Context.LS 5039 and LS I+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. Aims.The purpose is to 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 for explaining the observations of LS 5039 and LS I+61°303. Emission arises 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 interstellar medium. Methods.The timescales for acceleration and radiation of particles at the shock between the pulsar wind and stellar wind are calculated. Simple expectations for the spectral energy distribution (SED) are derived and are shown to depend on very few input parameters. Detailed modelling of the particle evolution is attempted and compared to the observations from radio to TeV energies. Results.Acceleration at the shock provides high-energy electrons that steadily emit synchrotron in X-rays and inverse Compton scatter stellar light to γ-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. Conclusions.The pulsar wind scenario provides a common, viable framework for interpreting the emission from all three γ-ray binaries.
Context.Gamma-ray emission in the TeV (1012 eV) 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 γ-rays. This results in a modulation of the γ-ray flux as the relative positions of the γ-ray source and companion star change with orbital phase for the observer. Aims.The extent to which this variable absorption can provide useful diagnostics for the location and nature of γ-ray emission is examined. Methods.The absorption spectrum and transmitted flux are calculated by integrating the $\gamma\gamma$ cross-section along the line-of-sight, taking into account the orbit, the spectrum and the finite size of the companion star in LS 5039, PSR B1259-63 and LSI +61°303, a system similar to LS 5039 but still undetected at TeV energies. Results.In LS 5039, emission close to a black hole or a neutron star primary is considered. In both cases, the transmitted flux >250 GeV drops by an order-of-magnitude near periastron (ϕ = 0). 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 (ϕ = 0.1) to 3 TeV (ϕ = 0.7). Only 20% of the flux is absorbed at ϕ = 0.7, allowing for an almost direct view of the intrinsic spectrum. Low variability will require emission on large scales, more than 0.7 AU away to have <50% absorption in a jet. In LSI +61°303, significant absorption (up to 90% of the 100 GeV flux) is predicted only slightly before periastron, accompanied by a spectral hardening above 1 TeV. In PSR B1259-63, although 40% of the flux is absorbed before periastron, the large variability seen by HESS is due to the γ-ray emission process. Conclusions.The predictions made here are essential to distinguish variability in the emission of γ-rays from that due to absorption. A modulation would provide a novel way to constrain the γ-ray source. Its absence would imply that γ-ray emission occurs on large scales.
Context.
The origin of ultra-rapid flares of very high-energy radiation from active galactic nuclei remains elusive. Magnetospheric processes, occurring in the close vicinity of the central black ...hole, could account for these flares.
Aims.
Our aim is to bridge the gap between simulations and observations by synthesizing gamma-ray light curves in order to characterize the activity of a black hole magnetosphere, using kinetic simulations.
Methods.
We performed global axisymmetric 2D general-relativistic particle-in-cell simulations of a Kerr black hole magnetosphere. We included a self-consistent treatment of radiative processes and plasma supply, as well as a realistic magnetic configuration, with a large-scale equatorial current sheet. We coupled our particle-in-cell code with a ray-tracing algorithm in order to produce synthetic light curves.
Results.
These simulations show a highly dynamic magnetosphere, as well as very efficient dissipation of the magnetic energy. An external supply of magnetic flux is found to maintain the magnetosphere in a dynamic state, otherwise the magnetosphere settles in a quasi-steady Wald-like configuration. The dissipated energy is mostly converted to gamma-ray photons. The light curves at low viewing angle (face-on) mainly trace the spark gap activity and exhibit high variability. On the other hand, no significant variability is found at high viewing angle (edge-on), where the main contribution comes from the reconnecting current sheet.
Conclusions.
We observe that black hole magnetospheres with a current sheet are characterized by a very high radiative efficiency. The typical amplitude of the flares in our simulations is lower than is detected in active galactic nuclei. These flares could result from the variation in parameters external to the black hole.
Context. On March 2010, the symbiotic binary V407 Cyg erupted as a result of a nova explosion. The event gave rise to a two-week long burst of ≥ 100 MeV γ-rays detected by Fermi/LAT, a unique ...observation testifying to particle acceleration in the system. Aims. The outburst can be considered a scaled-down supernova, with short dynamical time scale, and thus can constitute a test case for theories of the origin of Galactic cosmic rays. We aim at determining the properties of the accelerated particles and identifying the origin of the high-energy radiation. Methods. We developed a model for diffusive shock acceleration and non-thermal emission in V407 Cyg, complemented by an evaluation of the thermal emission from the shocked plasma. We considered both leptonic and hadronic contributions to the non-thermal processes, and investigated the effect of many binary and nova parameters. Results. The γ-ray emission is mostly of leptonic origin and arises predominantly from inverse-Compton scattering of the nova light. Matching the light curve requires gas accumulation in the vicinity of the white dwarf, as a consequence of wind accretion, while the spectrum imposes particle scattering close to the Bohm limit in the upstream equipartition magnetic field. The nova accelerated protons (respectively electrons) with energies up to ≃300 GeV (respectively ≃20 GeV), for a total non-thermal energy ≃10 erg after two weeks, representing ≃10% of the initial nova kinetic energy. The electron-to-proton ratio at injection is 6%. Conclusions. The V407 Cyg eruption can be understood from the same principles that are invoked for particle acceleration in supernova remnants, although without the need for strong magnetic field amplification. The population of novae in symbiotic systems is a negligible source of Galactic cosmic rays, and most likely not a class of TeV-emitters.
