Ralph P. Kraft

Determination of confidence limits for experiments with low numbers of counts. [Poisson-distributed photon counts from astrophysical sources]

Two different methods, classical and Bayesian, for determining confidence intervals involving Poisson-distributed data are compared. Particular consideration is given to cases where the number of counts observed is small and is comparable to the mean number of background counts. Reasons for preferring the Bayesian over the classical method are given. Tables of confidence limits calculated by the Bayesian method are provided for quick reference. 12 refs.

Reflections of Active Galactic Nucleus Outbursts in the Gaseous Atmosphere of M87

We combined deep Chandra, ROSAT HRI, and XMM-Newton observations of M87 to study the impact of active galactic nucleus (AGN) outbursts on its gaseous atmosphere. Many X-ray features appear to be a direct result of repetitive AGN outbursts. In particular, the X-ray cavities around the jet and counterjet are likely due to the expansion of radio plasma, while rings of enhanced emission at 14 and 17 kpc are probably shock fronts associated with outbursts that began 1-2 × 107 yr ago. The effects of these shocks are also seen in brightenings within the prominent X-ray arms. On larger scales, ~50 kpc from the nucleus, depressions in the surface brightness may be remnants of earlier outbursts. As suggested for the Perseus Cluster by Fabian and his coauthors, our analysis of the energetics of the M87 outbursts argues that shocks may be the most significant channel for AGN energy input into the cooling-flow atmospheres of galaxies, groups, and clusters. For M87, the mean power driving the shock outburst, 2.4 × 1043 ergs s-1, is 3 times greater than the radiative losses from the entire cooling flow. Thus, even in the absence of other energy inputs, outbursts every 3 × 107 yr are sufficient to quench the flow.

Chandra and Xmm-Newton observations of a sample of low-redshift FR I and FR II radio galaxy nuclei

We present spectral results from Chandra and XMM-Newton observations of a sample of 22 low-redshift (z < 0.1) radio galaxies and consider whether the core emission originates from the base of a relativistic jet, or an accretion flow, or contains contributions from both. We find correlations between the unabsorbed X-ray, radio, and optical fluxes and luminosities of FR I-type radio-galaxy cores, implying a common origin in the form of a jet. On the other hand, we find that the X-ray spectra of FR II-type radio galaxy cores are dominated by absorbed emission, with NH 1023 atoms cm-2, which is likely to originate in an accretion flow. We discuss several models that may account for the different nuclear properties of FR I- and FR II-type cores and also demonstrate that both heavily obscured, accretion-related and unobscured, jet-related components may be present in all radio galaxy nuclei. Any absorbed, accretion-related components in FR I-type galaxies have low radiative efficiencies.

Radio and X-Ray Observations of the Jet in Centaurus A

We present new, high dynamic range VLA images of the inner jet of the closest radio galaxy, Centaurus A. Over a 10 yr baseline we detect apparent subluminal motions (v � 0:5c) in the jet on scales of hundreds of parsecs. The inferred speeds are larger than those previously determined using VLBI on smaller scales and provide new constraints on the angle made by the jet to the line of sight if we assume jet-counterjet symmetry. The new images also allow us to detect faint radio counterparts to a number of previously unidentified X-ray knots in the inner part of the jet and counterjet, showing conclusively that these X-ray features are genuinely associated with the outflow. However, we find that the knots with the highest X-ray/radio flux density ratios do not have detectable proper motions, suggesting that they may be related to standing shocks in the jet; we consider some possible internal obstacles that the jet may encounter. Using new, high-resolution Chandra data, we discuss the radio to X-ray spectra of the jet and the discrete features that it contains, and we argue that the compact radio and X-ray knots are privileged sites for the in situ particle acceleration that must be taking place throughout the jet. We show that the offsets observed between the peaks of the radio and X-ray emission at several places in the Cen A jet are not compatible with the simplest possible models involving particle acceleration and downstream advection together with synchrotron and expansion losses. Subject headings: galaxies: active — galaxies: individual (Centaurus A, NGC 5128) — galaxies: nuclei — radio continuum: galaxies — X-rays: galaxies

