Supersoft X-ray Sources Research Articles

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.

The outburst of Nova CSS 081007:030559+054715 (HV Ceti)

Aims. The outburst of nova CSS 081007:030559+054715 (HV Cet) was extensively followed by Swift to study the evolution of its X-ray and UV emission.Methods. We obtained Swift X-ray and UV observations, at times with high cadence, over an interval of 266 days and analysed the temporal and spectral properties of the object. Optical polarisation measurements were also obtained, but no significant circular polarisation was identified. UV data from GALEX and AAVSO optical data were investigated in addition.Results. A variable supersoft source was discovered, with a 1.77 d periodic modulation in its X-ray, UV and optical emission, in phase across all three bands. High cadence observations revealed a declining trend of the X-ray modulation amplitude, while the UV oscillation became stronger over this interval; the UV data also show occasional secondary minima. The X-ray and UV flux declines sharply ~150 days after discovery (the date of optical outburst peak is uncertain). While bright, the UV and, to a lesser extent, the X-ray light curves show two cycles of a ~40–50 day modulation. We believe the 1.77 d modulation to be orbital, likely caused by structured disc-rim occultation. The X-ray spectra were soft at all times, with negligible flux above 1 keV. These spectra were fitted with an NLTE white dwarf model atmosphere of kT = 60–80 eV, with no spectral changes related to the 1.77 d or possible ~40-day periodicities seen. A Cloudy model demonstrated that it is possible to account for the UV flux through the reprocessing of X-rays. The UV flux is far in excess of the soft X-ray spectral component. We propose that the apparently sub-Eddington white dwarf luminosity is due to permanent obscuration by the accretion disc rim: we see only X-rays scattered by optically thin plasma interior to the disc rim, whereas the UV flux is mostly created by reprocessing in the disk rim interior surface. The in-phase 1.77 d UV and X-ray modulation is caused by the region in which the gas stream from the secondary raises the disc rim, the maximum X-ray obscuration coinciding with the minimum aspect of the UV reprocessing area. In summary, HV Cet had characteristics of both classical novae and persistent super-soft X-ray sources.

A VLT VIMOS IFU study of the ionisation nebula surrounding the supersoft X-ray source CAL 83

CAL83 is a prototype of the class of Super Soft X-ray Sources (SXS). It is a binary consisting of a low mass secondary that is transferring mass onto a white dwarf primary and is the only known SXS surrounded by an ionisation nebula, made up of the interstellar medium (ISM) ionised by the source itself. With the VIMOS IFU we obtained spectra over a 25\times25" field of view, encompassing one quarter of the nebula. Emission line maps - H I, He II, [O III], [N II], and [S II] - are produced in order to study the morphology of the ionised gas. We include CAL83 on diagrams of various diagnostic ion ratios to compare it to other X-ray ionised sources. Finally we computed some simple models of the ionised gas around CAL83 and compare the predicted to the observed spectra. CAL83 appears to have a fairly standard ionisation nebula as far as the morphology goes: the edges where H is recombining are strong in the low stage ionisation lines and the central, clumpy regions are stronger in the higher stage ionisation lines. But the He II emission is unusual in being confined to one side of CAL83 rather than being homogeneously distributed as with the other ions. We model the CAL83 nebula with cloudy using model parameters for SXSs found in the literature. The He II emission does not fit in with model predictions; in fact none of the models is able to fit the observed spectrum very well. The spectral line images of the region surrounding CAL83 are revealing and instructive. However, more modelling of the spectrum of the ionised gas is necessary, and especially for the high-ionisation level emission from CAL83. In particular, we wish to know if the He II emission and the other nebular lines are powered by the same ionising source.

Accretion process powering the supersoft X-ray sources: A test with the multiwavelength modeling the SED

AbstractRadiation of supersoft X-ray sources (SSS) dominates both the supersof X-ray and the far-UV domain, and can be detected also within the optical/near-IR wavelengths. To determine fundamental parameters of SSSs, a multiwavelength approach in modeling their spectra is essential. By this way, the basic physical parameters of a SSS (the temperature, radius, luminosity and column density of the neutral hydrogen) can be determined unambiguously. Here I demonstrate this case for the symbiotic X-ray binary RXJ0059.1-7505 (LIN 358) in the Small Magellanic Cloud (SMC).

