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NASA NTRS1994

The derived population of luminous supersoft X-ray sources

The existence of a new class of astrophysical object, luminous supersoft X-ray sources, has been established through ROSAT satellite observations and analysis during the past approximately 3 yr. Because most of the radiation emitted by supersoft sources spans a range of wavelengths readily absorbed by interstellar gas, a substantial fraction of these sources may not be detectable with present satellite instrumentation. It is therefore important to derive a reliable estimate of the underlying population, based on the approximately 30 sources that have been observed to date. The work reported here combines the observational results with a theoretical analysis, to obtain an estimate of the total number of sources likely to be present in M31, the Magellanic Clouds, and in our own Galaxy. We find that in M31, where approximately 15 supersoft sources have been identified and roughly an equal number of sources are being investigated as supersoft candidates, there are likely to be approximately 2500 active supersoft sources at the present time. In our own Galaxy, where about four supersoft sources have been detected in the Galactic plane, there are likely to be approximately 1000 active sources. Similarly, with about six and about four (nonforeground) sources observed in the Large (LMC) and Small Magellanic Clouds (SMC), respectively, there should be approximately 30 supersoft sources in the LMC, and approximately 20 in the SMC. The likely uncertainties in the numbers quoted above, and the properties of observable sources relative to those of the total underlying population, are also derived in detail. These results can be scaled to estimate the numbers of supersoft sources likely to be present in other galaxies. The results reported here on the underlying population of supersoft X-ray sources are in good agreement with the results of a prior population synthesis study of the white dwarf accretor model for luminous supersoft X-ray sources. It should be emphasized, however, that the questions asked in these two investigations are distinct, that the approaches taken to answer these questions are largely independent and that the findings of these two studies could in principle have been quite different.

Di Stefano, R.STEFANO↗
NASA NTRS1994

Formation and evolution of luminous supersoft X-ray sources

Luminous supersoft X-ray sources, with characteristic luminosities of approximately 10(exp 38) 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(exp 37) - 10(exp 38) ergs/s, (1-5) x 10(exp 5) 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.

Rappaport, S.↗
DOE OSTI2023

A helium-burning white dwarf binary as a supersoft X-ray source

Abstract Type Ia supernovae are cosmic distance indicators 1,2 , and the main source of iron in the Universe 3,4 , but their formation paths are still debated. Several dozen supersoft X-ray sources, in which a white dwarf accretes hydrogen-rich matter from a non-degenerate donor star, have been observed 5 and suggested as Type Ia supernovae progenitors 6–9 . However, observational evidence for hydrogen, which is expected to be stripped off the donor star during the supernova explosion 10 , is lacking. Helium-accreting white dwarfs, which would circumvent this problem, have been predicted for more than 30 years (refs. 7,11,12 ), including their appearance as supersoft X-ray sources, but have so far escaped detection. Here we report a supersoft X-ray source with an accretion disk whose optical spectrum is completely dominated by helium, suggesting that the donor star is hydrogen-free. We interpret the luminous and supersoft X-rays as resulting from helium burning near the surface of the accreting white dwarf. The properties of our system provide evidence for extended pathways towards Chandrasekhar-mass explosions based on helium accretion, in particular for stable burning in white dwarfs at lower accretion rates than expected so far. This may allow us to recover the population of the sub-energetic so-called Type Iax supernovae, up to 30% of all Type Ia supernovae 13 , within this scenario.

