ASCA SIS Research Articles

Constraining the role of Type Ia and Type II supernovae in galaxy groups by spatially resolved analysis of ROSAT and ASCA observations

We present the results of modelling the distribution of gas properties in the galaxy groups HCG 51, HCG 62 and NGC 5044, and in the poor cluster AWM 7, using both ASCA SIS and ROSAT data. The spectral quality of the ASCA data allows the radial distribution in the abundances of several elements to be resolved. In all systems apart from HCG 51, we see both central cooling flows and a general decline in metal abundances with radius. The ratio of iron to α-element abundances varies significantly, and, in comparison with theoretical supernova yields, indicates a significant contribution to the metal abundance of the intergalactic medium from Type Ia supernovae. This is seen both within the groups and throughout much of the cluster AWM 7. The total energy input into the IGM from supernovae can be calculated from our results, and is typically 20--40 per cent of the thermal energy of the gas, mostly from Type II supernovae. Our results support the idea that the SN II ejecta have been more widely distributed in the IGM, probably owing to the action of galaxy winds, and the lower iron mass-to-light ratio in groups suggests that some of this enriched gas has been lost altogether from the shallower potential wells of the smaller systems.

ASCAObservations of the Starburst‐driven Superwind Galaxy NGC 2146: Broadband (0.6–9 keV) Spectral Properties

We report ASCA GIS and SIS observations of the nearby (D=11.6 Mpc), nearly edge-on, starburst galaxy NGC 2146. These X-ray spectral data complement ROSAT PSPC and HRI imaging already discussed in the literature. The broadband (0.6-9 keV) X-ray spectrum of NGC 2146 is best described by a two-component model, the soft X-ray emission being modeled by a Raymond-Smith thermal plasma model with a temperature of kT~0.8 keV, while the hard X-ray emission is modeled by a thermal plasma model with kT~8 keV, or by a power-law model with a photon index of ~1.7. We do not find compelling evidence of substantial excess absorption above the Galactic value. The total luminosities of NGC 2146 in the soft (0.5-2.0 keV), hard (2-10 keV), and broad (0.5-10.0 keV) energy bands are ~1.3×1040, ~1.8×1040, and ~3.1×1040 ergs s-1 , respectively. The soft and hard thermal components provide about 30% and 70%, respectively, of the total luminosity in the 0.5-2.0 keV energy band, while in the 2-10 keV energy range only the hard component plays a major role. The spectral results allow us to set tighter constraints on the starburst-driven superwind model, which we show can satisfactorily account for the luminosity, mass, and energy content represented by the soft X-ray spectral component. We estimate that the mass outflow rate (~9 M☉ yr-1) is about an order of magnitude greater than the predicted rate at which supernova and stellar winds return mass into the interstellar medium, and therefore we argue that the flow is strongly "mass loaded" with material in and around the starburst. The estimated outflow velocity of the hot gas is close to the escape velocity from the galaxy, so the fate of the gas is not clear. We suggest that the hard X-ray spectral component is due to the combined emission of X-ray binaries and/or young supernova remnants associated with the starburst.

The Enrichment History of Hot Gas in Poor Galaxy Groups

We have analyzed the ASCA SIS and GIS data for 17 groups and determined the average temperature and abundance of the hot X-ray-emitting gas. For groups with gas temperatures less than 1.5 keV, we find that the abundance is correlated with the gas temperature and luminosity. We have also determined the abundance of the α-elements and iron independently for those groups with sufficient counts. We find that for the cool groups (i.e., kT<1.5 keV) the ratio of α-elements to iron is ~1, which is about half that seen in clusters. Spectral fits with the S, Si, and Fe abundances allowed to vary separately suggest the S/Fe ratio is similar to that seen in clusters, while the Si/Fe ratio in groups is half the value determined for richer systems. The mass of metals per unit of blue luminosity drops rapidly in groups as the temperature drops. There are two possible explanations for this decrease. One is that the star formation in groups is very different from that in rich clusters. The other explanation is that groups lose much of their enriched material via winds during the early evolution of ellipticals. If the latter is true, we find that poor groups will have contributed significantly (roughly one-third of the metals) to the enrichment of the intergalactic medium.

