ROSAT Position Sensitive Proportional Counter Research Articles

Optical discovery and multiwavelength investigation of supernova remnant MCSNR J0512– 6707 in the Large Magellanic Cloud

We present optical, radio and X-ray data that confirm a new supernova remnant (SNR) in the Large Magellanic Cloud (LMC) discovered using our deep H-alpha imagery. Optically, the new SNR has a somewhat filamentary morphology and a diameter of 56 x 64 arcsec (13.5 x 15.5 pc at the 49.9 kpc distance of the LMC). Spectroscopic follow-up of multiple regions show high [SII]/H-alpha emission-line ratios ranging from 0.66+/-0.02 to 0.93+/-0.01, all of which are typical of an SNR. We found radio counterparts for this object using our new Australia Telescope Compact Array (ATCA) 6cm pointed observations as well as a number of available radio surveys at 8 640 MHz, 4 850 MHz, 1 377 MHz and 843 MHz. With these combined data we provide a spectral index (alpha) = -0.5 between 843 and 8640 MHz. Both spectral line analysis and the magnetic field strength, ranging from 124 - 184 mG, suggest a dynamical age between 2,200 and 4,700 yrs. The SNR has a previously catalogued X-ray counterpart listed as HP 483 in the ROSAT Position Sensitive Proportional Counter (PSPC) catalogue.

X-ray photometry

Abstract I describe a method for synthesizing photometric passbands for use with current and future X-ray instruments. The method permits the standardization of X-ray passbands and thus X-ray photometry between different instruments and missions. The method is illustrated by synthesizing a passband in the XMM–Newton European Photon Imaging Camera pn which is similar to the ROSAT Position Sensitive Proportional Counter 0.5–2 keV band.

Radio observations of CTB80: detection of the snowplough in an old supernova remnant

New radio continuum and H i line observations of the old supernova remnant CTB80 are analysed. The radio continuum emission is more extended than previously known. We analyse archival ROSAT Position Sensitive Proportional Counter images of CTB80 and find extended X-ray emission associated with CTB80 over a 1°.2 diameter region. A new H i absorption spectrum towards the core of CTB80 yields a kinematic distance of ≃1.5 kpc. An analysis of the H i channel maps confirms the inner shell discovered by Koo et al. In addition, we discover an outer slowly moving shell centred on CTB80’s centre, with radius 76 arcmin and velocity 40 km s−1. The shell’s size and velocity are inconsistent with a stellar wind origin, but are consistent with what is expected for a cool dense shell in the cooling (snowplough) phase of a supernova remnant. Taking into account deceleration of the shell, its age also agrees with that of CTB80 (∼60 000 yr). The resulting picture of CTB80 is that of a large old supernova remnant, with an H i shell moving out slowly behind the slow outer shock and a hot interior which is still emitting X-rays and visible in the ROSAT images.

HARD X-RAY EMISSION FROM THE NGC 5044 GROUP

Observations made with the Rossi X-ray Timing Explorer Proportional Counter Array (PCA) to constrain the hard X-ray emission in the NGC 5044 group are reported here. Modeling a combined PCA and ROSAT position sensitive proportional counter spectrum with a 0.5–15 keV energy range shows excess hard emission above 4 keV. Addition of a power-law component with a spectral index of 2.6–2.8 and a luminosity of 2.6 × 1042 erg s−1 within 700 kpc in the observed energy band removes these residuals. Thus, there is a detection of a significant non-thermal component that is 32% of the total X-ray emission. Point-source emission makes up at most 14% of the non-thermal emission from the NGC 5044 group. Therefore, the diffuse, point-source-subtracted, non-thermal component is (2.2–3.0) × 1042 erg s−1. The cosmic-ray electron energy density is 3.6 × 10−12 erg cm−3 and the average magnetic field is 0.034 μG in the largest radio emitting region. The ratio of cosmic-ray electron energy density to magnetic field energy density, ∼2.5 × 104, is significantly out of equipartition and is therefore atypical of radio lobes. In addition, the group's small size and low non-thermal energy density strongly contradicts the size–energy relationship found for radio lobes. Thus, it is unlikely related to the active galaxy and is most likely a relic of the merger. The energy in cosmic rays and magnetic field is consistent with simulations of cosmic-ray acceleration by merger shocks.

