arXiv subjects

R. P. Kraft

Publications and source records attributed to R. P. Kraft.

At least 73 records · Page 4Linked to original sources

arXiv2009

Long Term Monitoring of the Dynamics and Particle Acceleration of Knots in the Jet of Centaurus A

We present new and archival multi-frequency radio and X-ray data for Centaurus A obtained over almost 20 years at the VLA and with Chandra, with which we measure the X-ray and radio spectral indices of jet knots, flux density variations in the jet knots, polarization variations, and proper motions. We compare the observed properties with current knot formation models and particle acceleration mechanisms. We rule out impulsive particle acceleration as a formation mechanism for all of the knots as we detect the same population of knots in all of the observations and we find no evidence of extreme variability in the X-ray knots. We find the most likely mechanism for all the stationary knots is a collision resulting in a local shock followed by a steady state of prolonged, stable particle acceleration and X-ray synchrotron emission. In this scenario, the X-ray-only knots have radio counterparts that are too faint to be detected, while the radio-only knots are due to weak shocks where no particles are accelerated to X-ray emitting energies. Although the base knots are prime candidates for reconfinement shocks, the presence of a moving knot in this vicinity and the fact that there are two base knots are hard to explain in this model. We detect apparent motion in three knots; however, their velocities and locations provide no conclusive evidence for or against a faster moving `spine' within the jet. The radio-only knots, both stationary and moving, may be due to compression of the fluid.

astro-ph.CO↗

arXiv2009

Luminosity functions of LMXBs in Centaurus A: globular clusters versus the field

We study the X-ray luminosity function (XLF) of low mass X-ray binaries (LMXB) in the nearby early-type galaxy Centaurus A, concentrating primarily on two aspects of binary populations: the XLF behavior at the low luminosity limit and comparison between globular cluster and field sources. The 800 ksec exposure of the deep Chandra VLP program allows us to reach a limiting luminosity of 8e35 erg/s, about 2-3 times deeper than previous investigations. We confirm the presence of the low luminosity break in the overall LMXB XLF at log(L_X)=37.2-37.6 below which the luminosity distribution follows a constant dN/d(ln L). Separating globular cluster and field sources, we find a statistically significant difference between the two luminosity distributions with a relative underabundance of faint sources in the globular cluster population. This demonstrates that the samples are drawn from distinct parent populations and may disprove the hypothesis that the entire LMXB population in early type galaxies is created dynamically in globular clusters. As a plausible explanation for this difference in the XLFs, we suggest that there is an enhanced fraction of helium accreting systems in globular clusters, which are created in collisions between red giants and neutron stars. Due to the 4 times higher ionization temperature of He, such systems are subject to accretion disk instabilities at approximately 20 times higher mass accretion rate, and therefore are not observed as persistent sources at low luminosities.

astro-ph.HE↗

arXiv2009

Nature of X-shaped sources

The nature of X-shaped sources is a matter of considerable debate: it has even been proposed that they provide evidence for black hole mergers$ / $spin reorientation, and therefore constrain the rate of strong gravitational wave events (Merritt & Ekers 2002). Based on morphological and spectral characteristics of these sources, currently a strong contender to explain the nature of these sources is the `alternative' model of Lal & Rao (2007), in which these sources consist of two pairs of jets, which are associated with two unresolved AGNs. Detailed morphological and spectral results on milliarcsecond-scales (mas) provide a crucial test of this model, and hence these sources are excellent candidates to study on mas; {\it i.e.}, to detect he presence/absence of double nuclei/AGNs, signs of helical/disrupted jets, thereby, to investigate spatially resolved/unresolved binary AGN systems and providing clues to understanding the physics of merging of AGNs on mas. We conducted a systematic study of a large sample of known X-shaped, comparison FR II radio galaxies, and newly discovered X-shaped candidate sources using Giant Metrewave Radio Telescope and Very Large Array at several radio frequencies. In our new observations of `comparison' FR II radio galaxies we find that almost all of our targets show standard spectral steepening as a function of distance from the hotspot. However, one source, 3C 321, has a low-surface-brightness extension that shows a flatter spectral index than the high-surface-brightness hotspots$ / $lobes, as found in `known' X-shaped sources.

