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Using Chandra X-ray observations of young, post-merger elliptical galaxies, we present X-ray characteristics of age-related observational results by comparing them with typical old elliptical galaxies in terms of metal abundances in the hot interstellar matter (ISM). While the absolute element abundances may be uncertain because of unknown systematic errors and partly because of the smaller amount of hot gas in young ellipticals, the relative abundance ratios (e.g., the α-element to Fe ratio, and most importantly the Si/Fe ratio) can be relatively well constrained. In two young elliptical galaxies (NGC 720 and NGC 3923) we find that the Si to Fe abundance ratio is super-solar (at a 99% significance level), in contrast to typical old elliptical galaxies where the Si to Fe abundance ratio is close to solar. Also, the O/Mg ratio is close to solar in the two young elliptical galaxies, as opposed to the sub-solar O/Mg ratio reported in old elliptical galaxies. Both features appear to be less significant outside the effective radius (roughly 30'' for the galaxies under study), consistent with the observations that confine to the centermost regions the signatures of recent star formation in elliptical galaxies. Observed differences between young and old elliptical galaxies can be explained by the additional contribution from SNe II ejecta in the former. In young elliptical galaxies, the later star formation associated with recent mergers would have a dual effect, resulting both in galaxy scale winds—and therefore smaller observed amounts of hot ISM—because of the additional SN II heating, and in different metal abundances, because of the additional SN II yields.

We have studied the X-ray emission within the inner 150 pc radius of NGC 4151 by constructing high spatial resolution emission line images of OVII, OVIII, and NeIX. These maps show extended structures that are spatially correlated with the radio outflow and optical [OIII] emission. We find strong evidence for jet--gas cloud interaction, including morphological correspondences with regions of X-ray enhancement, peaks of near-infrared [FeII] emission, and optical clouds. In these regions, moreover, we find evidence of elevated NeIX/OVII ratios; the X-ray emission of these regions also exceeds that expected from nuclear photoionization. Spectral fitting reveals the presence of a collisionally ionized component. The thermal energy of the hot gas suggests that >0.1% of the estimated jet power is deposited into the host interstellar medium through interaction between the radio jet and the dense medium of the circum-nuclear region. We find possible pressure equilibrium between the collisionally ionized hot gas and the photoionized line-emitting cool clouds. We also obtain constraints on the extended iron and silicon fluorescent emission. Both lines are spatially unresolved. The upper limit on the contribution of an extended emission region to the Fe Kalpha emission is, Accepted for publication in ApJ. 28 pages, 9 figures

We have selected a sample of 30 normal (non-cD) early-type galaxies, for all of which optical spectroscopy is available and which have been observed with Chandra to a depth such as to ensure the detection of bright low-mass X-ray binaries (LMXBs) with LX > 1038 erg s–1. This sample includes a larger fraction of gas-poor galaxies than previously studied samples and covers a wide range of stellar luminosity (LK ), velocity dispersion (σ*), globular cluster specific frequency (SN ), and stellar age. We derive X-ray luminosities (or upper limits) from the different significant X-ray components of these galaxies: nuclei, detected and undetected LMXBs, coronally active binaries (ABs), cataclysmic variables (CVs), and hot gas. The ABs and CVs contribution is estimated from the LX -LK scaling relation of M31 and M32. The contribution of undetected LMXBs is estimated both by fitting the spectra of the unresolved X-ray emission and by extrapolating the LMXB X-ray luminosity function. On average, the X-ray luminosity of LMXBs is a factor of ~10 higher than that of ABs+CVs. By spectral fitting the emission (also considering gas emission in the regions of point sources), we estimate the contribution of the hot gas. We find our sample equally divided among galaxies with LX (gas) > LX (LMXB), LX (ABCV) ≤ LX (gas) ≤ LX (LMXB), and LX (gas) 240 km s–1, where it may be related to the analogous correlation found in cD galaxies and groups/clusters. However, the gas-poor galaxies with the shallowest potentials (σ* < 200 km s–1) also follow this relation, contrary to the expected anti-correlation in a simple outflow/wind scenario. Galaxies with intermediate values of σ* instead tend to have the same kT, while LX (gas) spans a factor of ~20; among these galaxies, we find a moderate, positive correlation between LX (gas) and the average stellar age, possibly suggesting a transition from halo retention to outflow caused by rejuvenated star formation associated with recent mergers.

