Luminous Supersoft X-ray Sources Research Articles

Multiwavelength Modeling the SED of Luminous Supersoft X-Ray Sources in Large Magellanic Cloud and Small Magellanic Cloud

Classical supersoft X-ray sources (SSSs) are understood as close binary systems in which a massive white dwarf accretes from its companion at rates sustaining steady hydrogen burning on its surface generating bolometric luminosities of 1036–2 × 1038 erg s−1. Here, we perform for the first time the global supersoft X-rays to near-infrared (NIR) spectral energy distribution (SED) for the brightest SSSs in the Large Magellanic Cloud and Small Magellanic Cloud. We test a model in which the ultraviolet–NIR is dominated by emission from a compact (unresolved) circumstellar nebula represented by the ionized gas outflowing from the SSS. The SED models correspond to luminosities of SSSs of a few times 1038–1039 erg s−1, radiating at blackbody temperatures of ≈3 × 105 K, and indicate a nebular continuum, whose emission measure of ≳2 × 1060 cm−3 corresponds to a wind mass loss at rates ≳2 × 10−6 M ⊙ yr−1. Such extreme parameters suggest that the brightest SSSs could be unidentified optical novae in a post-nova SSS state sustained at a high long-lasting luminosity by resumed accretion, possibly at super-Eddington rates. New observations and theoretical multiwavelength modeling of the global SED of SSSs are needed to reliably determine their parameters, and thus understand their proper stage in stellar evolution.

Supersoft X-Ray Sources Identified with Be Binaries in the Magellanic Clouds*

Abstract We investigated four luminous supersoft X-ray sources (SSSs) in the Magellanic Clouds suspected to have optical counterparts of Be spectral type. If the origin of the X-rays is in a very hot atmosphere heated by hydrogen burning in accreted envelopes of white dwarfs (WDs), like in the majority of SSSs, these objects are close binaries with very massive WD primaries. Using the South African Large Telescope, we obtained the first optical spectra of the proposed optical counterparts of two candidate Be stars associated with SUZAKU J0105–72 and XMMU J010147.5–715550, respectively, a transient and a recurrent SSS, and confirmed the proposed Be classification and Small Magellanic Cloud membership. We also obtained new optical spectra of two other Be stars proposed as optical counterparts of the transient SSS XMMU J052016.0–692505 and MAXI J0158–744. The optical spectra with double-peaked emission-line profiles are typical of Be stars and present characteristics similar to many high-mass X-ray binaries with excretion disks truncated by the tidal interaction with a compact object. The presence of a massive WD that sporadically ignites nuclear burning, accreting only at certain orbital or evolutionary phases, explains the supersoft X-ray flares. We measured equivalent widths and distances between line peaks and investigated the variability of the prominent emission-line profiles. The excretion disks seem to be small in size and are likely to be differentially rotating. We discuss possible future observations and the relevance of these objects as a new class of SN Ia progenitors.

What We Learn from the X-Ray Grating Spectra of Nova SMC 2016

Abstract Nova SMC 2016 has been the most luminous nova known in the direction of the Magellanic Clouds. It turned into a very luminous supersoft X-ray source between days 16 and 28 after the optical maximum. We observed it with Chandra, the HRC-S camera, and the Low Energy Transmission Grating on 2016 November and 2017 January (days 39 and 88 after optical maximum), and with XMM-Newton on 2016 December (day 75). We detected the compact white dwarf (WD) spectrum as a luminous supersoft X-ray continuum with deep absorption features of carbon, nitrogen, magnesium, calcium, probably argon, and sulfur on day 39, and oxygen, nitrogen, and carbon on days 75 and 88. The spectral features attributed to the WD atmosphere are all blueshifted, by about 1800 km s−1 on day 39 and up to 2100 km s−1 in the following observations. Spectral lines attributed to low-ionization potential transitions in the interstellar medium are also observed. Assuming the distance to the Small Magellanic Cloud, the bolometric luminosity exceeded the Eddington level for at least three months. A preliminary analysis with atmospheric models indicates an effective temperature of around 700,000 K on day 39, peaking at the later dates in the 850,000–900,000 K range, as expected for a ≃1.25 M ⊙ WD. We suggest a possible classification as an oxygen–neon WD, but more precise modeling is needed to accurately determine the abundances. The X-ray light curves show a large, aperiodic flux variability, which is not associated with spectral variability. We detected red noise, but did not find periodic or quasiperiodic modulations.

