arXiv subjects

R. Kraft

Publications and source records attributed to R. Kraft.

At least 37 records · Page 2Linked to original sources

arXiv2019

Andreev reflection in ballistic normal metal/graphene/superconductor junctions

We report the study of ballistic transport in normal metal/graphene/superconductor junctions in edge-contact geometry. While in the normal state, we have observed Fabry-Pérot resonances suggesting that charge carriers travel ballistically, the superconducting state shows that the Andreev reflection at the graphene/superconductor interface is affected by these interferences. Our experimental results in the superconducting state have been analyzed and explained with a modified Octavio-Tinkham-Blonder-Klapwijk model taking into account the magnetic pair-breaking effects and the two different interface transparencies, \textit{i.e.}\,between the normal metal and graphene, and between graphene and the superconductor. We show that the transparency of the normal metal/graphene interface strongly varies with doping at large scale, while it undergoes weaker changes at the graphene/superconductor interface. When a cavity is formed by the charge transfer occurring in the vicinity of the contacts, we see that the transmission probabilities follow the normal state conductance highlighting the interplay between the Andreev processes and the electronic interferometer.

cond-mat.mes-hall↗

arXiv2019

Cluster Cosmology Constraints from the 2500 deg$^2$ SPT-SZ Survey: Inclusion of Weak Gravitational Lensing Data from Magellan and the Hubble Space Telescope

We derive cosmological constraints using a galaxy cluster sample selected from the 2500~deg$^2$ SPT-SZ survey. The sample spans the redshift range $0.25< z<1.75$ and contains 343 clusters with SZ detection significance $ξ>5$. The sample is supplemented with optical weak gravitational lensing measurements of 32 clusters with $0.29<z<1.13$ (from Magellan and HST) and X-ray measurements of 89 clusters with $0.25<z<1.75$ (from Chandra). We rely on minimal modeling assumptions: i) weak lensing provides an accurate means of measuring halo masses, ii) the mean SZ and X-ray observables are related to the true halo mass through power-law relations in mass and dimensionless Hubble parameter $E(z)$ with a-priori unknown parameters, iii) there is (correlated, lognormal) intrinsic scatter and measurement noise relating these observables to their mean relations. We simultaneously fit for these astrophysical modeling parameters and for cosmology. Assuming a flat $νΛ$CDM model, in which the sum of neutrino masses is a free parameter, we measure $Ω_\mathrm{m}=0.276\pm0.047$, $σ_8=0.781\pm0.037$, and $σ_8(Ω_\mathrm{m}/0.3)^{0.2}=0.766\pm0.025$. The redshift evolution of the X-ray $Y_\mathrm{X}$-mass and $M_\mathrm{gas}$-mass relations are both consistent with self-similar evolution to within $1σ$. The mass-slope of the $Y_\mathrm{X}$-mass relation shows a $2.3σ$ deviation from self-similarity. Similarly, the mass-slope of the $M_\mathrm{gas}$-mass relation is steeper than self-similarity at the $2.5σ$ level. In a $νw$CDM cosmology, we measure the dark energy equation of state parameter $w=-1.55\pm0.41$ from the cluster data. We perform a measurement of the growth of structure since redshift $z\sim1.7$ and find no evidence for tension with the prediction from General Relativity. We provide updated redshift and mass estimates for the SPT sample. (abridged)

astro-ph.CO↗

arXiv2019

Astro2020 Science White Paper: A Unification of the Micro and Macro Physics in the Intracluster Medium of Nearby Clusters

Clusters of galaxies are the most massive virialized structures in the Universe. The microphysical properties of the intracluster plasma can affect dynamical processes over an enormous range: from the feedback of active galactic nuclei to shock acceleration in merging clusters. All the major cosmological simulations assume the astrophysical plasma to be inviscid. It is critical to measure microphysical properties of the intracluster plasma to truly understand the physical processes that drive the cosmic evolution. Tremendous progress has been made by comparing high spatial resolution X-ray images to (magneto-)hydrodynamic simulations. Future X-ray missions with calorimeters promise a direct measurement of transport coefficients and gas motions, providing a more realistic benchmark for cosmological simulations.

