Browsing by Author "Bregman, JN"
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Item A 2 hour quasi period in an ultraluminous X-ray source in NGC 628(IOP Publishing, 2005-03-01) Liu, JF; Bregman, JN; Lloyd-Davies, E; Irwin, J; Espaillat, C; Seitzer, P; University of Michigan System; University of Michigan; University of Alabama TuscaloosaQuasi-periodic oscillations and X-ray spectroscopy are powerful probes of black hole masses and accretion disks, and here we apply these diagnostics to an ultraluminous X-ray source (ULX) in the spiral galaxy NGC 628 (M74). This object was observed four times over 2 years with the Chandra X-Ray Observatory and XMM-Newton, with three long observations showing dramatic variability, distinguished by a series of outbursts with a quasi period of 4000-7000 s. This is unique behavior among ULXs and Galactic X-ray binaries because of the combination of its burstlike peaks and deep troughs, its long quasi periods, its high variation amplitudes of >90%, and its substantial variability between observations. The X-ray spectra is fitted by an absorbed accretion disk plus a power-law component, suggesting the ULX was in a spectral state analogous to the low/hard state or the very high state of Galactic black hole X-ray binaries. A black hole mass of similar to(2-20) x 10(3) M-circle dot is estimated from the f(b)-M-circle scaling relation found in the Galactic X- ray binaries and active galactic nuclei.Item Chandra X-ray observations of the X-ray faint elliptical galaxy NGC 4697(IOP Publishing, 2001-08-01) Sarazin, CL; Irwin, JA; Bregman, JN; University of Virginia; University of Michigan System; University of Michigan; University of Alabama TuscaloosaA Chandra ACIS S3 observation of the X-ray faint elliptical galaxy NGC 4697 resolves much of the X-ray emission (61% of the counts from within one effective radius) into 90 point sources, of which similar to 80 are low-mass X-ray binaries (LMXBs) associated with this galaxy. The dominance of LMXBs indicates that X-ray faint early-type galaxies have lost much of their interstellar gas. On the other hand, a modest portion of the X-ray emission from NGC 4697 is due to hot gas. Of the unresolved emission, it is likely that about half is from fainter unresolved LMXBs, while the other half (similar to 23% of the total count rate) is from interstellar gas. The X-ray-emitting gas in NGC 4697 has a rather low temperature (kT = 0.29 keV). The emission from the gas is very extended, with a much flatter surface brightness profile than the optical light, and has an irregular, L-shaped morphology. The physical state of the hot gas is uncertain; the X-ray luminosity and extended surface brightness are inconsistent with a global supersonic wind, a partial wind, or a global cooling inflow. The gas may be undergoing subsonic inflation, rotationally induced outflow, or ram pressure stripping. X-ray spectra of the resolved sources and diffuse emission show that the soft X-ray spectral component, found in this and other X-ray faint ellipticals with ROSAT, is due to interstellar gas. The cumulative LMXB spectrum is well fitted by thermal bremsstrahlung at kT = 8.1 keV, without a significant soft component. NGC 4697 has a central X-ray source with a luminosity of L(X) = 8 x 10(38) ergs s(-1), which may be due to an active galactic nucleus and/or one or more LMXBs. At most, the massive black hole (BH) at the center of this galaxy is radiating at a very small fraction (less than or equal to4 x 10(-8)) of its Eddington luminosity. Three of the resolved sources in NGC 4697 are supersoft sources. In the outer regions of NGC 4697, seven of the LMXBs (about 20%) are coincident with candidate globular clusters, which indicates that globulars have a high probability of containing X-ray binaries compared to the normal stellar population. We suggest that all of the LMXBs may have been formed in globulars. The X-ray to optical luminosity ratio for the LMXBs in NGC 4697 is L(X)(LMXB, 0.3-10 keV)/L(B) = 8.1 X 10(29) ergs s(-1) L(B.)