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Chandra enables study of x-ray jets

Daniel Schwartz1

Smithsonian Astrophysical Observatory, Cambridge, MA

Edited by Neta A. Bahcall, Princeton University, Princeton, NJ, and approved February 2, 2010 (received for review December 3, 2009)

The exquisite angular resolution of the Chandra x-ray telescope has enabled the detection and study of resolved x-ray jets in a wide variety of astronomical systems. Chandra has detected extended jets in our galaxy from protostars, symbiotic binaries, neutron star pulsars, black hole binaries, extragalactic jets in radio sources, and quasars. The x-ray data play an essential role in deducing the emis- sion mechanism of the jets, in revealing the interaction of jets with the intergalactic or intracluster media, and in studying the energy generation budget of black holes.

black holes∣ quasars ∣ radio sources

I

n this article we show that the Chandra x-ray observatory has made possible a unique and rich field of study: extended x-ray jets, resolved on scales as fine as000:5. We discuss how the x-ray observations contribute significantly to the overall un- derstanding of jets, providing information on the energetics and interaction of the system with its environment. In the case of powerful extragalactic jets, the energy flux in the x-ray radiation is typically 10 times larger than that in the GHz radio emission.

Identifications of sources in some of the earliest radio astron- omy surveys, carried out at a few hundred MHz, revealed many cases of double sources separated by hundreds of kpc on opposite sides of a galaxy or quasar. The total energy content in many cases was estimated to be a minimum of1061 ergs. It was hypothesized that this energy was supplied from the central source via a flow of plasma and electromagnetic waves. These flows were recognized as the narrow radio structures which were revealed at GHz fre- quencies as the resolution of radio telescopes improved. The term“jet” was applied, and came to mean a structure which was at least four times longer than its transverse dimension. As very long baseline interferometry (VLBI) was developed, structured radio emission within pc distances of the central source was dis- covered, and in many objects appeared to be expanding at appar- ent speeds, vapp¼ βappc in excess of the velocity of light, (c)! In general these appeared to be in the same direction as the jets on kpc scales, and it was ultimately recognized that they were indeed emitted at relativistic velocities directed at an angleθ close to our line of sight (LOS) so that their apparent velocity, βapp¼ ðβ sin θÞ∕ð1 − β cos θÞ. More recently, narrow radio and optical structures have been detected in stellar systems and called“jets.”

Although the first jet was discovered in an optical image of the galaxy M87 (1) in 1918, radio astronomy dominated the study of jets at the time of the Chandra launch. Previous to the Chandra observatory, x-ray jets had been clearly detected only in the near- by giant radio galaxy Cen A (2, 3), the quasar 3C 273 (4), and in M87 at the center of the Virgo cluster (5). Since the jet is typically no more than a few percent of the intensity of the central source which emits it, one typically needs a separation several times the full width at half maximum resolution in order to separate the jet image from the extended point spread function of the central source. Fig. 1 illustrates how the 100-fold increase in imaging power of Chandra, compared to the x-ray telescopes on the Einstein and ROSAT missions, enabled detailed study of the pre- viously detected jet in 3C 273. Each of the three historical x-ray jets teaches us something new via their Chandra observations.

Galactic Jets

Pulsars.Pulsar jets are clearly revealed in x-ray images. Observa- tions of the Crab Nebula (6) and of the Vela Supernova Remnant (7) show narrow, two-sided structures emanating from the central pulsar. In the case of the Vela Pulsar jet, apparent proper motions of blobs in the jet correspond to velocities between 0.35 and 0.51 of the speed of light (7). In general, jets are assumed to emerge along the spin axis of their source. This is reasonable based on symmetry and on physical mechanisms. However, we do not have independent observational evidence of the spin-axis direction of the central object, or even that it is rotating in the case of extra- galactic sources. We know that the Crab and Vela are spinning, because we measure very precise and stable periods of their pulsed emission. They therefore provide direct evidence that the jet emerges from the spin axis, since otherwise we would see the jet emission smeared over the cone of the offset angle.