A recent re-analysis of Einstein data, and new ROSAT observations, have revealed the presence of at least two components in the X-ray spectra of X-ray faint early-type galaxies: a relatively hard component (kT>1.5 keV), and a very soft component (kT\sim 0.2-0.3 keV). We address the problem of the nature of the very soft component, and whether it can be due to a hot interstellar medium (ISM), or is most likely originated by the collective emission of very soft stellar sources. To this purpose, hydrodynamical evolutionary sequences for the secular behavior of gas flows in ellipticals have been performed, and the results are compared with the observational X-ray data: the very soft component could be entirely explained with a hot ISM only in galaxies where the depth of the potential well is quite shallow, otherwise the softest contribution to the X-ray emission comes certainly from stellar sources. As stellar soft X-ray emitters, we consider late-type stellar corone, supersoft sources such as those discovered by ROSAT in the Magellanic Clouds and M31, and RSCVn systems. We finally present a model for the X-ray emission of NGC4365, to reproduce in detail the results of the ROSAT pointed observation (PSPC spectrum and radial surface brightness distribution)., Postscript file, 28 pages. For hardcopy of figures contact spellegr@eso.org. BAP 12-1993-040-DDA

The X-ray luminosity of galaxies is not dominated by their stellar coronal emission. However, stars are there and their presence must account for a fraction of the X-ray emission. Stellar coronal emission in the Galaxy may explain the Galactic Ridge. In nearby spirals, a fraction of the unresolved emission must be ascribed to stars. Even in more distant X-ray faint elliptical galaxies, there may ber evidence for stellar emission.

We use a long (300 ksec), continuous Suzaku X-ray observation of the active nucleus in NGC1365 to investigate the structure of the circumnuclear BLR clouds through their occultation of the X-ray source. The variations of the absorbing column density and of the covering factor indicate that the clouds surrounding the black hole are far from having a spherical geometry (as sometimes assumed), instead they have a strongly elongated and cometary shape, with a dense head (n=10^11 cm^-3) and an expanding, dissolving tail. We infer that the cometary tails must be longer than a few times 10^13 cm and their opening angle must be smaller than a few degrees. We suggest that the cometary shape may be a common feature of BLR clouds in general, but which has been difficult to recognize observationally so far. The cometary shape may originate from shocks and hydrodynamical instabilities generated by the supersonic motion of the BLR clouds into the intracloud medium. As a consequence of the mass loss into their tail, we infer that the BLR clouds probably have a lifetime of only a few months, implying that they must be continuously replenished. We also find a large, puzzling discrepancy (two orders of magnitude) between the mass of the BLR inferred from the properties of the absorbing clouds and the mass of the BLR inferred from photoionization models; we discuss the possible solutions to this discrepancy., Accepted for publication in A&A. 11 pages, 9 figures

X-ray observations give us a direct view of galaxian components that would otherwise remain unknown. Such are the end products of stellar evolution, the hot component of the interstellar medium, and phenomena related to nuclear activity. Here a few examples are presented of how the X-ray data can be used to constrain galaxy evolution and structural properties, when used together with data from other observational windows.

In order to understand the relationship between the different phases of the ISM and the role of the ISM in fueling radio continuum sources and star formation, we have compared observational data from a wide range of wavelengths.

This paper starts with a brief review of the status of our knowledge of the X‐ray properties of normal galaxies after the Einstein Observatory survey. It then summarizes some of the currently open issues on this subject, and addresses how substantial increases in sensitivity, angular resolution, bandwidth, spectral capabilities, and the capability to do timing observations are needed to further our knowledge of the X‐ray properties of galaxies. It concludes by proposing a large throughput X‐ray lunar observatory as a goal for the high‐energy astrophysical community, but this should be preceded by a series of increasingly ambitious missions.

In a 14 ks {ital ROSAT} Position Sensitive Proportional Counter pointing we detected an extended (28{times}40 kpc) halo of hot tenuous gas around NGC 3628, with a flat emission distribution and a temperature of {ital T}{approx_equal}2{times}10{sup 6} K. The soft X-ray luminosity of the halo gas in the 0.1{endash}2.0 keV band is {approximately}8{times}10{sup 39} ergss{sup {minus}1}, and the total soft X-ray luminosity of NGC 3628 is approximately 2.1{times}10{sup 40} ergss{sup {minus}1}. The most likely heating source for the halo gas are the shocks resulting from a supernova-driven outflow from the circumnuclear starburst propagating through the halo interstellar medium. Both the morphology and the line ratios of associated optical emission-line gas are consistent with this interpretation. In many respects the soft X-ray halo of NGC 3628 resembles those of other nearby, edge-on starburst galaxies like, e.g., M82 and NGC 253. Its size and radiative cooling time indicate that halos like that of NGC 3628 have to be considered as potentially important contributors to the population of quasi-stellar object absorption-line systems. With two exceptions, most of the compact X-ray sources seen in projection within the halo of NGC 3628 are probably background active galactic nuclei. The remaining two sources might be condensationsmore » in the shock-heated halo gas. The soft X-ray properties of a few sources in the disk of NGC 3628 are consistent with those of high-mass X-ray binaries or powerful young supernova remnants. The PSPC spectrum of NGC 3627, which was within the field of view during our observations, also exhibits the typical characteristics of a starburst galaxy. It can be approximated by a power-law + Raymond-Smith model with a total soft X-ray luminosity of {ital L}{sub {ital x}}=1.1{times}10{sup 40} ergss{sup {minus}1}. {copyright} {ital 1996 The American Astronomical Society.}« less

