Roentgen Satellite Research Articles

Correlations in Chromospheric and Coronal Activity Indicators of Giant Stars

The main goal of this paper is to use ultraviolet photometry from the Galaxy Evolution Explorer (GALEX) satellite to study chromospheric activity among red giant and core-helium-burning (CHeB) stars. Correlations are sought between several chromospheric and coronal activity indicators for giants that were detected in soft X-rays by the ROentgen SATellite. There is an evident correlation between and , where L X is the X-ray luminosity and F Mg II is the flux from Mg ii h and k emission lines, although there is substantial scatter. Using GALEX far-ultraviolet (FUV) magnitudes, the relationship between an FUV-excess parameter (a proxy for chromospheric and transition-region emission lines) and X-ray luminosity is documented. Correlations found herein are not strong, and may be linked to binarity. There is a varied range of FUV emission among CHeB stars, indicating that giants in this phase may be exhibiting differences or cycles in outer atmospheric activity. Additionally, efforts were made to constrain a relationship between excess FUV and near-ultraviolet emission and the projected surface rotation velocity, , of giants. No clear relationship was found. Obscurities in this relationship may result from a spread in inclinations, and/or too few stars with km s−1.

A New X-Ray Tidal Disruption Event Candidate with Fast Variability

Abstract During a close encounter between a star and a supermassive black hole, the star can get disrupted by the black hole’s tidal forces, resulting in a tidal disruption event (TDE). The accretion of the star’s material onto the black hole produces strong emission in different wavelength regimes. Here we report the discovery with the Roentgen Satellite (ROSAT) of an X-ray-selected transient source in an optically non-active galaxy. At the location R.A.: 13h 31m57.ˢ66 and decl.: −32°43′19.″7 a sudden rise in X-ray luminosity by a factor of 8 within 8 days has been observed. Additionally, a very soft X-ray spectrum with a blackbody temperature kT = 0.1 keV and a peak luminosity of at least 1 × 1043 erg s−1 suggests a TDE interpretation, and the observed properties are very similar to previously identified soft X-ray (ROSAT) TDEs. An optical spectrum taken of the galaxy at the position of RXJ133157.6324319.7 six years after the X-ray outburst does not show any emission lines as would be expected from a persistent active galactic nucleus. The redshift of the galaxy is determined to be 0.051 based on absorption lines. It is therefore likely a member of the galaxy cluster A3560. The rise in X-ray luminosity happens within 8 days and thus appears to be fast for such an event. No X-ray emission was detected 170 days before and 165 days after the event, and none was detected 25 yr later with the Neil Gehrels Swift Observatory. The change in X-ray luminosity is at least a factor of 40.

Clustering of CODEX clusters

Context. The clustering of galaxy clusters links the spatial nonuniformity of dark matter halos to the growth of the primordial spectrum of perturbations. The amplitude of the clustering signal is widely used to estimate the halo mass of astrophysical objects. The advent of cluster mass calibrations enables using clustering in cosmological studies. Aims. We analyze the autocorrelation function of a large contiguous sample of galaxy clusters, the Constrain Dark Energy with X-ray (CODEX) sample, in which we take particular care of cluster definition. These clusters were X-ray selected using the ROentgen SATellite All-Sky Survey and then identified as galaxy clusters using the code redMaPPer run on the photometry of the Sloan Digital Sky Survey. We develop methods for precisely accounting for the sample selection effects on the clustering and demonstrate their robustness using numerical simulations. Methods. Using the clean CODEX sample, which was obtained by applying a redshift-dependent richness selection, we computed the two-point autocorrelation function of galaxy clusters in the 0.1 < z < 0.3 and 0.3 < z < 0.5 redshift bins. We compared the bias in the measured correlation function with values obtained in numerical simulations using a similar cluster mass range. Results. By fitting a power law, we measured a correlation length r0 = 18.7 ± 1.1 and slope γ = 1.98 ± 0.14 for the correlation function in the full redshift range. By fixing the other cosmological parameters to their nine-year Wilkinson Microwave Anisotropy Probe values, we reproduced the observed shape of the correlation function under the following cosmological conditions: Ωm0 = 0.22−0.03+0.04 and S8 = σ8(Ωm0/0.3)0.5 = 0.85−0.08+0.10 with estimated additional systematic errors of σΩm0 = 0.02 and σS8 = 0.20. We illustrate the complementarity of clustering constraints by combining them with CODEX cosmological constraints based on the X-ray luminosity function, deriving Ωm0 = 0.25 ± 0.01 and σ8 = 0.81−0.02+0.01 with an estimated additional systematic error of σΩm0 = 0.07 and σσ8 = 0.04. The mass calibration and statistical quality of the mass tracers are the dominant source of uncertainty.

