133 research outputs found
X-ray Observations of Gravitationally Lensed Quasars; Evidence for a Hidden Quasar Population
The large magnification factors of gravitationally lensed (GL) systems allow
us to investigate the properties of quasars with X-ray luminosities that are
substantially lower than those of unlensed ones and also provide an independent
means of estimating the contribution of faint quasars to the hard X-ray
component of the cosmic X-ray background. Our spectral analysis indicate a
flattening of the spectral index in the hard band (4-20keV restframe) for 2
radio-loud quasars in the GL quasar sample for which the data have moderate
signal-to-noise ratio. We have identified a large fraction of Broad Absorption
Line (BAL) quasars amongst the GL quasar population. We find that approximately
35% of radio-quiet GL quasars contain BAL features which is significantly
larger than the 10% fraction of BAL quasars presently found in optically
selected flux limited quasar samples. We present a simple model that estimates
the effects of attenuation and lens magnification on the luminosity function of
quasars and that explains the observed fraction of GL BAL quasars. These
observations suggest that a large fraction of BAL quasars are missed from flux
limited optical surveys. Modeling of several X-ray observations of the GL BAL
quasar PG1115+080 suggests that the observed large X-ray variability may be
caused in part by a variable intrinsic absorber consistent with previously
observed variability of the BAL troughs in the UV band. The observed large
X-ray flux variations in PG1115+080 offer the prospect of considerably reducing
errors in determining the time delay with future X-ray monitoring of this
system and hence constraining the Hubble constant H.Comment: 19 pages, 10 figures, 9 Tables, accepted for publication in Ap
Absolute Proper Motion of the Fornax Dwarf Spheroidal Galaxy from Photographic and HST WFPC2 Data
We have measured the absolute proper motion of the Fornax dwarf spheroidal
galaxy from a combination of photographic plate material and HST WFPC2 data
that provide a time baseline of up to 50 years. The extragalactic reference
frame consists of 8 QSO images and 48 galaxies. The absolute proper motion is
mu_alpha cos(delta) = 0.59 +-0.16 mas/yr and mu_delta = -0.15 +- 0.16 mas/yr.
The corresponding orbit of Fornax is polar, with an eccentricity of 0.27, and a
radial period of 4.5 Gyr. Fornax's current location is near pericenter. The
direction of the motion of Fornax supports the notion that Fornax belongs to
the Fornax-LeoI-LeoII-Sculptor-Sextans stream as hypothesized by Lynden-Bell
(1976, 1982) and Majewski (1994).
According to our orbit determination, Fornax crossed the Magellanic plane
\~190 Myr ago, a time that coincides with the termination of the star-formation
process in Fornax. We propose that ram-pressure stripping due to the passage of
Fornax through a gaseous medium denser than the typical intragalactic medium
left behind from the LMC may have caused the end of star formation in Fornax.
The excess, anomalous clouds within the South Galactic pole region of the
Magellanic Stream whose origin has long been debated in the literature as
constituents of either the Magellanic Stream or of the extragalactic Sculptor
group, are found to lie along the orbit of Fornax. We speculate that these
clouds are stripped material from Fornax as the dwarf crossed the Magellanic
Clouds' orbit.Comment: Accepted for publication in Astronomical Journal. The version with
high resolution figures can be found at
ftp://pegasus.astro.yale.edu/pub/dana/paper
On the Double Planet System Around HD 83443
The Geneva group has reported two Saturn-mass planets orbiting HD 83443 (K0V)
with periods of 2.98 and 29.8 d. The two planets have raised interest in their
dynamics because of the possible 10:1 orbital resonance and the strong
gravitational interactions. We report precise Doppler measurements of HD 83443
obtained with the Keck/HIRES and the AAT/UCLES spectrometers. These
measurements strongly confirm the inner planet with period of 2.985 d, with
orbital parameters in very good agreement with those of the Geneva group.