We report on the X-ray properties of Arp 22(#the most luminous object in the local universe and the nearest, brightest, and best-studied example of the class of "ultraluminous" infrared galaxies. New X-ray images and spectra obtained with ROSAT show that the X-ray emission in the 0.1-2.4 keV band is spatially extended with a size of ˜30 × 11 kpc, has a luminosity of between 4.3 × 1040 and 2.3 × 1041 ergs s-1 (depending on the amount of X-ray absorption), and can be fitted by a thermal spectrum with T ˜ 107 K. The total thermal energy associated with the hot gas is ˜2 x 1057 ergs. At most ˜20% of the observed X-ray emission can be associated with any single point source (e.g., a "buried" QSO). The X-ray nebula bears a strong morphological relationship to the expanding "double-bubble" system seen in optical emission-line images. We suggest that the X-ray and optical emission arise as the result of a bipolar "superwind" driven out from the nucleus by a starburst or a dust-shrouded QSO. A simple model of an expanding structure driven over a timescale of a few times 107 yr by a mechanical luminosity of ˜1043 ergs s-1 into a galactic halo with mean density ˜10-2 cm-3 satisfactorily accounts for all the observed global X-ray and optical properties of the nebula. Such a mechanical luminosity agrees with estimates for an ultraluminous starburst; however, a QSO-driven wind cannot be ruled out. We note that this energetic outflow in Arp 220 represents a local laboratory to study how newly formed galaxies and/or QSOs may have pumped energy and metals into the intergalactic medium at early epochs. Alternative models for the nebula are less attractive. Gas shock-heated as a result of a galactic merger will probably be too cool (T less than a few times 106 K) and would not be expected to have the regular, symmetric morphology we observe in Hα. Electron scattering of X-rays from a hidden QSO by warm (T < 106 K) halo gas underpredicts the total observed X-ray luminosity by more than an order of magnitude. However, it is possible that this mechanism may produce a bright central pair of pointlike X-ray sources that straddle the nucleus, which together account for ˜40% of the total X-ray emission. Our X-ray images also show a diffuse source located about 2' south-southwest of Arp 220. Optical images show that the source is probably associated with a group or poor cluster of galaxies at an estimated redshift of ˜0.1. This source contributes about half the emission seen in earlier Einstein X-ray observations of Arp 220.

After 12 years, during which its unabsorbed soft X-ray flux in the 0.1-2.0 keV band was almost constant at about f(sub x) approximately 10(exp -12) ergs/s/sq cm, the compact nuclear source in NGC 3628 was not detected in one of our ROSAT observations, with a limiting sensitivity of f(sub x) approximately 5 x 10(exp -14) ergs/s/sq cm. Our data can be explained in two ways. The source is either the most massive X-ray binary known so far, with a greater than and approximately equal to 75 solar mass black hole, or an unusual low-luminosity Active Galactic Nuclei (AGN). The X-ray spectrum is typical of a high-mass X-ray binary, while the luminosity of the source of L(sub x) is approximately equal to 10(exp 40) ergs/s is more similar to those of low-luminosity AGNs. If it is an AGN, variable obscuration might explain the observed light curve.

We have conducted bivariate and multivariate statistical analysis of data measuring the integrated luminosity, shape, and potential depth of the Einstein sample of early-type galaxies (presented by Fabbiano et al. 1992). We find significant correlations between the X-ray properties and the axial ratios (a/b) of our sample, such that the roundest systems tend to have the highest L(sub x) and L(sub x)/L(sub B). The most radio-loud objects are also the roundest. We confirm the assertion of Bender et al. (1989) that galaxies with high L(sub x) are boxy (have negative a(sub 4)). Both a/b and a(sub 4) are correlated with L(sub B), but not with IRAS 12 um and 100 um luminosities. There are strong correlations between L(sub x), Mg(sub 2), and sigma(sub nu) in the sense that those systems with the deepest potential wells have the highest L(sub x) and Mg(sub 2). Thus the depth of the potential well appears to govern both the ability to reatin an ISM at the present epoch and to retain the enriched ejecta of early star formation bursts. Both L(sub x)/L(sub B) and L(sub 6) (the 6 cm radio luminosity) show threshold effects with sigma(sub nu) exhibiting sharp increases at log sigma(sub nu) approximately = 2.2. Finally, there is clearly an interrelationship between the various stellar and structural parameters: The scatter in the bivariate relationships between the shape parameters (a/b and a(sub 4)) and the depth parameter sigma(sub nu) is a function of abundance in the sense that, for a given a(sub 4) or a/b, the systems with the highest sigma(sub nu) also have the highest Mg(sub 2). Furthermore, for a constant sigma(sun nu), disky galaxies tend to have higher Mg(sub 2) than boxy ones. Alternatively, for a given abundance, boxy ellipticals tend to be more massive than disky ellipticals. One possibility is that early-type galaxies of a given mass, originating from mergers (boxy ellipticals), have lower abundances than 'primordial' (disky) early-type galaxies. Another is that disky inner isophotes are due not to primordial dissipation collapse, but to either the self-gravitating inner disks of captured spirals or the dissipational collapse of new disk structures from the premerger ISM. The high measured nuclear Mg(sub 2) values would thus be due to enrichment from secondary bursts of star formation triggered by the merging event.