Auroral X‐ray emission at Jupiter: Depth effects

Auroral X‐ray emissions from Jupiter with a total power of about 1 GW have been observed by the Einstein Observatory, Roentgen satellite, Chandra X‐ray Observatory, and XMM‐Newton. Previous theoretical studies have shown that precipitating energetic sulfur and oxygen ions can produce the observed X‐rays. This study presents the results of a hybrid Monte Carlo (MC) model for sulfur and oxygen ion precipitation at high latitudes, looks at differences with the continuous slow‐down model, and compares the results to synthetic spectra fitted to observations. We concentrate on the effects of altitude on the observed spectrum. The opacity of the atmosphere to the outgoing X‐ray photons is found to be important for incident ion energies greater than about 1.2 MeV per nucleon for both sulfur and oxygen. Model spectra are calculated for intensities with and without any opacity effects. These synthetic spectra were compared with the results shown by Hui et al. (2010) which fit Chandra X‐ray Observatory observations for the north and south Jovian auroral emissions. Quenching of long‐lived excited states of the oxygen ions is found to be important. Opacity considerably diminishes the outgoing X‐ray intensity calculated, particularly when the viewing geometry is not favorable.

MULTIWAVELENGTH SEARCH FOR AND STUDIES OF ACTIVE GALAXIES: AN INTERNATIONAL COLLABORATION BETWEEN THE BYURAKAN OBSERVATORY AND OBSERVATOIRE DE HAUTE-PROVENCE

An international collaboration on the multiwavelength search for and studies of active galaxies between the Byurakan Astrophysical Observatory (Armenia) and Observatoire de Haute-Provence (France) has been ongoing since 1997. The blue stellar objects of the first Byurakan survey have been studied in order to reveal and build a sample of new bright quasistellar objects and to determine their surface density. On the basis of optical identifications of 1577 IRAS point sources, a new (Byurakan-IRAS galaxy) sample of IRAS galaxies has been constructed and studied. 90 bright Seyfert 1 galaxies have been observed with a high resolution to make a fine analysis of their emission line profiles and to study their emission line regions. A new programme of investigation of Roentgen Satellite (ROSAT) sources identified by the Hamburg quasar survey has been conducted to search for new narrow-line Seyfert 1 galaxies. Databases and observations in the optical, infrared, radio and X-ray ranges are being used.

ITAL]Chandra[/ITAL] Observations of Unresolved X-Ray Sources around Two Clusters of Galaxies

We have searched for unresolved X-ray sources in the vicinity of two rich clusters of galaxies: Abell 1995 (A1995) and MS 0451.6-0305 (MS0451), using the Chandra X-ray observatory. We detected significantly more unresolved sources around A1995 than expected based on the number of X-ray sources to the same flux limit detected in deep Chandra observations of blank fields. Previous studies have also found excess X-ray sources in the vicinity of several nearby clusters of galaxies using ROSAT (Roentgen Satellite), and recently in more distant (z is approximately 0.5) clusters (RXJ0030 and 3C295) using Chandra. In contrast, we detect only 14 unresolved X-ray sources near MS0451, which is consistent with the number expected from a cluster-free background. We determine the luminosity functions of the extra sources under the assumption that they are at the distance of their respective clusters. The characteristic luminosity of the extra sources around A1995 must be an order of magnitude fainter than that of the extra sources around RXJ0030 and 3C295. The apparent lack of extra sources around MS0451 is consistent with its greater distance and the same characteristic luminosity as the A1995 sources. Hardness ratios suggest that, on average, the extra sources in A1995 may have harder spectra than those of RXJ0030 and 3C295. These results indicate that different classes of objects may dominate in different clusters, perhaps depending on the formation history and/or dynamical state of the accompanying cluster.

