A Comparison of Intermediate Mass Black Hole Candidate ULXs and Stellar-Mass Black Holes

Cool thermal emission components have recently been revealed in the X-ray spectra of a small number of ultra-luminous X-ray (ULX) sources with L_X > 1 E+40 erg/s in nearby galaxies. These components can be well fitted with accretion disk models, with temperatures approximately 5-10 times lower than disk temperatures measured in stellar-mass Galactic black holes when observed in their brightest states. Because disk temperature is expected to fall with increasing black hole mass, and because the X-ray luminosity of these sources exceeds the Eddington limit for 10 Msun black holes (L_Edd = 1.3 E+39 erg/s), these sources are extremely promising intermediate-mass black hole candidates (IMBHCs). In this Letter, we directly compare the inferred disk temperatures and luminosities of these ULXs, with the disk temperatures and luminosities of a number of Galactic black holes. The sample of stellar-mass black holes was selected to include different orbital periods, companion types, inclinations, and column densities. These ULXs and stellar-mass black holes occupy distinct regions of a L_X -- kT diagram, suggesting these ULXs may harbor IMBHs. We briefly discuss the important strengths and weaknesses of this interpretation.Comment: 4 pages, 2 color figures, uses emulateapj.sty and apjfonts.sty, subm. to ApJ

L_X-T Relation and Related Properties of Galaxy Clusters

An observational approach is presented to constrain the global structure and evolution of the intracluster medium based on the ROSAT and ASCA distant cluster sample. From statistical analysis of the gas density profile and the connection to the LX-T relation under the beta-model, the scaled gas profile is nearly universal for the outer region and the LX(>0.2r500) is tightly related to the temperature through T^3 rather than T^2. On the other hand, a large density scatter exists in the core region and there is clearly a deviation from the self-similar scaling for clusters with a small core size. A direct link between the core size and the radiative cooling timescale suggest that t_cool is a parameter to control the gas structure and the appearance of small cores in regular clusters may be much connected with the thermal evolution. We derive the luminosity-ambient temperature (T') relation, assuming the universal temperature profile to find the dispersion around the relation significantly decreases: L_1keV is almost constant for a wide range of t_cool. We further examined the LX-Tbeta and LX-T'beta relations and showed a trend that merging clusters segregate from the regular clusters on the planes. A good correlation between t_cool and the X-ray morphology on the L_1keV-t_cool/t_age plane leads us to define three phases according to the different level of cooling, and draw a phenomenological picture: after a cluster collapses and t_cool falls below t_age, the core cools radiatively with quasi-hydrostatic balancing in the gravitational potential, and the central density gradually becomes higher to evolve from an outer-core-dominant cluster to inner-core-dominant cluster.Comment: 39 pages, 37 figures. Accepted for publication in ApJ. Version with high-quality color figures at http://cosmic.riken.jp/ota/publications/index.htm

X-ray Study of Seventy-nine Distant Clusters of Galaxies: Discovery of Two Classes of Cluster Size

We have performed a uniform analysis of 79 clusters of galaxies with the ROSAT HRI and ASCA to study the X-ray structure and evolution of clusters in the redshift range 0.1 < z < 1. We determined the average X-ray temperatures and the bolometric luminosities with ASCA and the spatial distributions of the X-ray brightness with the ROSAT HRI by utilizing the isothermal beta-model. We do not find any significant redshift dependence in the X-ray parameters including the temperature, beta-model parameters, and the central electron density. Among the parameters, the core radius shows the largest cluster-to-cluster dispersions. We discovered that the histogram of the core radius shows two peaks at 60 and 220 kpc. If we divide the cluster samples into two subgroups corresponding to the two peaks in the core radius distribution, they show differences in the X-ray and optical morphologies and in the X-ray luminosity temperature relation. From these observational results, we suggest that the clusters are divided into at least two subgroups according to the core radius.Comment: 12 pages, 3 figures. Accepted for publication in ApJ Letter

INTEGRAL spectral variability study of the atoll 4U 1820-30: first detection of hard X-ray emission

We study the 4-200 keV spectral and temporal behaviour of the low mass X-ray binary 4U 1820-30 with INTEGRAL during 2003-2005. This source as been observed in both the soft (banana) and hard (island) spectral states. A high energy tail, above 50 keV, in the hard state has been observed for the first time. This places the source in the category of X-ray bursters showing high-energy emission. The tail can be modeled as a soft power law component, with the photon index of ~2.4, on top of thermal Comptonization emission from a plasma with the electron temperature of kT_e~6 keV and optical depth of \tau~4. Alternatively, but at a lower goodness of the fit, the hard-state broad band spectrum can be accounted for by emission from a hybrid, thermal-nonthermal, plasma. During this monitoring the source spent most of the time in the soft state, usual for this source, and the >~4 keV spectra are represented by thermal Comptonization with kT_e~3 keV and \tau~6-7.Comment: 14 pages, 4 figures, accepted for publication by Ap

On the interpretation of the multicolour disc model for black hole candidates

We present a critical analysis of the usual interpretation of the multicolour disc model parameters for black hole candidates in terms of the inner radius and temperature of the accretion disc. Using a self-consistent model for the radiative transfer and the vertical temperature structure in a Shakura-Sunyaev disc, we simulate the observed disc spectra, taking into account doppler blurring and gravitational redshift, and fit them with multicolour models. We show not only that such a model systematically underestimates the value of the inner disc radius, but that when the accretion rate and/or the energy dissipated in the corona are allowed to change the inner edge of the disc, as inferred from the multicolour model, appears to move even when it is in fact fixed at the innermost stable orbit.Comment: 4 pages including 2 figures, accepted for publication in MNRA

