arXiv subjects

Guillaume Dubus

Publications and source records attributed to Guillaume Dubus.

64 records · Page 4Linked to original sources

arXiv2006

Gamma-ray binaries: pulsars in disguise ?

LS 5039 and LSI +61 303 are unique amongst high-mass X-ray binaries (HMXB) for their spatially-resolved radio emission and their counterpart at >GeV gamma-ray energies, canonically attributed to non-thermal particles in an accretion-powered relativistic jet. The only other HMXB known to emit very high energy (VHE) gamma-rays, PSR B1259-63, harbours a non-accreting millisecond pulsar. I investigate whether the interaction of the relativistic wind from a young pulsar with the wind from its stellar companion, as in PSR B1259-63, constitutes a viable scenario to explain the observations of LS 5039 and LSI +61 303. Emission would arise from the shocked pulsar wind material, which then flows away to large distances in a comet-shape tail, reproducing on a smaller scale what is observed in isolated, high motion pulsars interacting with the ISM. Simple expectations for the SED are derived and are shown to depend on few input parameters. Detailed modelling of the particle evolution is compared to the observations from radio to TeV energies. Acceleration at the shock provides high energy electrons that steadily emit synchrotron in X-rays and inverse Compton scatter stellar light to gamma-rays. Electrons streaming out of the system emit at IR frequencies and below. The overall aspect of the SEDs is adequately reproduced for standard values of the parameters. The morphology of the radio tail can mimic a microquasar jet. Good agreement is found with the published VLBI map of LS 5039 and predictions are made on the expected change in appearance with orbital phase. The pulsar wind scenario can provide a common, viable framework to interpret the emission from all three gamma-ray binaries.

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arXiv2006

Gamma-ray absorption in massive X-ray binaries

Gamma-ray emission in the TeV range has been detected by HESS from two X-ray binaries: PSR B1259-63 and LS 5039. In both, the early-type star provides large numbers of target photons for pair-production with TeV gamma-rays. This results in a modulation of the gamma-ray flux as the relative positions of the gamma-ray source and companion star change with orbital phase. The lightcurve and spectrum of absorption provide useful diagnostics for the location and nature of gamma-ray emission. The expected orbital variations of the absorption spectrum and flux are calculated for LS 5039, PSR B1259-63 and LSI +61 303, a system similar to LS 5039 but still undetected at TeV energies. In LS 5039, emission close to a black hole or a neutron star primary is considered. In both cases, the flux above 250 GeV drops by more than an order-of-magnitude near periastron due to absorption. A black hole yields a clear spectral signature in the average spectrum at 400 GeV. A neutron star yields more variability, with the spectral feature moving from 200 GeV to 2 TeV. The absorbed fraction of the flux can be as small as 20%, allowing for an almost direct view of the intrinsic spectrum. Low variability requires emission on large scales, more than 0.7 AU away to have <50% absorption in a jet. Such predictions are essential to distinguish between variability due to the gamma-ray emission process from that due to absorption. A modulation in LS 5039 would provide a novel way to constrain the gamma-ray source. Its absence would imply that gamma-ray emission occurs on large scales.

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arXiv2005

DIM light on Black Hole X-ray Transients

The current model for the outburst of stellar-mass black holes X-ray binaries is the disk instability model (DIM). An overview of this model and a discussion of its theoretical and observational challenges are given.

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arXiv2002

Chandra Observations of the Nucleus of M33

The nearby galaxy M33 hosts the most luminous steady X-ray source in the Local Group. The high spatial resolution of Chandra allows us to confirm that this ultra-luminous X-ray source is within the nucleus and rule out at the 4.6 sigma level a previously proposed possible counterpart located 1" away. The X-ray spectrum is well fitted by a disc blackbody with kT=1.18+-0.02 keV and R cos(theta)^(0.5}=57+-3 km, consistent with earlier results from ASCA and BeppoSAX. The source flux is steady between 1 and 1e4 s (<3% rms variability, 0.5-10 keV) with a 0.5-10 keV luminosity of 1.5e39 erg/s. The X-ray properties and an association with a radio source are reminiscent of the galactic microquasar GRS 1915+105.

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arXiv2002

Thin discs, thick discs and transition zones

Accretion onto a compact object must occur through a disc when the material has some initial angular momentum. Thin discs and the thicker low radiative efficiency accretion flows are solutions to this problem that have been widely studied and applied. This is an introduction to these accretion flows within the context of X-ray binaries and cataclysmic variables.

