X-ray Jet Research Articles

DeepChandraand MulticolorHSTFollow‐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 FR II 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 to derive 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 overlaid maps and the emissivity profiles along the jet show good correspondence between emission regions at the various wavelengths; a few show offsets between the knot 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 does 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 ~65 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 ΓX ~ 2 in the inner part to ΓX ~ 1.7 to the end of the jet. We interpret the jet behavior in 1136-135 in a scenario in which the relativistic flow suffers systematic deceleration along the jet, and we briefly discuss the major consequences of this scenario. The latter is discussed in more detail in our companion paper (Tavecchio et al.).

The X‐Ray Jet in Centaurus A: Clues to the Jet Structure and Particle Acceleration

We report detailed studies of the X-ray emission from the kiloparsec-scale jet in the nearest active galaxy, Centaurus A. By analyzing the highest quality X-ray data obtained with the Chandra ACIS-S, 41 compact sources (mostly bright jet knots) were found within the jet on angular scales less than 4'', 13 of which were newly identified. We construct the luminosity function for the detected jet knots and argue that the remaining emission is most likely to be truly diffuse, rather than resulting from the sum of many unresolved fainter knots. We subtracted the contributions of the bright knots from the total X-ray jet flux, and show that the remaining extended emission has a relatively flat-topped intensity profile in the transverse jet direction, with the intensity peaking at the jet boundaries between 50'' and 170''. We note that limb-brightened morphologies have been observed previously at radio frequencies in a few FR I and FR II jet sources, but never so clearly at higher photon energies. Our result therefore supports a stratified jet model, consisting of a relativistic outflow including a boundary layer with a velocity shear. In addition, we found that the X-ray spectrum of the diffuse component is almost uniform across and along the jet, with an X-ray energy spectral index of αX ≈ 1, similar to those observed in the compact knots. We discuss this spectral behavior within a framework of shock and stochastic particle acceleration processes, connected with the turbulent, supersonic, and nonsteady nature of the relativistic outflow. We note some evidence for a possible spectral hardening at the outer sheath of the jet, and manifesting itself in observed X-ray spectra of αX < 0.5 in the most extreme cases. Due to the limited photon statistics of the present data, further deep observations of Centaurus A are required to determine the reality of this finding; however, we note that the existence of the hard X-ray features at outer jet boundaries would provide an important challenge to theories for the evolution of ultrarelativistic particles within extragalactic jets.

AChandraSnapshot Survey of Representative High-Redshift Radio-Loud Quasars from the Parkes-MIT-NRAO Sample

We present the results of Chandra ACIS-S snapshot observations of six radio-loud quasars (RLQs) at z ≈ 3.5-4.7. These observations sample luminous RLQs with moderate to high radio loudness (R ≈ 200-9600) and aim to connect the X-ray properties of radio-quiet quasars (R 1000) at high redshift. This work extends a study by Bassett and coworkers that used similar methods to examine z > 4 RLQs with moderate radio loudness (R ≈ 40-400). All of our targets are clearly detected. A search for extended X-ray emission associated with kiloparsec-scale radio jets revealed only limited evidence for X-ray extension in our sample: three sources showed no evidence of X-ray extension, and the other three had 3%-30% of their total X-ray fluxes extended >1'' away from their X-ray cores. In addition, we do not observe any systematic flattening of the optical-to-X-ray spectral index (αox) compared to low-redshift quasars. These results suggest that kiloparsec-scale X-ray jet emission is not dominated by inverse Compton scattering of cosmic microwave background seed photons off jet electrons. We measured X-ray continuum shapes and performed individual and joint spectral fits of our data combined with eight archival RLQs. A single power-law model acceptably fitted the data, with best-fit photon indices of 1.72 for moderate-R sources and 1.47 for high-R sources. We added an intrinsic absorption component to our model, and neither the moderate-R nor the high-R fits set a lower bound on NH. The upper limits were NH < 3.0 × 1022 cm-2 and NH < 2.8 × 1022 cm-2, respectively. Our spectral results suggest that intrinsic absorption does not strongly depend on radio loudness, and high-R sources have flatter power laws than moderate-R sources. Overall, our high-redshift RLQs have basic X-ray properties consistent with similar RLQs in the local universe.

