arXiv subjects

Dimitrios Giannios

Publications and source records attributed to Dimitrios Giannios.

At least 73 records · Page 4Linked to original sources

arXiv2017

Radio Emission from Sgr A*: Pulsar Transits Through the Accretion Disc

Radiatively inefficient accretion flow models have been shown to accurately account for the spectrum and luminosity observed from Sgr A* in the X-ray regime down to mm wavelengths. However, observations at a few GHz cannot be explained by thermal electrons alone but require the presence of an additional non-thermal particle population. Here, we propose a model for the origin of such a population in the accretion flow via means of a pulsar orbiting the supermassive black hole in our Galaxy. Interactions between the relativistic pulsar wind with the disc lead to the formation of a bow shock in the wind. During the pulsar's transit through the accretion disc, relativistic pairs, accelerated at the shock front, are injected into the disc. The radio-emitting particles are long-lived and remain within the disc long after the pulsar's transit. Periodic pulsar transits through the disc result in regular injection episodes of non-thermal particles. We show that for a pulsar with spin-down luminosity $L_{\rm sd} \sim 3\times 10^{35}$ erg s$^{-1}$ and a wind Lorentz factor of $γ_{\rm w} \sim 10^4$ a quasi-steady synchrotron emission is established with luminosities in the $1-10$ GHz range comparable to the observed one.

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arXiv2016

The TeV emission of Ap Librae: a hadronic interpretation and prospects for CTA

Ap Librae is one out of a handful of low-frequency peaked blazars to be detected at TeV $γ$-rays and the only one with an identified X-ray jet. Combined observations of Fermi-LAT at high energies (HE) and of H.E.S.S at very high energies (VHE) revealed a striking spectral property of Ap Librae; the presence of a broad high-energy component that extends more than nine orders of magnitude in energy and is, therefore, hard to be explained by the usual single-zone synchrotron self-Compton model. We show that the superposition of different emission components related to photohadronic interactions can explain the $γ$-ray emission of Ap Librae without invoking external radiation fields. We present two indicative model fits to the spectral energy distribution of Ap Librae where the VHE emission is assumed to originate from a compact, sub-pc scale region of the jet. A robust prediction of our model is VHE flux variability on timescales similar to those observed at X-rays and HE $γ$-rays, which can be further used to distinguish between a sub-pc or kpc scale origin of the TeV emission. We thus calculate the expected variability signatures at X-rays, HE and VHE $γ$-rays and show that quasi-simultaneous flares are expected, with larger amplitude flares appearing at $γ$-rays. We assess the detectability of VHE variability from Ap Librae with CTA, next generation of IACTs. We show that $\sim$hr timescale variability at $E_γ>0.1$ TeV could be detectable at high significance with shorter exposure times than current Cherenkov telescopes.

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arXiv2016

GRB off-axis afterglows and the emission from the accompanying supernovae

Gamma-ray burst (GRB) afterglows are likely produced in the shock that is driven as the GRB jet interacts with the external medium. Long-duration GRBs are also associated with powerful supernovae (SNe). We consider the optical and radio afterglows of long GRBs for both blasts viewed along the jet axis ("on-axis" afterglows) and misaligned observes ("off-axis" afterglows). Comparing the optical emission from the afterglow with that of the accompanying SN, using SN 1998bw as an archetype, we find that only a few percent of afterglows viewed off-axis are brighter than the SN. For observable optical off-axis afterglows, the viewing angle is at most twice the half-opening angle of the GRB jet. Radio off-axis afterglows should be detected with upcoming radio surveys within a few hundred Mpc. We propose that these surveys will act as "radio triggers", and that dedicated radio facilities should follow-up these sources. Follow-ups can unveil the presence of the radio SN remnant, if present. In addition, they can probe the presence of a mildly relativistic component, either associated with the GRB jet or the SN ejecta, expected in these sources.

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arXiv2016

Radio SNRs in the Magellanic Clouds as probes of shock microphysics

A large number of supernova remnants (SNRs) in our Galaxy and galaxies nearby have been resolved in various radio bands. This radio emission is thought to be produced via synchrotron emission from electrons accelerated by the shock that the supernova ejecta drives into the external medium. Here we consider the sample of radio SNRs in the Magellanic Clouds. Given the size and radio flux of a SNR, we seek to constrain the fraction of shocked fluid energy in non-thermal electrons ($ε_e$) and magnetic field ($ε_B$), and find $ε_e ε_B \sim 10^{-3}$. These estimates do not depend on the largely uncertain values of the external density and the age of the SNR. We develop a Monte Carlo scheme that reproduces the observed distribution of radio fluxes and sizes of the population of radio SNRs in the Magellanic Clouds. This simple model provides a framework that could potentially be applied to other galaxies with complete radio SNRs samples.

