Symposium Proceedings

Normal Galaxies: X-Ray Populations

Three Distinct Classes of Ultraluminous X-Ray Sources

Hua Feng, Philip Kaaret (Univ. of Iowa)

We examined X-ray properties of ultraluminous X-ray sources (ULXs) in nearby galaxies in XMM-Newton archival data. There appear to be three distinct classes of spectra. One class shows emission from hot, diffuse plasma. This thermal emission is similar to that seen from recent supernovae; the temperatures are in the range 0.6-0.8 keV and the luminosities are the lowest in our sample, near $10^{39}$ erg/s. Three sources have spectra which are strongly curved at high energies and have the highest temperatures in our sample, 1.0-1.4 keV. These spectra are well fitted with a power-law plus multicolor disk blackbody model with the power-law dominant at low energies or a Comptonization model. The remainder of the sources are best fitted with a power-law plus multicolor disk blackbody model, as is commonly used to describe the spectra of accreting black holes. These sources have the lowest thermal component temperatures, 0.1-0.4 keV, and extend to the highest luminosities, above $10^{40}$ erg/s. This diversity of spectral shapes and the fact that the sources lie in three distinct temperature ranges suggests that the ULXs are a diverse population.

Chandra - VLA-FIRST Ultraluminous X-ray Sources

Kajal Ghosh, Douglas Swartz (USRA, NASA/MSFC), Lakshmi Saripall (ATNF/CSIRO), Allyn Tennant (NASA/MSFC)

We have searched the VLA-FIRST catalog for potential radio counterparts to Ultraluminous X-ray source (ULX) candidates. Five radio sources (two in NGC 4490 and one each in NGC 4631, NGC 5194 and NGC 5775) with offsets between 0.4 and 4 arcsec from their Chandra positions were identified. Analysis of new and archival radio, infrared, optical, and X-ray observations of these sources are presented. We conclude the object in NGC 5775 and one of the objects in NGC 4490 are likely recent supernovae while the remaining three objects lack distinct optical counterparts and their nature remains uncertain.

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Chandra X-ray Survey Results of Galactic Center Fields

JaeSub Hong, Jonathan Grindlay, Maureen van den Berg, Silas Laycock, Xavier Koenig, Ping Zhao, Eric Schlegel (Harvard-Smithsonian Center for Astrophysics)

We present the X-ray survey results of the Galactic Center (GC) fields, which include deep Chandra observations of three low extinction windows (see paper by van den Berg et al. for a detailed description of these three fields) near the GC, and the Chandra archival data of Sgr A (750 ksec) and Sgr B2 field (100 ksec). The X-ray source populations in the GC fields may provide insights on the formation and evolution of the Galaxy, the Galactic Bulge and the supermassive blackhole in the center. The low extinction windows allow unique opportunities to study the bulge population directly without absorption through the molecular clouds. We classify the X-ray sources by quantile analysis, and drive logN-logS and spatial distributions according to the spectral types of the X-ray sources in order to understand their nature.


Deep O/IR Survey of Chandra Galactic Bulge Fields

Silas Laycock, Jonathan Grindlay, Ping Zhao, Maureen van den Berg, JaeSub Hong , Xavier Koenig, Eric Schlegel (Harvard-Smithsonian Center for Astrophysics)

We report on progress in our deep infrared and optical survey of archival Chandra fields in the Galactic Bulge, which we are conducting as part of ChaMPlane (The Chandra Multiwavelength Plane Survey). From our analysis it appears that the extraordinary cluster of hard X-ray point sources in the inner Bulge (central 10x10 arcmin) are dominated by faint low-mass systems. Fewer than 10% can be associated with High Mass X-ray Binaries on statistical grounds. We will present preliminary results of a much larger survey of 1.5x0.5 degrees using the CTIO 4m telescope with ISPI and Mosaic imagers to conduct an O/IR counterpart search for Chandra X-ray sources in the Bulge as well as in the foreground Galactic Plane.

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ULXs in the Chandra Observations of the Interacting Galaxies NGC 7714/15

Michael Nowak (MIT-CXC), Beverly Smith (ETSU), Curt Struck (Iowa State Univ.)