Testing the beamed inverse-Compton model for jet X-ray emission: velocity structure and deceleration

By considering a small sample of core-dominated radio-loud quasars with X-ray jets, I show, as has been argued previously by others, that the observations require bulk jet deceleration if all of the X-ray emission is to be explained using the widely adopted beamed inverse-Compton model, and argue that jets even in these powerful objects must have velocity structure in order to reconcile their radio and X-ray properties. I then argue t hat the deceleration model has several serious weaknesses, and discuss the viability of al ternative models for the decline in X-ray/radio ratio as a function of position. Although inver se-Compton scattering from the jets is a required process and must come to dominate at high redshifts, adopting an alternative model for the X-ray emission of some nearby, well-studied objects can greatly alleviate some of the problems posed by these observations for the beamed inverse-Compton model.

X-rays from the radio-quiet quasar PG 1407+265: relativistic jet or accretion disc emission?

We present two XMM-Newton observations of the luminous (L_x > 10^46 erg/s), radio-quiet quasar, PG 1407+265, separated by eleven months. The data indicate two distinct states: a highly variable, bright state (first epoch); and a quiescent, low-flux one (second epoch). During the low-flux state the spectrum is consistent with a single, unabsorbed power law. However, during the brighter state a highly variable, steep component is statistically required. Contemporaneous UV data from the Optical Monitor allow an estimate of the optical-to-X-ray spectral index (alpha_ox), which appears typical of radio-quiet quasars during the low-flux state, but extremely flat during the high-flux state. The XMM-Newton data can be described as originating from a combination of jet and accretion disc processes, in which the (relativistic) X-ray jet only works intermittently. The scenario could help describe some of the complexities seen in the broadband spectral energy distribution of PG 1407+265, such as weak high-ionisation emission lines, strong Fe II, unbeamed continuum, and the weak radio emission relative to the optical.

Detection of Nonthermal X-ray structures near the Galactic Center with Chandra

We have discovered a number of nonthermal X-ray features within central 40 pc region of the Galactic center by analysing 600-ksec observations of Chandra archival data. Most of the detected X-ray structures exhibit small-scale knot-like morphologies and their spectra are well reproduced by single hard power-law with photon indices of 1-2. Among them, the most outstanding features are the three X-ray knots which are aligned on a straight line from the potition of Sgr A* to north-northwest direction. The X-ray properties of these knots lead us to suspect that they are X-ray jets ejected from Sgr A* in the recent past. In addition, we have obtained an indication that the summed flux of nonthermal diffuse X-rays within 30 pc of the GC seems to be smoothly connected to the 20-100 keV flux detected with INTEGRAL IBIS/ISGRI. These results suggest that the origin of GC hard X-rays (or High energy Gamma-rays) is not (or partly) from the Galactic nucleus.

The Chandra view of extended X-ray emission from Pictor A

We discuss the extended X-ray emission seen in three archival Chandraobservations, and one archival XMM-Newtonobservation, of the FRII radio galaxy Pictor A. The overall properties of the X-ray lobes are consistent with the conclusions of earlier workers that the extended X-ray emission is largely due to the inverse-Compton process, and the implied departure from equipartition is in the range seen by us in other sources. In d etail, we show that the X-ray/radio flux ratio varies quite strongly as a function of position thr oughout the source, and we discuss possible implications of this observation for the spatial v ariation of electron energy spectra and magnetic field strength through the lobe. We show that the rad io and X-ray properties of the lobe are not consistent with a simple model in which variations in the magnetic field strength alone are responsible for the observed differences between emission at different frequencies. We also discuss the origins of the extended emission seen around the eastern hotspot, arguing that it may be diffuse synchrotron radiation tracing a regio n of distributed particle acceleration, and the implications of a possible weak X-ray counterjet detection which, taken together with the other properties of the bright X-ray jet, leads us to suggest that the X-ray jet and possible counterjet are also produced by synchrotron emission.

