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J. F. C. Wardle

Publications and source records attributed to J. F. C. Wardle.

At least 19 recordsLinked to original sources
arXiv2020

Inverse-Compton scattering in the resolved jet of the high-redshift quasar PKS J1421-0643

Despite the fact that kpc-scale inverse-Compton (iC) scattering of cosmic microwave background (CMB) photons into the X-ray band is mandated, proof of detection in resolved quasar jets is often insecure. High redshift provides favourable conditions due to the increased energy density of the CMB, and it allows constraints to be placed on the radio synchrotron-emitting electron component at high energies that are otherwise inaccessible. We present new X-ray, optical and radio results from Chandra, HST and the VLA for the core and resolved jet in the $z=3.69$ quasar PKS J1421-0643. The X-ray jet extends for about $4.5''$ (32 kpc projected length). The jet's radio spectrum is abnormally steep and consistent with electrons being accelerated to a maximum Lorentz factor of about 5000. Results argue in favour of the detection of inverse-Compton X-rays for modest magnetic field strength of a few nT, Doppler factor of about 4, and viewing angle of about $15^\circ$, and predict the jet to be largely invisible in most other spectral bands including the far- and mid-infrared and high-energy gamma-ray. The jet power is estimated to be about $3 \times 10^{46}$ erg s$^{-1}$ which is of order a tenth of the quasar bolometric power, for an electron--positron jet. The jet radiative power is only about 0.07 per cent of the jet power, with a smaller radiated power ratio if the jet contains heavy particles, so most of the jet power is available for heating the intergalactic medium.

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arXiv2016

Detecting Relativistic X-ray Jets in High-Redshift Quasars

We analyze Chandra X-ray images of a sample of 11 quasars that are known to contain kiloparsec scale radio jets. The sample consists of five high-redshift (z >= 3.6) flat-spectrum radio quasars, and six intermediate redshift (2.1 < z < 2.9) quasars. The dataset includes four sources with integrated steep radio spectra and seven with flat radio spectra. A total of 25 radio jet features are present in this sample. We apply a Bayesian multi-scale image reconstruction method to detect and measure the X-ray emission from the jets. We compute deviations from a baseline model that does not include the jet, and compare observed X-ray images with those computed with simulated images where no jet features exist. This allows us to compute p-value upper bounds on the significance that an X- ray jet is detected in a pre-determined region of interest. We detected 12 of the features unambiguously, and an additional 6 marginally. We also find residual emission in the cores of 3 quasars and in the background of 1 quasar that suggest the existence of unresolved X-ray jets. The dependence of the X-ray to radio luminosity ratio on redshift is a potential diagnostic of the emission mechanism, since the inverse Compton scattering of cosmic microwave background photons (IC/CMB) is thought to be redshift dependent, whereas in synchrotron models no clear redshift dependence is expected. We find that the high-redshift jets have X-ray to radio flux ratios that are marginally inconsistent with those from lower redshifts, suggesting that either the X-ray emissions is due to the IC/CMB rather than the synchrotron process, or that high redshift jets are qualitatively different.

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arXiv2012

Discovery of a Kiloparsec Scale X-ray/Radio Jet in the z=4.72 Quasar GB 1428+4217

We report the discovery of a one-sided 3.6" (24 kpc, projected) long jet in the high-redshift, z=4.72, quasar GB 1428+4217 in new Chandra X-ray and VLA radio observations. This is the highest redshift kiloparsec-scale X-ray/radio jet known. Analysis of archival VLBI 2.3 and 8.6 GHz data reveal a faint one-sided jet extending out to ~200 parsecs and aligned to within ~30 deg of the Chandra/VLA emission. The 3.6" distant knot is not detected in an archival HST image, and its broad-band spectral energy distribution is consistent with an origin from inverse Compton scattering of cosmic microwave background photons for the X-rays. Assuming also equipartition between the radiating particles and magnetic field, the implied jet Lorentz factor is ~5. This is similar to the other two known z ~ 4 kpc-scale X-ray jet cases and smaller than typically inferred in lower-redshift cases. Although there are still but a few such very high-redshift quasar X-ray jets known, for an inverse Compton origin, the present data suggest that they are less relativistic on large-scales than their lower-redshift counterparts.