Protostars.Objects in the process of condensing to form nuclear burning stars may, depending on their characteristics, be called premain sequence stars, young stellar objects, or T Tauri stars.

Angular momentum and magnetic field must be expelled from the large gas cloud in order for it to contract to a size where the central density is high enough to ignite nuclear reactions. Out- flows from stellar winds and jets provide an expulsion mechanism.

Optical and radio jets are well known to be associated with premain sequence stars. However, the x-ray emission from these jets is apparently limited to distances of order 1000 AU

Fig. 1. X-ray images of 3C 273 as seen by the ROSAT (left) and Chandra (insert to the right) telescopes. Images are to the same scale, and registered top (N) to bottom. East is to the left. Even though the ROSAT resolution was 500compared to the extension of the jet from1000to2000from the quasar, only the end of the jet is clearly distinguished from the bright quasar. The Chandra image, which is to the same scale, clearly resolves the length of the radio and optical jet, and also shows a faint x-ray jet connecting to the nucleus. (The streak SE to NW in the Chandra image is the CCD readout artifact). Image Courtesy: NASA/SAO/D. Schwartz.

Author contributions: D.S. performed research, analyzed data, and wrote the paper.

The authors declare no conflict of interest.

This article is a PNAS Direct Submission.

1E-mail: das@head.cfa.harvard.edu.

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(AU:1 AU ¼ 1.496 × 1013 cm), which is only a few arcsecond at distances beyond 100 pc (pc:1 pc ¼ 3.086 × 1018cm). While the small size necessitates an x-ray telescope with resolution better than an arcsecond for detection, such resolution is required in any event in order to extract the key astrophysics of jet formation and collimation taking place near the proto-stellar object.

The first detection of a jet in a T Tauri star was made by (8, 9), for DG Tauri. The x-ray image in Fig. 2 shows the two-sided jet.

The weaker jet to the northeast is interpreted as being pointed away from us, and its x-rays attenuated by the proto-stellar disk relative to the southwest jet which is on the near side of the sys- tem relative to us. The star is estimated to be 140 pc away, so that the total x-ray luminosity of the jet is about2.4 × 1028 ergs s−1, and the 500 jets are about 700 AU in length. Because the proper motion is measured, the 500 distance implies a lifetime of≈20 years. Using optical estimates of the mass and velocity of outflow (10) allowed the kinetic energy power to be estimated as1.7 × 1033ergs s−1(9). This gives the interesting efficiency of order10−5 for conversion of kinetic flux to x-ray luminosity—a number similar to that occuring in relativistic AGN jets.

Symbiotic Star.A symbiotic star system is a spectroscopic binary system containing a red giant star, which is rapidly losing mass, and a hot white dwarf star. R Aquarii is a classical example. Fig. 3 shows two-sided x-ray jets from R Aqr at two epochs. Between the 2000.7 observation and the 2004.0 observation a new jet was ejected to the southwest. On larger scales, an x-ray jet extends to700:5 to the northeast and about 3000to the southwest, indicating alternating ejections in the past. The jets are clearly nonrelativis- tic, so the observation that the new ejection was only to the south- west is significant in view of the fact that jets are generally assumed to be intrinsically symmetric toward and away from the observer. Differences in the present case are that the binary system provides an asymmetry as the white dwarf orbits in a very eccentric path, and of course that the jets here do seem to be symmetric on time scales greater than 10 yr.

The X-radiation we see arises from the interaction of the jets with the material ejected from the red giant star. Fitting a thermal model for the x-rays allows in principle to determine the ioniza- tion time scale, and hence the age is inferred to be less than 10 yr for densities which are at least1 electron cm−3 (11).

Black Hole Binaries.Massive stars which collapse to remnants of greater than roughly 3 to 5 solar masses must end their life as black holes. When the black hole is in a binary system where it can capture mass from its main sequence companion, we see a luminous x-ray source which can undergo many complex changes of intensity and spectral shape. In correlation with these state changes, we observe the ejection of jets, specifically in the radio and x-ray bands. Because of the analogy to radio bright

quasars, which are powered by black holes of mass 106 to1010 times that of the sun, these ≈10 solar mass black hole systems are often called microquasars. Since we observe them in our own galaxy, instead of at cosmological distances, we can observe motions on arcsecond scales.