Context.
Dwarf novæ (DNe) and low mass X-ray binaries (LMXBs) show eruptions that are thought to be due to a thermal-viscous instability in their accretion disk. These eruptions provide constraints ...on angular momentum transport mechanisms.
Aims.
We explore the idea that angular momentum transport could be controlled by the dynamical evolution of the large-scale magnetic field. We study the impact of different prescriptions for the magnetic field evolution on the dynamics of the disk. This is a first step in confronting the theory of magnetic field transport with observations.
Methods.
We developed a version of the disk instability model that evolves the density, the temperature, and the large-scale vertical magnetic flux simultaneously. We took into account the accretion driven by turbulence or by a magnetized outflow with prescriptions taken, respectively, from shearing box simulations or self-similar solutions of magnetized outflows. To evolve the magnetic flux, we used a toy model with physically motivated prescriptions that depend mainly on the local magnetization
β
, where
β
is the ratio of thermal pressure to magnetic pressure.
Results.
We find that allowing magnetic flux to be advected inwards provides the best agreement with DNe light curves. This leads to a hybrid configuration with an inner magnetized disk, driven by angular momentum losses to an MHD outflow, sharply transiting to an outer weakly-magnetized turbulent disk where the eruptions are triggered. The dynamical impact is equivalent to truncating a viscous disk so that it does not extend down to the compact object, with the truncation radius dependent on the magnetic flux and evolving as
Ṁ
−2/3
.
Conclusions.
Models of DNe and LMXB light curves typically require the outer, viscous disk to be truncated in order to match the observations. There is no generic explanation for this truncation. We propose that it is a natural outcome of the presence of large-scale magnetic fields in both DNe and LMXBs, with the magnetic flux accumulating towards the center to produce a magnetized disk with a fast accretion timescale.
The eruptive cycles of dwarf novae are thought to be due to a thermal-viscous instability in the accretion disk surrounding the white dwarf. This model has long been known to imply enhanced angular ...momentum transport in the accretion disk during outburst. This is measured by the stress to pressure ratio α, with α ≈ 0.1 required in outburst compared to α ≈ 0.01 in quiescence. Such an enhancement in α has recently been observed in simulations of turbulent transport driven by the magneto-rotational instability (MRI) when convection is present, without requiring a net magnetic flux. We independently recover this result by carrying out PLUTO magnetohydrodynamic (MHD) simulations of vertically stratified, radiative, shearing boxes with the thermodynamics and opacities appropriate to dwarf novae. The results are robust against the choice of vertical boundary conditions. The thermal equilibrium solutions found by the simulations trace the well-known S-curve in the density-temperature plane that constitutes the core of the disk thermal-viscous instability model. We confirm that the high values of α ≈ 0.1 occur near the tip of the hot branch of the S-curve, where convection is active. However, we also present thermally stable simulations at lower temperatures that have standard values of α ≈ 0.03 despite the presence of vigorous convection. We find no simple relationship between α and the strength of the convection, as measured by the ratio of convective to radiative flux. The cold branch is only very weakly ionized so, in the second part of this work, we studied the impact of non-ideal MHD effects on transport. Ohmic dissipation is the dominant effect in the conditions of quiescent dwarf novae. We include resistivity in the simulations and find that the MRI-driven transport is quenched (α ≈ 0) below the critical density at which the magnetic Reynolds number Rm ≤ 104. This is problematic because the X-ray emission observed in quiescent systems requires ongoing accretion onto the white dwarf. We verify that these X-rays cannot self-sustain MRI-driven turbulence by photo-ionizing the disk and discuss possible solutions to the issue of accretion in quiescence.
