94 research outputs found
BL LAC PKSB1144-379 an extreme scintillator
Rapid variability in the radio flux density of the BL Lac object PKSB1144-379
has been observed at four frequencies, ranging from 1.5 to 15 GHz, with the VLA
and the University of Tasmania's Ceduna antenna. Intrinsic and line of sight
effects were examined as possible causes of this variability, with interstellar
scintillation best explaining the frequency dependence of the variability
timescales and modulation indices. This scintillation is consistent with a
compact source 20-40 microarcseconds, or 0.15-0.3 pc in size. The inferred
brightness temperature for PKSB1144-379 (assuming that the observed variations
are due to scintillation) is 6.2e12 K at 4.9 GHz, with approximately 10 percent
of the total flux in the scintillating component. We show that scintillation
surveys aimed at identifying variability timescales of days to weeks are an
effective way to identify the AGN with the highest brightness temperatures.Comment: 6 pages, 3 figures, accepted for publication in ApJ Letter
VLBA images of High Frequency Peakers
We propose a morphological classification based on the parsec scale structure
of fifty-one High Frequency Peakers (HFPs) from the ``bright'' HFP sample. VLBA
images at two adjacent frequencies (chosen among 8.4, 15.3, 22.2 and 43.2 GHz)
have been used to investigate the morphological properties of the HFPs in the
optically thin part of their spectrum. We confirm that there is quite a clear
distinction between the pc-scale radio structure of galaxies and quasars: the
78% of the galaxies show a ``Double/Triple'' morphology, typical of Compact
Symmetric Objects (CSOs), while the 87% of the quasars are characterised by
Core-Jet or unresolved structure. This suggests that most HFP candidates
identified with quasars are likely blazar objects in which a flaring
self-absorbed component at the jet base was outshining the remainder of the
source at the time of the selection based on the spectral shape. Among the
sources classified as CSOs or candidates it is possible to find extremely young
radio sources with ages of about 100 years or even less.Comment: 21 pages, 8 figures; accepted for pubblication in A&A. Paper version
with full resolution images is available at
http://www.ira.inaf.it/~ddallaca/orienti.p
Discovery of an X-ray Jet and Extended Jet Structure in the Quasar PKS 1055+201
This letter reports rich X-ray jet structures found in the Chandra
observation of PKS 1055+201. In addition to an X-ray jet coincident with the
radio jet we detect a region of extended X-ray emission surrounding the jet as
far from the core as the radio hotspot to the North, and a similar extended
X-ray region along the presumed path of the unseen counterjet to the Southern
radio lobe. Both X-ray regions show a similar curvature to the west, relative
to the quasar. We interpret this as the first example where we separately
detect the X-ray emission from a narrow jet and extended, residual jet plasma
over the entire length of a powerful FRII jet.Comment: Accepted for publication in Ap. J. Letters. 4 pages, 3 figure
Observations and properties of candidate high frequency GPS radio sources in the AT20G survey
We used the Australia Telescope Compact Array (ATCA) to obtain 40 GHz and 95
GHz observations of a number of sources that were selected from the Australia
Telescope Compact Array 20 GHz (AT20G) survey . The aim of the observations was
to improve the spectral coverage for sources with spectral peaks near 20 GHz or
inverted (rising) radio spectra between 8.6 GHz and 20 GHz. We present the
radio observations of a sample of 21 such sources along with optical spectra
taken from the ANU Siding Spring Observatory 2.3m telescope and the ESO-New
Technology Telescope (NTT). We find that as a group the sources show the same
level of variability as typical GPS sources, and that of the 21 candidate GPS
sources roughly 60% appear to be genuinely young radio galaxies. Three of the
21 sources studied show evidence of being restarted radio galaxies. If these
numbers are indicative of the larger population of AT20G radio sources then as
many as 400 genuine GPS sources could be contained within the AT20G with up to
25% of them being restarted radio galaxies.Comment: 21 pages, 24 figures, Table 1 truncated at 11 column
Far-infrared observations of a massive cluster forming in the Monoceros R2 filament hub
We present far-infrared observations of Monoceros R2 (a giant molecular cloud at approximately 830 pc distance, containing several sites of active star formation), as observed at 70 ÎŒm, 160 ÎŒm, 250 ÎŒm, 350 ÎŒm, and 500 ÎŒm by the Photodetector Array Camera and Spectrometer (PACS) and Spectral and Photometric Imaging Receiver (SPIRE) instruments on the Herschel Space Observatory as part of the Herschel imaging survey of OB young stellar objects (HOBYS) Key programme. The Herschel data are complemented by SCUBA-2 data in the submillimetre range, and WISE and Spitzer data in the mid-infrared. In addition, C18O data from the IRAM 30-m Telescope are presented, and used for kinematic information. Sources were extracted from the maps with getsources, and from the fluxes measured, spectral energy distributions were constructed, allowing measurements of source mass and dust temperature. Of 177 Herschel sources robustly detected in the region (a detection with high signal-to-noise and low axis ratio at multiple wavelengths), including protostars and starless cores, 29 are found in a filamentary hub at the centre of the region (a little over 1% of the observed area). These objects are on average smaller, more massive, and more luminous than those in the surrounding regions (which together suggest that they are at a later stage of evolution), a result that cannot be explained entirely by selection effects. These results suggest a picture in which the hub may have begun star formation at a point significantly earlier than the outer regions, possibly forming as a result of feedback from earlier star formation. Furthermore, the hub may be sustaining its star formation by accreting material from the surrounding filaments
