establish the precise registration of the A and B lensed image positions.
We present the results of phase-referenced VLBA+Effelsberg observations at five frequencies of the double-image gravitational lens JVAS B0218+357, made to establish the precise registration of the A and B lensed image positions. The motivation behind these observations is to investigate the anomalous variation of the image flux density ratio (A/B) with frequency - this ratio changes by almost a factor of two over a frequency range from 1.65 GHz to 15.35 GHz. We investigate whether frequency dependent image positions, combined with a magnification gradient across the image field, could give rise to the anomaly. Our observations confirm the variation of image flux ratio with frequency. The results from our phase-reference astrometry, taken together with the lens mass model of Wucknitz et al. (2004), show that shifts of the image peaks and centroids are too small to account for the observed frequency-dependent ratio.
We present results on multifrequency Very Long Baseline Array (VLBA) monitoring observations of the double-image gravitationally lensed blazar JVAS B0218+357. Multi-epoch observations started less than one month after the gamma-ray flare detected in 2012 by the Large Area Telescope on board Fermi, and spanned a 2-month interval. The radio light curves did not reveal any significant flux density variability, suggesting that no clear correlation between the high energy and low-energy emission is present. This behaviour was confirmed also by the long-term Owens Valley Radio Observatory monitoring data at 15 GHz. The milliarcsecond-scale resolution provided by the VLBA observations allowed us to resolve the two images of the lensed blazar, which have a core-jet structure. No significant morphological variation is found by the analysis of the multi-epoch data, suggesting that the region responsible for the gamma-ray variability is located in the core of the AGN, which is opaque up to the highest observing frequency of 22 GHz.
The gravitational lens toward B0218+357 offers the unique possibility to study cool moderately dense gas with high sensitivity and angular resolution in a cloud that existed half a Hubble time ago. Observations of the radio continuum and six formaldehyde (H2CO) lines were carried out with the VLA, the Plateau de Bure interferometer, and the Effelsberg 100-m telescope. Three radio continuum maps indicate a flux density ratio between the two main images, A and B, of ~ 3.4 +/- 0.2. Within the errors the ratio is the same at 8.6, 14.1, and 43 GHz. The 1_{01}-0_{00} line of para-H2CO is shown to absorb the continuum of image A. Large Velocity Gradient radiative transfer calculations are performed to reproduce the optical depths of the observed two cm-wave K-doublet and four mm-wave rotational lines. These calculations also account for a likely frequency-dependent continuum cloud coverage. Confirming the diffuse nature of the cloud, an n(H2) density of < 1000 cm^{-3} is derived, with the best fit suggesting n(H2) ~ 200 cm^{-3}. The H2CO column density of the main velocity component is ~5 * 10^{13} cm^{-2}, to which about 7.5 * 10^{12} cm^{-2} has to be added to also account for a weaker feature on the blue side, 13 km/s apart. N(H2CO)/N(NH3) ~ 0.6, which is four times less than the average ratio obtained from a small number of local diffuse (galactic) clouds seen in absorption. The ortho-to-para H2CO abundance ratio is 2.0 - 3.0, which is consistent with the kinetic temperature of the molecular gas associated with the lens of B0218+357. With the gas kinetic temperature and density known, it is found that optically thin transitions of CS, HCN, HNC, HCO+, and N2H+ (but not CO) will provide excellent probes of the cosmic microwave background at redshift z=0.68.
In this paper we present new observations of the gravitational lens system JVAS B0218+357 made with a global VLBI network at a frequency of 8.4 GHz. Our maps have an rms noise of 30 microJy/beam and with these we have been able to image much of the extended structure of the radio jet in both the A and B images at high resolution (~1 mas). The main use of these maps will be to enable us to further constrain the lens model for the purposes of H0 determination. We are able to identify several sub-components common to both images with the expected parity reversal, including one which we identify as a counter-jet. We have not been successful in detecting either the core of the lensing galaxy or a third image. Using a model of the lensing galaxy we have back-projected both of the images to the source plane and find that they agree well. However, there are small, but significant, differences which we suggest may arise from multi-path scattering in the ISM of the lensing galaxy. We also find an exponent of the radial mass distribution of approximately 1.04, in agreement with lens modelling of published 15-GHz VLBI data. Polarisation maps of each image are presented which show that the distributions of polarisation across images A and B are different. We suggest that this results from Faraday rotation and associated depolarisation in the lensing galaxy.
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