Supersoft X-ray Sources Research Articles

SINGLE-DEGENERATE TYPE Ia SUPERNOVAE ARE PREFERENTIALLY OVERLUMINOUS

Recent observational and theoretical progress has favored merging and helium-accreting sub-Chandrasekhar mass white dwarfs in the double-degenerate and the double-detonation channels, respectively, as the most promising progenitors of normal Type Ia supernovae (SNe Ia). Thus the fate of rapidly-accreting Chandrasekhar mass white dwarfs in the single-degenerate channel remains more mysterious then ever. In this paper, we clarify the nature of ignition in Chandrasekhar-mass single-degenerate SNe Ia by analytically deriving the existence of a characteristic length scale which establishes a transition from central ignitions to buoyancy-driven ignitions. Using this criterion, combined with data from three-dimensional simulations of convection and ignition, we demonstrate that the overwhelming majority of ignition events within Chandrasekhar-mass white dwarfs in the single-degenerate channel are buoyancy-driven, and consequently lack a vigorous deflagration phase. We thus infer that single-degenerate SNe Ia are generally expected to lead to overluminous 1991T-like SNe Ia events. We establish that the rates predicted from both the population of supersoft X-ray sources and binary population synthesis models of the single-degenerate channel are broadly consistent with the observed rates of overluminous SNe Ia, and suggest that the population of supersoft X-ray sources are the dominant stellar progenitors of SNe 1991T-like events. We further demonstrate that the single-degenerate channel contribution to the normal and failed 2002cx-like rates is not likely to exceed 1% of the total SNe Ia rate. We conclude with a range of observational tests of overluminous SNe Ia which will either support or strongly constrain the single-degenerate scenario.

PHOTOIONIZATION HEATING OF NOVA EJECTA BY THE POST-OUTBURST SUPERSOFT SOURCE

The expanding ejecta from a classical nova remains hot enough ($\sim10^{4}\, {\rm K}$) to be detected in thermal radio emission for up to years after the cessation of mass loss triggered by a thermonuclear instability on the underlying white dwarf (WD). Nebular spectroscopy of nova remnants confirms the hot temperatures observed in radio observations. During this same period, the unstable thermonuclear burning transitions to a prolonged period of stable burning of the remnant hydrogen-rich envelope, causing the WD to become, temporarily, a super-soft X-ray source. We show that photoionization heating of the expanding ejecta by the hot WD maintains the observed nearly constant temperature of $(1-4)\times10^4\mathrm{~K}$ for up to a year before an eventual decline in temperature due to either the cessation of the supersoft phase or the onset of a predominantly adiabatic expansion. We simulate the expanding ejecta using a one-zone model as well as the Cloudy spectral synthesis code, both incorporating the time-dependent WD effective temperatures for a range of masses from $0.60\ M_{\odot}$ to $1.10\ M_{\odot}$. We show that the duration of the nearly isothermal phase depends most strongly on the velocity and mass of the ejecta and that the ejecta temperature depends on the WD's effective temperature, and hence its mass.

500 days of Stromgren b, y and narrow-band [OIII], H[formula omitted] photometric evolution of gamma-ray Nova Del 2013 (=V339 Del)

We present and discuss highly accurate photometry obtained through medium Stromgren y,b bands and narrow [OIII], Hα bands covering 500days of the evolution of Nova Del 2013 since its maximum brightness. This is by far the most complete study of any nova observed in such photometric systems. The nova behavior in these photometric bands is very different from that observed with the more conventional broad bands like UBVRCIC or u′g′r′i′z′, providing unique information about extent and ionization of the ejecta, the onset of critical phases like the transition between optically thick and thin conditions, and re-ionization by the central super-soft X-ray source. The actual transmission profiles of the y,b, [OIII] and Hα photometric filters have been accurately measured at different epochs and different illumination angles, to evaluate in detail their performance under exact operating conditions. The extreme smoothness of both the Hα and [OIII] lightcurves argues for absence of large and abrupt discontinuities in the ejecta of Nova Del 2013. Should they exist, glitches in the lightcurves would have appeared when the ionization and/or recombination fronts overtook them. During the period of recorded very large variability (up to 100× over a single day) in the super-soft X-ray emission (from day +69 to +86 past V maximum), no glitch in excess of 1% was observed in the optical photometry, either in the continuum (Stromgren y) or in the lines ([OIII] and Hα filters), or in a combination of the two (Stromgren b, Johnson B and V). Considering that the recombination time scale in the ejecta was one week at that time, this excludes global changes of the white dwarf as the source of the X-ray variability and supports instead clumpy ejecta passing through the line of sight to us as its origin.

