Marshall W. Bautz

Chandra X-Ray Observations of the Hydra A Cluster: An Interaction between the Radio Source and the X-Ray-emitting Gas

We present Chandra X-ray observations of the Hydra A cluster of galaxies, and we report the discovery of structure in the central 80 kpc of the clusters X-ray-emitting gas. The most remarkable structures are depressions in the X-ray surface brightness, approximately 25-35 kpc in diameter, that are coincident with Hydra As radio lobes. The depressions are nearly devoid of X-ray-emitting gas, and there is no evidence for shock-heated gas surrounding the radio lobes. We suggest that the gas within the surface brightness depressions was displaced as the radio lobes expanded subsonically, leaving cavities in the hot atmosphere. The gas temperature declines from 4 keV at 70 kpc to 3 keV in the inner 20 kpc of the brightest cluster galaxy (BCG), and the cooling time of the gas is approximately 600 Myr in the inner 10 kpc. These properties are consistent with the presence of an approximately 34 M middle dot in circle yr-1 cooling flow within a 70 kpc radius. Bright X-ray emission is present in the BCG surrounding a recently accreted disk of nebular emission and young stars. The star formation rate is commensurate with the cooling rate of the hot gas within the volume of the disk, although the sink for the material that may be cooling at larger radii remains elusive. A bright, unresolved X-ray source is present in the BCGs nucleus, coincident with the radio core. Its X-ray spectrum is consistent with a power law absorbed by a foreground NH approximately 4x1022 cm-2 column of hydrogen. This column is roughly consistent with the hydrogen column seen in absorption toward the less, similar24 pc diameter VLBA radio source. Apart from the point source, no evidence for excess X-ray absorption above the Galactic column is found.

Rapid X-ray flaring from the direction of the supermassive black hole at the Galactic Centre

The nuclei of most galaxies are now believed to harbour supermassive black holes. The motions of stars in the central few light years of our Milky Way Galaxy indicate the presence of a dark object with a mass of about 2.6 × 106 solar masses (refs 2, 3). This object is spatially coincident with the compact radio source Sagittarius A* (Sgr A*) at the dynamical centre of the Galaxy, and the radio emission is thought to be powered by the gravitational potential energy released by matter as it accretes onto a supermassive black hole. Sgr A* is, however, much fainter than expected at all wavelengths, especially in X-rays, which has cast some doubt on this model. The first strong evidence for X-ray emission was found only recently. Here we report the discovery of rapid X-ray flaring from the direction of Sgr A*, which, together with the previously reported steady X-ray emission, provides compelling evidence that the emission is coming from the accretion of gas onto a supermassive black hole at the Galactic Centre.

The Chandra Deep Survey of the Hubble Deep Field-North Area. II. Results from the Caltech Faint Field Galaxy Redshift Survey Area*

A deep X-ray survey of the Hubble Deep Field-North (HDF-N) and its environs is performed using data collected by the Advanced CCD Imaging Spectrometer (ACIS) on board the Chandra X-Ray Observatory. Currently a 221.9 ks exposure is available, the deepest ever presented, and here we give results on X-ray sources located in the 86 × 87 area covered by the Caltech Faint Field Galaxy Redshift Survey (the Caltech area). This area has (1) deep photometric coverage in several optical and near-infrared bands; (2) extensive coverage at radio, submillimeter, and mid-infrared wavelengths; and (3) some of the deepest and most complete spectroscopic coverage ever obtained. It is also where the X-ray data have the greatest sensitivity; the minimum detectable fluxes in the 0.5-2 keV (soft) and 2-8 keV (hard) bands are ≈1.3 × 10-16 and ≈6.5 × 10-16 ergs cm-2 s-1, respectively. More than ≈80% of the extragalactic X-ray background in the hard band is resolved. The 82 Chandra sources detected in the Caltech area are correlated with more than 25 multiwavelength source catalogs, and the results of these correlations as well as spectroscopic follow-up results obtained with the Keck and Hobby-Eberly Telescopes are presented. All but nine of the Chandra sources are detected optically with R 26.5. Redshifts are available for 39% of the Chandra sources, including 96% of the sources with R 5.0) objects. A total of 16 of the 67 1.4 GHz μJy sources in the Caltech area are detected in the X-ray band, and the detection rates for starburst-type and AGN-candidate μJy sources are comparable. Only two of the 17 red, optically faint (I > 25) μJy sources are detected in X-rays. While many of the starburst-type μJy sources appear to contain obscured active galactic nuclei (AGNs), the Chandra data are consistent with the majority of the μJy radio sources being powered by star formation. A total of 11 of the ≈100 ISO mid-infrared sources found in and near the HDF-N are detected in X-rays. In the HDF-N itself, where both the infrared coverage and the X-ray coverage are deepest, it is notable that six of the eight Chandra sources are detected by ISO; most of these are known to be AGNs where the X-ray and infrared detections reveal both the direct and indirect accretion power being generated. The high X-ray-to-infrared matching rate bodes well for future sensitive infrared observations of faint X-ray sources. Four of the 33 very red objects that have been identified in the Caltech area are detected in X-rays; these four are among our hardest Chandra sources, and we argue that they contain moderately luminous obscured AGNs. Overall, however, the small Chandra detection fraction suggests a relatively small AGN content in the optically selected very red object population. A stacking analysis of the very red objects not detected individually by Chandra yields a soft-band detection with an average soft-band X-ray flux of ≈1.9 × 10-17 ergs cm-2 s-1; the observed emission may be associated with the hot interstellar media of moderate-redshift elliptical galaxies. Constraints on AGN candidates, extended X-ray sources, and Galactic objects in the Caltech area are also presented.

