298 research outputs found
New ATCA, ALMA and VISIR observations of the candidate LBV SK-67266 (S61): the nebular mass from modelling 3D density distributions
We present new observations of the nebula around the Magellanic candidate
Luminous Blue Variable S61. These comprise high-resolution data acquired with
the Australia Telescope Compact Array (ATCA), the Atacama Large
Millimetre/Submillimetre Array (ALMA), and VISIR at the Very Large Telescope
(VLT). The nebula was detected only in the radio, up to 17 GHz. The 17 GHz ATCA
map, with 0.8 arcsec resolution, allowed a morphological comparison with the
H Hubble Space Telescope image. The radio nebula resembles a spherical
shell, as in the optical. The spectral index map indicates that the radio
emission is due to free-free transitions in the ionised, optically thin gas,
but there are hints of inhomogeneities. We present our new public code RHOCUBE
to model 3D density distributions, and determine via Bayesian inference the
nebula's geometric parameters. We applied the code to model the electron
density distribution in the S61 nebula. We found that different distributions
fit the data, but all of them converge to the same ionised mass, ~0.1 , which is an order of magnitude smaller than previous estimates. We
show how the nebula models can be used to derive the mass-loss history with
high-temporal resolution. The nebula was probably formed through stellar winds,
rather than eruptions. From the ALMA and VISIR non-detections, plus the derived
extinction map, we deduce that the infrared emission observed by space
telescopes must arise from extended, diffuse dust within the ionised region.Comment: 17 pages, 9 figures. Authors list corrected. In press in MNRAS.
RHOCUBE code available online ( https://github.com/rnikutta/rhocube
The Luminous Blue Variable RMC127 as seen with ALMA and ATCA
We present ALMA and ATCA observations of the luminous blue variable \rmc. The
radio maps show for the first time the core of the nebula and evidence that the
nebula is strongly asymmetric with a Z-pattern shape. Hints of this morphology
are also visible in the archival \emph{HST} image, which overall
resembles the radio emission. The emission mechanism in the outer nebula is
optically thin free-free in the radio. At high frequencies, a component of
point-source emission appears at the position of the star, up to the ALMA
frequencies. The rising flux density distribution () of this object suggests thermal emission from the ionized
stellar wind and indicates a departure from spherical symmetry with
. We examine different scenarios to explain this excess
of thermal emission from the wind and show that this can arise from a bipolar
outflow, supporting the suggestion by other authors that the stellar wind of
\rmc is aspherical. We fit the data with two collimated ionized wind models and
we find that the mass-loss rate can be a factor of two or more smaller than in
the spherical case. We also fit the photometry obtained by IR space telescopes
and deduce that the mid- to far-IR emission must arise from extended, cool
() dust within the outer ionized nebula. Finally we discuss two
possible scenarios for the nebular morphology: the canonical single star
expanding shell geometry, and a precessing jet model assuming presence of a
companion star.Comment: Accepted for publication in ApJ (minor revision included
SCORPIO-II: Spectral indices of weak Galactic radio sources
In the next few years the classification of radio sources observed by the
large surveys will be a challenging problem, and spectral index is a powerful
tool for addressing it. Here we present an algorithm to estimate the spectral
index of sources from multiwavelength radio images. We have applied our
algorithm to SCORPIO (Umana et al. 2015), a Galactic Plane survey centred
around 2.1 GHz carried out with ATCA, and found we can measure reliable
spectral indices only for sources stronger than 40 times the rms noise. Above a
threshold of 1 mJy, the source density in SCORPIO is 20 percent greater than in
a typical extra-galactic field, like ATLAS (Norris et al. 2006), because of the
presence of Galactic sources. Among this excess population, 16 sources per
square degree have a spectral index of about zero, suggesting optically thin
thermal emission such as Hii regions and planetary nebulae, while 12 per square
degree present a rising spectrum, suggesting optically thick thermal emission
such as stars and UCHii regions.Comment: 12 pages, 11 figures, accepted by MNRA
The polarization mode of the auroral radio emission from the early-type star HD142301
We report the detection of the auroral radio emission from the early-type
magnetic star HD142301. New VLA observations of HD142301 detected highly