On double-degenerate type Ia supernova progenitors as supersoft X-ray sources

Context: The nature of the progenitors of type Ia supernova (SN) progenitors remains unclear. While it is usually agreed that single-degenerate progenitor systems would be luminous supersoft X-ray sources (SSSs), it was recently suggested that double-degenerate progenitors might also go through a SSS phase. Aims: We aim to examine the possibility of double-degenerate progenitor systems being SSSs, and place stringent upper limits on the maximally possible durations of any SSS phases and expected number of these systems in a galactic population. Method: We employ the binary population synthesis code SeBa to examine the mass-transfer characteristics of a possible SSS phase of double-degenerate type Ia SN progenitor systems for 1) the standard SeBa assumptions, and 2) an optimistic best-case scenario. The latter case establishes firm upper limits on the possible population of supersoft source double-degenerate type Ia SN progenitor systems. Results: Our results indicate that unlike what is expected for single-degenerate progenitor systems, the vast majority of the material accreted by either pure wind mass transfer or a combination of wind and RLOF mass transfer is helium rather than hydrogen. Even with extremely optimistic assumptions concerning the mass-transfer and retention efficiencies, the average mass accreted by systems that eventually become double-degenerate type Ia SNe is small. Consequently, the lengths of time that these systems may be SSSs are short, even under optimal conditions, and the expected number of such systems in a galactic population is negligible. Conclusions: The population of double-degenerate type Ia SN progenitors that are SSSs is at least an order of magnitude smaller than the population of single-degenerate progenitors expected to be SSSs, and the supersoft X-ray behaviour of double-degenerate systems typically ceases long before the SN explosions.

ANALYSIS OF A STATE CHANGING SUPERSOFT X-RAY SOURCE IN M31

We report on observations of a luminous supersoft X-ray source (SSS) in M31, r1-25, that has exhibited spectral changes to harder X-ray states. We document these spectral changes. In addition, we show that they have important implications for modeling the source. Quasisoft states in a source that has been observed as an SSS represent a newly discovered phenomenon. We show how such state changers could prove to be examples of unusual black hole or neutron star accretors. Future observations of this and other state changers can provide the information needed to determine the nature(s) of these intriguing new sources.

Upper limits on bolometric luminosities of 10 Type Ia supernova progenitors fromChandraobservations

We present an analysis of Chandra observations of the position of 10 nearby (<25 Mpc) Type Ia supernovae, taken before the explosions. No sources corresponding to progenitors were found in any of the observations. We calculated upper limits on the bolometric luminosities of the progenitors assuming blackbody X-ray spectra with temperatures of 30–150 eV. This is inspired by the fact that luminous super-soft X-ray sources have been suggested as the direct progenitors of Type Ia supernovae. The upper limits of two supernovae in our sample are comparable to the luminosities of the brightest observed super-soft sources, ruling out such sources as the progenitors of these supernovae. In contrast to Liu et al., we find that for SN2011fe we can rule out Eddington luminosity systems for blackbody temperatures as low as 40 eV. Our findings are consistent with statistical studies comparing the observed Type Ia supernova rate to the number of super-soft sources or the integrated X-ray luminosity in external galaxies. This suggests that either the progenitors of Type Ia supernovae are not accreting, nuclear burning white dwarfs or that they do not look like the classical super-soft sources, e.g. because they are obscured.