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arXiv2001

HST Spectroscopy of the Nucleus of M33

We have used Hubble Space Telescope to obtain moderate resolution 1150-5700 Ang. spectroscopy of the nucleus of M33 and a blue star ~1 arcsec NNW of the nucleus in an attempt to find the optical counterpart of the nuclear X-ray source and to characterize stellar populations in the nuclear region. The STIS spectra of the nucleus can be modelled in terms of two starbursts, one with a mass of about 9000 solar masses at 40 Myrs and the other with a mass of about 76,000 solar masses at 1 Gyrs. The blue star is a late type O giant, with no obvious spectral anomalies to indicate that it is associated with the luminous X-ray source. The nuclear region is not heavily reddened; 2200 Ang absorption features in the spectra of both the nucleus and the star are weak. The data and the star formation history support the hypothesis that the M33 nuclear source, the brightest persistent source in the Local group, is an ~10 solar mass black hole binary.

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arXiv2001

Optical Spectroscopy Of The X-Ray Transient XTE J1118+480 In Outburst

We report on optical spectroscopic observations of the X-ray Transient XTE J1118+480 covering the period from April 7, 2000 to July 4, 2000. The spectrum is characterized by weak, broad, double-peaked Balmer and He lines on top of a blue continuum of slope p ~ 1/3, as expected for an optically-thick accretion disk. The weak Bowen blend seen in our spectra may indicate a low intrinsic metallicity for the source. The presence of a partial S-wave pattern in the HeII lambda-4686 line appears consistent with the reported photometric orbital period Porb = 4.1 hr for XTE J1118+480. By using a combination of Doppler mapping and various theoretical arguments, we constrain plausible orbital parameters for the system: a mass ratio 0.02 <~ q <~ 0.1, an inclination i >~ 70 deg for a neutron star primary, or 30 deg <~ i <~ 50 deg for a black hole primary with a mass between 4 and 10 Msun. CaII absorption features observed at very high resolution constrain the interstellar hydrogen absorption column log[N_HI (cm^{-2})] ~ 20.45 +/- 0.2 and the identification of three absorbing clouds indicate a distance to the source <~ 1 kpc, assuming the line-of-sight to XTE J1118+480 has average high-latitude properties. These results are discussed in the context of previous multiwavelength observations of this unusual system.

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arXiv1999

Disc instability models, evaporation and radius variations

We show that the outcome of disc instability models is strongly influenced by boundary conditions such as the position of the inner and outer disc edges. We discuss other sources of uncertainties, such as the tidal torque, and we conclude that disc illumination, disk size variations and a proper prescription for the tidal torque must be included in models if one wishes to extract meaningful physical information on e.g. viscosity from the comparison of predicted and observed lightcurves.

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arXiv1998

Evidence for evolving accretion disk structure in 4U 1957+115

We present results of the first multicolour photometric study of 4U1957+115, one of the optically less studied low mass X-ray binaries (LMXB). Our quasi-simultaneous UBVRI observations reveal that the light curve pulse shape over the 9.33 hr period discovered by Thorstensen (1987) has changed substantially since his earlier V band studies. The light curve now shows clear asymmetry and an amplitude more than double of that seen by Thorstensen (1987). The light curve also shows colour dependence that seems to rule out the X-ray heating model as an origin for photometric variability. We believe that the changes observed in the light curve shape are indicative of evolving accretion disk structure. We discuss implications of this on the origin of optical emission in 4U1957+115 and disk accreting binaries in general.

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arXiv1998

Accretion disc outbursts: a new version of an old model

We have developed 1D time-dependent numerical models of accretion discs, using an adaptive grid technique and an implicit numerical scheme, in which the disc size is allowed to vary with time. The code fully resolves the cooling and heating fronts propagating in the disc. We show that models in which the radius of the outer edge of the disc is fixed produce incorrect results, from which probably incorrect conclusions about the viscosity law have been inferred. In particular we show that outside-in outbursts are possible when a standard bimodal behaviour of the Shakura-Sunyaev viscosity parameter alpha is used. We also discuss to what extent insufficient grid resolutions have limited the predictive power of previous models. We find that the global properties (magnitudes, etc. ...) of transient discs can be addressed by codes using a high, but reasonable, number of fixed grid points. However, the study of the detailed physical properties of the transition fronts generally requires resolutions which are out of reach of fixed grid codes. It appears that most time-dependent models of accretion discs published in the literature have been limited by resolution effects, improper outer boundary conditions, or both.

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