Testing the Blazar Paradigm: ASCA Observations of FSRQs with Steep Soft X-ray Spectra
We present the first observations at medium-hard X-rays with ASCA in 1998
August--November of four Flat Spectrum Radio Quasars (FSRQs), characterized by
unusually steep soft X-ray spectra (photon index, \Gamma_{0.2-2.4 keV} \sim
2-2.5), as previously measured with ROSAT. Such steep X-ray slopes are similar
to those observed in synchrotron-dominated BL Lacs and are unexpected in the
context of the recent blazar paradigm, where sources with strong emission lines
(such as FSRQs) are dominated in soft X-rays by a flat inverse Compton tail. We
find that the ASCA spectra of the four FSRQs are consistent with a power law
model with \Gamma_{2-10 keV} \sim 1.8, flatter than their ROSAT spectra. This
indicates the onset of an inverse Compton component at energies \gtrsim 2 keV,
in agreement with the blazar unification scheme. However, these objects are
still anomalous within the blazar class for their steep soft X-ray continua
which, together with non-simultaneous data at longer wavelengths, hint at the
possibility that the synchrotron emission extends to soft X-rays. This would
imply an anomalously high synchotron peak frequency for a quasar with luminous
broad lines, challenging current blazar unification schemes. Alternatively, a
plausible explanation for the steep optical-to-soft X-ray continua of the four
FSRQs is thermal emission from the accretion disk, similar to the blazars
3C~273 and 3C~345. In the Appendix, we present fits to the SIS data in an
effort to contribute to the ongoing calibration of the the time-dependence of
the SIS response at low energies.Comment: accepted for publication in The Astrophysical Journa
Sub-milliarcsecond Imaging of Quasars and AGN
We have used the VLBA at 15 GHz to image the structure of 132 strong compact
AGN and quasars with a resolution better than one milliarcsecond and a dynamic
range typically exceeding 1000 to 1. These observations were made as part of a
program to investigate the sub-parsec structure of quasars and AGN and to study
the changes in their structure with time. Many of the sources included in our
study, particularly those located south of +35 degrees, have not been
previously imaged with milliarcsecond resolution. Each of the sources has been
observed at multiple epochs. In this paper we show images of each of the 132
sources which we have observed. For each source we present data at the epoch
which had the best quality data. The milliarcsecond jets generally appear
one-sided but two-sided structure is often found in lower luminosity radio
galaxies and in high luminosity quasars with gigahertz peaked spectra. Usually
the structure is unresolved along the direction perpendicular to the jet, but a
few sources have broad plumes. In some low luminosity radio galaxies, the
structure appears more symmetric at 2 cm than at long wavelengths. The apparent
long wavelength symmetry in these sources is probably due to absorption by
intervening material. A few sources contain only a single component with any
secondary feature at least a thousand times weaker. We find no obvious
correlation of radio morphology and the detection of gamma-ray emission by
EGRET.Comment: 19 pages, 3 tables, 3 figures. Figure 2 (132 contour diagrams) is
long and is omitted here. Figure 2 may be viewed at
http://www.cv.nrao.edu/2cmsurvey/ In press, Astronomical Journal, April 199
Detection of Intra-day Variability Timescales of Four High Energy Peaked Blazars with XMM-Newton
We selected a sample of 24 XMM-Newton light curves (LCs) of four high energy
peaked blazars, PKS 0548-322, ON 231, 1ES 1426+428 and PKS 2155-304. These data
comprise continuous light curves of 7.67h to 18.97h in length. We searched for
possible quasi-periodic oscillations (QPO) and intra-day variability (IDV)
timescales in the LCs of these blazars. We found a likely QPO in one LC of PKS
2155-304 which was reported elsewhere (Lachowicz et al. 2009). In the remaining
23 LCs we found hints of possible weak QPOs in one LC of each of ON 231 and PKS
2155-304, but neither is statistically significant. We found IDV timescales
that ranged from 15.7 ks to 46.8 ks in 8 LCs. In 13 LCs any variability
timescales were longer than the length of the data. Assuming the possible weak
QPO periods in the blazars PKS 2155-304 and ON 231 are real and are associated
with the innermost portions of their accretion disk, we can estimate that their
central black hole masses exceed 1.2 10 M. Emission
models for radio-loud active galactic nuclei (AGN) that could explain our
results are briefly discussed.Comment: 13 emulateapj pages, 2 tables, 4 figures, accepted for publication in
The Astrophysical Journa
A Uniform CO Survey of the Molecular Clouds in Orion and Monoceros
We report the results of a new large scale survey of the Orion-Monoceros
complex of molecular clouds made in the J = 1->0 line of CO with the
Harvard-Smithsonian 1.2m millimetre-wave telescope. The survey consists of
52,288 uniformly spaced spectra that cover an area of 432 square degrees on the
sky and is the most sensitive large-scale survey of the region to date.
Distances to the constituent molecular clouds of the complex, estimated from an
analysis of foreground and background stars, have provided information on the
three dimensional structure of the entire complex.Comment: Accepted for publication in Astronomy and Astrophysics. 19 pages with
17 colour figures - 39 if you count the sub-figures separately. The figures
here have been bit-mapped with some loss of quality and beauty. The paper
version in A&A will be in greyscale with the on-line version in colour. In
the meantime the colour version can be obtained by following links at
http://www.star.bris.ac.uk/mrwm . The 9MB PostScript is recommended if you
have appropriate bandwidth or otherwise the 2.3MB PDF is usabl
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