Joshua C. Kempner

Radio Halo and Relic Candidates from the Westerbork Northern Sky Survey

We have undertaken a systematic search for diffuse radio halos and relics in all of the Abell clusters that are visible in the Westerbork Northern Sky Survey. In this survey we found 18 candidates, 11 of which are already known from the literature and seven for which we provide the first evidence of diffuse radio emission. All the clusters in this sample show other evidence for a recent or ongoing merger. We also investigate the correlation between cluster X-ray luminosity and radio power of halos. We develop a very simple model for merger shocks that reproduce the sense of this correlation, although it is probably not as steep as the correlation in the data. We discuss the implications of X-ray-radio correlations for future detections of radio halos.

NONTHERMAL BREMSSTRAHLUNG AND HARD X-RAY EMISSION FROM CLUSTERS OF GALAXIES

We have calculated nonthermal bremsstrahlung (NTB) models for the hard X-ray (HXR) tails recently observed by BeppoSAX in clusters of galaxies. In these models, the HXR emission is caused by supra- thermal electrons with energies of D10¨200 keV. We consider models in which these transrelativistic suprathermal particles are the low energy end of a population of electrons being accelerated to high energies by shocks or turbulence (ii accelerating electron ˇˇ models). We also consider a model in which these electrons are the remnant of an older nonthermal population that is losing energy and rejoining the thermal distribution as a result of Coulomb interactions (ii cooling electron ˇˇ models). The supra- thermal populations are assumed to start at an electron kinetic energy 3kT , where T is the temperature of the thermal intracluster medium (ICM). The nonthermal bremsstrahlung spectra —atten at low photon energies because of the lack of low-energy nonthermal particles. The accelerating electron models have HXR spectra that are nearly power laws from D20¨100 keV. However, the spectra are brighter and —atter than given by the nonrelativistic bremsstrahlung cross section because of transrelativistic eUects. The HXR spectrum of the cooling electron model is very —at, and most of the X-ray emission in the HXR energy range (10¨100 keV) actually arises from electrons with much higher energies (D100 MeV). Under the assumption that the suprathermal electrons form part of a continuous spectrum of electrons including highly relativistic particles, we have calculated the inverse Compton (IC) extreme-ultraviolet (EUV), HXR, and radio synchrotron emission by the extensions of the same populations. For acceler- ating electron models with power-law momentum spectra (N(p) P p~k) with which are those k ( 2.7, expected from strong shock acceleration, the IC HXR emission exceeds that caused by NTB. Thus, these models are of interest only if the electron population is cut oU at some upper energy GeV. Similarly, (1 —at-spectrum accelerating electron models produce more radio synchrotron emission than is observed from clusters if the ICM magnetic —eld is kG. The cooling electron model produces vastly too B Z 1 much EUV emission as compared to the observations of clusters. We have compared these NTB models to the observed HXR tails in Coma and Abell 2199. The NTB models require a nonthermal electron population that contains about 3% of the number of electrons in the thermal ICM. If the suprathermal electron population is cut oU at some energy above 100 keV, then the models can —t the observed HXR —uxes and spectral indices in both clusters easily. For accelerating electron models without a cutoU, the electron spectrum must be rather steep, to avoid producing too much IC HXR emission. The Z2.9, model HXR spectra are then rather steep but are marginally consistent with observations of the HXR spectrum in Abell 2199 and Coma or the radio spectrum in Coma. These models can account for the HXR and radio properties of these two clusters but do not produce enough EUV emission. Subject headings: acceleration of particlescosmic raysgalaxies: clusters: general ¨ intergalactic mediumradiation mechanisms: nonthermalX-rays: general

Five Years of Pulsar Flux Density Monitoring: Refractive Scintillation and the Interstellar Medium

We have monitored the radio flux density of 21 pulsars on a daily basis for five years. The 610 MHz flux density time series for these pulsars range from nearly constant for the most distant and heavily scattered pulsars to rapidly varying, saturated time series for more nearby pulsars. The measured stability of the flux density from the most distant pulsars (variations less than 5%) implies that the average radio emission from pulsars, before it has been affected by propagation through the interstellar medium, is constant in strength on timescales of a few hours to several years. The modulation index of the flux density variations never exceeds 0.5, ruling out a density inhomogeneity spectrum with a steep power-law exponent (β > 4). The flux density variations for 15 of the pulsars are consistent with a Kolmogorov turbulence spectrum over a range of more than 5 orders of magnitude in scattering strength, with no detectable presence of an inner scale. For these lines of sight we constrain the inhomogeneity slope to be in the range 3.5 ≤ β ≤ 3.7, which brackets the Kolmogorov value of β = 3.67. The flux density variations are greater than predicted by this model for six pulsars—including the Crab and Vela—but this group is consistent with a Kolmogorov spectrum and an inner scale of ≈1010 cm. The lines of sight to three of the other pulsars in this group pass through H II regions around young, hot stars. For six pulsars we have found a change in the slope of the intensity structure function, which could be connected with a change in the slope of the inhomogeneity power spectrum at a scale of ≈1013 cm.

A Chandra view of the multiple merger in Abell 2744

We present a Chandra observation of the merging cluster of galaxies Abell 2744. The cluster shows strong evidence for an ongoing major merger which we believe to be responsible for the radio halo. X-ray emission and temperature maps of the cluster, combined with the spatial and redshift distribution of the galaxies, indicate a roughly north-south axis for the merger, with a significant velocity component along the line of sight. The merger is occurring at a very large velocity, with M = 2-3. We estimate the mass ratio of the merging subclusters to be of the order of unity. They are nearing their closest approach to each other, but with a significant impact parameter. In addition, there is a small merging subcluster toward the northwest, unrelated to the major merger, which shows evidence of a bow shock. A hydrodynamical analysis of the subcluster indicates a merger velocity corresponding to a Mach number of ∼1.2, consistent with a simple infall model. This infalling subcluster may also be re-exciting electrons in the radio halo. Its small Mach number lends support to turbulent reacceleration models for radio halo formation.