astro-ph.CO↗

arXiv2009

The Jet-Heated X-ray Filament in the Centaurus A Northern Middle Radio Lobe

We present results from a 40 ks {\em XMM-Newton} observation of the X-ray filament coincident with the southeast edge of the Centaurus A Northern Middle Radio Lobe (NML). We find that the X-ray filament consists of five spatially resolved X-ray knots embedded in a continuous diffuse bridge. The spectrum of each knot is well fitted by a thermal model with temperatures ranging from 0.3-0.7 keV and subsolar elemental abundances. In four of the five knots, non-thermal models are a poor fit to the spectra, conclusively ruling out synchrotron or IC/CMB mechanisms for their emission. The internal pressures of the knots exceed that of the ambient ISM or the equipartition pressure of the NML by more than an order of magnitude, demonstrating that they must be short lived ($\sim3\times10^6$ yrs). Based on energetic arguments, it is implausible that these knots have been ionized by the beamed flux from the active galactic nucleus of Cen A or that they have been shock-heated by supersonic inflation of the NML. In our view, the most viable scenario for the origin of the X-ray knots is that they are the result of cold gas shock heated by a direct interaction with the jet. The lifetimes of the X-ray knots in the NML are roughly the same as the age of the strong shock around the SW inner lobe, suggesting they were created in the same AGN outburst. The most plausible model of the NML is that it is a bubble from a previous outburst that is being re-energized by the current outburst. We also report the discovery of a large scale (at least 35 kpc radius) gas halo around Cen A.

astro-ph.HE↗

arXiv2009

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. 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 synchrotron emitting energies, similar to supernova remnants such as SN1006. This interpretation resolves a problem of our earlier, purely thermal, interpretation for this emission, namely that the density compression across the shock was required to be much larger than the theoretically expected factor of 4. We estimate that the lobe is expanding to the southwest with a velocity of ~2600 km/s, 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 gamma_max for the accelerated particles to be 10^8 at the strongest part of the shock, consistent with expectations 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 strengths.

astro-ph.HE↗

arXiv2008

Chandra evidence for AGN feedback in the spiral galaxy NGC 6764

We report the Chandra detection of X-ray emission spatially coincident with the kpc-scale radio bubbles in the nearby (D_L ~ 31 Mpc) AGN-starburst galaxy NGC 6764. The X-ray emission originates in hot gas (kT ~ 0.75 keV), which may either be contained within the radio bubbles, or in a shell of hot gas surrounding them. We consider three models for the origin of the hot gas: (1) a starburst-driven galactic wind, (2) shocked gas associated with the expanding radio bubbles, and (3) gas heated and entrained into the bubbles by jet/ISM interactions in the inner AGN outflow. We rule out a galactic wind based on significant differences from known galactic wind systems. The tight correspondence between the brightest X-ray emission and the radio emission in the inner outflow from the Seyfert nucleus, as well as a correlation between X-ray and radio spectral features suggestive of shocks and particle acceleration, lead us to favour the third model; however, we cannot firmly rule out a model in which the bubbles are driving large-scale shocks into the galaxy ISM. In either AGN-driven heating scenario, the total energy stored in the hot gas is high, ~10^56 ergs, comparable to the energetic impact of low-power radio galaxies such as Centaurus A, and will have a dramatic impact on the galaxy and its surroundings.

astro-ph↗

arXiv2008

Evidence for Non-Hydrostatic Gas Motions in the Hot ISM of Centaurus A

We present preliminary results from a deep (600 ks) {\em Chandra} observation of the hot interstellar medium of the nearby early-type galaxy Centaurus A (Cen A). We find a surface brightness discontinuity in the gas $\sim$3.5 kpc from the nucleus spanning a 120$^\circ$ arc. The temperature of the gas is 0.60$\pm$0.05 and 0.68$\pm$0.10 keV, interior and exterior to the discontinuity, respectively. The elemental abundance is poorly constrained by the spectral fits, but if the abundance is constant across the discontinuity, there is a factor of 2.3$\pm$0.4 pressure jump across the discontinuity. This would imply that the gas is moving at 470$\pm$100 km s$^{-1}$, or Mach 1.0$\pm$0.2 (1.2$\pm$0.2) relative to the sound speed of the gas external (internal) to the discontinuity. Alternatively, pressure balance could be maintained if there is a large (factor of $\sim$7) discontinuity in the elemental abundance. We suggest that the observed discontinuity is the result of non-hydrostatic motion of the gas core (i.e. sloshing) due to the recent merger. In this situation, both gas motions and abundance gradients are important in the visibility of the discontinuity. Cen A is in the late stages of merging with a small late-type galaxy, and a large discontinuity in density and abundance across a short distance demonstrates that the gas of the two galaxies remains poorly mixed even several hundred million years after the merger. The pressure discontinuity may have had a profound influence on the temporal evolution of the kpc-scale jet. The jet could have decollimated crossing the discontinuity and thereby forming the northeast radio lobe.