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arXiv2016

Blazar flares powered by plasmoids in relativistic reconnection

Powerful flares from blazars with short ($\sim$ min) variability timescales are challenging for current models of blazar emission. Here, we present a physically motivated ab initio model for blazar flares based on the results of recent particle-in-cell (PIC) simulations of relativistic magnetic reconnection. PIC simulations demonstrate that quasi-spherical plasmoids filled with high-energy particles and magnetic fields are a self-consistent by-product of the reconnection process. By coupling our PIC-based results (i.e., plasmoid growth, acceleration profile, particle and magnetic content) with a kinetic equation for the evolution of the electron distribution function we demonstrate that relativistic reconnection in blazar jets can produce powerful flares whose temporal and spectral properties are consistent with the observations. In particular, our model predicts correlated synchrotron and synchrotron self-Compton flares of duration of several hours--days powered by the largest and slowest moving plasmoids that form in the reconnection layer. Smaller and faster plasmoids produce flares of sub-hour duration with higher peak luminosities than those powered by the largest plasmoids. Yet, the observed fluence in both types of flares is similar. Multiple flares with a range of flux-doubling timescales (minutes to several hours) observed over a longer period of flaring activity (days or longer) may be used as a probe of the reconnection layer's orientation and the jet's magnetization. Our model shows that blazar flares are naturally expected as a result of magnetic reconnection in a magnetically-dominated jet.

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arXiv2016

Flares from Galactic centre pulsars: a new class of X-ray transients?

Despite intensive searches, the only pulsar within 0.1 pc of the central black hole in our Galaxy, Sgr A*, is a radio-loud magnetar. Since magnetars are rare among the Galactic neutron star population, and a large number of massive stars are already known in this region, the Galactic centre (GC) should harbor a large number of neutron stars. Population syntheses suggest several thousand neutron stars may be present in the GC. Many of these could be highly energetic millisecond pulsars which are also proposed to be responsible for the GC gamma-ray excess. We propose that the presence of a neutron star within 0.03~pc from Sgr~A* can be revealed by the shock interactions with the disk around the central black hole. As we demonstrate, these interactions result in observable transient non-thermal X-ray and gamma-ray emission over timescales of months, provided that the spin down luminosity of the neutron star is L_{sd}~10^{35} erg/s. Current limits on the population of normal and millisecond pulsars in the GC region suggest that a number of such pulsars are present with such luminosities.

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arXiv2015

An anisotropic minijets model for the GRB prompt emission

In order to explain rapid light curve variability without invoking a variable source, several authors have proposed "minijets" that move relativistically relative to the main flow of the jet. Here we consider the possibility that these minijets, instead of being isotropically distributed in the comoving frame of the jet, form primarily perpendicular to the direction of the flow, as the jet dissipates its energy at a large emission radius. This yields two robust features. First, the emission is significantly delayed compared with the isotropic case. This delay allows for the peak of the afterglow emission to appear while the source is still active, in contrast to the simplest isotropic model. Secondly, the flux decline after the source turns off is steeper than the isotropic case. We find that these two features are realized in gamma-ray bursts (GRBs): 1. The peak of most GeV light curves (ascribed to the external shock) appears during the prompt emission phase. 2. Many X-ray light curves exhibit a period of steep decay, which is faster than that predicted by the standard isotropic case. The gamma-ray generation mechanism in GRBs, and possibly in other relativistic flows, may therefore be anisotropic.

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arXiv2015

Radio rebrightening of the GRB afterglow by the accompanying supernova

The gamma-ray burst (GRB) jet powers the afterglow emission by shocking the surrounding medium, and radio afterglow can now be routinely observed to almost a year after the explosion. Long-duration GRBs are accompanied by supernovae (SNe) that typically contain much more energy than the GRB jet. Here we consider the fact that the SN blast wave will also produce its own afterglow (supernova remnant emission), which will peak at much later time (since it is non-relativistic), when the SN blast wave transitions from a coasting phase to a decelerating Sedov-Taylor phase. We predict that this component will peak generally a few tens of years after the explosion and it will outshine the GRB powered afterglow well-before its peak emission. In the case of GRB 030329, where the external density is constrained by the $\sim 10$-year coverage of the radio GRB afterglow, the radio emission is predicted to start rising over the next decade and to continue to increase for the following decades up to a level of $\sim$ mJy. Detection of the SN-powered radio emission will greatly advance our knowledge of particle acceleration in $ \sim 0.1$c shocks.

