Jimmy A. Irwin

CHANDRA X-RAY OBSERVATIONS OF THE X-RAY FAINT ELLIPTICAL GALAXY NGC 4697

A 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 ~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 (~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 LX = 8 ? 1038 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 (?4 ? 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 LX(LMXB, 0.3-10 keV)/LB = 8.1 ? 1029 ergs s-1 L, which is about 35% higher than the value for the bulge of M31. Other comparisons suggest that there are significant variations (factor of 2) 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 ~5 ? 1037 to ~2.5 ? 1039 ergs s-1. The luminosity function of the LMXBs has a knee at 3.2 ? 1038 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.

X-Ray Spectral Properties of Low-Mass X-Ray Binaries in Nearby Galaxies

We 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 1039 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 Γ = 1.56 ± 0.02 (90% confidence), and using a thermal bremsstrahlung model yielded kTbrem = 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) × 1039 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 ~10-15 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 × 1039 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.

Low-Mass X-Ray Binaries and Globular Clusters in Early-Type Galaxies

A 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 (fX-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 fGC-X ~ 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 ? 10-7 LMXBs per L?,I for LX 1 ? 1038 ergs s-1 (0.3-10 keV) and rises to about 2.0 ? 10-7 LMXBs per L?,I at lower X-ray luminosities, LX 3 ? 1037 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.

X-Ray Evidence for the Interaction of the Giant Elliptical Galaxy NGC 4472 with its Virgo Cluster Environment

We analyze X-ray spatial and spectral data on the giant elliptical galaxy NGC 4472, the brightest galaxy in the Virgo cluster. The X-ray contours of NGC 4472 are elongated in the northeast-southwest direction, perhaps as a result of motion through the Virgo intracluster gas. A bow shock-like structure is evident on the galaxys north side. The temperature at a given radius in this bow shock region is slightly higher than the temperature at the same radius on the galaxys southwest side. Away from this bow shock region, the surface brightness profile of NGC 4472 can be traced out to a radius of 260 kpc in the southwest direction. Beyond 260 kpc, we find evidence for emission from both the Virgo cluster and the Galactic North Polar Spur (believed to be the rim of a hot Galactic superbubble). NGC 4472 is interacting with the dwarf irregular galaxy UGC 7636. We do not detect any excess or deficit in the X-ray emission toward this galaxy. An H i cloud, detected previously in the 21 cm line and located midway between the two galaxies, appears to have been removed from the irregular galaxy through either tidal interaction or ram pressure stripping. We find a marginally significant hole in the ROSAT HRI and PSPC X-ray images at the position of this cloud, suggesting that the cloud lies at the front side of NGC 4472. If the hole in the X-ray images is due to soft X-ray absorption, the total gaseous mass of the cloud must be at least 1.7 × 109 M☉, far greater than its 21 cm H I mass. This suggests that the majority of the material in the cloud is molecular.

Diffuse Gas and Low-Mass X-Ray Binaries in the Chandra Observation of the S0 Galaxy NGC 1553

We have spatially and spectrally resolved the sources of X-ray emission from the X-ray-faint S0 galaxy NGC 1553 using an observation from the Chandra X-Ray Observatory. The majority (70%) of the emission in the 0.3-10.0 keV band is diffuse, and the remaining 30% is resolved into 49 discrete sources. Most of the discrete sources associated with the galaxy appear to be low-mass X-ray binaries (LMXBs). The luminosity function of the LMXB sources is well fitted by a broken power law with a break luminosity comparable to the Eddington luminosity for a 1.4 M☉ neutron star. It is likely that those sources with luminosities above the break are accreting black holes, and those below are mostly neutron stars in binary systems. Spectra were extracted for the total emission, diffuse emission, and sum of the resolved sources; the spectral fits for all require a model including both a soft and hard component. The diffuse emission is predominately soft, while the emission from the sources is mostly hard. Approximately 24% of the diffuse emission arises from unresolved LMXBs, with the remainder resulting from thermal emission from hot gas. There is a very bright source at the projected position of the nucleus of the galaxy. The spectrum and luminosity derived from this central source are consistent with it being an active galactic nucleus (AGN); the galaxy also is a weak radio source. Finally, the diffuse emission exhibits significant substructure with an intriguing spiral feature passing through the center of the galaxy. The X-ray spectrum and surface brightness of the spiral feature are consistent with adiabatic or shock compression of ambient gas but not with cooling. This feature may be due to compression of the hot interstellar gas by radio lobes or jets associated with the AGN.

