Kimberly Ann Weaver

Old and Young X-Ray Point Source Populations in Nearby Galaxies

We have analyzed Chandra ACIS observations of 32 nearby spiral and elliptical galaxies and present the results of 1441 X-ray point sources that were detected in these galaxies. The total point-source X-ray (0.3-8.0 keV) luminosity LXP is well correlated with the B-band, K-band, and FIR+UV luminosities of spiral host galaxies and is well correlated with the B-band and K-band luminosities of elliptical galaxies. This suggests an intimate connection between LXP and both the old and young stellar populations, for which K and FIR+UV luminosities are reasonable proxies for the galaxy mass M and star formation rate SFR. We derive proportionality constants ? = 1.3 ? 1029 ergs s-1 M and ? = 0.7 ? 1039 ergs s-1 (M? yr-1)-1, which can be used to estimate the old and young components from M and SFR, respectively. The cumulative X-ray luminosity functions for the point sources have significantly different slopes. For the spiral and starburst galaxies, ? ? 0.6-0.8, and for the elliptical galaxies, ? ? 1.4. This implies that the most luminous point sources?those with LX 1038 ergs s-1?dominate LXP for the spiral and starburst galaxies. Most of the point sources have X-ray colors that are consistent with soft-spectrum (photon index ? ~ 1-2) low-mass X-ray binaries, accretion-powered black hole high-mass X-ray binaries (BH HMXBs), or ultraluminous X-ray sources (ULXs, also known as IXOs). We rule out hard-spectrum neutron star HMXBs (e.g., accretion-powered X-ray pulsars) as contributing much to LXP. Thus, for spirals, LXP is dominated by ULXs and BH HMXBs. We find no discernible difference between the X-ray colors of ULXs (LX ? 1039 ergs s-1) in spiral galaxies and point sources with LX ? 1038-1039 ergs s-1. We estimate that 20% of all ULXs found in spirals originate from the older (Population II) stellar populations, indicating that many of the ULXs that have been found in spiral galaxies are in fact Population II ULXs, like those in elliptical galaxies. We find that LXP depends linearly (within uncertainties) on both M and SFR for our sample galaxies (M 1011 M? and SFR 10 M? yr-1).

An X-Ray Minisurvey of Nearby Edge-on Starburst Galaxies. I. The Data

We have analyzed all available ROSAT PSPC and HRI and ASCA data for a small far-infrared flux-limited sample of seven nearby edge-on starburst galaxies in order to search for hot gas in their halos. We find that all five normal-sized spiral galaxies (NGC 253, NGC 3079, NGC 3628, NGC 4631, and NGC 4666) have hot gas in their halos, as does the small peculiar galaxy M82. NGC 55, a nearby Magellanic irregular, shows signs of hot gas beyond its thin disk only near the most actively star-forming region that is associated with a giant H? bubble. All fits to joint PSPC + ASCA spectra, except NGC 55, indicate the presence of two gas phases, one at 0.2-0.4 keV and another at 0.65-0.9 keV, which in general appear to dominate the emission of the hot gas. In NGC 4666, for which only ROSAT data are available, we currently find only a gas component near 0.3 keV. The data of NGC 4631 hint at the presence of a third gas component in this galaxy, at a very low temperature of kT 0.05 keV. One of our general results is that the joint PSPC + ASCA spectra allow us to identify different spectral components. Based on the additional imaging information and on consistency checks, we could fit all spectra with similar composite models including both the above thermal plasmas and a harder power-law component. The results of these fits are statistically good, consistent with each other, and physically meaningful. However, they are not unique, and it is extremely difficult to constrain the metallicities of the different components of hot gas. Our results show that all data can be fitted by composite model spectra with near-solar metallicities. Extremely low metallicities, as previously fitted by others based on individual ROSAT or ASCA observations, are thus not required. As a by-product of our investigations, spectra of other X-ray emitters in the target galaxies were obtained. We determined the spectral properties of the hot gas and compact sources (presumably mostly high-mass X-ray binaries) in their disks. Integral spectra, indicating thermal emission characteristics, of those compact sources outside the galaxy disks with soft hardness ratios, suggest that they might be associated with the halo gas.