astro-ph.HE↗
arXiv2009

Full Polarization Spectra of 3C 279

We report the results of parsec-scale, multi-frequency VLBA observations of the core region of 3C 279 in Stokes I, linear polarization, and circular polarization. These full polarization spectra are modeled by radiative transfer simulations to constrain the magnetic field and particle properties of the parsec-scale jet in 3C 279. The polarization properties of the core region, including the amount of linear polarization, the amount and sign of Faraday rotation, and the amount and sign of circular polarization can be explained by a consistent physical picture. The base of the jet is modeled as an inhomogeneous Blandford-Konigl style conical jet dominated by a vector-ordered poloidal magnetic field along the jet axis, and we estimate its net magnetic flux. This poloidal field is responsible for the linear and circular polarization from this inhomogeneous component. Farther down the jet the magnetic field in two homogeneous features is dominated by local shocks and a smaller fraction of vector-ordered poloidal field remains along the jet axis. In this picture, we find the jet to be kinetically dominated by protons with the radiating particles being dominated by electrons at an approximate fraction of >~ 75%. Based on the amounts of Faraday conversion deduced for the homogeneous components, we find a plausible range for the lower cutoff in the relativistic particle energy spectrum to be 5 <~ gamma_l <~ 35. The physical picture described here is not unique if the observed Faraday rotation and depolarization occur in screens external to the jet; however, we find the joint explanation of linear and circular polarization observations from a single set of magnetic fields and particle properties internal to the jet to be compelling evidence for this picture. (Abridged)

astro-ph.CO↗
arXiv2007

The Highest Redshift Relativistic Jets

We describe our efforts to understand large-scale (10's-100's kpc) relativistic jet systems through observations of the highest-redshift quasars. Results from a VLA survey search for radio jets in ~30 z>3.4 quasars are described along with new Chandra observations of 4 selected targets.

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arXiv2006

Large-Scale Radio and X-ray Jets in the Highest Redshift Quasars

We describe our program to search for and study the kilo-parsec scale radio jets in a sample of high-redshift (greater than 3.4), flat spectrum quasars using new and archival VLA data. Two of these radio jets have been imaged with Chandra, and have X-ray counterparts and are briefly discussed. These high-redshift sources are important targets for testing current X-ray jet emission models for kpc-scale jets and follow-up multi-wavelength observations will shed light on this problem.

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arXiv2005

Deceleration from Entrainment in the jet of the quasar 1136-135?

Modeling the multiwavelength emission of successive regions in the jet of the quasar PKS 1136-135 we find indication that the jet suffers deceleration near its end on a (deprojected) scale of ~400 kpc. Adopting a continuous flow approximation we discuss the possibility that the inferred deceleration from a Lorentz factor Gamma=6.5 to Gamma=2.5 is induced by entrainment of external gas. Some consequences of this scenario are discussed.

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arXiv2005

Deep Chandra and multicolor HST follow-up of the jets in two powerful radio quasars

We present deep (70-80 ks) Chandra and multicolor HST ACS images of two jets hosted by the powerful quasars 1136-135 and 1150+497, together with new radio observations. The sources have an FRII morphology and were selected from our previous X-ray and optical jet survey for detailed follow up aimed at obtaining better constraints on the jet multiwavelength morphology and X-ray and optical spectra of individual knots, and to test emission models deriving physical parameters more accurately. All the X-ray and optical knots detected in our previous short exposures are confirmed, together with a few new faint features. The overlayed maps and the emissivity profiles along the jet show good correspondence between emission regions at the various wavelengths; a few show offsets between the knots peaks of <1". In 1150+497 the X-ray, optical, and radio profiles decrease in similar ways with distance from the core up to ~7", after which the radio emission increases more than the X-ray one. No X-ray spectral variations are observed in 1150+497. In 1136-135 an interesting behavior is observed, whereby, downstream of the most prominent knot at ~ 6.5" from the core, the X-ray emission fades while the radio emission brightens. The X-ray spectrum also varies, with the X-ray photon index flattening from ~2 in the inner part to ~1.7 to the end of the jet. We intepret the jet behavior in 1136-135 in a scenario where the relativistic flow suffers systematic deceleration along the jet, and briefly discuss the major consequences of this scenario (abridged).

