Shock corrugation to the rescue of the internal shock model in microquasars: The single-scale MHD view

Questions regarding energy dissipation in astrophysical jets are open to date, despite of numerous attempts to limit the diversity of models. Some of the most popular models assume that energy is transferred to particles via internal shocks, which develop as a consequence of non-uniform velocity of the jet matter. In this context, we study the structure and energy deposition of colliding plasma shells, focusing our attention on the case of initially inhomogeneous shells. This leads to formation of distorted (corrugated) shock fronts -- a setup that has recently been shown to revive particle acceleration in relativistic magnetized perpendicular shocks. Our studies show that the radiative power of the far downstream of non-relativistic magnetized perpendicular shocks is moderately enhanced with respect to the flat shock cases. Based on the decay rate of downstream magnetic field, we make predictions for multiwavelength polarization properties.Comment: 17 pages, 8 figures; Accepted for publication in Ap

IGR J17451--3022: constraints on the nature of the donor star

We constrain the binary parameters of the eclipsing accreting X-ray binary IGR J17451--3022 and the nature of its donor star. The donor mass, its radius, and the system inclination angle are computed based on the system orbital period and eclipse duration recently reported by Bozzo et al.\ (2016). We find that the donor is most likely a main-sequence star with the mass comprised within the range $\sim$(0.5--$0.8){\rm M}_{\odot}$ and the radius of $\sim\! 0.7{\rm R}_{\odot}$. Assuming that the accreting compact object in IGR J17451--3022 is a neutron star, the duration of the nearly total rectangular eclipses yields the inclination angle of the system of $71^\circ\lesssim i\lesssim 76^\circ$, compatible with the presence of dips in this system. We rule out the presence of either brown or white dwarf. However, we find an alternative possibility that the donor star in IGR J17451--3022 could be a partially stripped giant with a very low mass, $\sim\! 0.2{\rm M}_{\odot}$. This case requires a substantial mass loss prior to the formation of the giant-star He core. According to that solution, the radius would be $\sim\! 0.4{\rm R}_{\odot}$, at $i\sim 80^\circ$. We additionally show that the well-known approximate dependence of the giant-star radius exclusively on its core mass breaks down below $\sim\!0.3{\rm M}_{\odot}$.Comment: Astronomy & Astrophysics, in pres

Circumnuclear Structures in Megamaser Host Galaxies

Using HST, we identify circumnuclear ($100$-$500$ pc scale) structures in nine new H$_2$O megamaser host galaxies to understand the flow of matter from kpc-scale galactic structures down to the supermassive black holes (SMBHs) at galactic centers. We double the sample analyzed in a similar way by Greene et al. (2013) and consider the properties of the combined sample of 18 sources. We find that disk-like structure is virtually ubiquitous when we can resolve $<200$ pc scales, in support of the notion that non-axisymmetries on these scales are a necessary condition for SMBH fueling. We perform an analysis of the orientation of our identified nuclear regions and compare it with the orientation of megamaser disks and the kpc-scale disks of the hosts. We find marginal evidence that the disk-like nuclear structures show increasing misalignment from the kpc-scale host galaxy disk as the scale of the structure decreases. In turn, we find that the orientation of both the $\sim100$ pc scale nuclear structures and their host galaxy large-scale disks is consistent with random with respect to the orientation of their respective megamaser disks.Comment: 24 pages, 16 figures, 4 tables; Resubmitted to ApJ after referee's comment

Cataclysmic Variables in MHD

We present results of global magnetohydrodynamic (MHD) simulations of accretion disks in Cataclysmic Variables fed by Roche lobe overflow, including vertical stratification, in order to investigate the roles of spiral shocks, magnetorotational instability (MRI), and the accretion stream on disk structure and evolution. In preparation for these global models, we test the effects of numerical diffusion relevant to studies of accretion disks. We perform one- and two-dimensional simulations of a dense cold Keplerian ring surrounded by a hot hydrostatic atmosphere, varying the Riemann solver, resolution, and the reconstruction method used. Finding HLLC/Roe Riemann solver and PLM reconstruction to be the best fit for our needs, we build our global models of CV disks. We include a simple treatment of gas thermodynamics designed to approximate conditions typical of dwarf nova outbursts, with orbital Mach numbers at the inner edge of the disk M_in of 5 and 10. We find mass accretion rates in our global models to vary considerably on all time scales, with only the Mach 5 model reaching a clear quasi-stationary state. For Mach 10, the model undergoes an outside-in magnetically-driven accretion event occurring on a time scale of ~10 orbital periods of the binary. Both models exhibit spiral shocks inclined with respect to the binary plane, with their position and inclination changing rapidly. However, the time-averaged location of these shocks in the equatorial plane is well-fit by simple linear models. MRI turbulence in the disk generates toroidal magnetic field patterns (butterfly diagrams) that are in some cases irregular, perhaps due to interaction with spiral structure. While many of our results are in good agreement with local studies, we find some features (most notably those related to spiral shocks) can only be captured in global models such as studied here. Thus, while global studies remain computationally expensive, we find them essential (along with more sophisticated treatment of radiation transport and disk thermodynamics) for furthering our understanding of accretion in binary systems. Finally, we discuss future improvements to our framework including the use of orbital advection, where early tests indicate 50-100% improvement in code performance

Jet contributions to the broad-band spectrum of Cyg X-1 in the hard state

We apply the jet model developed in the preceding paper of Zdziarski et al. to the hard-state emission spectra of Cyg X-1. We augment the model for the analytical treatment of the particle evolution beyond the energy dissipation region, and allow for various forms of the acceleration rate. We calculate the resulting electron and emission spectra as functions of the jet height, along with the emission spectra integrated over the outflow. The model accounts well for the observed radio, infrared and GeV fluxes of the source, although the available data do not provide unique constraints on the model free parameters. The contribution of the jet emission in the UV–X-ray range turns out to be in all the cases negligible compared to the radiative output of the accretion component. Nevertheless, we find out that it is possible to account for the observed flux of Cyg X-1 at MeV energies by synchrotron jet emission, in accord with the recent claims of the detection of strong linear polarization of the source in that range. However, this is possible only assuming a very efficient particle acceleration leading to the formation of flat electron spectra, and jet magnetic fields much above the equipartition level