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arXiv2015

Relativistic Jets Shine through Shocks or Magnetic Reconnection?

Observations of gamma-ray-bursts and jets from active galactic nuclei reveal that the jet flow is characterized by a high radiative efficiency and that the dissipative mechanism must be a powerful accelerator of non-thermal particles. Shocks and magnetic reconnection have long been considered as possible candidates for powering the jet emission. Recent progress via fully-kinetic particle-in-cell simulations allows us to revisit this issue on firm physical grounds. We show that shock models are unlikely to account for the jet emission. In fact, when shocks are efficient at dissipating energy, they typically do not accelerate particles far beyond the thermal energy, and vice versa. In contrast, we show that magnetic reconnection can deposit more than 50% of the dissipated energy into non-thermal leptons as long as the energy density of the magnetic field in the bulk flow is larger than the rest mass energy density. The emitting region, i.e., the reconnection downstream, is characterized by a rough energy equipartition between magnetic fields and radiating particles, which naturally accounts for a commonly observed property of blazar jets.

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arXiv2014

Relativistic magnetic reconnection in pair plasmas and its astrophysical applications

This review discusses the physics of magnetic reconnection, a process in which the magnetic field topology changes and magnetic energy is converted to kinetic energy, in pair plasmas in the relativistic regime. We focus on recent progress in the field driven by theory advances and the maturity of particle-in-cell codes. This work shows that fragmentation instabilities at the current sheet can play a critical role in setting the reconnection speed and affect the resulting particle acceleration, anisotropy, bulk flows, and radiation. Then, we discuss how this novel understanding of relativistic reconnection can be applied to high-energy astrophysical phenomena, with an emphasis on pulsars, pulsar wind nebulae, and active galactic nucleus jets.

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arXiv2014

Black-hole jets without large-scale net magnetic flux

We propose a scenario for launching relativistic jets from rotating black holes, in which small-scale magnetic flux loops, sustained by disc turbulence, are forced to inflate and open by differential rotation between the black hole and the accretion flow. This mechanism does not require a large-scale net magnetic flux in the accreting plasma. Estimates suggest that the process could operate effectively in many systems, and particularly naturally and efficiently when the accretion flow is retrograde. We present the results of general-relativistic force-free electrodynamic simulations demonstrating the time evolution of the black hole's magnetosphere, the cyclic formation of jets, and the effect of magnetic reconnection. The jets are highly variable on timescales ~ 10-10^3 r_ g/c, where r_g is the black hole's gravitational radius. The reconnecting current sheets observed in the simulations may be responsible for the hard X-ray emission from accreting black holes.

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arXiv2014

Magnetic Flux of Progenitor Stars Sets Gamma-ray Burst Luminosity and Variability

Long-duration gamma-ray bursts (GRBs) are thought to come from the core-collapse of Wolf-Rayet stars. Whereas their stellar masses $M_*$ have a rather narrow distribution, the population of GRBs is very diverse, with gamma-ray luminosities $L_γ$ spanning several orders of magnitude. This suggests the existence of a "hidden" stellar variable whose burst-to-burst variation leads to a spread in $L_γ$. Whatever this hidden variable is, its variation should not noticeably affect the shape of GRB lightcurves, which display a constant luminosity (in a time-average sense) followed by a sharp drop at the end of the burst seen with Swift/XRT. We argue that such a hidden variable is progenitor star's large-scale magnetic flux. Shortly after the core collapse, most of stellar magnetic flux accumulates near the black hole (BH) and remains there. The flux extracts BH rotational energy and powers jets of roughly a constant luminosity, $L_j$. However, once BH mass accretion rate $\dot M$ falls below $\sim L_j/c^2$, the flux becomes dynamically important and diffuses outwards, with the jet luminosity set by the rapidly declining mass accretion rate, $L_j\sim \dot M c^2$. This provides a potential explanation for the sharp end of GRBs and the universal shape of their lightcurves. During the GRB, gas infall translates spatial variation of stellar magnetic flux into temporal variation of $L_j$. We make use of the deviations from constancy in $L_j$ to perform stellar magnetic flux "tomography". Using this method, we infer the presence of magnetised tori in the outer layers of progenitor stars for GRB 920513 and GRB 940210.