Production of cometary X rays by electron impact and Bremsstrahlung processes

Bremsstrahlung and electron impact excitation are suggested in a few papers to be the main processes for generation of X rays in comets. To check this hypothesis, we determined a peak X‐ray volume emission rate in comet Hyakutake during the Roentgen satellite (ROSAT) observation. We found that scaling of the electron fluxes observed in comet Halley from the Vega probe to the known properties of the solar wind electrons agrees with theory of electron acceleration by lower hybrid waves in comets [Shapiro et al., 1999]. Using those scaled electron fluxes, we calculated Bremsstrahlung at the peak brightness in comet Hyakutake at a level of 0.2–0.3% of the observed peak emission. Bremsstrahlung in comet Hale‐Bopp constitutes 4% of the X‐ray emission observed with the EUVE, and the Bremsstrahlung spectrum, calculated for that comet by Shapiro et al. [1999], needs corrections. Emissions of the K‐lines of O 525 eV and C 277 eV, excited by energetic electron impact in comet Hyakutake, are at a level of 1.4–2.2% of the observed X‐ray emission. The calculation of energetic electron impact excitation in cometary oxygen by Shapiro et al. [1999] does not result in purely X‐ray emission but mostly results in visible and ultraviolet emission with just a small contribution at 525 eV which is equal to 1.7% of the total emission. With the Giotto electron fluxes the X‐ray emissions from Bremsstrahlung and electron impact would be smaller by orders of magnitude than the above values. The X‐ray luminosity associated with the energetic electron mechanism depends on the gas production rate and on the heliocentric distance in a different way than does the luminosity associated with the charge transfer mechanism. A test for these dependences using our EUVE observations rules out the energetic electron excitation mechanism on this basis. There are no papers that have adequately justified energetic electrons as the main source of X rays in comets.

Multifrequency Studies of Bright Radio Supernova Remnants. III. X‐Ray and Radio Observations of 3C 397

Radio-bright, presumably young supernova remnants offer the opportunity of studying strong-shock physics and the nature of the interaction of ejected material with the surrounding medium. The relation between radio and X-ray morphology varies considerably among supernova remnants, with important implications for the physics of the emission processes at different wavelengths. We use Very Large Array (VLA) and Roentgen Satellite (ROSAT) images of the radio-bright supernova remnant 3C 397 (G41.1-0.3) to examine the shock structure in both thermal X-ray emission and nonthermal radio emission. The unusual rectangular morphology can be seen in VLA maps at 20 and 6 cm wavelength at a resolution of 6'' and in ROSAT HRI images. The X-ray images resemble the radio strongly, except for a small, possibly unresolved X-ray hot spot near the center. There is no variation in the X-ray hardness ratio from ROSAT Position Sensitive Proportional Counter data across the remnant, suggesting that at least between 0.4 and 2 keV the interior emission is not different in character from that in the bright shell regions. The remnant is unpolarized at 20 cm and has a mean fractional polarization of 1.5% ± 0.1% at 6 cm. The polarized flux, and polarized fraction, peak inside the remnant at a location not coincident with either an internal maximum in total-intensity radio emission or with the X-ray hot spot. Spectral index maps between 6 and 20 cm do not show any systematic differences associated with interior emission; there appears to be no plerionic or pulsar-driven component in 3C 397, at least as normally characterized by high polarization and a flat radio spectrum. Spectral index values spread about the mean by about Δα ~ 0.2, a result consistent with previous work. The steep total-intensity profile off the southwest edge of 3C 397 allows an inference of the upstream electron diffusion coefficient and implies a mean free path for electron scattering shorter than in the general interstellar medium but longer than that similarly inferred for Tycho and SN 1006. A simple analysis based on the observed X-ray flux gives an estimate of the mean density in 3C 397 of about 4 cm-3, which would also be enough to depolarize the 20 cm emission completely, as observed. The remnant age is then of order 103 yr, and the current shock velocity is about 1600 km s-1. Finally, we speculate on possible mechanisms producing the X-ray hot spot.