astro-ph↗

arXiv2007

Where Centaurus A gets its X-ray knottiness

We report an X-ray spectral study of the transverse structure of the Centaurus A jet using new data from the Chandra Cen A Very Large Project. We find that the spectrum steepens with increasing distance from the jet axis, and that this steepening can be attributed to a change in the average spectrum of the knotty emission. Such a trend is unexpected if the knots are predominantly a surface feature residing in a shear layer between faster and slower flows. We suggest that the spectral steepening of the knot emission as a function of distance from the jet axis is due to knot migration, implying a component of transverse motion of knots within the flow.

astro-ph↗

arXiv2007

The Active Nucleus of IC4970: A Nearby Example of Merger-Induced Cold-Gas Accretion

We present results from Chandra X-ray and Spitzer mid-infrared observations of the interacting galaxy pair NGC6872/IC4970 in the Pavo galaxy group and show that the smaller companion galaxy IC4970 hosts a highly obscured active galactic nucleus (AGN). The 0.5-10 keV X-ray luminosity of the nucleus is variable, increasing by a factor 2.9 to 1.7 x 10^{42} erg/s (bright state) on ~100 ks timescales. The X-ray spectrum of the is heavily absorbed (N_H = 3 x 10^{23} cm^{-2}) for power law models with Gamma = 1.5-2.0 and shows a clear 6.4 keV Fe Kalpha line with equivalent width of 144-195 eV. Limits on the diffuse emission in IC4970 from Chandra X-ray data suggest that the available power from Bondi accretion of hot interstellar gas may be an order of magnitude too small to power the AGN. Spitzer images show that 8 micron nonstellar emission is concentrated in the central 1 kpc of IC4970, consistent with high obscuration in this region. The mid-infrared colors of the nucleus are consistent with those expected for a highly obscured AGN. Taken together these data suggest that the nucleus of IC4970 is a Seyfert 2, triggered and fueled by cold material supplied to the central supermassive black hole as a result of the off-axis collision of IC4970 with the cold-gas rich spiral galaxy NGC6872.

astro-ph↗

arXiv2007

New results on particle acceleration in the Centaurus A jet and counterjet from a deep Chandra observation

We present new deep Chandra observations of the Centaurus A jet, with a combined on-source exposure time of 719 ks. These data allow detailed X-ray spectral measurements to be made along the jet out to its disappearance at 4.5 kpc from the nucleus. We distinguish several regimes of high-energy particle acceleration: while the inner part of the jet is dominated by knots and has properties consistent with local particle acceleration at shocks, the particle acceleration in the outer 3.4 kpc of the jet is likely to be dominated by an unknown distributed acceleration mechanism. In addition to several compact counterjet features we detect probable extended emission from a counterjet out to 2.0 kpc from the nucleus, and argue that this implies that the diffuse acceleration process operates in the counterjet as well. A preliminary search for X-ray variability finds no jet knots with dramatic flux density variations, unlike the situation seen in M87.

astro-ph↗

arXiv2007

A Chandra study of particle acceleration in the multiple hotspots of nearby radio galaxies

We present Chandra observations of a small sample of nearby classical double radio galaxies which have more than one radio hotspot in at least one of their lobes. The X-ray emission from the hotspots of these comparatively low-power objects is expected to be synchrotron in origin, and therefore to provide information about the locations of high-energy particle acceleration. In some models of the relationship between the jet and hotspot the hotspots that are not the current jet termination point should be detached from the energy supply from the active nucleus and therefore not capable of accelerating particles to high energies. We find that in fact some secondary hotspots are X-ray sources, and thus probably locations for high-energy particle acceleration after the initial jet termination shock. In detail, though, we show that the spatial structures seen in X-ray are not consistent with naive expectations from a simple shock model: the current locations of the acceleration of the highest-energy observable particles in powerful radio galaxies need not be coincident with the peaks of radio or even optical emission.

astro-ph↗

arXiv2007

A Chandra Study of the Lobe/ISM Interactions Around the Inner Radio Lobes of Centaurus A: Constraints on the Temperature Structure and Transport Processes