We have conducted bivariate and multivariate statistical analysis of data measuring the luminosity and interstellar medium of the Einstein sample of early-type galaxies (presented by Fabbiano, Kim, & Trinchieri 1992). We find a strong nonlinear correlation between L(sub B) and L(sub X), with a power-law slope of 1.8 +/- 0.1, steepening to 2.0 +/- if we do not consider the Local Group dwarf galaxies M32 and NGC 205. Considering only galaxies with log L(sub X) less than or equal to 40.5, we instead find a slope of 1.0 +/- 0.2 (with or without the Local Group dwarfs). Although E and S0 galaxies have consistent slopes for their L(sub B)-L(sub X) relationships, the mean values of the distribution functions of both L(sub X) and L(sub X)/L(sub B) for the S0 galaxies are lower than those for the E galaxies at the 2.8 sigma and 3.5 sigma levels, respectively. We find clear evidence for a correlation between L(sub X) and the X-ray color C(sub 21), defined by Kim, Fabbiano, & Trinchieri (1992b), which indicates that X-ray luminosity is correlated with the spectral shape below 1 keV in the sense that low-L(sub X) systems have relatively large contributions from a soft component compared with high-L(sub X) systems. We find evidence from our analysis of the 12 micron IRAS data for our sample that our S0 sample has excess 12 micron emission compared with the E sample, scaled by their optical luminosities. This may be due to emission from dust heated in star-forming regions in S0 disks. This interpretation is reinforced by the existence of a strong L(sub 12)-L(sub 100) correlation for our S0 sample that is not found for the E galaxies, and by an analysis of optical-IR colors. We find steep slopes for power-law relationships between radio luminosity and optical, X-ray, and far-IR (FIR) properties. This last point argues that the presence of an FIR-emitting interstellar medium (ISM) in early-type galaxies is coupled to their ability to generate nonthermal radio continuum, as previously argued by, e.g., Walsh et al. (1989). We also find that, for a given L(sub 100), galaxies with larger L(sub X)/L(sub B) tend to be stronger nonthermal radio sources, as originally suggested by Kim & Fabbiano (1990). We note that, while L(sub B) is most strongly correlated with L(sub 6), the total radio luminosity, both L(sub X) and L(sub X)/L(sub B) are more strongly correlated with L(sub 6 CO), the core radio luminosity. These points support the argument (proposed by Fabbiano, Gioia, & Trinchieri 1989) that radio cores in early-type galaxies are fueled by the hot ISM.

ROSAT Position Sensitive Proportional Counter (PSPC) observations of NGC 4636 have provided a deep image of the galaxy and its surroundings, which reveal the presence of emission well outside the optical galaxy. The source emission is measured out to r approximately 18 min, where the instrument support structure prevents us at this time from following it farther out. The nature of this emission is not fully understood, but could be evidence that the Virgo cluster gas extends as far out as this galaxy (greater than 3 Mpc from M87). Spectral analysis of the X-ray emission suggests a relatively cool (kT approximately 0.5 to 0.9 keV) interstellar medium, with temperatures increasing with radius. However, the detailed properties of the interstellar gas cannot be unambiguously determined with the present data, since the results we obtain depend strongly on the choice of the spectral model. Comparison of the spectral distributions in concentric annuli clearly indicates significant differences with radius, which can be parameterized as a general increase of the temperature. For low cosmic abundance models, kT varies from approximately 0.55 keV in the inner 1 min to approximately 0.8 keV at r approximately 6 to 8 min. Outside 8 min the average temperature is higher than in the inner region (kT approximately 0.8 to 1.2 keV) and the low-energy absorption is significantly lower. For 100% cosmic abundance and galactic line-of-sight absorption, multi-temperature fits are required, suggesting the possibility that the interstellar medi um is inhomogeneous.

The X-ray spectra of normal galaxies is investigated systematically by using the Imaging Proportional Counter (IPC) data in the Einstein data bank. In addition to the standard model fitting technique, X-ray colors are introduced in order to extract spectral information from the fainter X-ray sources. Spectral parameters for 43 galaxies and X-ray colors for 127 galaxies are presented.