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arXiv2014

Implications of a PeV neutrino spectral cutoff in GRB models

The recent discovery of extragalactic PeV neutrinos opens a new window to the exploration of cosmic-ray accelerators. The observed PeV neutrino flux is close to the Waxman-Bahcall upper bound implying that gamma-ray bursts (GRBs) may be the source of ultra-high energy cosmic rays (UHECRs). Starting with the assumption of the GRB-UHECR connection, we show using both analytical estimates and numerical simulations that the observed neutrinos can originate at the jet as a result of photopion interactions with the following implications: the neutrino spectra are predicted to have a cutoff at energy $\le 10$ PeV; the dissipation responsible for the GRB emission and cosmic-ray acceleration takes place at distances $r_{\rm diss} \approx 3 \times 10^{11}-3 \times 10^{13} \ {\rm cm}$ from the central engine; the Thomson optical depth at the dissipation region is $τ_{\rm T} \sim 1$; the jet carries a substantial fraction of its energy in the form of Poynting flux at the dissipation region, and has a Lorentz factor $Γ\simeq 100-500$. The non-detection of $\sim$PeV neutrinos coincident with GRBs will indicate that GRBs are either poor cosmic accelerators or the dissipation takes place at small optical depths in the jet.

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arXiv2014

Testing the neutrino annihilation model for launching GRB jets

The mechanism behind the launching of gamma-ray-burst (GRB) jets remains debated resulting in large uncertainty over the jet composition. Both magnetohydrodynamical and neutrino annihilation models have been proposed for the energy extraction in a black hole/accretion-disc central engine. In particular, for the extreme accretion rates $\dot M\sim 0.1-1$~ M$_\odot$s$^{-1}$ expected for bursts of duration $T\lesssim 100$~s, the disc can be an efficient neutrino emitter. Neutrino-antineutrino annihilation results in an energy deposition rate at the jet that can, in principle, account for the burst's energetics. Recent discoveries of X-ray flares hours after the burst and of ultra-long GRBs suggest that GRB activity can last for $\sim 10^4$~s or longer. These long-lived events have fluence similar to that of classical GRBs. In view of these findings, we re-evaluate the neutrino annihilation model. We derive the maximum possible energy of a neutrino-powered jet as a function of the burst duration and show that the available energy drops fast for longer bursts. For a standard choice of the parameters, the model falls short by three to four orders of magnitude in explaining the observed energetics of events that last longer than $\sim 10^3$~s.

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arXiv2014

Hadronic supercriticality as a trigger for GRB emission

We explore a one-zone hadronic model that may be able to reproduce $γ$-ray burst (GRB) prompt emission with a minimum of free parameters. Assuming only that GRBs are efficient high-energy proton accelerators and without the presence of an ab initio photon field, we investigate the conditions under which the system becomes supercritical, i.e. there is a fast, non-linear transfer of energy from protons to secondary particles initiated by the spontaneous quenching of proton-produced $γ$-rays. We first show analytically that the transition to supercriticality occurs whenever the proton injection compactness exceeds a critical value, which favours high proton injection luminosities and a wide range of bulk Lorentz factors. The properties of supercriticality are then studied with a time-dependent numerical code that solves concurrently the coupled equations of proton, photon, electron, neutron and neutrino distributions. For conditions that drive the system deep into the supercriticality we find that the photon spectra obtain a Band-like shape due to Comptonization by cooled pairs and that the energy transfer efficiency from protons to $γ$-rays and neutrinos is high reaching $\sim 0.3$. Although some questions concerning its full adaptability to the GRB prompt emission remain open, supercriticality is found to be a promising process in that regard.