Rossi X‐Ray Timing ExplorerObservation of PSR B0656+14

PSR B0656+14 was observed by the Rossi X-Ray Timing Explorer (RXTE) with the proportional counter array (PCA) and the high-energy X-ray timing experiment (HEXTE) for 160 ks during 1997 August 22-September 3. No pulsation was firmly found in the timing analysis, during which the contemporaneous radio ephemeris and various statistical tests were applied in searching for evidence of pulsation. A marginal detection of pulsation at a confidence level of 95.5% based on the H test was found with data in the whole HEXTE energy band. In the energy band of 2-10 keV the RXTE PCA upper limits are about 1 order of magnitude lower than that from ASCA GIS data. If the Compton Gamma Ray Observatory EGRET detection of this pulsar is real, considering the common trait that most EGRET-detected pulsars have a cooling spectrum in hard X-ray and gamma-ray energy bands, the estimated RXTE upper limits indicate a deviation (low-energy turnover) from a cooling spectrum starting from 20 keV or higher. This in turn suggests an outer magnetospheric synchrotron radiation origin for high-energy emissions from PSR B0656+14. The RXTE PCA upper limits also suggest that a reported power-law component based on ASCA SIS data in 1-10 keV fitted jointly with ROSAT data, if real, should be mainly unpulsed.

The Active Corona of HD 35850 (F8 V)

We present Extreme Ultraviolet Explorer (EUVE) spectroscopy and photometry of the nearby F8 V star HD 35850 (HR 1817). The EUVE short-wavelength 75-175 A and medium-wavelength 160-365 A spectra reveal 28 emission lines from Fe IX and Fe XV to Fe XXIV. The Fe XXI λλ102, 129 ratio yields an upper limit for the coronal electron density, logne<11.6 cm−3. The EUVE short-wavelength spectrum shows a small but clearly detectable continuum. The 75-150 A line-to-continuum ratio indicates approximately solar Fe abundances, with 0.8<Z<1.6 (90% confidence interval). Upper limits have been derived for a dozen high-emissivity Fe X through Fe XIV lines. The resulting emission measure distribution is characterized by two broad temperature components at logT of 6.8 and 7.4. Over the course of the 1 week observation, large-amplitude, long-duration flares were not seen in the EUVE Deep Survey light curve, although the light curve does show signs of persistent, low-level flaring and possible rotational modulation. The EUVE spectra have been compared with nonsimultaneous ASCA SIS spectra of HD 35850 obtained in 1995. The SPEX DEM analysis of the SIS spectrum indicates the same temperature distribution as the EUVE DEM analysis. However, the SIS spectra suggest subsolar abundances, 0.34<Z<0.81. Although some of the discrepancy may be the result of incomplete X-ray line lists, we cannot explain the disagreement between the EUVE line-to-continuum ratio and the ASCA-derived Fe abundance. The X-ray surface flux on HD 35850 is comparable to that of cooler dwarfs of comparable age and rotation like EK Draconis (G0 V) and AB Doradus (K1 V). Given its youth (t≈100 Myr), its rapid rotation (vsini≈50 km s−1), and its high X-ray activity (LX≈1.5×1030 ergs s−1), HD 35850 may represent an activity extremum for single, main-sequence F-type stars. The variability and emission measure distribution can be reconstructed using the continuous flaring model of Gudel provided that the flare distribution has a power-law index α≈1.8. Similar results obtained for other young solar analogs suggest that continuous flaring is a viable coronal heating mechanism on rapidly rotating, late-type, main-sequence stars.

