Photometric monitoring of the blazar 3C 345 for the period 1996 - 2006

We present the results of the blazar 3C 345 monitoring in Johnson-Cousins BVRI bands for the period 1996 - 2006. We have collected 29 V and 43 R data points for this period; the BI light curves contain a few measurements only. The accuracy of our photometry is not better than 0.03 mag in the VR bands. The total amplitude of the variability obtained from our data is 2.06 mag in the V band and 2.25 mag in the R one. 3C 345 showed periods of flaring activity during 1998/99 and 2001: a maximum of the blazar brightness was detected in 2001 February - 15.345 mag in the V band and 14.944 mag in the R one. We confirm that during brighter stages 3C 345 becomes redder; for higher fluxes the colour index seems to be less dependent on the magnitude. The intra-night monitoring of 3C 345 in three consecutive nights in 2001 August revealed no significant intra-night variability; 3C 345 did not show evident flux changes over timescales of weeks around the period of the intra-night monitoring. This result supports the existing facts that intra-night variability is correlated with rapid flux changes rather than with specific flux levels

Spectroscopy of the Lens Galaxy of Q0957+561A,B. Implications of a possible central massive dark object

We present new long-slit William Herschel Telescope spectroscopic observations of the lens galaxy G1 associated with the double-imaged QSO 0957+561A,B. The obtained central stellar velocity dispersion, sigma_l = 310 +/- 20 km/s, is in reasonable agreement with other measurements of this dynamical parameter. Using all updated measurements of the stellar velocity dispersion in the internal region of the galaxy (at angular separations < 1".5) and a simple isotropic model, we discuss the mass of a possible central massive dark object. It is found that the data of Falco et al. (1997) suggest the existence of an extremely massive object of (0.5-2.1) x 10E10/h M_\odot (80% confidence level), whereas the inclusion of very recent data (Tonry & Franx 1998, and this paper) substantially changes the results: the compact central mass must be $\le$ 6 x10E9/h M_\odot at the 90% confidence level. We note that, taking into account all the available dynamical data, a compact nucleus with a mass of 10E9/h M_\odot (best fit) cannot be ruled out.Comment: 20 pages, 10 figures ApJ, in pres

A Multiwavelength Investigation of Unidentified EGRET Sources

Statistical studies indicate that the 271 point sources of high-energy gamma rays belong to two groups: a Galactic population and an isotropic extragalactic population. Many unidentified extragalactic sources are certainly blazars, and it is the intention of this work to uncover gamma-ray blazars missed by previous attempts. Until recently, searches for blazar counterparts to unidentified EGRET sources have focused on finding AGN that have 5-GHz radio flux densities S_5 near or above 1 Jy. However, the recent blazar identification of 3EG J2006-2321 (S_5 = 260 mJy) and other work suggest that careful studies of weaker flat-spectrum sources may be fruitful. In this spirit, error circles of 4 high-latitude unidentified EGRET sources have been searched for 5-GHz sources. The gamma-ray sources are 3EG J1133+0033, 3EG J1212+2304, 3EG J1222+2315, and 3EG J1227+4302. Within the error contours of each of the four sources are found 6 radio candidates; by observing the positions of the radio sources with the 0.81-m Tenagra II telescope it is determined that 14 of these 24 radio sources have optical counterparts with R < 22. Eight of these from two different EGRET sources have been observed in the B, V, and R bands in more than one epoch and the analysis of these data is ongoing. Any sources that are found to be variable will be the objects of multi-epoch polarimetry studies.Comment: 6 pages, 2 tables. To appear in Astrophysics & Space Scienc

Microlensing of gamma ray bursts by stars and MACHOs

The microlensing interpretation of the optical afterglow of GRB 000301C seems naively surprising, since a simple estimate of the stellar microlensing rate gives less than one in four hundred for a flat Omega_Lambda=0.7 cosmology, whereas one event was seen in about thirty afterglows. Considering baryonic MACHOs making up half of the baryons in the universe, the microlensing probability per burst can be roughly 5% for a GRB at redshift z=2. We explore two effects that may enhance the probability of observing microlensed gamma-ray burst afterglows: binary lenses and double magnification bias. We find that the consideration of binary lenses can increase the rate only at the ~15% level. On the other hand, because gamma-ray bursts for which afterglow observations exist are typically selected based on fluxes at widely separated wavebands which are not necessarily well correlated (e.g. localization in X-ray, afterglow in optical/infrared), magnification bias can operate at an enhanced level compared to the usual single-bias case. We find that existing estimates of the slope of the luminosity function of gamma-ray bursts, while as yet quite uncertain, point to enhancement factors of more than three above the simple estimates of the microlensing rate. We find that the probability to observe at least one microlensing event in the sample of 27 measured afterglows can be 3-4% for stellar lenses, or as much as 25 Omega_lens for baryonic MACHOs. We note that the probability to observe at least one event over the available sample of afterglows is significant only if a large fraction of the baryons in the universe are condensed in stellar-mass objects. (ABRIDGED)Comment: 22 pages, 4 figures, 2 table

Photometric observations of selected, optically bright quasars for Space Interferometry Mission and other future celestial reference frames

Photometric observations of 235 extragalactic objects that are potential targets for the Space Interferometry Mission (SIM) are presented. Mean B, V, R, I magnitudes at the 5% level are obtained at 1 - 4 epochs between 2005 and 2007 using the 1-m telescopes at Cerro Tololo Inter-American Observatory and Naval Observatory Flagstaff Station. Of the 134 sources which have V magnitudes in the Veron & Veron-Cetty catalog a difference of over 1.0 mag is found for the observed-catalog magnitudes for about 36% of the common sources, and 10 sources show over 3 mag difference. Our first set of observations presented here form the basis of a long-term photometric variability study of the selected reference frame sources to assist in mission target selection and to support in general QSO multi-color photometric variability studies.Comment: 40 pages, 13 figures, 4 table