X-RAY EMISSION FROM THE HOT INTERSTELLAR MEDIUM AND SOUTHWEST RADIO LOBE OF THE NEARBY RADIO GALAXY CENTAURUS A

We present results from two Chandra/ACIS-I observations and one XMM-Newton observation of X-ray emission from the interstellar medium (ISM) and the inner radio lobes of the nearby radio galaxy Centaurus A. The ISM has an average radial surface brightness profile that is well described by a � -model profile with index � ¼ 0:40 � 0:04 and a temperature of kBTISM � 0:29 keV beyond 2 kpc from the nucleus. We find that diffuse X-ray emission is coincident with the outer half of the southwest radio lobe, and a bright X-ray enhancement is detected along the edge of the lobe. On the basis of energetic and lifetime arguments, we reject a nonthermal explanation for this emission. We model this emission as a thin, hot shell or cap of X-ray– emitting plasma surrounding the radio lobe that was created by the supersonic inflation of the lobe. This plasma shell is both hotter than (kBTSH � 2:9 keV) and greatly overpressurized relative to the ambient ISM, indicating supersonic expansion. We estimate that the lobe is expanding into the ISM at approximately Mach 8.5, or 2400 km s � 1 . We are not directly observing the bow shock, but rather the cooler, denser material that is accumulating ahead of the contact discontinuity. The thermal energy in the shell is a significant fraction of the thermal energy of the hot ISM, demonstrating the possibility that the hot ISM of early galaxies can be reenergized by outflows from nuclear activity. Interestingly, no similarly bright X-ray emission is detected in or along the edge of the northeast lobe, implying that there are differences in the dynamics and evolution of the kiloparsec-scale radio components. Subject headings: galaxies: active — galaxies: individual (Centaurus A, NGC 5128) — galaxies: ISM — galaxies: jets — radio continuum: galaxies — X-rays: galaxies On-line material: color figures

Shocks and Cavities from Multiple Outbursts in the Galaxy Group NGC 5813: A Window to Active Galactic Nucleus Feedback

We present results from new Chandra, GMRT, and SOAR observations of NGC 5813, the dominant central galaxy in a nearby galaxy group. The system shows three pairs of collinear cavities at 1 kpc, 8 kpc, and 20 kpc from the central source, from three distinct outbursts of the central active galactic nucleus (AGN), which occurred 3 × 106, 2 × 107, and 9 × 107 yr ago. The Hα and X-ray observations reveal filaments of cool gas that has been uplifted by the X-ray cavities. The inner two cavity pairs are filled with radio-emitting plasma, and each pair is associated with an elliptical surface brightness edge, which we unambiguously identify as shocks (with measured temperature jumps) with Mach numbers of M ≈ 1.7 and M ≈ 1.5 for the inner and outer shocks, respectively. Such clear signatures from three distinct AGN outbursts in an otherwise dynamically relaxed system provide a unique opportunity to study AGN feedback and outburst history. The mean power of the two most recent outbursts differs by a factor of six, from (1.5-10)×1042 erg s–1, indicating that the mean jet power changes significantly over long (~107 yr) timescales. The total energy output of the most recent outburst is also more than an order of magnitude less than the total energy of the previous outburst (1.5 × 1056 erg versus 4 × 1057 erg), which may be a result of the lower mean power, or may indicate that the most recent outburst is ongoing. The outburst interval implied by both the shock and cavity ages (~107 yr) indicates that, in this system, shock heating alone is sufficient to balance radiative cooling close to the central AGN, which is the relevant region for regulating feedback between the intracluster medium and the central supermassive black hole.

Chandra and XMM-Newton Observations of the Nucleus of Centaurus A

We present X-ray spectra of the nucleus of the nearby radio galaxy Centaurus A from observations with the XMM-Newton EPIC CCD cameras (two exposures separated by 12 months) and the Chandra HETGS. For the first time in an FR I type galaxy, we resolve fluorescent Kemission from cold, neutral, or near-neutral iron at 6.4 keV, with an rms line width of � 20 eV. The Fe line parameters observed are consistent with fluorescent emission from material at a large distance from the active galactic nucleus, either in the form of an absorber that nearly completely surrounds the central engine or a torus that lies predominantly out of the line of sight. Unresolved emission lines from neutral Si Kat 1.74 keV and neutral S Kat 2.30 keV are also detected. We find no evidence in the data for a previously reported 6.8 keV broadened Fe line. The continuum spectrum is well fitted with a combination of a heavily absorbed power-law component that we relate, using Bondi theory, to accretion phenomena in the form of a standard, geometrically thin, optically thick disk, and a second, less absorbed, power-law component that we associate with emission from the subparsec VLBI radio jet.