The jets in microquasars can show superluminal expansion, with a typical value of the apparent velocityβapp≈ 2–3. The black hole binary XTE J1550-564 had a large x-ray flare in September 1997, followed by radio observations of superluminal jets. An x-ray emission region was discovered to be moving along the direction of the radio jet to the east in observations during 2000 and 2002 (12). The distance to the system can be estimated from observations of the main sequence companion star, so that the apparent angular motion of the jets can be transformed to an apparent velocity. X-ray observations of the eastern jet in 2002 give a mean apparent velocity of 1.0 c since the ejection in 1998, while the apparent motion between 2000 and 2002 gives a velocity only 0.32 c (12).

This is direct evidence for gradual deceleration of the jet.

Considering that the jet decelerates due to interaction with the interstellar medium, Hao and Zhang (13) create a model where the bulk Lorentz factor of the initial ejection is Γ ¼ 3, and the interstellar medium is asymmetric with density 0.034 cm−3 to the east and0.12 cm−3 to the west. The smaller apparent velocity of the western jet leads to the higher density estimate. Although the modeled numbers have limited precision, it shows the wealth of information which the x-ray jets can reveal.

The radio spectrum can be extrapolated directly to the x-ray flux, leading to the inference that the x-rays are also produced by synchrotron radiation as is known for the radio emission.

From an estimate of the magnetic field strength, electrons must be accelerated to energies of 10 TeV, or Lorentz factors γ ≈ 2 × 107, to produce the keV x-ray emission.

Extragalactic Jets

The Fanaroff and Riley (FR) (14) morphological classification of radio sources has been extremely useful in considering the phe- nomenology and physics of extragalactic sources. The original definition took the ratio of the distance between the highest brightness regions on the two sides of the source to the overall length of the source. For FR class I the brightest points were con- centrated within half the total source extent, and for FR class II the ratio exceeded one half. This simple classification remarkably divides sources into a group of low power (FR I) vs. high power (FR II) emitters. We will see that their x-ray jets further distin- guish these two classes.

FR I Jets.The mechanism of x-ray emission in FR I radio source jets is well explained in general as arising from synchrotron ra- diation (15), from an extension of the relativistic electron spec- trum which gives rise to the radio and (when observed) optical emissions. The radio and optical radiation are known to be

Fig. 2. Chandra image of the bipolar x-ray jet in DG Tau (from Fig. 1 of (9)). N is up, E to the left. The northeast and southwest jets contain 9 and 18 x-ray counts, respectively, in the 0.6 to 1.7 keV band, in contrast to 1,083 counts from the star. Left shows x-rays counts sorted into00049 bins, while right shows a smoothed representation of the data. Image Courtesy: NASA/Paul Scherrer Institut/M.Güdel et al.

Fig. 3. Smoothed Chandra images of the two-sided jet in the R Aquarii symbiotic binary system (Fig. 2 from (34)). The small white cross marks the position of R Aqr. Left is an observation from 2000.7 and right from 2004.0, showing expansion of the newly ejected jet. The vertical100scale cor- responds to 200 AU, given the distance estimate of 200 pc (35) to the star.

Image Courtesy: NASA/SAO/J. Nichols et al.

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polarization can be measured from a jet by any means currently.

Estimates of the magnetic fields based on the radio observations typically show that the magnetic field energy density, B2∕8π, ex- ceeds any possible photon energy density, considering the central black hole radiation, the ambient synchrotron radio photons, or the local cosmic microwave background (CMB). That energy density argument shows that inverse Compton (IC) radiation is not a major contributor.

Relativistic electrons of energies upward of 1013 eV are re- quired to produce keV x-rays via synchrotron radiation in the magnetic fields of order10 μG estimated in jets. Because the life- times of such electrons are only 10 to 100 yr, their synchrotron x-ray emission is important to show us the location where the particle acceleration is taking place.