We recently have obtained a 60 ksec Chandra observation of the interacting galaxy pair NGC 7714/7715 (Arp 284), which at only 37 Mpc ( $H_0 = 75 {\rm km~s^{-1}~Mpc^{-1}}$) is relatively nearby (significantly closer, for example, than the Cartwheel galaxy). In this paper, we concentrate on our results for the ultraluminious X-ray source (ULX) population. In addition to an unresolved starburst nucleus, we detect a variable, $L_{\rm x} \approx 10^{40}~{\rm ergs~s^{-1}}$ point source 270 pc to the northwest of the nucleus, coincident with a blue, extremely optically luminous ( $M_V \approx -14.1$) point source on Hubble Space Telescope images. Eleven other candidate pointlike ULXs were also detected in the vicinity of NGC 7714/7715. Two of these exceed $10^{40}
{\rm ergs~s^{-1}}$, and are among the brightest ULXs ever observed, and one of which, when compared to archival XMM observations, shows evidence of long term (month), large amplitude changes in flux. Additionally, one of the ULXs exhibits evidence for a short time scale (20 sec) flare. Ten of the ULXs appear to be associated with interaction-induced features, but interestingly only two are associated with currently active star formation regions. We then briefly discuss the X-ray emission associated with four extranuclear ${\rm H_{II}}$ region complexes. In galaxies much more distant than NGC 7714, for example, the Cartwheel galaxy, similar ${\rm H_{II}}$ region complexes would be unresolved by Chandra, and could mimic ULXs.

Chandra Observations of the Nearby Face-on Sd Spiral Galaxy NGC 45

Thomas Pannuti (Spitzer Science Center/JPL/Caltech), Eric Schlegel (Univ. of Texas-San Antonio), Douglas Swartz (USRA/MSFC), Christina Lacey (Univ. of South Carolina)

We present the results of our Chandra observations fo the nearby face-on spiral Sd galaxy NGC 45. We have observed this galaxy as part of our study of supernova remnants in a sampe of nearby galaxies. The effective total exposure time of the observations was approximately 50 kiloseconds and we have detected approximately 20 discrete sources within the optical extent of the galaxy. We have searched for counterparts to the detected sources at multiple wavelengths: this search has included comparisons with known young star clusters and HII regions associated with this galaxy, foreground Galactic stars and background galaxies seen through the disk of NGC 45. We have also performed an analysis of the spectral properties of the brightest X-ray sources seen in this galaxy that were sampled by the observations. Initial results of this work will be presented and discussed.

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Chandra Observations of the X-Ray Point Source Population in NGC 4636

Jennifer Posson-Brown, Christine Jones, William Forman (SAO), R. Hank Donnelly (SAO/CNA), Somak Raychaudhury (Univ. of Birmingham), Stephen Murray (SAO)

We present results from four Chandra X-ray Observatory observations of the X-ray point source population in the nearby Virgo elliptical galaxy NGC 4636. These observations, totaling roughly 210 ks, were taken with the Advanced CCD Imaging Camera over a four year period. Using a wavelet decomposition detection algorithm, we detect over 200 individual point sources above a limiting luminosity of approximately $3\cdot10^{37}$ ergs $s^{-1}$ (outside the central bright core). Approximately 20% of sources in the common field of view were not detected in all observations, implying long-term variability or transience. We present a luminosity function for NGC 4636, as well as a radial source density profile, hardness ratios for the sources, and lightcurves for several sources which display short-term variability. We estimate the number of AGNs in the field based on Chandra Deep Field studies, and give optical identifications for sources associated with globular clusters.

This work was supported by NASA contract NAS8-39073, the CXC and the Smithsonian Institution.

Multi-colored X-ray Sources in Spiral Galaxies

Andrea Prestwich, Roy Kilgard (SAO)

Spiral galaxies contain a multitude of X-ray sources. In this paper I show that the brightest sources can be classified according to X-ray color into two main categories: those that have X-ray colors characteristic of Low Mass X-ray Binaries and a population of softer sources that are clearly associated with star forming regions. The differential luminosity functions of these two populations are different. The "LMXB" sources have an X-ray Luminosity Function (XLF) very similar to the XLF of sources in elliptical galaxies, whereas the XLF of the "soft" sources is a single power law with a slope consistent with the cannonical value for High Mass X-ray Binaries. These latter sources are probably black hole/neutron star binaries with a high mass donor in a high/soft state.

