14,718 research outputs found
INTEGRAL-ISGRI observations of the CygOB2 region: earching for hard X-ray point sources in a region containing several non-thermal emitting massive stars
Aims: We analyze INTEGRAL-ISGRI data in order to probe the hard X-ray
emission (above 20 keV) from point sources in the Cyg OB2 region and to
investigate the putative non-thermal high-energy emission from early-type stars
(Wolf-Rayet and O-type stars). Among the targets located in the field of view,
we focus on the still unidentified EGRET source 3EG 2033+4118 that may be
related to massive stars known to produce non-thermal emission in the radio
domain, and on the wide colliding-wind binary WR 140. Methods: Using a large
set of data obtained with the IBIS-ISGRI imager onboard INTEGRAL, we run the
OSA software package in order to find point sources in the fully coded field of
view of the instrument. Results: Our data do not allow the detection of a
lower-energy counterpart of 3EG J2033+4118 nor of any other new point sources
in the field of view, and we derive upper limits on the high-energy flux for a
few targets: 3EG J2033+4118, TeV J2032+4130, WR140, WR146 and WR147. The
results are discussed in the context of the multiwavelength investigation of
these objects. Conclusions: The upper limits derived are valuable constraints
for models aimed at understanding the acceleration of particles in non-thermal
emitting massive stars, and of the still unidentified very-high gamma-ray
source TeV J2032+4130.Comment: 6 page, 2 figures including one figure in GIF format, accepted for
publication by A&
The investigation of particle acceleration in colliding-wind massive binaries with SIMBOL-X
An increasing number of early-type (O and Wolf-Rayet) colliding wind binaries
(CWBs) is known to accelerate particles up to relativistic energies. In this
context, non-thermal emission processes such as inverse Compton (IC) scattering
are expected to produce a high energy spectrum, in addition to the strong
thermal emission from the shock-heated plasma. SIMBOL-X will be the ideal
observatory to investigate the hard X-ray spectrum (above 10 keV) of these
systems, i.e. where it is no longer dominated by the thermal emission. Such
observations are strongly needed to constrain the models aimed at understanding
the physics of particle acceleration in CWB. Such systems are important
laboratories for investigating the underlying physics of particle acceleration
at high Mach number shocks, and probe a different region of parameter space
than studies of supernova remnants.Comment: 2 pages, 2 figures, to appear in the proceedings of the workshop
"Simbol-X: the hard X-ray universe in focus", held in Bologna, Italy (14-16
May 2007
Heterotic String Compactifications on Half-flat Manifolds II
In this paper, we continue the analysis of heterotic string compactifications
on half-flat mirror manifolds by including the 10-dimensional gauge fields. It
is argued, that the heterotic Bianchi identity is solved by a variant of the
standard embedding. Then, the resulting gauge group in four dimensions is still
E6 despite the fact that the Levi-Civita connection has SO(6) holonomy. We
derive the associated four-dimensional effective theories including matter
field terms for such compactifications. The results are also extended to more
general manifolds with SU(3) structure.Comment: 31 page
Phase resolved spectroscopy of the Vela pulsar with XMM-Newton
The ~10^4 y old Vela Pulsar represents the bridge between the young Crab-like
and the middle-aged rotation powered pulsars. Its multiwavelength behaviour is
due to the superposition of different spectral components. We take advantage of
the unprecedented harvest of photons collected by XMM-Newton to assess the Vela
Pulsar spectral shape and to study the pulsar spectrum as a function of its
rotational phase. As for the middle-aged pulsars Geminga, PSR B0656+14 and PSR
B1055-52 (the "Three Musketeers"), the phase-integrated spectrum of Vela is
well described by a three-component model, consisting of two blackbodies
(T_bb1=(1.06+/-0.03)x10^6 K, R_bb1=5.1+/-0.3 km, T_bb2=(2.16+/-0.06)x10^6 K,
R_bb2=0.73+/-0.08 km) plus a power-law (gamma=2.2+/-0.3). The relative
contributions of the three components are seen to vary as a function of the
pulsar rotational phase. The two blackbodies have a shallow 7-9% modulation.