M87 gives us some independent evidence that we see synchro- tron x-ray emission. The spatial resolution of Chandra has allowed the independent observation of different regions of its jet. Repeated observations over time (Fig. 4) show that a knot observed by the Hubble Space Telescope (HST) and only 000:86 from the core has brightened significantly over the years on time scales as short as months, but has also dimmed somewhat on those time scales (16, 17). The rapid decreases must reflect the lifetimes of the emitting electrons, and match the time scales expected for electrons with Lorentz factorsγ ≈ 107which are re- quired to produce x-rays in the magnetic fields of order hundreds ofμG estimated to be present from equipartition considerations.

This establishes synchrotron radiation as the mechanism of x-ray emission for this jet.

Centaurus A is the nearest giant radio galaxy, and Feigelson et al. (3) detected its northeast jet using the High Resolution Imager on the Einstein X-ray Observatory. The spectral energy distribu- tion (SED) and energetic considerations argue that the x-rays arise from synchrotron radiation. The Chandra image (Fig. 5, from (18)) reveals a great deal of structure and irregularity in the jet, and resolves it in the transverse direction. The jet shows

”knotty” structures in the radio and x-ray, but also continuous radiation. This indicates that the x-ray emitting electrons must be accelerated continuously throughout the volume of the jet, due to their short lifetimes.

We also take a lesson from the irregular structure. Here we are resolving 100 irregularities, which correspond to linear scales of 17 pc. But in the distant quasar jets we typically have 100¼ 4–8 kpc. Nonetheless the initial models of those regions typically assume uniformity throughout, since obviously only a limited number of parameters can be extracted from the data.

We can measure only smoothed out properties for the distant sources.

FR II and Radio Loud Quasar Jets.The first target at which Chandra pointed was the radio loud quasar PKS 0637-752. That target was chosen as a moderate strength point source, to be used for performing the initial focusing of the telescope. Remarkably, an extended x-ray jet was found (19) coincident with the straight, inner portion of a previously known radio jet. Radio loud quasars are generally assumed to be FR II type radio galaxies, but with a relativistic jet extending from pc to tens of kpc (and inferred to be present in all FR II sources) pointed relatively close to our line of sight, so that its emission is enhanced by special relativistic effects at all frequencies. Quasars give us a very different picture of jet x-ray emission compared to the FR I sources; namely, their emission is likely due to IC scattering of relativistic electrons on the ambient CMB photons (the IC/CMB mechanism).

Fig. 6 shows images of the jet to the same scale in x-rays, and in the 8.6 GHz and 20.16 GHz bands. For PKS 0637-752 an archival HST observation had detected optical emission from the three brightest x-ray and radio knots, K3 and K4 in Fig. 6. That optical emission was so weak that the broad band spectral energy distri- bution clearly did not allow the radio spectrum to be extended into the x-ray region. This ruled out synchrotron emission from a single population of relativistic electrons. In an IC scenario, there was no obvious source of target photons to be scattered:

The knots were not compact enough to give a high density of synchrotron photons, they were too far from the central quasar for its radiation density to compete with the magnetic field energy density, and the CMB energy density, even though enhanced by a factorð1 þ zÞ4was also much less than the minimum energy mag- netic field density. The dilemma was resolved by recognizing that the jet was in bulk relativistic motion with Lorentz factor Γ.

Evidence for this included the known superluminal motion in the core of the quasar, and the fact that the extended jet was one-sided. Since the CMB energy density is enhanced by a factor Γ2 in the jet rest frame (20), it was realized that IC/CMB was indeed the most likely mechanism for x-ray production (21, 22).

This mechanism provides the best explanation for x-ray emis- sion from most of the quasar jets (23–26). If correct, there is a profound consequence: that similar jet structures will be seen anywhere in the universe that they may form (27), this is because the jet emissivity increases with theð1 þ zÞ4dependence of the

Fig. 4. Chandra x-ray images of the central region of M87, showing the variability of knot HST-1. X-rays from 0.5 to 7 keV are binned in000:2 bins.

Image Courtesy: NASA/SAO/D.Schwartz.

