42 research outputs found
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
What Do Gravitational Lens Time Delays Measure?
Gravitational lens time delays depend on the Hubble constant, the observed
image positions, and the surface mass density of the lens in the annulus
between the images. Simple time delay lenses like PG1115+080, SBS1520+530,
B1600+434, PKS1830-211 and HE2149-2745 have H0 = A(1-)+B(e-1) where the
two coefficients A ~ 90km/s Mpc and B ~ 10km/s Mpc depend on the measured
delays and the observed image positions, is the mean surface density in the
annulus between the images, and there is a small correction from the
logarithmic slope e ~ 2 of the surface density profile, k ~ R^(1-e), in the
annulus. These 5 systems are very homogeneous, since for fixed H0=100h km/s Mpc
they must have the same surface density, =1.11-1.22 h +/- 0.04, with an
upper bound of 0.07 on any dispersion in beyond those due to the
measurement errors. If the lenses have their expected dark halos, ~ 0.5 and
H0=51+/-5 km/s Mpc, while if they have constant mass-to-light ratios, ~
0.1-0.2 and H0=73+/-8 km/s Mpc. More complicated lenses with multiple
components or strong perturbations from nearby clusters, like RXJ0911+0551 and
Q0957+561, are easily recognized because they have significantly different
coefficients.Comment: Submitted to ApJ. 18 pages, no figure
Detecting and interpreting statistical lensing by absorbers
We propose a method for detecting gravitational magnification of distant
sources, like quasars, due to absorber systems detected in their spectra. We
first motivate the use of metal absorption lines rather than Lyman-alpha lines,
then we show how to relate the observed moments of the source magnitude
distribution to the mass distribution of absorbers. In order to illustrate the
feasibility of the method, we use a simple model to estimate the amplitude of
the effect expected for MgII absorption lines, and show that their lensing
signal might already be detectable in large surveys like the SDSS. Our model
suggests that quasars behind strong MgII absorbers are in average brightened by
-0.05 to -0.2 magnitude due to magnification. One must therefore revisit the
claim that, in magnitude limited surveys, quasars with strong absorbers tend to
be missed due to extinction effects. In addition to constraining the mass of
absorber systems, applying our method will allow for the quantification of this
bias.Comment: 21 pages, 7 figures, submitted to Ap
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
High redshift AGNs from the 1Jy catalogue and the magnification bias
We have found a statistically significant (99.1 \%) excess of red ()
galaxies with photographic magnitudes , taken from the APM Sky
Catalogue around 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
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
Two New Low Redshift 21cm Absorbers
As part of a larger program to identify low redshift radio analogues of the
damped Lyman-alpha (DLA) absorbers seen in the spectra of high redshift
quasars, Westerbork Synthesis Radio Telescope (WSRT) observations have
discovered two new HI 21cm absorption lines at z=0.394 and z=0.437 in the
spectra of the radio sources B 0248+430 and B 1243-072 respectively. These
sightlines and redshifts were selected for study on the basis of the previously
known low ionization absorption lines of MgII, and neither has been observed in
the Lyman-alpha line. The 21cm line observations provide information on column
densities, temperatures and kinematics of the thickest cold neutral clouds in
the absorbers.Comment: 6 pages incl. 2 figures, accepted for publication in the
Astrophysical Journal Letters, missing reference adde
Imaging Photometry of Seyfert 1 AGNs and Quasars II: Observation of Long-Term Variability
Observations of 226 AGNs in the near-infrared , , and bands are
presented along with the analysis of the observations for variability. Our
sample consists mainly of Seyfert 1 AGNs and QSOs. About a quarter of the
objects in each category are radio loud. The AGNs in the entire sample have the
redshifts spanning the range from to 1, and the absolute magnitudes from
to -18. All the objects were observed twice and their variability was
measured by differential photometry.
A reduction method of differential photometry, optimized to the analysis of
extended images, has been developed. The systematic error in variability
arising from AGNs of highly extended images is estimated to be less than 0.01
mag in each of the , , and bands. The systematic error arising from
the flat fielding is negligible for most AGNs, although it is more than 0.1 mag
for some particular cases. The overall average flat fielding error is 0.03 mag
for the image pairs. We find that these systematic errors are superseded by
statistical errors, and the overall average total systematic and statistical
errors amounts to 0.05 mag in the measured variability in each band.
We find that 58% of all the AGNs in the entire sample show variability of
more than , and 44% of more than . This result holds
independent of the , , and bands. The detection rate of variability
is higher for a subsample of higher photometric accuracy, and there appears no
limit to this tendency. In particular, when we consider a subsample with small
photometric errors of mag, the rate of detection is
80%, and 64% for detection. This suggests that most AGNs are variable
in the near-infrared
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