Browsing by Author "Irwin, JA"
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Item Chandra observation of diffuse gas and low-mass X-ray binaries in the elliptical galaxy NGC 4649 (M60)(IOP Publishing, 2004-01-10) Randall, SW; Sarazin, CL; Irwin, JA; University of Virginia; University of Michigan System; University of Michigan; University of Alabama TuscaloosaWe present a Chandra X-ray observation of the X-ray bright E2 elliptical galaxy NGC 4649. In addition to bright diffuse emission, we resolve 165 discrete sources, most of which are presumably low-mass X-ray binaries. As found in previous studies, the luminosity function of the resolved sources is well-fitted by a broken power law. In NGC 4697 and NGC 1553, the break luminosity was comparable to the Eddington luminosity of a 1.4 M-. neutron star. One possible interpretation of this result is that those sources with luminosities above the break are accreting black holes and those below are mainly accreting neutron stars. The total X-ray spectrum of the resolved sources is well fitted by a hard power law, while the diffuse spectrum requires a hard and a soft component, presumably due to the relatively soft diffuse gas and the harder unresolved sources. We also find evidence for structure in the diffuse emission near the center of NGC 4649. Specifically, there appear to be bright "fingers'' of emission extending from the center of the galaxy and a 500 long bar at the center of the galaxy. The fingers are morphologically similar to radial features seen in two-dimensional hydrodynamic simulations of cooling flows in elliptical galaxies, and although their other properties do not match the predictions of the particular simulations used, we conclude that the radial fingers might be due to convective motions of hot outflowing gas and cooler inflowing gas. The bar is coincident with the central extended radio source; we conclude that the bar may be caused by weak shocks in the diffuse gas from an undetected low-luminosity active galactic nucleus.Item Chandra observations of low-mass X-ray binaries and diffuse gas in the early-type galaxies NGC 4365 and NGC 4382 (M85)(IOP Publishing, 2003-12-10) Sivakoff, GR; Sarazin, CL; Irwin, JA; University of Virginia; University of Michigan System; University of Michigan; University of Alabama TuscaloosaWe used the Chandra X-Ray Observatory ACIS-S3 to image the X-ray-faint elliptical galaxy NGC 4365 and lenticular galaxy NGC 4382. The observations resolve much of the X-ray emission into 99 and 58 sources, respectively, most of which are low-mass X-ray binaries (LMXBs) associated with each of the galaxies. Within one effective radius of NGC 4365, about 45% of the counts are resolved into sources, 30% are attributed to unresolved LMXBs, and 25% are attributed to diffuse gas. Within two effective radii of NGC 4382, about 22% of the counts are resolved into sources, 33% are attributed to unresolved LMXBs, and 45% are attributed to diffuse gas. We identify 18 out of the 37 X-ray sources in a central field in NGC 4365 with globular clusters. The luminosity functions of the resolved sources for both galaxies are best fitted with cutoff power laws whose cutoff luminosity is approximate to (0.9-3.1) x 10(39) ergs s(-1). These luminosities are much larger than those previously measured for similar galaxies; we do not find evidence for a break in the luminosity function at the Eddington luminosity of a 1.4 M-. neutron star. The spatial distributions of the resolved sources for both galaxies are broader than the distribution of optical stars. In both galaxies, a hard powerlaw model fits the summed spectrum of all of the sources. The unresolved emission is best fitted by the sum of a soft MEKAL model, representing emission from diffuse gas, and a hard power law, presumed to be from unresolved LMXBs. There is some evidence that the temperature of the diffuse gas increases with increasing radius. A standard beta model fits the radial distribution of the diffuse gas in both galaxies. In the elliptical galaxy NGC 4365, the best-fit core radius is very small, while the SO galaxy NGC 4382 has a larger core radius. This may indicate that the gas in NGC 4382 is rotating significantly.