SHOCK SPEED, COSMIC RAY PRESSURE, AND GAS TEMPERATURE IN THE CYGNUS LOOP

Upper limits on the shock speeds in supernova remnants can be combined with post-shock temperatures to obtain upper limits on the ratio of cosmic ray to gas pressure (P CR/PG ) behind the shocks. We constrain shock speeds from proper motions and distance estimates, and we derive temperatures from X-ray spectra. The shock waves are observed as faint Hα filaments stretching around the Cygnus Loop supernova remnant in two epochs of the Palomar Observatory Sky Survey (POSS) separated by 39.1 years. We measured proper motions of 18 nonradiative filaments and derived shock velocity limits based on a limit to the Cygnus Loop distance of 576 ± 61 pc given by Blair et al. for a background star. The Position Sensitive Proportional Counter (PSPC) instrument on-board ROSAT observed the X-ray emission of the post-shock gas along the perimeter of the Cygnus Loop, and we measure post-shock electron temperature from spectral fits. Proper motions range from 27 to 54 over the POSS epochs and post-shock temperatures range from kT ~ 100-200 eV. Our analysis suggests a cosmic ray to post-shock gas pressure consistent with zero, and in some positions P CR is formally smaller than zero. We conclude that the distance to the Cygnus Loop is close to the upper limit given by the distance to the background star and that either the electron temperatures are lower than those measured from ROSAT PSPC X-ray spectral fits or an additional heat input for the electrons, possibly due to thermal conduction, is required.

X-ray spectrum variations in the south-west Cygnus Loop

The Cygnus Loop is one of the nearest supernova remnants (440 pc), allowing studies of spatial variations to be carried out at high physical resolution compared with other supernova remnants. Chandra Advanced CCD Imaging Spectrometer (ACIS) observations were obtained covering the bright V-shaped region on the south-west limb. This region has strong 0.2–2 keV spectral variations seen with the ROSAT Position Sensitive Proportional Counter. The nature of the spectral variations is analysed using the ACIS observations. It is found that temperature, abundance variations and column density variations all have a significant contribution to the observed spectral variations. Fits with C, N, O (O group) elements and Ne, Na, Mg, Al, Si, S, Ar (Ne group) elements varied together are better than fits with just O and Ne varied. Large spatial variations in abundances are seen, yet the Ne group to O group ratio and the Fe group (Ca, Fe, Ni) to O group ratio are nearly constant for all of the spectra. The most natural explanation is that the elements in the region are due to supernova explosion of a star of ∼18 M⊙ which predated the Cygnus Loop explosion.

Searching for periods in X-ray observations using Kuiper's test

We use Kuiper's test to detect periodicities in X-ray and gamma-ray observations. Like Rayleigh's test, it uses the individual photon arrival times, and is therefore well suited to the analysis of faint sources. Our method makes it possible to take into account the discontinuities in the observation, and to completely get rid of the contamination that results from them. This makes it particularly adapted to the search of periods long compared to the total observation duration. We propose a semi-analytical approach to determine the effective number of trial frequencies when searching for unknown periods over a frequency range. This approach can be easily adapted to other tests. We show that, using Kuiper's test, we can recover periods in frequency domains where other tests are completely confused by contamination. We finally search the entire ROSAT Position-Sensitive Proportional Counter (PSPC) archive for long periods, and find 28 new periodic-source candidates.