(-1), which is about 35% higher than the value for the bulge of M31. Other comparisons suggest that there are significant variations (factor of greater than or similar to2) in the LMXB X-ray-to-optical ratios of early-type galaxies and spiral bulges. The X-ray luminosity function of NGC 4697 is also flatter than that found for the bulge of M31. The X-ray luminosities (0.3-10 keV) of the resolved LMXBs range from similar to5 x 10(37) to similar to2.5 X 10(39) ergs s(-1). The luminosity function of the LMXBs has a "knee" at 3.2 X 10(38) ergs s(-1), which is approximately the Eddington luminosity of a 1.4 M. neutron star (NS). This knee appears to be a characteristic feature of the LMXB population of early-type galaxies, and we argue that it separates BH and NS binaries. This characteristic luminosity could be used as a distance estimator. If they are Eddington limited, the brightest LMXBs contain fairly massive accreting BHs. The presence of this large population of NS and massive BH stellar remnants in this elliptical galaxy shows that it (or its progenitors) once contained a large population of massive main-sequence stars.Item The cosmological unimportance of low surface brightness galaxies(IOP Publishing, 2005-12-20) Hayward, CC; Irwin, JA; Bregman, JN; University of Michigan System; University of Michigan; University of Alabama TuscaloosaWe have searched for Type Ia supernovae (SNe Ia) in the local (d less than or similar to 60 Mpc) universe using Northern Sky Variability Survey (NSVS) data collected from the nightly optical surveys of the Robotic Optical Transient Search Experiment (ROTSE) Telescope. It was hoped that SNe Ia would provide a means to find previously unknown low surface brightness (LSB) galaxies or displaced stars that would otherwise be very difficult to detect. The ROTSE data allowed us to survey 19,000 deg(2) at declinations north of 0 degrees, but we did not find a single SN Ia in a period of time covering roughly 1 year. Using known SNe Ia rates in bright galaxies, we set an upper limit on the optical luminosity density, LB, of LSBs in the local universe. Using mean LSB baryonic and dynamical mass-to-light ratios, we find 95% upper limits for LSBs of L-B <= 2.53 x 10(8)L(B,circle dot) Mpc(-3), Omega(b) <= 0: 0040, and Omega(m) <= 0.036. We conclude that LSBs and displaced stars are not a major constituent of matter in the local universe.Item Galactic globular clusters with luminous X-ray binaries(IOP Publishing, 2006-03-20) Bregman, JN; Irwin, JA; Seitzer, P; Flores, M; University of Michigan System; University of Michigan; University of Alabama TuscaloosaLuminous X-ray binaries (LMXBs; > 10(34) ergs s(-1)) have a neutron star or black hole primary, and in globular clusters, most of these close binaries are expected to have evolved from wider binaries through dynamical interactions with other stars. We attempt to find a predictor of this formation rate that is representative of the initial properties of globular clusters rather than of the highly evolved core quantities. Models indicate the half-light quantities best reflect the initial conditions, so we examine whether the associated dynamical interaction rate, proportional to L-1.5/r(h)(2.5), is useful for understanding the presence of luminous LMXBs in the Galactic globular cluster system. We find that while LMXB clusters with large values of L-1.5/r(h)(2.5) preferentially host LMXBs, the systems must also have half-mass relaxation times below t(h,relax) similar to 10(9) yr. This relaxation time effect probably occurs because several relaxation times are required to modify binary separations, a timescale that must be shorter than cluster ages. The likelihood of finding an LMXB cluster is enhanced if the cluster is metal-rich and if it is close to the bulge region. The dependence on metallicity is most likely either due to differing initial mass functions at the high-mass end or because bulge systems evolve more rapidly from tidal interactions with the bulge. This approach can be used to investigate globular cluster systems in external galaxies, where core properties are unresolved.Item Iron abundance profiles of 12 clusters of galaxies observed with BeppoSAX(IOP Publishing, 2001-01-01) Irwin, JA; Bregman, JN; University of Michigan System; University of Michigan; University of Alabama TuscaloosaWe have derived azimuthally averaged radial iron-abundance profiles of the X-ray gas contained within 12 clusters of galaxies with redshift 0.03 less than or equal to z less than or equal to 0.2 observed