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Register a new userTidal torques, disc radius variations, and instabilities in dwarf novae and soft X-ray transients
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Authors
Jean-Marie Hameury
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The study of outer disc radius variations in close binary systems is important for understanding the structure and evolution of accretion discs. These variations are predicted by models of both quasi steady and time-dependent discs, and these predictions can be confronted with observations. We consider theoretical and observational consequences of such variations in cataclysmic variables and low mass X-ray binaries. We find that the action of tidal torques, that determine the outer radius at which the disc is truncated, must be important also well inside the tidal radius. We conclude that it is doubtful that the tidal-thermal instability is responsible for the superoutburst/superhump phenomena in dwarf novae, and confirm that it cannot be the reason for the outbursts of soft X-ray transients. It is likely that tidal torques play a role during superoutbursts of very-low mass-ratio systems but they cannot be the main and only cause of superhumps.
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I review the basics of the disc instability model (DIM) for dwarf novae and soft-X-ray transients and its most recent developments, as well as the current limitations of the model, focusing on the dwarf nova case. Although the DIM uses the Shakura-Sunyaev prescription for angular momentum transport, which we know now to be at best inaccurate, it is surprisingly efficient in reproducing the outbursts of dwarf novae and soft X-ray transients, provided that some ingredients, such as irradiation of the accretion disc and of the donor star, mass transfer variations, truncation of the inner disc, etc., are added to the basic model. As recently realized, taking into account the existence of winds and outflows and of the torque they exert on the accretion disc may significantly impact the model. I also discuss the origin of the superoutbursts that are probably due to a combination of variations of the mass transfer rate and of a tidal instability. I finally mention a number of unsolved problems and caveats, among which the most embarrassing one is the modelling of the low state. Despite significant progresses in the past few years both on our understanding of angular momentum transport, the DIM is still needed for understanding transient systems.
Transient short-period <100s oscillations have been found in the X-ray light curves of three novae during their SSS phase and in one persistent SSS. We pursue an observational approach to determine possible driving mechanisms and relations to fundamental system parameters such as the white dwarf mass. We performed a systematic search for short-period oscillations in all available XMM-Newton and Chandra X-ray light curves of persistent SSS and novae during their SSS phase. To study time evolution, we divided each light curve into short time segments and computed power spectra. We then constructed dynamic power spectra from which we identified transient periodic signals even when only present for a short time. From all time segments of each system, we computed fractions of time when periodic signals were detected. In addition to the previously known systems with short-period oscillations, RS Oph (35s), KT Eri (35s), V339 Del (54s), and Cal 83 (67s), we found one additional system, LMC 2009a (33s), and also confirm the 35s period from Chandra data of KT Eri. The amplitudes of oscillations are of order <15% of the respective count rates and vary without any clear dependence on the X-ray count rate. The fractions of the time when the respective periods were detected at 2-sigma significance (duty cycle) are 11.3%, 38.8%, 16.9%, 49.2%, and 18.7% for LMC 2009a, RS Oph, KT Eri, V339 Del, and Cal 83, respectively. The respective highest duty cycles found in a single observation are 38.1%, 74.5%, 61.4%, 67.8%, and 61.8%.
(Abridged) Narrow Line Seyfert 1 (NLS1) galaxies have low mass black holes and mass accretion rates close to (or exceeding) Eddington, so a standard blackbody accretion disc should peak in the EUV. However, the lack of true absorption opacity in the disc means that the emission is better approximated by a colour temperature corrected blackbody, and this colour temperature correction is large enough ($sim 2.4$) that the bare disc emission from a zero spin black hole can extend into the soft X-ray bandpass. Part of the soft X-ray excess seen in these objects must be intrinsic emission from the disc unless the vertical structure is very different to that predicted. However, the soft excess is much broader than predicted by a bare disc spectrum, indicating some Compton upscattering by cool, optically thick material. We associate this with the disc itself, so it must ultimately be powered by mass accretion. We build an energetically self consistent model assuming that the emission thermalises at large radii, but that at smaller radii the gravitational energy is split between powering optically thick Comptonised disc emission (forming the soft X-ray excess) and an optically thin corona above the disc (forming the tail to higher energies). We show examples of this model fit to the extreme NLS1 REJ1034+396, and to the much lower Eddington fraction Broad Line Seyfert 1 PG1048+231. We use these to guide our fits and interpretations of three template spectra made from co-adding multiple sources to track out a sequence of AGN spectra as a function of $L/L_{Edd}$. The new model is publically available within the {sc xspec} spectral fitting package.
We study the effect of physics input on thermal evolution of neutron stars in soft X-ray transients (SXTs). In particular, we consider different modern models of the sources of deep crustal heating during accretion episodes and the effects brought about by impurities embedded in the crust during its formation. We simulate thermal structure and evolution of episodically accreting neutron stars under different assumptions on the crust composition and on the distribution of heat sources and impurities. For the nonaccreted crust, we consider the nuclear charge fluctuations that arise at crust formation. For the accreted crust, we compare different theoretical models of composition and internal heating. We also compare results of numerical simulations with observations of the crust cooling in SXT MXB 1659-29. We found that the nonaccreted part of the inner crust of a neutron star can have a layered structure, with almost pure crystalline layers interchanging with layers composed of mixtures of different nuclei. The latter layers have relatively low thermal conductivities, which affects thermal evolution of the transients. The impurity distribution in the crust strongly depends on the models of the dense matter and the crust formation scenario. The shallow heating that is needed to reach agreement between the theory and observations depends on characteristics of the crust and envelope.
We perform the first high-sensitivity soft X-ray long-term monitoring with Swift/XRT of three relatively unexplored Supergiant Fast X-ray Transients (SFXTs), IGR J08408-4503, IGR J16328-4726, and IGR J16465-4507, whose hard X-ray duty cycles are the lowest measured among the SFXT sample, and compare their properties with those of the prototypical SFXTs. The behaviour of J08408 and J16328 resembles that of other SFXTs, and it is characterized by a relatively high inactivity duty cycle (IDC) and pronounced dynamic range (DR) in the X-ray luminosity. Like the SFXT prototypes, J08408 shows two distinct populations of flares, the first one associated with the brightest outbursts ($L_{rm X}gtrsim 10^{35-36}$ erg s$^{-1}$), the second one comprising less bright events with $L_{rm X}lesssim$10$^{35}$ erg s$^{-1}$. This double-peaked distribution seems to be a ubiquitous feature of the extreme SFXTs. The lower DR of J16328 suggests it is an intermediate SFXT. We find J16465 is characterized by IDC$sim$5% and DR$sim$40, reminiscent of classical supergiant HMXBs. The duty cycles measured with XRT are found to be comparable with those reported previously by BAT and INTEGRAL, when the higher limiting sensitivities of these instruments are taken into account and sufficiently long observational campaigns are available. We prove that no clear correlation exists between the duty cycles of the SFXTs and their orbital periods, which makes it difficult to interpret the SFXT peculiar variability by only using arguments related to the properties of supergiant star winds. Our findings favour the idea that a correct interpretation of the SFXT phenomenology requires a mechanism to strongly reduce the mass accretion rate onto the compact object during most of its orbit around the companion, as proposed in a number of theoretical works. [Abridged]
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