SEVERAL OBSERVED FEATURES AND THEIR IMPLICATIONS FOR GAMMA-RAY-LOUD BLAZAR PKS 1510-089

The new results of spectrophotometry, and X-ray imaging spectroscopy observations for the high-polarization, radio-loud and Gev gamma-ray-loud blazar PKS 1510-089 have been observed. The main results show that: (1) A double-horned broad Hβ line in the spectrum of PKS 1510-089 has been discovered. The Hβ double-peaked emission profiles are impressive with the line asymmetry that the red peak seems to be higher than the blue peak; (2) The observation of X-rays, performed with the ACIS-S detector aboard the Chandra X-ray observatory, led to the discovery of a bending X-ray jet, coincident with the radio arcsecond jet of PKS 1510-089; (3) According to the reverberation mapping method and the empirical relation between the broad line region (BLR) size and the optical continuum luminosity at 5100 Å (the rest frame), we obtained the Virial mass of the central primary black hole, which coincides with our previous result by extremely rapid optical variability method. In addition, based on these new observational results mentioned above and the other previous observations, we have discussed the theoretical models for PKS 1510-089.

The Large-Scale, Decelerating X-ray Jets from the Microquasar Xte J1550−564: Evidence for External Shocks Caused by the Jet-Ism Interaction?

Large-scale, decelerating, relativistic X-ray jets from microquasar XTE J1550—564 has been recently discovered with Chandra by Corbel et al. (2002). We find that the dynamical evolution of the approaching jet at the late time is consistent with the well-known Sedov evolutionary phase R ∝ t2/5. A trans-relativistic external shock dynamic model by analogy with the evolution of gammaray burst remnants, is shown to be able to fit the proper-motion data of the approaching jet reasonably well. The inferred interstellar medium density around the source is well below the canonical value nISM ∼ 1 cm−3. The rapidly fading X-ray emission can be interpreted as synchrotron radiation from the non-thermal electrons in the adiabatically expanding ejecta. These electrons were accelerated by the reverse shock (moving back into the ejecta) which becomes important when the inertia of the swept external matter leads to an appreciable slowing down of the original ejecta.

Constraints on jet X-ray emission in low/hard-state X-ray binaries

Abstract We show that the similarities between the X-ray properties of low-luminosity accreting black holes and accreting neutron stars, combined with the differences in their radio properties, argue that the X-rays from these systems are unlikely to be formed in the relativistic jets. Specifically, the spectra of extreme island-state neutron stars and low/hard-state black holes are known to be quite similar, while the power spectra from these systems are known to show only minor differences beyond what would be expected from scaling the characteristic variability frequencies by the mass of the compact object. The spectral and temporal similarities thus imply a common emission mechanism that has only minor deviations from having all key parameters scaling linearly with the mass of the compact object, while we show that this is inconsistent with the observations that the radio powers of neutron stars are typically about 30 times lower than those of black holes at the same X-ray luminosity. We also show that an abrupt luminosity change would be expected when a system makes a spectral state transition from a radiatively inefficient jet-dominated accretion flow to a thin disc-dominated flow, but that such a change is not seen.

Evidence for an X-Ray Jet in DG Tauri A?

We present evidence for an X-ray jet in the T Tauri star DG Tau A based on Chandra ACIS data. DG Tau A, a jet-driving classical T Tauri star with a flat infrared spectrum, reveals an unusual X-ray spectrum that requires two thermal components with different intervening absorption column densities. The softer component shows a low temperature of T ≈ 2.9 MK, and its absorption is compatible with the stellar optical extinction (hydrogen column density NH ≈ 5 × 1021 cm-2). In contrast, the harder component reveals a temperature (22 MK) characteristic of active T Tauri stars, but its emission is more strongly absorbed (NH ≈ 2.8 × 1022 cm-2). Furthermore, the high-resolution ACIS-S image reveals a weak excess of soft (0.5-2 keV) counts at distances of 2''-4'' from the star precisely along the optical jet, with a suggestive concentration at 4'', where a bow shock-like structure has previously been identified in optical line observations. The energy distribution of these photons is similar to those of the stellar soft component. We interpret the soft spectral component as originating from shocks at the base of the jet, with shock-heating continuing out to a distance of at least 500 AU along the jet, whereas the hard component is most likely coronal or magnetospheric as in other young stellar systems.