astro-ph.HE↗

arXiv2019

Unveiling the Galaxy Cluster - Cosmic Web Connection with X-ray observations in the Next Decade

In recent years, the outskirts of galaxy clusters have emerged as one of the new frontiers and unique laboratories for studying the growth of large scale structure in the universe. Modern cosmological hydrodynamical simulations make firm and testable predictions of the thermodynamic and chemical evolution of the X-ray emitting intracluster medium. However, recent X-ray and Sunyaev-Zeldovich effect observations have revealed enigmatic disagreements with theoretical predictions, which have motivated deeper investigations of a plethora of astrophysical processes operating in the virialization region in the cluster outskirts. Much of the physics of cluster outskirts is fundamentally different from that of cluster cores, which has been the main focus of X-ray cluster science over the past several decades. A next-generation X-ray telescope, equipped with sub-arcsecond spatial resolution over a large field of view along with a low and stable instrumental background, is required in order to reveal the full story of the growth of galaxy clusters and the cosmic web and their applications for cosmology.

astro-ph.CO↗

arXiv2018

Valley subband splitting in bilayer graphene quantum point contact

We report a study of one-dimensional subband splitting in a bilayer graphene quantum point contact in which quantized conductance in steps of $4\,e^2/h$ is clearly defined down to the lowest subband. While our source-drain bias spectroscopy measurements reveal an unconventional confinement, we observe a full lifting of the valley degeneracy at high magnetic fields perpendicular to the bilayer graphene plane for the first two lowest subbands where confinement and Coulomb interactions are the strongest and a peculiar merging/mixing of $K$ and $K'$ valleys from two non-adjacent subbands with indices $(N,N+2)$ which are well described by our semi-phenomenological model.

cond-mat.mes-hall↗

arXiv2018

Baryon Content in a Sample of 91 Galaxy Clusters Selected by the South Pole Telescope at 0.2 < z < 1.25

We estimate total mass ($M_{500}$), intracluster medium (ICM) mass ($M_{\mathrm{ICM}}$) and stellar mass ($M_{\star}$) in a Sunyaev-Zel'dovich effect (SZE) selected sample of 91 galaxy clusters with masses $M_{500}\gtrsim2.5\times10^{14}M_{\odot}$ and redshift $0.2 < z < 1.25$ from the 2500 deg$^2$ South Pole Telescope SPT-SZ survey. The total masses $M_{500}$ are estimated from the SZE observable, the ICM masses $M_{\mathrm{ICM}}$ are obtained from the analysis of $Chandra$ X-ray observations, and the stellar masses $M_{\star}$ are derived by fitting spectral energy distribution templates to Dark Energy Survey (DES) $griz$ optical photometry and $WISE$ or $Spitzer$ near-infrared photometry. We study trends in the stellar mass, the ICM mass, the total baryonic mass and the cold baryonic fraction with cluster mass and redshift. We find significant departures from self-similarity in the mass scaling for all quantities, while the redshift trends are all statistically consistent with zero, indicating that the baryon content of clusters at fixed mass has changed remarkably little over the past $\approx9$ Gyr. We compare our results to the mean baryon fraction (and the stellar mass fraction) in the field, finding that these values lie above (below) those in cluster virial regions in all but the most massive clusters at low redshift. Using a simple model of the matter assembly of clusters from infalling groups with lower masses and from infalling material from the low density environment or field surrounding the parent halos, we show that the measured mass trends without strong redshift trends in the stellar mass scaling relation could be explained by a mass and redshift dependent fractional contribution from field material. Similar analyses of the ICM and baryon mass scaling relations provide evidence for the so-called "missing baryons" outside cluster virial regions.

astro-ph.CO↗

arXiv2017

Partitioning the Outburst Energy of a Low Eddington Accretion Rate AGN at the Center of an Elliptical Galaxy: the Recent 12 Myr History of the Supermassive Black Hole in M87