polarized amplified emission occurring at fixed stellar orientations. The
coherent emission mechanism responsible for the stellar auroral radio emission
amplifies the radiation within a narrow beam, making the star where this
phenomenon occurs similar to a radio lighthouse. The elementary emission
process responsible for the auroral radiation mainly amplifies one of the two
magneto-ionic modes of the electromagnetic wave. This explains why the auroral
pulses are highly circularly polarized. The auroral radio emission of HD142301
is characterized by a reversal of the sense of polarization as the star
rotates. The effective magnetic field curve of HD142301 is also available
making it possible to correlate the transition from the left to the right-hand
circular polarization sense (and vice-versa) of the auroral pulses with the
known orientation of the stellar magnetic field. The results presented in this
letter have implications for the estimation of the dominant magneto-ionic mode
amplified within the HD142301 magnetosphere.Comment: 5 pages, 4 figures; accepted to MNRAS Letter
A combined multiwavelength VLA/ALMA/Chandra study unveils the complex magnetosphere of the B-type star HR5907
We present new radio/millimeter measurements of the hot magnetic star HR5907
obtained with the VLA and ALMA interferometers. We find that HR5907 is the most
radio luminous early type star in the cm-mm band among those presently known.
Its multi-wavelength radio light curves are strongly variable with an amplitude
that increases with radio frequency. The radio emission can be explained by the
populations of the non-thermal electrons accelerated in the current sheets on
the outer border of the magnetosphere of this fast rotating magnetic star. We
classify HR5907 as another member of the growing class of strongly magnetic
fast rotating hot stars where the gyro-synchrotron emission mechanism
efficiently operates in their magnetospheres. The new radio observations of
HR5907 are combined with archival X-ray data to study the physical condition of
its magnetosphere. The X-ray spectra of HR5907 show tentative evidence for the
presence of non-thermal spectral component. We suggest that non-thermal X-rays
originate a stellar X-ray aurora due to streams of non-thermal electrons
impacting on the stellar surface. Taking advantage of the relation between the
spectral indices of the X-ray power-law spectrum and the non-thermal electron
energy distributions, we perform 3-D modeling of the radio emission for HR5907.
The wavelength-dependent radio light-curves probe magnetospheric layers at
different heights above the stellar surface. A detailed comparison between
simulated and observed radio light-curves leads us to conclude that the stellar
magnetic field of HR5907 is likely non-dipolar, providing further indirect
evidence of the complex magnetic field topology of HR5907.Comment: 18 pages, 13 figures; accepted to MNRA
Multifrequency monitoring of the blazar 0716+714 during the GASP-WEBT-AGILE campaign of 2007
Since the CGRO operation in 1991-2000, one of the primary unresolved
questions about the blazar gamma-ray emission has been its possible correlation
with the low-energy (in particular optical) emission. To help answer this
problem, the Whole Earth Blazar Telescope (WEBT) consortium has organized the
GLAST-AGILE Support Program (GASP) to provide the optical-to-radio monitoring
data to be compared with the gamma-ray detections by the AGILE and GLAST
satellites. This new WEBT project started in early September 2007, just before
a strong gamma-ray detection of 0716+714 by AGILE. We present the GASP-WEBT
optical and radio light curves of this blazar obtained in July-November 2007,
about various AGILE pointings at the source. We construct NIR-to-UV spectral
energy distributions (SEDs), by assembling GASP-WEBT data together with UV data
from the Swift ToO observations of late October. We observe a contemporaneous
optical-radio outburst, which is a rare and interesting phenomenon in blazars.
The shape of the SEDs during the outburst appears peculiarly wavy because of an
optical excess and a UV drop-and-rise. The optical light curve is well sampled
during the AGILE pointings, showing prominent and sharp flares. A future
cross-correlation analysis of the optical and AGILE data will shed light on the
expected relationship between these flares and the gamma-ray events.Comment: 5 pages, 5 figures, to be published in A&A (Letters); revised to
match the final version (changes in Fig. 5 and related text
The WEBT Campaign on the Blazar 3C279 in 2006
The quasar 3C279 was the target of an extensive multiwavelength monitoring
campaign from January through April 2006, including an optical-IR-radio
monitoring campaign by the Whole Earth Blazar Telescope (WEBT) collaboration.
