arXiv subjects

Dimitrios Giannios

Publications and source records attributed to Dimitrios Giannios.

101 records · Page 6Linked to original sources

arXiv2008

Soft X-ray components in the hard state of accreting black holes

Recent observations of two black hole candidates (GX 339-4 and J1753.5-0127) in the low-hard state (L_X/L_Edd ~ 0.003-0.05) suggest the presence of a cool accretion disk very close to the innermost stable orbit of the black hole. This runs counter to models of the low-hard state in which the cool disk is truncated at a much larger radius. We study the interaction between a moderately truncated disk and a hot inner flow. Ion-bombardment heats the surface of the disk in the overlap region between a two-temperature advection-dominated accretion flow and standard accretion disk, producing a hot (kT_e ~70 keV) layer on the surface of the cool disk. The hard X-ray flux from this layer heats the inner parts of the underlying cool disk, producing a soft X-ray excess. Together with interstellar absorption these effects mimic the thermal spectrum from a disk extending to the last stable orbit. The results show that soft excesses in the low-hard state are a natural feature of truncated disk models.

astro-ph↗

arXiv2008

An RMHD study of transition between prompt and afterglow GRB phases

We study the afterglow phases of a GRB through relativistic magnetohydrodynamic simulations. The evolution of a relativistic shell propagating into a homogeneous external medium is followed. We focus on the effect of the magnetization of the ejecta on the initial phases of the ejecta-external medium interaction. In particular we are studying the condition for the existence of a reverse shock into the ejecta, the timescale for the transfer of the energy from the shell to the shocked medium and the resulting multiwavelength light curves. To this end, we have developed a novel scheme to include non-thermal processeses which is coupled to the relativistic magnetohydrodynamic code MRGENESIS in order to compute the non-thermal synchrotron radiation.

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arXiv2007

Neutron-rich gamma-ray burst flows: dynamics and particle creation in neutron - proton collisions

We consider gamma-ray burst outflows with a substantial neutron component that are either dominated by thermal energy (fireballs) or by magnetic energy. In the latter case, we focus on the recently introduced `AC' model which relies on magnetic reconnection to accelerate the flow and power the prompt emission. For both the fireball and the AC model, we investigate the dynamical importance of neutrons on the outflow. We study particle creation in inelastic neutron - proton collisions and find that in both models the resulting neutrino emission is too weak to be detectable. The inelastic collisions also produce gamma-rays, which create pairs in interactions with soft photons carried with the flow. In magnetically driven outflows, the energy of these pairs is radiated away as synchrotron emission. The bulk of the emission takes place at a few hundred keV, which makes it difficult to disentangle this signal from the prompt emission. In fireballs, however, pair cascading leads to the emission of gamma-rays with observer energy in the range of 2 - 20 GeV and a fluence well above the GLAST threshold. Therefore this emission can be a useful diagnostic of the nature of the outflow.

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arXiv2007

Spectral and timing properties of a dissipative GRB photosphere

We explore the observational appearance of the photosphere of an ultrarelativistic flow with internal dissipation of energy as predicted by the magnetic reconnection model. Previous study of the radiative transfer in the photospheric region has shown that gradual dissipation of energy results in a hot photosphere. There, inverse Compton scattering of the thermal radiation advected with the flow leads to powerful photospheric emission with spectral properties close to those of the observed prompt GRB emission. Here, we build on that study by calculating the spectra for a large range of the characteristics of the flow. An accurate fitting formula is given that provides the photospheric spectral energy distribution in the ~10 keV to ~10 MeV energy range (in the central engine frame) as a function of the basic physical parameters of the flow. It facilitates the direct comparison of the model predictions with observations, including the variability properties of the lightcurves. We verify that the model naturally accounts for the observed clustering in peak energies of the E*f(E) spectrum. In this model, the Amati relation indicates a tendency for the most luminous bursts to have more energy per baryon. If this tendency also holds for individual GRB pulses, the model predicts the observed narrowing of the width of pulses with increasing photon energy.

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arXiv2007

Neutrino-cooled accretion and GRB variability

For accretion rates Mdot~0.1 Msun/s to a few solar mass black hole the inner part of the disk is expected to make a transition from advection dominance to neutrino cooling. This transition is characterized by sharp changes of the disk properties. I argue here that during this transition, a modest increase of the accretion rate leads to powerful enhancement of the Poynting luminosity of the GRB flow and decrease of its baryon loading. These changes of the characteristics of the GRB flow translate into changing gamma-ray spectra from the photosphere of the flow. The photospheric interpretation of the GRB emission explains the observed narrowing of GRB pulses with increasing photon energy and the luminosity-spectral peak relation within and among bursts.

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arXiv2006

Prompt emission spectra from the photosphere of a GRB

I explore the observational appearance of the photosphere of an ultrarelativistic flow with internal dissipation of energy (``dissipative'' GRB models). As a case study, I use the magnetic reconnection model (AC model) that makes robust predictions on the energy dissipation rates at different radii in the flow. With analytical and numerical tools for the radiative transfer problem, I show that the flow develops a hot photosphere where inverse Compton scattering leads to highly non-thermal spectra. For a wide range of luminosities and baryon loadings of the flow, this spectrum is very close to the observed prompt GRB emission. Its luminosity ranges from ~3 to 20% of that of the total energy input.