Chandra Observations of NGC 253: New Insights into the Nature of Starburst-driven Superwinds

Arcsecond-resolution X-ray imaging of the nucleus of the nearby starburst galaxy NGC 253 with Chandra reveals a well-collimated, strongly limb-brightened, kiloparsec-scale conical outflow from the central starburst region. The outflow is very similar in morphology to the known Hα outflow cone, on scales down to 20 pc. This provides, for the first time, robust evidence that both X-ray and Hα emission come from low volume filling factor regions of interaction between the fast energetic wind of SN ejecta and the denser ambient interstellar medium and not from the wind fluid itself. We provide estimates of the (observationally and theoretically important) filling factor of the X-ray–emitting gas, of between ~4% and 40%, consistent with an upper limit of ~40%, based directly on the observed limb-brightened morphology of the outflow. Only 20% of the observed X-ray emission can come from the volume-filling, metal-enriched, wind fluid itself. Spatially resolved spectroscopy of the soft diffuse thermal X-ray emission reveals that the predominant source of spectral variation along the outflow cones is due to strong variation in the absorption on scales of ~60 pc, there being little change in the characteristic temperature of the emission. We show that these observations are easily explained by, and fully consistent with, the standard model of a superwind driven by a starburst of the observed power of NGC 253. If these results are typical of all starburst-driven winds, then we do not directly see all the energy and gas (in particular the hot metal-enriched gas) transported out of galaxies by superwinds, even in X-ray emission.

Fe XXV AND Fe XXVI DIAGNOSTICS OF THE BLACK HOLE AND ACCRETION DISK IN ACTIVE GALAXIES: CHANDRA TIME-RESOLVED GRATING SPECTROSCOPY OF NGC 7314

We report the detection of Fe XXV and Fe XXVI Kα emission lines from a Chandra High Energy Transmission Grating Spectrometer observation of the narrow-line Seyfert 1 galaxy NGC 7314, made simultaneously with the Rossi X-Ray Timing Explorer. The lines are redshifted (cz ~ 1500 km s-1) relative to the systemic velocity and unresolved by the gratings. We argue that the lines originate in a near face-on (<7°) disk having a radial line emissivity flatter than r-2. Line emission from ionization states of Fe in the range ~Fe I up to Fe XXVI is observed. The ionization balance of Fe responds to continuum variations on timescales of less than 12.5 ks, supporting an origin of the lines close to the X-ray source. We present additional, detailed diagnostics from this rich data set. These results identify NGC 7314 as a key source to study in the future if we are to pursue reverberation mapping of spacetime near black hole event horizons. This is because it is first necessary to understand the ionization structure of accretion disks and the relation between the X-ray continuum and Fe Kα line emission. However, we also describe how our results are suggestive of a means of measuring black hole spin without a knowledge of the relation between the continuum and line emission. Finally, these data emphasize that one can study strong gravity with narrow (as opposed to very broad) disk lines. In fact, narrow lines offer higher precision, given sufficient energy resolution.

