17,214 research outputs found
Local sublattice-symmetry breaking in rotationally faulted multilayer graphene
Interlayer coupling in rotationally faulted graphene multilayers breaks the
local sublattice-symmetry of the individual layers. We present a theory of this
mechanism, which reduces to an effective Dirac model with space-dependent mass
in an important limit. It thus makes a wealth of existing knowledge available
for the study of rotationally faulted graphene multilayers. We demonstrate
quantitative agreement between our theory and a recent experiment.Comment: Valley dependence in Eqs. (2) and (7) corrected; coordinates x and y
interchanged in the appendi
The z>4 Quasar Population Observed by Chandra and XMM-Newton
The current status of our Chandra and XMM-Newton project on high-redshift
(z>4) quasars is briefly reviewed. We report the main results obtained in the
last few years for the detected quasars, along with a few (~10%) intriguing
cases where no detection has been obtained with Chandra snapshot observations.Comment: 4 pages, to appear in the proceedings of 'Multiwavelength AGN
surveys' (Cozumel, December 8-12 2003), ed. R. Maiolino and R. Mujic
ROSAT PSPC and Hri Observations of the Composite Starburst/Seyfert 2 Galaxy NGC 1672
The nearby barred spiral galaxy NGC 1672 is thought to have a weak Seyfert
nucleus in addition to its strong starburst activity. Observations with the
PSPC and HRI instruments on board the ROSAT X-ray satellite show that three
X-ray sources with luminosities (1--2)\times 10^{40} erg/s are clearly
identified with NGC 1672. The strongest X-ray source lies at the nucleus, and
the other two lie near the ends of the prominent bar, locations that are also
bright in H-alpha and near-infrared images. The nuclear source is resolved by
the HRI on about the scale of the recently identified nuclear ring, and one of
the sources at the ends of the bar is also probably resolved. The X-ray
spectrum of the nuclear source is quite soft, having a Raymond--Smith plasma
temperature of about 0.7 keV and little evidence for intrinsic absorption. The
ROSAT band X-ray flux of the nuclear source appears to be dominated not by
X-ray binary emission but rather by diffuse gas emission. While the properties
of the nuclear source are generally supportive of a superbubble interpretation,
its large density and emission measure stretch the limits that can be
comfortably accommodated by such models. We do not detect direct emission from
the putative Seyfert nucleus, although an alternative model for the nuclear
source is thermal emission from gas that is photoionized by a hidden Seyfert
nucleus. The spectra of the other two X-ray sources are harder than that of the
nuclear source, and superbubble models for them have the same strengths and
weaknesses.Comment: 11 pages, uuencoded compressed postscript, MNRAS in pres
X-ray Lighthouses of the High-Redshift Universe. II. Further Snapshot Observations of the Most Luminous z>4 Quasars with Chandra
We report on Chandra observations of a sample of 11 optically luminous
(Mb<-28.5) quasars at z=3.96-4.55 selected from the Palomar Digital Sky Survey
and the Automatic Plate Measuring Facility Survey. These are among the most
luminous z>4 quasars known and hence represent ideal witnesses of the end of
the "dark age ''. Nine quasars are detected by Chandra, with ~2-57 counts in
the observed 0.5-8 keV band. These detections increase the number of X-ray
detected AGN at z>4 to ~90; overall, Chandra has detected ~85% of the
high-redshift quasars observed with snapshot (few kilosecond) observations. PSS
1506+5220, one of the two X-ray undetected quasars, displays a number of
notable features in its rest-frame ultraviolet spectrum, the most prominent
being broad, deep SiIV and CIV absorption lines. The average optical-to-X-ray
spectral index for the present sample (=-1.88+/-0.05) is steeper than
that typically found for z>4 quasars but consistent with the expected value
from the known dependence of this spectral index on quasar luminosity.
We present joint X-ray spectral fitting for a sample of 48 radio-quiet
quasars in the redshift range 3.99-6.28 for which Chandra observations are
available. The X-ray spectrum (~870 counts) is well parameterized by a power
law with Gamma=1.93+0.10/-0.09 in the rest-frame ~2-40 keV band, and a tight
upper limit of N_H~5x10^21 cm^-2 is obtained on any average intrinsic X-ray
absorption. There is no indication of any significant evolution in the X-ray
properties of quasars between redshifts zero and six, suggesting that the
physical processes of accretion onto massive black holes have not changed over
the bulk of cosmic time.Comment: 15 pages, 7 figures, accepted for publication in A
El Roque de Los Muchachos Site Characteristics. II. Analysis of Wind, Relative Humidity and Air Pressure
In this paper we present an analysis of wind speed, wind direction, relative
humidity and air pressure taken at TNG, CAMC and NOT at Observatorio del Roque
de Los Muchachos, in the Canary Islands. Data are compared in order to check
local variations and both long term and short term trends of the microclimate.
Furthermore, influence of wind speed on the astronomical seeing is estimated to
the aim to better understand the influence of wide scale parameters on local
meteorological data. The three telescopes show different prevailing wind
direction, wind speed, relative humidity and air pressure confirming
differences in local microclimate. We found that seeing deteriorates when wind
speed is lower than 3.3 m/s. Comparison in terms of wind speed and high
relative humidity (> 90%) shows that TNG seems to have optimal observational
conditions with respect to CAMC and NOT. Air pressure analysis shows that ORM
is dominated by high pressure. Short time variations of pressure anticipate
temperature variations tipically by 2-3 hours, this property vanishes in time
scales higher than some hours and disappear in longer time scales.Comment: 10 pages, 9 figures, accepted for publication in PAS
- …