The bright transient AT2018cow has been unlike any other known type of transient. Its high brightness, rapid rise and decay and initially nearly featureless spectrum are unprecedented and difficult to explain using models for similar burst sources. We present evidence for faint γ-ray emission continuing for at least 8 days, and featureless spectra in the ultraviolet bands -both unusual for eruptive sources. The X-ray variability of the source has a burst-like character. The UV-optical spectrum does not show any CNO line but is well described by a blackbody. We demonstrate that a model invoking the tidal disruption of a 0.1 − 0.4 M Helium White Dwarf (WD) by a 10 5 − 10 6 M Black Hole (BH) located in the outskirts of galaxy Z 137-068 could provide an explanation for most of the characteristics shown in the multi-wavelength observations. A blackbody-like emission is emitted from an opaque photosphere, formed by the debris of the WD disruption. Broad features showing up in the optical/infrared spectra in the early stage are probably velocity broadened lines produced in a transient high-velocity outward moving cocoon. The asymmetric optical/infrared lines that appeared at a later stage are emission from an atmospheric layer when it detached from thermal equilibrium with the photosphere, which undergoes more rapid cooling. The photosphere shrinks when its temperature drops, and the subsequent infall of the atmosphere produced asymmetric line profiles. Additionally, a non-thermal jet might be present, emitting X-rays in the 10 − 150 keV band.
Binary millisecond pulsars (MSPs) are believed to have descended from low-mass X-ray binaries (LMXBs), which have experienced substantial mass transfer and tidal circularization. Therefore, they should have very circular orbits. However, the discovery of several eccentric binary MSPs (with eccentricity e ∼ 0.01–0.1) challenges this standard picture. Three models have been proposed thus far based on accretion-induced collapse of massive white dwarfs (WDs), neutron star–strange star transition, and formation of circumbinary disks. All of them are subject to various uncertainties and are not entirely consistent with observations. Here, we propose an alternative model that takes into account the influence of thermonuclear flashes on proto-WDs. We assume that the flashes lead to asymmetrical mass ejection, which imparts a mild kick to the proto-WDs. By simulating orbital changes of binary MSPs with multiple shell flashes, we show that it is possible to reproduce the observed eccentricities, provided that the kick velocities are around a few km s−1.
We report a detailed analysis of the orbital properties of binary millisecond pulsar (MSP) with a white dwarf (WD) companion. Positive correlations between the orbital period P b and eccentricity ǫ are found in two classes of MSP binaries with a He WD and with a CO/ONeMg WD, though their trends are different. The distribution of P b is not uniform. Deficiency of sources at P b ∼ 35 − 50 days (Gap 1) have been mentioned in previous studies. On the other hand, another gap at P b ∼ 2.5 − 4.5 days (Gap 2) is identified for the first time. Inspection of the relation between P b and the companion masses M c revealed the subpopulations of MSP binaries with a He WD separated by Gap 1, above which P b is independent of M c (horizontal branch) but below which P b correlates strongly with M c (lower branch). Distinctive horizontal branch and lower branch separated by Gap 2 were identified for the MSP binaries with a CO/ONeMg WD at shorter P b and higher M c . Generally, M c are higher in the horizontal branch than in the lower branch for the MSP binaries with a He WD. These properties can be explained in terms of a binary orbital evolution scenario in which the WD companion was ablated by a pulsar wind in the post mass-transfer phase.
SS433 is an extraordinary X-ray binary that is ejecting bipolar jets at 26% the speed of light. Associated with the supernova-like shell W50, SS433 is also probably one of the youngest X-ray binaries with an age of ≲105 yr. However, the masses of the two components in SS433 and even the nature of the compact object are still under debate. In this work, assuming that the compact object is a black hole, we employ a binary population synthesis method to study the formation of SS433. We use previous estimates of the age of W50 and the duration of the jet activity to constrain the evolutionary history. Our calculations suggest that SS433 likely harbored a Hertzsprung gap star at the beginning of the current Roche-lobe overflow phase. The masses of the black hole and the optical/donor star in SS433 suggested by the simulations are around 8 M ☉ and 24 M ☉, respectively. Future measurement of the donor mass and radius can help infer not only the origin of the binary but also the nature of the nebula W50.
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