An X-ray catalog and atlas of galaxies observed with the Einstein Observatory imaging instruments (IPC and HRI) are presented. The catalog comprises 493 galaxies, including targets of pointed observations, and RSA or RC2 galaxies serendipitously included in Einstein fields. A total of 450 of these galaxies were imaged well within the instrumental fields, resulting in 238 detections and 2123 sigma upper limits. The other galaxies were either at the edge of the visible field of view or confused with other X-ray sources. For these a rough measure of their X-ray emission is also given. The atlas shows X-ray contour maps of detected galaxies superposed on optical photographs and gives azimuthally averaged surface brightness profiles of galaxies detected with a high signal-to-noise ratio.

Observations of the cD galaxy NGC 1399 in the Fornax cluster of galaxies with Ginga have detected extended X-ray emission out to a radius of more than about 360 kpc. The energy spectrum of this emission is well fitted with an optically thin thermal spectrum with kT = 1.46 +0.05/-0.21 keV with a strong iron emission line corresponding to an iron abundance of 1.1 +1.3/-0.5 times solar. The mass of the hot gas responsible for X-ray emission is nearly the same as the total stellar mass of the cluster. Therefore the presence of iron at near-solar abundance suggests that the mass of the hot gas ejected from galaxies is comparable to the total stellar mass. This result indicates that most of the hot gas in a very poor cluster is created through ejections from galaxies, rather than being primordial.

Results of an analysis of the complete set of Einstein imaging observations of M31 are reported. One-hundred-eight individual X-ray sources are detected, with (0.2-4.0 keV) luminosities ranging from 5 x 10 exp 36 ergs/s to above 10 exp 38 ergs/s at the distance of M31. Fourteen of these sources are luminous enough to perform a spectral fit of the X-ray data. Fourteen sources are found to vary between Einstein observations, in addition to the two sources already reported by Collura, Reale, and Peres (1990). Contrary to earlier reports, no substantial difference is found between the luminosity distributions of X-ray sources in the bulge and in the disk. Contour maps of the X-ray emission in the central region is presented, and the radial distribution of the surface brightness is compared with the optical profiles. It is inferred that the disk is relatively underluminous in X-rays, relative to the stellar content. The spatial distribution of bulge sources is compared with that of optical novae and with emission-like images of the bulge. These comparisons constrain models of the formation of low-mass X-ray binaries.

New evidence is presented for collimated outflow from a starburst nucleus in the edge-on galaxy NGC 3628. A plume of X-ray emission along the minor axis of this galaxy is suggested by the distribution of counts in the softer energy channels of the Einstein Observatory IPC. Plumes of H-alpha emission are visible in a CCD image of the southern minor axis region, at a position angle consistent with that of the X-ray plume, while extended, possibly filamentary, H-alpha emission is visible in the north. Optical spectroscopy along the northern minor axis clearly detects line emission, with line ratios of H-alpha, N II, and S II changing from those characteristic of normal H II regions near the nucleus, to a low-ionization state, consistent with shock heating, farther out. All this evidence, and the IR and radio continuum properties of this galaxy, make NGC 3628 another member of the growing class of galaxies with starburst nuclear activity and outflowing winds, to which NGC 253 and M82 already belong.

The Einstein X-ray observations of the face-on spiral galaxy M101 are presented. The global X-ray luminosity L(x) of M101 is about 1.2 x 10 to the 40th ergs/s for D = 7.2 Mpc, consistent with the expected X-ray luminosity of normal spiral galaxies of its optical magnitude. The X-ray emission is mostly due to very luminous individual sources, with L(x) greater than 10 to the 38th ergs/s each, most likely very massive accreting binary systems. The data suggest a deficiency of sources in the luminosity range of L(x) from about 10 to the 37th to about 10 to the 38th ergs/s, which would indicate that the luminosity distribution of the X-ray sources in M101 might be different from that of M31 or M33.

Some results of an Einstein Observatory survey of X-ray emitting spiral galaxies are reported. The sources were normally 10-100 times further away than M31. Luminosities of 10 to the 38th to 10 to the 40th erg/sec were calculated and could have been generated by Pop I and Pop II binary systems, supernova remnants and small nuclear sources. Highly positive correlations were found among properties such as the integrated X-ray, optical and radio emissions. The data will also be compared with similar data from star burst and elliptical galaxies.

Imaging x-ray observations of a complete sample of 3CR radio galaxies led to the detection of 26 of them with (0.5–3.0 keV) Lx ∼10 41 -10 45 erg s −1 . A statistical analysis of the sample, in conjunction with radio and optical data reveals that nuclear activity is likely to be responsible for most of the x-ray emission. Moreover we find that the 3CR emission line galaxies are similar to both Seyfert galaxies and quasars with double radio morphology in their x-ray properties, strongly reinforcing an unified picture of active nuclei. The properties of double radio sources and flat spectrum quasars however differ. The non-emission line galaxies (like 3C264) are also likely to contain a nuclear x-ray source.