ASCA X-Ray Observations of Gamma Cassiopeiae

We present the results of X-ray observations of γ Cassiopeiae made with the ASCA satellite. Since the discovery of X-ray emission from this system, several possible models for the origin of X-rays have been proposed, including the corona of γ Cas, itself, a binary neutron-star companion, and a binary white-dwarf companion. However, there has been no clear way to distinguish which, if any, of these models is correct. In this paper we exploit the spectral resolution of the ASCA SIS detectors which, for the first time, enables the iron-line emission from this system to be studied in detail. The line and continuous spectrum of γ Cas is compared with the X-ray spectra of hot coronae of early type stars, low-mass X-ray binaries, high-mass X-ray binaries, and white-dwarf binaries. The observed properties of rapid fluctuations on timescales down to 10 s and a spectrum with a temperature of 10.6 keV, together with iron line emission, are very similar to those of binary white-dwarf systems. Assuming a white-dwarf binary origin for the X-rays, we estimated the luminosity of this system as a function of the binary separation using recent measurements of the circumstellar environment of γ Cas. Despite previous arguments that the X-ray luminosity is too large to be accounted for by accretion onto a white dwarf, we find that such a model is, in fact, consistent with the observed properties of the system.

Deep hard X-ray source counts from a fluctuation analysis of ASCA SIS images

An analysis of the spatial fluctuations in 15 deep ASCA SIS0 images has been conducted in order to probe the 2–10 keV X-ray source counts down to a flux limit ∼ 2 × 10−14 erg cm−2 s−1. Special care has been taken in modelling the fluctuations in terms of the sensitivity maps of every one of the 16 regions (5.6 × 5.6 arcmin2 each) into which the SIS0 has been divided, by means of ray-tracing simulations with improved optical constants in the X-ray telescope. The very extended ‘sidelobes’ (extending up to a couple of degrees) exhibited by these sensitivity maps make our analysis sensitive to both faint on-axis sources and brighter off-axis ones, the former being dominant. The source counts in the range (2−12) × 10−14 erg cm−2 s−1 are found to be close to a Euclidean form which extrapolates well to previous results from higher fluxes and are in reasonable agreement with some recent ASCA surveys. However, our results disagree with the deep survey counts by Georgantopoulos et al. The possibility that the source counts flatten to a sub-Euclidean form, as is observed at soft energies in ROSAT data, is only weakly constrained to happen at a flux < 1.8 × 10−12 erg cm−2 s−1 (90 per cent confidence). Down to the sensitivity limit of our analysis, the integrated contribution of the sources the imprint of which is seen in the fluctuations amounts to ∼ 35 ± 13 per cent of the extragalactic 2–10 keV X-ray background.

The Complex X-Ray Spectrum of 3C 273: ASCA Observations

The results obtained from 9 X-ray observations of 3C 273 performed by ASCA are presented (total exposure time ∼ 160000 s). The analysis and interpretation of the data was complicated by the fact that 4 of these observations were used for the on-board calibration of the CCDs spectral response. The present standard analysis shows that, in agreement with official recommendations, a conservative systematic error (at low energies) of 2–3 × 1020 cm−2 must be assumed when analyzing ASCA SIS data. A soft-excess, with variable flux and/or shape, has been clearly detected as well as flux and spectral variability. An anti-correlation has been found between the spectral index and the flux in the 2–10 keV energy range. Fitting the data with the latest available calibration matrices, we also detect an emission line at 5.4–5.7 keV (6.3–6.6 keV in the quasar frame) in only the two observations with lowest fluxes where it is weak (EW ∼ 20–30 eV), narrow and consistent with being produced by Fe Kα emission from neutral matter. Overall, the observations are qualitatively consistent with a variable, non-thermal X-ray continuum emission (a power law with Γ ∼ 1.6, possibly produced in the innermost regions of the radio-optical jet), plus underlying “Seyfert-like” features (a soft-excess and Fe Kα line emission). When the continuum (jet) emission is in a low state, the Seyfert-like spectral features (soft-excess, iron line, and possibly a steep power law plus reflection continuum) are more easily seen.