Science & Technology - Other Topics↗
NASA NTRS2002

Chandra Discovery of Luminous Supersoft X-Ray Sources in M81

A Chandra ACIS-S imaging observation of the nearby galaxy M81 (NGC 3031) reveals 9 luminous soft X-ray sources. The local environments, X-ray spectral properties, and X-ray light curves of the sources are presented and discussed in the context of prevailing physical models for supersoft sources. It is shown that the sample falls within expectations based on population synthesis models taken from the literature though the high observed luminosities (L approx.2e36 to approx.3e38 ergs in the 0.2--2.0-keV band) and equivalent blackbody temperatures (T approx.40 to 80 eV) place the brightest detected M81 objects at the high luminosity end of the class of supersoft sources defined by previous ROSAT and Einstein studies of nearby galaxies. This is interpreted as a natural consequence of the higher sensitivity of Chandra to hotter and more luminous systems. Most of the sources can be explained as canonical supersoft sources, secreting white dwarfs powered by steady surface nuclear burning, with X-ray spectra well-fit by hot white dwarf local thermodynamic equilibrium atmosphere models. An exceptionally bright source is scrutinized in greater detail as its estimated barometric luminosity, L approx. 1.5e39 ergs, greatly exceeds theoretical estimates for supersoft sources. This source may be beyond the stability limit and undergoing a phase of mass outflow under extreme conditions. Alternatively, a model in which the observed X-ray spectrum arises from an accretion disk around a blacklists of mass approx.1200/sqrt(cosi) solar masses (viewed at an inclination angle 1) cannot be excluded.

Swartz, Douglas A.↗
NASA NTRS1994

Ionization nebulae surrounding supersoft X-ray sources

In this work we carry out a theoretical investigation of a new type of astrophysical gaseous nebula, viz., ionized regions surrounding supersoft X-ray sources. Supersoft X-ray sources, many of which have characteristic luminosities of approximately 10(exp 37)-(10(exp 38) ergs/s and effective temperatures of approximately 4 x 10(exp 5) K, were first discovered with the Einstein Observatory. These sources have now been shown to constitute a distinct class of X-ray source and are being found in substantial numbers with ROSAT. We predict that these sources should be surrounded by regions of ionized hydrogen and helium with properties that are distinct from other astrophysical gaseous nebulae. We present caluations of the ionization and temperature structure of these ionization nebulae, as well as the expected optical line fluxes. The ionization profiles for both hydrogen and helium exhibit substantially more gradual transitions from the ionized to the unionized state than is the case for conventional H II regions. The calculated optical line intensitites are presented as absolute fluxes from sources in the Large Magellanic Cloud and as fractions of the central source luminosity. We find, in particular, that (O III) lambda 5008 and He II lambda 4686 are especially prominent in these ionization nebulae as compared to other astrophysical nebulae. We propose that searches for supersoft X-rays via their characteristic optical lines may reveal sources in regions where the soft X-rays are nearly completely absorbed by the interstellar medium.

Rappaport, S.↗
NASA NTRS2003

A XMM-Newton Observation of Nova LMC 1995, a Bright Supersoft X-ray Source

Nova LMC 1995, previously detected during 1995-1998 with ROSAT, was observed again as a luminous supersoft X-ray source with XMM-Newton in December of 2000. This nova offers the possibility to observe the spectrum of a hot white dwarf, burning hydrogen in a shell and not obscured by a wind or by nebular emission like in other supersoft X-ray sources. Notwithstanding uncertainties in the calibration of the EPIC instruments at energy E<0.5 keV, using atmospheric models in Non Local Thermonuclear Equilibrium we derived an effective temperature in the range 400,000-450,000 K, a bolometric luminosity Lbolabout equal to 2.3 times 10 sup37 erg s sup-l, and we verified that the abundance of carbon is not significantly enhanced in the X-rays emitting shell. The RGS grating spectra do not show emission lines (originated in a nebula or a wind) observed for some other supersoft X-ray sources. The crowded atmospheric absorption lines of the white dwarf cannot be not resolved. There is no hard component (expected from a wind, a surrounding nebula or an accretion disk), with no counts above the background at E>0.6 keV, and an upper limit Fx,hard = 10 sup-14 erg s sup-l cm sup-2 to the X-ray flux above this energy. The background corrected count rate measured by the EPIC instruments was variable on time scales of minutes and hours, but without the flares or sudden obscuration observed for other novae. The power spectrum shows a peak at 5.25 hours, possibly due to a modulation with the orbital period. We also briefly discuss the scenarios in which this nova may become a type Ia supernova progenitor.