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arXiv2013

Relativistic Pair Beams from TeV Blazars: A Source of Reprocessed GeV Emission rather than IGM Heating

The interaction of TeV photons from blazars with the extragalactic background light produces a relativistic beam of electron-positron pairs streaming through the intergalactic medium (IGM). The fate of the beam energy is uncertain. By means of two- and three-dimensional particle-in-cell simulations, we study the non-linear evolution of dilute ultra-relativistic pair beams propagating through the IGM. We explore a wide range of beam Lorentz factors gamma_b>>1 and beam-to-plasma density ratios alpha<<1, so that our results can be extrapolated to the extreme parameters of blazar-induced beams (gamma_b~10^6 and alpha~10^(-15), for the most powerful blazars). For cold beams, we show that the oblique instability governs the early stages of evolution, but its exponential growth terminates - due to self-heating of the beam in the transverse direction - when only a negligible fraction ~(alpha/gamma_b)^(1/3)~10^(-7) of the beam energy has been transferred to the IGM plasma. Further relaxation of the beam proceeds through quasi-longitudinal modes, until the momentum dispersion in the direction of propagation saturates at DeltaP_{b,parallel}/gamma_b m_e c ~ 0.2. This corresponds to a fraction ~10% of the beam energy being ultimately transferred to the IGM plasma, irrespective of gamma_b or alpha. If the initial dispersion in beam momentum satisfies DeltaP_{b,parallel}/gamma_b m_e c > 0.2 (as typically expected for blazar-induced beams), the fraction of beam energy deposited into the IGM is much smaller than ~10%. It follows that at least ~90% of the beam energy is still available to power the GeV emission produced by inverse Compton up-scattering of the Cosmic Microwave Background by the beam pairs.

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arXiv2013

A Late-Time Flattening of Light Curves in Gamma-Ray Burst Afterglows

The afterglow emission from Gamma-Ray Bursts (GRBs) is usually interpreted as synchrotron radiation from relativistic electrons accelerated at the GRB external shock, that decelerates from ultra-relativistic to non-relativistic speeds as it sweeps up the surrounding medium. We investigate the temporal decay of the emission from GRB afterglows at late times, when the bulk of the shock-accelerated electrons are non-relativistic. For a uniform circumburst medium, we show that such "deep Newtonian phase" begins at t_{DN} ~ 3 epsilon_{e,-1}^{5/6} t_{ST}, where t_{ST} marks the transition of the blast wave to the non-relativistic spherically-symmetric Sedov-Taylor solution, and epsilon_e = 0.1 epsilon_{e,-1} quantifies the amount of shock energy transferred to the post-shock electrons. For typical parameters, the deep Newtonian stage starts ~ 0.5-several years after the GRB. The radio flux in this phase decays as F_nu ~ t^{-3(p+1)/10}, with decay slope between -0.9 and -1.2, for a power-law distribution of shock-accelerated electrons with index 2 < p < 3. This is shallower than the commonly assumed scaling F_nu ~ t^{-3(5p-7)/10} (with decay slope between -0.9 and -2.4) derived by Frail et al. (2000), which only applies if the GRB shock is non-relativistic, but the electron distribution still peaks at ultra-relativistic energies (a regime that is relevant for a narrow time interval, and only if t_{DN} > t_{ST}, or epsilon_e > 0.03). We discuss how the deep Newtonian phase can be reliably used for GRB calorimetry, and we comment on the good detection prospects of trans-relativistic blast waves at 0.1-10 GHz with EVLA and LOFAR.

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arXiv2013

Reconnection-driven plasmoids in blazars: fast flares on a slow envelope

Minute-timescale TeV flares have been observed in several blazars. The fast flaring requires compact regions in the jet that boost their emission towards the observer at an extreme Doppler factor of delta>50. For TeV photons to avoid annihilation in the broad line region of PKS 1222+216, the flares must come from large (pc) scales challenging most models proposed to explain them. Here we elaborate on the magnetic reconnection minijet model for the blazar flaring, focusing on the inherently time-dependent aspects of the process of magnetic reconnection. I argue that, for the physical conditions prevailing in blazar jets, the reconnection layer fragments leading to the formation a large number of plasmoids. Occasionally a plasmoid grows to become a large, "monster" plasmoid. I show that radiation emitted from the reconnection event can account for the observed "envelope" of ~ day-long blazar activity while radiation from monster plasmoids can power the fastest TeV flares. The model is applied to several blazars with observed fast flaring. The inferred distance of the dissipation zone from the black hole and the typical size of the reconnection regions are R_diss~0.3-1 pc and l' \simless 10^{16} cm, respectively. The required magnetization of the jet at this distance is modest: sigma ~a few. Such dissipation distance R_diss and reconnection size l' are expected if the jet contains field structures with size of the order of the black-hole horizon.

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