M31N 2008-05d: a M 31 disk nova with a dipping supersoft X-ray light curve

Classical novae (CNe) represent a major class of supersoft X-ray sources (SSSs) in the central region of our neighbouring galaxy M 31. Significantly different SSS properties of CNe in the M 31 bulge and disk were indicated by recent X-ray population studies, which however considered only a small number of disk novae. We initiated a target of opportunity (ToO) program with XMM-Newton to observe the SSS phases of CNe in the disk of M 31 and improve the database for further population studies. We analysed two XMM-Newton ToO observations triggered in Aug 2011 and Jan 2012, respectively, and extracted X-ray spectra and light curves. We report the discovery of an X-ray counterpart to the M 31 disk nova M31N 2008-05d. The X-ray spectrum of the object allows us to classify it as a SSS parametrised by a blackbody temperature of 32+/-6 eV. More than three years after the nova outburst, the X-ray light curve of the SSS exhibits irregular, broad dip features. These dips affect primarily the very soft part of the X-ray spectrum, which might indicate absorption effects. Dipping SSS light curves are rarely observed in M 31 novae. As well as providing an unparalleled statistical sample, the M 31 population of novae with SSS counterparts produces frequent discoveries of unusual objects, thereby underlining the importance of regular monitoring.

A new super-soft X-ray source in the Small Magellanic Cloud: Discovery of the first Be/white dwarf system in the SMC?

The Small Magellanic Cloud (SMC) hosts a large number of Be/X-ray binaries, however no Be/white dwarf system is known so far, although population synthesis calculations predict that they might be more frequent than Be/neutron star systems. XMMUJ010147.5-715550 was found as a new faint super-soft X-ray source (SSS) with a likely Be star optical counterpart. We investigate the nature of this system and search for further high-absorbed candidates in the SMC. We analysed the XMM-Newton X-ray spectrum and light curve, optical photometry, and the I-band OGLE III light curve. The X-ray spectrum is well represented by black-body and white dwarf atmosphere models with highly model-dependent temperature between 20 and 100 eV. The likely optical counterpart AzV 281 showed low near infrared emission during X-ray activity, followed by a brightening in the I-band afterwards. We find further candidates for high-absorbed SSSs with a blue star as counterpart. We discuss XMMUJ010147.5-715550 as the first candidate for a Be/white dwarf binary system in the SMC.

The Wendelstein Calar Alto Pixellensing Project (WeCAPP): the M 31 nova catalogue

We present light curves from the novae detected in the long-term, M31 monitoring WeCAPP project. The goal of WeCAPP is to constrain the compact dark matter fraction of the M31 halo with microlensing observations. As a by product we have detected 91 novae benefiting from the high cadence and highly sensitive difference imaging technique required for pixellensing. We thus can now present the largest CCD and optical filters based nova light curve sample up-to-date towards M31. We also obtained thorough coverage of the light curve before and after the eruption thanks to the long-term monitoring. We apply the nova taxonomy proposed by Strope et al. (2010) to our nova candidates and found 29 S-class novae, 10 C-class novae, 2 O-class novae and 1 J-class nova. We have investigated the universal decline law advocated by Hachichu and Kato (2006) on the S-class novae. In addition, we correlated our catalogue with the literature and found 4 potential recurrent novae. Part of our catalogue has been used to search for optical counter-parts of the super soft X-ray sources detected in M31 (Pietsch et al. 2005). Optical surveys like WeCAPP, and coordinated with multi-wavelength observation, will continue to shed light on the underlying physical mechanism of novae in the future.

ECLIPSES DURING THE 2010 ERUPTION OF THE RECURRENT NOVA U SCORPII

The eruption of the recurrent nova U Scorpii on 28 January 2010 is now the all-time best observed nova event. We report 36,776 magnitudes throughout its 67 day eruption, for an average of one measure every 2.6 minutes. This unique and unprecedented coverage is the first time that a nova has any substantial amount of fast photometry. With this, two new phenomena have been discovered: the fast flares in the early light curve seen from days 9-15 (which have no proposed explanation) and the optical dips seen out of eclipse from days 41-61 (likely caused by raised rims of the accretion disk occulting the bright inner regions of the disk as seen over specific orbital phases). The expanding shell and wind cleared enough from days 12-15 so that the inner binary system became visible, resulting in the sudden onset of eclipses and the turn-on of the supersoft X-ray source. On day 15, a strong asymmetry in the out-of-eclipse light points to the existence of the accretion stream. The normal optical flickering restarts on day 24.5. For days 15-26, eclipse mapping shows that the optical source is spherically symmetric with a radius of 4.1 R_sun. For days 26-41, the optical light is coming from a rim-bright disk of radius 3.4 R_sun. For days 41-67, the optical source is a center-bright disk of radius 2.2 R_sun. Throughout the eruption, the colors remain essentially constant. We present 12 eclipse times during eruption plus five just after the eruption.

Supersoft X-ray sources

Supersoft X-ray sources constitute a fascinating class of X-ray systems. Some of these sources consist of a compact object accreting material from a binary companion. The analysis of X-ray and optical spectra discloses valuable information on the exact nature and temporal variation of the extremely powerful physical processes involved.