X-ray Grating Observations of Recent Recurrent Novae

AbstractTwo recurrent novae (RNe) that do not host red giants were observed in outburst at the beginning of 2009 and 2010, respectively. The first nova was LMC 2009a, and the second one was U Scorpii. Nova LMC 2009a was a relatively slow RN, and it was quite luminous both at optical and X-ray wavelengths. U Sco is the fastest nova ever recorded. Its supersoft X-ray phase started a couple of weeks after optical maximum and lasted for about a month, while for Nova LMC 2009a this phase started four months after maximum and lasted for 5 months. For both novae, the first X-ray spectrum taken while the luminous supersoft X-ray source was beginning to emerge is remarkably similar with broad and prominent emission lines of nitrogen and carbon accounting for at least 30% of the X-ray flux. Blue-shifted absorption and red-shifted emission form apparent P-Cyg profiles. We attribute the emission features to the ejecta and show evidence that they are largely due to collisional ionization. In the case of U Sco the absorption lines were embedded in the emission features after the first observation. We find evidence that in U Scorpii we were observing the Thomson reflected spectrum at a distance of ≃ 3 R⊙ from the white dwarf rather than the atmosphere itself. For both novae, the peak temperature was remarkably high, probably close to 900,000 K for U Sco, and about 600,000 K for Nova LMC 2009a. We suggest that these two objects represent different stages of RNe secular evolution.

The Appearance of Type Ia Supernova Progenitors: If Not SSSs, then What Do They Look Like?

Abstract“What do the progenitors of Type Ia supernovae (SNe Ia) look like? How can we hope to find them?” We focus on the epoch during which mass is incident on a white dwarf (WD) at high rates (&gt; 10−7M⊙ yr−1). Such epochs are expected in single-degenerate (SD) progenitors, double-degenerate (DD) progenitors, and in a wide range of binaries with WDs that will not achieve the Chandrasekhar mass, MCh. High-rate accretion onto a WD produces high luminosities through accretion alone; in addition, most calculations show that quasisteady or episodic nuclear burning can occur, increasing the luminosity by more than an order of magnitude. If the photosphere is not much larger than the WD, the emission will have values of kT in the range of tens of eV, and the source will appear as a luminous supersoft x-ray source (SSS). Studies of local SSSs that are good candidates for nuclear-burning WDs (NBWDs) suggest that many have low duty cycles of SSS activity. This is consistent with the fact that binary WD models predict about 100 times as many SSSs in external galaxies of all types as are actually detected. Interstellar absorption does not appear to be the problem. Instead, it is likely that the ~1037−1038 erg s−1 emitted by NBWDs emerges in other wavebands. The challenge we face is to search for highly luminous systems within the Milky Way and nearby galaxies that have unusual properties consistent with NBWDs, and inconsistent with other physical models. Model tests can then be conducted for individual candidates, allowing us to identify large numbers of progenitors years before explosion.

THE PROGENITORS OF TYPE Ia SUPERNOVAE. I. ARE THEY SUPERSOFT SOURCES?

In a canonical model, the progenitors of Type Ia supernovae (SNe Ia) are accreting, nuclear-burning white dwarfs (NBWDs), which explode when the white dwarf reaches the Chandrasekhar mass, M_C. Such massive NBWDs are hot (kT ~100 eV), luminous (L ~ 10^{38} erg/s), and are potentially observable as luminous supersoft X-ray sources (SSSs). During the past several years, surveys for soft X-ray sources in external galaxies have been conducted. This paper shows that the results falsify the hypothesis that a large fraction of progenitors are NBWDs which are presently observable as SSSs. The data also place limits on sub-M_C models. While Type Ia supernova progenitors may pass through one or more phases of SSS activity, these phases are far shorter than the time needed to accrete most of the matter that brings them close to M_C.

Ten facts of life for distant supersoft sources

First discovered in the Magellanic Clouds and in the Milky Way, the largest pools of luminous supersoft X-ray sources (SSSs) now known lie in M31 and in more distant galaxies. Hundreds of newly-discovered SSSs are helping us to test models for Type Ia supernovae and to identify SSSs that may represent a wider range of physical systems, including accreting intermediate-mass black holes. In this short report we list ten intriguing facts about distant SSSs.