In this paper we focus on the results of the WEBT campaign. The source
exhibited substantial variability of optical flux and spectral shape, with a
characteristic time scale of a few days. The variability patterns throughout
the optical BVRI bands were very closely correlated with each other. In
intriguing contrast to other (in particular, BL Lac type) blazars, we find a
lag of shorter- behind longer-wavelength variability throughout the RVB ranges,
with a time delay increasing with increasing frequency. Spectral hardening
during flares appears delayed with respect to a rising optical flux. This, in
combination with the very steep IR-optical continuum spectral index of ~ 1.5 -
2.0, may indicate a highly oblique magnetic field configuration near the base
of the jet. An alternative explanation through a slow (time scale of several
days) acceleration mechanism would require an unusually low magnetic field of <
0.2 G, about an order of magnitude lower than inferred from previous analyses
of simultaneous SEDs of 3C279 and other FSRQs with similar properties.Comment: Accepted for publication in Ap
Study of the Galactic radio sources in the SCORPIO survey resolved by ATCA at 2.1 GHz
We present a catalogue of a large sample of extended radio sources in the
SCORPIO field, observed and resolved by the Australia Telescope Compact Array.
SCORPIO, a pathfinder project for addressing the early operations of the
Australia SKA Pathfinder, is a survey of ~5 square degrees between 1.4 and 3.1
GHz, centered at l=343.5{\deg}, b=0.75{\deg} and with an angular resolution of
about 10 arcsec. It is aimed at understanding the scientific and technical
challenges to be faced by future Galactic surveys. With a mean sensitivity
around 100 Jy/beam and the possibility to recover angular scales at least
up to 4 arcmin, we extracted 99 extended sources, 35 of them detected for the
first time. Among the 64 known sources 55 had at least a tentative
classification in literature. Studying the radio morphology and comparing the
radio emission with infrared we propose as candidates 6 new H II regions, 2 new
planetary nebulae, 2 new luminous blue variable or Wolf--Rayet stars and 3 new
supernova remnants. This study provides an overview of the potentiality of
future radio surveys in terms of Galactic source extraction and
characterization and a discussion on the difficulty to reduce and analyze
interferometric data on the Galactic plane
Another look at the BL Lacertae flux and spectral variability
The GLAST-AGILE Support Program (GASP) of the Whole Earth Blazar Telescope
(WEBT) monitored BL Lacertae in 2008-2009 at radio, near-IR, and optical
frequencies. During this period, high-energy observations were performed by
XMM-Newton, Swift, and Fermi. We analyse these data with particular attention
to the calibration of Swift UV data, and apply a helical jet model to interpret
the source broad-band variability. The GASP-WEBT observations show an optical
flare in 2008 February-March, and oscillations of several tenths of mag on a
few-day time scale afterwards. The radio flux is only mildly variable. The UV
data from both XMM-Newton and Swift seem to confirm a UV excess that is likely
caused by thermal emission from the accretion disc. The X-ray data from
XMM-Newton indicate a strongly concave spectrum, as well as moderate flux
variability on an hour time scale. The Swift X-ray data reveal fast (interday)
flux changes, not correlated with those observed at lower energies. We compare
the spectral energy distribution (SED) corresponding to the 2008 low-brightness
state, which was characterised by a synchrotron dominance, to the 1997 outburst
state, where the inverse-Compton emission was prevailing. A fit with an
inhomogeneous helical jet model suggests that two synchrotron components are at
work with their self inverse-Compton emission. Most likely, they represent the
radiation from two distinct emitting regions in the jet. We show that the
difference between the source SEDs in 2008 and 1997 can be explained in terms
of pure geometrical variations. The outburst state occurred when the
jet-emitting regions were better aligned with the line of sight, producing an
increase of the Doppler beaming factor. Our analysis demonstrates that the jet
geometry can play an extremely important role in the BL Lacertae flux and
spectral variability.Comment: 12 pages, 10 figures, accepted for publication in A&
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