We present results from deeper {\em Chandra} observations of the southwest radio lobe of Centaurus A, first described by Kraft et al. (2003). We find that the sharp X-ray surface brightness discontinuity extends around $\sim$75% of the periphery of the radio lobe, and detect significant temperature jumps in the brightest regions of this discontinuity nearest to the nucleus. This demonstrates that this discontinuity is indeed a strong shock which is the result of an overpressure which has built up in the entire lobe over time. Additionally, we demonstrate that if the mean free path for ions to transfer energy and momentum to the electrons behind the shock is as large as the Spitzer value, the electron and proton temperatures will not have equilibrated along the SW boundary of the radio lobe where the shock is strongest. Thus the proton temperature of the shocked gas could be considerably larger than the observed electron temperature, and the total energy of the outburst correspondingly larger as well. We investigate this using a simple one-dimensional shock model for a two-fluid (proton/electron) plasma. We find that for the thermodynamic parameters of the Cen A shock the electron temperature rises rapidly from $\sim$0.29 keV (the temperature of the ambient ISM) to $\sim$3.5 keV at which point heating from the protons is balanced by adiabatic losses. The proton and electron temperatures do not equilibrate in a timescale less than the age of the lobe. We note that the measured electron temperature of similar features in other nearby powerful radio galaxies in poor environments may considerably underestimate the strength and velocity of the shock.

astro-ph↗

arXiv2007

The Disturbed 17 keV Cluster Associated with the Radio Galaxy 3C 438

We present results from a {\em Chandra} observation of the cluster gas associated with the FR II radio galaxy 3C 438. This radio galaxy is embedded within a massive cluster with gas temperature $\sim$17 keV and bolometric luminosity of 6$\times10^{45}$ ergs s$^{-1}$. It is unclear if this high temperature represents the gravitational mass of the cluster, or if this is an already high ($\sim$ 11 keV) temperature cluster that has been heated transiently. We detect a surface brightness discontinuity in the gas that extends $\sim$600 kpc through the cluster. The radio galaxy 3C 438 is too small ($\sim$110 kpc across) and too weak to have created this large disturbance in the gas. The discontinuity must be the result of either an extremely powerful nuclear outburst or the major merger of two massive clusters. If the observed features are the result of a nuclear outburst, it must be from an earlier epoch of unusually energetic nuclear activity. However, the energy required ($\sim10^{63}$ ergs) to move the gas on the observed spatial scales strongly supports the merger hypothesis. In either scenario, this is one of the most extreme events in the local Universe.

astro-ph↗

arXiv2007

X-ray Constraints on Galaxy-Gas-Jet Interactions in the Dumbbell Galaxies NGC4782 and NGC4783 in the LGG316 Group

We present results from a 49.3 ks Chandra X-ray observation of the strongly interacting dumbbell galaxies NGC4782(3C278) and NGC4783 that constrain the kinematics of the interaction and models for bending the radio jets associated with NGC4782. The galaxies are embedded in an approximately spherical distribution of group gas, centered on NGC4782,that is nearly isothermal with mean kT ~ 1.4+/- 0.4 keV. The X-ray morphology suggests that NGC4783 is infalling into a single, massive galaxy group (LGG316) with NGC4782 nearly at rest at the center of the group potential.NGC4783 shows a sharp X-ray surface brightness edge (cold front) to the east and a ~15 kpc ram-pressure-stripped tail to the west. Analysis of this cold front indicates NGC4783 is moving east with a total velocity ~870+270-400 km/s (Mach ~1.4+0.5-0.7) at an inclination angle ~46 deg.(>33 deg.)towards us with respect to theplane of the sky. A ~45Myr old X-ray cavity, with enthalpy of 4.4 x 10^{57}ergs, coincides with the eastern radio lobe of 3C278. X-ray knots are found on both the radio jet and counter-jet, coincident with peaks in the radio emission. Assuming a light, mildly relativistic jet in 3C278, ram pressure velocities of 100-200 km/s impacting the eastern jet and ~170 km/s acting on the western radio lobe are sufficient to produce their observed bending. These velocities may be caused by bulk motions established in the IGM by the high velocity interaction between the galaxies, by the motion of NGC4782 relative to the IGM, or both.

astro-ph↗

arXiv2007

The interaction between radio lobes and hot gas in the nearby radio galaxies 3C285 and 3C442A

We present Chandra observations of two nearby radio galaxies in group environments, 3C285 and 3C442A. The host galaxies of both sources are involved in mergers with nearby massive galaxies, and the hot gas in the systems is extended along lines joining the interacting galaxies. Both sources show strong evidence for interactions between the radio lobes and the asymmetrical hot gas. We argue that the structure in the hot gas is independent of the existence of the radio lobes in these systems, and argue that hot gas shaped by an ongoing massive galaxy merger may play an important role in the dynamics of radio lobes in other objects. For 3C442A, our observations show that gas is being driven out of both members of the host interacting galaxy pair, and the implied constraints on galaxy velocities are consistent with mildly supersonic motions with respect to the group-scale hot gas. The previously known filamentary radio structure in the center of 3C442A may be a result of the interaction between hot gas expelled from these galaxies and pre-existing radio-emitting plasma. In 3C285, where there is no ongoing galaxy merger, the powerful radio source is probably having a significant effect on the energetics of the host group.