Multialtitude Observations of a Coronal Jet during the Third Whole Sun Month Campaign

On 1999 August 26, a coronal jet occurred at the northwest limb near a sigmoid active region (AR 8668) that was the target for a joint observation plan (SOHO joint observing program 106) during the third Whole Sun Month Campaign. This jet was observed by several instruments at the limb (SOHO/CDS, SOHO/EIT, TRACE, and Mauna Loa Solar Observatory CHIP and PICS) and at 1.64 R☉ (SOHO/UVCS). At the limb, this jet event displayed both low- and high-temperature components. Both high- and low-temperature components were evident during the early phase (first 20 minutes) of the event. However, the low-temperature component is maintained for ~1 hr after the higher temperature component is gone. There is a second brightening (a possible second jet) seen by EIT and TRACE about 50 minutes after the onset of the first jet. The line-of-sight motion at the limb began with a 300 km s-1 redshift and evolved to a 200 km s-1 blueshift. At 1.64 R☉, the intensities of Lyα and Lyβ in the jet increased by a factor of several hundred compared with the background corona. The C III λ977 line also brightened significantly. This indicates low-temperature [~(1-2) × 105 K] emission in the jet, while the intensities of O VI λ1032 and O VI λ1037 increased by as much as a factor of 8. The UVCS data show evidence of heating at the early phase of the event. The Doppler shift in the lines indicates that the line-of-sight (LOS) velocity in the jet started from ~150 km s-1 in blueshift and ended at ~100 km s-1 in redshift. This LOS motion seen at 1.64 R☉ was apparently opposite to what was observed when the jet emerged from the limb. The Doppler dimming analysis indicates that the radial outflow speed correlates with the magnitude of the LOS speed. Interestingly, UVCS observations at 2.33 and 2.66 R☉ show no trace of the jet and SOHO/LASCO observations also yield no firm detection. We find that a simple ballistic model can explain most of the dynamical properties of this jet, while the morphology and the thermal properties agree well with reconnection-driven X-ray jet models.

Chandra and XMM-Newton observations of NGC 6251

We present new X-ray observations of the nucleus, jet and extended emission of the nearby radio galaxy NGC 6251 using the Chandra/ACIS-S camera, together with a reanalysis of archival Chandra/ACIS-I and XMM–Newton/EPIC data. We find that the nuclear X-ray spectrum is well fitted with an absorbed power law, and that there is tentative, but not highly significant, evidence for Fe Kα emission. We argue that the observed nuclear X-ray emission is likely to originate in a relativistic jet, based on the double-peaked nature, and our synchrotron self-Compton modelling, of the radio to X-ray spectral energy distribution. However, we cannot rule out a contribution from an accretion flow. We resolve X-ray jet emission in three distinct regions, and argue in favour of a synchrotron origin for all three; inverse Compton emission models are possible but require extreme parameters. We detect thermal emission on both galaxy and group scales, and demonstrate that hot gas can confine the jet, particularly if relativistic beaming is important. We show evidence that the radio lobe has evacuated a cavity in the X-ray-emitting gas, and suggest that the lobe is close to the plane of the sky, with the jet entering the lobe close to the surface nearest to the observer.

