Hot white dwarfs and the UV delay in dwarf novae

We calculate the effect of illumination of dwarf nova accretion discs by radiation from a hot, central, white dwarf. We show that only for very hot white dwarfs (Teff ~ 40 000$ K) the inner region of quiescent dwarf nova discs are partially depleted so that the delay between the rise to outburst of the optical and UV fluxes would be increased as suggested recently by King (1997). This depletion, however, must create several small outbursts between main outbursts, contrary to observations. Lower white dwarf temperatures may cause the outburts to be of the `inside-out' type removing the UV delay. We conclude that white dwarf irradiation of dwarf nova discs is not very efficient for example because the UV radiation from the hot white dwarf does not penetrate deep enough in the disc atmosphere. The total ablation of the inner disc by e.g. evaporation (possibly related to illumination) appears to be a very promising possibility, accounting for both the EUV delay and the general lightcurves properties.Comment: 6 pages, 8 figures; accepted for publication in MNRA

X-ray irradiation in low mass binary systems

We calculate self-consistent models of X-ray irradiated accretion discs in close binary systems. We show that a point X-ray source powered by accretion and located in the disc plane cannot modify the disc structure, mainly because of the self-screening by the disc of its outer regions. Since observations show that the emission of the outer disc regions in low mass X-ray binaries is dominated by the reprocessed X-ray flux, accretion discs in these systems must be either warped or irradiated by a source above the disc plane, or both. We analyse the thermal-viscous stability of irradiated accretion discs and derive the stability criteria of such systems. We find that, contrary to the usual assumptions, the critical accretion rate below which a disc is unstable is rather uncertain since the correct formula describing irradiation is not well known.Comment: to be published in MNRAS, uses epsfig.st

The Compact UV Nucleus of M33

The most luminous X-ray source in the Local Group is associated with the nucleus of M33. This source, M33 X-8, appears modulated by ~20% over a ~106 day period, making it unlikely that the combined emission from unresolved sources could explain the otherwise persistent ~1e39 erg/s X-ray flux (Dubus et al. 1997, Hernquist et al. 1991). We present here high resolution UV imaging of the nucleus with the Planetary Camera of the HST undertaken in order to search for the counterpart to X-8. The nucleus is bluer and more compact than at longer wavelength images but it is still extended with half of its 3e38 erg/s UV luminosity coming from the inner 0.14". We cannot distinguish between a concentrated blue population and emission from a single object.Comment: 3 figures, accepted for publication in ApJ Letter

Approximate Solutions of the Boltzmann Equation for Secondary Electron Emission: Results and Comparisons to Experiments

The aim of the present paper is to survey the theoretical work performed in Brussels about secondary electron emission (SEE). Two new approximate solutions of the Boltzmann equation for internal secondary electrons are applied to both electron and ion induced SEE. Using a realistic set of interaction cross sections, most calculated characteristics of electron emission compare fairly well to experiments. The improved age-diffusion model can be used to calculate the electron yield, the energy and angular spectrum and also the depth and radial distributions of outgoing electrons for incident electrons and ions. The transport-albedo model assumes an uniform internal electron source in a semiinfinite medium and gives the electron yield and the energy spectrum of secondary electrons for incident light ions. Taking into account the anisotropy of the internal electron source, the ratio of the forward and backward yields and the influence of the angle of incidence have been calculated for thin targets

The disc instability model for X-ray transients: evidence for truncation and irradiation

We study the prospect of explaining the outbursts of Soft X-ray Transients (SXTs) by the thermal-viscous instability in a thin disc. This instability is linked to hydrogen ionization and is significantly changed when irradiation of the disc by X-rays from the inner regions is included. We present the first numerically reliable, physically consistent calculations of the outburst cycles which include the effects of accretion disc irradiation. The decay from outburst is governed by irradiation, as pointed out by King & Ritter (1998), leading to slow exponential decays. At the end of the outburst, the disc is severely depleted, which lengthens the time needed to rebuild mass to the critical density for an outburst. Despite this, the long recurrence times and quiescent X-ray luminosities of SXTs still require the inner disc to be replaced by another type of flow in quiescence, presumably a hot advection dominated accretion flow (ADAF). We include the effects of such truncation and show that the resulting lightcurves are conclusively similar to those of SXTs like A0620-00. We conclude that the two-alpha disc instability model provides an adequate description of the outbursts of SXTs when both truncation and irradiation are included. The values for the viscosities in outburst and in quiescence are comparable to those used in CVs. We discuss the model in the context of present observations