Variable Iron Kα Lines in Seyfert 1 Galaxies

We find that variability of the iron K alpha line is common in Seyfert 1 galaxies. Using data from the ASCA archive for objects that have been observed more than once during the mission, we study the time-averaged spectra from individual observations, thereby probing variability on timescales that range from days to years. Since the statistics of the data do not warrant searches for line variability in terms of a complex physical model, we use a simple Gaussian to model the gross shape of the line and then use the centroid energy, intensity, and equivalent width as robust indicators of changes in the line profile. We find that approximately 70% of Seyfert 1 galaxies (10 out of 15) show variability in at least one of these parameters: the centroid energy, intensity, and equivalent width vary in six, four, and eight sources, respectively. Because of the low signal-to-noise ratio, limited sampling, and time averaging, we consider these results to represent lower limits to the rate of incidence of variability. In most cases changes in the line do not appear to track changes in the continuum. In particular, we find no evidence for variability of the line intensity in NGC 4151, suggesting an origin in a region larger than the putative accretion disk, where most of the iron line has been thought to originate. Mrk 279 is investigated on short timescales. The time-averaged effective line energy (as measured by the Gaussian center energy, which is weighted by emission in the entire line profile) is 6.5 keV in the galaxy rest frame. As the continuum flux increases by 20% in a few hours, the Fe K line responds within approximately 10,000 seconds with the effective line energy increasing by 0.22 keV (approximately 10,500 kilometers per second). We also examine the ROSAT PSPC spectrum of Mrk 279 but find inconsistencies with ASCA. Problems with the ASCA and ROSAT calibration that affect simultaneous spectral fits at low energies are discussed in an appendix.

The Seyfert-Starburst Connection in X-Rays. II. Results and Implications

We present the results of X-ray imaging and spectroscopic analysis of a sample of Seyfert 2 galaxies that contain starbursts, based on their optical and UV characteristics. These composite galaxies exhibit extended, soft, thermal X-ray emission, which we attribute to their starburst components. Comparing their X-ray and far-infrared properties with ordinary Seyfert and starburst galaxies, we identify the spectral characteristics of their various intrinsic emission sources. The observed far-infrared emission of the composite galaxies may be associated almost exclusively with star formation, rather than the active nucleus. The ratio of the hard X-ray luminosity to the far-infrared and [O III] λ5007 luminosity distinguishes most of these composite galaxies from pure Seyfert 2 galaxies, while their total observed hard X-ray luminosity distinguishes them from pure starbursts. The hard nuclear X-ray source is generally heavily absorbed (NH > 1023 cm-2) in the composite galaxies. Based on these results, we suggest that the interstellar medium of the nuclear starburst is a significant source of absorption. The majority of the sample are located in groups or are interacting with other galaxies, which may trigger the starburst or allow rapid mass infall to the central black hole or both. We conclude that starbursts are energetically important in a significant fraction of active galaxies and that starbursts and active galactic nuclei may be part of a common evolutionary sequence.

Relativistic Iron K Emission and absorption in the Seyfert 1.9 galaxy MCG-5-23-16

We present the results of the simultaneous deep XMM-Newton and Chandra observations of the bright Seyfert 1.9 galaxy MCG -5-23-16, which is thought to have one of the best known examples of a relativistically broadened iron Kα line. The time-averaged spectral analysis shows that the iron K-shell complex is best modeled with an unresolved narrow emission component (FWHM < 5000 km s-1, EW ~ 60 eV) plus a broad component. This latter component has FWHM ~ 44,000 km s-1 and EW ~ 50 eV. Its profile is well described by an emission line originating from an accretion disk viewed with an inclination angle ~40°, with the emission arising from within a few tens of gravitational radii of the central black hole. The time-resolved spectral analysis of the XMM-Newton EPIC pn spectrum shows that both the narrow and broad components of the Fe K emission line appear to be constant in time within the errors. We detected a narrow sporadic absorption line at 7.7 keV, which appears to be variable on a timescale of 20 ks. If associated with Fe XXVI Lyα, this absorption is indicative of a possibly variable, high-ionization, high-velocity outflow. The variability of this absorption feature appears to rule out a local (z = 0) origin. The analysis of the XMM-Newton RGS spectrum reveals that the soft X-ray emission of MCG -5-23-16 is likely dominated by several emission lines superimposed on an unabsorbed scattered power-law continuum. The lack of strong Fe L-shell emission, together with the detection of a strong forbidden line in the O VII triplet, is consistent with a scenario in which the soft X-ray emission lines are produced in a plasma photoionized by the nuclear emission.