A Deep Chandra Catalog of X-Ray Point Sources toward the Galactic Center

(abridged) We present a catalog of 2357 point sources detected during 590 ks of Chandra observations of the 17-by-17 arcminute field around Sgr A*. This field encompasses a physical area of 40 by 40 pc at a distance of 8 kpc. The completeness limit of the sample at the Galactic center is 10^{31} erg s^{-1} (2.0--8.0 keV), while the detection limit is an order of magnitude lower. The 281 sources detected below 1.5 keV are mainly in the foreground of the Galactic center, while comparisons to the Chandra deep fields at high Galactic latitudes suggest that only about 100 of the observed sources are background AGN. The surface density of absorbed sources (not detected below 1.5 keV) falls off as 1/theta away from Sgr A*, in agreement with the distribution of stars in infrared surveys. Point sources brighter than our completeness limit produce 10% of the flux previously attributed to diffuse emission. The log(N)-log(S) distribution of the Galactic center sources is extremely steep (power-law slope alpha = 1.7). If this distribution extends down to a flux of 10^{-17} erg cm^{-1} s^{-1} (10^{29} erg s^{-1} at 8 kpc, 2.0--8.0 keV) with the same slope, then point sources would account for all of the previously reported diffuse emission. Therefore, the 2.0--8.0 keV luminosity distribution must flatten between 10^{29} - 10^{31} erg s^{-1}. Finally, the spectra of more than half of the Galactic center sources are very hard, and can be described by a power law (

An x-ray, infrared, and submillimeter flare of Sagittarius A*

E^{-Gamma}) with photon index Gamma < 1. Such hard spectra have been seen previously only from magnetically accreting white dwarfs and wind-accreting neutron stars, suggesting that there are large numbers of these systems in our field.

An abuttable CCD imager for visible and X-ray focal plane arrays

Energetic flares are observed in the Galactic supermassive black hole Sagittarius A* from radio to X-ray wavelengths. On a few occasions, simultaneous flares have been detected in IR and X-ray observations, but clear counterparts at longer wavelengths have not been seen. We present a flare observed over several hours on 2006 July 17 with the Chandra X-Ray Observatory, the Keck II telescope, the Caltech Submillimeter Observatory, and the Submillimeter Array. All telescopes observed strong flare events, but the submillimeter peak is found to occur nearly 100 minutes after the X-ray peak. Submillimeter polarization data show linear polarization in the excess flare emission, increasing from 9% to 17% as the flare passes through its peak, consistent with a transition from optically thick to thin synchrotron emission. The temporal and spectral behavior of the flare require that the energetic electrons responsible for the emission cool faster than expected from their radiative output. This is consistent with adiabatic cooling in an expanding emission region, with X-rays produced through self-Compton scattering, although not consistent with the simplest model of such expansion. We also present a submillimeter flare that followed a bright IR flare on 2005 July 31. Compared to 2006, this event had a larger peak IR flux and similar submillimeter flux, but it lacked measurable X-ray emission. It also showed a shorter delay between the IR and submillimeter peaks. Based on these events we propose a synchrotron and self-Compton model to relate the submillimeter lag and the variable IR/X-ray luminosity ratio.