SPECTROSCOPIC STUDIES OF AN ULTRALUMINOUS SUPERSOFT X-RAY SOURCE IN M81

Ultraluminous supersoft X-ray sources (ULSs) exhibit supersoft X-ray spectra with blackbody temperatures below 0.1 keV and bolometric luminosities above 1039 ergs s−1. In this Letter, we report the first optical spectroscopic observations of a ULS in M81 using the LRIS spectrograph on the Keck I telescope. The detected Balmer emission lines show a mean intrinsic velocity dispersion of 400 ± 80 km s−1, which is consistent with that from an accretion disk. The spectral index of the continuum on the blue side is also consistent with the multi-color disk model. The Hα emission line exhibits a velocity of ∼180 km s−1 relative to the local stellar environment, suggesting that this ULS may be a halo system in M81 belonging to an old population. No significant shift is found for the Hα emission line between two observations separated by four nights.

X-Ray Observations of VY Scl-Type Nova-Like Binaries in the High and Low State

Four VY Scl-type nova-like systems were observed in X-rays both during the low and the high optical states. They are BZ Cam, MV Lyr, TT Ari, and V794 Aql. Using archival ROSAT, <em>Swift</em> and <em>SUZAKU</em> observations we found that the X-ray flux for BZ Cam is higher during the low state, but there is no supersoft X-ray source (SSS) that would indicate the thermonuclear burning predicted in a previous article. The X-ray flux is lower by a factor 2–10 in the low than the high state in other systems, and does not reflect the drop in <em>˙M</em> inferred from optical and UV data. The best fit model for the X-ray spectra is a collisionally ionized plasma model. The X-ray flux may originate in a shocked wind or in accretion onto polar caps in intermediate polar systems that continues even during the low state.

AE Aquarii: A Short Review

The nova-like variable AE Aquarii has been continuously studied since its discovery on photographic plates in 1934. In this short review the peculiar multi-wavelength properties of AE Aquarii will be reviewed and explained in context of its evolution from a high mass transfer phase, during which period it could have been a supersoft X-ray source (SSS).

Model Atmosphere Spectrum Fit to the Soft X-Ray Outburst Spectrum of SS Cyg

The X-ray spectrum of SS Cyg in outburst has a very soft component that can be interpreted as the fast-rotating optically thick boundary layer on the white dwarf surface. This component was carefully investigated by Mauche (2004) using the Chandra LETG spectrum of this object in outburst. The spectrum shows broad ( ≈5 °A) spectral features that have been interpreted as a large number of absorption lines on a blackbody continuum with a temperature of ≈250 kK. Because the spectrum resembles the photospheric spectra of super-soft X-ray sources, we tried to fit it with high gravity hot LTE stellar model atmospheres with solar chemical composition, specially computed for this purpose. We obtained a reasonably good fit to the 60–125 °A spectrum with the following parameters: T<sub>eff</sub> = 190 kK, log g = 6.2, and NH = 8 · 10<sup>19</sup> cm<sup>−2</sup>, although at shorter wavelengths the observed spectrum has a much higher flux. The reasons for this are discussed. The hypothesis of a fast rotating boundary layer is supported by the derived low surface gravity.

GLORIA and Study of Cataclysmic Variables

We report here on the ongoing EU FP7 Project GLORIA (GLObal Robotic-telescopes Intelligent Array) with emphasis on possibility of investigation of cataclysmic variables by users. GLORIA will enable the first free and open-access network of robotic telescopes in the world. We show several examples of the not often used topics (but suitable for GLORIA) for the studies of activity of cataclysmic variables, e.g. search for outbursts in intermediate polars and the fluctuations of brightness in their quiescence, and investigation of the optical counterparts of supersoft X-ray sources.