Filamentary structure on the Sun from the magnetic Rayleigh–Taylor instability

Magnetic flux emerges from the solar surface as dark filaments connecting small sunspots with opposite polarities. The regions around the dark filaments are often bright in X-rays and are associated with jets. This implies plasma heating and acceleration, which are important for coronal heating. Previous two-dimensional simulations of such regions showed that magnetic reconnection between the coronal magnetic field and the emerging flux produced X-ray jets and flares, but left unresolved the origin of filamentary structure and the intermittent nature of the heating. Here we report three-dimensional simulations of emerging flux showing that the filamentary structure arises spontaneously from the magnetic Rayleigh-Taylor instability, contrary to the previous view that the dark filaments are isolated bundles of magnetic field that rise from the photosphere carrying the dense gas. As a result of the magnetic Rayleigh-Taylor instability, thin current sheets are formed in the emerging flux, and magnetic reconnection occurs between emerging flux and the pre-existing coronal field in a spatially intermittent way. This explains naturally the intermittent nature of coronal heating and the patchy brightenings in solar flares.

Rapid variability of the arcsec-scale X-ray jets of SS 433

We present X-ray images of all the available Chandra observations of the galactic jet source SS 433. We have studied the morphology of the X-ray images and inspected the evolution of the arcsec X-ray jets, recently found to be manifestations of in situ reheating of the relativistic gas downstream in the jets. The Chandra images reveal that the arcsec X-ray jets are not steady long-term structures; the structure varies, indicating that the reheating processes have no preference for a particular precession phase or distance from the binary core. Three observations made within about five days in 2001 May, and a 60-ks observation made in 2003 July, show that the variability of the jets can be very rapid, from time-scales of days to (possibly) hours. The three 2001 May images show two resolved knots in the east jet getting brighter one after the other, suggesting that a common phenomenon might be at the origin of the sequential reheatings of the knots. We discuss possible scenarios and propose a model to interpret these brightenings in terms of a propagating shock wave, revealing a second, faster outflow in the jet.

A Chandra Survey of Quasar Jets: First Results

We present results from Chandra X-ray imaging and spectroscopy of a flux-limited sample of flat-spectrum radio-emitting quasars with jetlike extended structure. Twelve of 20 quasar jets are detected in 5 ks ACIS-S exposures. The quasars without X-ray jets are not significantly different from those in the sample with detected jets except that the extended radio emission is generally fainter. New radio maps are combined with the X-ray images in order to elucidate the relation between radio and X-ray emission in spatially resolved structures. We find a variety of morphologies, including long straight jets and bends up to 90°. All X-ray jets are one-sided although the radio images used for source selection often show lobes opposite the X-ray jets. The FR II X-ray jets can all be interpreted as inverse Compton scattering of cosmic microwave background photons by electrons in large-scale relativistic jets although deeper observations are required to test this interpretation in detail. Applying this interpretation to the jets as a population, we find that the jets would be aligned to within 30° of the line of sight generally, assuming that the bulk Lorentz factor of the jets is 10.

The Energy Density of Soft Photons at the Jet Center: the Case with Velocity Structures

On the basis of observational facts and theoretical speculation, it has been suggested that bulk velocity structures along and perpendicular to the jet axis are present in relativistic jets. There are some clues that these velocity structures influence the energy density of seed photons from synchrotron emission. In this paper, we adopt a rigorous method to investigate the energy density in the case of continuous velocity structures. As shown by numerical results, the energy densities are similar in accelerating and stationary jets. However, in a decelerating jet or a transversely structured jet the energy density is much higher, sometimes reaching similar to20-200 times the value estimated for a uniform blob. The emitting region that produces most of the observed radiation flux is determined by the location of the dissipation and the area in which the peak bulk Lorentz factor Gamma(p) appears. The inverse Compton radiation in the cases Gamma(0)less than or equal toGamma(p) and Gamma(c)less than or equal toGamma(p) is studied. We think that, in the comoving frame, the inverse Compton scattering process in a structured jet has properties similar to the external Compton (EC) model; i.e., soft photons are injected just from the slower, downstream part. In this regard, the EC models in which seed photons come from outside the jet are not necessary if the velocity structures exist in regions very close to the core and the structure index is suitable. In particular, the jet model in which the two kinds of velocity structures coexist can be applied to TeV blazars, large-scale X-ray jets, and possibly gamma-ray bursts.

ChandraObservations of the Quiescent Nuclear Black Hole of NGC 821: Evidence of Nuclear Activity?

