4,031 research outputs found
Discovery of a new radio galaxy within the error box of the unidentified gamma-ray source 3EG J1735-1500
We report the discovery of a new radio galaxy within the location error box
of the gamma-ray source 3EG J1735-1500. The galaxy is a double-sided jet source
forming a large angle with the line of sight. Optical observations reveal a V ~
18 magnitude galaxy at the position of the radio core. Although the association
with the EGRET source is not confirmed at the present stage, because there is a
competing, alternative gamma-ray candidate within the location error contours
which is also studied here, the case deserves further attention. The new radio
galaxy can be used to test the recently proposed possibility of gamma-ray
emitting radio galaxies beyond the already known case of Centaurus A.Comment: 12 pages, 3 figures. Accepted for publication in Ap
Gravitational microlensing of gamma-ray blazars
We present a detailed study of the effects of gravitational microlensing on
compact and distant -ray blazars. These objects have -ray
emitting regions which are small enough as to be affected by microlensing
effects produced by stars lying in intermediate galaxies. We analyze the
temporal evolution of the gamma-ray magnification for sources moving in a
caustic pattern field, where the combined effects of thousands of stars are
taken into account using a numerical technique. We propose that some of the
unidentified -ray sources (particularly some of those lying at high
galactic latitude whose gamma-ray statistical properties are very similar to
detected -ray blazars) are indeed the result of gravitational lensing
magnification of background undetected Active Galactic Nuclei (AGNs).Comment: 30 pages, 27 figures. Four figures are being submitted only as .gif
files, and should be printed separately. The abstract below has been
shortened from the actual version appearing in the pape
Microquasar models for 3EG J1828+0142 and 3EG J1735-1500
Microquasars are promising candidates to emit high-energy gamma-rays.
Moreover, statistical studies show that variable EGRET sources at low galactic
latitudes could be associated with the inner spiral arms. The variable nature
and the location in the Galaxy of the high-mass microquasars, concentrated in
the galactic plane and within 55 degrees from the galactic center, give to
these objects the status of likely counterparts of the variable low-latitude
EGRET sources. We consider in this work the two most variable EGRET sources at
low-latitudes: 3EG J1828+0142 and 3EG J1735-1500, proposing a microquasar model
to explain the EGRET data in consistency with the observations at lower
energies (from radio frequencies to soft gamma-rays) within the EGRET error
box.Comment: (1)Universitat de Barcelona, (2)Instituto Argentino de
Radioastronomia (3) Facultad de Ciencias Astronomicas y Geofisicas
(4)Lawrence Livermore National Laboratory 6 pages, 2 figures. Presented as a
poster at the V Microquasar Workshop, Beijing, June 2004. Accepted for
publication in the Chinese Journal of Astronomy & Astrophysic
Precessing microblazars and unidentified gamma-ray sources
The recent discovery by Paredes et al. (2000) of a persistent microquasar
that is positionally coincident with an unidentified gamma-ray source has open
the possibility that other sources in the Third EGRET Catalog could be
interpreted as microquasars as well. In this letter we show that some variable
unidentified EGRET sources in the galactic plane could be produced by faint,
otherwise undetected microquasars with precessing jets. When the jet points
towards the observer, gamma-ray emission resulting from upscattered stellar
photons could be detectable yielding a variable source with weak or
undetectable counterpart at longer wavelengths. Strategies for detecting
these``microblazars'' with forthcoming satellites are briefly discussed.Comment: 4 pages, 3 figures, Astronomy & Astrophysics Letters in press, typing
errors correctio
INTEGRAL and XMM-Newton observations towards the unidentified MeV source GRO J1411-64
The COMPTEL unidentified source GRO J1411-64 was observed by INTEGRAL, and
its central part, also by XMM-Newton. The data analysis shows no hint for new
detections at hard X-rays. The upper limits in flux herein presented constrain
the energy spectrum of whatever was producing GRO J1411-64, imposing, in the
framework of earlier COMPTEL observations, the existence of a peak in power
output located somewhere between 300-700 keV for the so-called low state. The
Circinus Galaxy is the only source detected within the 4 location error
of GRO J1411-64, but can be safely excluded as the possible counterpart: the
extrapolation of the energy spectrum is well below the one for GRO J1411-64 at
MeV energies. 22 significant sources (likelihood ) were extracted and
analyzed from XMM-Newton data. Only one of these sources, XMMU
J141255.6-635932, is spectrally compatible with GRO J1411-64 although the fact
the soft X-ray observations do not cover the full extent of the COMPTEL source
position uncertainty make an association hard to quantify and thus risky. The
unique peak of the power output at high energies (hard X-rays and gamma-rays)
resembles that found in the SED seen in blazars or microquasars. However, an
analysis using a microquasar model consisting on a magnetized conical jet
filled with relativistic electrons which radiate through synchrotron and
inverse Compton scattering with star, disk, corona and synchrotron photons
shows that it is hard to comply with all observational constrains. This and the
non-detection at hard X-rays introduce an a-posteriori question mark upon the
physical reality of this source, which is discussed in some detail
Hadronic gamma-ray emission from windy microquasars
The jets of microquasars with high-mass stellar companions are exposed to the
dense matter field of the stellar wind. We present estimates of the gamma-ray
emission expected from the jet-wind hadronic interaction and we discuss the
detectability of the phenomenon at high energies. The proposed mechanism could
explain some of the unidentified gamma-ray sources detected by EGRET instrument
on the galactic plane.Comment: Accepted for publication in Astronomy and Astrophysics as a Letter to
the Edito
A broadband leptonic model for gamma-ray emitting microquasars
Observational and theoretical studies point to microquasars (MQs) as possible
counterparts of a significant fraction of the unidentified gamma-ray sources
detected so far. At present, a proper scenario to explain the emission beyond
soft X-rays from these objects is not known, nor what the precise connection is
between the radio and the high-energy radiation. We develop a new model where
the MQ jet is dynamically dominated by cold protons and radiatively dominated
by relativistic leptons. The matter content and power of the jet are both
related with the accretion process. The magnetic field is assumed to be close
to equipartition, although it is attached to and dominated by the jet matter.