Supersoft Sources

Supersoft X-ray sources are a new class of luminous X-ray binaries discovered with the X-ray telescopes of the Einstein and ROSAT satellites and extensively studied in the optical with ground based telescopes, in the UV with IUE and HST and in X-rays with ROSAT, Beppo-SAX and ASCA (cf. Kahabka &amp; van den Heuvel 1997, van Teeseling 1997). The luminosities derived for a first sample of supersoft sources studied with moderate resolution X-ray spectroscopy (using Beppo-SAX LECS and ASCA SIS detectors, Parmar et al. 1997, Ebisawa et al. 1997) have been predicted to follow Iben’s stability line (Iben 1982), i.e. the location in the Hertzsprung-Russell diagram which separates the plateau phase from the cooling phase. This is not unreasonable as any system experiencing steady-state accretion, i.e. accretion at a rate equalling about the nuclear burning rate will be found close to the stability line. If the accretion rate exceeds this limit then the white dwarf gets bloated and disappears in X-rays. If the accretion rate falls below this limit the white dwarf envelope cools, the luminosity as well as the temperature ceases and the source enters unstable recurrent nuclear burning. From the population synthesis calculations of Yungelson (1996) follows that there exit for the Milky Way a few sources at any epoch which are more massive than 1.2 MQ. They are expected to be extremely X-ray bright and may be standard candles (cf. Table 1 and Figure 1 for the brightest known supersoft sources per galaxy Milky Way to NGC 55). Their spectral distribution is expected to be similar to that of the extremely hot galactic source RXJ0925.7-4758 (it peaks at 1 keV and the flux is distributed from 0.5 to 2 keV, see Figure 2 for the ASCA spectrum of RX J0925.7-4758 (and CAL 87) as derived by Ebisawa et al. 1997).

Complex Spectra of the Galactic Ridge X‐Rays Observed withASCA

X-ray spectra of the Galactic ridge emission in the Scutum arm region have been obtained with ASCA GIS and SIS in the energy range 0.7-10 keV. The observed spectra are basically of thermal emission from thin hot plasmas, and individual K emission lines from helium-like Mg, Si, S, and Fe ions are confirmed in both the GIS and SIS spectra. This means that the Galactic ridge X-ray emission cannot be explained by a single-temperature ionization-equilibrium plasma model. It cannot, however, be reproduced even if a nonequilibrium ionization model is introduced; thus multiple plasma components are required. The GIS spectra are fairly well fitted by a double-temperature nonequilibrium ionization plasma model with temperatures of kT ~ 0.8 keV and kT ~ 7 keV. The softer component is found to be in an extremely low ionization state, with net ~ 109 cm-3 s, while the harder component is in a relatively high ionization state, though not yet in a full equilibrium. The GRXE properties obtained with the GIS are carefully reexamined by the highly resolved spectral-line features with the SIS. The soft and hard components are absorbed by equivalent hydrogen columns of 0.7 × 1022 cm-2 and 4.6 × 1022 cm-2, respectively. The surface brightness of the soft and hard components at b ~ 0° are estimated to be 1.9 × 10-6 and 5.3 × 10-7 ergs cm-2 s-1 sr-1 respectively, both in the 0.5-10 keV band. The surface brightness of the softer component extends toward significantly higher (~2°) Galactic latitudes than the harder component, although their actual scale heights may be similar at ~100 pc if the differences in their observable depths are taken into account. Spectral properties of the two components are seen to depend on the latitude; the most noticeable effect is a rapid decrease in the Fe K line equivalent width seen in the hard component. Attempts are made to interpret the two components in terms of diffuse hot plasmas filling the interstellar space.