The Einstein Observatory, Exosat, and Ginga observations of X-rays from normal galaxies are reviewed. Local spirals and bright spiral galaxies are discussed, emphasizing the nature of the X-ray sources and addressing average sample properties and correlation with other wavebands. Starburst activity and low-activity nuclei are examined. The hot ISM, halos, and cooling flows in elliptical galaxies are addressed, and what X-ray observations reveal about the mass of early-type galaxies is considered. The relationship between radio sources and gaseous halos and between galaxies and the X-ray background is discussed.

Spectra of AM Herculis taken with IUE show a continuum which is partially eclipsed in phase with the X-ray eclipse and can be separated into two components; a black-body component (kTBB = 25 − 30 eV) which accounts for the ¼ keV X-rays and the eclipsed UV continuum; and a component roughly described by Fν ∂ ν−1 which is not eclipsed.

Hard X-ray emission from a 15 min by 1.5 deg area containing SS Cygni had previously been reported and identified with SS Cygni. This paper reports a 0.4 sq arc min measurement of the position of the X-ray source with the HEAO 1 modulation collimator experiment which leads to an unequivocal identification of SS Cygni as an X-ray emitter in the 2-10 keV energy range. The historical light curve in hard X-rays was also investigated by analyzing relevant Uhuru data, and two instances of possible source detection in the 2-6 keV band were noted. From Ariel 5 observations it is calculated that the X-ray luminosity of SS Cygni in the 2-10 keV energy range is of the order of 10 to the 33rd erg/sec in the optical low state and probably at least an order of magnitude less in the high state. By contrast, the soft X-ray luminosity increases by an order of magnitude between the optical low and high states. A coronal model or accretion onto the white dwarf can account for the inverse correlation of the soft and hard X-rays.

We have studied a sample of 81 E and SO galaxies (taken mostly from the literature) that were observed with the Einstein Observatory. Fifty-five galaxies are detected in X-rays, most of which come from a hot interstellar medium. It is possible that discrete sources make a significant contribution to the X-ray emission for 21 of the detected galaxies with the lowest X-ray luminosity, L X , for a given optical luminosity, L B - We examine the L X vs. L B distribution (see Figure 1), and derive approximate values of the central electron density, central cooling time and total mass in gas for the sample. Typical values are ~ 0.1cm−3, ~ 5 x 106 yr and 5 x 1O9M⊙, respectively. The short cooling times suggest the presence of cooling flows, and we consider heating by supernovae and by gravitational processes. Supernovae at the accepted rate would overproduce the observed X-ray luminosity: either the true rate is many times lower or the supernova energy is not well coupled to the hot gas. There are also difficulties in explaining the suppression of a strong galactic wind if supernova heating dominates, unless an external pressure confines the gas. Gravitational heating gives L X ∝ L B σ2, where σ is the line-of- sight velocity dispersion in the galaxy, and we find that this is roughly the case. Gravitational heating exceeds the mean observed L X by a factor of ~ 3 − 4 if one uses the accepted rate of stellar mass loss and assumes that the gas falls all the way to the center of the galaxy. This could be reconciled if the mass injection rate were lower or if matter drops out of the flow at all radii. The sample properties provide no additional information about the presence or absence of heavy halos in early type galaxies.

As for almost any other aspect of X-ray Astronomy, the sensitivity, the arcsecond angular resolution and the spectral resolution achieved with the Einstein Observatory are giving new surprising results in the observation of Supernova Remnants (SNR’s). We will concentrate here on the results obtained with the imaging experiments on the galactic SNR’s.

The results of an analysis of a sample of 84 elliptical and S0 galaxies, observed in X-rays with the Einstein Observatory and in radio continuum at 5 GHz, are reported. Radio flux densities result in some of the lowest radio powers yet reported for early-type galaxies. Radio structures extending beyond the optical radius are found only in galaxies with 5 GHz radio power greater than about 10 exp 29.5 ergs/s/Hz. Radio and X-ray luminosities are correlated, although with large intrinsic scatter, suggesting that more than one mechanism may be involved. A correlation between core radio power and the X-ray-to-optical ratio suggests a connection between the hot ISM and nuclear radio sources and points to accreting cooling flows as the fuel for the radio sources. For the same radio core power, extended radio lobes tend to be associated with galaxies with relatively smaller X-ray-to-optical ratios, pointing to the importance of the hot ISM in disrupting the radio jets and confining extended radio structures.