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arXiv2005

Discovery of Optical Emission in the Hotspots of Three 3CR Quasars: High-Energy Particle Acceleration in Powerful Radio Hotspots

Archival Hubble Space Telescope WFPC2 images were used to search for optical emission associated with the radio jets of a number of luminous quasars. From this search, we report new optical hotspot detections in the well-known blazar 3C~454.3 and the lobe-dominated quasars 3C~275.1 and 3C~336. We also find optical emission in the vicinity of the hotspot in 3C~208, but believe this is a chance alignment. Optical emission from the arcsecond-scale jet in 3C~454.3 is also detected. Multi-frequency archival radio data from the VLA and MERLIN are analyzed, and the synchrotron spectra of these high-power hotspots are presented. We estimate that their break frequencies are in the range of $10^{10}-10^{11}$ Hz, with large uncertainties due to the wide gap in frequency coverage between the radio and optical bands. We also calculate their equipartition magnetic fields, and find that the anti-correlation between break frequency and magnetic field found by Brunetti et al. for lower power hotspots extends to these high power hotspots. This supports their model of hot-spots based on shock acceleration and synchrotron losses.

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arXiv2004

Probing the polarization characteristics of SS433 on mas scales

We present a status report of dedicated circular polarization (CP) global VLBI observations of SS433. The total intensity image of the source is presented. We do not detect linear polarization in the core region of the source. We estimate the errors in D-terms for the EVN, and its effect on the final right-left circular polarization gain calibration. The data from this and earlier observations are analyzed with the "zero-V" self-calibration technique, in order to obtain indications for detectable CP in different regions of the source. Our data show no evidence for strong gyro-synchrotron (GS) emission on the size scale probed by our beam.

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arXiv2004

High Levels of Circularly Polarized Emission from the Radio Jet in NGC 1275 (3C 84)

We present multi-frequency, high resolution VLBA circular polarization images of the radio source 3C 84 in the center of NGC 1275. Our images reveal a complex distribution of circular polarization in the inner parsec of the radio jet, with local levels exceeding 3% polarization, the highest yet detected with VLBI techniques. The circular polarization changes sign along the jet, making 3C 84 also the first radio jet to show both signs of circular polarization simultaneously. The spectrum and changing sign of the circular polarization indicate that it is unlikely to be purely intrinsic to the emitted synchrotron radiation. The Faraday conversion process makes a significant and perhaps dominant contribution to the circular polarization, and the observed spectrum suggests the conversion process is near saturation. The sign change in the circular polarization along the jet may result from this saturation or may be due to a change in magnetic field order after an apparent bend in the jet. From the small spatial scales probed here, ~ 0.15 pc, and the comparably high levels of circular polarization inferred for the intra-day variable source PKS 1519-273, we suggest a connection between small spatial scales and efficient production of circular polarization.

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arXiv2002

New Detections of Optical Emission from Kiloparsec-scale Quasar Jets

We report initial results from the detection of optical emission in the arcsecond-scale radio jets of two quasars utilizing images from the {\it Hubble Space Telescope} archive. The optical emission has a very knotty appearance and is consistent with synchrotron emission from highly relativistic electrons in the jet. Combining these observations with those of previously reported features in other quasars, an emerging trend appears to be that their radio-to-optical spectral indices are steeper than those of similar features in jets of lower power radio sources.

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arXiv2002

Chandra Observations of Three SDSS Quasars at z~6

Chandra snapshot observations of the three most distant quasars then known, at redshifts 5.82, 5.99, and 6.28, gave signficant detections even in the short, 6 -- 8 ks, observations. The X-ray to optical luminosity ratios indicate that quasars will be detectable in X-rays if they exist at even larger redshifts. The present observations hint at two exciting discoveries. An extended X-ray source 23 arcsec from SDSS1306+0356 may be a jet emitting inverse Compton radiation from the Cosmic Microwave Background. SDSS 1030+0524 does not appear to be a point source, and may be a gravitationally lensed system, or contain a small scale X-ray jet.