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 The dependence of the soft X-ray properties of low-mass X-ray binaries on the metallicity of their environment(University of Chicago Press, 1999-01-01) Irwin, JA; Bregman, JR; University of Michigan System; University of Michigan; University of Alabama TuscaloosaWe determine the X-ray spectral properties of a sample of low-mass X-ray binaries (LMXBs) that reside in globular clusters of M31 as well as five LMXBs in Galactic globular clusters and in the Large Magellanic Cloud using the ROSAT Position Sensitive Proportional Counter. We find a trend in the X-ray properties of the LMXBs as a function of globular cluster metallicity. The spectra of LMXBs become progressively softer as the metallicity of their environment increases. The one M31 globular cluster LMXB in our sample that has a metallicity greater than solar has spectral properties similar to those of LMXBs in the bulge of M31 bur markedly different from those that reside in low-metallicity globular clusters, both in M31 and the Galaxy. The spectral properties of this high-metallicity LMXB are also similar to those of X-ray-faint, early-type galaxies. This lends support to the claim that a majority of the X-ray emission from these X-ray-faint, early-type galaxies results from LMXBs and not hot gas, as is the case in their X-ray-bright counterparts.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 Heating of the intracluster gas in the Triangulum Australis cluster(University of Chicago Press, 1996-11-20) Markevitch, ML; Sarazin, CL; Irwin, JA; University of Alabama TuscaloosaASCA and ROSAT X-ray data are used to obtain two-dimensional maps of the gas temperature, pressure, and specific entropy in the Triangulum Australis cluster of galaxies. We find that this hot (T-e = 10.3 +/- 0.8 keV) system probably has a temperature peak (T-e > 12 keV) at the cluster core, which approximately corresponds to the adiabatic relation. An underdense gas sector, found in the ROSAT cluster image eastward of the core, has a higher temperature than average at that radius. At this higher temperature, the gas pressure in this sector is equal to that of the rest of the cluster at the same radius, but the specific entropy of this gas is significantly higher (although the temperature difference itself is only marginally significant). We speculate that the existence of this region of underdense high-entropy gas, as well as the adiabatic central temperature peak, indicate recent or ongoing heating of the intergalactic medium in this cluster. The most probable source of such heating is a subcluster merger, for which the hydrodynamic simulations predict a qualitatively similar temperature structure. We point out that entropy maps can provide a physically meaningful way of diagnosing merging clusters and comparing the predictions of merger simulations to the data.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 Is there molecular gas in the H I cloud between NGC 4472 and UGC 7636?(American Institute of Physics, 1997-05) Irwin, JA; Frayer, DT; Sarazin, CL; University of Toronto; University of Alabama TuscaloosaWe present CO observations of the H I cloud located between the Virgo elliptical galaxy NGC 4472 and its dwarf irregular companion galaxy UGC 7636. ROSAT observations of the X-ray halo of NGC 4472 show a hole in the X-ray emission at the position of the H I cloud (Irwin gi Sarazin 1996), If the hole is assumed to be the result of soft X-ray absorption by some absorbing material within the cloud, the total implied mass of the absorbing material is much larger than the measured H I mass of the cloud, This implies that a large fraction of the cloud is composed of molecular hydrogen. However, the CO observations fail to find the required amount of molecular gas if a Galactic CO-to-H-2 conversion factor is assumed. While a low density in the molecular gas might increase the CO-to-H-2 conversion factor enough to make the CO upper limit consistent with the X-ray absorption prediction too low a density would allow the H-2 to be dissociated by UV radiation. We conclude that it is unlikely that the required absorbing mass is present. In any case, these observations provide a strict upper limit on the CO content of the H I cloud. (C) 1997 American Astronomical Society.