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Register a new userWhere are all of the nebulae ionized by supersoft X-ray sources?
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Accreting, steadily nuclear-burning white dwarfs are associated with so-called close-binary supersoft X-ray sources (SSSs), observed to have temperatures of a few$times 10^{5}$K and luminosities on the order of $10^{38}$erg/s. These and other types of SSSs are expected to be capable of ionizing their surrounding circumstellar medium, however, to date only one such nebula was detected in the Large Magellanic Cloud (of its 6 known close-binary SSSs), surrounding the accreting, nuclear-burning WD CAL 83. This has led to the conclusion that most SSSs cannot have been both luminous ($gtrsim 10^{37}$erg/s) and hot ($gtrsim$ few $times 10^{4}$K) for the majority of their past accretion history, unless the density of the ISM surrounding most sources is much less than that inferred for the CAL 83 nebula (4--10$rm{cm}^{-3}$). Here we demonstrate that most SSSs must lie in much lower density media than CAL 83. Past efforts to detect such nebulae have not accounted for the structure of the ISM in star-forming galaxies and, in particular, for the fact that most of the volume is occupied by low density warm & hot ISM. CAL 83 appears to lie in a region of ISM which is at least $sim 40$-fold overdense. We compute the probability of such an event to be $approx 18%$, in good agreement with observed statistics. We provide a revised model for the typical SSS nebula, and outline the requirements of a survey of the Magellanic clouds which could detect the majority of such objects. We then briefly discuss some of the possible implications, should there prove to be a large population of previously undiscovered ionizing sources.
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We present the results of deep optical spectroscopic observations using the LRIS spectrograph on the Keck I 10-m telescope of three ultra-luminous X-ray sources (ULXs), Ho IX X-1; M81 X-6; and Ho II X-1. Our observations reveal the existence of large (100 - 200 pc diameter) highly-ionized nebulae, identified by diffuse He II (4686 Angstrom) emission, surrounding these sources. Our results are the first to find highly-ionized nebulae of this extent, and the detection in all three objects indicates this may be a common feature of ULXs. In addition to the extended emission, Ho IX X-1 has an unresolved central component containing about one-third of the total He II flux, with a significant velocity dispersion of ~ 370 km/s, suggestive of the existence of a photo-ionized accretion disk or an extremely hot early-type stellar counterpart. Most of the He II emission appears to be surrounded by significantly more extended Hbeta emission, and the intensity ratios between the two lines, which range from 0.12 - 0.33, indicate that photo-ionization is the origin of the He II emission. Sustaining these extended nebulae requires substantial X-ray emission, in the range ~ 10^{39} - 10^{40} ergs/s, comparable to the measured X-ray luminosities of the sources. This favors models where the X-ray emission is isotropic, rather than beamed, which includes the interpretation that ULXs harbor intermediate-mass black holes.
Supersoft X-ray sources are stellar objects which emit X-rays with temperatures of about 1 million Kelvin and luminosities well in excess of what can be produced by stellar coronae. It has generally been presumed that the objects in this class are binary star systems in which mass transfer leads to nuclear fusion on the surface of a white dwarf. Classical novae, the runaway fusion events on the surfaces of white dwarfs, generally have supersoft phases, and it is often stated that the bright steady supersoft X-ray sources seen from white dwarfs accreting mass at a high rate are undergoing steady nuclear fusion. In this letter, we report the discovery of a transient supersoft source in the Small Magellanic Cloud without any signature of nuclear fusion having taken place. This discovery indicates that the X-ray emission probably comes from a spreading layer - a belt on the surface of the white dwarf near the inner edge of the accretion disk in which a large fraction of the total accretion energy is emitted - and (albeit more tentatively) that the accreting white dwarf is relatively massive. We thus establish that the presence of a supersoft source cannot always be used as a tracer of nuclear fusion, in contradiction with decades-old consensus about the nature of supersoft emission.
Classical novae occur on the surface of an accreting white dwarf in a binary system. After ejection of a fraction of the envelope and when the expanding shell becomes optically thin to X-rays, a bright source of supersoft X-rays arises, powered by residual H burning on the surface of the white dwarf. While the general picture of the nova event is well established, the details and balance of accretion and ejection processes in classical novae are still full of unknowns. The long-term balance of accreted matter is of special interest for massive accreting white dwarfs, which may be promising supernova Ia progenitor candidates. V5116 Sgr was observed as a bright and variable supersoft X-ray source by XMM-Newton 610~days after outburst. The light curve showed a periodicity consistent with the orbital period. During one third of the orbit the luminosity was a factor of seven brighter than during the other two thirds of the orbital period. In the present work we aim to disentangle the X-ray spectral components of V5116 Sgr and their variability. We present the high resolution spectra obtained with XMM-Newton RGS and Chandra LETGS/HRC-S in March and August 2007. The grating spectrum during the periods of high-flux shows a typical hot white dwarf atmosphere dominated by absorption lines of N VI and N VII. During the low-flux periods, the spectrum is dominated by an atmosphere with the same temperature as during the high-flux period, but with several emission features superimposed. Some of the emission lines are well modeled with an optically thin plasma in collisional equilibrium, rich in C and N, which also explains some excess in the spectra of the high-flux period. No velocity shifts are observed in the absorption lines, with an upper limit set by the spectral resolution of 500 km/s, consistent with the expectation of a non-expanding atmosphere so late in the evolution of the post-nova.
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.
We utilize observations of 16 white dwarf stars to calculate and analyze the oxidation states of the parent bodies accreting onto the stars. Oxygen fugacity, a measure of overall oxidation state for rocks, is as important as pressure and temperature in determining the structure of a planet. We find that most of the extrasolar rocky bodies formed under oxidizing conditions, but approximately 1/4 of the polluted white dwarfs have compositions consistent with more reduced parent bodies. The difficulty in constraining the oxidation states of relatively reduced bodies is discussed and a model for the time-dependent evolution of the apparent oxygen fugacity for a hypothetical reduced body engulfed by a WD is investigated. Differences in diffusive fluxes of various elements through the WD envelope yield spurious inferred bulk elemental compositions and oxidation states of the accreting parent bodies under certain conditions. The worst case for biasing against detection of reduced bodies occurs for high effective temperatures. For moderate and low effective temperatures, evidence for relatively reduced parent bodies is preserved under most circumstances for at least several characteristic lifetimes of the debris disk.
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