with BeppoSAX. We find evidence for a negative metal-abundance gradient in most of the clusters, particularly significant in clusters that possess cooling flows. The composite profile from the 12 clusters resembles that of cluster simulations of Metzler & Evrard. This abundance gradient could be the result of the spatial distribution of gas-losing galaxies within the cluster being more centrally condensed than the primordial hot gas. Both inside and outside the core region we find a higher abundance in cooling-flow clusters than in non-cooling-flow clusters. Outside of the cooling region this difference cannot be the result of more efficient sputtering of metals into the gaseous phase in cooling-flow clusters but might be the result of the mixing of low-metallicity gas from the outer regions of the cluster during a merger.Item The lack of very ultraluminous X-ray sources in early-type galaxies(IOP Publishing, 2004-02-01) Irwin, JA; Bregman, JN; Athey, AE; University of Michigan System; University of Michigan; Carnegie Institution for Science; University of Alabama TuscaloosaWe have searched for ultraluminous X-ray sources (ULXs) in a sample of 28 elliptical and S0 galaxies observed with Chandra. We find that the number of X-ray sources detected at a flux level that would correspond to a 0.3-10 keV X-ray luminosity of similar to2 x 10(39) ergs s(-1) or greater (for which we have used the designation very ultraluminous X-ray sources [VULXs]) at the distance of each galaxy is equal to the number of expected foreground/background objects. In addition, the VULXs are uniformly distributed over the Chandra field of view rather than distributed like the optical light of the galaxies, strengthening the argument that the high-flux sources are unassociated with the galaxies. We have also taken the VULX candidate list of Colbert and Ptak and determined the spatial distribution of VULXs in early-type galaxies and late-type galaxies separately. While the spiral galaxy VULXs are clearly concentrated toward the centers of the galaxies, the early-type galaxy VULXs are distributed randomly over the ROSAT HRI field of view, again indicating that they are not associated with the galaxies themselves. We conclude that with the exception of two rare high-luminosity objects within globular clusters of the elliptical galaxy NGC 1399, VULXs are generally not found within old stellar systems. However, we do find a significant population of sources with luminosities of (1 - 2) x 10(39) ergs s(-1) that reside within the sample galaxies that can be explained by accretion onto 10 - 20 M, black holes. Given our results, we propose that ULXs be defined as X-ray sources with L-X(0.3 - 10 keV) > 2 x 10(39) ergs s(-1).Item OVI in elliptical galaxies: Indicators of cooling flows(IOP Publishing, 2005-12-20) Bregman, JN; Miller, ED; Athey, AE; Irwin, JA; University of Michigan System; University of Michigan; Massachusetts Institute of Technology (MIT); Carnegie Institution for Science; University of Alabama TuscaloosaEarly-type galaxies often contain a hot X-ray-emitting interstellar medium [(3-8) x 10(6) K] with an apparent radiative cooling time much less than a Hubble time. If unopposed by a heating mechanism, the gas will radiatively cool to temperatures less than or similar to 10(4) K at a rate proportional to L-X/T-X, typically 0.03-1 M-circle dot yr(-1). We can test whether gas is cooling through the 3 x 10(5) K range by observing the O VI doublet, whose luminosity is proportional to the cooling rate. Here we report on a study of an unbiased sample of 24 galaxies, obtaining Far Ultraviolet Spectroscopic Explorer spectra to complement the X-ray data of ROSAT and Chandra. The O VI line emission was detected in about 40% of the galaxies and at a luminosity level similar to the prediction from the cooling flow model. There is a correlation between. M-O (VI) and M-X, although there is significant dispersion about the relationship, where the O VI is brighter or dimmer than expected by a factor of 3 or more. If the cooling flow picture is to be retained, then this dispersion requires that cooling flows be time-dependent, as might occur by the activity of an AGN. However, of detected objects, those with the highest or lowest values of. M-O (VI)/M-X are not