Supersoft X-Ray Source CAL 83: A Possible AE Aqr-like System

CAL83 is a close binary supersoft X-ray source in the Large Magellanic Cloud. A ~67 s periodicity detected in supersoft X-rays is most probably associated with the spin period of a highly spun-up white dwarf (WD). The variability in the period is ascribed to the obscuration of the WD by the hydrogen burning envelope surrounding it, rotating with a period that is close to, but not quite synchronized with, the WD rotation period. Optical spectra obtained with SALT exhibit accretion disc emission lines with broad wing structures and P Cyg profiles, indicating mass outflows. Timing analysis of photometrical observations performed at the South African Astronomical Observatory (SAAO) revealed variable signals at ≤1 mHz which are thought to be associated with quasi-periodic oscillations from an accretion disc. The short spin period inferred for CAL83 can be the result of spin-up by accretion disc torques during a long mass transfer history, placing this source on a similar evolutionary track as the cataclysmic variable AE Aqr.

Next generation population synthesis of accreting white dwarfs – I. Hybrid calculations using bse + mesa

Accreting, nuclear-burning white dwarfs have been deemed to be candidate progenitors of type Ia supernovae, and to account for supersoft X-ray sources, novae, etc. depending on their accretion rates. We have carried out a binary population synthesis study of their populations using two algorithms. In the first, we use the binary population synthesis code \textsf{BSE} as a baseline for the "rapid" approach commonly used in such studies. In the second, we employ a "hybrid" approach, in which we use \textsf{BSE} to generate a population of white dwarfs (WD) with non-degenerate companions on the verge of filling their Roche lobes. We then follow their mass transfer phase using the detailed stellar evolution code \textsf{MESA}. We investigate the evolution of the number of rapidly accreting white dwarfs (RAWDs) and stably nuclear-burning white dwarfs (SNBWDs), and estimate the type Ia supernovae (SNe Ia) rate produced by "single-degenerate" systems (SD). We find significant differences between the two algorithms in the predicted numbers of SNBWDs at early times, and also in the delay time distribution (DTD) of SD SNe Ia. Such differences in the treatment of mass transfer may partially account for differences in the SNe Ia rate and DTD found by different groups. Adopting 100\% efficiency for helium burning, the rate of SNe Ia produced by the SD-channel in a Milky-way-like galaxy in our calculations is $2.0\times10^{-4}\rm{yr}^{-1}$, more than an order of magnitude below the observationally inferred value. In agreement with previous studies, our calculated SD DTD is inconsistent with observations.

X-ray observations of VY Scl-type nova-like binaries in the high and low state

Four VY Scl-type nova-like systems were observed in X-rays during both the low and the high optical states. We examined Chandra, ROSAT, Swift and Suzaku archival observations of BZ Cam, MV Lyr, TT Ari, and V794 Aql. The X-ray flux of BZ Cam is higher during the low state, but there is no supersoft X-ray source (SSS) as hypothesized in previous articles. No SSS was detected in the low state of the any of the other systems, with the X-ray flux decreasing by a factor between 2 and 50. The best fit to the Swift X-ray spectra is obtained with a multi-component model of plasma in collisional ionization equilibrium. The high state high resolution spectra of TT Ari taken with Chandra ACIS-S and the HETG gratings show a rich emission line spectrum, with prominent lines of in Mg, Si, Ne, and S. The complexity of this spectrum seems to have origin in more than one region, or more than one single physical mechanism. While several emission lines are consistent with a cooling flow in an accretion stream, there is at least an additional component. We discuss the origin of this component, which is probably arising in a wind from the system. We also examine the possibility that the VY Scl systems may be intermediate polars, and that while the boundary layer of the accretion disk emits only in the extreme ultraviolet, part of the X-ray flux may be due to magnetically driven accretion.