We report the discovery of a ultraluminous X-ray source (ULX; CXO J133815.6+043255) in NGC 5252. This ULX is an off-nuclear point-source, which is 22$^{\prime\prime}$ away from the center of NGC 5252, and has an X-ray luminosity of 1.5 $\times$ $10^{40}$erg s$^{-1}$. It is one of the rare examples of ULX, which exhibits clear counterparts in radio, optical, UV bands. Follow-up optical spectrum of the ULX shows strong emission lines. The redshift of [O III] emission line coincides with the systematic velocity of NGC 5252, suggesting the ULX is gravitationally bound to NGC 5252. The flux of [O III] appears to be correlated with both X-ray and radio luminosity in the same manner as ordinary AGNs, indicating that the [O III] emission is intrinsically associated with the ULX. Based on the multiwavelength data, we argue that the ULX is unlikely to be a background AGN. A more likely option is an accreting BH with a black hole mass of $\geq 10^4M_\odot$, which might be a stripped remnant of a merging dwarf galaxy., To appear in ApJ

The X-ray survey of the galaxies, 40 in all, was made with the Einstein Observatory. By comparing the distributions of X-ray luminosities, it is found that 3CR galaxies with double radio morphology (FR 2) and optical emission-line spectra tend to be the more powerful X-ray emitters, with broad-line galaxies at the top of the distribution. It is also found that the X-ray luminosity is strongly correlated with the 5 GHz radio nuclear luminosity. Through an analysis of the complete optical and radio sample with the Spearman partial rank correlation technique, it is determined that nuclear radio luminosity at 5 GHz is correlated with both total radio luminosity at 178 MHz and with galaxy optical luminosity. Other weaker correlations are found of the X-ray luminosity with the total radio luminosity at 178 MHz and the optical luminosity of the galaxy. The results are seen as underlining the importance of nuclear phenomena in radio galaxies and indicating a nuclear origin of their X-ray emission. In addition, it is found that the 3CR emission-line galaxies are similar to both Seyfert galaxies and quasars with double radio morphology in their X-ray properties, strongly reinforcing a unified picture of active nuclei.

In three years of observation, the X-ray source Cen X-3 appears to alternate between 'high states', with an intensity of 150 counts/s (2-6 keV) or greater, and 'low states', where the source is barely detectable. The time scale of this behavior is of the order of months, and no apparent periodicity has been observed. Analysis of two transitions between these states is reported. During two weeks in July 1972, the source increased from about 20 counts/s to 150 counts/s. The detailed nature of this turn-on is interpreted in terms of a model in which the supergiant's stellar wind decreases in density. A second transition, a turnoff in February 1973, is similarly analyzed and found to be consistent with a simple decrease in accretion rate. The presence of absorption dips during transitions at orbital phases 0.4-0.5 as well as at phase 0.75 is discussed. The data are consistent with a stellar-wind accretion model and with different kinds of extended lows caused by increased wind density masking the X-ray emission or by decreased wind density lowering the accretion rate.

X-ray observations of M81 made with the Einstein satellite are analyzed, and the results are reported. Eight bright sources other than the nucleus were detected, with luminosities in excess of about 2 x 10 to the 38th ergs/s. These are probably young, massive, accreting binary systems. A comparison of M81 with the morphologically similar galaxy M31 shows that the former is overluminous in both the radio continuum and the X-ray emission and to a lesser extent the far-infared emission, relative to the optical luminosity. This points to a difference in the star-formation histories of the two galaxies. The individual X-ray sources detected in M81 are all more luminous than the most luminous sources of M31. The nuclear source in M81 has a very soft and absorbed X-ray spectrum which cannot be explained by either inverse Compton radiation of the compact nuclear radio source or by an extension of the radio synchrotron emission.

The Tycho SNR was observed with the scanning modulation collimator experiment on board the HEAO 1 satellite. A soft extended source was detected. The extended X-ray source is best fitted with a thin spherical shell model of approximately 3.8 arc minute radius. The presence of a brighter, less extended structure in the western side of the remnant is suggested by our data.

The long- and short-term variability of the 4.8-s pulsation and the 2.1-day orbital periods of Centaurus X-3 are studied. The pulsation period decreases over 4 yr with a fractional change of -0.00028 per yr, but with rms fluctuations of 0.0002 s. In August-September 1972, a continuous transition from speedup to slowdown was observed. The orbital period also decreases over 4 yr with decrease of approximately 8 millionths per yr, and with significant fluctuations of the order of 0.00001 day over months. The orbital eccentricity is found to be about 0.0008. The pulsation-period variability is found to be consistent with a near balance between the Alfven and corotation radii in an accretion-disk model. The orbital-period variability is interpreted in terms of tidal circularization and possible mass transfer and loss.