systematically hot or cool, as one might predict from AGN heating.Item OVI observations of galaxy clusters: Evidence for modest cooling flows(University of Chicago Press, 2006-05-10) Bregman, JN; Fabian, AC; Miller, ED; Irwin, JA; University of Michigan System; University of Michigan; University of Cambridge; Massachusetts Institute of Technology (MIT); University of Alabama TuscaloosaA prediction of the galaxy-cluster cooling flow model is that as gas cools from the ambient cluster temperature, emission lines are produced in gas at subsequently decreasing temperatures. Gas passing through 10(5.5) K emits in the lines of O (VI) lambda lambda 1032, 1035, and here we report a FUSE study of these lines in three cooling flow clusters, Abell 426, Abell 1795, and AWM 7. No emission was detected from AWM 7, but O vi is detected from the centers of Abell 426 and Abell 1795, and possibly to the south of the center in Abell 1795, where X-ray and optical emission line filaments lie. In Abell 426 these line luminosities imply a cooling rate of 32 +/- 6M(circle dot) yr(-1) within the central r = 6.2 kpc region, while for Abell 1795 the central cooling rate is 26 +/- 7 M-circle dot yr(-1) (within r = 22 kpc), and about 42 +/- 9 M-circle dot yr(-1) including the southern pointing. Including other studies, three of six clusters have O vi emission, and they also have star formation as well as emission lines from 104 K gas. These observations are generally consistent with the cooling flow model, but at a rate closer to 30 M-circle dot yr(-1) than to the originally suggested values of 10(2)-10(3) M-circle dot yr(1).Item OVI observations of galaxy clusters: Evidence for modest cooling flows(University of Chicago Press, 2006-01-01) Bregman, JN; Fabian, AC; Miller, ED; Irwin, JA; University of Michigan System; University of Michigan; University of Cambridge; Massachusetts Institute of Technology (MIT); University of Alabama TuscaloosaA prediction of the galaxy-cluster cooling flow model is that as gas cools from the ambient cluster temperature, emission lines are produced in gas at subsequently decreasing temperatures. Gas passing through 10(5.5) K emits in the lines of O (VI) lambda lambda 1032, 1035, and here we report a FUSE study of these lines in three cooling flow clusters, Abell 426, Abell 1795, and AWM 7. No emission was detected from AWM 7, but O vi is detected from the centers of Abell 426 and Abell 1795, and possibly to the south of the center in Abell 1795, where X-ray and optical emission line filaments lie. In Abell 426 these line luminosities imply a cooling rate of 32 +/- 6M(circle dot) yr(-1) within the central r = 6.2 kpc region, while for Abell 1795 the central cooling rate is 26 +/- 7 M-circle dot yr(-1) (within r = 22 kpc), and about 42 +/- 9 M-circle dot yr(-1) including the southern pointing. Including other studies, three of six clusters have O vi emission, and they also have star formation as well as emission lines from 104 K gas. These observations are generally consistent with the cooling flow model, but at a rate closer to 30 M-circle dot yr(-1) than to the originally suggested values of 10(2)-10(3) M-circle dot yr(1).Item Radial temperature profiles of 11 clusters of galaxies observed with BeppoSAX(University of Chicago Press, 2000-08-01) Irwin, JA; Bregman, JN; University of Michigan System; University of Michigan; University of Alabama TuscaloosaWe have derived azimuthally averaged radial temperature profiles of the X-ray gas contained within 11 clusters of galaxies with redshift z = 0.03-0.2 observed with BeppoSAX. Each of the 11 clusters have had their radial temperature profiles previously determined with ASCA. We find that the temperature profiles of these clusters are generally flat or increase slightly out to similar to 30% of the virial radius and that a decline in temperature of 14% out to 30% of the virial radius is ruled out at the 99% confidence level. This is in accordance with a previous ROSAT PSPC study and an ASCA study by White but in disagreement with an ASCA study by Markevitch et al. that found on average that cluster temperature profiles decreased significantly with radius.Item Radial temperature profiles of X-ray-emitting gas within clusters of galaxies(University of Chicago Press, 