We report the results of the Chandra ACIS-S observations of the elliptical galaxy NGC 821, which harbors a supermassive nuclear black hole (3.5 × 107 M☉) but does not show signs of active galactic nucleus activity. A small, 85 long (~1 kpc at the galaxy's distance of 23 Mpc), possibly S-shaped, jetlike feature centered on the nucleus is detected in the 38 ks ACIS-S integrated exposure of this region. The luminosity of this feature is LX ~ 2.6 × 1039 ergs s-1 (0.3-10 keV), and its spectrum is hard (described by a power law of Γ = 1.8 or by thermal emission with kT > 2 keV). We discuss two possibilities for the origin of this feature: (1) a low-luminosity X-ray jet, or (2) a hot shocked gas. In either case, it is a clear indication of nuclear activity, detectable only in the X-ray band. Steady spherical accretion of the mass losses from the central stellar cusp within the accretion radius, when coupled with a high radiative efficiency, already provides a power source exceeding the observed radiative losses from the nuclear region. A third possibility, that this feature may arise from a fortuitous distribution of luminous X-ray binaries in NGC 821, is also discussed.

Magnetohydrodynamic Simulation of Solar Coronal Chromospheric Evaporation Jets Caused by Magnetic Reconnection Associated with Magnetic Flux Emergence

We studied solar coronal X-ray jets by MHD numerical simulations with heat conduction effects based on a magnetic reconnection model. Key physical processes are included, such as the emergence of magnetic flux from the convection zone, magnetic reconnection with the coronal magnetic fields, heat conduction to the chromosphere, and chromospheric evaporation. Radiation, however, has been neglected. High-density evaporation jets were successfully reproduced in the simulations. The mass of the evaporation jets M is described as M = 6.8 × 1012g(B/10 G)15/7(Tcor/106 K)5/14(L/5000 km)12/7(t/400 s), where B is the strength of magnetic fields, Tcor is the coronal temperature, L is the loop height, and t is the duration of ejection, respectively. We also derived a theoretical model of the Mach number of the reconnection jets as a function of ambient coronal variables. Numerical simulations also show that two different types of jets (evaporation jets and low-density jets) exist simultaneously around the emerging flux region, and the energy of evaporation jets is somewhat larger than that of the low-density jets.

The Highly Relativistic Kiloparsec‐Scale Jet of the Gamma‐Ray Quasar 0827+243

We present Chandra X-ray (0.2-8 keV) and Very Large Array radio (15 and 5 GHz) images of the γ-ray-bright, superluminal quasar 0827+243. The X-ray jet bends sharply—by ~90°, presumably amplified by projection effects—5'' from the core. Only extremely weak radio emission is detected between the nuclear region and the bend. The X-ray continuum spectrum of the combined emission of the knots is rather flat, with a slope of -0.4 ± 0.2, while the 5-15 GHz spectra are steeper for knots detected in the radio. These characteristics, as well as nondetection of the jet in the optical band by the Hubble Space Telescope, pose challenges to models for the spectral energy distributions (SEDs) of the jet features. The SEDs could arise from pure synchrotron emission from either a single or dual population of relativistic electrons only if the minimum electron energy per unit mass γmin 1000. In the case of a single population, the radiative energy losses of the X-ray-emitting electrons must be suppressed owing to inverse Compton scattering in the Klein-Nishina regime, as proposed by Dermer & Atoyan. Alternatively, the X-ray emission could result from inverse Compton scattering of the cosmic microwave background photons by electrons with Lorentz factors as low as γ ~ 15. In all models, the bulk Lorentz factor of the jet flow Γ 20 found on parsec scales must continue without substantial deceleration out to 800 kpc (deprojected) from the nucleus, and the magnetic field is very low, 2 μG, until the bend. Deceleration does appear to occur at and beyond the sharp bend, such that the flow could be only mildly relativistic at the end of the jet. Significant intensification of the magnetic field occurs downstream of the bend, where there is an offset between the projected positions of the X-ray and radio features.