The GEMS project: X-ray analysis and statistical properties of the group sample

The Group Evolution Multiwavelength Study (GEMS) involves a multiwavelength study of a sample of 60 galaxy groups, chosen to span a wide range of group properties. Substantial ROSAT Position Sensitive Proportional Counter (PSPC) observations, available for all of these groups, are used to characterize the state of the intergalactic medium in each. We present the results of a uniform analysis of these ROSAT data and a statistical investigation of the relationship between X-ray and optical properties across the sample. Our analysis improves in several respects on previous work: (i) we distinguish between systems in which the hot gas is a group-scale medium and those in which it appears to be just a hot halo associated with a central galaxy; (ii) we extrapolate X-ray luminosities to a fixed overdensity radius (r 500 ) using fitted surface brightness models, in order to avoid biases arising from the fact that cooler systems are detectable to smaller radii, and (iii) optical properties have been rederived in a uniform manner from the NASA Extragalactic Database, rather than relying on the data in the disparate collection of group catalogues from which our systems are drawn. The steepening of the L X -T X relation in the group regime reported previously is not seen in our sample, which fits well on to the cluster trend, albeit with large non-statistical scatter. A number of biases affect the fitting of regression lines under these circumstances, and until the impact of these has been thoroughly investigated it seems best to regard the slope of the group L X -T X relation as being poorly determined. A significant problem in comparing the properties of groups and clusters is the derivation of system radii, to allow different systems to be compared within regions having the same overdensity. We find evidence that group velocity dispersion (σ v ) provides a very unreliable measure of system mass (and hence radius), with a number of groups having remarkably low values of σ v , given that they appear from their X-ray properties to be collapsed systems. We confirm that the surface brightness profiles of groups are significantly flatter than those of clusters - the maximum value of the β fit parameter for our sample is 0.58, lower than the typical value of 0.67 seen in clusters - however, we find no significant tendency within our sample for cooler groups to show flatter profiles. This result is inconsistent with simple universal pre-heating models. The morphology of the galaxies in the GEMS groups is correlated to their X-ray properties in a number of ways: we confirm the very strong relationship between X-ray emission and a dominant early-type central galaxy, which has been noted since the early X-ray studies of groups, and also find that spiral fraction is correlated with the temperature of the hot gas and hence the depth of the gravitational potential. A class of spiral-rich groups with little or no X-ray emission probably corresponds to groups that have not yet fully collapsed.

Periodicities in the X-Ray Intensity Variations of TV Columbae: An Intermediate Polar

We present results from a temporal analysis of the longest and the most sensitive X-ray observations of TV Columbae—a cataclysmic variable classified as an intermediate polar. The observations were carried out with the Rossi X-Ray Timing Explorer (RXTE) Proportional Counter Array, ROSAT Position Sensitive Proportional Counter, and ASCA. Data were analyzed using a one-dimensional CLEAN and Bayesian algorithms. The presence of a nearly sinusoidal modulation due to the spin of the white dwarf is seen clearly in all the data, confirming the previous reports based on the EXOSAT data. An improved period of 1909.7 ± 2.5 s is derived for the spin from the RXTE data. The binary period of 5.5 hr is detected unambiguously in X-rays for the first time. Several sidebands due to the interaction of these periods are observed in the power spectra, thereby suggesting contributions from both the disk-fed and the stream-fed accretion for TV Col. The accretion disk could perhaps be precessing, as sidebands due to the influence of 4 day period on the orbital period are seen. The presence of a significant power at certain sidebands of the spin frequency indicates that the emission poles are asymmetrically located. The strong power at the orbital sidebands seen in both the RXTE and ROSAT data gives an indication for an absorption site fixed in the orbital frame. Both the spin and the binary modulation are found to be energy dependent. The fact that the spin modulation amplitude in TV Col decreases with energy is confirmed. Hardness ratio variations and the energy-dependent modulation depth during the spin modulation can be explained by partially covered absorbers in the path of the X-ray emission region in the accretion stream. Increased hardness ratio during a broad dip in the intensity at binary phase of 0.75–1.0 confirms the presence of a strong attenuation due to additional absorbers, probably from an impact site of the accretion stream with the disk or magnetosphere.

Timescales of Soft X‐Ray Variability and Physical Constraints in Active Galactic Nuclei

We present soft X-ray variability timescales for 65 active galactic nuclei (AGNs) derived from ROSAT Position Sensitive Proportional Counter pointing data. For these 65 objects with obvious exponential increasing or decreasing patterns in their light curves, we use the exponential timescales and find they are more suitable for analyzing ROSAT light-curve data. We also discuss some physical constraints on the central engine implied by our results. Assuming that this soft X-ray variability exponential timescale is approximately equal to the thermal timescale of the standard accretion disk, we obtain the accretion rate, the size of the soft X-ray radiation region, and the compactness parameter for 37 AGNs, using their recently estimated central black hole masses. For 12 of these 37 AGNs, the radii of the gravitational instability in the standard thin accretion disks are obtained using the central black hole masses and the calculated accretion rates. These are consistent with the results from the reverberation mapping method. These results provide supporting evidence that such gravitational instabilities contribute to the formation of the broad-line regions in AGNs.

