21,142 research outputs found
Massive non-thermal radio emitters: new data and their modelling
During recent years some non-thermal radio emitting OB stars have been
discovered to be binary, or multiple systems. The non-thermal emission is due
to synchrotron radiation that is emitted by electrons accelerated up to high
energies. The electron acceleration occurs at the strong shocks created by the
collision of radiatively-driven winds. Here we summarize the available radio
data and more recent observations for the binary Cyg OB2 No. 9. We also show a
new emission model which is being developed to compare the theoretical total
radio flux and the spectral index with the observed radio light curves. This
comparison will be useful in order to solve fundamental questions, such as the
determination of the stellar mass loss rates, which are perturbed by clumping.Comment: 3 pages, 1 figure, poster at Four Decades of Research on Massive
Stars-A Scientific Meeting in Honour of Anthony F.J.Moffa
HST Proper Motion confirms the optical identification of the nearby pulsar PSR 1929+10
We report on the proper motion measurement of the proposed optical
counterpart of the X-ray/radio pulsar PSR 1929+10. Using images obtained with
the HST/STIS (average epoch 2001.73) we computed a yearly displacement of +97
+/- 1 mas yr^{-1} in RA and +46 +/- 1 mas yr^{-1} in Dec since the epoch
(1994.52) of the original HST/FOC detection. Both the magnitude and direction
of the optical proper motion components are found to be fully consistent with
the most recent VLBA radio measurements. This result provides an unambiguous
confirmation of the pulsar optical identification. In addition, we have used
the combined STIS/FOC datasets to derive information on the pulsar spectrum,
which seems characterized by a power law component, apparently unrelated to the
X-ray emission.Comment: 12 pages, 3 figures, submitted to ApJ Letter
On the X-ray feature associated with the Guitar Nebula
Context: A mysterious X-ray nebula, showing a remarkably linear geometry, was
recently discovered close to the Guitar Nebula, the bow-shock nebula associated
with B2224+65, which is the fastest pulsar known. The nature of this X-ray
feature is unknown, and even its association with pulsar B2224+65 is unclear.
Aims: We attempt to develop a self-consistent scenario to explain the complex
phenomenology of this object. Methods: We assume that the highest energy
electrons accelerated at the termination shock escape from the bow shock and
diffuse into the ambient medium, where they emit synchrotron X-rays. The linear
geometry should reflect the plane-parallel geometry of its ambient field.
Results: We estimate the Lorentz factor of the X-ray emitting electrons and the
strength of the magnetic field. The former (~10^8) is close to its maximum
possible value, while the latter, at ~45 uG, is higher than typical
interstellar values and must have been amplified in some way. The magnetic
field must also be turbulent to some degree to trap the electrons sufficiently
for synchrotron X-ray emission to occur effectively. We propose a
self-consistent scenario in which, by some streaming instability, the electrons
themselves generate a turbulent field in which they then diffuse. Some
numerical coincidences are explained, and tests are proposed to verify our
scenario. Conclusions: Electron leaking may be common in the majority of pulsar
bow-shock nebulae, even though the X-ray nebulosity in general is too diffuse
to be detectable.Comment: 4 pages. Accepted for publication in A&A, Letters to the Edito
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 long period eccentric orbit of the particle accelerator HD167971 revealed by long baseline interferometry
Using optical long baseline interferometry, we resolved for the first time
the two wide components of HD167971, a candidate hierarchical triple system
known to efficiently accelerate particles. Our multi-epoch VLTI observations
provide direct evidence for a gravitational link between the O8 supergiant and
the close eclipsing O + O binary. The separation varies from 8 to 15 mas over
the three-year baseline of our observations, suggesting that the components
evolve on a wide and very eccentric orbit (most probably e>0.5). These results
provide evidence that the wide orbit revealed by our study is not coplanar with
the orbit of the inner eclipsing binary. From our measurements of the
near-infrared luminosity ratio, we constrain the spectral classification of the
components in the close binary to be O6-O7, and confirm that these stars are
likely main-sequence objects. Our results are discussed in the context of the
bright non-thermal radio emission already reported for this system, and we
provide arguments in favour of a maximum radio emission coincident with
periastron passage. HD167971 turns out to be an efficient O-type particle
accelerator that constitutes a valuable target for future high angular
resolution radio imaging using VLBI facilities.Comment: 8 pages, including 4 figures, accepted by Monthly Notices of the
Royal Astronomical Societ
Star-Formation in Low Radio Luminosity AGN from the Sloan Digital Sky Survey
We investigate faint radio emission from low- to high-luminosity Active
Galactic Nuclei (AGN) selected from the Sloan Digital Sky Survey (SDSS). Their
radio properties are inferred by co-adding large ensembles of radio image
cut-outs from the FIRST survey, as almost all of the sources are individually
undetected. We correlate the median radio flux densities against a range of
other sample properties, including median values for redshift, [OIII]
luminosity, emission line ratios, and the strength of the 4000A break. We
detect a strong trend for sources that are actively undergoing star-formation
to have excess radio emission beyond the ~10^28 ergs/s/Hz level found for
sources without any discernible star-formation. Furthermore, this additional
radio emission correlates well with the strength of the 4000A break in the
optical spectrum, and may be used to assess the age of the star-forming
component. We examine two subsamples, one containing the systems with emission
line ratios most like star-forming systems, and one with the sources that have
characteristic AGN ratios. This division also separates the mechanism
responsible for the radio emission (star-formation vs. AGN). For both cases we
find a strong, almost identical, correlation between [OIII] and radio
luminosity, with the AGN sample extending toward lower, and the star-formation
sample toward higher luminosities. A clearer separation between the two
subsamples is seen as function of the central velocity dispersion of the host
galaxy. For systems with similar redshifts and velocity dispersions, the
star-formation subsample is brighter than the AGN in the radio by an order of
magnitude. This underlines the notion that the radio emission in star-forming
systems can dominate the emission associated with the AGN.Comment: Accepted for publication in Astronomical Journal; 15 pages, 8 color
figure
The 2.35 year itch of Cyg OB2 #9. II. Radio monitoring
Cyg OB2 #9 is one of a small set of non-thermal radio emitting massive O-star
binaries. The non-thermal radiation is due to synchrotron emission in the
colliding-wind region. Cyg OB2 #9 was only recently discovered to be a binary
system and a multi-wavelength campaign was organized to study its 2011
periastron passage. We report here on the results of the radio observations
obtained in this monitoring campaign. We used the Expanded Very Large Array
(EVLA) radio interferometer to obtain 6 and 20 cm continuum fluxes. The
observed radio light curve shows a steep drop in flux sometime before
periastron. The fluxes drop to a level that is comparable to the expected
free-free emission from the stellar winds, suggesting that the non-thermal
emitting region is completely hidden at that time. After periastron passage,
the fluxes slowly increase. We introduce a simple model to solve the radiative
transfer in the stellar winds and the colliding-wind region, and thus determine
the expected behaviour of the radio light curve. From the asymmetry of the
light curve, we show that the primary has the stronger wind. This is somewhat
unexpected if we use the astrophysical parameters based on theoretical
calibrations. But it becomes entirely feasible if we take into account that a
given spectral type - luminosity class combination covers a range of
astrophysical parameters. The colliding-wind region also contributes to the
free-free emission, which can help to explain the high values of the spectral
index seen after periastron passage. Combining our data with older Very Large
Array (VLA) data allows us to derive a period P = 860.0 +- 3.7 days for this
system. With this period, we update the orbital parameters that were derived in
the first paper of this series.Comment: 10 pages, 4 figures, accepted for publication in 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
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
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