Archival ROSAT Research Articles

X‐Ray Properties of B2 1028+313: A Quasar at the Center of the Abell Cluster A1030

X-ray observations with the ROSAT HRI and with ASCA are presented for the nearby radio quasar B2 1028+313, which is located in the cD galaxy at the center of the Abell cluster A1030. We also analyze archival ROSAT PSPC observations. We find that the X-ray emission is dominated by the quasar. The flux varied by a factor of about 2 between the ROSAT HRI and ASCA observations, which were about 1 yr apart. The X-ray spectrum of the quasar is fitted by a single power law, except at low energies where there is a soft excess. Although the shape of the soft excess is not strongly constrained, it can be fitted by a blackbody with a temperature of about 30 eV. There was evidence for extended X-ray emission, which contributed about 25% of the total flux. However, this emission does not appear to be normal X-ray emission from intracluster gas or a central cooling flow. The extended X-ray emission appears to be quite soft; if its spectrum is modeled as thermal emission, the temperature is ~0.2 keV, rather than the 5-10 keV expected for intracluster medium (ICM) emission. The radial surface distribution of the emission was not fitted by either the beta model that usually describes ICM emission or by a cooling-flow model. The ASCA and ROSAT spectra showed no convincing evidence for a thermal component with a cluster-like temperature, either in the overall spectral shape or in emission lines. In addition, the ROSAT PSPC image showed that the extended X-ray emission was highly elongated to the north-northwest and the south-southeast, in the same direction as the extended radio emission from the quasar. We suggest that the extended emission is inverse Compton emission from the extended radio lobes.

On the Nature of the X‐Ray Emission from M32

We have obtained the -rst broadband X-ray spectra of the nearby compact elliptical galaxy M32 by using the ASCA satellite. The extracted spectra and X-ray luminosity are consistent with the properties of the hard spectral component measured in giant elliptical galaxies believed to originate from X-ray binaries. Two ASCA observations were performed two weeks apart; a 25% Nux decrease and spectral softening occurred in the interval. We have also analyzed archival ROSAT HRI data, and discovered that the X-ray emission is dominated by a single unresolved source o†set from the nucleus of M32. We argue that this o†set, combined with the extremely rapid large magnitude variations, and hard X-ray spectrum combine to weakly favor a (single) X-ray binary over an AGN origin for the X-rays from M32. The nuclear black hole in M32 must be fuel-starved and/or accreting from a radiatively inefficient advection-dominated disk: the product of the accretion rate and the radiative efficiency must be less than D10~10 yr~1 if the X-ray source is indeed an X-ray binary. M _ Subject headings: galaxies: elliptical and lenticular, cD E galaxies: individual (M32) E galaxies: nuclei E X-rays: galaxies E X-rays: stars

Discovery of the 198 Second X‐Ray Pulsar GRO J2058+42

GRO J2058+42, a transient 198 second x-ray pulsar, was discovered by the Burst and Transient Source Experiment (BATSE) on the Compton Gamma-Ray Observatory (CGRO), during a "giant" outburst in 1995 September-October. The total flux peaked at about 300 mCrab (20-50 keV) as measured by Earth occultation. The pulse period decreased from 198 s to 196 s during the 46-day outburst. The pulse shape evolved over the course of the outburst and exhibited energy dependent variations. BATSE observed five additional weak outbursts from GRO J2058+42, each with two week duration and peak pulsed flux of about 15 mCrab (20-50 keV), that were spaced by about 110 days. An observation of the 1996 November outburst by the Rossi X-ray Timing Explorer (RXTE) Proportional Counter Array (PCA) localized the source to within a 4\arcmin radius error circle (90 % confidence) centered on R.A. = 20h 59m.0, Decl. = 41 deg 43 arcmin (J2000). Additional shorter outbursts with peak pulsed fluxes of about 8 mCrab were detected by BATSE halfway between the first four 15 mCrab outbursts. The RXTE All-Sky Monitor detected all 8 weak outbursts with approximately equal durations and intensities. GRO J2058+42 is most likely a Be/X-ray binary that appears to outburst at periastron and apastron. No optical counterpart has been identified to date and no x-ray source was present in the error circle in archival ROSAT observations.