The cooler blackbody, possibly related to the bulk of the neutron star surface,
has a complex modulation, with two peaks per period, separated by ~0.35 in
phase, the radio pulse occurring exactly in between. The hotter blackbody,
possibly originating from a hot polar region, has a nearly sinusoidal
modulation, with a single, broad maximum aligned with the second peak of the
cooler blackbody, trailing the radio pulse by ~0.15 in phase. The non thermal
component, magnetospheric in origin, is present only during 20% of the pulsar
phase and appears to be opposite to the radio pulse. XMM-Newton phase-resolved
spectroscopy unveils the link between the thermally emitting surface of the
neutron star and its charge-filled magnetosphere, probing emission geometry as
a function of the pulsar rotation. This is a fundamental piece of information
for future 3-dimensional modeling of the pulsar magnetosphere.Comment: 27 pages, 9 figures. Accepted for publication in Ap
Quantum Gravity Corrections for Schwarzschild Black Holes
We consider the Matrix theory proposal describing eleven-dimensional
Schwarzschild black holes. We argue that the Newtonian potential between two
black holes receives a genuine long range quantum gravity correction, which is
finite and can be computed from the supergravity point of view. The result
agrees with Matrix theory up to a numerical factor which we have not computed.Comment: 14 pages, Tex, no figure
Long Term Variability of SDSS Quasars
We use a sample of 3791 quasars from the Sloan Digital Sky Survey (SDSS)
Early Data Release (EDR), and compare their photometry to historic plate
material for the same set of quasars in order to study their variability
properties. The time base-line we attain this way ranges from a few months to
up to 50 years. In contrast to monitoring programs, where relatively few
quasars are photometrically measured over shorter time periods, we utilize
existing databases to extend this base-line as much as possible, at the cost of
sampling per quasar. Our method, however, can easily be extended to much larger
samples. We construct variability Structure Functions and compare these to the
literature and model functions. From our modeling we conclude that 1) quasars
are more variable toward shorter wavelengths, 2) their variability is
consistent with an exponentially decaying light-curve with a typical time-scale
of ~2 years, 3) these outbursts occur on typical time-scales of ~200 years.
With the upcoming first data release of the SDSS, a much larger quasar sample
can be used to put these conclusions on a more secure footing.Comment: 16 pages, accepted for publication in AJ, Sept issu
Recurrent Neutrino Emission from Supermassive Black Hole Mergers
The recent detection of possible neutrino emission from the blazar TXS
0506+056 was the first high-energy neutrino associated with an astrophysical
source, making this special type of active galaxies promising neutrino
emitters. The fact that two distinct episodes of neutrino emission were
detected with a separation of around 3 years suggests that emission could be
periodic. Periodic emission is expected from supermassive binary black hole
systems due to jet precession close to the binary's merger. Here we show that
if TXS 0506+056 is a binary source then the next neutrino flare could occur
before the end of 2021. We derive the binary properties that would lead to the
detection of gravitational waves from this system by LISA. Our results for the
first time quantify the time scale of these correlations for the example of TXS
0506+056, providing clear predictions for both the neutrino and
gravitational-wave signatures of such sources.Comment: 6 pages, 3 figures, submitte
The Projected Rotational Velocity Distribution of a Sample of OB stars from a Calibration based on Synthetic He I lines
We derive projected rotational velocities (vsini) for a sample of 156
Galactic OB star members of 35 clusters, HII regions, and associations. The HeI
lines at 4026, 4388, and 4471A were analyzed in order to define
a calibration of the synthetic HeI full-widths at half maximum versus stellar
vsini. A grid of synthetic spectra of HeI line profiles was calculated in
non-LTE using an extensive helium model atom and updated atomic data. The
vsini's for all stars were derived using the He I FWHM calibrations but also,
for those target stars with relatively sharp lines, vsini values were obtained
from best fit synthetic spectra of up to 40 lines of CII, NII, OII, AlIII,
MgII, SiIII, and SIII. This calibration is a useful and efficient tool for
estimating the projected rotational velocities of O9-B5 main-sequence stars.
The distribution of vsini for an unbiased sample of early B stars in the
unbound association Cep OB2 is consistent with the distribution reported
elsewhere for other unbound associations.Comment: Accepted for publication in The Astronomical Journa
On the Supergravity Gauge theory Correspondence and the Matrix Model
We review the assumptions and the logic underlying the derivation of DLCQ
Matrix models. In particular we try to clarify what remains valid at finite
, the role of the non-renormalization theorems and higher order terms in the
supergravity expansion. The relation to Maldacena's conjecture is also
discussed. In particular the compactification of the Matrix model on is
compared to the super Yang-Mills duality, and
the different role of the branes in the two cases is pointed out.Comment: 19 pages, Late
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