M87, the active galaxy at the center of the Virgo cluster, is ideal for studying the interaction of a supermassive black hole (SMBH) with a hot, gas-rich environment. A deep Chandra observation of M87 exhibits an approximately circular shock front (13 kpc radius, in projection) driven by the expansion of the central cavity (filled by the SMBH with relativistic radio-emitting plasma) with projected radius $\sim$1.9 kpc. We combine constraints from X-ray and radio observations of M87 with a shock model to derive the properties of the outburst that created the 13 kpc shock. Principal constraints for the model are 1) the measured Mach number ($M$$\sim$1.2), 2) the radius of the 13 kpc shock, and 3) the observed size of the central cavity/bubble (the radio-bright cocoon) that serves as the piston to drive the shock. We find an outburst of $\sim$5$\times$$10^{57}$ ergs that began about 12 Myr ago and lasted $\sim$2 Myr matches all the constraints. In this model, $\sim$22% of the energy is carried by the shock as it expands. The remaining $\sim$80% of the outburst energy is available to heat the core gas. More than half the total outburst energy initially goes into the enthalpy of the central bubble, the radio cocoon. As the buoyant bubble rises, much of its energy is transferred to the ambient thermal gas. For an outburst repetition rate of about 12 Myrs (the age of the outburst), 80% of the outburst energy is sufficient to balance the radiative cooling.

astro-ph.GA↗

arXiv2016

Frontier Fields Clusters: Deep Chandra Observations of the Complex Merger MACS J1149.6+2223

The HST Frontier Fields cluster MACS J1149.6+2223 is one of the most complex merging clusters, believed to consist of four dark matter halos. We present results from deep (365 ks) Chandra observations of the cluster, which reveal the most distant cold front (z=0.544) discovered to date. In the cluster outskirts, we also detect hints of a surface brightness edge that could be the bow shock preceding the cold front. The substructure analysis of the cluster identified several components with large relative radial velocities, thus indicating that at least some collisions occur almost along the line of sight. The inclination of the mergers with respect to the plane of the sky poses significant observational challenges at X-ray wavelengths. MACS J1149.6+2223 possibly hosts a steep-spectrum radio halo. If the steepness of the radio halo is confirmed, then the radio spectrum, combined with the relatively regular ICM morphology, could indicate that MACS J1149.6+2223 is an old merging cluster.

astro-ph.CO↗

arXiv2015

Frontier Fields Clusters: Chandra and JVLA View of the Pre-Merging Cluster MACS J0416.1-2403

Merging galaxy clusters leave long-lasting signatures on the baryonic and non-baryonic cluster constituents, including shock fronts, cold fronts, X-ray substructure, radio halos, and offsets between the dark matter and the gas components. Using observations from Chandra, the Jansky Very Large Array, the Giant Metrewave Radio Telescope, and the Hubble Space Telescope, we present a multiwavelength analysis of the merging Frontier Fields cluster MACS J0416.1-2403 (z=0.396), which consists of a NE and a SW subclusters whose cores are separated on the sky by ~250 kpc. We find that the NE subcluster has a compact core and hosts an X-ray cavity, yet it is not a cool core. Approximately 450 kpc south-south west of the SW subcluster, we detect a density discontinuity that corresponds to a compression factor of ~1.5. The discontinuity was most likely caused by the interaction of the SW subcluster with a less massive structure detected in the lensing maps SW of the subcluster's center. For both the NE and the SW subclusters, the dark matter and the gas components are well-aligned, suggesting that MACS J0416.1-2403 is a pre-merging system. The cluster also hosts a radio halo, which is unusual for a pre-merging system. The halo has a 1.4 GHz power of (1.06 +/- 0.09) x 10^{24} W Hz^{-1}, which is somewhat lower than expected based on the X-ray luminosity of the cluster. We suggest that we are either witnessing the birth of a radio halo, or have discovered a rare ultra-steep spectrum halo.

astro-ph.CO↗

arXiv2015

A Very Deep Chandra Observation of the Galaxy Group NGC 5813: AGN Shocks, Feedback, and Outburst History