Orio, Marina↗
NASA NTRS1995

Ionization nebulae surrounding CAL 83 and other supersoft X-ray sources

We present the results of an optical search for ionized gaseous nebulae surrounding luminous, 'supersoft' X-ray sources in the Large and Small Magellanic Clouds. This relatively new and mysterious X-ray class has characteristic luminosities approximately 10(exp 37)-10(exp 38) ergs/s with effective temperatures in the range of 2-6 x 10(exp 5) K. The presence of a large flux of UV and soft X-ray photons from these objects has led to predictions of bright optical emission lines from the local interstellar medium. One such object, CAL 83 in the LMC, was known to have an associated nebula, and we quantify here the asymmetry and luminosity of this remarkable nebula. Deep images were made using narowband filters to isolate the emission lines of H alpha and (O III) (lambda 5007). In these emission lines, the nebula is detected out to distances as far as 25 pc from the central object, and the integrated luminositu in each line is of order approximately 100 solar luminosity. Model calculations of such nebulae for chemical abundances characteristic of the LMC indicate that approximately 1% of the X-ray luminosity of the central source is reprocessed into the nebular H-alpha and (O III) lambda 5007 emission lines, from which we conclude that the time-averaged X-ray luminosity of the central source, CAL 83, is greater than 3 x 10(exp 37) ergs/s. The bright inner nebula contains approximately 150 solar mass within 7.5 pc of CAL 83, which clearly indicates that the nebular material has its origin in the interstelar medium. In sharp contrast, there were null detections for nebulae associated with nine other luminous, supersoft X-ray sources in the LMC and SMC, with upper limits for the (O III) luminosity that are a factor of approximately 10 below that for CAL 83. For eight of these latter sources, we conclude that either their time-averaged luminosity is substantially below that of CAL 83, or that the local interstellar medium is much less dense. The latter effect may be enhanced by expansion of the ionized nebula during the last several million years.

Remillard, R. A.↗
NASA NTRS1995

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.

White, N. E.↗
NASA NTRS1996

Accretion Disks in Supersoft X-ray Sources

We examine the role of the accretion disk in the steady-burning white dwarf model for supersoft sources. The accretion luminosity of the disk is quite small compared to the nuclear burning luminosity of the central source. Thus, in contrast to standard accretion disks, the main role of the disk is to reprocess the radiation from the white dwarf. We calculate models of accretion disks around luminous white dwarfs and compare the resulting disk fluxes to optical and UV observations of the LMC supersoft sources CAL 83, CAL 87, and RX J0513.9-6951. We find that if the white dwarf luminosity is near the upper end of the steady-burning region, and the flaring of the disk is included, then reprocessing by the disk can account for the UV fluxes and a substantial fraction of the optical fluxes of these systems. Reprocessing by the companion star can provide additional optical flux, and here too the disk plays an important role: since the disk is fairly thick, it shadows a significant fraction of the companion's surface.