However these Doppler measurements show no evidence of the outer planet, at
thresholds of 1/4 (3 m/s) of the reported velocity amplitude of 13.8 m/s. Thus,
the existence of the outer planet is in question. Indeed, the current Doppler
measurements reveal no evidence of any second planet with periods less than a
year.Comment: 26 pages incl. 3 tables and 8 figures; uses AASTE
AGN in the XMM-Newton first-light image as probes for the interstellar medium in the LMC
The XMM-Newton first-light image revealed X-ray point sources which show
heavily absorbed power-law spectra. The spectral indices and the probable
identification of a radio counterpart for the brightest source suggest AGN
shining through the interstellar gas of the Large Magellanic Cloud (LMC). The
column densities derived from the X-ray spectra in combination with HI
measurements will allow to draw conclusions on HI to H_2 ratios in the LMC and
compare these with values found for the galactic plane.Comment: 4 pages, LaTex, 4 figures, Accepted for publication in A&A Letter
Two extra-solar planets from the Anglo-Australian Planet Search
We report the detection of two new extra-solar planets from the
Anglo-Australian Planet Search around the stars HD142 and HD23079. The planet
orbiting HD142 has an orbital period of just under one year, while that
orbiting HD23079 has a period of just under two years. HD142 falls into the
class of "eccentric" gas giants. HD23079 lies in the recently uncovered class
of "epsilon Ret-like" planets - extra-solar gas giant planets with
near-circular orbits outside 0.1 a.u. The recent discovery of several more
members of this class provides new impetus for the extension of existing planet
searches to longer periods, in the search for Jupiter-like planets in
Jupiter-like orbits.Comment: 6 pages, 4 figures and 3 tables include
Probing Dust in the Atmosphere of Brown Dwarfs Through Polarization
Theoretical analysis and observational evidences indicate that a brown dwarf
with effective temperature greater than 1400 K would have dust cloud in its
atmosphere. In this letter, we show that dust scattering should yield polarized
continuum radiation from the relatively warm brown dwarfs and the polarized
flux profile could be a potential diagnosis tool for the optical and the
physical properties of dust grains. The degree of polarization due to multiple
scattering will be more in the optical region if the particle size is small
while significant polarization should be detected in the infra-red region if
the particle size is large. It is pointed out that the departure from
sphericity in the shape of the object due to rapid rotation and due to tidal
effect by the companion in a binary system ensures the disc integrated
polarization to be non-zero.Comment: 9 pages (Latex AAS v4.0), 2 postscript figures, Accepted by The
Astrophysical Journal Letter
A Synoptic X-ray Study of M31 with the Chandra-HRC
We have obtained 17 epochs of Chandra High Resolution Camera (HRC) snapshot
images, each covering most of the M31 disk. The data cover a total baseline of
2.5 years and contain a mean effective exposure of 17 ks. We have measured the
mean fluxes and long-term lightcurves for 166 objects detected in these data.
At least 25% of the sources show significant variability. The cumulative
luminosity function (CLF) of the disk sources is well-fit by a power-law with a
slope comparable to those observed in typical elliptical galaxies. The CLF of
the bulge is a broken power law similar to measurements made by previous
surveys. We note several sources in the southwestern disk with L_X > 10^{37}
erg/s . We cross-correlate all of our sources with published optical and radio
catalogs, as well as new optical data, finding counterpart candidates for 55
sources. In addition, 17 sources are likely X-ray transients. We analyze
follow-up HST WFPC2 data of two X-ray transients, finding F336W (U-band
equivalent) counterparts. In both cases, the counterparts are variable. In one
case, the optical counterpart is transient with F336W = 22.3 +/- 0.1 mag. The
X-ray and optical properties of this object are consistent with a ~10 solar
mass black hole X-ray nova with an orbital period of ~20 days. In the other
case, the optical counterpart varies between F336W = 20.82 +/- 0.06 mag and
F336W = 21.11 +/- 0.02 mag. Ground-based and HST observations show this object
is bright (V = 18.8 +/- 0.1) and slightly extended. Finally, the frequency of
bright X-ray transients in the M31 bulge suggests that the ratio of neutron
star to black hole primaries in low-mass X-ray binaries (NS/BH) is ~1.Comment: 68 pages (27 text), 8 tables, 16 figures, 1 appendix, accepted by
ApJ; accepted version contains reorganized text, new tables and figures, and
updated result
On the Nature of Stars with Planets
We consider the metallicities and kinematics of nearby stars known to have
planetary-mass companions in the general context of the overall properties of
the local Galactic Disk. We have used Stromgren photometry to determine
abundances for both the extrasolar-planet host stars and for a volume-limited
sample of 486 F, G and K stars selected from the Hipparcos catalogue. The
latter data show that the Sun lies near the modal abundance of the disk, with
over 45% of local stars having super-solar metallicities. Twenty of the latter
stars (4.1%) are known to have planetary-mass companions. Using that ratio to
scale data for the complete sample of planetary host stars, we find that the
fraction of stars with extrasolar planets rises sharply with increasing
abundance, confirming previous results. However, the frequency remains at the
3-4% level for stars within 0.15 dex of solar abundance, and falls to ~1% only
for stars with abundances less than half solar. Given the present observational
constraints, both in velocity precision and in the available time baseline,
these numbers represent a lower limit to the frequency of extrasolar planetary
systems. A comparison between the kinematics of the planetary host stars and a
representative sample of disk stars suggests that the former have an average
age which is ~60% of the latter.Comment: 46 pages, 13 figures; accepted for PAS
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