The correlations between the emission in the far-IR, H-alpha, and blue in a sample of normal spiral galaxies are investigated. It is found that the luminosities in these three bands are all tightly correlated, although both the strength of the correlations and their functional dependencies are a function of the galaxies' morphological types. The best-fit power laws to these correlations are different for the comparison of different quantities and deviate significantly from linearity in some cases, implying the presence of additional emission mechanisms not related to the general increase of luminosity with galactic mass. Clear evidence is found of two independent effects in the incidence of warm far-IR emission in late-type spirals. One is a luminosity effect shown by the presence of excess far-IR relative to H-alpha or optical emission in the more luminous galaxies. The other is a dependence on widespread star-formation activity.

The results of X-ray observations with the Einstein Observatory of 33 galaxies are presented. Almost all of these galaxies, some of which lie in groups and in interactive pairs, display disturbed morphologies. No uniform, well-defined selection criterion can describe the sample galaxies, but almost all lie in a region of the optical color/color diagram that suggests that they are experiencing bursts of star formation. This region is compatible with the locus of Markarian galaxies. The observed sample and the X-ray data are presented, and the X-ray data are discussed and compared with the optical and radio properties of the galaxies. It is concluded that nuclear activity is not the predominant source of X-rays in late-type peculiar galaxies and that the X-ray emission is likely to originate from the Population I galactic component, in particular binary sources and possible young SNR's.

The results of X-ray observations carried out with the Einstein satellite of 14 nearby field spiral and irregular galaxies are reported. The integrated properties of the galaxies are examined in the luminosity range from 10 to the 38th and 10 to the 40th ergs per sec at 0.5-3.0 keV. It is found that the X-ray luminosities of the galaxies are correlated with both the optical (blue) and the radio luminosities (1.4 GHz). The linear relationship found between the X-ray and the optical blue luminosities suggests a relationship to be between the X-ray luminosity and the stellar content of spiral galaxies exhibiting blue luminosities. On the basis of the correlations, it is proposed that most of the spiral galaxies are relatively young systems of not less than about 10 to the 9th years. Data obtained from the Einstein satellite and those of a previous investigation of the blue galaxies are compared in a table.

X-ray observation of M83 with the Einstein Observatory indicates an extent and shape for this emission that are comparable to those in the blue band, although the inner 2-arcmin region shows a relative X-ray excess that may be accounted for by a clustering of bulge-type X-ray sources in the inner region of the plane of the galaxy. A lack of correlation in high resolution X-ray observations is noted between X-ray emission and the spiral arm pattern, suggesting that most of the X-ray emission from the plane of M83 is due to unresolved sources, belonging to the 'smooth' disk population, whose ages are in the 100 million-1 billion year range. Attention is given to six bright sources in the plane and nuclear region of M83, whose high X-ray luminosities suggest that they are close accreting binaries.

The detection of soft and hard X rays from VV Pup is reported, and an orbital light curve is presented for both. The light curves are found to be in good agreement with the orbital light curve observed in the optical (Warner and Nather, 1972), suggesting that both X rays and optical light arise from the vicinity of a bright spot near one magnetic pole of the white dwarf. The radius of the soft X-ray-emitting bright spot is in the range 20-120 km, and its temperature is in the range (2-6) x 10 to the 5th K. The observed ratio of soft X-ray to hard X-ray fluxes is approximately 15, the highest of all the AM Her stars.

A statistically significant association is found between X-ray bursters and ''soft'' X-ray transients. Some of the observational evidences about galactic X-ray sources are reviewed. The data support the picture of galactic X-ray sources as due to compact objects in a variety of binary system environments.

The results of a variability study of the X-ray sources detected by the Einstein Observatory in the galaxy M33 are reported. Two of the 15 known sources are variable above the 99.73 percent confidence level. The light curve of one of these sources, M33 X-7, exhibits a variability pattern of high and low states, suggesting an eclipsing binary X-ray source. Such a finding would be the first identification of a close accreting binary system with an X-ray source in an external galaxy other than the Magellanic Clouds. The data suggest a binary period of 1.7857 day and an eclipse duration of about 0.4 day. The nuclear source M33 X-8 varies only in the softest part of the spectrum. The observations suggest a rapid variability and show a rapid flare with a rise time shorter than three days together with longer timescale variability.

A previous analysis of the X-ray data on M33 has been extended to include a detailed study of the morpholgoy and spectral characteristics of the X-ray emission, and the results are reported. A low surface brightness, extended emission in the plane of the galaxy is detected. The X-ray luminosity of this component, about 10 to the 38th egs/s, is comparable to the total luminosity of the bright sources observed in the same region. Its radial distribution is similar to that of the blue light. The spectrum of the extended emission shows two distinct components: a hard one, with a temperature above 3 keV and a soft one with a temperature below 1 keV. The X-ray spectrum of the nuclear source, which is inconsistent with any of the known spectra of X-ray binary sources, can be fitted with either a low-temperature thermal emission or a steep power law model.