Hard X-Ray Emission Associated with White Dwarfs. II.

We have previously conducted a search for X-ray sources coincident with white dwarfs using the white dwarf catalog compiled by McCook & Sion and the ROSAT sources in the WGA Catalog (Paper I). To include the white dwarfs discovered since 1999 and to include the X-ray sources detected in ROSAT Position Sensitive Proportional Counter (PSPC) observations made with a boron filter, we have carried out another search using an updated list of white dwarfs and the final catalogs of the ROSAT PSPC observations with and without a boron filter. Forty-seven new X-ray sources convincingly coincident with white dwarfs are found and reported in this paper. Among these, only five show hard X-ray emission: three possess confirmed or suggested late-type companions, one is apparently single, and the other is likely a misclassified BL Lac object. The apparently single white dwarf with hard X-ray emission, KPD 0005+5106, was discussed extensively in Paper I. Photospheric origin for the hard X-ray emission from hot DO and DQZO white dwarfs remains a tantalizing possibility, but high-quality near-IR spectroscopic observations and monitoring of the Hα emission line are needed to rule out the existence of a faint dMe companion.

ROSAT PSPC view of the hot interstellar medium of the Magellanic Clouds

Diffuse X-ray emission from the Magellanic Clouds (MCs) is studied by using all the archival data of pointed ROSAT Position Sensitive Proportional Counter (PSPC) observations. For this purpose, contributions from the point and point-like sources in the ROSAT High Resolution Imager (HRI) and PSPC source catalogues are eliminated and periods of high solar activity are excluded. The spectral analysis yielded characteristic temperatures of 10 6 -10 7 K for the hot thin plasma of the ISM which extends over the whole Large Magellanic Cloud (LMC) and the Small Magellanic Cloud (SMC). The total unabsorbed luminosity in the 0.1-2.4 keV band within the observed area amounts to 3.2 x 10 3 8 erg s - 1 in the LMC and 1.1 × 10 3 7 erg s - 1 in the SMC, each with an uncertainty of ∼-40%, +100%. The X-ray luminosity of the LMC is comparable to that of other nearby galaxies with pronounced star formation. In the LMC, hot regions were found especially around the supergiant shell (SGS) LMC 4 and in the field covering SGS LMC 2 and LMC 3. Highest temperatures for the SMC were derived in the southwestern part of the galaxy. The diffuse X-ray emission is most likely a superposition of the emission from the hot gas in the interior of shells and supershells as well as from the halo of these galaxies.

ROSATX‐Ray Observations of the Dwarf Galaxy Holmberg II

We present a study of the irregular dwarf galaxy Holmberg II based on ROSAT Position Sensitive Proportional Counter observations (total exposure time: 22 ks). Holmberg II is a nearby (3.2 Mpc), well-studied dwarf irregular galaxy. It is famous for its interstellar medium (ISM), which is dominated by expanding structures such as H I holes and shells. We search for X-ray emission from point sources as well as for diffuse emission, down to the detection limit of the ROSAT data. Using X-ray hardness ratio diagrams we differentiate between thermal plasma and power-law X-ray spectra, which helps to determine the nature of the individual sources. Correlating the X-ray data with complementary observations ranging from the far-ultraviolet to the radio regime, we increase the probability of correctly identifying sources belonging to Holmberg II. We did not detect soft X-ray emission originating from hot gas within supergiant H I shells above our luminosity sensitivity limit of Llimit(0.1-2.1 keV) ≥ 1037ergs s-1. This finding can probably be attributed to blowout in the case of the largest holes and insufficient sensitivity (owing to strong photoelectric absorption) in the case of the smaller H I holes. However, we find faint X-ray sources well beyond the stellar body but within the H I distribution of Holmberg II, which suggests the presence of X-ray binaries. This indicates that star formation has taken place across the entire gaseous disk of Holmberg II in the past, some of which may have created the structures seen in the ISM at large galactocentric radii.

