Dimitrios Giannios

A Possible Relativistic Jetted Outburst from a Massive Black Hole Fed by a Tidally Disrupted Star

A recent bright emission observed by the Swift satellite is due to the sudden accretion of a star onto a massive black hole. Gas accretion onto some massive black holes (MBHs) at the centers of galaxies actively powers luminous emission, but most MBHs are considered dormant. Occasionally, a star passing too near an MBH is torn apart by gravitational forces, leading to a bright tidal disruption flare (TDF). Although the high-energy transient Sw 1644+57 initially displayed none of the theoretically anticipated (nor previously observed) TDF characteristics, we show that observations suggest a sudden accretion event onto a central MBH of mass about 106 to 107 solar masses. There is evidence for a mildly relativistic outflow, jet collimation, and a spectrum characterized by synchrotron and inverse Compton processes; this leads to a natural analogy of Sw 1644+57 to a temporary smaller-scale blazar.

Fast TeV variability in blazars: jets in a jet

The fast TeV variability of the blazars Mrk 501 and PKS 2155−304 implies a compact emitting region that moves with a bulk Lorentz factor of Γem∼ 100 towards the observer. The Lorentz factor is clearly in excess of the jet Lorentz factors Γj≲ 10 measured on sub-pc scales in these sources. We propose that the TeV emission originates from compact emitting regions that move relativistically within a jet of bulk Γj∼ 10. This can be physically realized in a Poynting flux-dominated jet. We show that if a large fraction of the luminosity of the jet is prone to magnetic dissipation through reconnection, then material outflowing from the reconnection regions can efficiently power the observed TeV flares through synchrotron-self-Compton emission. The model predicts simultaneous far-ultraviolet/soft X-ray flares.

Prompt GRB emission from gradual energy dissipation

I calculate the emission expected from a Poynting-flux-dominated gamma-ray burst (GRB) flow in which energy is dissipated gradually by magnetic reconnection. In this picture, the energy of the radiating particles is determined by heating and cooling balance (slow heating model). Detailed radiative transfer calculations show that, at Thomson optical depths of order of unity, the dominant radiative process is inverse Compton scattering. Synchrotron-self-absorbed emission and inverse Compton dominate in the Thomson thin parts of the flow. The electrons stay thermal throughout the dissipation region because of Coulomb collisions (Thomson thick part of the flow) and exchange of synchrotron photons (Thomson thin part). The resulting spectrum naturally explains the observed sub-MeV break of the GRB emission and the spectral slopes above and below the break. The model predicts that the γ-ray power-law tail has a high-energy cutoff typically in the ∼0.1−1 GeV energy range that should be observable with GLAST. The model also predicts a prompt emission component in the optical and UV associated with the GeV emission. Observations of the prompt emission of GRB 061121 that cover the energy range from the optical to ∼1 MeV are explained by the model.

Unveiling the origin of X-ray flares in gamma-ray bursts

We present an updated catalogue of 113 X-ray flares detected by Swift in the similar to 33 per cent of the X-ray afterglows of gamma-ray burst (GRB). 43 flares have a measured redshift. For the first time the analysis is performed in four different X-ray energy bands, allowing us to constrain the evolution of the flare temporal properties with energy. We find that flares are narrower at higher energies: their width follows a power-law relation w proportional to E-0.5 reminiscent of the prompt emission. Flares are asymmetric structures, with a decay time which is twice the rise time on average. Both time-scales linearly evolve with time, giving rise to a constant rise-to-decay ratio: this implies that both time-scales are stretched by the same factor. As a consequence, the flare width linearly evolves with time to larger values: this is a key point that clearly distinguishes the flare from the GRB prompt emission. The flare 0.3-10 keV peak luminosity decreases with time, following a power-law behaviour with large scatter: L(pk) proportional to t-2.7 +/- 0.5(pk). When multiple flares are present, a global softening trend is established: each flare is on average softer than the previous one. The 0.3-10 keV isotropic energy distribution is a lognormal peaked at 1051 erg, with a possible excess at low energies. The flare average spectral energy distribution is found to be a power law with spectral energy index beta similar to 1.1. These results confirmed that the flares are tightly linked to the prompt emission. However, after considering various models we conclude that no model is currently able to account for the entire set of observations.