No Periodicity Revealed for an “Eclipsing” Ultraluminous Supersoft X‐Ray Source in M81

Luminous supersoft X-ray sources found in the Milky Way and Magellanic Clouds are likely white dwarfs that steadily or cyclically burn accreted matter on their surface, which are promising Type Ia supernova progenitors. Observations of distant galaxies with Chandra and XMM-Newton have revealed supersoft sources that are generally hotter and more luminous, including some ultraluminous supersoft sources (ULSs) that are possibly intermediate-mass black holes of a few thousand solar masses. In this paper we report our X-ray spectral and timing analysis for M81-ULS 1, a ULS in the nearby spiral galaxy M81. M81-ULS 1 has been persistently supersoft in 17 Chandra ACIS observations spanning 6 years, and its spectrum can be described by either a kTbb ≈ 70 eV blackbody for a ~1.2 M☉ white dwarf, or a kTin ≈ 80 eV multicolor accretion disk for a 103 M☉ intermediate-mass black hole. In two observations, the light curves exhibited dramatic flux drop/rise on timescales of 103 s, reminiscent of eclipse ingress/egress in eclipsing X-ray binaries. However, the exhaustive search for periodicity in the reasonable range of 50 ks to 50 days failed to reveal an orbital period. The failure to reveal any periodicity is consistent with the long period (≥30 yr) predicted for this system given the optical identification of the secondary with an asymptotic giant star. In addition, the eclipse-like dramatic flux changes in hours are hard to explain under the white dwarf model, but can in principle be explained by disk temperature changes induced by accretion rate variations under the intermediate-mass black hole model.

Thermal Stability of White Dwarfs Accreting Hydrogen‐rich Matter and Progenitors of Type Ia Supernovae

We revisit the properties of white dwarfs accreting hydrogen-rich matter by constructing steady-state models, in which hydrogen shell burning consumes hydrogen at the same rate as the white dwarf accretes it. We obtain such steady-state models for various accretion rates and white dwarf masses. We confirm that these steady models are thermally stable only when the accretion rate is higher than \sim 10^{-7} M_sun/yr. We show that recent models of ``quiescent burning'' in the ``surface hydrogen burning'' at a much wider range of accretion rates results from the too large zone mass in the outer part of the models; hydrogen burning must occur in a much thinner layer. A comparison of the positions on the HR diagram suggests that most of the luminous supersoft X-ray sources are white dwarfs accreting matter at rates high enough that the hydrogen burning shell is thermally stable. Implications on the progenitors of Type Ia supernovae are discussed.

A new luminous supersoft X-ray source in NGC 300

Context. We report the discovery of a new luminous supersoft source, XMMU J005455.0−374117, in the nearby spiral galaxy GC 300, in XMM-Newton observations performed on 2005 May 22 and on 2005 November 25. The source is not present in ROSAT data nor in the previous XMM-Newton observations of 2000 December/2001 January. The unique luminous supersoft source,XMMU J005510.7−373855, detected in the 1992 May/June ROSAT data and in the 2000/2001 XMM-Newton data, fell below detectability. This source already appeared highly variable in ROSAT observations. Aims. We report on the temporal and spectral analysis of this new supersoft source (SSS) and compare its properties with the previous known SSS. Methods. We present the light curves of the SSS, model its spectrum and estimate the corresponding flux and luminosities. Results. The light curve of XMMU J005455.0−374117 does not show large fluctuations in any of the observations and its spectrum can be modelled with an absorbed blackbody with kT ∼ 60 eV. The corresponding bolometric luminosity is 8.1+1.4 −4.5 × 1038 erg s−1 in the first observation and drops to 2.2+0.5 −1.4 × 1038 erg s−1 six months later. No optical source brighter than mV ∼ 21.7 mag is found coincident with its position. Conclusions. The luminosity of these two SSSs is higher than what has been found for “classical” SSSs. Their nature could be explained by beamed emission from steady nuclear burning of hydrogen onto white dwarfs, or accretion onto stellar-mass black hole with matter outflow or observed at high inclination angle. The presence of an intermediate-mass black hole seems unlikely in our case.