An X-Ray–Selected Galaxy Cluster at [ITAL][CLC]z[/CLC][/ITAL] = 1.11 in the [ITAL]ROSAT[/ITAL] Deep Cluster Survey

We report the discovery of an X-ray–luminous galaxy cluster at z = 1.11. RDCS J0910+5422 was selected as an X-ray cluster candidate in the ROSAT Deep Cluster Survey on the basis of its spatial extent in a ROSAT Position Sensitive Proportional Counter image. Deep optical and near-IR imaging reveal a red galaxy overdensity around the peak of the X-ray emission, with a significant excess of objects with J-K and I-K colors typical of elliptical galaxies at z ~ 1. Spectroscopic observations at the Keck II telescope secured nine galaxy redshifts in the range 1.095 1.

The X-ray emission and the magnetic fields of late-type stars

We present the results of X-ray luminosities of some active late-type stars, based on data primarily from the ROSAT position sensitive proportional counter (PSPC). According to the observations, we divide the stars into four groups: single, wide binary, binary and RS CVn. We investigated the correlation between the X-ray emission and the hardness ratio, coronal temperature, magnetic field strength, magnetic flux density. Our results suggest that the magnetic field plays a very important role in stellar X-ray emission.

Position and Variability of 2A 1704+241

ABSTRACTWe present results of analyses of observations of the X‐ray source 2A 1704+241 with the ROSAT Position Sensitive Proportional Counter (PSPC) and the High Resolution Imager (HRI). The source 2A 1704+241 was first associated with the M giant star HD 154791 based on observations with the HEAO 1 scanning modulation collimator and the Einstein IPC and analysis of a spectrum of HD 154791 obtained with the International Ultraviolet Explorer. This identification was unusual because there are few bright X‐ray binaries associated with an M giant star. We observed 2A 1704+241 with the PSPC and the HRI in order to determine more accurately the position of the X‐ray source and in order to study the previously seen 900 s variability in the Einstein data. Based on the previous identification and determination of the position of MS 1703.7+2417, an active galactic nucleus in the field, and the position of three previously unreported X‐ray sources that we have associated with stars in the USNO‐A2.0 catalog, we have greatly reduced the X‐ray positional error of 2A 1704+241. HD 154791 remains the prime candidate as the optical counterpart of the X‐ray source. While the 50% modulation in the X‐ray flux seen by the Einstein IPC is apparent in the ROSAT data, it appears to be at a slightly different frequency.

The Sun as an X‐Ray Star. IV. The Contribution of Different Regions of the Corona to Its X‐Ray Spectrum

We study X-ray-synthesized spectra of solar regions as templates to interpret analogous stellar spectra. We define three classes of coronal structures of different brightness, low (background quiet corona), medium (active regions), and high (active region cores), and determine their contribution to the solar X-ray emission measure versus temperature, EM(T), luminosity, and spectrum. This study defines the extent of the solar analogy quantitatively and accurately. To this end, we have selected a large sample of full-disk Yohkoh soft X-ray telescope observations taken between the maximum and the minimum of solar cycle 22, obtaining the contribution of each class to the whole Sun's EM(T). From the EM(T) distributions, we synthesize the X-ray spectra of the Sun and of the single classes of solar coronal regions as they would be collected with the ROSAT Position Sensitive Proportional Counter (PSPC) and ASCA Solid-State Imaging Spectrometer. We find that the Sun during the cycle fits well in the stellar scenario as a low-activity star. The ROSAT PSPC hardness ratio (HR) and surface X-ray flux, FPSPC, both increase going from the background corona to the active regions and the cores of the active regions, and range between the values of low and intermediate activity stars. We suggest that the coronae of these stars may be explained as the effect of structures similar to those present on the Sun and that the various levels of X-ray luminosity, HR, and FPSPC are achieved by changing the surface coverage of the different classes of coronal regions.