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 Low-mass X-ray binaries and globular clusters in early-type galaxies(University of Chicago Press, 2003-10-01) Sarazin, CL; Kundu, A; Irwin, JA; Sivakoff, GR; Blanton, EL; Randall, SW; University of Virginia; Michigan State University; University of Michigan System; University of Michigan; University of Alabama TuscaloosaA high fraction of the low-mass X-ray binaries (LMXBs) in early-type galaxies are associated with globular clusters (GCs). Here we discuss the correlations between LMXBs and GCs in a sample of four early-type galaxies with X-ray source lists determined from Chandra observations. There is some evidence that the fraction of LMXBs associated with GCs (f(X-GC)) increases along the Hubble sequence from spiral bulges ( or spheroids) to S0s to Es to cDs. On the other hand, the fraction of GCs that contain X-ray sources appears to be roughly constant at f(GC-X) similar to 4%. There is a strong tendency for the X-ray sources to be associated with the optically more luminous GCs. However, this correlation is consistent with a constant probability of finding an LMXB per unit optical luminosity; that is, it seems to result primarily from the larger number of stars in optically luminous GCs. The probability of finding a bright LMXB per unit optical luminosity in the GCs is about 1.5 x 10(-7) LMXBs per L-.,L-I for L-X greater than or similar to 1 x 10(38) ergs s(-1) (0.3 - 10 keV) and rises to about 2.0 x 10(-7) LMXBs per L-.; I at lower X-ray luminosities, L-X greater than or similar to 3 x 10(37) ergs s(-1). This frequency appears to be roughly constant for different galaxies, including the bulges of the Milky Way and M31. There is a tendency for the X-ray sources to be found preferentially in redder GCs, which is independent of optical luminosity correlation. This seems to indicate that the evolution of X-ray binaries in a GC is affected by either the metallicity or the age of the GC, with younger and/or more metal rich GCs having more LMXBs. There is no strong difference in the X-ray luminosities of GC and non-GC LMXBs. There is a weak tendency for the brightest LMXBs, whose luminosities exceed the Eddington luminosity for a 1.4 M-. neutron star, to avoid GCs. That may indicate that black hole X-ray binaries are somewhat less likely to be found in GCs, as seems to be true in our Galaxy. On the other hand, there are some luminous LMXBs associated with GCs. There is no clear evidence that the X-ray spectra or variability of GC and non-GC X-ray sources differ. We also find no evidence for a difference in the spatial distribution of GC and non-GC LMXBs. Many of these results are similar to those found in NGC 1399 and NGC 4472 by Angelini et al. and Kundu et al., respectively.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 ROSAT HRI X-ray observations of the open globular cluster NGC 288(IOP Publishing, 1999-10-10) Sarazin, CL; Irwin, JA; Rood, RT; Ferraro, FR; Paltrinieri, B; University of Virginia; University of Michigan System; University of Michigan; Istituto Nazionale Astrofisica (INAF); European Southern Observatory; Sapienza University Rome; University of Alabama TuscaloosaA ROSAT HRI X-ray image was obtained of the open globular cluster NGC 288, which is located near the South Galactic Pole. This is the first deep X-ray image of this system. We detect a low-luminosity globular cluster X-ray source (LLGCX) RX J005245.0-263449 with an X-ray luminosity of (5.5 +/- 1.4) x 10(32) ergs s(-1) (0.1-2.0 keV), which is located very close to the cluster center. There is evidence for X-ray variability on a timescale of less than or similar to 1 day. The presence of this LLGCX in such an open globular cluster suggests that dense stellar systems with high interaction rates are not needed to form LLGCXs. We also searched for diffuse X-ray emission from NGC 288. Upper limits on the X-ray luminosities are L-X(h) < 9.5 x 10(32) ergs s(-1) (0.52-2.02 keV) and L-X(s) < 9.3 x 10(32) ergs s(-1) (0.11-0.41 keV), These imply upper limits to the diffuse X-ray-to-optical light ratios in NGC 288, which are lower than the values observed for X-ray faint early-type galaxies. This indicates that the soft X-ray emission in these galaxies is due either to a component that is not present in globular clusters (e.g., interstellar gas or a stellar component that is not found