X-ray monitoring of classical novae in the central region of M 31

[Abridged] Classical novae (CNe) represent the major class of supersoft X-ray sources (SSSs) in the central region of our neighbouring galaxy M 31. We performed a dedicated monitoring of the M 31 central region with XMM-Newton and Chandra between Nov 2007 and Feb 2008 and between Nov 2008 and Feb 2009 respectively, in order to find SSS counterparts of CNe, determine the duration of their SSS phase and derive physical outburst parameters. We systematically searched our data for X-ray counterparts of CNe and determined their X-ray light curves and spectral properties. We detected in total 17 X-ray counterparts of CNe in M 31, only four of which were known previously. These latter sources are still active 12.5, 11.0, 7.4 and 4.8 years after the optical outburst. From the 17 X-ray counterparts 13 were classified as SSSs. Four novae displayed short SSS phases (< 100 d). Based on these results and previous studies we compiled a catalogue of all novae with SSS counterparts in M 31 known so far. We used this catalogue to derive correlations between the following X-ray and optical nova parameters: turn-on time, turn-off time, effective temperature (X-ray), t2 decay time and expansion velocity of the ejected envelope (optical). Furthermore, we found a first hint for the existence of a difference between SSS parameters of novae associated with the stellar populations of the M 31 bulge and disk. Additionally, we conducted a Monte Carlo Markov Chain simulation on the intrinsic fraction of novae with SSS phase. This simulation showed that the relatively high fraction of novae without detected SSS emission might be explained by the inevitably incomplete coverage with X-ray observations in combination with a large fraction of novae with short SSS states, as expected from the WD mass distribution. In order to verify our results with an increased sample further monitoring observations are needed.

Binary population synthesis study of the supersoft X-ray phase of single degenerate type Ia supernova progenitors

In the single degenerate (SD) scenario for type Ia supernovae (SNe Ia), a mass-accreting white dwarf is expected to experience a supersoft X-ray source (SSS) phase. However, some recent observations showed that the expected number of mass-accreting WDs is much lower than that predicted from theory, regardless of whether they are in spiral or elliptical galaxies. In this paper, we performed a binary population synthesis study on the relative duration of the SSS phase to their whole mass-increasing phase of WDs leading to SNe Ia. We found that for about 40% of the progenitor systems, the relative duration is shorter than 2% and the evolution of the mean relative duration shows that it is always smaller than 5%, both for young and old SNe Ia. In addition, before the SNe Ia explosions, more than 55% of the progenitor systems were experiencing a dwarf novae phase and no more than 10% were staying in the SSS phase. These results are consistent with the recent observations and imply that both in early- and late-type galaxies, only a small fraction of mass-accreting WDs resulting in SNe Ia contributes to the supersoft X-ray flux. So, although our results are not directly related to the X-ray output of the SN Ia progenitor, the low supersoft X-ray luminosity observed in early type galaxies may not be able to exclude the validity of the SD model. On the contrary, it is evidence to support the SD scenario.

Obscuring Supersoft X-ray Sources in Stellar Winds

AbstractWe investigate the possibility of obscuring supersoft X-ray sources in the winds of companion stars. We derive limits on the amount of circumstellar material needed to fully obscure a ‘canonical’ supersoft X-ray source in the Large Magellanic Cloud, as observed with the Chandra X-ray Observatory.

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.

Novae and Accreting White Dwarfs as Progenitors of Type Ia Supernovae

AbstractI review various phenomena associated with mass-accreting white dwarfs (WDs) in relation to progenitors of Type Ia supernovae (SNe Ia). The WD mass can be estimated from light curve analysis in multiwavelength bands based on the theory of optically thick winds. In the single degenerate scenario of SNe Ia, two main channels are known, i.e., WD + main sequence (MS) channel and WD + red giant (RG) channel. In each channel, a typical binary undergoes three evolutionary stages before explosion, i.e., the wind phase, supersoft X-ray source (SSS) phase, and recurrent nova phase, in this order because the accretion rate decreases with time as the companion mass decreases. For some accreting WDs we can identify the corresponding stage of evolution. Intermittent supersoft X-ray sources like RX J0513.9−6951 and V Sge correspond to wind phase objects. For the SSS phase, CAL 87-type objects correspond to the WD+MS channel. For the WD + RG channel, soft X-ray observations of early type galaxies give statistical evidence of SSS phase binaries. Recurrent novae of U Sco-type and RS Oph-type correspond to the WD + MS channel and WD + RG channel, respectively. The majority of recurrent novae host a very massive WD (≳ 1.35 M⊙) and often show a plateau phase in their optical light curves corresponding to the long-lasting supersoft X-ray phase. These properties are indications of increasing WD masses.