The re-analysis of the ROSAT data of Nova Mus 1983 using white dwarf atmosphere emission models

The analyses of X-ray emission from classical novae during the outburst stage have shown that the soft X-ray emission below 1 keV, which is thought to originate from the photosphere of the white dwarf, is inconsistent with the simple blackbody model of emission. Thus, ROSAT Position Sensitive Proportional Counter (PSPC) archival data of the classical Nova Mus 1983 (GQ Mus) have been re-analysed in order to understand the spectral development in the X-ray wavelengths during the outburst stage. The X-ray spectra are fitted with the hot white dwarf (WD) atmosphere emission models developed for the remnants of classical novae near the Eddington luminosity. The post-outburst X-ray spectra of the remnant white dwarf are examined in the context of evolution on the Hertzsprung–Russell diagram using C–O enhanced atmosphere models. The data obtained in 1991 August (during the ROSAT All Sky Survey) indicate that the effective temperature is . The 1992 February data show that the white dwarf had reached an effective temperature in the range with an unabsorbed X-ray flux (i.e. ∼ bolometric flux) between and . We show that the H burning at the surface of the WD had most likely ceased at the time of the X-ray observations. Only the 1991 August data show evidence for ongoing H burning.

A ROSAT medium-sensitivity Galactic plane survey at 180 < l < 280

We have performed a moderately deep soft X-ray (0.1–2 keV) survey of the Galactic plane using pointed observations with the ROSAT Position Sensitive Proportional Counter (PSPC). The survey is more than an order of magnitude more sensitive than previous X-ray surveys near the Galactic plane. The data consist of nine fields each of ≈10 ks exposure, pointed at positions on or very close to the Galactic plane (ǀbǀ<0°.3) in the longitude range 180°≲l≲280°. This region has relatively low X-ray absorbing material out to distances of several hundred pc and presents fewer source-confusion problems than at other longitudes. The total sky area surveyed was 2.5 deg2; this yielded 93 sources, 89 of which were detected in the ‘hard’(0.4–2.0 keV) band. Nine sources were detected in both ‘soft’(0.1–0.4 keV) and hard bands. In the hard band, the survey coverage is ≳90 per cent for sources brighter than 0.002 count s−1(∼2×10−14 erg cm−2 s−1), but falls steeply below this value, with the weakest sources being ∼0.001 count s−1. The median limiting flux is ≈0.0013 count s−1(∼1.3×10−14erg cm−2 s−1). There are 64 sources with hard-band count rates > 0.002 count s−1. We present the catalogue of X-ray sources and the number–flux relations (log N–log S). Eighteen sources have possible identifications from the SIMBAD data base. We have searched the Tycho-2 and USNO-A2.0 catalogues to find all possible optical counterparts brighter than ∼ 19th magnitude, and attempt to classify these on the basis of log (fX/fopt) versus optical colour diagrams and near-infrared photometry from the 2MASS Second Incremental Data Release. Hence, we have found the majority of these sources to be consistent with being late-type main-sequence stars, as previous studies have proposed from incompletely identified surveys. Comparison of the measured number–flux relations with predictions of Galactic (stellar) and extragalactic populations supports the view that the population of young stars in the plane is denser than previously thought.

A Richness Study of 14 Distant X‐Ray Clusters from the 160 Square Degree Survey

We have measured the surface density of galaxies toward 14 X-ray-selected cluster candidates at redshifts z(sub i) 0.46, and we show that they are associated with rich galaxy concentrations. These clusters, having X-ray luminosities of Lx(0.5-2 keV) approx. (0.5 - 2.6) x 10(exp 44) ergs/ sec are among the most distant and luminous in our 160 deg(exp 2) ROSAT Position Sensitive Proportional Counter cluster survey. We find that the clusters range between Abell richness classes 0 and 2 and have a most probable richness class of 1. We compare the richness distribution of our distant clusters to those for three samples of nearby clusters with similar X-ray luminosities. We find that the nearby and distant samples have similar richness distributions, which shows that clusters have apparently not evolved substantially in richness since redshift z=0.5. There is, however, a marginal tendency for the distant clusters to be slightly poorer than nearby clusters, although deeper multicolor data for a large sample would be required to confirm this trend. We compare the distribution of distant X-ray clusters in the L(sub X)-richness plane to the distribution of optically selected clusters from the Palomar Distant Cluster Survey. The optically selected clusters appear overly rich for their X-ray luminosities, when compared to X-ray-selected clusters. Apparently, X-ray and optical surveys do not necessarily sample identical mass concentrations at large redshifts. This may indicate the existence of a population of optically rich clusters with anomalously low X-ray emission, More likely, however, it reflects the tendency for optical surveys to select unvirialized mass concentrations, as might be expected when peering along large-scale filaments.