Subscribe to the gold package and get unlimited access to Shamra Academy
Register a new userVLBA and Chandra Observations of Jets in FRI radio galaxies: Constraints on Jet Evolution
103
0
0.0
(
0
)
Authors
P. Kharb
Ask ChatGPT about the research
No Arabic abstract
(ABRIDGED) We present here the results from new Very Long Baseline Array observations at 1.6 and 5 GHz of 19 galaxies of a complete sample of 21 UGC FRI radio galaxies. New Chandra data of two sources, viz., UGC00408 and UGC08433, are combined with the Chandra archival data of 13 sources. The 5 GHz observations of ten core-jet sources are polarization-sensitive, while the 1.6 GHz observations constitute second epoch total intensity observations of nine core-only sources. Polarized emission is detected in the jets of seven sources at 5 GHz, but the cores are essentially unpolarized, except in M87. Polarization is detected at the jet edges in several sources, and the inferred magnetic field is primarily aligned with the jet direction. This could be indicative of magnetic field shearing due to jet-medium interaction, or the presence of helical magnetic fields. The jet peak intensity $I_ u$ falls with distance $d$ from the core, following the relation, $I_ upropto d^a$, where $a$ is typically -1.5. Assuming that adiabatic expansion losses are primarily responsible for the jet intensity dimming, two limiting cases are considered: [1] the jet has a constant speed on parsec-scales and is expanding gradually such that the jet radius $rpropto d^0.4$; this expansion is however unobservable in the laterally unresolved jets at 5 GHz, and [2] the jet is cylindrical and is accelerating on parsec-scales. Accelerating parsec-scale jets are consistent with the phenomenon of magnetic driving in Poynting flux dominated jets. Chandra observations of 15 UGC FRIs detect X-ray jets in nine of them. The high frequency of occurrence of X-ray jets in this complete sample suggests that they are a signature of a ubiquitous process in FRI jets.
rate research
Read More
There is now unequivocal evidence that the jets in FR I radio galaxies are initially relativistic, decelerating flows. On the assumption that they are axisymmetric and intrinsically symmetrical (a good approximation close to the nucleus), we can make models of their geometry, velocity, emissivity and field structure whose parameters can be determined by fitting to deep VLA observations. Mass entrainment - either from stellar mass loss within the jet volume or via a boundary layer at the jet surface - is the most likely cause for deceleration. This idea is quantitatively consistent with the velocity field and geometry inferred from kinematic modelling and the external gas density and pressure profiles derived from X-ray observations. The jets must initially be very light, perhaps with an electron-positron composition.
(Abridged) We present the results from new 15 ks Chandra-ACIS and 4.9 GHz Very Large Array observations of 13 galaxies hosting low luminosity AGN. This completes the multiwavelength study of a sample of 51 nearby early-type galaxies described in Capetti & Balmaverde (2005, 2006); Balmaverde & Capetti (2006). The aim of the three previous papers was to explore the connection between the host galaxies and AGN activity in a radio-selected sample. We detect nuclear X-ray emission in eight sources and radio emission in all but one (viz., UGC6985). The new VLA observations improve the spatial resolution by a factor of ten: the presence of nuclear radio sources in 12 of the 13 galaxies confirms their AGN nature. As previously indicated, the behavior of the X-ray and radio emission in these sources depends strongly on the form of their optical surface brightness profiles derived from Hubble Space Telescope imaging, i.e., on their classification as core, power-law or intermediate galaxies. With more than twice the number of power-law and intermediate galaxies compared to previous work, we confirm with a much higher statistical significance that these galaxies lie well above the radio-X-ray correlation established in FRI radio galaxies and the low-luminosity core galaxies. This result highlights the fact that the radio-loud/radio-quiet dichotomy is a function of the host galaxys optical surface brightness profile. We present radio-optical-X-ray spectral indices for all 51 sample galaxies. Survival statistics point to significant differences in the radio-to-optical and radio-to-X-ray spectral indices between the core and power-law galaxies (Gehans Generalized Wilcoxon test probability p for the two classes being statistically similar is <10^-5), but not in the optical-to-X-ray spectral indices (p=0.25).