On the Enigmatic X‐Ray Source V1408 Aquilae (=4U 1957+11)

Models for the characteristically soft X-ray spectrum of the compact X-ray source V1498 Aql (=4U 1957+11) have ranged from optically thick Comptonization to multicolor accretion disk models. We critically examine the X-ray spectrum of V1408 Aql via archival Advanced Satellite for Cosmology and Astrophysics (ASCA) data, archival Roentgensatellit (ROSAT) data, and recent Rossi X-Ray Timing Explorer (RXTE) data. Although we are able to fit a variety of X-ray spectral models to these data, we favor an interpretation of the X-ray spectrum as being due to an accretion disk viewed at large inclination angles. Evidence for this hypothesis includes long term (117 day, 235 day, 352 day) periodicities seen by the RXTE All Sky Monitor (ASM), which we interpret as being due to a warped precessing disk, and a 1 keV feature in the ASCA data, which we interpret as being the blend of L fluorescence features from a disk atmosphere or wind. We also present timing analysis of the RXTE data and find upper limits of 4% for the root mean square (rms) variability between f=0.001-16 Hz. The situation of whether the compact object is a black hole or neutron star is still ambiguous; however, it now seems more likely that an X-ray emitting, warped accretion disk is an important component of this system.

Excitation of X rays in comet Hyakutake (C/1996 B2)

There is great disagreement between current evaluations of the role of bremsstrahlung and line excitation by electron impact and recombination in cometary X rays. To study the problem in detail, we use the exact values of the Roentgen satellite (ROSAT) high‐resolution imager and wide‐field camera instrument response functions and apply them to all nine exposure intervals in the ROSAT observation of comet Hyakutake to determine X ray two‐parameter power law and thermal bremsstrahlung spectra. Thermal bremsstrahlung is a convenient form for two‐parameter spectrum and not a physical reality in comets. Then spectral index is α = 1.97 ± 0.18 ≈ 2, spectral temperature is kT = 360 eV, and the exposure‐mean photon emission is 3 × 1025 ph s−1 and 1.3 × 1016 ergs s−1 in a range of 90–2000 eV and in an aperture radius of 1.2 × 105 km. The exposure‐mean ROSAT spectrum is in excellent agreement with the Extreme Ultarviolet Explorer (EUVE) observation of 7.5 × 1024 ph s−1 in a range of 100–165 eV from comet Hyakutake. Then we follow the same methods as Northrop et al. [1997] to calculate a volume emission rate at the comet brightness maximum and to calculate that from bremsstrahlung with the Vega 2 electron flux. Yet we obtain a much lower emission than Northrop et al., at a level of 0.5% of the observed emission in accordance with results of Krasnopolsky [1996, 1997a] and Bingham et al. [1997]. Electron impacts of molecular, atomic, and ion species in comets produce mostly excitation of the X ray lines C 277 eV and O 525 eV at a level of 2% of the total X ray emission from comet Hyakutake. This value is calculated for the Vega 2 electron flux. The value for the Giotto electron flux is 3 × 10−6 of the measured emission. Processes which are responsible for impurity radiation in fusion plasma either are inefficient or produce ultraviolet photons in cometary plasma, and the method of Bingham et al. is inapplicable to excitation of X ray lines in comets. The calculations show that the expected effects of bremsstrahlung and electron impact are very low in cometary X rays. The obtained spectral index α = 2 differs significantly from α ∼ 5 for scattering of solar X rays by attogram dust. These facts favor charge transfer of heavy solar wind ions as the dominant process of X ray excitation in comet Hyakutake. Charge transfer is not efficient in production of X rays on the Moon and other solid bodies.

Multiwavelength Observations of a Dramatic High‐Energy Flare in the Blazar 3C 279