We present results from a very deep (650 ks) Chandra X-ray observation of the galaxy group NGC~5813, the deepest Chandra observation of a galaxy group to date. Earlier observations showed two pairs of cavities distributed roughly collinearly, with each pair associated with an elliptical shock front. The new observations confirm a third pair of outer cavities, collinear with the other pairs, and reveal an associated outer outburst shock at ~30 kpc. This system is therefore unique in exhibiting three cavity pairs, each associated with an unambiguous AGN outburst shock front. The implied mean kinetic power is roughly the same for each outburst, demonstrating that the average AGN kinetic luminosity can remain stable over long timescales (~50 Myr). The two older outbursts have larger, roughly equal total energies as compared with the youngest outburst, implying that the youngest outburst is ongoing. We find that the radiative cooling rate and the mean shock heating rate of the gas are well balanced at each shock front, suggesting that AGN outburst shock heating alone is sufficient to offset cooling and establish AGN/ICM feedback within at least the central 30 kpc. This heating takes place roughly isotropically and most strongly at small radii, as is required for feedback to operate. We suggest that shock heating may play a significant role in AGN feedback at smaller radii in other systems, where weak shocks are more difficult to detect. We find non-zero shock front widths that are too large to be explained by particle diffusion. Instead, all measured widths are consistent with shock broadening due to propagation through a turbulent ICM with a mean turbulent speed of ~70 km/s. Finally, we place lower limits on the temperature of any volume-filling thermal gas within the cavities that would balance the internal cavity pressure with the external ICM.

astro-ph.HE↗

arXiv2012

The Chandra X-ray point source catalog in the DEEP2 Galaxy Redshift Survey fields

We present the X-ray point-source catalog produced from the Chandra Advanced CCD Imaging Spectrometer (ACIS-I) observations of the combined \sim3.2 deg2 DEEP2 (XDEEP2) survey fields, which consist of four ~0.7-1.1 deg2 fields. The combined total exposures across all four XDEEP2 fields range from ~10ks-1.1Ms. We detect X-ray point-sources in both the individual ACIS-I observations and the overlapping regions in the merged (stacked) images. We find a total of 2976 unique X-ray sources within the survey area with an expected false-source contamination of ~30 sources (~1%). We present the combined logN-logS distribution of sources detected across the XDEEP2 survey fields and find good agreement with the Extended Chandra Deep Field and Chandra-COSMOS fields to f_{X,0.5-2keV}\sim2x10^{-16} erg/cm^2/s. Given the large survey area of XDEEP2, we additionally place relatively strong constraints on the logN-logS distribution at high fluxes (f_{X,0.5-2keV}\sim3x10^{-14} erg/cm^2/s), and find a small systematic offset (a factor ~1.5) towards lower source numbers in this regime, when compared to smaller area surveys. The number counts observed in XDEEP2 are in close agreement with those predicted by X-ray background synthesis models. Additionally, we present a Bayesian-style method for associating the X-ray sources with optical photometric counterparts in the DEEP2 catalog (complete to R_AB < 25.2) and find that 2126 (~71.4\pm2.8%) of the 2976 X-ray sources presented here have a secure optical counterpart with a <6% contamination fraction. We provide the DEEP2 optical source properties (e.g., magnitude, redshift) as part of the X-ray-optical counterpart catalog.

astro-ph.HE↗

arXiv2011

ICM Signatures from Multiple Outbursts in the Galaxy Group NGC 5813: A Window to AGN Feedback

We present results from Chandra observations of NGC 5813, the dominant central galaxy in a nearby galaxy group. We focus on three main results. 1) The diffuse gas shows clear signatures from three distinct outbursts of the central AGN, with three pairs of roughly collinear cavities. The inner two cavity pairs are associated with unambiguous elliptical shock fronts, with Mach numbers M~1.7 and M~1.5 for the inner and outer shocks, respectively. 2) The mean power of the two most recent outbursts differs by a factor of six, indicating that the mean jet power varies over long (~10^7 yr) time scales. 3) The heating from the shocks alone is sufficient to balance radiative cooling of the gas within at least the central 10 kpc, allowing feedback to operate isotropically at small radii.