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arXiv2001

Making Movies from Radio Astronomical Images with AIPS

We present a detailed recipe for making movies from multi-epoch radio observations of astronomical sources. Images are interpolated linearly in time to create a smooth succession of frames so that a continuous movie can be compiled. Here, we outline the procedure, and draw attention to specific details necessary for making a successful movie. In particular, we discuss the issues pertaining specifically to making polarization movies. The procedure described here has been implemented into scripts in NRAO's AIPS package (Brandeis AIPS Movie Maker -- BAMM) that are available for public use (http://www.astro.brandeis.edu).

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arXiv2000

Parsec-Scale Blazar Monitoring: Proper Motions

We present proper motions obtained from a dual frequency, six-epoch, VLBA polarization experiment monitoring a sample of 12 blazars. The observations were made at 15 GHz and 22 GHz at bi-monthly intervals over 1996. Ten of the eleven sources for which proper motion could be reliably determined are superluminal. Only J2005+77 has no superluminal components. Three sources (OJ287, J1224+21, and J1512-09) show motion faster than 10h^{-1}c, requiring $γ_{pattern}$ of at least 10h^{-1}c (H_0 = 100h km/s/Mpc). We compare our results to those in the literature and find motions outside the previously observed range for four sources. While some jet components exhibit significant non-radial motion, most motion is radial. In at least two sources there are components moving radially at significantly different structural position angles. In five of six sources (3C120, J1224+21, 3C273, 3C279, J1512-09, and J1927+73) that have multiple components with measurable proper motion, the innermost component is significantly slower than the others, suggesting that acceleration occurs in the jet. In the motions of individual components we observe at least one decelerating motion and two ``bending'' accelerations which tend to align their motions with larger scale structure. We also discuss in detail our procedures for obtaining robust kinematical results from multi-frequency VLBI data spanning several epochs.

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arXiv2000

Detection and Measurement of Parsec-Scale Circular Polarization in Four AGN

We present five epochs of 15 GHz VLBA observations of 13 AGN. These observations were specially calibrated to detect parsec scale circular polarization and our calibration techniques are discussed and analyzed in detail. We obtained reliable detections of parsec scale circular polarization in the radio jets of 4 AGN: 3C84, PKS 0528+134, 3C273, and 3C279. For each of these objects our detections are at the level of ~ 0.3-1% local fractional circular polarization. Our observations are consistent across multiple epochs (and different calibration techniques) in the sign and magnitude of the circular polarization observed. 3C273 and 3C279 both undergo core outbursts during our observations and changes in the circular polarization of both sources are correlated with these outbursts. In general, we observe the circular polarization to be nearly coincident with the strong VLBI cores of these objects; however, in 3C84 the circular polarization is located a full milli-arcsecond south of the source peak, and in the 1996.73 epoch of 3C273 the circular polarization is predominately associated with the newly emerging jet component. Circular polarization may be produced as an intrinsic component of synchrotron radiation or by the Faraday conversion of linear to circular polarization. Our single frequency observations do not easily distinguish between these possible mechanisms, but independent of mechanism, the remarkable consistency across epoch of the sign of the observed circular polarization suggests the existence of a long term, stable, uni-directional magnetic field. Single dish observations of 3C273 and 3C279 at 8 GHz by Hodge and Aller suggest that this stability may persist for decades in our frame of observation.

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arXiv2000

Direct Distance Measurements to Superluminal Radio Sources

We present a new technique for directly measuring the distances to superluminal radio sources. By comparing the observed proper motions of components in a parsec scale radio jet to their measured Doppler factors, we can deduce the distance to the radio source independent of the standard rungs in the cosmological distance ladder. This technique requires that the jet angle to the line of sight and the ratio of pattern to flow velocities are sufficiently constrained. We evaluate a number of possibilities for constraining these parameters and demonstrate the technique on a well defined component in the parsec scale jet of the quasar 3C279 (z = 0.536). We find an angular size distance to 3C279 of greater than 1.8 (+0.5,-0.3) n^{1/8} Gpc, where n is the ratio of the energy density in the magnetic field to the energy density in the radiating particles in that jet component. For an Einstein-de Sitter Universe, this measurement would constrain the Hubble constant to be H < 65 n^{-1/8} km/s/Mpc at the two sigma level. Similar measurements on higher redshift sources may help discriminate between cosmological models.

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