The blazar 3C 279, one of the brightest identified extragalactic objects in the γ-ray sky, underwent a large (factor of ~10 in amplitude) flare in γ-rays toward the end of a 3 week pointing by Compton Gamma Ray Observatory (CGRO), in 1996 January-February. The flare peak represents the highest γ-ray intensity ever recorded for this object. During the high state, extremely rapid γ-ray variability was seen, including an increase of a factor of 2.6 in ~8 hr, which strengthens the case for relativistic beaming. Coordinated multifrequency observations were carried out with Rossi X-Ray Timing Explorer (RXTE), Advanced Satellite for Cosmology and Astrophysics (ASCA; or, Astro-D), Roentgen Satellite (ROSAT), and International Ultraviolet Explorer (IUE) and from many ground-based observatories, covering most accessible wavelengths. The well-sampled, simultaneous RXTE light curve shows an outburst of lower amplitude (factor of simeq3) well correlated with the γ-ray flare without any lag larger than the temporal resolution of ~1 day. The optical-UV light curves, which are not well sampled during the high-energy flare, exhibit more modest variations (factor of ~2) and a lower degree of correlation. The flux at millimetric wavelengths was near a historical maximum during the γ-ray flare peak, and there is a suggestion of a correlated decay. We present simultaneous spectral energy distributions of 3C 279 prior to and near to the flare peak. The γ-rays vary by more than the square of the observed IR-optical flux change, which poses some problems for specific blazar emission models. The synchrotron self-Compton (SSC) model would require that the largest synchrotron variability occurred in the mostly unobserved submillimeter/far-infrared region. Alternatively, a large variation in the external photon field could occur over a timescale of a few days. This occurs naturally in the mirror model, wherein the flaring region in the jet photoionizes nearby broad emission line clouds, which, in turn, provide soft external photons that are Comptonized to γ-ray energies.

An Explanation of the [ITAL]ROSAT[/ITAL] High-Resolution Imager Ultraviolet Sensitivity

The UV sensitivity to bright stars observed during operations of the high-resolution imager (HRI) on the X-ray-XUV Roentgen Satellite (ROSAT) has been commonly interpreted as a result of enhanced transmission solely in the wavelength range 1500-1700 A of the HRI's UV/ion shield. We report on recent laboratory UV transmission measurements of HRI flight spare filters that show transmissions much higher than previously assumed for wavelengths longward of approximately 2000 A. We present a model for the UV response of the HRI based on these measurements. Using this model and the known UV spectra of the A stars Vega (α Lyrae, V = +0.03) and β Carinae (V = +1.68), we successfully predict the observed HRI count rates for these two stars.

LUMINOUS SUPERSOFT X-RAY SOURCES

Luminous supersoft X-ray sources were discovered with the Einstein observatory and have been established as an important new class of X-ray binaries on the basis of observations with the Roentgen Satellite (ROSAT). They have extremely soft spectra (equivalent blackbody temperatures of ∼15–80 eV) and are highly luminous (bolometric luminosities of 1036–1038erg s−1). Correcting for the heavy extinction of soft X rays by interstellar neutral hydrogen, their numbers in the disks of ordinary spiral galaxies like our own and M31 are estimated to be of the order of 103. Their observed characteristics are consistent with those of white dwarfs, which are steadily or cyclically burning hydrogen-rich matter accreted onto the surface at a rate of order 10−7[Formula: see text] year−1. The required high accretion rates can be supplied by mass transfer on a thermal time scale (106–107years) from close companion stars that are more massive than the white dwarf accretor, typically 1.3–2.5 [Formula: see text]. Steady burning can also occur in a post-nova stage, but for shorter time scales, and it has been observed in a few classical novae and symbiotic novae. A few supersoft sources have been found to be recurrent transients. They are possibly connected with very massive white dwarfs accreting at high rates. Luminous supersoft sources may make a considerable contribution to the Type Ia supernova rate in spiral and irregular galaxies.

Hot white dwarfs: An observational viewpoint

Abstract In the last two decades, space and ground-based observations have revolutionized our understanding of the remnants of stellar evolution, the white dwarf stars. We now have a detailed picture of the evolution of the surface composition of hot white dwarfs, those objects with temperatures above about 20 000 K, as they cool. Significant quantities of heavy elements are supported by radiation pressure against the downward pull of gravity in the very hottest examples (above 40 000 K) but, once the efficiency of the mechanism diminishes below 40 000 K, they fall rapidly out of the atmosphere. However, while the observational results are qualitatively consistent with the predictions of theoretical calculations, the measured element abundances differ substantially from those expected. A viable explanation, supported by good observational evidence, is that the surface composition is further modified through the loss of material in a weak wind. This paper reviews recent developments, made in particular with the International Ultraviolet Explorer, Roentgen Satellit and Extreme Ultraviolet Explorer, setting them in the context of our earlier picture of white dwarf evolution to show the important advances made.