Fast Radio Bursts (FRBs) are bright radio transients with millisecond duration at cosmological distances. Since compact dark matter/objects (COs) could act as lenses and cause split of this kind of very short duration signals, Mu$rm{tilde{n}}$oz et al. (2016) has proposed a novel method to probe COs with lensing of FRBs. In this Letter, we for the first time apply this method to real data and give constraints of the nature of COs with currently available FRB observations. We emphasize the information from dynamic spectra of FRBs is quite necessary for identifying any lensed signals and find no echoes in the existing data. The null search gives a constraint comparable to that from galactic wide binaries, though the methods of redshift inference from dispersion measure would impact a little. Furthermore, we make an improved forecast basing on the distributions of real data for the ongoing and upcoming telescopes. Finally, we discuss the situation where one or more lensed signals will be detected. In such a case, the parameter space of COs can be pinned down very well since the lens mass can be directly determined through the observed flux ratio and time delay between split images.
We discuss acceleration measurements for a large sample of extragalactic radio jets from the MOJAVE program which studies the parsec-scale jet structure and kinematics of a complete, flux-density-limited sample of Active Galactic Nuclei (AGN). Accelerations are measured from the apparent motion of individual jet features or components which may represent patterns in the jet flow. We find that significant accelerations are common both parallel and perpendicular to the observed component velocities. Parallel accelerations, representing changes in apparent speed, are generally larger than perpendicular acceleration that represent changes in apparent direction. The trend for larger parallel accelerations indicates that a significant fraction of these changes in apparent speed are due to changes in intrinsic speed of the component rather than changes in direction to the line of sight. We find an overall tendency for components with increasing apparent speed to be closer to the base of their jets than components with decreasing apparent speed. This suggests a link between the observed pattern motions and the underlying flow which, in some cases, may increase in speed close to the base and decrease in speed further out; however, common hydro-dynamical processes for propagating shocks may also play a role. About half of the components show non-radial motion, or a misalignment between the components structural position angle and its velocity direction, and these misalignments generally better align the component motion with the downstream emission. Perpendicular accelerations are closely linked with non-radial motion. When observed together, perpendicular accelerations are usually in the correct direction to have caused the observed misalignment.
Wide-field surveys are a commonly-used method for studying thousands of objects simultaneously, to investigate, e.g., the joint evolution of star-forming galaxies and active galactic nuclei. VLBI observations can yield valuable input to such studies because they are able to identify AGN. However, VLBI observations of large swaths of the sky are impractical using standard methods, because the fields of view of VLBI observations are of the order of 10 or less. We have embarked on a project to carry out Very Long Baseline Array (VLBA) observations of all 96 known radio sources in one of the best-studied areas in the sky, the Chandra Deep Field South (CDFS). The challenge was to develop methods which could significantly reduce the amount of observing (and post-processing) time. We have developed an extension to the DiFX software correlator which allows one to correlate hundreds of positions within the primary beams. This extension enabled us to target many sources, at full resolution and high sensitivity, using only a small amount of observing time. The combination of wide fields-of-view and high sensitivity across the field in this survey is unprecedented. We have observed with the VLBA a single pointing containing the Chandra Deep Field South, in which 96 radio sources were known from previous observations with the ATCA. From our input sample, 20 were detected with the VLBA. The majority of objects have flux densities in agreement with arcsec-scale observations, implying that their radio emission comes from very small regions. One VLBI-detected object had earlier been classified as a star-forming galaxy. Comparing the VLBI detections to sources found in sensitive, co-located X-ray observations we find that X-ray detections are not a good indicator for VLBI detections. Wide-field VLBI survey science is now coming of age.
Log in to be able to interact and post comments
comments
Fetching comments
Sign in to be able to follow your search criteria