The 1993 August Minioutburst of GRO J0422+32

We report on observations of the soft X-ray transient (SXT) GRO J0422+32 made with the ROSAT Position Sensitive Proportional Counter (PSPC) and the ASCA GIS and SIS X-ray imaging spectrometers during its 1993 August outburst. This event occurred approximately one year after the primary outburst of the source and was characterized by an X-ray-to-optical luminosity ratio much smaller than that typically seen in SXT outbursts or in steady, low-mass X-ray binaries. Our X-ray observations with the ROSAT PSPC, spread over 6.2 days and totalling 28.8 ks, combined with our archival analysis of 17 ks of ASCA observations, indicate an approximately 1 mcrab source that turned and subsequently back on over a 10 day period, with distinct spectral characteristics in each case. In addition to our analysis of the X-ray spectra, we report on optical photometry and spectroscopy bracketing the X-ray observations. It is noteworthy that the optical light curves remain approximately constant, at least in our data's almost daily sampling, during the turning and turning on sequence seen in the X-rays. Optical spectroscopy reveals large, asymmetric redshifted and blueshifted components in the double-peaked emission lines He II λ4686 and Hβ λ4861, which can be interpreted as evidence for a wind emanating from the secondary. The phasing of these shifts ruled out a wind originating from the disk. On the other hand, the X-ray power illuminating the star seems insufficient for driving such a wind. Comparisons are made with the primary (1992) outburst and decay phase of GRO J0422+32. Implications for X-ray reprocessing scenarios of the production of optical and UV emission and for other possible scenarios, such as advection-dominated accretion, are briefly considered. An upper limit to the quiescent X-ray luminosity is estimated using data acquired during the X-ray off state. A similarity between the X-ray-to-optical behavior of GRO J0422+32 during this minioutburst and previously documented optical-to-EUV behavior of certain SU Ursae Majoris system outbursts is noted. A physical interpretation of this analogous behavior, suggesting that the hot and cold branches of the S-shaped stability curve may be relatively closely spaced in GRO J0422+32, is offered.

ITAL]ASCA[/ITAL] Observations of the Supernova Remnant VRO 42.05.01

We present the results of our ASCA SIS and GIS observations of the supernova remnant VRO 42.05.01. Our spectral fits indicate that the SNR is isothermal (~8.3 × 106 K), consistent with our ROSAT analysis results. The absorbing column density (~2.9 × 1021 cm-2) obtained from these spectral fits is much smaller than expected for the nominal distance of 5 kpc, which indicates that the line of sight toward VRO 42.05.01 has an unusually low gas density. The spectral resolution of ASCA allows us to determine elemental abundances for the hot X-ray-emitting plasma in the bright wing component of this remnant. We find that Mg, Si, and Fe are underabundant and attribute these low abundances to the Galactic metallicity gradient and to the location of the remnant in the outer Milky Way.

ASCA SIS X-ray Observations of the Wind Blown Bubble NGC 6888

AbstractWe present ASCA SIS observations of the wind-blown bubble NGC 6888. Because the ASCA SIS is sensitive to higher energy photons and has a higher spectral resolution compared to the ROSAT PSPC, we are able to detect a T ≈ 8×106 K plasma component besides the T ≈ 1.5×106 K component known from previous PSPC observations. The existence of a high-temperature component, the observed limb-brightened X-ray surface brightness profile, and the observed level of X-ray surface brightness cannot be satisfactorily explained by currently available models. Reducing heat conduction at the contact discontinuity may raise the central temperature and produce a limb-brightening; however, the expected X-ray surface brightness is still considerably higher than the observed surface brightness.