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 >1000 rg 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 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.

Fast TeV variability from misaligned minijets in the jet of M87

The jet of the radio galaxy M87 is misaligned, resulting in a Doppler factor δ~1 for emission of plasma moving parallel to the jet. This makes the observed fast TeV flares on time-scales of t v ~ 5 R g /c harder to understand as emission from the jet. In previous work, we have proposed a jets-in-a-jet model for the ultrafast TeV flares with t v « R g /c seen in Mrk 501 and PKS 2155-304. Here, we show that about half of the minijets beam their emission outside the jet cone. Minijets emitting off the jet axis result in rapidly evolving TeV (and maybe lower energy) flares that can be observed in nearby radio galaxies. The TeV flaring from M87 fits well into this picture, if M87 is a misaligned blazar.

Swift J1644+57 gone MAD: the case for dynamically-important magnetic flux threading the black hole in a jetted tidal disruption event

The unusual transient Swift J1644+57 likely resulted from a collimated relativistic jet, powered by the sudden onset of accretion onto a massive black hole (BH) following the tidal disruption (TD) of a star. However, several mysteries cloud the interpretation of this event, including (1) the extreme flaring and ‘plateau’ shape of the X-ray/ -ray light curve during the first t ttrig 10 days after the ray trigger; (2) unexpected rebrightening of the forward shock radio emission at t ttrig months; (3) lack of obvious evidence for jet precession, despite the misalignment typically expected between the angular momentum of the accretion disk and BH; (4) recent abrupt shut-o in the jet X-ray emission at t ttrig 1:5 years. Here we show that all of these seemingly disparate mysteries are naturally resolved by one assumption: the presence of strong magnetic flux threading the BH. Just after the TD event, is dynamically weak relative to the high rate of fall-back accretion ˙ M, such that the accretion

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 e ffi ciency 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 effi cient at dissipating energy, they typically do not accelera te 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 tha n 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 partic les, which naturally accounts for a commonly observed property of blazar jets.

The peak energy of dissipative gamma-ray burst photospheres

ABSTRACT The radiation released at the transparency radius of an ultrarelativistic flow can account forthe observed properties of gamma-ray bursts (GRBs) provided that sufficient energy is dissi-pated in the sub-photospheric region. Here, I investigate how the peak energy of the E · f(E)spectrum and its overall shape depend on the properties of the jet for various “dissipative pho-tospheres”. I find that continuous energy release over a wide range of distances may be thekey to explain the GRB emission. In this picture, the peak of the spectrum forms at a Thom-son optical depth of several tens. The peak depends mainly on the bulk Lorentz factor Γ ofthe flow and can, therefore, be used to determine it. The Γ can range from ∼ 10 to 1000 fromX-ray flashes to the brightest observed GRBs. The Amati relation can be understood if thebrightest bursts are the least baryon loaded ones. Implications from this interpretation of theGRB emission for the central engine are discussed.Key words: Gamma rays: bursts – radiation mechanisms: general – method s: statistical

A Strong Optical Flare Before the Rising Afterglow of GRB 080129

We report on GROND observations of a 40 s duration (rest-frame) optical flare from GRB 080129 at redshift 4.349. The rise and decay times follow a power law with indices +12 and –8, respectively, inconsistent with a reverse shock and a factor 105 faster than variability caused by interstellar material interaction. While optical flares have been seen in the past (e.g., GRB 990123, 041219B, 060111B, and 080319B), for the first time, our observations not only resolve the optical flare into subcomponents, but also provide a spectral energy distribution (SED) from the optical to the near-infrared once every minute. The delay of the flare relative to the gamma-ray burst (GRB), its SED as well as the ratio of pulse widths suggest it to arise from residual collisions in GRB outflows. If this interpretation is correct and can be supported by a more detailed modeling or observation in further GRBs, the delay measurement provides an independent determination of the Lorentz factor Γ of the outflow.