Popham, Robert↗
NASA NTRS1995

Ionization Nebulae Surrounding CAL 83 and Other Supersoft X-ray Sources

We present the results of an optical search for ionized gaseous nebulae surrounding luminous, "supersoft" X-ray sources in the Large and Small Magellanic Clouds. This relatively new and mysterious X-ray class has characteristic luminosities approximately 10(exp 37) - 10(exp 38) ergs/s with effective temperatures in the range of 2 - 6 x 10(exp 5) K. The presence of a large flux of UV and soft X-ray photons from these objects has led to predictions of bright optical emission lines from the local interstellar medium. One such object, CAL 83 in the LMC, was known to have an associated nebula, and we quantify here the asymmetry and luminosity of this remarkable nebula. Deep images were made using narrowband filters to isolate the emission lines of H.alpha and [O III] (lamda5007). In these emission lines, the nebula is detected out to distances as far as 25 pc from the central object, and the integrated luminosity in each line is of order approximately 100 solar luminosity. Model calculations of such nebulae for chemical abundances characteristic of the LMC indicate that approximately 1% of the X-ray luminosity of the central source is reprocessed into the nebular H.alpha and [O III] lamda5007 emission lines, from which we conclude that the time-averaged X-ray luminosity of the central source, CAL 83, is greater than 3 x 10(exp 37) ergs/s. The bright inner nebula contains approximately 150 solar mass within 7.5 pc of CAL 83, which clearly indicates that the nebular material has its origin in the interstellar medium. In sharp contrast, there were null detections for nebulae associated with nine other luminous, supersoft X-ray sources in the LMC and SMC, with upper limits for the [O III] luminosity that are a factor of approximately 10 below that for CAL 83. For eight of these latter sources, we conclude that either their time-averaged luminosity is substantially below that of CAL 83, or that the local interstellar medium is much less dense. The latter effect may be enhanced by expansion of the ionized nebula during the last several million years.

Remillard, R. A.↗
DOE OSTI2024

Supersoft Norbornene–Based Thermoplastic Elastomers with High Strength and Upper Service Temperature

With over 6 million tons produced annually, thermoplastic elastomers (TPEs) have become ubiquitous in modern society, due to their unique combination of elasticity, toughness, and reprocessability. Nevertheless, industrial TPEs display a tradeoff between softness and strength, along with low upper service temperatures, typically ≤100 °C. This limits their utility, such as in bio-interfacial applications where supersoft deformation is required in tandem with strength, in addition to applications that require thermal stability (e.g., encapsulation of electronics, seals/joints for aeronautics, protective clothing for firefighting, and biomedical devices that can be subjected to steam sterilization). Thus, combining softness, strength, and high thermal resistance into a single versatile TPE has remained an unmet opportunity. Through de novo design and synthesis of novel norbornene-based ABA triblock copolymers, this gap is filled. Ring-opening metathesis polymerization is employed to prepare TPEs with an unprecedented combination of properties, including skin-like moduli (<100 kPa), strength competitive with commercial TPEs (>5 MPa), and upper service temperatures akin to high-performance plastics (≈260 °C). Here, the materials are elastic, tough, reprocessable, and shelf stable (≥2 months) without incorporation of plasticizer. Structure–property relationships identified herein inform development of next-generation TPEs that are both biologically soft yet thermomechanically durable.

36 MATERIALS SCIENCE↗
NASA NTRS1993

Detection of supersoft X-ray emission from GQ Muscae nine years after a nova outburst

The detection by the Rosat satellite of GQ Muscae as a very soft blackbodylike source is reported. If the observed X-ray flux is being radiated at the Eddington luminosity from a one solar mass white dwarf, its effective temperature must be about 350,000 K. It is concluded that the white dwarf is burning hydrogen-rich material near its surface. GQ Mus is, however, the only one of 26 recent novae detected in the all-sky Rosat study; this suggests that either most novae eject all their accreted material during outburst or GQ Mus is now burning recently accreted material. GQ Mus appears identical to the supersoft X-ray sources CAL83, CAL87, and RX J0527.8-6954, lending support to the suggestion that these sources are white dwarfs accreting and burning material from a companion.

Oegelman, Hakki↗
NASA NTRS1993

A supersoft variable low-luminosity X-ray source in the globular cluster M3

The globular cluster M3 (NGC 5272) was observed twice with the ROSAT high-resolution imager in order to study the low-luminosity X-ray source 1E 1339.8 + 2837. In 1992 January 1E 1339.8 + 2837 had an X-ray luminosity of 2 x 10 exp 35 ergs/s over an order of magnitude brighter than it was when observed with the Einstein Observatory. The source was unresolved and very soft; such supersoft outbursts would be difficult to detect in the vast majority of globular clusters which are more heavily absorbed than M3. In 1992 June the source was too faint to be detected. The soft outburst luminosity and the blackbody radius suggest that 1E 1339.8 + 2837 is a cataclysmic variable in which much of the luminosity is generated by steady nuclear burning of accreted material on the surface of the white dwarf primary.