X-ray emission from comet Hale-Bopp

The discovery of X-ray emission from comets has created a number of questions about the physical mechanism producing the radiation. There are now a variety of explanations for the emission, from thermal bremsstrahlung of electrons off neutrals or dust, to charge exchange induced emission from solar wind ions, to scattering of solar X-rays from attogram dust, to reconnection of solar magnetic field lines. In an effort to understand this new phenomenon, we observed but failed to detect in the X-ray the very dusty and active comet C/Hale-Bopp 1995 O1 over a two year period, September 1996 to December 1997, using the ROSAT HRI imaging photometer at 0.1–2.0 keV and the ASCA SIS imaging spectrometer at 0.5–10.0 keV. The results of our Hale-Bopp non-detections, when combined with spectroscopic imaging 0.08–1.0 keV observations of the comet by EUVE and BeppoSAX, show that the emission has the same spectral shape and strong variability seen in other comets. Comparison of the ROSAT photometry of the comet to our ROSAT database of 8 comets strongly suggests that the overall X-ray faintness of the comet was due to an emission mechanism coupled to gas, and not dust, in the comet’s coma.

The X-Ray Spectrum of a Metal-poor Corona

We present an ASCA SIS spectrum of the Population II binary star HD 89499. This is the most metal poor star ([Fe/H] = -2.1) known to have a corona. The ASCA data confirm initial results from ROSAT which suggested a very hot, single-component plasma temperature, with the temperature derived from the ASCA data being slightly higher (2.6 × 107 K) than the one determined by ROSAT. The ASCA spectrum clearly shows the absence of emission lines and can be best modeled as thermal bremsstrahlung, although strong absorption at photon energies less than 0.5 keV is indicated. We suggest that such an extremely high coronal temperature results from the lack of an efficient cooling mechanism due to the absence of metallic (mostly Fe) emission lines.

Photoionization-Driven X-Ray Line Emission in Cygnus X-3

The ASCA SIS spectrum of the X-ray binary Cygnus X-3 is shown to contain a narrow recombination continuum of hydrogenic sulfur near 3.5 keV, which indicates that the temperature of the X-ray emission-line region is far below the coronal equilibrium temperature required to support S15+. The presence of this feature immediately confirms that X-ray photoionization produces extreme overionization in the circumsource medium, and that the X-ray emission features are energized by the hard X-ray continuum. Therefore, we conclude that spectral analyses of Cygnus X-3 and, most likely, other accretion-powered X-ray sources must be conducted in the context of plasma emission models that account for population kinetics dominated by recombination. Furthermore, the temperature dependence of the shapes of recombination continua provides a simple diagnostic of the electron temperature. We reanalyze the SIS data using a spectral model developed to account for recombination cascades, and, using these diagnostics, derive constraints on the temperatures in the S15+, Si13+, and Mg11+ ionization zones.

ASCA observations of the iron K complex of Circinus X-1 near zero phase: spectral evidence for partial covering

We report on ASCA energy spectra of Cir X-1 taken near its zero phase on 1994 August 4-5. The ASCA SIS detectors allow a more detailed study of the iron K complex than has been possible before. We find that prior to a sudden upward flux transition the dominant iron K feature appears to consist of a large edge from neutral or nearly-neutral iron. The depth of the edge corresponds to an absorption column of about 1.5E24 cm^{-2}, while little absorption over that expected from the Galaxy is seen at lower X-ray energies. The differential absorption at high and low X-ray energies combined with the iron edge energy are strong evidence that partial covering is a crucial determinant of the behaviour observed from Cir X-1. The continuum spectral variability observed by ASCA can also be understood naturally in terms of partial covering column changes. There is evidence for a relatively weak emission line from neutral iron with an equivalent width of only about 65 eV. After the flux transition, the strength of the edge feature is greatly reduced, suggesting a large reduction in the amount of partial covering. For a large region of statistically acceptable chi-squared parameter space, the luminosity of Cir X-1, after correction for partial covering, need not change during the transition. We discuss models for the partial covering and suggest that X-ray scattering by electrons may be important. Aspects of the Cir X-1 spectrum are very similar to those of Seyfert 2 galaxies with Compton-thin tori.