Fig. 5. Chandra x-ray image (false color) of the first kpc length of the Cen A NE jet. Contours show the 8.4 GHz radio emission, which closely, but not iden- tically, correlates with the x-ray emission. X-rays are emitted continuous throughout the jet, showing that they are locally accelerated everywhere.

Image Courtesy: NASA/University of Hertfordshire/M. Hardcastle et al.

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CMB energy density, compensating for the cosmological diminu- tion of surface brightness asð1 þ zÞ−4. The IC/CMB mechanism is not universally accepted, with many researchers invoking models based on synchrotron radiation. Problems with the IC/CMB interpretation include the fact that often the x-ray emission peaks just upstream of a radio enhancement. Since the IC/CMB x-rays would be from lower energy, hence longer lived electrons, they should persist at least as far downstream as the higher energy, radio emitting electron. Also, there is some resistance to accept- ing the higher bulk Lorentz factors need for the IC/CMB. In support of the IC/CMB, it is very hard to explain the very close correspondence of x-ray and radio emission, which typically track each other within a factor of 2 over≈100 kpc scales, if the x-rays arise from a separate population of relativistic electrons. The controversy is exciting, because it means that its resolution will reveal important information on the internal jet dynamics.

We point out a very important exception which is manifested by the jet in 3C 273, one of the jets detected prior to the Chandra mission (4). As one of the nearest quasars, its jet is very bright at radio, optical, and x-ray wavelengths. Figure 1 in reference (28) gives a detailed x-ray image of the jet, compared to optical and radio images. The jet is well resolved into many distinct spatial elements. As in other quasars, the SED of these elements do not allow an extrapolation of the radio spectrum through the op- tical to connect to the x-ray flux. This shows that a single electron spectrum does not produce the x-rays via synchrotron emission.

However, the fact that the x-ray spectra are steeper than the radio spectra (at least beyond the first two bright x-ray knots) shows that they are not due to electrons of lower energy than those pro- ducing the radio emission. The conclusion is that there is a sepa- rate population of relativistic electrons, accelerated throughout the jet to energies above1013eV, which produce the x-rays via synchrotron radiation (29).

Why is the separate population considered an exception, and not as evidence that the same circumstances prevail in all qua- sars? As noted above, the IC/CMB mechanism is controversial.

However, the 3C 273 jet has two key phenomenological differ- ences from most quasar jets. First, the jet x-ray luminosity is only about 0.3% of the quasar x-ray luminosity. This is a factor of

about 10 less than typical quasar jets, for which the x-ray lumi- nosity is 1% to 10% of the quasar core value. So the same con- tribution of x-ray synchrotron emission relative to the core could indeed be present in most x-ray jets, but would be dwarfed by the IC/CMB contribution. Second, and probably more significant, the relative x-ray and radio morphology is grossly different for 3C 273 compared to most quasar jets. PKS 0637-752 is the usual case, with the x-ray and radio profiles tracking each other within a fac- tor of about two over500to1000scales, corresponding to tens to hundreds of kpc linear scales. In contrast, for the 3C 273 jet the x-rays decrease away from the quasar while the radio increases, with overall change in the ratio greater than a factor of 100, as shown in Fig. 7.

Environment. A 237 ks Chandra observation of the system PKS 1055 þ 201 is shown in Fig. 8 (30). This quasar has a jet clearly coincident with the 1.46 GHz radio jet image, up to the apparent terminal hot-spot where the radio jet undergoes abrupt changes of direction. There is x-ray emission extended≈  800surrounding the northern jet, and emission of similar size and surface bright- ness surrounding the unseen jet assumed to extend to the south (31). This is important evidence supporting the usual assumption that equal jets are emitted in both directions along the black hole spin axis, with the approaching jet being enhanced by special relativity effects and the receding jet rendered undetectable by the same effects. Both jets curve to the East, as if the quasar were moving through the medium of a cluster of galaxies. Indeed, there does seem to be additional x-ray emission surrounding the system (Fig. 8 right) which could be emission from the cluster gas.

However, there is no other supporting evidence for a cluster:

The numbers of x-rays are too few to test the spectrum of extended emission for a thermal bremsstrahlung signature.