1999-07-10) Irwin, JA; Bregman, JN; Evrard, AE; University of Michigan System; University of Michigan; University of Alabama TuscaloosaPrevious analyses of ASCA data of clusters of galaxies have found conflicting results regarding the slope of the temperature profile of the hot X-ray gas within clusters, mainly because of the large, energy-dependent point-spread function (PSF) of the ASCA mirrors, We present a summary of all ASCA-determined cluster temperature profiles found in the literature and find a discrepancy in the radial temperature trend of clusters based on which PSF-correction routine is used, This uncertainty in the cluster temperature profile in turn can lead to large uncertainties in the amount of dark matter in clusters. In this study, we have used ROSAT PSPC data to obtain independent relative temperature profiles for 26 clusters, most of which have had their temperature profiles determined by ASCA. Our aim is not to measure the actual temperature values of the clusters but to use X-ray color profiles to search for a hardening or softening of the spectra as a function of radius for comparison to ASCA-derived profiles. The radial color profiles indicate that outside of the cooling flow region, the temperature profiles of clusters are in general constant. Within 35% of the virial radius, we find that a temperature drop of 20% at 10 keV and 12% at 5 keV can be ruled out at the 99% confidence level. A subsample of non-cooling flow clusters shows that the condition of isothermality applies at very small radii too, although cooling gas complicates this determination in the cooling how cluster subsample. The colors predicted from the temperature profiles of a series of hydrodynamical cluster simulations match the data very well, although they cannot be used to discriminate among different cosmologies. An additional result is that the color profiles show evidence for a central peak in metallicity in low-temperature clusters.Item Resolving the mystery of X-ray-faint elliptical galaxies: Chandra X-ray observations of NGC 4697(University of Chicago Press, 2000-12-01) Sarazin, CL; Irwin, JA; Bregman, JN; University of Virginia; University of Michigan System; University of Michigan; University of Alabama TuscaloosaChandra observations of the X-ray-faint elliptical galaxy NGC 4697 resolve much of the X-ray emission (61% within one effective radius) into similar to 80 point sources, of which most are low-mass X-ray binaries (LMXBs). These LMXBs provide the bulk of the hard emission and much of the soft emission as well. Of the remaining unresolved emission, it is likely that about half is from fainter LMXBs, while the other half (similar to 23% of the total emission) is from interstellar gas. Three of the resolved sources are supersoft sources. In the outer regions of NGC 4697, eight of the LMXBs (about 25%) are coincident with candidate globular clusters, indicating that globulars have a high probability of containing X-ray binaries compared with the normal stellar population. The X-ray luminosities (0.3-10 keV) of the resolved LMXBs range from similar to5 x 10(37) to similar to2.5 x 10(39) ergs s(-1). The luminosity function of the LMXBs has a "knee" at 3.2 x 10(38) ergs s(-1), which is roughly the Eddington luminosity of an 1.4 M-. neutron star (NS); this knee might be useful as a distance indicator. The highest luminosity source has the Eddington luminosity of an similar to 20 M-. black hole (BH). The presence of this large population of NS and massive BH stellar remnants in this elliptical galaxy shows that it (or its progenitors) once contained a large population of massive main-sequence stars.Item A shadow of the extragalactic X-ray background(University of Chicago Press, 2002-01-20) Bregman, JN; Irwin, JA; University of Michigan System; University of Michigan; University of Alabama TuscaloosaThe majority of baryons in the local universe are expected to lie outside of galaxies, in modest overdensity regions of the universe. This gas is predicted to be at a temperature of 10(5)-10(7) K and to emit X-rays that can contribute significantly to the X-ray background near 0.75 keV. If present, the gas in an edge-on galaxy would absorb this diffuse background emission, causing a shadow. We carried out this test using Chandra observations