ROSATTiming of the LMC Pulsar 0540−69

We present a timing study of the young rotation-powered pulsar 0540-69 in the Large Magellanic Cloud, based on 130 kiloseconds of archival ROSAT data spanning a $\sim 3$-year period. We use ``$f- \dot f$'' techniques to measure the pulsar frequency as a function of frequency derivative at 17 independent epochs. From these measurements we derive a timing solution with a braking index $n = 2.5^{+0.6}_{-0.7}$, and we compare this solution to previous timing studies of 0540-69. Using this frequency-based solution, we create 27 pulse profiles and perform a time-of-arrival (TOA) analysis to investigate further the pulsar's timing behavior. While we can successfully fit smooth spin-down models to subsets of the TOAs spanning up to 2 years, we are unable to obtain acceptable phase-coherent fits to the entire 3-year set of TOAs. This behavior provides the first clear evidence for timing noise in 0540-69. We discuss the implications of these results for understanding previous studies of the timing behavior of 0540-69.

X‐Ray Spectrum of the High‐Polarization Quasar PKS 1510−089

We present results from the X-ray spectra of the radio-loud, high-polarization quasar PKS 1510-089, based on new data obtained using ASCA and from archival ROSAT data. We find the X-ray spectrum obtained by ASCA to be unusually hard, with the photon index Γ = 1.30 ± 0.06, while the (nonsimultaneous) ROSAT data indicate a steeper spectrum with Γ = 1.9 ± 0.3. However, we find the X-ray flux at 1 keV to be within ~10% during both observations. Thus, we suggest the most likely explanation is that there is a break (with ΔΓ ≥ 0.6) in the underlying continuum at ~0.7 keV. Although the sample of high-polarization quasars for which high-quality X-ray spectra are available is small, flat X-ray spectra seem to be characteristic of these objects, and they also appear to be harder than that of the other radio-loud but low-polarization quasars. The multiwavelength spectrum of PKS 1510-089 is similar to many other γ-ray blazars, suggesting the emission is dominated by that from a relativistic jet. A big blue bump is also seen in its multiwavelength spectrum, suggesting the presence of a strong thermal component as well.

Ultraviolet Emission from the Liner Nucleus of NGC 6500

As a step towards clarifying the ionization mechanism of LINERs, we have used the Hubble Space Telescope Faint Object Spectrograph to obtain an ultraviolet spectrum of the nucleus of NGC 6500. Compared with more luminous Seyfert nuclei, the emission-line spectrum of NGC 6500 is remarkable for its low-excitation character: C III], C II], and Mg II are the strongest collisionally excited lines in the UV, and C IV is not detected. The fast shock models of Dopita & Sutherland (1995, 1996) are a poor match to the observed spectrum, as they predict much stronger emission than is observed for high-excitation lines such as C IV. Photoionization by an obscured nonstellar continuum source, or possibly ionization by slower (100 km/s) shocks, are more likely explanations for the emission-line ratios. The origin of the UV continuum is unclear; its overall spectral shape is reasonably well matched by spectral evolution models for single-burst populations with ages in the range 70-100 Myr, but no definite absorption features from young stars are present. Alternately, the UV continuum (or some fraction of it) may be scattered radiation from a hidden active nucleus. We tentatively detect a broad He II 4686 emission feature, which may be due to Wolf-Rayet stars, although the UV spectrum does not show any clear Wolf-Rayet signatures. We also find that NGC 6500 is weakly detected in an archival ROSAT HRI image with a luminosity of 5 x 10^40 erg/s in the 0.1-2.4 keV band, within the normal range for LINERs.

A 106 Day Period in the Nuclear Source X-8 in M33

With an X-ray luminosity of about 1039 ergs s−1, the source X-8, coincident with the optical center of M33, is the most luminous X-ray source in the Local Group. However, its nature remains a mystery. We present here new and archival ROSAT observations of X-8 spread over 6 yr that show variability and a periodicity of ~106 days. This implies that (most of) the emission from M33 X-8 arises from a single object, perhaps a binary system with a black hole primary of ~10 M☉. We suggest that the companion is a giant orbiting with a period of ~10 days and that the observed modulation is superorbital, analogous to that seen in Cyg X-2 and X1820-30.