X-Ray Emission from the Sun in Its Youth and Old Age

We have obtained ROSAT PSPC (Roentgen Satellite Position Sensitive Proportional Counter) pointed observations of two nearby G stars of ages 70 Myr and 9.5 Gyr that are of unique importance as proxies for the Sun at the two extremes of its main-sequence evolutionary lifetime. The younger star, HD 129333 (EK Dra; G0 V), a rapid rotator with a 2.7 day period, is a strong source with an X-ray luminosity L(x)(0.2-2.4 keV) = (7.5-11.5) x 10(exp 29) erg/s. Modeling suggests a two-temperature corona with T(1) = (2.0 +/- 0.3) x 10(exp 6) K and T(2) = (9.7 +/- 0.3) x 10(exp 6) K (formal uncertainties). A continuous emission measure distribution, increasing to higher temperatures and with a cutoff at (20-30) x 10(exp 6) K, yields even better fits to the data. The old star, beta Hyi (HR 98; G2 IV), represents the Sun in the future, near the end of its hydrogen-core burning stage, when it should be rotating more slowly (present P(rot) = 25.4 day) and should have lower levels of activity. The ROSAT measurements yield L(x) = (0.9-3.0) x 10(exp 27) ergs/s and a rather cool, single coronal temperature of T = (1.7 +/- 0.4) x 10(exp 6) K. For comparison, the Sun has L(x) approx. equal to 2 x 10(exp 27) ergs/s and a coronal temperature of about T = 2 x 10(exp 6) K. These stars provide information on the decline of the stellar (and specifically solar) magnetic activity from extreme youth to old age. HD 129333 is also important in that it yields an estimate of the solar soft X-ray flux in the early solar system at the epoch of the terminal stages of planetary accretion.

The RIASS coronathon: Joint X-ray and ultraviolet observations of normal F-K stars

Between 1990 August and 1991 January the ROSAT/IUE All Sky Survey (RIASS) coordinated pointings by the International Ultraviolet Explorer (IUE) with the continuous X-ray/EUV mapping by the Roentgensatellit (ROSAT). The campaign provided an unprecedented multiwavelength view of a wide variety of cosmic sources. We report findings for F-K stars, a large proportion of the RIASS targets. Forty-eight of our 91 'Coronathon' candidates were observed by the IUE during the campaign. For stars missed by the IUE, we supplemented the ROSAT survey fluxes with archival UV spectra and/or follow-on observations.

X-ray emission from the historical supernovae in the spiral galaxy NGC 6946: SN 1980K and SN 1968D recovered ?

This paper describes a study of the Roentgen Satellite (ROSAT) Position Sensitive Proportional Counter (PSPC) observation of the spiral galaxy NGC 6946 to search for x-ray emission from the six historical supernovae that have occurred in that galaxy. At the age of these supernovae, all could be emitting x rays due to circumstellar interactions. Supernova (SN) 1980K may be present based upon the hardness ratio and the coincidence of an x-ray source with the optical position. Its luminosity agrees with a recent prediction by Chevalier & Fransson. The situation for SN 1968D, apparently recovered in Very Large Array (VLA) data, is less clear due to source confusion, but suggests SN 1968D has not been recovered. Only upper limits are available for the remaining four supernovae that have occurred in this galaxy.

X-ray emission associated with radio galaxies in the Perseus cluster

In this paper, we report on new x-ray observations of the Perseus cluster made using four separate pointings of the Roentgen Satellite (ROSAT) Positron Sensitive Proportional Counter (PSPC). We searched for x-ray emission associated with 16 radio galaxies and detected six above 3 sigma. We made use of the PSPC spectra to determine if the x-ray emission associated with radio galaxies in Perseus is thermal or nonthermal in origin (i.e., hot gas or an active galactic nuclei (AGN)). For the head-tail radio galaxy IC 310, we find that the data are best fit by a power law model with an unusually large spectral index alpha = 2.7. This is consistent with its unresolved spatial structure. On the other hand, a second resolved x-ray source associated with another radio galaxy 2.3 Mpc from the Perseus center (V Zw 331) is best fit by a thermal model. For three sources with insufficient flux for a full spectral analysis, we calculated hardness ratios. On this basis, the x-ray emission associated with the well known head-tail source NGC 1265 is consistent with thermal radiation. The x-ray spectra of UGC 2608 and UGC 2654 probably arise from hot gas, although very steep power-law spectra (alpha greater than 3.2) are also possible. The spectrum of NGC 1275 is quite complex due to the presence of an AGN and the galaxy's location at the center of a cluster cooling flow.