For the relativistic particles in the jet, their maximum energy depends on both
the acceleration efficiency and the energy losses. The model takes into account
the interaction of the relativistic jet particles with the magnetic field and
all the photon and matter fields. Such interaction produces significant amounts
of radiation from radio to very high energies through synchrotron, relativistic
Bremsstrahlung, and inverse Compton (IC) processes. Variability of the emission
produced by changes in the accretion process (e.g. via orbital eccentricity) is
also expected. The effects of the gamma-ray absorption by the external photon
fields on the gamma-ray spectrum have been taken into account, revealing clear
spectral features that might be observed. This model is consistent to the
accretion scenario, energy conservation laws, and current observational
knowledge, and can provide deeper physical information of the source when
tested against multiwavelength data.Comment: 15 pages, 12 figures, A&A, in press (text and plots improved after
minor corrections in calculations, text improved also by referee comments
Deep Chandra observations of TeV binaries I: LSI +61 303
We report on a 95ks Chandra observation of the TeV emitting High Mass X-ray
Binary LSI +61 303, using the ACIS-S camera in Continuos Clocking mode to
search for a possible X-ray pulsar in this system. The observation was
performed while the compact object was passing from phase 0.94 to 0.98 in its
orbit around the Be companion star (hence close to the apastron passage). We
did not find any periodic or quasi-periodic signal (at this orbital phase) in a
frequency range of 0.005-175 Hz. We derived an average pulsed fraction 3 sigma
upper limit for the presence of a periodic signal of ~10% (although this limit
is strongly dependent on the frequency and the energy band), the deepest limit
ever reached for this object. Furthermore, the source appears highly variable
in flux and spectrum even in this very small orbital phase range, in particular
we detect two flares, lasting thousands of seconds, with a very hard X-ray
spectrum with respect to the average source spectral distribution. The X-ray
pulsed fraction limits we derived are lower than the pulsed fraction of any
isolated rotational-powered pulsar, in particular having a TeV counterpart. In
this scenario most of the X-ray emission of LSI +61 303 should necessarily come
from the interwind or inner-pulsar wind zone shock rather than from the
magnetosphere of the putative pulsar. Furthermore, we did not find evidence for
the previously suggested extended X-ray emission (abridged).Comment: 9 pages, 6 figures, MNRAS in pres
Accretion vs colliding wind models for the gamma-ray binary LS I +61 303: an assessment
LS I +61 303 is a puzzling Be/X-ray binary with variable gamma-ray emission
at up TeV energies. The nature of the compact object and the origin of the
high-energy emission are unclear. One family of models invokes particle
acceleration in shocks from the collision between the B-star wind and a
relativistic pulsar wind, while another centers on a relativistic jet powered
by accretion. Recent high-resolution radio observations showing a putative
"cometary tail" pointing away from the Be star near periastron have been cited
as support for the pulsar-wind model. We wish here to carry out a quantitative
assessment of these competing models for this extraordinary source. We apply a
3D SPH code for dynamical simulations of both the pulsar-wind-interaction and
accretion-jet models. The former yields a description of the shape of the
wind-wind interaction surface. The latter provides an estimation of the
accretion rate. The results allow critical evaluation of how the two distinct
models confront the data in various wavebands under a range of conditions. When
one accounts for the 3D dynamical wind interaction under realistic constraints
for the relative strength of the B-star and pulsar winds, the resulting form of
the interaction front does not match the putative "cometary tail" claimed from
radio observations. On the other hand, dynamical simulations of the
accretion-jet model indicate that the orbital phase variation of accretion
power includes a secondary broad peak well away from periastron, thus providing
a plausible way to explain the observed TeV gamma ray emission toward apastron.
We conclude that the colliding-wind model is not clearly established for LS I
+61 303, while the accretion-jet model can reproduce many key characteristics
of the observed TeV gamma-ray emission.Comment: Accepted for publication in A&A. The resolution of the figures is
lower than in the journal paper to minimize file sizes. Seven pages, 5
figure
Extended X-ray emission in the vicinity of the microquasar LS 5039: pulsar wind nebula?
LS 5039 is a high-mass binary with a period of 4 days, containing a compact
object and an O star, one of the few high-mass binaries detected in gamma-rays.
Our Chandra ACIS observation of LS 5039 provided a high-significance (~10sigma)
detection of extended emission clearly visible for up to 1' from the point
source. The spectrum of this emission can be described by an absorbed power-law
model with photon index Gamma=1.9pm0.3, somewhat softer than the point source
spectrum Gamma=1.44pm0.07, with the same absorption, N_H=(6.4pm0.6)e21 /cm2.
The observed 0.5-8 keV flux of the extended emission is 8.8e-14 erg/s/cm2, or
5% of the point source flux; the latter is a factor of ~2 lower than the lowest
flux detected so far. Fainter extended emission with comparable flux and a
softer (Gamma~3) spectrum is detected at even greater radii (up to 2'). Two
possible interpretations of the extended emission are a dust scattering halo
and a synchrotron nebula powered by energetic particles escaping the binary. We
discuss both of these scenarios and favor the nebula interpretation, although
some dust contribution is possible. We have also found transient sources
located within a narrow stripe south of LS 5039. We discuss the likelihood of
these sources to be related to LS 5039.Comment: 28 pages. Accepted for publication in Ap
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