Reprint of: Multiwavelength modeling the SED of supersoft X-ray sources III. RS Ophiuchi: The supersoft X-ray phase and beyond

I modeled the 14Å–37μm SED of the recurrent symbiotic nova RS Oph during its supersoft source (SSS) phase and the following quiescent phase. During the SSS phase, the model SEDs revealed the presence of a strong stellar and nebular component of radiation in the spectrum. The former was emitted by the burning WD at highly super-Eddington rate, while the latter represented a fraction of its radiation reprocessed by the thermal nebula. During the transition phase, both the components were decreasing and during quiescence the SED satisfied radiation produced by a large, optically thick disk (Rdisk>10R⊙). The super-Eddington luminosity of the burning WD during the SSS phase was independently justified by the high quantity of the nebular emission. The emitting material surrounded the burning WD, and its mass was (1.6±0.5)×10-4(d/1.6kpc)5/2M⊙. The helium ash, deposited on the WD surface during the whole burning period, was around of 8×10-6(d/1.6kpc)2M⊙, which yields an average growing rate of the WD mass, ṀWD∼4×10-7(d/1.6kpc)2M⊙yr-1. The mass accreted by the WD between outbursts, macc∼1.26×10-5M⊙, constrains the average accretion rate, Ṁacc∼6.3×10-7M⊙yr-1. During quiescence, the accretion rate from the model SED of ∼2.3×10-7M⊙yr-1 requires a super-Eddington accretion from the disk at ∼3.6×10-5M⊙yr-1 during the outburst. Such a high accretion can be responsible for the super-Eddington luminosity during the whole burning phase. Simultaneous presence of jets supports this scenario. If the wind from the giant is not sufficient to feed the WD at the required rate, the accretion can be realized from the disk-like reservoir of material around the WD. In this case the time between outbursts will extend, with the next explosion beyond 2027. In the opposite case, the wind from the giant has to be focused to the orbital plane to sustain the high accretion rate at a few ×10-7M⊙yr-1. Then the next explosion can occur even prior to 2027.

The orbital period of the V Sge star candidate QU Carinae★

Close Binary Supersoft X-ray Sources (CBSS) are considered strong candidates to SN Ia progenitors, but very few CBSS are known in our Galaxy. The galactic counterparts of the CBSS may be the V Sge stars, not detected in X-rays due to the strong absorption by the interstellar gas. Nevertheless, the number of members in the V Sge class is still small. In order to find new members we selected QU Car for photometric and spectroscopic observations. The orbital period of this system was published in the literature as 10.9 h, determined from radial velocity data taken in 1979-1980, but posterior analysis of data taken in 2006-2007 did not confirm this period. We analysed the high variability of its emission line profiles with the Temporal Variance Spectrum (TVS) technique. Besides, we recovered the 10.9 h orbital period from the radial velocities of the He II 4686 A emission line and, for the first time, detected what may be the orbital modulation in the photometric data. This photometric modulation is present only in the lower brightness state data, when the flickering is attenuated. The inclusion of QU Car in the V Sge class is supported by many features like high/low states, strong winds, nebular lines and He II 4686 A/Hbeta line ratios. However, the non-detection of the characteristic O VI 3811-34 A lines in its spectrum claims against this classification. These lines, though, may be highly variable so additional spectra analysed with the TVS technique can, possibly, solve this question.

THE X-RAY ECLIPSE GEOMETRY OF THE SUPER-SOFT X-RAY SOURCE CAL 87

We explore XMM-{\it Newton} observations of the eclipsing super-soft X-ray source CAL~87 in order to map the accretion structures of the system. { Indirect imaging techniques were applied in X-ray light curves to provide eclipse maps}. The surface brightness distribution exhibits an extended and symmetric emission, { and from the hardest X-rays is revealed a feature that is likely due to a bright spot}. A rate of $\dot{P} = (+6\pm2) \times 10^{-10}$ for changes in the orbital period of the system was derived from the eclipses. There is no significant variation of the emission lines even during eclipses, arguing that the lines are formed in an extended region. The continuum emission dominates the decrease in flux which is observed during eclipses. The O\,{\small VIII} Ly$\alpha$ line reveals a broadening velocity { which is} estimated in 365$^{+65}_{-69}$ km\,s$^{-1}$ (at 1$\sigma$) and marginal evidence for asymmetry in its profile, and sometimes shows evidence of double-peaked emission. Together, the results support that the wind-driven mass transfer scenario is running in CAL~87.