Context. Gamma-ray burst emission in the prompt phase is often interpreted as synchrotron radiation from high-energy electrons accelerated in internal shocks. Fast synchrotron cooling of a power-law ...distribution of electrons leads to the prediction that the slope below the spectral peak has a photon index α = −3/2 (N(E) ∝ Eα). However, this differs significantly from the observed median value α ≈ −1. This discrepancy has been used to argue against this scenario. Aims. We quantify the influence of inverse Compton (IC) and adiabatic cooling on the low energy slope to understand whether these processes can reconcile the observed slopes with a synchrotron origin. Methods. We use a time-dependent code developed to calculate the GRB prompt emission within the internal shock model. The code follows both the shock dynamics and electron energy losses and can be used to generate lightcurves and spectra. We investigate the dependence of the low-energy slope on the parameters of the model and identify parameter regions that lead to values α > −3/2. Results. Values of α between −3/2 and −1 are reached when electrons suffer IC losses in the Klein-Nishina regime. This does not necessarily imply a strong IC component in the Fermi/LAT range (GeV) because scatterings are only moderately efficient. Steep slopes require that a large fraction (10−30%) of the dissipated energy is given to a small fraction (≲1%) of the electrons and that the magnetic field energy density fraction remains low (≲0.1%). Values of α up to −2/3 can be obtained with relatively high radiative efficiencies (>50%) when adiabatic cooling is comparable with radiative cooling (marginally fast cooling). This requires collisions at small radii and/or with low magnetic fields. Conclusions. Amending the standard fast cooling scenario to account for IC cooling naturally leads to values α up to −1. Marginally fast cooling may also account for values of α up to −2/3, although the conditions required are more difficult to reach. About 20% of GRBs show spectra with slopes α > −2/3. Other effects, not investigated here, such as a thermal component in the electron distribution or pair production by high-energy gamma-ray photons may further affect α. Still, the majority of observed GRB prompt spectra can be reconciled with a synchrotron origin, constraining the microphysics of mildly relativistic internal shocks.
Context. Gamma-ray emission at energies ≥100 MeV has been detected from nine novae using the Fermi Large Area Telescope (LAT), and can be explained by particle acceleration at shocks in these ...systems. Eight out of these nine objects are classical novae in which interaction of the ejecta with a tenuous circumbinary material is not expected to generate detectable gamma-ray emission. Aim. We examine whether particle acceleration at internal shocks can account for the gamma-ray emission from these novae. The shocks result from the interaction of a fast wind radiatively-driven by nuclear burning on the white dwarf with material ejected in the initial runaway stage of the nova outburst. Methods. We present a one-dimensional model for the dynamics of a forward and reverse shock system in a nova ejecta, and for the associated time-dependent particle acceleration and high-energy gamma-ray emission. Non-thermal proton and electron spectra are calculated by solving a time-dependent transport equation for particle injection, acceleration, losses, and escape from the shock region. The predicted emission is compared to LAT observations of V407 Cyg, V1324 Sco, V959 Mon, V339 Del, V1369 Cen, and V5668 Sgr. Results. The ≥100 MeV gamma-ray emission arises predominantly from particles accelerated up to ~100 GeV at the reverse shock and undergoing hadronic interactions in the dense cooling layer downstream of the shock. The emission rises within days after the onset of the wind, quickly reaches a maximum, and its subsequent decrease reflects mostly the time evolution of the wind properties. Comparison to gamma-ray data points to a typical scenario where an ejecta of mass 10−5–10−4 M⊙ expands in a homologous way with a maximum velocity of 1000–2000 km s−1, followed within a day by a wind with a velocity <2000 km s−1 and a mass-loss rate of 10−4–10−3 M⊙ yr−1 declining over a time scale of a few days. Because of the large uncertainties in the measurements, many parameters of the problem are degenerate and/or poorly constrained except for the wind velocity, the relatively low values of which result in the majority of best-fit models having gamma-ray spectra with a high-energy turnover below ~10 GeV. Our typical model is able to account for the main features in the observations of the recent gamma-ray nova ASASSN-16ma. Conclusions. The internal shock model can account for the gamma-ray emission of the novae detected by Fermi LAT. Gamma-ray observations hold potential for probing the mechanism of mass ejection in novae, but should be combined to diagnostics of the thermal emission at lower energies to be more constraining.
Context.
In dwarf novæ (DNe) and low-mass X-ray binaries, the tidal potential excites spiral waves in the accretion disc. Spiral-wave-driven accretion may be important in quiescent discs, where the ...angular momentum transport mechanism has yet to be identified. Previous studies were limited to unrealistically high temperatures for numerical studies or to specific regimes for analytical studies.
Aims.
We perform the first numerical simulation of spiral-wave-driven accretion in the cold temperature regime appropriate to quiescent discs, which have Mach numbers ≳100.
Methods.
We used the new GPU-accelerated finite-volume code I
DEFIX
to produce global hydrodynamics 2D simulations of the accretion discs of DN systems with a sufficiently fine spatial resolution to capture the short scale-height of cold, quiescent discs with Mach numbers ranging from 80 to 370.
Results.
Running the simulations on timescales of tens of binary orbits shows transient angular momentum transport that decays as the disc relaxes from its initial conditions. We find the angular momentum parameter
α
drops to values of ≪10
−2
, too weak to drive accretion in quiescence.
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