The Southern SHARC Survey: the [CLC][ITAL]z[/ITAL][/CLC] = 0.3–0.7 Cluster X-Ray Luminosity Function

We present the z = 0.3-0.7 cluster X-ray luminosity function (XLF) determined from the Southern Serendipitous High-Redshift Archival ROSAT Cluster (SHARC) survey. Over the luminosity range L ~ (0.3-3) × 1044 ergs s-1 (0.5-2.0 keV), the XLF is in close agreement with that of the low-redshift X-ray cluster population. This result greatly strengthens our previous claim of no evolution of the cluster population, at these luminosities, at a median redshift of z=0.44.

X-ray Emissions from Comets Detected in the Röntgen X-ray Satellite All-Sky Survey

After the unexpected discovery of x-rays emitted from comet C/1996 B2 (Hyakutake) with the Röntgen X-ray Satellite (ROSAT) in March 1996, x-ray emissions from comets C/1990 K1 (Levy), C/1990 N1 (Tsuchiya-Kiuchi), 45P (Honda–Mrkos–Pajdušáková), and C/1991 A2 (Arai), optically 300 to 30,000 times fainter than Hyakutake, were discovered in archival ROSAT data. These findings establish comets as a class of x-ray sources and allow their properties to be studied over a wide optical brightness range. The results indicate that charge exchange between highly charged heavy ions in the solar wind and cometary neutrals is the dominant process for the x-ray emission. Comets may thus be used as probes for monitoring the heavy-ion content of the solar wind.

The nature of the X-ray point source in the bar of NGC 6822.

We have analysed archival ROSAT PSPC and Einstein HRI data for the x-ray point source in the bar of NGC 6822. The source decreased in x-ray flux by at least half an order of magnitude in the 13 years between the HRI and PSPC observations. It has a PSPC flux of f_X = 8 x 10E-14 erg/cm^2/s, giving L_X = 2.4 x 10E36 erg/s for a distance of 500 kpc. The source appears to be related to the optical emission-line object Ho 12. It is unclear if Ho 12 is an HII region or a supernova remnant, although the latter interpretation is better supported by the available optical data. The x-rays are unlikely to be direct emission from a supernova remnant, due to the observed strong x-ray variability. The x-ray spectrum of the source is very soft. Acceptable fits are found for either a Raymond-Smith thermal model with kT = 0.56 keV, or a blackbody model with kT = 0.06 keV. The latter model would place the source amongst the class of Super-Soft x-ray sources first found in the Magellanic Clouds. If the thermal model is correct, the source appears similar to Galactic and Magellanic Cloud stellar-mass black hole binary candidates. We have identified a list of potential optical counterparts from published photometry. However, for either model discussed above, we expect the apparent magnitude of the optical counterpart to be in the range 21.5 < V < 24.0. Thus the optical counterpart may be below the limit of published photometry.

ROSATObservations of the Sombrero Galaxy: Discovery of an X‐Ray Active Nucleus

The analysis of a high-resolution X-ray image of NGC 4594, obtained with the ROSAT HRI, shows at least three components of the X-ray emission. The most striking one is a pointlike source associated with the LINER nucleus, with (0.1-2.4) keV LX ~ 3.5 × 1040 ergs s-1. Comparison of the X-ray, Hα, and radio emission of this source with those of active galactic nuclei suggests that it may be a low-luminosity version of the latter. If the X-ray emission is due to accretion onto the 5 × 108 M☉ black hole that may be present at the nucleus of NGC 4594, the accretion rates must be very low, since the luminosity is extremely sub-Eddington. Alternatively, the source may be heavily obscured and only reprocessed photons are detected in the ROSAT range. We also detect clumpy emission associated with the disk of NGC 4594, and diffuse (or not resolved) emission from the bulge. With the present data, including a reanalysis of the archival ROSAT PSPC observation of this galaxy, we cannot determine how much of this diffuse emission is due to a hot interstellar medium, and how much is instead due to the integrated emission of a population of evolved stellar sources.

