Mass and dust in the disk of a spiral lens galaxy

Gravitational lensing is a potentially important probe of spiral galaxy structure, but only a few cases of lensing by spiral galaxies are known. We present Hubble Space Telescope and Magellan observations of the two-image quasar PMN J2004-1349, revealing that the lens galaxy is a spiral galaxy. One of the quasar images passes through a spiral arm of the galaxy and suffers 3 magnitudes of V-band extinction. Using simple lens models, we show that the mass quadrupole is well-aligned with the observed galaxy disk. A more detailed model with components representing the bulge and disk gives a bulge-to-disk mass ratio of 0.16 +/- 0.05. The addition of a spherical dark halo, tailored to produce an overall flat rotation curve, does not change this conclusion.Comment: ApJ, in press [9pp, 7 figs

Probing The Dust-To-Gas Ratio of z > 0 Galaxies Through Gravitational Lenses

We report the detection of differential gas column densities in three gravitational lenses, MG0414+0534, HE1104-1805, and PKS1830-211. Combined with the previous differential column density measurements in B1600+434 and Q2237+0305 and the differential extinction measurements of these lenses, we probe the dust-to-gas ratio of a small sample of cosmologically distant normal galaxies. We obtain an average dust-to-gas ratio of E(B-V)/NH =(1.4\pm0.5) e-22 mag cm^2/atoms with an estimated intrinsic dispersion in the ratio of ~40%. This average dust-to-gas ratio is consistent with the average Galactic value of 1.7e-22 mag cm^2/atoms and the estimated intrinsic dispersion is also consistent with the 30% observed in the Galaxy.Comment: 14 pages, 4 figures, Accepted by Ap

PKS 1830-211: A Face-On Spiral Galaxy Lens

We present new Hubble Space Telescope images of the gravitational lens PKS 1830-211, which allow us to characterize the lens galaxy and update the determination of the Hubble constant from this system. The I-band image shows that the lens galaxy is a face-on spiral galaxy with clearly delineated spiral arms. The southwestern image of the background quasar passes through one of the spiral arms, explaining the previous detections of large quantities of molecular gas and dust in front of this image. The lens galaxy photometry is consistent with the Tully-Fisher relation, suggesting the lens galaxy is a typical spiral galaxy for its redshift. The lens galaxy position, which was the main source of uncertainty in previous attempts to determine H_0, is now known precisely. Given the current time delay measurement and assuming the lens galaxy has an isothermal mass distribution, we compute H_0 = 44 +/- 9 km/s/Mpc for an Omega_m = 0.3 flat cosmological model. We describe some possible systematic errors and how to reduce them. We also discuss the possibility raised by Courbin et al. (2002), that what we have identified as a single lens galaxy is actually a foreground star and two separate galaxies.Comment: 21 pp., 4 figs., accepted by ApJ, section added to discuss related work by Courbin et al. (astro-ph/0202026

Optical and Radio observations of the bright GRB010222 afterglow: evidence for rapid synchrotron cooling?

We report photometric observations of the optical afterglow of GRB010222 in V,R and I passbands carried out at UPSO, Naini Tal between 22-27 Feb 2001. We determine CCD Johnson BV and Cousins RI photometric magnitudes for 31 stars in the field of GRB010222 and use them to calibrate our measurements as well as other published BVRI photometric magnitudes of GRB010222 afterglow. We construct the light curve in V,R,I passbands and from a broken power-law fit determine the decay indices of 0.74+/-0.05 and 1.35+/-0.04 before and after the break at 0.7 days. Using reported X-ray flux measurements at 0.35 and 9.13 days after the burst we determine X-ray to opt/IR spectral index of 0.61+/-0.02 and 0.75+/-0.02 on these two days. We also report upper limits to the radio flux obtained from the RATAN-600 telescope and the GMRT, and millimeter-wave upper limits obtained from the Plateau de Bure Millimeter interferometer. We argue that the synchrotron cooling frequency is below the optical band for most of the observing period. We also estimate an initial jet opening angle of about 2.0n^(1/8) degrees, where n is the number density of the ambient medium.Comment: 16 pages, 4 postscript figures, minor revisions according to referee's comments, millimeter upper limit added, accepted for publication in Bulletin of the Astronomical Society of Indi

On the origin of the various types of radio emission in GRS 1915+105

We investigate the association between the radio ``plateau'' states and the large superluminal flares in GRS 1915+105 and propose a qualitative scenario to explain this association. We identify several candidate superluminal flare events from available monitoring data on this source and analyze the contemporaneous RXTE pointed observations. We detect a strong correlation between the average X-ray flux during the ``plateau'' state and the total energy emitted in radio during the subsequent radio flare. We find that the sequence of events is similar for all large radio flares with a fast rise and exponential decay morphology. Based on these results, we propose a qualitative scenario in which the separating ejecta during the superluminal flares are observed due to the interaction of the matter blob ejected during the X-ray soft dips, with the steady jet already established during the ``plateau'' state. This picture can explain all types of radio emission observed from this source in terms of its X-ray emission characteristics.Comment: Corrected typo in the author names, contents unchanged, accepted in Ap

GMRT Observations of Microquasar V4641 Sgr

We report the GMRT observations of V4641 Sgr during the May 2002 outburst at radio frequencies of 610 and 244 MHz. This is the lowest frequency radio detection of this source. The present low frequency radio observations clearly showed spectral evolution from the optically thick to thin state. This behavior is broadly consistent with the expanding bubble model. However, the flux densities observed at lower frequencies are much higher than predicted by this model. In the conical jet model, this discrepancy could be reconciled.Comment: 5 pages, 3 figure

Low Frequency Radio Observations of GRS1915+105 with GMRT

We present the first detailed low frequency radio measurements of the galactic microquasar GRS1915+105 with GMRT. Simultaneous observations were carried out at 610 and 244 MHz. Our data does not show any signature of spectral turn over even at low radio frequency of 244 MHz. We propose that while the radio emission at high radio frequencies could predominantly come from compact jets, the emission at lower frequency originates in the lobes at the end of the jet which acts like a reservoir of low energy electrons.Comment: 7 pages, 3 figure