astro-ph.CO↗

arXiv2010

Shocks and Cavities from Multiple Outbursts in the Galaxy Group NGC 5813: A Window to AGN 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 AGN, which occurred 3x10^6, 2x10^7, and 9x10^7 yr ago. The H-alpha 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--10x10^42 erg/s, indicating that the mean jet power changes significantly over long (~10^7 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 (1.5x10^56 erg versus 4x10^57 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 (~10^7 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 ICM and the central SMBH.

astro-ph.CO↗

arXiv2009

The unusual X-ray morphology of NGC4636 revealed by deep Chandra observations: cavities and shocks created by past AGN outbursts

We present Chandra ACIS-I and ACIS-S observations (~200ks in total) of the X-ray luminous elliptical galaxy NGC4636, located in the outskirts of the Virgo cluster. A soft band (0.5-2 keV) image shows the presence of a bright core in the center surrounded by an extended X-ray corona and two pronounced quasi-symmetric, 8 kpc long, arm-like features. Each of this features defines the rim of an ellipsoidal bubble. An additional bubble-like feature, whose northern rim is located ~2 kpc south of the north-eastern arm, is detected as well. We present surface brightness and temperature profiles across the rims of the bubbles, showing that their edges are sharp and characterized by temperature jumps of about 20-25%. Through a comparison of the observed profiles with theoretical shock models, we demonstrate that a scenario where the bubbles were produced by shocks, probably driven by energy deposited off-center by jets, is the most viable explanation to the X-ray morphology observed in the central part of NGC4636. As a confirmation to this scenario, radio jets extending towards the bubbles and a central weak X-ray and radio source are detected and are most likely the signs of AGN activity which was more intense in the past. A bright dense core of ~1 kpc radius is observed at the center of NGC4636. A sharp decline in surface brightness from the core to the ambient gas is observed and is not accompanied by a variation in the temperature and thus could not be in thermal pressure equilibrium. However the bright core could be a long lived feature if the radio jets are acting as a balancing factor to thermal pressure or if the bright core is produced by steep abundance gradients.

astro-ph.CO↗

arXiv2009

The unusual X-ray morphology of NGC4636 revealed by deep Chandra observations: cavities and shocks created by past AGN outbursts

We present Chandra ACIS-I and ACIS-S observations ($\sim$200 ks in total) of the X-ray luminous elliptical galaxy NGC 4636, located in the outskirts of the Virgo cluster. A soft band (0.5-2 keV) image shows the presence of a bright core in the center surrounded by an extended X-ray corona and two pronounced quasi-symmetric, 8 kpc long, arm-like features. Each of this features defines the rimof an ellipsoidal bubble. An additional bubble-like feature, whose northern rim is located $\sim2$ kpc south of the north-eastern arm, is detected as well. We present surface brightness and temperature profiles across the rims of the bubbles, showing that their edges are sharp and characterized by temperature jumps of about 20-25%. Through a comparison of the observed profiles with theoretical shock models, we demonstrate that a scenario where the bubbles were produced by shocks, probably driven by energy deposited off-center by jets, is the most viable explanation to the X-ray morphology observed in the central part of NGC 4636.

astro-ph.CO↗

arXiv2009

Merging Cold Fronts in the Galaxy Pair NGC 7619 and NGC 7626

We present results from {\it Chandra} observations of the galaxy pair NGC 7619 and NGC 7626, the two dominant members of the Pegasus group. The X-ray images show a brightness edge associated with each galaxy, which we identify as merger cold fronts. The edges are sharp, and the axes of symmetry of the edges are roughly anti-parallel, suggesting that these galaxies are falling towards one another in the plane of the sky. The detection of merger cold fronts implies a merging subgroup scenario, since the alternative is that the galaxies are falling into a pre-existing $\sim1$ keV halo without a dominant galaxy of its own, and such objects are not observed. We estimate the 3D velocities from the cold fronts and show that the velocity vectors are indeed most likely close to the plane of the sky, with a relative velocity of $\sim1190\kms$. The relative velocity is consistent with what is expected from the infall of two roughly equal mass subgroups whose total viral mass equals that of the Pegasus group. We conclude that the Pegasus cluster is currently forming from a major merger of two subgroups, dominated by NGC 7619 and NGC 7626. NGC 7626 contains a strong radio source, a core with two symmetric jets and radio lobes. Although we find no associated structure in the X-ray surface brightness map, the temperature map reveals a clump of cool gas just outside the southern lobe, presumably entrained by the lobe, and an extension of cooler gas into the lobe itself. The jet axis is parallel with the projected direction of motion of NGC 7626 (inferred from the symmetry axis of the merger cold front), and the southern leading jet is foreshortened as compared to the northern trailing one, possibly due to the additional ram pressure the forward jet encounters.

