Ralph Porter Kraft

We present a 13 yr Chandra High Resolution Camera (HRC-I) study of the M87 jet, using point-spread function deconvolution to achieve unprecedented subarcsecond X-ray resolution of its kiloparsec-scale structure and temporal evolution. The deconvolved images reveal previously blended structures within the jet, including two components in HST-1 and complex morphology in downstream knots. Flux measurements reveal a global decrease in X-ray emission across the jet of up to 84%, consistent with synchrotron cooling. Modeling the fading yields minimum magnetic field strengths of ∼324–1006 μ G for HST-1 and ∼41–115 μ G for knot A. Consistent with synchrotron cooling, multiwavelength comparisons with the Atacama Large Millimeter Array, James Webb Space Telescope, and Hubble Space Telescope show that the principal X-ray structures closely match the jet width and knot locations observed at lower energies, while the X-ray emission is generally shifted upstream. Proper-motion measurements show that jet features exhibit both quasistationary and superluminal apparent motions, reaching up to 4.8 c for HST-1, and also demonstrate that unresolved component blending can substantially bias inferred velocities. These results demonstrate the unique capability of high-resolution Chandra X-ray imaging over long temporal baselines to resolve the evolving substructure of relativistic jets and to probe the particle acceleration and energy dissipation processes that shape their dynamics.

We update the X-ray information given in A. Maselli et al. for five bright radio sources in the SMS4 catalog, thanks to additional observations with the X-Ray Telescope on board the Neil Gehrels Swift Observatory (hereafter Swift) carried out through 2026 February. X-ray emission from MRC B1754−597, previously based only on data from the eROSITA-DE DR1 catalogs, is now detected also by Swift, which yields a more precise positional uncertainty. Thanks to ∼7 ks of additional exposure with Swift, we are now able to detect MRC B1817−391, a source for which no X-ray counterpart is found in eROSITA-DE DR1. Another source that is not detected in the eROSITA-DE DR1 catalogs, and that was not observed by Swift earlier than 2025, is MRC B1827−360: with a ∼6 ks X-ray observation, the source is detected. Finally, based on ∼8 ks of additional exposure, we detect X-ray emission for MRC B2032−350, a source that lies out of the DR1 footprint. In contrast, additional exposure (∼2 ks) just gives an upper limit on the X-ray emission of MRC B1814−519. The analysis of the extent and the hardness ratio of the four detected X-ray sources suggests the presence of soft, diffuse X-ray emission as expected from galaxy group cores or hot galaxy coronae. Using the positional uncertainty of the X-ray detections to constrain the search for counterparts at lower energies, we provide a new infrared/optical counterpart for MRC B1817−391 and confirm the counterparts previously reported in the literature for the three remaining sources.

A recent 2023 paper by Massaro et al. introduced the G4Jy-3CRE, a new catalog of the brightest radio sources in the Southern Hemisphere that serve as a southern equivalent to the Third Cambridge Catalog Revised (3CR). The G4Jy-3CRE catalog selected 264 sources from the GLEAM-4Jy survey based on the same criteria used to select the sources in the 3CR. In this paper, we present new Australian Square Kilometre Array Pathfinder (ASKAP) continuum imaging of the G4Jy-3CRE catalog. We use the three most recent data releases from the Rapid ASKAP Continuum Survey (RACS), covering the sky south of +30° decl.: RACS-low1, RACS-mid, and RACS-high. Together, these data releases cover a range of frequencies from 600 to 1800 MHz. The RACS surveys have improved spatial resolution and sensitivity over archival surveys at the same frequency, enabling us to classify 173 sources (66% of the sample) with morphologies indicative of the presence of jets, 37 of which did not show jet activity on archival radio maps. We were able to effectively classify FRI/FRII galaxies up to a redshift of z = 1.35. Moreover, we identified six optical counterparts for sources that were either previously unidentified or unconfirmed.

With rapid improvements in the assembly of large samples of galaxy clusters, we are approaching the ability to study clusters at z ≳ 2. Evolutionary studies comparing these distant clusters to the clusters in our local Universe depend heavily on the reliability of low-redshift cluster samples, most of which are subject to X-ray selection effects, biasing them to relaxed, cool-core clusters. Here, we introduce the Cluster Evolutionary Reference Ensemble at Low- z (CEREAL) sample, composed of Chandra X-ray observations of 169 galaxy clusters that have been selected from the Planck Sunyaev–Zel’dovich catalog. CEREAL has a simple and well-understood selection function, spans an order of magnitude in mass at z ∼ 0.15, and has uniform, high-resolution X-ray follow-up. We present the full sample and provide results based on X-ray surface brightness properties, finding significantly more non-cool-core systems than in X-ray-selected samples. We use surface brightness concentration ( c _SB ) as a proxy for cool-core strength and centroid shift ( w ) to measure dynamical state. Over the full sample, we find a cool-core ( c _SB > 0.075) fraction of 0.3 9 − 0.04 + 0.04 , a strong cool-core ( c _SB > 0.155) fraction of 0.1 3 − 0.03 + 0.03 , and a dynamically relaxed ( w < 0.01) fraction of 0.4 2 − 0.04 + 0.04 . We find no mass dependence in the fraction of clusters that appear relaxed or have cool cores. We quantify the rarity of X-ray-bright central point sources ( L _{nuc, 2−10 keV} > 10 ⁴³ erg s ⁻¹ ), finding them to be intrinsically rare ( 0 . 7 − 0.5 + 1.2 % of massive, low- z clusters) with a notable increase in occurrence rate at the centers of cool cores.