The average x-ray spectrum of the jet appears as a power law with energy index 0.95. The average lobe spectral index is flatter, 0.70. Since we expect the lobe material to be older, we may have expected its spectral index to be steeper due to greater energy loss by the radiating electrons. The data indicates that electrons are being accelerated prior to injection into the lobes. This presum-

Fig. 7. Images of 3C273 in x-rays (lower left) and 1.6 GHz (lower right).

The upper figures show projections perpendicular to the1200long axes of the boxes in the lower figures. The x-ray counts per bin (green) are com- pared to the 1.6 GHz projection (disjoint red dots). On the upper left, the radio data are arbitrarily renormalized over each of the different sections to show that they never track the x-ray data over more than an arcsec length.

On the upper right, the x-ray and radio data are plotted in physical units to the same scale, showing that the x-ray emission dominates up until near the very end of the jet. Image Courtesy: NASA/SAO/D. Schwartz.

Fig. 6. Chandra (top) and Australian Telescope Compact Array (center:

8.6 GHz; bottom: 20.16 GHz) images of PKS 0637-752. The x-rays are closely correlated with both radio bands in the straight part of the jet, but termi- nate when the radio jet undergoes a large apparent angular change and where the 20.16 GHz polarization changes direction. All figures are to same

scale,3100× 1500. Image Courtesy: NASA/SAO/D. Schwartz. ASTRONO

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ably takes place at the terminal hotspots, which are detected in both radio and x-ray at the ends of both jets.

Kinetic Energy Flux.The kinetic energy flux carried by a jet can be calculated from the enthalpy density (32). This quantity can be comparable to the radiative output of the black hole, and is thus a significant part of the overall accretion energy budget. Deter- mining the kinetic power requires knowledge of the internal magnetic and relativistic particle energy densities, and of the bulk Lorentz factor of the jet. Using x-ray data in conjunction with the radio flux, and a rather large number of reasonable assumptions, allows calculation of these quantities.

Basically, the apparent magnetic field in the observer frame can be calculated in two different ways, Bmefrom the radio data assuming minimum energy conditions, and Bicfrom the x-ray to radio flux density ratio assuming that the x-rays are produced via IC/CMB from an extension to lower energy of the relativistic electrons producing the radio emission. Typically these two quan- tities differ by up to a factor of 100. However the transformations to the intrinsic magnetic field in the jet rest frame go as Brest¼ Bme∕δ while Brest¼ Bic× Γ. The Doppler factor δ ¼ 1∕ðΓð1 − β cos θÞÞ where Γ is the bulk Lorentz factor, θ is the an- gle of the jet to the observer’s line of sight, and β ¼ ffiffi

1 − 1∕Γ2Þ.

Typically we do not know the true angle to the line of sight, so one either assumes a fiducial value ofΓ, typically the round num- ber 10, or it is assumed thatΓ ¼ δ. The latter is reasonable in that 2Γ is the maximum value of δ for large values of Γ and for θ near zero. It corresponds to the assumption thatθ ¼ 1∕Γ, and gives the largest possibleθ for a fixed δ, and hence gives the largest solid angle in which a priori the jet could have been found by a fixed observer. Such assumptions for the Chandra quasar jets typically give rest frame magnetic fields of order 5–20 μG, and Γ of order 3–15. Although these are reasonable estimates, the underlying assumption ofΓ ¼ δ or Γ ¼ 10 cannot be correct. Although such estimates can determineΓ or B within a reasonable factor of 2 (31), the kinetic energy flux depends onΓ2× B2, and it is that flux which relates to the basic physics of the black hole energy budget, and of interaction of the jet with the external interstellar or intergalactic medium. Typical kinetic power in these quasar jets is of order1046–1047ergs s−1. Since the kinetic power is an intrinsic property of the jet, i.e., not dependent on orientation, it is clear that when such black hole jets occur in clusters of galaxies they can produce sufficient energy to halt cooling flows, as has been deduced from the bolometric energy content of x-ray void regions coincident with radio lobes in clusters (33).

