Chak-chung Cheung

A Survey of Extended Radio Jets with Chandra and the Hubble Space Telescope

We present the results from an X-ray and optical survey of a sample of 17 radio jets in AGN performed with Chandra and HST. The sample was selected from the radio and is unbiased toward detection at shorter wavelengths, but preferentially it includes beamed sources. We find that X-ray emission is common on kpc-scales, with over half radio jets exhibiting at least one X-ray knot on the Chandra images. The distributions of the radio-to-X-ray and radio-to-optical spectral indices for the detected jets are similar to the limits for the non-detections,suggesting all bright radio jets have X-ray counterparts which will be visible in longer observations. Comparing the radio and X-ray morphologies shows that the majority of the X-ray jets have structures that closely map the radio. Analysis of the SED of the jet knots suggest the knots in which the X-ray and radio morphologies track each other produce X-rays by IC scattering of the Cosmic Microwave Background. The remaining knots produce X-rays by the synchrotron process. Spectral changes are detected along the jets, with the ratio of the X-ray-to-radio and optical-to-radio flux densities decreasing from the inner to the outer regions. This suggests the presence of an additional contribution to the X-ray flux in the jets inner part, either from synchrotron or IC of the stellar light. Alternatively, in a pure IC/CMB scenario, the plasma decelerates as it flows from the inner to the outer regions. Finally, the X-ray spectral indices for the brightest knots are flat, indicating that the bulk of the luminosity of the jets is emitted at GeV energies, and raising the interesting possibility of future detections with GLAST.

A Survey of Extended Radio Jets in Active Galactic Nuclei with Chandra and the Hubble Space Telescope: First Results

We present the first results from an X-ray and optical survey of a sample of radio jets in active galactic nuclei with Chandra and the Hubble Space Telescope (HST). We focus here on the first six sources observed in X-ray, in four of which a bright X-ray jet was detected for the first time. In three out of the four cases, optical emission from the jet is also detected in our HST images. We compare the X-ray morphology with the radio as derived from improved processing of archival Very Large Array data, and we construct spectral energy distributions (SEDs) for the most conspicuous emission knots. In most cases, the SEDs, together with the similarity of the X-ray and radio morphologies, favor an inverse Compton origin for the X-rays. The most likely origin of the seed photons is the cosmic microwave background, implying the jets are still relativistic on kiloparsec scales. However, in the first knot of the PKS 1136-135 jet, X-rays are likely produced via the synchrotron process. In all four cases, bulk Lorentz factors of a few are required. The radio maps of the two jets not detected by either Chandra or HST suggest that they are less beamed at large scales than the other four detected sources. Our results demonstrate that, at the sensitivity and resolution of Chandra, X-ray emission from extragalactic jets is common, yielding essential information on their physical properties.We present the first results from an X-ray and optical survey of a sample of AGN radio jets with Chandra and HST. We focus here on the first six sources observed at X-rays, in four of which a bright X-ray jet was detected for the first time. In three out of four cases optical emission from the jet is also detected in our HST images. We compare the X-ray morphology with the radio as derived from improved processing of archival VLA data and we construct spectral energy distributions (SED) for the most conspicuous emission knots. In most cases the SEDs, together with the similarity of the X-ray and radio morphologies, favor an inverse Compton origin of the X-rays. The most likely origin of the seed photons is the Cosmic Microwave Background, implying the jets are still relativistic on kiloparsec scales. However, in the first knot of the PKS 1136-135 jet, X-rays are likely produced via the synchrotron process. In all four cases bulk Lorentz factors of a few are required. The radio maps of the two jets not detected by either Chandra or HST suggest that they are less beamed at large scales than the other four detected sources. Our results demonstrate that, at the sensitivity and resolution of Chandra, X-ray emission from extragalactic jets is common, yielding essential information on their physical properties.

Chandra observations of nuclear X-ray emission from a sample of radio sources

We present the X-ray properties of a sample of 17 radio sources observed with the Chandra X-ray Observatory as part of a project aimed at studying the X-ray emission from their radio jets. In this paper, we concentrate on the X-ray properties of the unresolved cores. The sample includes 16 quasars (11 core-dominated and 5 lobe-dominated) in the redshift range z= 0:30-1.96, and one low-power radio-galaxy at z= 0:064. No diuse X-ray emission is present around the cores of the quasars, except for the nearby low-power galaxy that has diuse emission on a scale and with a luminosity consistent with other FRIs. No high-amplitude, short-term variability is detected within the relatively short Chandra exposures. However, 1510 089 shows low-amplitude flux changes with a timescale of 25 min. The X-ray spectra of the quasar cores are generally well described by a single power law model with Galactic absorption. However, in six quasars we find soft X-ray excess emission below 1.6 keV. Interestingly, we detect an Fe K-shell emission line, consistent with fluorescent K emission from cold iron, in one lobe- and two core-dominated sources. The average X-ray photon index for the quasars in the sample is sample= 1:66 and dispersionsample= 0:23. The average spectral slope for our sample is flatter than the slope found for radio-quiet quasars and for radio-loud AGNs with larger jet orientations; this indicates that beaming aects the X-ray emission from the cores in our sample of quasars.

Detection of an X-Ray Jet in 3C 371 with Chandra

We report the detection at X-rays of the radio/optical jet of 3C 371 from a short (10 ks) Chandra exposure in 2000 March. We also present a new MERLIN observation at 1.4 GHz together with a reanalysis of the archival Hubble Space Telescope Wide Field Planetary Camera 2 F555W image. Despite the limited signal-to-noise ratio of the Chandra data, the X-ray morphology is clearly different from that of the radio/optical emission, with the brightest X-ray knot at 17 from the nucleus and little X-ray emission from the brightest radio/optical knot at 31. We construct the spectral energy distributions for the two emission regions at 17 and 31. Both show that the X-ray flux is below the extrapolation from the radio-to-optical continuum, suggesting a moderately beamed synchrotron from an electron population with a decreasing high-energy cutoff as a plausible emission mechanism.