of the edge-on galaxy NGC 891 and using a cool, gas-rich region defined by high optical extinction, and we detect a shadow in the 0.4-1.0 keV band at the 99% confidence level. This shadow corresponds to approximately one-half of the total X-ray background in this energy range, brighter than the mean predicted value for emission from diffuse baryons but within the range of model values.Item The size of the cooling region of hot gas in two elliptical galaxies(University of Chicago Press, 2006-05-10) Bregman, JN; Otte, B; Miller, ED; Irwin, JA; University of Michigan System; University of Michigan; University of Alabama TuscaloosaSome early-type galaxies show O VI emission, a tracer of gas at 105: 5 K, and a predicted product of gas cooling from the X-ray-emitting temperatures. We studied the spatial extent and velocity structure of this cooling gas by obtaining spectra of the O VI doublet in NGC 4636 and NGC 5846 with the Far Ultraviolet Spectroscopic Explorer. For NGC 4636, the central LWRS pointing shows that the O VI lines are double-peaked and symmetrical about the systemic velocity of the galaxy, with a separation of 210 km s(-1). An LWRS observation 3000 from the center failed to show additional O vi emission. For NGC 5846, three spectra were obtained with 400; 2000 apertures (MDRS) at the center and 400 to the east and west of the center. The O VI line flux seen in the previous LWRS is contained in the sum of the smaller apertures, with most of the flux in a single noncentral MDRS aperture, suggesting a size for the emission <= 0.5 kpc; the emission consists of a blue and red peak. For both galaxies, the O vi velocity structure is similar to that of the optical [N II] emission and is consistent with rotation. The compactness and velocity structure of the O VI emission rules out cooling flow models with broadly distributed mass dropout but is consistent with cooling flow models in which the cooling occurs primarily in the central region. The 104 K gas may be the end state of the O vi emitting gas.Item An ultraluminous X-ray object with a 2 hour period in M51(University of Chicago Press, 2002-12-20) Liu, JF; Bregman, JN; Irwin, J; Seitzer, P; University of Michigan System; University of Michigan; University of Alabama TuscaloosaUltraluminous X-ray objects (ULXs) are off-nucleus point sources with L-X = 10(39)-10(41) ergs s(-1), but the nature 39 of such systems is largely unidentified. Here we report a 2.1 hr period observed in a Chandra ACIS observation for ULX M51 X-7, which is located on the edge of a young star cluster in the star-forming region in a spiral arm. In two ACIS observations separated by 1 yr, the ULX changed from a high-hard to a low-soft spectral state, in contrast to most Galactic low-mass X-ray binaries. On the basis of its period and spectral behaviors, we suggest that this ULX is a low-mass X-ray binary system, with a dwarf companion of 0.2-0.3 M. and a compact accretor, either a neutron star or a black hole, whose mass is not well constrained. Relativistic beaming effects are likely involved to produce the observed high X-ray luminosities, given its low accretion rate as inferred from a sustainable accretion scenario via Roche lobe overflow.Item Ultraluminous X-ray sources in nearby galaxies from ROSAT HRI observations. II. Statistical properties(IOP Publishing, 2006-05-01) Liu, JF; Bregman, JN; Irwin, J; University of Michigan System; University of Michigan; University of Alabama TuscaloosaThe statistical properties of the nonnuclear X-ray point sources from the ROSAT HRI survey of nearby galaxies in Paper I are studied, with particular attention to the contamination from background and/or foreground objects. This study reveals a statistical preference for the ultraluminous X-ray sources (ULXs) to occur in late-type galaxies over early-type galaxies, and in starburst/H (II) galaxies over nonstarburst galaxies. There is a trend of greater occurrence frequencies and ULX rates for galaxies with increasing star formation rates, confirming the connection between the ULX phenomenon and the star formation. A nonlinear correlation is found between the number of ULXs and the star formation rate, with significantly more ULXs at low star formation rates than the ULX population expected from the high-mass