astro-ph↗

arXiv2007

The effect of a Chandra-measured merger-related gas component on the lobes of a dead radio galaxy

We use Chandra data to infer that an X-ray bright component of gas is in the process of separating the radio lobes of 3C 442A. This is the first radio galaxy with convincing evidence that central gas, overpressured with respect to the lobe plasma and not simply a static atmosphere, is having a major dynamical effect on the radio structure. We speculate that the expansion of the gas also re-excites electrons in the lobes of 3C 442A through compression and adiabatic heating. Two features of 3C 442A contribute to its dynamical state. Firstly, the radio source is no longer being powered by a detected active jet, so that the dynamical state of the radio plasma is at the mercy of the ambient medium. Secondly the two early-type galaxies, NGC 7236 and NGC 7237, one of which was the original host of 3C 442A, are undergoing a merger and have already experienced a close encounter, suggesting that the X-ray bright gas is mostly the heated combined galaxy atmospheres. The lobes have been swept apart for about 10^8 yrs by the pressure-driven expansion of the X-ray bright inner gas.

astro-ph↗

arXiv2007

Shock heating in the nearby radio galaxy NGC 3801

We report the Chandra detection of shock-heated shells of hot gas surrounding the radio lobes of the nearby (D_L ~ 53 Mpc) low-power radio galaxy NGC 3801. The shells have temperatures of 1 keV and 0.7 keV, compared to an ISM temperature of 0.23 keV. The estimated expansion speed of the shells is 850 km/s, corresponding to a Mach number of ~ 4. This is the second X-ray detection of strong shocks produced by a low-power radio galaxy, and allows us to measure directly the contribution of shock heating to the radio galaxy's total energetic input to the ISM. We show that the gas properties of the shells and surrounding ISM are consistent with the Rankine-Hugoniot shock jump conditions. We estimate the energy stored in the hot gas shells (thermal + kinetic energy) to be 1.7x10^56 ergs, which is equivalent to the thermal energy of the ISM within ~11 kpc of the galaxy centre, and a factor of ~25 larger than the inferred PdV work required to inflate the lobe cavities, indicating that energy transfer from the AGN to its environment is dominated by shock heating during this stage of radio-source evolution. Our results provide direct evidence that shock heating in the early supersonic phase of FRI radio-source expansion can have important long-term effects on the properties of the host galaxy ISM. Finally, we discuss the merger history of NGC 3801, the fuelling of its AGN and the role of this type of system in feedback models.

astro-ph↗

arXiv2007

A Radio through X-ray Study of the Hot Spots, Active Nucleus, and Environment of the Nearby FR II Radio Galaxy 3C 33

We present results from {\em Chandra}/ACIS-S, {\em Spitzer}, {\em XMM-Newton}, {\em HST}, and VLA observations of the radio hot spots, extended environment, and nucleus of the nearby ($z$=0.0597) FR II radio galaxy 3C 33. This is a relatively low-power FR II radio galaxy, and so we expect, {\it a priori}, to detect a significant X-ray synchrotron component to the emission from the hot spots. We detect X-ray emission coincident with the two knots of polarized optical emission from the southern hot spot (SHS), as well as along the northwest arm of this hot spot. We also detect X-ray emission from two compact regions of the northern hot spot (NHS), as well as diffuse emission behind the radio peak. The X-ray flux density of the region at the tip of the southern hot spot, the most compact radio feature of the southern lobe, is consistent with the synchrotron self-Compton (SSC) process. The X-ray flux densities of the other three regions of the SHS and the two compact regions of the NHS are an order of magnitude or more above the predictions from either the SSC and inverse-Compton scattering of the CMB (IC/CMB) mechanisms, thus strongly disfavoring these scenarios unless they are far from equipartition ($B$ $\sim$4-14 times smaller than the equipartition values). The X-ray flux from the diffuse region behind the NHS is consistent with the IC/CMB prediction assuming a small departure from equipartition. We conclude that the X-ray emission is synchrotron emission from multiple populations of ultrarelativistic electrons unless these regions are far from equipartition. There must therefore be unresolved substructure within each knot, similar to that which is found in Chandra observations of nearby FR I jets such as Centaurus A and 3C 66B.

astro-ph↗