Jets from Subparsec to Kiloparsec Scales: A Physical Connection

The Chandra discovery of bright X-ray emission from kiloparsec-scale jets allows insight into the physical parameters of the jet flow at large scales. At the opposite extreme, extensive studies of the inner relativistic jets in blazars with multiwavelength observations yield comparable information on subparsec scales. In the framework of simple radiation models for the emission regions we compare the physical parameters of jets on these two very different scales in the only two well-studied blazars for which large-scale emission has been resolved by Chandra. Notably, we find that the relativistic Doppler factors and powers derived independently at the two scales are consistent, suggesting that the jet does not suffer severe deceleration or dissipation. Moreover, the internal equipartition pressures in the inner jet and in the external X-ray-bright knots scale inversely with the jet cross section as expected in the simple picture of a freely expanding jet in equipartition.

The XMM-Newton view of the X-ray halo and jet of NGC 6251

We present an XMM-Newton observation of the radio jet and diffuse halo of the nearby radio galaxy NGC 6251. The EPIC spectrum of the galaxys halo is best-fitted by a thermal model with temperature kT ∼ 1.6 keV and sub-solar abundances. Interestingly, an additional hard X-ray component is required to fit the EPIC spectra of the halo above 3 keV, and is independently confirmed by an archival Chandra observation. However, its physical origin is not clear. Contribution from a population of undetected Low Mass X-ray Binaries seems unlikely. Instead, the hard X-ray component could be due to inverse Compton scattering of the CMB photons (IC/CMB) off relativistic electrons scattered throughout the halo of the galaxy, or non-thermal bremsstrahlung emission. The IC/CMB interpretation, together with limits on the diffuse radio emission, implies a very weak magnetic field, B � 1 µGauss, while a non-thermal bremsstrahlung origin implies the presence of a large number of very energetic electrons. We also detect X-ray emission from the outer (∼3.5 � ) jet, confirming previous ROSAT findings. Both the EPIC and ACIS spectra of the jet are best-fitted by a power law with photon index Γ ∼ 1.2, fixed Galactic column density, and 1 keV flux F 1k eV = 2.1 nJy. A thermal model is formally ruled out by the data. Assuming an origin of the X-rays from the jet via IC/CMB, as suggested by energetic arguments, and assuming equipartition implies a large Doppler factor (δ ∼ 10). Alternatively, weaker beaming is possible for magnetic fields several orders of magnitude lower than the equipartition field.

NEAR-INFRARED OBSERVATIONS OF BL LACERTAE HOST GALAXIES

Multiband near-infrared images of 12 BL Lacertae objects were obtained with the 2.5 m telescope at the Las Campanas Observatory in order to determine the properties of their underlying host galaxies. Resolved emission was clearly detected in eight of the lowest redshift targets (up to z ~ 0.3) and was modeled with a de Vaucouleurs r1/4 surface brightness law. We find that the morphologies match the elliptical galaxy profiles well and that the BL Lac objects reside in large and luminous, but otherwise normal, hosts?consistent with previous studies done predominantly at optical wavelengths. The median absolute K-band magnitude of the galaxies in this study is -26.2, the average half-light radius is 4.2 ? 2.3 kpc, and their average integrated R-K color is 2.7 ? 0.3 mag. These are well within the range of values measured previously in the H band by Kotilainen et al. and Scarpa et al. in a comparable number of targets. Taking their data together with our results, we find a best-fit K-band Kormendy relation of ?e = 4.3 log10 (re/kpc) + 14.2 mag arcsec-2, virtually identical to that obtained for normal ellipticals. Finally, the near-infrared colors determined for five galaxies (average J-K = 0.8 ? 0.3 mag) are the first such measurements for BL Lac hosts and match those expected from old stellar populations at the BL Lac redshifts.

Radio Identification of the X-Ray Jet in the z = 4.3 Quasar GB 1508+5714

The recent discovery of an X-ray jet in the z = 4.3 quasar GB 1508+5714 by Yuan et al. and Siemiginowska et al. prompted a search for its radio counterpart. Here, we report the successful discovery of faint radio emission from the jet at 1.4 GHz using archival Very Large Array data. The X-ray emission is best interpreted as inverse Compton (IC) emission off the cosmic microwave background (CMB) as discussed by the previous investigators. In this scenario, its high X-ray-to-radio monochromatic luminosity ratio, compared to previously detected IC/CMB X-ray jets at lower redshift, is a natural consequence of its high redshift.

Detection of Optical Synchrotron Emission from the Radio Jet of 3C 279

We report the detection of optical and ultraviolet emission from the kiloparsec-scale jet of the well-known quasar 3C 279. A bright knot, discovered in archival V- and U-band Hubble Space Telescope Faint Object Camera images, is coincident with a peak in the radio jet ~06 from the nucleus. The detection was also confirmed in Wide Field Planetary Camera 2 images. Archival Very Large Array and MERLIN radio data are also analyzed, and these data help to show that the high-energy optical/UV continuum and the spectrum are consistent with a synchrotron origin from the same population of relativistic electrons responsible for the radio emission.

New detections of optical emission from kiloparsec-scale quasar jets ☆

Abstract We report initial results from the detection of optical emission in the arcsecond-scale radio jets of two quasars utilizing images from the 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.