43 research outputs found
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
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
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
Radio Loud and Radio Quiet Active Galactic Nuclei
We generated a sample of 409 AGNs for which both the radio luminosity at 5
GHz and the line luminosity in [OIII] 5007 have been measured. The radio
luminosity spans a range of ten orders of magnitude, and the [OIII] line
luminosity spans a range of eight orders of magnitude --- both considerably
larger than the ranges in previous studies. We show that these two quantities
are correlated in a similar way for both radio-loud and radio-quiet AGNs. We
demonstrate that the observed correlation can be explained in terms of a model
in which jets are accelerated and collimated by a vertical magnetic field.Comment: 45 pages inc. 7 figures, 1 table of 15 pages in ps-format. Accept to
AJ September 199
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 μm, 160 μm, 250 μm, 350 μm, and 500 μm 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
X-ray measurements of the field and particle energy distributions in the west lobe of the radio galaxy NGC 1316 (Fornax A)
A follow-up X-ray study was made of the west lobe of the radio galaxy Fornax
A, (NGC 1316) based on new ASCA observations made in 1997 for 98 ks, and
incorporating the previous observation in 1994 for 39 ks. The 0.7--10 keV
spectrum of the emission can be described by a power-law of energy index 0.74
+/- 0.10, which agrees with the synchrotron radio index of 0.9 +/- 0.2.
Therefore, the X-rays are reconfirmed to arise via inverse-Compton scattering
of the cosmic microwave photons, as Kaneda et al. (1995) and Feigelson et al.
(1995) concluded. The surface brightness of the inverse-Compton X-rays exhibits
a relatively flat distribution over the west lobe, indicative of an
approximately spherical emissivity distribution with a radius of ~11' (75 kpc).
n contrast, the 1.4 GHz radio image by Ekers et al. (1983) exhibits a rim-
brightened surface brightness, consistentwith a shell-like emissivity
distribution whose inner and outer boundaries are 4' and 11', respectively.
These morphological differences between radio and X-rays suggest that the
relativistic electrons are distributed homogeneously over the lobe volume,
whereas the magnetic field is amplified toward the lobe rim region.Comment: 17 pages, 4 figures, 1 table, to appear in ApJ Lette
Optical Spectra of Four Objects Identified with Variable Radio Sources
We obtained optical spectra of four objects identified with variable radio
sources. Three objects (0029+0554, 0400+0550, 2245+0500) were found to be
quasars with redshifts of 1.314, 0.761, and 1.091. One object (2349+0534) has a
continuum spectrum characteristic of BL Lac objects. We analyze spectra of the
radio sources in the range 0.97-21.7 GHz for the epoch 1997 and in the range
3.9-11.1 GHz for the epoch 1990, as well as the pattern of variability of their
flux densities on time scales of 1.5 and 7 years.Comment: 4 pages, PD
New Multiwavelength Observations of PKS 2155-304 and Implications for the Coordinated Variability Patterns of Blazars
The TeV blazar PKS 2155--304 was the subject of an intensive 2 week optical
and near-infrared observing campaign in 2004 August with the CTIO 0.9m
telescope. During this time, simultaneous X-ray data from RXTE were also
obtained. We compare the results of our observations to the results from two
previous simultaneous multiwavelength campaigns on PKS 2155-304. We conclude
that the correlation between the X-ray and UV/optical variability is strongest
and the time lag is shortest (only a few hours) when the object is brightest.
As the object becomes fainter, the correlations are weaker and the lags longer,
increasing to a few days. Based on the results of four campaigns, we find
evidence for a linear relationship between the mean optical brightness and lag
time of X-ray and UV/optical events. Furthermore, we assert that this behavior,
along with the different multiwavelength flare lag times across different flux
states is consistent with a highly relativistic shock propagating down the jet
producing the flares observed during a high state. In a quiescent state, the
variability is likely to be due to a number of factors including both the jet
and contributions outside of the jet, such as the accretion disk.Comment: 23 pages, 8 figures, to be published in the Astrophysical Journal,
volume 67
Renewed activity in the radio galaxy PKS B1545-321: twin edge-brightened beams within diffuse radio lobes
Australia Telescope Compact Array (ATCA) images of the giant radio galaxy PKS
B1545-321 show a pair of oppositely directed beams emerging from a radio core
and ending in bright components that are symmetrically located on either side.
These inner beams are embedded within edge-brightened outer lobes of lower
surface brightness and the bright ends of the inner beams are well recessed
from the ends of the outer lobes. The inner beams and diffuse surrounding lobes
share a common central core and radio axis. We propose that the observed inner
beams are double lobes which have been created within relic outer lobes as a
consequence of a restarting of the central activity; therefore, PKS B1545-321
is a rare opportunity for examining the development of restarted beams within a
relic synchrotron plasma cocoon. The inner double representing the new episode
has among the highest axial ratios found in typical edge-brightened radio
galaxies. The low radio luminosity of the inner double, the narrow and constant
transverse extent of its cocoon and the relatively low brightness of the
hotspots at its ends are consistent with the almost ballistic propagation
expected for a beam that has a low density contrast and is advancing within a
relatively light ambient medium.Comment: 30 pages, 12 figures, to appear in ApJ June, 200
The Molonglo Southern 4 Jy sample (MS4). II. ATCA imaging and optical identification
Of the 228 sources in the Molonglo Southern 4 Jy Sample (MS4), the 133 with
angular sizes < 35 arcsec have been imaged at 5 GHz at 2-4 arcsec resolution
with the Australia Telescope Compact Array. More than 90% of the sample has
been reliably optically identified, either on the plates of the UK Schmidt
Southern Sky Survey or on R-band CCD images made with the Anglo-Australian
Telescope. A subsample of 137 sources, the SMS4, defined to be a close southern
equivalent of the northern 3CRR sample, was found to have global properties
mostly consistent with the northern sample. Linear sizes of MS4 galaxies and
quasars were found to be consistent with galaxy-quasar unification models of
orientation and evolution.Comment: 102 pages; 6 figures in 21 Postscript files. To appear in
Astronomical Journal. For higher-resolution versions of some figures, see
http://www.physics.usyd.edu.au/astrop/rwh/ms4