in low-metallicity Population II systems) or to a relatively small number of bright low-mass X-ray binaries.Item Rosat X-ray colors and emission mechanisms in early-type galaxies(IOP Publishing, 1998-06-01) Irwin, JA; Sarazin, CL; University of Virginia; University of Alabama TuscaloosaThe X-ray colors and X-ray-to-optical luminosity ratios (L-X/L-B) of 61 early-type galaxies observed with the ROSAT PSPC are determined. The colors indicate that the X-ray spectral properties of galaxies vary as a function of L-X/L-B. The brightest X-ray galaxies have colors that are consistent with thermal emission from hot gas with roughly the same metallicity of 50% solar. The spatial variation of the colors indicates that the gas temperature in these galaxies increases radially. Galaxies with medium L-X/L-B also have spectral properties consistent with emission from hot gas. If a simple one-component thermal model is assumed to describe the 0.1-2.0 keV X-ray emission in these galaxies, then one possible explanation for the progressive decrease in L-X/L-B among galaxies of this class could be the progressive decrease in metal abundance of the X-ray-emitting gas contained by the galaxies. However, stellar X-ray emission may become a complicating factor for the fainter galaxies in this medium-L-X/L-B class. Galaxies with the lowest L-X/L-B values appear to be lacking a hot interstellar component. Their X-ray colors are consistent with those derived from the bulges of the spiral galaxies M31 and NGC 1291. In M31, the X-ray emission is resolved into discrete sources and is apparently due primarily to low-mass X-ray binaries (LMXBs). We therefore suggest that the bulk of the X-ray emission in the faintest elliptical galaxies is also due to LMXBs. Previously, the X-ray spectra of X-ray-faint galaxies had been found to be described by a hard component, which was attributed to LMXB emission, and a very soft component of unknown origin. We show that the very soft component also likely results from LMXBs, as a very soft component is seen in the X-ray spectra of the nearby LMXB Her X-1 and LMXBs in the bulge of M31. If the X-ray emission in X-ray-faint galaxies is primarily from stellar sources, then a range in L-X/L-B among these galaxies suggests that the stellar X-ray luminosity does not scale with optical luminosity, at least for galaxies of low optical luminosities. This could be the result of a decrease in the proportion of LMXBs with decreasing optical luminosity and/or the effects of fluctuations in the small number of LMXBs expected.Item ROSAT X-RAY OBSERVATIONS OF THE 2A-0335+096 CLUSTER OF GALAXIES(University of Chicago Press, 1995-12-20) Irwin, JA; Sarazin, CL; California Institute of Technology; University of Texas System; University of Texas Austin; University of Alabama TuscaloosaThe 2A 0335+096 cluster of galaxies was observed in X-rays with the Position Sensitive Proportional Counter (PSPC) and the High Resolution Imager (HRI) on ROSAT. Spatially resolved X-ray spectra show a decrease in the ambient temperature of the X-ray emitting gas in the inner regions of the cluster, indicative of a cooling flow. The spectra also confirm the presence of an absorbing column density in excess of the measured Galactic value toward the cooling how region, which must be intrinsic to the cluster. The mass of this absorbing matter is 7.4 x 10(11) M. within 170 kpc of the center of the cluster. A deprojection of the surface brightness profile combined with the observed temperature profile yields electron density, gas and gravitational mass, and mass deposition rate profiles. Within 1.2 Mpc the total gravitating mass is 1.9 x 10(14) M.. A cooling rate of 400(-65)(+90) M. yr(-1) is found within the cooling radius of 166(-18)(+28) kpc. This value is consistent with the cooling rate obtained by fitting a cooling flow model to the spectrum of the central 166 kpc. Although the cooling rate is proportional to radius for the inner 60 kpc, it is essentially constant beyond this point, which suggests that the gas cools homogeneously in this region. A similar result was found for the NGC 5044 group of galaxies by David et al. (1994). This disagrees with earlier observations of other clusters, which suggested that the gas within the entire cooling flow region was inhomogeneous.