32 research outputs found
Understanding the Fanaroff-Riley radio galaxy classification
The simple, yet profoundly far-reaching classification scheme based on
extended radio morphologies of radio galaxies, the Fanaroff-Riley
classification has been a cornerstone in our understanding of radio galaxies.
Over the decades since the recognition that there are two basic types of radio
galaxy morphologies there have been several findings in different wavebands
that have reported properties on different scales. Although it is realized that
there may be intrinsic as well external causes an overarching view of how we
may understand the two morphological types is missing. With the radio
power-absolute magnitude relation (the Owen-Ledlow diagram) as backdrop we
review and develop an understanding of the two radio galaxy types in the light
of what is known about them. We have for the first time included the dust
properties of the two FR classes together with the relative orientations of
dust, host major axis and the radio axis to present a qualitative framework
within which to understand the conditions under which they form. (Abridged).Comment: 15 pages, To appear in Astronomical Journa
Simultaneous X-ray and optical observations of thermonuclear bursts in the LMXB EXO 0748-676
We report the detection of a large number of optical bursts in the Low Mass
X-ray Binary (LMXB) EXO 0748-676 simultaneous with the thermonuclear X-ray
bursts. The X-ray and the optical bursts are detected in a long observation of
this source with the XMM-Newton observatory. This has increased the number of
thermonuclear X-ray bursts in the LMXBs with simultaneous optical detection by
several factors. The optical bursts are found to have a linear rise followed by
a slow, somewhat exponential decay. Most of the optical bursts have longer rise
and decay timescale compared to the corresponding X-ray bursts. We have
determined the X-ray and optical excess photon counts in the bursts that allow
us to look at the optical to X-ray burst fluence ratio for each burst and the
ratio as a function of the X-ray burst intensity and as a function of the
orbital phase. The delay between the onset of the X-ray bursts and the onset of
the optical bursts have also been measured and is found to have an average
value of 3.25 seconds. We do not find any convincing evidence of orbital phase
dependence of the following parameters: X-ray to optical delay, rise time of
the optical bursts, and optical to X-ray burst intensity ratio as would be
expected if the optical bursts were produced by reprocessing from the surface
of the secondary star that is facing the compact star. On the other hand, if
the optical bursts are produced by reprocessing of the X-rays in the accretion
disk, the onset of the bursts is not expected to have a sharp, linear shape as
is observed in a few of the bursts in EXO 0748-676. We emphasise the fact that
simultaneous optical observations of the X-ray bursts in multiple wavelength
bands will enable further detailed investigations of the reprocessing
phenomena, including any non-linear effect of the X-ray irradiation.Comment: 12 pares, 5 figures (Submitted to the Bulletin of the Astronomical
Society of India
The Abundance of X-Shaped Radio Sources: Implications for the Gravitational Wave Background
Coalescence of super massive black holes (SMBH's) in galaxy mergers is
potentially the dominant contributor to the low frequency gravitational wave
background (GWB). IIt was proposed by Merritt and Ekers (2002) that X-shaped
radio galaxies are signposts of such coalescences, and that their abundance
might be used to predict the magnitude of the gravitational wave background. In
Roberts et al. (2015) we present radio images of all 52 X-shaped radio source
candidates out of the sample of 100 selected by Cheung (2007) for which
archival VLA data were available. These images indicate that at most 21% of the
candidates might be genuine X-shaped radio sources that were formed by a
restarting of beams in a new direction following a major merger. This suggests
that fewer than 1.3% of extended radio sources appear to be candidates for
genuine axis reorientations ("spin flips"), much smaller than the 7% suggested
by Leahy and Parma (1992). Thus the associated gravitational wave background
may be substantially smaller than previous estimates. These results can be used
to normalize detailed calculations of the SMBH coalescence rate and the GWB
Physical conditions in the intergalactic medium
Measurements of the spectrum of the cosmic microwave background radiation by COBE (Cosmic Background Explorer) limit the line integral of the electron pressure in intergalactic gas out to cosmological distances. The internal physical properties of diffuse bridges in the lobes of giant radio galaxies are also expected to reflect directly the pressure in this intergalactic medium that forms the environment of the radio structure. Surprisingly, the COBE limits rule out an intergalactic medium with the pressure indicated by the radio structures. This suggests that these regions are not thermally confined. However, an intergalactic medium composed of a mixture of hot nuclei and cold electrons which are not in thermal equilibrium is allowed by the COBE constraints and processes during galaxy formation could provide a natural cause for an intergalactic medium with an energy density of the value indicated by the observed radio structures