Classical Novae as Supersoft X-ray Sources in the Andromeda Galaxy

AbstractWe review the first results of a dedicated, on-going monitoring project for supersoft X-ray source (SSS) states of classical novae (CNe) in the central region of the Andromeda galaxy (M 31). So far, during three high-cadence observation campaigns with the X-ray telescopes XMM-Newton and Chandra from 2006 until 2009 we detected 21 CNe, only four of which were seen in X-rays before. This increased the total number of known CNe with X-ray counterparts in M 31 to 60, which is significantly larger than for any other galaxy, including the Milky Way. We compiled a catalog of X-ray and optical properties for these 60 sources which allowed us to perform source statistics and population studies of CNe with SSS state in M 31 for the first time. Here we review the results of this analysis: (i) several correlations between X-ray and optical parameters, (ii) an analysis of the intrinsic fraction of CNe with SSS state, and (iii) the first nova population study in X-rays. Our first results are promising, however a continuing monitoring of the unique target M 31 is strongly needed to extend this valuable data base and to refine the early statements and predictions.

Type Ia Supernovae and Supersoft X-ray Sources

AbstractThe rates of SNe Ia for double-degenerate and single-degenerate scenarios are computed for the models of spiral and elliptical galaxies. The number of nuclear burning white dwarfs (NBWDs) is traced. The data favors the double-degenerate scenario and suggests a lower number of NBWDs per unit mass in ellipticals. Their lower average mass is one of the reasons for the difference in the number of supersoft X-ray sources observed in the galaxies of different types.

The Mass of the White Dwarf in the Recurrent Nova CI Aquilae

AbstractIf recurrent novae (RNe) are progenitors of Type Ia Supernovae (SNe Ia), their white dwarfs (WD) must have masses close to the Chandrasekhar limit. The most reliable means of determining WD masses in RNe is dynamically, via radial-velocity and rotational-broadening measurements of the companion star. Such measurements require the system to be both eclipsing and to show absorption features from the secondary star. The only other reliable RNe mass estimate is for U Sco, which has a WD mass of 1.55 ± 0.24 M⊙ (Thoroughgood et al. 2001).We present new time-resolved, intermediate-resolution spectroscopy of the eclipsing RN CI Aql during quiescence. We find the mass of the WD to be 1.02 ± 0.08 M⊙ and the mass of the secondary star to be 2.41 ± 0.2 M⊙. We estimate the radius of the secondary to be 2.10 ± 0.07 R⊙. The high mass ratio of q=2.37 ± 0.15 and the high secondary star mass suggests that mass transfer occurs on a thermal timescale. We suggest that CI Aql is evolving into a supersoft x-ray source, and ultimately will explode as an SN Ia.

Multiwavelength Modeling the SED of Strongly Interacting Binaries

AbstractThe spectrum of strongly interacting binaries, as for example, high and low mass X-ray binaries, symbiotic (X-ray) binaries and/or classical and recurrent novae, consists of more components of radiation contributing from hard X-rays to radio wavelengths. To understand the basic physical processes responsible for the observed spectrum we have to disentangle the composite spectrum into its individual components, i.e. to determine their physical parameters. In this short contribution I demonstrate the method of modeling the multiwavelength SED on the example of the extragalactic super-soft X-ray source RX J0059.1-7505 (LIN 358).

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.

On the Progenitor of SNR 0509-67.5

AbstractWe have used three independent methods to determine an accurate and precise geometric center of SNR 0509-67.5, at RA=05:09:31.208, DEC=−67:31:17.48 (J2000). This supernova, which occurred approximately 400 years ago in the Large Magellanic Cloud, was confirmed to be a Type Ia by Rest et al. (2005), Rest et al. (2008) based on spectra of a light echo from the eruption. If this supernova had a single-degenerate progenitor system, we would see the “leftover” companion star within a certain distance of the remnant's center. Accounting for an offset due to enhanced ISM in the west-southwest quadrant of the remnant, we find the eruption position to be at RA=05:09:30.976, DEC=−67:31:17.90; the error circle which should contain any possible ex-companion star has a radius of 1.60″ for 99.73% (3-sigma) containment. This accounts for the proper motion of the stars, the possibility of kicks from the supernova, and asymmetries in the explosion and remnant expansion. We find no possible ex-companion stars within this ellipse, to a limiting magnitude of V=26.9: there are no red giants, which precludes symbiotic progenitors, no subgiants, which when combined with the lack of red giants precludes recurrent nova progenitors, and no main sequence stars with mass greater than 1.16 solar masses (V brighter than 22.7 mag), which precludes persistent supersoft X-ray source progenitors. Indeed, all published SD models are eliminated, so we conclude that this particular Type Ia supernova had a double-degenerate progenitor.