Hertz, P.↗
NASA NTRS1996

Luminous Supersoft X-Ray Sources as Progenitors of Type Ia Supernovae

In some luminous supersoft X-ray sources, hydrogen accretes onto the surface of a white dwarf at rates more-or-less compatible with steady nuclear burning. The white dwarfs in these systems therefore have a good chance to grow in mass. Here we review what is known about the rate of Type la supernovae that may be associated with SSSS. Observable consequences of the conjecture that SSSs can be progenitors of Type Ia supernovae are also discussed.

DiStefano, R.↗
NASA NTRS1996

The Integrated X-Ray Spectrum of Galactic Populations of Luminous Supersoft X-Ray Sources

We compute the composite X-ray spectrum of a population of unresolved SSS's in a spiral galaxy such as our own or M31. The sources are meant to represent the total underlying population corresponding to all sources which have bolometric luminosities in the range of 10(exp 37) - 10(exp 38) ergs/s and kT on the order of tens of eV. These include close-binary supersoft sources, symbiotic novae, and planetary nebulae, for example. In order to determine whether the associated X-ray signal would be detectable, we also 'seed' the galaxy with other types of X-ray sources, specifically low-mass X-ray binaries (LMXB's) and high-mass X-ray binaries (HMXB's). We find that the total spectrum due to SSS's, LMXB's, and HMXB's exhibits a soft peak which owes its presence to the SSS population. Preliminary indications are that this soft peak may be observable.

DiStefano, R.↗
NASA NTRS1996

On the Evolution, Numbers, and Characteristics of Close-Binary Supersoft Sources

The ability to perform detailed evolutionary calculations is essential to the development of a well-defined and testable binary model. Unfortunately, traditional evolutionary calculations cannot be used to follow a significant fraction of possible close-binary supersoft sources (CBSS's). It is therefore important to examine the in-put physics carefully, to be sure that all relevant and potentially important physical processes are included. In this paper we continue a line of research begun last year, and explore the role that winds are expected to play in the evolution of CBSS's. We find that at least a subset of the systems that seemed to be candidates for common envelope evolution may survive, if radiation emitted by the white dwarf drives winds from the system. We study the effects of winds on the binary evolution of CBSS's, and compute the number and characteristics of CBSS's expected to be presently active in galaxies such as our own or M31.

DiStefano, R.↗
NASA NTRS2002

Luminous Binary Supersoft X-Ray Sources

This grant was for the study of Luminous Supersoft X-Ray Sources (SSSs). During the first year a number of projects were completed and new projects were started. The projects include: 1) Time variability of SSSs 2) SSSs in M31; 3) Binary evolution scenarios; and 4) Acquiring new data.

DiStefano, Rosanne↗
NASA NTRS2003

Luminous Binary Supersoft X-Ray Sources

We have made remarkable progress in the study of luminous supersoft X-ray sources during the past year. We have begun to discover a population of ultraluminous SSSs (e.g., in NGC 300 [Kong & Di Stefano 20031 as well as in Ml0l [Di Stefano & Kong 2003]), which may be accreting intermediate-mass (50-100 solar mass) black holes. This work follows from an algorithm we have developed (Di Stefano & Kong 2003) to identify SSSs in external galaxies, selecting them from among each galaxy s total population of X-ray sources. We have applied the algorithm to approximately one dozen galaxies and will make it public after it has been published in its entirety. Through our own application of the algorithm, we have discovered SSSs in every galaxy, mapping their spatial distribution, to obtain important clues to their fundamental natures. We have discovered that there is a large population of X-ray sources which are slightly hotter (100-250 eV) than standard SSSs. Some of these may be accreting BHs with masses between roughly 50 anf 100 solar masses. To explore this possibility, we are working on theoretical models for the formation and evolution of such systems (Di Stefano 2003).

Oliversen, Ronald J.↗