Chandra Observations of the Lensing Cluster EMSS 1358+6245: Implications for Self-interacting Dark Matter

A frame-transfer silicon charge-coupled-device (CCD) imager has been developed that can be closely abutted to other imagers on three sides of the imaging array. It is intended for use in multichip arrays. The device has 420*420 pixels in the imaging and frame-store regions and is constructed using a three-phase triple-polysilicon process. Particular emphasis has been placed on achieving low-noise charge detection for low-light-level imaging in the visible and maximum energy resolution for X-ray spectroscopic applications. Noise levels of 6 electrons at 1-MHz and less than 3 electrons at 100-kHz data rates have been achieved. Imagers have been fabricated on 1000- Omega cm material to maximize quantum efficiency and minimize split events in the soft X-ray regime. >

The Chandra Deep Survey of the Hubble Deep Field North Area. IV. An Ultradeep Image of the HDF-N

We present Chandra observations of EMSS 1358+6245, a relaxed cooling flow cluster of galaxies at z = 0.328. We employ a new deprojection technique to construct temperature, gas, and dark matter profiles. We confirm the presence of cool gas in the cluster core, and our deprojected temperature profile for the hot component is isothermal over 30 kpc < r < 0.8 Mpc. Fitting the mass profile to an NFW model yields r_s = 153 [+161,-83] kpc and c = 8.4 [+3.4,-2.3]. We find good agreement between our dark matter profile and weak gravitational lensing measurements. We place an upper limit of 42 kpc (90% confidence limit) on the size of any constant density core. We compare this result to recent simulations and place a conservative upper limit on the dark matter particle scattering cross section of 0.1 cm^2/g. This limit implies that the cross-section must be velocity dependent if the relatively shallow core mass profiles of dwarf galaxies are a direct result of dark matter self-interaction.

A massive, cooling-flow-induced starburst in the core of a luminous cluster of galaxies

We present results from a 479.7 ks Chandra exposure of the Hubble Deep Field North (HDF-N) and its immediate vicinity. In this X-ray image, the deepest ever reported with a 0.5¨2.0 keV —ux limit of B4.9 ] 10~17 ergs cm~2 s~1, four new HDF-N X-ray sources are detected, bringing the total number of such sources to 12. The new sources include two optically bright (R \ 18.3¨18.8) low-redshift (z \ 0.15) galaxies, a FanaroU-Riley I radio galaxy, and an edge-on spiral galaxy hosting either a powerful starburst and/or a low-luminosity active galactic nucleus (AGN). Notably, X-ray emission has now been detected from all luminous galaxies with z \ 0.15 known in the HDF-N. We have also (M V \ [18) detected the remarkable microjansky radio source VLA J123642.09]621331.4, which is located just outside the HDF-N and has a likely redshift of z \ 4.424. The observed X-ray emission supports the

Soft-X-ray CCD imagers for AXAF

In the cores of some clusters of galaxies the hot intracluster plasma is dense enough that it should cool radiatively in the cluster’s lifetime, leading to continuous ‘cooling flows’ of gas sinking towards the cluster centre, yet no such cooling flow has been observed. The low observed star-formation rates and cool gas masses for these ‘cool-core’ clusters suggest that much of the cooling must be offset by feedback to prevent the formation of a runaway cooling flow. Here we report X-ray, optical and infrared observations of the galaxy cluster SPT-CLJ2344-4243 (ref. 11) at redshift z = 0.596. These observations reveal an exceptionally luminous (8.2 × 1045 erg s−1) galaxy cluster that hosts an extremely strong cooling flow (around 3,820 solar masses a year). Further, the central galaxy in this cluster appears to be experiencing a massive starburst (formation of around 740 solar masses a year), which suggests that the feedback source responsible for preventing runaway cooling in nearby cool-core clusters may not yet be fully established in SPT-CLJ2344-4243. This large star-formation rate implies that a significant fraction of the stars in the central galaxy of this cluster may form through accretion of the intracluster medium, rather than (as is currently thought) assembling entirely via mergers.