The quasar jet PKS1354 þ 195 provides an ideal system to test alternative assumptions. Fig. 9 shows the 200 ks Chandra x-ray image of a portion of the jet which appears to be extremely straight over a distance of 1800. Because it is straight we can assume that the unknown direction to our LOS is constant (or nearly so) along the jet. Because of the length we can divide the image into 11 independent spatial elements (numbered 4 to 14 in Fig. 9). Element 14 is too close to the quasar, so we perform the IC/CMB analysis on the ten elements further down the jet. In Fig. 10 we consider the angle to the line of sight as an unknown free parameter, and for each angle from 1° to 11° we plot the resultingδ and Γ values for each region. Our intuition would have liked all the values ofΓ to be constant for some angle. At angles less that 6° this is roughly the case, whereas beyond 7° the values ofΓ start diverging strongly.

To constrain the LOS parameter we explore the hypothesis that the kinetic energy flux is constant along the jet. We know that jets are extremely efficient in carrying energy to radio lobes, so it is reasonable that the kinetic power is not being lost as it propagates down the jet. Of course, we know quasars are variable in their radiative output, so inputs to the jet may vary in time. The spatial elements we can resolve on distant objects must corre-

jet curving to the north and coincident with the radio jet. The center uses100 bins and shows the lobe-like x-ray structures surrounding the northern and the unseen southern jet. The right uses200bins and brings out further ex- tended structure which might be associated with a cluster of galaxies. The cyan contours are of the 1.46 GHz radio emission. The horizontal streak is an artifact of the ACIS instrument, namely, since there is no shutter, events occuring during the CCD readout have erroneous positions. Image Courtesy:

NASA/SAO/D. Schwartz et al.

Fig. 9. Chandra x-ray image of the jet in PKS1354 þ 195 ¼ 4C 19.44. Red numerals to the left of each box indicate the number of 0.5 to 7 keV x-rays detected in that region. The length of the jet is1800corresponding to 130 kpc in the plane of the sky, and to 1250 kpc when deprojected according to the IC/CMB modeling. Image Courtesy: NASA/SAO/D. Schwartz et al.

Fig. 10. Values ofΓ and δ for regions 4–13 along the jet, for different possible values of the unknown angle to the line of sight. Image Courtesy:

NASA/SAO/D. Schwartz et al.

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spond to outbursts over≈104years, so it might well be that the average quasar output is constant over such time scales.

In Fig. 11 we plot the derived kinetic energy flux through each of the 10 spatial regions as a function of the unknown LOS angle.

Again, we would expect that for some angle we would find that all 10 elements gave the same kinetic energy flux value. Indeed, for3° ≤ θ ≤ 7° we do have such agreement possible within the statistical errors of the x-ray and radio observations. (Systematic uncertainties allow more leeway.) Ignoring all errors, with a mean LOS anywhere between 5° and 7° the elements could all sustain a constant kinetic energy flux between 7 and12 × 1046 ergs s−1, if there were deviations of1° in the jet along its length. We con- clude that the hypothesis of constant kinetic power is viable, although it remains to be validated on a wider sample of deep observations of jets.

Summary

Chandra observations of x-ray jets have provided information about their energetics and interactions with their environment.

Major problems remain in determining the content of the jets (protons vs. positrons), the jet confinement, and the acceleration of particles in the jet. X-ray jets were not widely anticipated prior to Chandra and their study has been relatively immature and commanded proportionately modest observing time in the first decade of Chandra. We expect that the second decade of Chandra science will recognize the unprecedented opportunity to study x-ray jets, and produce results dwarfing those reported here.

ACKNOWLEDGMENTS. This work was supported by the National Aeronautics and Space Administration contract NAS8-03060 and Smithsonian Astro- physical Observatory Grants GO9-0121B and GO7-8107X.

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Fig. 11. Kinetic power through the 10 regions of the PKS1354 þ 195 jet, as a function of the unknown angle to the line of sight. If the angle is6°  1°, the kinetic flux may be constant (dashed line) at a value ofð7–12Þ × 1046ergs s−1. Image Courtesy: NASA/SAO/D. Schwartz et al.

ASTRONOMYSPECIALFEATURE

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