Corona(e) of AR Lacertae. I. The Temperature and Abundance Distribution

view Abstract Citations (92) References (43) Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS Corona(e) of AR Lacertae. I. The Temperature and Abundance Distribution Singh, K. P. ; White, N. E. ; Drake, S. A. Abstract X-ray spectra of the RS CVn binary AR Lac, obtained from simultaneous observations with the ROSAT PSPC and the ASCA SIS and GIS detectors, have been analyzed to study the coronal temperature and abundance distribution. The spectra were jointly fitted with plasma emission models to the following possible temperature distributions: (a) one with discrete multitemperature emission components, (b) a continuous emission measure with a power-law dependence on temperature, and (c) a continuous emission distribution parameterized by the sum of a sixth-order Chebyshev polynomial. We find that (i) solar abundance plasma models with either discrete or continuous emission measure (CEM) distributions are rejected, (ii) the best fit is obtained with a two-temperature (2T) plasma emission model with an underabundance of the elements 0, Mg, Si, S, Ar, Ca, and Fe by a factor of 3-4 relative to the solar photospheric values, and (iii) the best-fit CEM distribution also has similarly reduced abundances but fits the data less well than the 2T model. These results are confirmed even when the Fe-L region, which is subject to uncertainties in the atomic physics, is excluded from the fit. We consider optical depth effects as unlikely to be the explanation for the observed weakness of the line complexes relative to the continuum. Analysis of the spectral data during the primary and secondary eclipses shows that the emission measure of the high-temperature component in the 2T models appears to be more affected by the primary eclipse than the low-temperature component, suggesting that part of the former is concentrated in structures that are spatially more compact. Publication: The Astrophysical Journal Pub Date: January 1996 DOI: 10.1086/176695 Bibcode: 1996ApJ...456..766S Keywords: STARS: BINARIES: ECLIPSING; STARS: ABUNDANCES; STARS: CORONAE; STARS: INDIVIDUAL CONSTELLATION NAME: AR LACERTAE; X-RAYS: STARS full text sources ADS | data products SIMBAD (5) HEASARC (1) Related Materials (1) Part 2: 1996ApJ...473..470S

X-ray color analysis of the spectra of active galactic nuclei

The identification and detection of X-ray absorption and emission features depends on the resolution and the signal-to-noise ratio (S/N) of the observation, the understanding of the instrument response, and the Galactic line-of-sight absorption. Since many of the active galactic nucleus (AGN) data sets are limited in their S/N and full modeling of the physical conditions is rather complicated, we suggest a new analysis method based on "X-ray colors." The two sets of X-ray colors, defined for low (ROSAT PSPC) and medium (BBXRT and ASCA SIS) resolution experiments, are used to separate regions of different physical conditions in a two- dimensional color-color plane. They are similar but superior to previous methods using the X-ray "hardness ratio" in being able to reveal more of the physical properties of the source. We illustrate the use of such diagrams by studying a number of AGNs suspected of showing absorption features. A sample of 14 AGNs observed by the ROSAT PSPC is presented which includes several objects with suspected "warm absorbers" along the line-of-sight to the nucleus, several others exhibiting intrinsic continuum variations, and a number of control objects thought to be featureless. Our new observations show, for the first time, the color variation as a function of time for three of the Seyfert 1 sources: NGC 4051, Mrk 335, and Mrk 766. The variations suggest that in two sources we are witnessing real changes in continuum shape, while one (NGC 4051) is consistent with having a warm absorber. Four of the objects observed by BBXRT are reanalyzed using our X-ray colors. Out of these, we discuss in detail the case of NGC 4151 and show that the color-color analysis agrees very well with previous, detailed spectral fitting methods. In particular, we confirm that the observed BBXRT observation of this source is not consistent with the warm absorber explanation and must be interpreted as a combination of a variable hard component and another component. We explain the limitations in measuring low-energy emission features in ASCA SIS observations and present color-color diagrams which can be used as a powerful diagnostic tool in the analysis of data collected by this experiment.