ROSAT all-sky survey observations of X-ray variability in cool giant stars

view Abstract Citations (21) References (27) Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS ROSAT All-Sky Survey Observations of X-Ray Variability in Cool Giant Stars Haisch, Bernhard ; Schmitt, J. H. M. M. Abstract We have identified 24 active late-type giant stars, including 11 RS CVn systems, with soft X-ray count rates high enough to allow the detection of statistically significant variability on a Roentgen Satellite (ROSAT) orbital timescale (96 minutes) as observed by the Position Sensitive Proportional Counter (PSPC) during the all-sky survey. Our sensitivity typically lies in the range of 10% - 25%, depending on the source count rate. Comparison is made to the daily, nonflare solar soft X-ray variability as observed by the Solrad satellites during solar minimum in 1969 and solar maximum in 1975. Seven of the 24 stars show significant variability; in two of these cases (HR 3922 and HR 8448) major flares were observed in which the peak count rate is enhanced by at least a factor of 3 above quiescent. While HR 3922 (G5 III) is not (yet) classified as an RS CVn star, its flare is more energetic (3 x 1031 ergs/s) than previously observed RS CVn flares. The apparently single giant HR 8167 (G8 III) also shows two flares. While one might expect to find an anticorrelation between saturated coronae and variability, we find no evidence of this: the two stars in our sample with the highest ratio of fx/fv both show variability. We also point out that Capella (G6 III + F9 III) is one of the stars manifesting variability. Publication: The Astrophysical Journal Pub Date: May 1994 DOI: 10.1086/174108 Bibcode: 1994ApJ...426..716H Keywords: Cool Stars; Giant Stars; Sky Surveys (Astronomy); Spectral Emission; Stellar Coronas; Stellar Flares; X Rays; Proportional Counters; Rosat Mission; Stellar Physics; Variations; X Ray Astronomy; Astrophysics; STARS: CORONAE; STARS: FLARE; STARS: GIANTS; X-RAYS: STARS full text sources ADS | data products SIMBAD (24)

Formation and evolution of luminous supersoft X-ray sources

Luminous supersoft X-ray sources, with characteristic luminosities of approximately 10<SUP>38</SUP> ergs/s and temperatures, kT, of approximately 35 eV, have been established as a new and distinct class of X-ray source through recent Roentgen Satellite (ROSAT) observations. Several possible physical models have been proposed for these sources. One promising scenario (van den Heuvel et al. 1992) involves mass transfer, which is unstable on a thermal timescale, from a main-sequence or subgiant donor star onto the surface of a white dwarf. For a narrow range of accretion rates, steady nuclear burning of the accreted matter can take place. This process can provide the high luminosities and the correct range of temperatures observed in the supersoft sources. However, given the limited range of mass transfer rates that are consistent with this phenomenon, it is far from obvious that a sufficient population of such systems exists in galaxies such as our own, M31, and the Magellanic Clouds, in order to account for the large number of supersoft sources which can be inferred from present observations. This work addresses the population question in detail, through a Monte Carlo simulation of the formation and evolution of such systems, which starts with zero-age primordial binaries. In order to evolve into close binary systems which contain a white dwarf component and a companion transferring mass at a rate within the requisite narrow range, a binary system must undergo a specific progression of evolutionary steps. We find that a sufficient subset of our initial binaries evolve to become systems with the requisite properties, so that they can account for the population of supersoft sources that is inferred from observations. In particular, we find that there should be more than 1000 systems in the Galaxy today with properties that very closely match those of the observed supersoft sources. From our models, we find expected luminosities, white dwarf effective temperatures, and orbital periods in the ranges of 10<SUP>37</SUP> - 10<SUP>38</SUP> ergs/s, (1-5) x 10<SUP>5</SUP> K, and 8 hr-1.4 days, respectively. The masses of the white dwarf and donor star are expected to lie in the range of 0.7-1.05 solar mass and 1.3-2.7 solar mass. Finally, we discuss the role that supersoft X-ray sources may play as the progenitors of Type Ia supernovae, and estimate the rate of production via this channel.