X-ray binaries (HMXBs) as an indicator of the star formation and the accompanying young stellar population, suggestive of another population of ULXs associated with the low-mass X-ray binaries (LMXBs) and the old stellar population. There are no breaks around 10(39) ergs s(-1) in the luminosity functions of ULXs in all galaxies or in late-type galaxies, suggesting the regular ULXs below 10(40) ergs s(-1) are a high-luminosity extension of the ordinary HMXB/LMXB populations below 10(39) ergs s(-1). There is evidence that the extreme ULXs above 10(40) ergs s(-1) might be a different ULX class from the regular ULXs below 10(40) ergs s(-1), although a larger sample with more ULXs is needed to establish the statistical properties of the extreme ULXs as a class.Item Untangling the X-ray emission from the Sa galaxy NGC 1291 with Chandra(IOP Publishing, 2002-05-01) Irwin, JA; Sarazin, CL; Bregman, JN; University of Michigan System; University of Michigan; University of Virginia; University of Alabama TuscaloosaWe present a Chandra ACIS-S observation of the nearby bulge-dominated Sa galaxy NGC 1291. The X-ray emission from the bulge resembles the X-ray emission from a subclass of elliptical and S0 galaxies with low X-ray-to-optical luminosity ratios. The X-ray emission is composed of a central pointlike nucleus, similar to50 point sources that are most likely low-mass X-ray binaries (LMXBs), and diffuse gas detectable out to a radius of 120" (5.2 kpc). The diffuse gas has a global temperature of 0.32(-0.03)(+0.04) and metallicity of 0.06+/-0.02 solar, and both quantities marginally decrease with increasing radius. The hot gas fills the hole in the H I distribution, and the softening of the spectrum of the X-ray gas with radius might indicate a thermal coupling of the hot and cold phases of the interstellar medium as previously suggested. The integrated X-ray luminosity of the LMXBs, once normalized by the optical luminosity, is a factor of 1.4 less than in the elliptical galaxy NGC 4697 or S0 galaxy NGC 1553. The difference in L-X,L-stellar/L-B between the galaxies appears to be because of a lack of very bright sources in NGC 1291. No sources above 3x10(38) ergs s(-1) were found in NGC 1291 when similar to7 were expected from scaling from NGC 4697 and NGC 1553. The cumulative L-X,L-stellar/L-B value including only sources below 1.0x10(38) ergs s(-1) is remarkably similar between NGC 1291 and NGC 4697, if a recent surface brightness fluctuation-determined distance is assumed for NGC 4697. If this is a common feature of the LMXB population in early-type systems, it might be used as a distance indicator. Finally, a bright, variable [(1.6=3.1) x 10(39) ergs s(-1)] source was detected at the optical center of the galaxy. Its spectrum shows excess soft emission superposed on a highly absorbed power-law component, similar to what has been found in several other low-luminosity active galactic nuclei. However, the soft component does not vary in intensity like the hard component, indicating that the soft component is not reprocessed hard component emission.Item Using the bulge of M31 as a template for the integrated X-ray emission from low-mass X-ray binaries(University of Chicago Press, 1999-12-10) Irwin, JA; Bregman, JN; University of Michigan System; University of Michigan; University of Alabama TuscaloosaWe have performed joint ASCA + ROSAT PSPC spectral fitting on the inner 5' of the bulge of M31. We find that single-component spectral models provide an inadequate fit to the spectrum, in contrast to previous studies by Einstein and Ginga. Although the 2-10 keV spectrum can be fitted adequately with a bremsstrahlung model with kT = 7.4 +/- 0.3 keV, an additional soft component with kT = 0.38 +/- 0.03 keV is required to fit the spectrum below 2 keV. This soft component comprises 38% +/- 6% of the total emission in the 0.1-2.0 keV band, and possibly more depending on the absorption value used in the lit. Since previous spatial studies of the bulge of M31 indicate that less than 25% of the X-ray emission from the bulge in this band is from a diffuse gaseous component, this implies that stellar sources, namely, low-mass X-ray binaries (LMXBs), are responsible for some if not all of the soft component. The spectral properties of M31 are