Multiwavelength search for counterparts of supersoft X-ray sources in M31

We searched optical/UV/IR counterparts of seven supersoft X-ray sources (SSS) in M31 in the Hubble Space Telescope (HST) "Panchromatic Hubble Andromeda Treasury" (PHAT) archival images and photometric catalog. Three of the SSS were transient, the other four are persistent sources. The PHAT offers the opportunity to identify SSS hosting very massive white dwarfs that may explode as type Ia supernovae in single degenerate binaries, with magnitudes and color indexes typical of symbiotic stars, high mass close binaries, or systems with optically luminous accretion disks. We find evidence that the transient SSS were classical or recurrent novae; two likely counterparts we identified are probably symbiotic binaries undergoing mass transfer at a very high rate. There is a candidate accreting white dwarf binary in the error circle of one of the persistent sources, r3-8. In the spatial error circle of the best studied SSS in M31, r2-12, no red giants or AGB stars are sufficiently luminous in the optical and UV bands to be symbiotic systems hosting an accreting and hydrogen burning white dwarf. This SSS has a known modulation of the X-ray flux with a 217.7 s period, and we measured an upper limit on its derivative, 0.82 x 10(-11). This limit can be reconciled with the rotation period of a white dwarf accreting at high rate in a binary with a few-hours orbital period. However, there is no luminous counterpart with color indexes typical of an accretion disk irradiated by a hot central source. Adopting a semi-empirical relationship, the upper limit for the disk optical luminosity implies an upper limit of only 169 minutes for the orbital period of the white dwarf binary.

Upper limits on the luminosity of the progenitor of Type Ia supernova SN 2014J

We analysed archival data of Chandra pre-explosion observations of the position of SN2014J in M82. No X-ray source at this position was detected in the data, and we calculated upper limits on the luminosities of the progenitor. These upper limits allow us to firmly rule out an unobscured supersoft X-ray source progenitor with a photospheric radius comparable to the radius of white dwarf near the Chandrasekhar mass (~1.38 M_sun) and mass accretion rate in the interval where stable nuclear burning can occur. However, due to a relatively large hydrogen column density implied by optical observations of the supernova, we cannot exclude a supersoft source with lower temperatures, kT < 80 eV. We find that the supernova is located in the centre of a large structure of soft diffuse emission, about 200 pc across. The mass, ~3x10^4 M_sun and short cooling time of the gas, tau_cool ~ 8 Myrs, suggest that it is a supernova-inflated super-bubble, associated with the region of recent star formation. If SN2014J is indeed located inside the bubble, it likely belongs to the prompt population of type Ia supernovae, with a delay time as short as ~ 50 Myrs. Finally, we analysed the one existing post-supernova Chandra observation and placed upper limit of ~ (1-2) 10^37 erg/s on the X-ray luminosity of the supernova itself.

Multiwavelength modeling the SED of supersoft X-ray sources III. RS Ophiuchi: The supersoft X-ray phase and beyond

I modeled the 14Å–37μm SED of the recurrent symbiotic nova RS Oph during its supersoft source (SSS) phase and the following quiescent phase. During the SSS phase, the model SEDs revealed the presence of a strong stellar and nebular component of radiation in the spectrum. The former was emitted by the burning WD at highly super-Eddington rate, while the latter represented a fraction of its radiation reprocessed by the thermal nebula. During the transition phase, both the components were decreasing and during quiescence the SED satisfied radiation produced by a large, optically thick disk (Rdisk>10R⊙). The super-Eddington luminosity of the burning WD during the SSS phase was independently justified by the high quantity of the nebular emission. The emitting material surrounded the burning WD, and its mass was (1.6±0.5)×10-4(d/1.6kpc)5/2M⊙. The helium ash, deposited on the WD surface during the whole burning period, was around of 8×10-6(d/1.6kpc)2M⊙, which yields an average growing rate of the WD mass, ṀWD∼4×10-7(d/1.6kpc)2M⊙yr-1. The mass accreted by the WD between outbursts, macc∼1.26×10-5M⊙, constrains the average accretion rate, Ṁacc∼6.3×10-7M⊙yr-1. During quiescence, the accretion rate from the model SED of ∼2.3×10-7M⊙yr-1 requires a super-Eddington accretion from the disk at ∼3.6×10-5M⊙yr-1 during the outburst. Such a high accretion can be responsible for the super-Eddington luminosity during the whole burning phase. Simultaneous presence of jets supports this scenario. If the wind from the giant is not sufficient to feed the WD at the required rate, the accretion can be realized from the disk-like reservoir of material around the WD. In this case the time between outbursts will extend, with the next explosion beyond 2027. In the opposite case, the wind from the giant has to be focused to the orbital plane to sustain the high accretion rate at a few ×10-7M⊙yr-1. Then the next explosion can occur even prior to 2027.