High redshift AGNs from the 1Jy catalogue and the magnification bias

We have found a statistically significant (99.1 \%) excess of red ($O-E>2$) galaxies with photographic magnitudes $E<19.5$, $O< 21$ taken from the APM Sky Catalogue around $z \sim 1$ radiosources from the 1Jy catalogue. The amplitude, scale and dependence on galaxy colours of the observed overdensity are consistent with its being a result of the magnification bias caused by the weak gravitational lensing of large scale structures at redshift $z \approx 0.2-0.4$ and are hardly explained by other causes, as obscuration by dust.Comment: uuencoded file containing 3 ps files: the main text, a table and a figure. To appear in ApJ Letter

Rotating Nuclear Rings and Extreme Starbursts in Ultraluminous Galaxies

New high resolution interferometer data of 10 IR ultraluminous galaxies shows the molecular gas is in rotating nuclear rings or disks with radii 300 to 800 pc. Most of the CO flux comes from a moderate-density, warm, intercloud medium rather than self-gravitating clouds. Gas masses of ~ 5 x 10^9 Msun, 5 times lower than the standard method are derived from a model of the molecular disks. The ratio of molecular gas to dynamical mass, is M_gas/M_dyn ~ 1/6 with a maximum ratio of gas to total mass surface density of 1/3. For the galaxies VIIZw31, Arp193, and IRAS 10565+24, there is good evidence for rotating molecular rings with a central gap. In addition to the rotating rings a new class of star formation region is identified which we call an Extreme Starburst. They have a characteristic size of only 100 pc., about 10^9 Msun of gas and an IR luminosity of ~3 x 10^11 Lsun. Four extreme starbursts are identified in the 3 closest galaxies in the sample Arp220, Arp193 and Mrk273. They are the most prodigious star formation events in the local universe, each representing about 1000 times as many OB stars as 30 Doradus. In Arp220, the CO and 1.3 mm continuum maps show the two ``nuclei'' embedded in a central ring or disk and a fainter structure extending 3 kpc to the east, normal to the nuclear disk. There is no evidence that these sources really are the pre-merger nuclei. They are compact, extreme starburst regions containing 10^9 Msun of dense molecular gas and new stars, but no old stars. Most of the dust emission and HCN emission arises in the two extreme starbursts. The entire bolometric luminosity of Arp~220 comes from starbursts, not an AGN. In Mrk231, the disk geometry shows that the molecular disk cannot be heated by the AGN; the far IR luminosity of Mrk~231 is powered by a starburst, not the AGN. (Abridged)Comment: 97 pages Latex with aasms.sty, including 29 encapsulated Postscript figures. Figs 18 and 23 are GIFs. 31 figures total. Text and higher quality versions of figures available at http://sbastk.ess.sunysb.edu/www/RINGS_ESB_PREPRINT.html To be published in Ap. J., 10 Nov. 199

A Survey for Large Separation Lensed FIRST Quasars

Little is known about the statistics of gravitationally lensed quasars at large (7''-30'') image separations, which probe masses on the scale of galaxy clusters. We have carried out a survey for gravitationally-lensed objects, among sources in the FIRST 20cm radio survey that have unresolved optical counterparts in the digitizations of the Palomar Observatory Sky Survey. From the statistics of ongoing surveys that search for quasars among FIRST sources, we estimate that there are about 9100 quasars in this source sample, making this one of the largest lensing surveys to date. Using broad-band imaging, we have isolated all objects with double radio components separated by 5''-30'', that have unresolved optical counterparts with similar BVI colours. Our criteria for similar colours conservatively allow for observational error and for colour variations due to time delays between lensed images. Spectroscopy of these candidates shows that none of the pairs are lensed quasars. This sets an upper limit (95% confidence) on the lensing fraction in this survey of 3.3x10^-4, assuming 9100 quasars. Although the source redshift distribution is poorly known, a rough calculation of the expected lensing frequency and the detection efficiencies and biases suggests that simple theoretical expectations are of the same order of magnitude as our observational upper limit. Our procedure is novel in that our exhaustive search for lensed objects does not require prior identification of the quasars in the sample as such. Characterization of the FIRST-selected quasar population will enable using our result to constrain quantitatively the mass properties of clusters.Comment: 10 pages, accepted for publication in MNRA

Gravitational Waveguides in Cosmology

We discuss the possibility that, besides the usual gravitational lensing, there may exist a sort of gravitational waveguiding in cosmology which could explain some anomalous phenomena which cannot be understood by the current gravitational lensing models as the existence of "brothers" objects having different brilliancy but similar spectra and redshifts posed on the sky with large angular distance. Furthermore, such a phenomena could explain the huge luminosities coming from quasars using the cosmological structures as selfoc-type or planar waveguide. We describe the gravitational waveguide theory and then we discuss possible realizations in cosmology.Comment: 14 pages, latex, submitted to Int. Jou. Mod. Phys.

Cosmological waveguides for gravitational waves

We study the linearized equations describing the propagation of gravitational waves through dust. In the leading order of the WKB approximation, dust behaves as a non-dispersive, non-dissipative medium. Taking advantage of these features, we explore the possibility that a gravitational wave from a distant source gets trapped by the gravitational field of a long filament of galaxies of the kind seen in the large scale structure of the Universe. Such a waveguiding effect may lead to a huge magnification of the radiation from distant sources, thus lowering the sensitivity required for a successful detection of gravitational waves by detectors like VIRGO, LIGO and LISA.Comment: 19 pages, compressed Latex fil