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arXiv2006

Flares in GRB afterglows from delayed magnetic dissipation

One of the most intriguing discoveries made by the Swift satellite is the flaring activity in about half of the afterglow lightcurves. Flares have been observed on both long and short duration GRBs and on time scales that range from minutes to ~1 day after the prompt emission. The rapid evolution of some flares led to the suggestion that they are caused by late central engine activity. Here, I propose an alternative explanation that does not need reviving of the central engine. Flares can be powered by delayed magnetic dissipation in strongly magnetized (i.e. with initial Poynting to kinetic flux ratio $\simmore 1$) ejecta during its deceleration due to interaction with the external medium. A closer look at the length scales of the dissipation regions shows that magnetic dissipation can give rise to fast evolving and energetic flares. Multiple flares are also expected in the context of the model.

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arXiv2006

The role of kink instability in Poynting-flux dominated jets

The role of kink instability in magnetically driven jets is explored through numerical one-dimensional steady relativistic MHD calculations. The instability is shown to have enough time to grow and influence the dynamics of Poynting-flux dominated jets. In the case of AGN jets, the flow becomes kinetic flux dominated at distances larger than ~1000 r_g because of the rapid dissipation of Poynting flux. When applied to GRB outflows, the model predicts more gradual Poynting dissipation and moderately magnetized flow at distances of ~10^{16} cm where the deceleration of the ejecta due to its interaction with the external medium is expected. The energy released by the instability can power the compact ``blazar zone'' emission and the prompt emission of GRB outflows with high radiative efficiencies.

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arXiv2005

Spherically symmetric, static spacetimes in a tensor-vector-scalar theory

Recently, a relativistic gravitation theory has been proposed [J. D. Bekenstein, Phys. Rev. D {\bf 70}, 083509 (2004)] that gives the Modified Newtonian Dynamics (or MOND) in the weak acceleration regime. The theory is based on three dynamic gravitational fields and succeeds in explaining a large part of extragalactic and gravitational lensing phenomenology without invoking dark matter. In this work we consider the strong gravity regime of TeVeS. We study spherically symmetric, static and vacuum spacetimes relevant for a non-rotating black hole or the exterior of a star. Two branches of solutions are identified: in the first the vector field is aligned with the time direction while in the second the vector field has a non-vanishing radial component. We show that in the first branch of solutions the βand γPPN coefficients in TeVeS are identical to these of general relativity (GR) while in the second the βPPN coefficient differs from unity violating observational determinations of it (for the choice of the free function $F$ of the theory made in Bekenstein's paper). For the first branch of solutions, we derive analytic expressions for the physical metric and discuss their implications. Applying these solutions to the case of black holes, it is shown that they violate causality (since they allow for superluminal propagation of metric, vector and scalar waves) in the vicinity of the event horizon and/or that they are characterized by negative energy density carried by the fields.

gr-qc↗

arXiv2005

Spectra of black-hole binaries in the low/hard state: from radio to X-rays

We propose a jet model for the low/hard state of Galactic black-hole X-ray sources that can explain the energy spectra from radio to X-rays. The model assumes that i) there is a magnetic field along the axis of the jet; ii) the electron density in the jet drops inversely proportional to distance; and iii) the electrons in the jet follow a power law distribution function. We have performed Monte Carlo simulations of Compton upscattering of soft photons from the accretion disk and have found power-law high-energy spectra with photon-number index in the range 1.5 - 2 and cutoff at a few hundred keV. The spectrum at long wavelengths (radio, infrared, optical) is modeled to come from synchrotron radiation of the energetic electrons in the jet. We find flat to inverted radio spectra that extend from the radio up to about the optical band. For magnetic field strengths of the order of 10^5-10^6G at the base of the jet, the calculated spectra agree well in slope and flux with the observations. Our model has the advantage over other existing models that it also explains many of the existing timing data such as the time lag spectra, the hardening of the power density spectra and the narrowing of the autocorrelation function with increasing photon energy.

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arXiv2004

Excitation of low-frequency QPOs in black-hole accretion

We study possible mechanisms of excitation of quasiperiodic oscillations in the accretion flow of black hole accreters in their hard spectral states, in the context of the `truncated disk' model. Quasi-spherical oscillations of the inner ion-supported accretion flow (ISAF) can be excited by the interaction of this hot flow with the cool disk extending outward from it. The fundamental mode of (p-mode) oscillation is most easily excited, and has a frequency near the Kepler frequency at the inner edge of the cool disk. The strongest excitation mechanism is a feedback loop involving cooling of the ISAF by soft photons from the cool disk and heating of the cool disk by the ISAF, while synchrotron emission can be a relatively strong damping effect. Growth times are computed by detailed Comptonization calculations of the interaction of an idealized ISAF with a cool disk. Typical growth times as short as a few dynamical times are found, while amplitudes can be as large as 10%.

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