Chemoenzymatic elaboration of monosaccharides using engineered cytochrome P450_(BM3) demethylases

Polysaccharides comprise an extremely important class of biopolymers that play critical roles in a wide range of biological processes, but the synthesis of these compounds is challenging because of their complex structures. We have developed a chemoenzymatic method for regioselective deprotection of monosaccharide substrates using engineered Bacillus megaterium cytochrome P450 (P450_(BM3)) demethylases that provides a highly efficient means to access valuable intermediates, which can be converted to a wide range of substituted monosaccharides and polysaccharides. Demethylases displaying high levels of regioselectivity toward a number of protected monosaccharides were identified using a combination of protein and substrate engineering, suggesting that this approach ultimately could be used in the synthesis of a wide range of substituted mono- and polysaccharides for studies in chemistry, biology, and medicine

Advection-Dominated Accretion and Black Hole Event Horizons

The defining characteristic of a black hole is that it possesses an event horizon through which matter and energy can fall in but from which nothing escapes. Soft X-ray transients (SXTs), a class of X-ray binaries, appear to confirm this fundamental property of black holes. SXTs that are thought to contain accreting black holes display a large variation of luminosity between their bright and faint states, while SXTs with accreting neutron stars have a smaller variation. This difference is predicted if the former stars have horizons and the latter have normal surfaces.Comment: 11 pages, including 2 tables and 2 figures. To appear in The Astrophysical Journal Letter

X-ray Spectroscopy of the Core of the Perseus Cluster with Suzaku: Elemental Abundances in the Intracluster Medium

The results from Suzaku observations of the central region of the Perseus cluster are presented. Deep exposures with the X-ray Imaging Spectrometer provide high quality X-ray spectra from the intracluster medium. X-ray lines from helium-like Cr and Mn have been detected significantly for the first time in clusters. In addition, elemental abundances of Ne, Mg, Si, S, Ar, Ca, Fe, and Ni are accurately measured within 10' (or 220 kpc) from the cluster center. The relative abundance ratios are found to be within a range of 0.8-1.5 times the solar value. These abundance ratios are compared with previous measurements, those in extremely metal-poor stars in the Galaxy, and theoretical models.Comment: 10 pages, 3 figures, accepted for ApJ

Going with the flow: can the base of jets subsume the role of compact accretion disk coronae?

The hard state of X-ray binaries (XRBs) is characterized by a power law spectrum in the X-ray band, and a flat/inverted radio/IR spectrum associated with occasionally imaged compact jets. It has generally been thought that the hard X-rays result from Compton upscattering of thermal accretion disk photons by a hot, coronal plasma whose properties are inferred via spectral fitting. Interestingly, these properties-especially those from certain magnetized corona models-are very similar to the derived plasma conditions at the jet footpoints. Here we explore the question of whether the `corona' and `jet base' are in fact related, starting by testing the strongest premise that they are synonymous. In such models, the radio through the soft X-rays are dominated by synchrotron emission, while the hard X-rays are dominated by inverse Compton at the jet base - with both disk and synchrotron photons acting as seed photons. The conditions at the jet base fix the conditions along the rest of the jet, thus creating a direct link between the X-ray and radio emission. We also add to this model a simple iron line and convolve the spectrum with neutral reflection. After forward-folding the predicted spectra through the detector response functions, we compare the results to simultaneous radio/X-ray data obtained from the hard states of the Galactic XRBs GX339-4 and Cygnus X-1. Results from simple Compton corona model fits are also presented for comparison. We demonstrate that the jet model fits are statistically as good as the single-component corona model X-ray fits, yet are also able to address the simultaneous radio data.Comment: Accepted to the Astrophysical Journal. 14 pages, emulateapj.st

X-ray Spectroscopic Evidence for Intermediate Mass Black Holes: Cool Accretion Disks in Two Ultra--Luminous X-ray Sources

We have analyzed an XMM-Newton observation of the nearby spiral galaxy NGC 1313, which contains two "ultra-luminous" X-ray (ULX) sources. We measure isotropic luminosities of L_X = 2.0 * 10^(40) erg/s and L_X = 6.6 * 10^(39) erg/s for NGC 1313 X-1 and X-2 (0.2-10.0 keV, assuming a distance of 3.7 Mpc). The spectra statistically require soft and hard spectral components to describe the continuum emission; some prior studies of ULXs have claimed cool soft components with lower statistics. The improvement over several single-component models exceeds the 8 sigma level of confidence for X-1; the improvement for X-2 is significant at the 3 sigma level. The soft components in these ULX spectra are well-fit by multi-color disk blackbody models with color temperatures of kT = 150 eV. This temperature differs markedly from those commonly measured in the spectra of stellar-mass (10 M_sun) black holes in their brightest states (kT ~ 1 keV). It is expected that the temperature of an accretion disk orbiting a black hole should decrease with increasing black hole mass. If the soft components we measure are due to emission from the inner region of an accretion disk, and disks extend close to the innermost stable circular orbit at the accretion rates being probed, the low color temperatures may be interpreted as spectroscopic evidence of black holes with intermediate masses: M_BH ~ 10^(3) M_sun. Simple Eddington scaling arguments suggest a mimum mass of M_BH ~ 10^(2) M_sun. NGC 1313 X-1 and X-2 are found in optical nebulae (Pakull and Mirioni 2002), which may indicate that anisotropic emission geometries are unlikely to account for the fluxes observed.Comment: 5 pages, 3 color figures, uses emulateapj.sty and apjfonts.sty, ApJL accepte