Progenitor constraints on the Type-Ia supernova SN2011fe from pre-explosion Hubble Space Telescope He ii narrow-band observations

Abstract We present Hubble Space Telescope (HST) imaging observations of the site of the Type-Ia supernova SN2011fe in the nearby galaxy M101, obtained about 1 yr prior to the event, in a narrow band centred on the He ii λ4686 Å emission line. In a ‘single-degenerate’ progenitor scenario, the hard photon flux from an accreting white dwarf (WD), burning hydrogen on its surface over ∼1 Myr should, in principle, create a He iii Strömgren sphere or shell surrounding the WD. Depending on the WD luminosity, the interstellar density, and the velocity of an outflow from the WD, the He iii region could appear unresolved, extended, or as a ring, with a range of possible surface brightnesses. We find no trace of He ii λ4686 Å line emission in the HST data. Using simulations, we set 2σ upper limits on the He ii λ4686 Å luminosity of LHeii &amp;lt; 3.4 × 1034 erg s−1 for a point source, corresponding to an emission region of radius r &amp;lt; 1.8 pc. The upper limit for an extended source is LHeii &amp;lt; 1.7 × 1035 erg s−1, corresponding to an extended region with r ∼ 11 pc. The largest detectable shell, given an interstellar-medium density of 1 cm−3, has a radius of ∼6 pc. Our results argue against the presence, within the ∼105 yr prior to the explosion, of a supersoft X-ray source of luminosity Lbol ≳ 3 × 1037 erg s−1, or of a super-Eddington accreting WD that produces an outflowing wind capable of producing cavities with radii of 2–6 pc.

Identification of new Galactic symbiotic stars with SALT – I. Initial discoveries and other emission line objects★

We introduce the first results from an ongoing, systematic survey for new symbiotic stars selected from the AAO/UKST SuperCOSMOS H$\alpha$ Survey (SHS). The survey aims to identify and characterise the fainter population of symbiotic stars underrepresented in extant catalogues. The accreting white dwarfs (WDs) in symbiotic stars, fuelled by their red giant donors with high mass loss rate winds, make them promising candidates for type Ia supernovae. Several candidates were observed spectroscopically with the Southern African Large Telescope (SALT). A total of 12 bona-fide and 3 possible symbiotic stars were identified. The most remarkable example is a rare carbon-rich symbiotic star that displays coronal [Fe X] $\lambda$6375 emission, suggesting it may be a supersoft X-ray source with a massive WD. Several other emission line objects with near-infrared colours similar to symbiotic stars are listed in an appendix, including 6 B[e] stars, 4 planetary nebulae (PNe), 2 possible Be stars, one [WC9] Wolf-Rayet (WR) central star of a PN and one WC9 WR star. These initial discoveries will help shape and refine the candidate selection criteria that we expect will uncover several more symbiotic stars as the survey progresses.

A remarkable recurrent nova in M 31: The X-ray observations

Another outburst of the recurrent M31 nova M31N 2008-12a was announced in late November 2013. Optical data suggest an unprecedentedly short recurrence time of approximately one year. In this Letter we address the X-ray properties of M31N 2008-12a. We requested Swift monitoring observations shortly after the optical discovery. We estimated source count rates and extracted X-ray spectra from the resulting data. The corresponding ultraviolet (UV) data was also analysed. M31N 2008-12a was clearly detected as a bright supersoft X-ray source (SSS) only six days after the well-constrained optical discovery. It displayed a short SSS phase of two weeks duration and an exceptionally hot X-ray spectrum with an effective blackbody temperature of ~97 eV. During the SSS phase the X-ray light curve displayed significant variability that might have been accompanied by spectral variations. The very early X-ray variability was found to be anti-correlated with simultaneous variations in the UV flux. The X-ray properties of M31N 2008-12a coherently point towards a high-mass white dwarf in the nova system. This object might be a promising Type Ia supernova progenitor. We re-discovered additional X-ray detections of M31N 2008-12a that are consistent with our data and increase the number of known nova outbursts to seven. This nova is an exceptional object that merits further attention in the future.