The extended X-ray emission observed in the direction of several molecular clouds in the central molecular zone (CMZ) of our Galaxy exhibits spectral and temporal properties consistent with the X-ray echo scenario. This concept postulates that the observed signal is a light-travel-time delayed reflection of a short ( δt < 1.5 yr) and bright ( L _X > 10 ³⁹ erg s ⁻¹ ) flare that was most probably produced a few hundred years ago by Sgr A*. This scenario also predicts a distinct polarization signature for the reflected X-ray continuum, with the polarization vector being perpendicular to the direction toward the primary source and the polarization degree being determined by the scattering angle. We report the results of two deep observations of the currently brightest (in reflected emission) molecular complex Sgr A taken with the Imaging X-ray Polarimetry Explorer (IXPE) in 2022 and 2023. We confirm the previous polarization measurement for a large region encompassing the Sgr A complex with high significance. We also reveal an inconsistent polarization pattern for the brightest reflection region in its center. Specifically, the X-ray polarization from this region is almost perpendicular to the expected direction in the case of Sgr A* illumination, and it shows a smaller degree of polarization compared to the large region. Taken at face value, this could indicate the simultaneous propagation of several illumination fronts throughout the CMZ, with the origin of one of them not being Sgr A*. The primary source could be associated with the Arches stellar cluster or a currently unknown source located closer to the illuminated cloud, potentially lowering the required luminosity of the primary source. Although significantly deeper observations with IXPE would be required to unequivocally distinguish between the scenarios, a combination of high-resolution imaging and micro-calorimetric spectroscopy offers an additional promising path forward.

We present Chandra X-ray Observatory and Hubble Space Telescope (HST) follow-up observations of 12 dwarf galaxies from A. E. Reines et al. that are potential hosts of radio-selected active galactic nuclei (AGNs), eight of which are nonnuclear and possible “wandering” black holes (BHs). Our multiwavelength analysis indicates a heterogeneous sample with five radio sources detected at both X-ray and optical wavelengths within the positional uncertainties and nondetections for the remaining objects. Of the radio objects detected in the X-ray and optical, three have multiwavelength evidence for hosting nuclear massive BHs and one object is consistent with an extreme compact starburst. Only one of the off-nuclear radio sources has a significant optical counterpart and we present Palomar spectroscopy that identifies this object as a background AGN. We cannot definitively determine if the seven remaining off-nuclear radio sources are wandering massive BHs in the target dwarf galaxies or background AGNs, although the three sources with the largest offsets have compact radio cores detected with the Very Long Baseline Array and are consistent with expectations for background AGNs. Our HST sensitivity limits also allow for wandering massive BHs in the target dwarf galaxies that are hosted by stellar clusters with masses ≲ 10 ⁶ M _⊙ .

The Virgo Cluster is the nearest cool core cluster that features two well-studied sloshing cold fronts at radii of r ≈ 30 kpc and r ≈ 90 kpc, respectively. In this work, we present results of XMM- Newton mosaic observations of a third, southwestern, cold front at a radius of r ≈ 250 kpc, originally discovered with Suzaku . All three cold fronts are likely to be parts of an enormous swirling pattern, rooted in the core. The comparison with a numerical simulation of a binary cluster merger indicates that these cold fronts were produced in the same single event – likely the infall of M49 from the northwest of Virgo and it is now re-entering the cluster from the south. This outermost cold front has probably survived for 2 – 3 Gyr since the disturbance. We identified single sharp edges in the surface brightness profiles of the southern and southwestern sections of the cold front, whereas the western section is better characterized with double edges. This implies that magnetic fields have preserved the leading edge of the cold front, while its western side is beginning to split into two cold fronts likely due to Kelvin-Helmholtz instabilities. The slopes of the 2D power spectrum of the X-ray surface brightness fluctuations, derived for the brighter side of the cold front, are consistent with the expectation from Kolmogorov turbulence. Our findings highlight the role of cold fronts in shaping the thermal dynamics of the intracluster medium beyond the cluster core, which has important implications for cluster cosmology. Next-generation X-ray observatories, such as the proposed AXIS mission, will be ideal for identifying and characterizing ancient cold fronts.

SDSS-C4 3028 is a galaxy cluster at z = 0.061 , notable for its unusually high fraction of star-forming galaxies with 19 star-forming and 11 quiescent spectroscopically-confirmed member galaxies. From Subaru/HSC imaging, we derived a weak lensing mass of , indicating a low-mass cluster. This is in excellent agreement with its dynamical mass of , derived from SDSS spectroscopic data. XMM-Newton observations reveal that its X-ray emission is uniform and fully consistent with the astrophysical X-ray background, with no evidence for an intracluster medium (ICM). The 3 σ upper limit of erg s − 1 on the cluster’s X-ray luminosity falls below the value expected from the scaling relation of nearby galaxy clusters. We derived star-formation histories for its member galaxies using the photometric spectral energy distribution from SDSS, 2MASS, and WISE data. Most of its quiescent galaxies reside within the central 300 kpc, while star-forming ones dominate the outer region (300 kpc – 1 Mpc). The core region has formed the bulk of its stellar mass approximately 1.5 Gyr earlier than the outskirts. We infer a long quenching time of > 3 Gyr for its quiescent galaxies, consistent with slow quenching mechanisms such as galaxy-galaxy interaction or strangulation. These findings suggest that SDSS-C4 3028 may have undergone an “inside-out” formation and quenching process. Its ICM may have been expelled by intense AGN feedback after core formation but before full cluster assembly. The high fraction ( ∼ 0.63 ) of star-forming members likely results from the absence of ram pressure stripping in this blue cluster, supporting the important role of ram pressure stripping in quenching galaxies in clusters.