Properties of Warm Absorbers and Constraints from their Optical and UV Line Emission

We have studied the warm absorbers in several Seyfert galaxies using both PI and archival ROSAT data. The parameters that govern the emissivity of the ionized material, ionization parameter U, warm-absorber column density Nwa, and the source intrinsic power-law photon index Γx, are determined from X-ray spectral fits. These are then used to predict the expected IR to UV line emission of the ionized material in the individual objects. In particular, the possibility of a warm-absorber origin of one of the known emission-line regions in AGN is assessed. Good ‘candidate components’ are, e.g., the coronal-line region, and/or the high-ionization component of the NLR, or the component responsible for broad wings in the Balmer lines. All calculations have been carried out using the code CLOUDY (Ferland 1993).

X-Ray Emission from M32: X-Ray Binaries or a μAGN?

We have analyzed archival ROSAT PSPC data for M32 in order to study the X-ray emission from this nearest elliptical galaxy. We fitted spectra from three long exposures with Raymond-Smith, thermal bremsstrahlung, and power-law models. All models give excellent fits. The thermal fits have kT ≈ 4 keV, the Raymond-Smith iron abundance is 0.4−0.3+0.7 solar, the power-law fit has α = 1.6 ± 0.1, and all fits have NH consistent with the Galactic column. The source is centered on M32 to an accuracy of 9'', and unresolved at 27'' FWHM (~90 pc). M32 is X-ray variable by a factor of 3-5 on timescales of a decade down to minutes, with evidence for a possible period of ~1.3 days.

A Possible X-Ray Detection of the Binary Millisecond Pulsar J1012+5307

A possible X-ray detection of the newly discovered binary millisecond radio pulsar PSR J1012+5307 was obtained from an archival ROSAT observation. The 80 +/- 24 photons detected correspond to a 0.1 - 2.4 keV luminosity of approx. = 2.5 x 10(exp 30) erg/s at the nominal dispersion-measure distance of 520 pc. This luminosity is a factor of 2 less than that of PSR J0437-4715, a near twin of PSR J1012+5307 in its spin parameters and energetics, and the only millisecond pulsar from which pulsed X-rays have definitely been detected. PSR J1012+5307 is also within 6 deg of the HI hole in Ursa Major, providing a new estimate of the electron column density through this region which confirms that the ionized column density is also low. The small neutral column density to PSR J1012+5307, N(sub H) less than 7.5 x 10(exp 19)/sq cm, will facilitate future soft X-ray study, which will help to discriminate between thermal and nonthermal origins of the X-ray emission in millisecond pulsars.

AB Dor: A single star with RSCVn like activity in X-ray band

Using the archival ROSAT PSPC observations, AB Dor is found to be variable in X-rays. The periodic variations are consistent with previously reported rotational period of 0d.514. The average spectrum of AB Dor is best represented with two-temperature Raymond-Smith model with kT values of 0.19±0.07 and 1.17±0.02 keV. The quiescent luminosity of the system is found to be 4.36±0.6×1030 ergs s−1. A flare with a rise time of ∼350 seconds is detected during which X-ray luminosity rises from 5.8±1.6×1030 to 15.8±4.9×1030 ergs s−1. We conclude that AB Dor is very similar to the active components of RS CVn binaries and other active classes. In view of the wide separation from the binary companion Rst 137B, this activity must be intrinsic to the active star.

X-Ray Spectra of BL Lacertae Objects from the ROSAT Archive

Our sample comprises all BL Lac objects listed in the catalogue of Véron-Cetty &amp; Véron (1993) and which are detected in a ROSAT PSPC observation with at least 50 source counts: 74 objects in total. We reduced the data from the ROSAT archives at MPE and GSFC and fitted single power-law models with photoelectric absorption to the spectra. We calculated the broad band spectral indices αrx, αro, and αox from the ROSAT 1 keV fluxes, 5 GHz radio, and optical V band fluxes (Véron-Cetty &amp; Véron 1993).