Testing the Paradigm that Ultraluminous X-Ray Sources as a Class Represent Accreting Intermediate-Mass Black Holes

To test the idea that ultraluminous X-ray sources (ULXs) in external galaxies represent a class of accreting intermediate-mass black holes (IMBHs), we have undertaken a program to identify ULXs and a lower luminosity X-ray comparison sample with the highest quality data in the Chandra archive. We establish as a general property of ULXs that the most X-ray-luminous objects possess the flattest X-ray spectra (in the Chandra bandpass). No prior sample studies have established the general hardening of ULX spectra with luminosity. This hardening occurs at the highestluminosities (absorbed luminosity � 5 ; 10 39 ergs � 1 ) and isin line withrecent modelsarguingthat ULXs are actually stellar mass black holes. From spectral modeling, we show that the evidence originally taken to mean that ULXs are IMBHs—i.e., the ‘‘simple IMBH model’’—is nowhere near as compelling when a large sample of ULXs is looked at properly. During the last couple of years,XMM-Newtonspectroscopy of ULXs has to a large extent begun to negate the simple IMBH model based on fewer objects. We confirm and expand these results, which validates the XMM-Newton work in a broader sense with independent X-ray data. We find that (1) cool-disk components are present withroughlyequalprobabilityandtotalfluxfractionforanygivenULX,regardlessof luminosity,and(2)cool-diskcomponentsextendbelowthestandardULXluminositycutoff of 10 39 ergs � 1 ,downtooursamplelimitof 10 38.3 ergs � 1 .The

X-Ray Emission from Seyfert 2 Galaxies with Optical Polarized Broad Lines

We analyze the 0.5-10 keV spectra of six Seyfert 2 galaxies observed with the X-ray satellite ASCA: Mrk 3, Mrk 348, Mrk 1210, Mrk 477, NGC 7212, and Was 49b. These galaxies were selected based on their possession of optical polarized broad lines. In the 2-10 keV band, their spectra are heavily absorbed, with 2-10 keV absorption-corrected X-ray luminosities ranging from 1042 to 1043 ergs s-1. The observed X-ray emission is generally only about 1/10 that predicted based on their known infrared and Hβ luminosities. This apparent X-ray weakness can be understood if a considerable fraction of the nuclear activity is completely blocked from view by thick matter along our line of sight to the nucleus. All of these galaxies possess significant soft X-ray emission whose origin appears to be scattered light from their nuclear emission. Based on this hypothesis, we estimate a typical scattering efficiency for X-rays to be about 10%. This efficiency is larger than the few percent found for ordinary Seyfert 2 galaxies with no report of optical polarized broad lines. A large scattering efficiency is best explained by an apparent weakness of the hard X-ray luminosity rather than unusually strong scattered light in the soft band. When we estimate the scattering efficiency using the intrinsic luminosity derived assuming that Compton scattering dominates the hard X-ray spectrum, as opposed to a purely absorbed nuclear continuum, the efficiency can be less and is similar to that of ordinary Seyfert 2 galaxies. Since the difference between our sample and ordinary Seyfert 2 galaxies can be explained by the difference of viewing angle, the similar scattering efficiency suggests the existence of a scattering region that is larger than the putative dusty torus.

A new X-ray spectral observation of NGC 1068

A new X-ray observation of NGC 1068, in which improved spectral resolution (R is approximately equal to 40) and broad energy range provide important new constraints on models for this galaxy, is reported. The observed X-ray continuum of NGC 1068 from 0.3 to 10 keV is well fitted as the sum of two power-law spectra with no evidence for absorption intrinsic to the source. Strong Fe K emission lines with a total equivalent width of 2700 eV were detected due to iron less ionized than Fe XX and to iron more ionized than Fe XXIII. No evidence was seen for lines due to the recombination of highly ionized oxygen with an upper limit for the O Ly-alpha emission line of 40 eV. The discovery of multiple Fe K and Fe L emission lines indicates a broad range of ionization states for this gas. The X-ray emission from the two components is modeled for various geometries using a photoionization code that calculates the temperature and ionization state of the gas. Typical model parameters are a total Compton depth of a few percent, an inner boundary of the hot component of about 1 pc, and an inner boundary of the warm component of about 20 pc.