X-Ray Observations of Classical and Recurrent Novae in Outburst

AbstractI review X-ray observations of classical and recurrent novae in outburst, some of them recently done with Chandra and XMM-Newton for 12 objects. Significant X-ray flux is emitted by the nova shell, with a peak luminosity up to Lx = 1035 erg s–1 in the 0.2-10 keV range. In recurrent nova systems, or in novae hosting a red giant, the source of X-rays may be previous circumstellar matter shocked by the nova wind. However, for most classical novae, X-rays originate inside the nebula ejected in the outburst. The data indicate a very high fraction of shocked material, and a non-smooth, varying wind outflow. A nebular emission line spectrum is also observed at late phases. In about half of the observed novae, the central white dwarf appears as a very luminous supersoft X-ray source for 1 to 9 years after the outburst. It is the best type of object to study the characteristics of shell hydrogen burning on white dwarfs in single degenerate systems. Still incomplete statistics indicate that the duration of the supersoft X-ray phase is peaked around ≃2 years. The correlation of the X-ray light curve with the nova properties is not quite clear. Recently, “template grating spectra” with high S/N have been obtained for V4743 Sgr. The X-ray light curve of this nova reveals a rich and complex power spectrum, with signatures of non-radial g-mode oscillations of the white dwarf. The oscillations and the spectra allow to determine the properties of the shell hydrogen burning white dwarf.

Luminous supersoft X-ray sources

We discuss possible evolution channels that lead to the formation of luminous supersoft X-ray sources, subclasses of which may be progenitors of type Ia supernovae. We carry out full evolution calculations from the zero-age main sequence to the supersoft source. A novel feature of our calculations is the inclusion of thermohaline mixing after mass transfer during binary evolution. The main effect of this is to produce secondaries of non-solar composition. Candidate initial progenitors are intermediate-mass donors of about with companions in the range . We concentrate on early case-C evolution, which means that the primary fills its Roche lobe when it ascends the Early Asymptotic Giant Branch while its core is highly evolved and massive enough to form a CO white dwarf. A crucial role, established by observations in this part of HR diagram, is played by mass loss in winds and we treat winds with a new approach. Since common-envelope evolution (CE) is generally invoked to explain the formation of close binaries with one or two degenerate components, we assume that the progenitors undergo severe mass and angular momentum loss through such a phase. We further study how the configurations of the post-CE systems, composed of a massive white dwarf and a companion, depend on the parameters of CE-evolution and mass-loss rates in various phases of evolution. Under these general assumptions a new path for the formation of SSSs is found which differs from that of the, usually assumed, solar composition donors. Our results may explain supersoft systems with enhanced helium abundances such as U Sco and very luminous extragalactic supersoft sources such as CAL 83 in the LMC and possibly the CHANDRA source (N1) in M 81.

A New Interpretation of the Low Radial Velocity Determined from the HeII Emission Line in the Galactic Supersoft X-Ray Source RX J0019.8+2156

We propose a new interpretation for the low radial velocity of He II emission line in the Galactic supersoft X-ray source RX J0019.8+2156 based on a 2-D numerical simulation of dynamical mass transfer in binary systems. A more massive main-sequence (MS) companion of ~ 2.0 M⊙ is derived for a ~ 0.6 M⊙ white dwarf (WD) primary.The Galactic supersoft X-ray source RX J0019.8+2156 (hereafter, RX J0019) with a 12.2 mag optical counterpart was discovered by ROSAT. The source is a binary with the period of 0.66 days and its X-ray luminosity is fairly constant, at least, for 7 yrs. Therefore RX J0019 seems to be a typical luminous supersoft X-ray source (SSS) with steady hydrogen shell-burning on a mass-accreting white dwarf (WD).