A deep survey of the low-surface-brightness radio sky
We have made a radio survey--the Australia Telescope Low Brightness Survey
(ATLBS)--of 8.4 square degrees sky area, using the Australia Telescope Compact
Array in the 20-cm band, in an observing mode designed to provide wide-field
images with exceptional sensitivity in surface brightness, and thereby explore
a new parameter space in radio source populations. The goals of this survey are
to quantify the distribution in angular sizes, particularly at weak surface
brightness levels: this has implications for the confusion in deep surveys with
the SKA. The survey is expected to lead to a census of the radio emission
associated with low-power radio galaxies at redshifts 1-3, without any missing
extended emission, and hence a study of the cosmic evolution of low-power radio
galaxies to higher redshift and a comprehensive study of the AGN feedback
during the intense black hole growth phase during this redshift range.Comment: 5 pages, includes 2 figures and 1 table. To appear in the proceedings
of "From Planets to Dark energy: the modern radio universe" in the online
journal Proceedings of Science - Po
0319 - 454: an FR II giant radio galaxy with twin jets
We present radio images of the edge-brightened giant radio galaxy 0319 – 454 made with the Australia Telescope at frequencies 1.5, 2.4 and 4.8 GHz. The observations show a jet and a counterjet out to exceptional distances of 380 and 590 kpc, respectively, from the radio core. 0319–454 is therefore one of only three powerful radio galaxies observed to date to have twin quasi-continuous jets. The NE lobe of the double radio structure has an extremely rare configuration of five compact hotspots that may be classified into a primary and a cluster of four secondaries. Optical imaging of the parent galaxy reveals a prominent, warped dust lane and a highly disturbed structure that is indicative of past merger activity
The Abundance of X-Shaped Radio Sources I. VLA Survey of 52 Sources With Off-Axis Distortions
Cheung identified a sample of 100 candidate X-shaped radio galaxies using the
NRAO FIRST survey; these are small-axial-ratio extended radio sources with
off-axis emission. Here we present radio images of 52 of these sources that
have been made from archival Very Large Array data with resolution of about 1
arcsec. Fifty-one of the 52 were observed at 1.4 GHz, seven were observed at
1.4 GHz and 5 GHz, and one was observed only at 5 GHz. We also present overlays
of the SDSS red images for 48 of the sources, and DSS II overlays for the
remainder. Optical counterparts have been identified for most sources, but
there remain a few empty fields. Our higher resolution VLA images along with
FIRST survey images of the sources in the sample reveal that extended
extragalactic radio sources with small axial ratios are largely (60%) cases of
double radio sources with twin lobes that have off-axis extensions, usually
with inversion-symmetric structure. The available radio images indicate that at
most 20% sources might be genuine X-shaped radio sources that could have formed
by a restarting of beams in a new direction following an interruption and axis
flip. The remaining 20% are in neither of these categories. The implications of
this result for the gravitational wave background are discussed in Roberts,
Saripalli, and Subrahmanyan.Comment: ApJ Supplements (accepted
The X-shaped Radio Galaxy J0725+5835 is Associated with an AGN Pair
X-shaped radio galaxies (XRGs) are those that exhibit two pairs of unaligned radio lobes (main radio lobes and wings). One of the promising models for the peculiar morphology is jet reorientation. To clarify this, we conducted a 5 GHz observation with the European VLBI Network (EVN) of XRG J0725+5835, which resembles the archetypal binary active galactic nuclei (AGNs) 0402+379 in radio morphology, but it is larger in angular size. In our observation, two milliarcsecond-scale radio components with nonthermal radio emission are detected. Each of them coincides with an optical counterpart with similar photometric redshift and (optical and infrared) magnitude, corresponding to dual active nuclei. Furthermore, with the improved Very Large Array (VLA) images, we find a bridge between the two radio cores and a jet bending in the region surrounding the companion galaxy. This further supports the interplay between the main and companion galaxies. In addition, we also report the discovery of an arcsecond-scale jet in the companion. Given the projected separation of similar to 100 kpc between the main and companion galaxies, XRG J0725+5835 is likely associated with a dual jetted-AGN system. In both EVN and VLA observations, we find signatures that the jet is changing its direction, which is likely responsible for the X-shaped morphology. For the origin of jet reorientation, several scenarios are discussed