very similar to those of the X-ray-faint early-type galaxy NGC 4382. This supports the claim that the unexplained soft X-ray emission seen previously in these galaxies also emanates from LMXBs.Item X-ray spectral properties of low-mass X-ray binaries in nearby galaxies(University of Chicago Press, 2003-04-10) Irwin, JA; Athey, AE; Bregman, JN; University of Michigan System; University of Michigan; University of Alabama TuscaloosaWe have investigated the X-ray spectral properties of a collection of low-mass X-ray binaries (LMXBs) within a sample of 15 nearby early-type galaxies using proprietary and archival data from the Chandra X-Ray Observatory. We find that the spectrum of the sum of the sources in a given galaxy is remarkably similar from galaxy to galaxy when only sources with X-ray luminosities less than 10(39) ergs s(-1) (0.3-10 keV) are considered. Fitting these lower luminosity sources in all galaxies simultaneously with a power-law model led to a best-fit power-law exponent of Gamma = 1.56 +/- 0.02 (90% confidence), and using a thermal bremsstrahlung model yielded kT(brem) = 7.3 +/- 0.3 keV. This is the tightest constraint to date on the spectral properties of LMXBs in external galaxies. The spectral properties of the LMXBs do not vary with galactic radius out to three effective radii. There is also no apparent difference between the spectral properties of LMXBs that reside within globular clusters and those that do not. We demonstrate how the uniformity of the spectral properties of LMXBs can lead to more accurate determinations of the temperature and metallicity of the hot gas in galaxies that have comparable amounts of X-ray emission from hot gas and LMXBs. Although few in number in any given galaxy, sources with luminosities of (1-2) x 10(39) ergs s(-1) are present in 10 of the galaxies. The spectra of these luminous sources are softer than the spectra of the rest of the sources and are consistent with the spectra of Galactic black hole X-ray binary candidates when they are in their very high state. The spatial distribution of these sources is much flatter than the optical light distribution, suggesting that a significant portion of them must reside within globular clusters. The simplest explanation of these sources is that they are similar to10-15 M. black holes accreting near their Eddington limit. The spectra of these sources are very different from those of ultraluminous X-ray sources (ULXs) that have been found within spiral galaxies, suggesting that the two populations of X-ray-luminous objects have different formation mechanisms. The number of sources with apparent luminosities above 2 x 10(39) ergs s(-1) when determined using the distance of the galaxy is equal to the number of expected background active galactic nuclei and thus appears not to be associated with the galaxy, indicating that very luminous sources are absent or very rare in early-type galaxies. The lack of ULXs within elliptical galaxies strengthens the argument that ULXs are associated with recent star formation.Item The X-ray-faint early-type galaxy NGC 4697(IOP Publishing, 2000-11-20) Irwin, JA; Sarazin, CL; Bregman, JN; University of Michigan System; University of Michigan; University of Virginia; University of Alabama TuscaloosaWe analyze archival ROSAT HRI, ROSAT PSPC, and ASCA data of the X-ray-faint early-type galaxy NGC 4697. The joint ROSAT PSPC + ASCA spectrum is fitted by a two-component thermal model, a MEKAL model with kT(MEKAL) = 0.26(-0.03)(+0.04) keV with low metallicity and a bremsstrahlung model with kT(BREM) = 5.2(-1.6)(+3.0) keV. A similar model was found to fit the spectra of another faint early-type galaxy (NGC 4382) and the bulge of M31. We interpret this soft emission as a combination of emission from a soft component of low-mass X-ray binaries (LMXBs) and from a low-temperature interstellar medium, although the relative contributions of the two components could not be determined. Twelve point sources were identified within 4 ' of NGC 4697, of which 11 are most likely LMXBs associated with the galaxy. The soft X-ray colors of four of the LMXBs in NGC 4697 support the claim that LMXBs possess a soft spectral component. Finally, we present a simulation of what we believe the Chandra data of NGC 4697 will look like.