The Low-Mass X-ray Binary - Globular Connection in the HST ACS Virgo Cluster Survey

Gregory Sivakoff, Craig Sarazin (Univ. of Virginia), Andres Jordan (ESO / Oxford Univ.), Elizabeth Blanton (Boston Univ.), Patrick Cote, Laura Ferrarese (HIA / Rutgers Univ.), Jimmy Irwin (Univ. of Michigan), Adrienne Juett (Univ. of Virginia)

With the sub-arcsecond resolution of Chandra, a strong association between low-mass X-ray binaries (LMXBs) and globular clusters (GCs) has been established in early-type galaxies (20-70 % of LMXBs reside in GCs). Most of this association has been based on GC lists generated from ground-based observations, which can not resolve GCs at Virgo, or, HST-WFPC2 observations, which have small field-of-views. With the HST ACS Virgo Cluster Survey (VCS), a uniform sample of the GCs in the central 3 arcmin times 3 arcmin of 100 early-type galaxies has been created that contains not only positions and magnitudes, but also structural parameters. We present initial results of a study using this superior list of GCs and archival Chandra observations of LMXBS in a sub-sample of the HST ACS VCS to probe aspects of the LMXB-GC connection.

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Chandra/HST Survey of Low-extinction Windows in the Galactic Bulge

Maureen van den Berg, Josh Grindlay, JaeSub Hong, Silas Laycock, Ping Zhao, Xavier Koenig (Harvard-Smithsonian Center for Astrophysics), Haldan Cohn, Phyllis Lugger (Indiana Univ.), Eric Schlegel (Univ. of Texas)

We present results of our deep, near-simultaneous Chandra/HST observations of three low-extinction windows in the Galactic Bulge. With positions progressively closer to the Galactic Center (GC), these windows are used to constrain the radial gradient of low-luminosity point sources towards the GC. We detect a group of hard sources - some with candidate optical counterparts - out to Baade's Window, and discuss this population in the context of our extensive optical/infrared survey to understand the nature of the numerous low-luminosity, hard sources in the GC region.

The X-ray Transient Population of M31

Benjamin Williams, Michael Garcia, Jeffrey McClintock, Frank Primini, Stephen Murray (SAO)

Through an ongoing Chandra and HST snapshot campaign, we have been studying the X-ray transient population of M31. The observations have provided 5 years of X-ray variability information and high-resolution optical photometry coordinated with several bright transient events. Many Galactic X-ray binaries that exhibit outbursts similar to these events are known to contain black holes. These sources in M31 therefore offer an excellent opportunity to significantly expand the sample of known black hole binaries. The combined power of X-ray spectra and optical photometry provides new clues about the physical properties of the binary systems responsible for these outbursts in M31, and studies of the entire sample of M31 transient sources yield some of the first comparisons between the Galactic population and that of another galaxy with similar size and morphology.

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X-ray Binary Populations in Nearby Star-forming Galaxies

Andreas Zezas, Giuseppina Fabbiano (Harvard-Smithsonian Center for Astrophysics), Francois Schweizer (OCIW), Vicky Kalogera (Northwestern University), Martin Ward (Univ. of Durham), Roy Kilgard (Harvard-Smithsonian Center for Astrophysics),

We present results from our Chandra monitoring campaign of the Antennae and M82 galaxies as well as a sample of nearby galaxies (NGC55, NGC1569, NGC4214 and NGC5253) spaning a range of star-formation histories. We discuss the spectral and temporal characteristics of the sources in these different galaxies, and their implications for the sources' nature. We find that the X-ray sources follow diverse spectral variability patterns suggesting that they belong to different populations or long term states. This diversity is also observed within the class of Ultra-luminous sources. The discrete sources are in their majority associated with young star clusters, indicating that they are High Mass X-ray binaries. Finally, we present the X-ray luminosity functions (XLF) of the discrete sources from the individual as well as the co-added observations and we discuss the effect of source variability on the shape of the luminosity function. By comparing the XLF of the Antennae, M82, and the sample of nearby galaxies spaning a range of star-formation histories (from young to post star-bursts), we discuss how the XLF depends on the stellar populations of different ages.