astro-ph↗

arXiv2008

Chandra observations of the galaxy group AWM 5: cool core re-heating and thermal conduction suppression

We present an analysis of a 40 ksec Chandra observation of the galaxy group AWM 5. It has a small ($\sim8$ kpc) dense cool core with a temperature of $\sim1.2$ keV and the temperature profile decreases at larger radii, from $\sim3.5$ keV just outside the core to $\sim2$ keV at $\sim300$ kpc from the center. The abundance distribution shows a "hole" in the central $\sim10$ kpc, where the temperature declines sharply. An abundance of at least a few times solar is observed $\sim15-20$ kpc from the center. The deprojected electron density profile shows a break in slope at $\sim13$ kpc and can be fit by two $β$-models, with $β=0.72_{-0.11}^{+0.16}$ and $r_c=5.7_{-1.5}^{+1.8}$ kpc, for the inner part, and $β=0.34\pm0.01$ and $r_c=31.3_{-5.5}^{+5.8}$ kpc, for the outer part. The mass fraction of hot gas is fairly flat in the center and increases for $r>30$ kpc up to a maximum of $\sim6.5%$ at $r\sim380$ kpc. The gas cooling time within the central 30 kpc is smaller than a Hubble time, although the temperature only declines in the central $\sim8$ kpc region. This discrepancy suggests that an existing cooling core has been partially re-heated. In particular, thermal conduction could have been a significant source of re-heating. In order for heating due to conduction to balance cooling due to emission of X-rays, the conductivity must be suppressed by a large factor (at least $\sim100$). Past AGN activity (still visible as a radio source in the center of the group) is however the most likely source that re-heated the central regions of AWM 5.

astro-ph↗

arXiv2007

Filaments, Bubbles, and Weak Shocks in the Gaseous Atmosphere of M87

We present the first results from a 500 ksec Chandra ACIS-I observation of M87. At soft energies (0.5-1.0 keV), we detect filamentary structures associated with the eastern and southwestern X-ray and radio arms. Many filaments are spatially resolved with widths of ~300 pc. This filamentary structure is particularly striking in the eastern arm where we suggest the filaments are outer edges of a series of plasma-filled, buoyant bubbles whose ages differ by ~6 x 10^6 years. These X-ray structures may be influenced by magnetic filamentation. At hard energies (3.5-7.5 keV), we detect a nearly circular ring of outer radius 2.8' (13 kpc) which provides an unambiguous signature of a weak shock, driven by an outburst from the SMBH. The density rise in the shock is ~1.3 (Mach number, M~1.2). The observed spectral hardening in the ring corresponds to a temperature rise T_shock / T_0 ~ 1.2, or M~1.2, in agreement with the Mach number derived independently from the gas density. Thus, for the first time, we detect gas temperature and density jumps associated with a classical shock in the atmosphere around a supermassive black hole. We also detect two additional surface brightness edges and pressure enhancements at radii of ~0.6' and ~1'. The ~0.6' feature may be over-pressurized thermal gas surrounding the relativistic plasma in the radio cocoon, the ``piston'', produced by the current episode of AGN activity. The over-pressurized gas is surrounded by a cool gas shell. The ~1' feature may be an additional weak shock from a secondary outburst. In an earlier episode, the ``piston'' was responsible for driving the 2.8' shock.

astro-ph↗