CLASSIFYING X-RAY SOURCES IN EXTERNAL GALAXIES FROM X-RAY COLORS

The X-ray populations of Local Group galaxies have been classified in detail by Einstein, ROSAT, and ASCA, revealing a mix of binaries, supernova remnants (SNRs), and H II regions. However, these observatories were unable to resolve X-ray sources in galaxies beyond the Local Group. With Chandras exquisite spatial resolution, we are able to resolve sources in a sample of nearby galaxies. We show that there are highly significant differences in the X-ray colors of sources in bulge and disk systems. In particular, we find that there is a population of X-ray-soft, faint sources in disk galaxies not seen in bulges and a smaller population of hard sources also seen preferentially in disk systems. These differences can be used as a basis to classify sources as low- and high-mass X-ray binaries, SNRs, and supersoft sources. We suggest that the soft sources seen preferentially in disks are probably dominated by SNRs, although we cannot rule out the possibility that they are a new population of absorbed, faint, supersoft accretion sources associated with the young stellar population. The hard sources seen in disks but not bulges we identify as high-mass X-ray binaries. While it is impossible to classify any individual source on the basis of X-ray color alone, the observations presented here suggest that it is possible to separate sources into groups dominated by one or two source types. This classification scheme is likely to be very useful in population studies, where it is crucial to distinguish between different classes of objects.The X-ray populations of Local group galaxies have been classified in detail by Einstein, ROSAT and ASCA revealing a mix of binaries, supernova remnants and HII regions. However, these observatories were unable to resolve X-ray sources in galaxies beyond the local group. With Chandras exquisite spatial resolution we are able to resolve sources in a sample of nearby galaxies. We show that there are highly significant differences in the X-ray colors of sources in bulge and disk systems. These differences can be used as a basis to classify sources as low and high mass X-ray binaries, supernova remnants and supersoft sources. While it is impossible to identify any individual source on the basis of X-ray color alone, the observations presented here suggest that it is possible to separate sources into groups dominated by a single type of source. This classification scheme is likely to be very useful in population studies, where it is crucial to distinguish between different classes of objects.

Radial Temperature Profiles of 11 Clusters of Galaxies Observed with BEPPOSAX

We 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 ~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.

Radial Temperature Profiles of X-Ray-emitting Gas within Clusters of Galaxies

Previous 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 flow 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.

Flows of X-ray gas reveal the disruption of a star by a massive black hole

Tidal forces close to massive black holes can violently disrupt stars that make a close approach. These extreme events are discovered via bright X-ray and optical/ultraviolet flares in galactic centres. Prior studies based on modelling decaying flux trends have been able to estimate broad properties, such as the mass accretion rate. Here we report the detection of flows of hot, ionized gas in high-resolution X-ray spectra of a nearby tidal disruption event, ASASSN-14li in the galaxy PGC 043234. Variability within the absorption-dominated spectra indicates that the gas is relatively close to the black hole. Narrow linewidths indicate that the gas does not stretch over a large range of radii, giving a low volume filling factor. Modest outflow speeds of a few hundred kilometres per second are observed; these are below the escape speed from the radius set by variability. The gas flow is consistent with a rotating wind from the inner, super-Eddington region of a nascent accretion disk, or with a filament of disrupted stellar gas near to the apocentre of an elliptical orbit. Flows of this sort are predicted by fundamental analytical theory and more recent numerical simulations.

O VI Observations of Galaxy Clusters: Evidence for Modest Cooling Flows

A 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 105.5 K emits in the lines of O VI λλ1032, 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 ± 6 M☉ yr-1 within the central r = 6.2 kpc region, while for Abell 1795 the central cooling rate is 26 ± 7 M☉ yr-1 (within r = 22 kpc), and about 42 ± 9 M☉ 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☉ yr-1 than to the originally suggested values of 102-10 3 M☉ yr-1.