High-energy particle acceleration at the radio-lobe shock of Centaurus A

We present new results on the shock around the southwest radio lobe of Centaurus A using data from the Chandra Very Large Programme observations (740 ks total observing time). The X-ray spectrum of the emission around the outer southwestern edge of the lobe is well described by a single power-law model with Galactic absorption ‐ thermal models are strongly disfavoured, except in the region closest to the nucleus. We conclude that a significant fraction of the X-ray emission around the southwest part of the lobe is synchrotron, not thermal. We infer that in the region where the shock is strongest and the ambient gas density lowest, the inflation of the lobe is accelerating particles to X-ray sync hrotron emitting energies, similar to supernova remnants such as SN1006. This interpretation resolves a problem of our earlier, purely thermal, interpretation for this emission, namely t hat the density compression across the shock was required to be much larger than the theoretically expected factor of 4. We describe a self-consistent model for the lobe dynamics and shock properties using the shell of thermal emission to the north of the lobe to estimate the lobe pressure. Based on this model, we estimate that the lobe is expanding to the southwest with a velocity of �2600 km s 1 , roughly Mach 8 relative to the ambient medium. We discuss the spatial variation of spectral index across the shock region, concluding that our observations constrain γmax for the accelerated particles to be �10 8 at the strongest part of the shock, consistent with expectat ions from diffusive shock acceleration theory. Finally, we consider the implications of these results for the production of ultra-high energy cosmic rays (UHECRs) and TeV emission from Centaurus A, concluding that the shock front region is unlikely to be a significant source of UHECRs, but that TeV emission from this region is expected at levels comparable to current limits at TeV energies, for plausible assumed magnetic field strength s.

A Chandra study of the multicomponent X-ray emission from the X-shaped radio galaxy 3C 403

We present results from a 49.4 ks Chandra ACIS-S observation of the nearby (z ¼ 0:059) X-shaped FR type II (FR II)radiogalaxy3C403.ThisisthefirstChandraobservationofanX-shapedradiogalaxy,andoneofthegoals of this pioneering study is to determine the relationship between the X-ray‐emitting gas and the X-shaped radio morphology. We find that the X-ray isophotes of the hot gas within � 3B5 of the central galaxy are highly elliptical (eccentricity � 0.57) and coaligned with the elliptical optical isophotes. This supports the hypothesis that X-shaped radio sources are created by propagation of jets through asymmetric density distributions. Within large uncertainties, there is no evidence that the lobes or wings are overpressurized relative to the interstellar medium (ISM), supporting the scenario in which the wings are the result of strong backflows of material from the jet head and subsequent buoyant evolution. We have detected X-ray emission from several of the radio knots to the east of the active nucleus and diffuse emission from the radio lobe to the west. The X-ray emission from the eastern knots cannot be explained by an inverse Compton model unless they are far from equipartition. Using archival Hubble Space Telescope (HST) data, optical emission is detected from two knots, and the radio/optical/X-ray spectra are well fitted by simple synchrotron models. This is one of the strongest examples to date of X-ray synchrotron emissionfrommultipleknotsinthejetofanFRIIradiogalaxy.X-rayemissionisalsodetectedfromtheradiowings at a flux consistent with inverse Compton scattering of cosmic microwave background (CMB) photons from relativistic electrons if the wings are near equipartition. The nuclear spectrum is well described by a multicomponent model that includes a heavily absorbed power law (NH � 4 ;10 23 cm � 2 ) and a bright (EW � 244 eV), broadened Fe line. A second, less absorbed, power-law component, likely representing unresolved emission from a parsec-scale jet, is also required. Subject headings: galaxies: active — galaxies: individual (3C 403) — galaxies: ISM — X-rays: galaxies

Chandra Observations of the X-Ray Jet in Centaurus A

We present high angular resolution X-ray images and spectra from two Chandra AXAF CCD Imaging Spectrometer (ACIS-I) observations of the X-ray jet in the nearby radio galaxy Cen A. We find that the X-ray emission from the jet is composed of a low surface brightness diffuse component extending continuously from within at least 60 pc of the active nucleus into the northeast radio lobe 4 kpc from the nucleus, along with 31 discrete compact knots, most of which are extended at the resolution of our observation. We find that there are small but significant differences between the X-ray and radio morphologies of the inner jet at the arcsecond level, making it unlikely that a single, spatially coincident population of ultrarelativistic electrons is responsible for the emission in both energy regimes. We suggest that the X-ray knots of the inner jet are indeed the sites of particle acceleration and shocks and that the X-ray and radio knot offsets are caused by a combination of particle diffusion and energy loss. These offsets may be a common feature of all jets in radio galaxies, or at least jets in FR I galaxies, and may be fundamental to the physics of such jets. They are best observed in Cen A because the source is so close. Even though the X-ray and radio knots are offset in position and there are variations of more than a factor of 3 in the ratio of X-ray to radio flux density in the inner jet, the radio to X-ray two-point spectral indices at the X-ray knots are not unusually flat and are consistent with those observed in other X-ray jets seen in FR I galaxies such as M87 and 3C 66B. We find the width of the jet in the X-ray bandpass to be narrower than that measured in the radio along most of its length. The X-ray spectra of several regions of the jet are well fitted by absorbed power-law models with photon indices � 2.2– 2.5, although the spectrum of one bright knot located � 1 kpc from the nucleus (knot B) is harder (photon index = 2.0). Subject headings: galaxies: active — galaxies: individual (Centaurus A, NGC 5128) — galaxies: jets — X-rays: galaxies