A Collision of Subclusters in Abell 754

We present direct evidence of a collision of subclusters in the galaxy cluster Abell 754. Our comparison of new optical data and archival ROSAT PSPC X-ray data reveals three collision signatures predicted by n-body/hydrodynamical simulations of hierarchical cluster evolution. First, there is strong evidence of a nonhydrostatic process; neither of the two major clumps in the galaxy distribution lies on the off-center peak of the X-ray emission from the intracluster gas. Second, the peak of the X-ray emission is elongated perpendicular to the collision axis defined by connecting the centroids of the two galaxy clumps. Third, there is evidence of compression-heated gas; one of A754's two X-ray temperature components (Henry & Briel 1995) is among the hottest observed in any cluster and hotter than that inferred from the velocity dispersion of the associated galaxy clump. These signatures are consistent with the qualitative features of simulations (Evrard 1990a, b) in which two subclusters have collided in the plane of the sky during the last ~1 Gyr. The detection of such collisions is crucial for understanding both the dynamics of individual clusters and the underlying cosmology. First, for systems like A754, estimating the cluster X-ray mass from assumptions of hydrostatic equilibrium and isothermality is incorrect and may produce the discrepancies sometimes found between X-ray masses and those derived from gravitational lens models (Babul & Miralda-Escude 1994). Second, the fraction of nearby clusters in which subclusters have collided in the last ~1 Gyr is especially sensitive to the mean mass density parameter Ω0 (see Richstone, Loeb, & Turner 1992; Evrard et al. 1993; Lacey & Cole 1993). As we show for A754, we now have the means to identify recent collisions. With a large, well-defined cluster sample, it will be possible to place a new and powerful constraint on cosmological models.

RX J0045.4+4154: A recurrent supersoft x-ray transient in M31

Using data extracted from the ROSAT archive we have discovered a recurrent supersoft X-ray transient RX J0045.4+4154 in M31. The first outburst began on 1992 February 2 and continued for at least 4 days, until the end of the observation sequence. A second outburst that lasted more than 6 days was seen to begin on 1993 January 7. The X-ray spectrum on both occasions yields a characteristic blackbody temperature of approximately 90 eV. For a range of plausible continuum models, the hydrogen column density is (0.8-1.5) x 10(exp 21)/sq cm and is consistent with the source being located in M31. This implies an unabsorbed 0.1-2.0 keV peak luminosity of approximately 10(exp 38) ergs/sec. This is the first recurrent X-ray transient to be found in M31 and is particularly notable because it is much softer than the bright X-ray transients seen in our Galaxy. The spectrum is characteristic of the supersoft class of X-ray sources, which are thought to be accreting white dwarfs that have a hydrogen-burning surface layer. A fit to a white dwarf model atmosphere gives a temperature of 10(exp 6)K, the hottest found so far. This high temperature is consistent with a white dwarf mass of 1.3-1.4 solar mass, approaching the Chandrasekhar limit, and burning close to the nuclear stability limit.

ROSAT observations of compact groups of galaxies

We have systematically analyzed a sample of 13 new and archival ROSAT PSPC observations of compact groups of galaxies: 12 Hickson Compact Groups plus the NCG 2300 group. We find that approximately two-thirds of the groups have extended X-ray emission and, in four of these, the emission is resolved into diffuse emission from gas at a temperature of $k$T $\sim 1$ keV in the group potential. All but one of the groups with extended emission have a spiral fraction of less than 50\%. The baryon fraction of groups with diffuse emission is 5--19\%, similar to the values in clusters of galaxies. However, with a single exception (HCG 62), the gas-to-stellar-mass ratio in our groups has a median value near 5\%, somewhat greater than the values for individual early-type galaxies and two orders of magnitude lower than in clusters of galaxies. The X-ray luminosities of individual group galaxies are comparable to those of similar field galaxies, although the L$_X$-L$_B$ relation for early-type galaxies may be flatter in compact groups than in the field.

OeBe-stars with possible compact companions observed at X-rays

A survey of bright O- and B-type stars has been conducted from the ROSAT All Sky Survey data, aiming at the detection of compact companions expected for Oe and Be stars. Concurrent Hα spectroscopy has been used to establish the level of activity of these stars around the time of the X-ray observations. The detection rate of OB and OeBe stars as well as their average X-ray over bolometric luminosities suggest little difference between these two groups of objects. Instead of accretion onto a compact object (NS, WD), the X-ray emission from OeBe stars may be produced by stellar winds as has been proposed for O- and early B-type stars in general. Flexibilities in the available models for X-ray emission from accreting WDs may allow the X-ray emission from such objects to remain below a detectable level. These investigations are now being extended to pointed observations in the ROSAT Archive.