Long Term X-ray Monitoring Of The TeV Binary LS I +61 303 with RXTE

We report on the results of a long term X-ray monitoring campaign of the galactic binary LS I +61 303 performed by the Rossi X-ray Timing Explorer. This dataset consists of 1 ks pointings taken every other day between 2007 August 28 until 2008 February 2. The observations covered six full cycles of the 26.496 day binary period and constitute the largest continuous X-ray monitoring dataset on LS I +61 303 to date with this sensitivity. There is no statistically strong detection of modulation of flux or photon index with orbital phase; however, we do find a strong correlation between flux and photon index, with the spectrum becoming harder at higher fluxes. The dataset contains three large flaring episodes, the largest of these reaching a flux level of 7.2 (+0.1,-0.2)*10^-11 erg cm^-2 s^-1 in the 3-10 keV band, which is a factor of three times larger than flux levels typically seen in the system. Analysis of these flares shows the X-ray emission from LS I +61 303 changing by up to a factor of six over timescales of several hundred seconds as well as doubling times as fast as 2 seconds. This is the fastest variability ever observed from LS I +61 303 at this wavelength and places constraints on the size of the X-ray emitting region.Comment: 24 pages, 7 figures, 2 tables. Accepted for publication in Ap

Infrared study of H 1743-322 in outburst: a radio-quiet and NIR-dim microquasar

International audienceContext. Microquasars are accreting Galactic sources that are commonly observed to launch relativistic jets. One of the most important issues regarding these sources is the energy budget of ejections relative to the accretion of matter.Aims. The X-ray binary, black hole candidate, and microquasar H 1743-322 exhibited a series of X-ray outbursts between 2003 and 2008. We took optical and near-infrared (OIR) observations with the ESO/NTT telescope during three of these outbursts (2003, 2004, and 2008). The goals of these observations were to investigate the presence of a jet, and to disentangle the various contributions constituting the spectral energy distribution (SED): accretion, ejection, and stellar emission.Methods. Photometric and spectroscopic OIR observations allowed us to produce a high time-resolution lightcurve in Ks-band, to analyze emission lines present in the IR spectra, to construct a multiwavelength SED including radio, IR, and X-ray data, and to complete the OIR vs. X-ray correlation of black hole binaries with H 1743-322 data points.Results. We detect rapid flares of duration ~5 min in the high time-resolution IR lightcurve. We identify hydrogen and helium emission lines in the IR spectra, coming from the accretion disk. The IR SED exhibits the spectral index typically associated with the X-ray high, soft state in our observations taken during the 2003 and 2004 outbursts, while the index changes to one that is typical of the X-ray low, hard state during the 2008 outburst. During this last outburst, we detected a change of slope in the NIR spectrum between the J and Ks bands, where the JH part is characteristic of an optically thick disk emission, while the HKs part is typical of optically thin synchrotron emission. Furthermore, the comparison of our IR data with radio and X-ray data shows that H 1743-322 exhibits a faint jet both in radio and NIR domains. Finally, we suggest that the companion star is a late-type main sequence star located in the Galactic bulge.Conclusions. These OIR photometric and spectroscopic observations of the microquasar H 1743-322, which are the first of this source to be published in a broad multiwavelength context, allow us to unambiguously identify two spectra of different origins in the OIR domain, evolving from optically thick thermal emission to optically thin synchrotron emission toward longer wavelengths. Comparing these OIR observations with other black hole candidates suggests that H 1743-322 behaves like a radio-quiet and NIR-dim black hole in the low, hard state. This study will be useful when quantitatively comparing the overall contribution of the compact jet and accretion flow in the energy budget of microquasars

Stability of helium accretion discs in ultracompact binaries

Stellar companions of accreting neutron stars in ultra compact X-ray binaries (UCXBs) are hydrogen-deficient. Their helium or C/O accretion discs are strongly X-ray irradiated. Both the chemical composition and irradiation determine the disc stability with respect to thermal and viscous perturbations. At shorter periods, UCXBs are persistent, whereas longer-period systems are mostly transient. To understand this behaviour one has to derive the stability criteria for X-ray irradiated hydrogen-poor accretion discs. We use a modified and updated version of the Dubus et al. code describing time-dependent irradiated accretion discs around compact objects. We obtained the relevant stability criteria and compared the results to observed properties of UCXBs. Although the general trend in the stability behaviour of UCXBs is consistent with the prediction of the disc instability model, in a few cases the inconsistency of theoretical predictions with the system observed properties is weak enough to be attributed to observational and/or theoretical uncertainties. Two systems might require the presence of some amount of hydrogen in the donor star.Comment: Astronomy & Astrophysics, in pres