X‐Ray Point Sources in the Sombrero Galaxy: Very Soft Sources, the Globular Cluster/Low‐Mass X‐Ray Binary Connection, and an Overview

We report on the population of point sources discovered during an 18.5 ks Chandra ACIS-S observation of the Sombrero galaxy. We present the luminosity function and the spectra of the six brightest sources, consider correlations with globular clusters (GCs) and with planetary nebulae, and study the galaxy's population of very soft sources. We detected 122 sources. Twenty-two sources are identified as very soft; of these, five appear to be classical luminous supersoft X-ray sources (SSSs), while 17 may belong to the slightly harder class referred to as quasi-soft (QSSs). There is an overdensity of very soft sources within 2 kpc of the nucleus, which is itself the brightest X-ray source. Very soft sources are also found in the disk and halo, with one QSS in a globular cluster (GC). This source is somewhat harder than most SSSs; the energy distribution of its photons is consistent with what is expected from an accreting intermediate-mass black hole. Several sources in the Sombrero's halo are good candidates for SSS models in which the accretor is a nuclear-burning white dwarf. In total, 32 X-ray sources are associated with GCs. The majority of sources with luminosity greater than 1038 ergs s-1 are in GCs. These results for M104, an Sa galaxy, are similar to what has been found for elliptical galaxies and for the late-type spiral M31. We find that those optically bright GCs with X-ray sources house only the brightest X-ray sources. We find that, in common with other galaxies, there appears to be a positive connection between young (metal-rich) GCs and X-ray sources but that the brightest X-ray sources are equally likely to be in metal-poor GCs. The luminosity function of X-ray sources in GCs has a cut-off near the Eddington luminosity for a 1.4 M☉ object. We propose a model that can explain the trends seen in the data sets from the Sombrero and other galaxies. Thermal timescale mass transfer can occur in some of the younger clusters in which the turnoff mass is slightly greater than 0.8 M☉; multiplicity may play a role in some of the most massive clusters; accretion from giant stars may be the dominant mechanism in some older, less massive and less centrally concentrated clusters. Key elements of the model can be tested.

XMM--Newton observation of the X-ray point source population of the starburst galaxy IC 342

We present the results of an XMM–Newton observation of the starburst galaxy IC 342. Thirty-seven X-ray point sources were detected down to a luminosity limit of ∼1037 erg s−1. Most of the sources are located near the spiral arms. The X-ray point source luminosity function is consistent with a power-law shape with a slope of 0.55, typical of starburst galaxies. We also present the energy spectra of several ultraluminous X-ray sources (ULXs), including the luminous X-ray source in the galactic nucleus. Except for the nucleus and a luminous supersoft X-ray source, other ULXs can generally be fitted with a simple power-law spectral model. The nucleus is very luminous (∼1040 erg s−1 in 0.2–12 keV) and requires disc blackbody and power-law components to describe the X-ray emission. The spectral fit reveals a cool accretion disc (kT= 0.11 keV) and suggests that the source harbours either an intermediate-mass black hole or a stellar-mass black hole with outflow.

AnXMM‐NewtonObservation 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 0.6 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.

A Luminous Recurrent Supersoft X-Ray Source in NGC 300

We report the results of XMM-Newton observations for an especially luminous supersoft X-ray source (SSS) with a bolometric luminosity of 1039 ergs s-1 in the spiral galaxy NGC 300. The source was detected as an SSS in 1992 and disappeared in subsequent X-ray observations. The source was active again during recent XMM-Newton observations. It appeared to be very soft (kT ~ 60 eV) and very luminous (~1038-1039 ergs s-1). The two XMM-Newton observations also reveal that the source went from a high state to a state in 6 days. We also found a 5.4 hr periodicity during the low state. We consider white dwarf, black hole, and neutron star models in order to explain the nature of the source.