AWM 4 – an isothermal cluster observed with XMM–Newton

We present an analysis of an XMM-Newton observation of the poor cluster AWM 4. The cluster is relaxed and its X-ray halo is regular with no apparent substructure. Azimuthally averaged radial spectral profiles suggest that the cluster is isothermal to a radius of at least 160 kpc, with no evidence of a central cooling region. Spectral mapping shows some significant temperature and abundance substructure, but no evidence of strong cooling in the cluster core. Abundance increases in the core, but not to the extent expected, and we find some indication of gas mixing. Modelling the three-dimensional properties of the system, we show that ongoing heating by an active galactic nuclei (AGN) in the dominant elliptical, NGC 6051, is likely to be responsible for the lack of cooling. We also compare AWM 4 to MKW 4, a cluster of similar mass observed recently with XMM-Newton. While the two systems have similar gravitational mass profiles, MKW 4 has a cool core and a somewhat steeper gas density profile, which leads to a lower core entropy. AWM 4 has a considerably larger gas fraction at 0.1R 200 , and we show that these differences result from the difference in mass between the two dominant galaxies and the activity cycles of their AGN. We estimate the energy required to raise the temperature profile of MKW 4 to match that of AWM 4 to be 9 x 10 58 erg or 3 x 10 43 erg s -1 for 100 Myr, comparable to the likely power output of the AGN in AWM 4.

XMM-Newton Observation of the Merging Galaxy Cluster A1644

We present an XMM-Newton imaging spectroscopy analysis of the galaxy cluster A1644. A1644 is a complex merging system consisting of a main cluster and a subcluster. A trail of cool, metal-rich gas has been discovered close to the subcluster. The combination of results from X-ray, optical, and radio data and a comparison to a hydrodynamic simulation suggest that the subcluster has passed by the main cluster off-axis and that a fraction of its gas has been stripped off during this process. Furthermore, for this merging system simple effects are illustrated that can affect the use of clusters as cosmological probes. Specifically, double clusters may affect estimates of the cluster number density when treated as a single system. Mergers, as well as cool cores, can alter the X-ray luminosity and temperature measured for clusters, causing these values to differ from those expected in equilibrium.

Chandra Observation of Abell 2065: An Unequal Mass Merger?

We present an analysis of a 41 ks Chandra ACIS-I observation of the merging cluster Abell 2065. Previous ROSAT and ASCA observations provided evidence for an ongoing merger and suggested that there were two surviving cooling cores, which were associated with the two cD galaxies at the center of the cluster. The Chandra observation reveals only one X-ray surface brightness peak. Its gas is cool and displaced slightly from the more luminous, southern cD galaxy. Combined with the surface brightness profile, this suggests that this cool material has formed a cold front. In the higher spatial resolution Chandra image, the second feature to the north is not associated with the second cD; rather, it appears to be a trial of gas behind the main cD. We argue that only one of the two cooling cores has survived the merger. The survival of the cool core constrains the merger kinematics, placing an upper limit of 1900 km s-1 on the relative merger velocity. A surface brightness discontinuity, found at ~140 kpc from the southern cD, has a Mach number of 1.66; the nature of the discontinuity (shock or cold front) is not clear fron the data. We argue that Abell 2065 is an unequal mass merger. The more massive southern cluster has driven a shock into the ICM of the infalling northern cluster, which has disrupted the cool core of the latter. We estimate that core crossing occurred a few hundred Myr ago, probably for the first time.

ANALYSIS OF THE Si IV ULTRAVIOLET SPECTRA OF U SAGITTAE

Archival IUE spectra of U Sge have been used to study hot (D105 K) circumstellar gas in this system. The observed spectra contain several UV resonance lines, of which the Si IV lines (jj1394 and 1405) are the strongest. These lines are observed in absorption, so we must remove the photospheric absorption line contribution to the spectra in order to see the emission produced by the circumstellar gas. We have developed a robust method for creating such diUerence spectra from IUE data. In the observed spectra we see a variation in the line pro—les and line centers with an orbital phase suggesting the presence of circumstellar gas. The residual emission seen in the diUerence spectra is strongest between phases / \ 0.3 and / \ 0.7, with a strength of up to 0.2 of the continuum —ux. This is con—rmed by the presence of an emission region visible in the Doppler tomogram of the j1394 line. This is consistent with the location of an Ha ii absorption zone ˇˇ seen in tomograms of U Sge and U CrB, and with theoretical predictions of a high temperature accretion region. Subject headings : accretion, accretion disksbinaries : eclipsingcircumstellar matter ¨ stars : individual (U Sagittae) ¨ ultraviolet : stars

A Chandra Study of the Core of the Nearby Cluster A576

We present data from a Chandra observation of the nearby cluster of galaxies A576. The core of the cluster shows a significant departure from dynamical equilibrium. We show that this core gas is most likely the remnant of a merging subcluster, which has been stripped of much of its gas, depositing a stream of gas behind it in the main cluster. The unstripped remnant of the subcluster is characterized by a different temperature, density, and metallicity than that of the surrounding main cluster, suggesting its distinct origin. Continual dissipation of the kinetic energy of this minor merger may be sufficient to counteract most cooling in the main cluster over the lifetime of the merger event.