The detection of rapidly variable TeV emission on timescales of ∼ 1 day in M87 implies a source size of a few Schwarzschild radii r s , and suggests that the TeV emission may originate from unscreened electric fields in the black hole magnetospheres. The presence of the VLBA jet, on the other hand, indicates that a force-free (ideal) MHD flow is established on scales < 50 r s. We show that inverse Compton scattering of ambient photons by electrons accelerated in the gap can initiate a pair cascade above the gap, that results in a sufficiently large multiplicity to ensure a force-free flow. Photons having energies below a few TeV can escape freely. The VHE power radiated by the gap can easily account for the luminosity of the TeV source detected by, e.g., H.E.S.S. Intermittencies of the cascade process will give rise to the observed variability of the TeV emission, and the resultant force-free outflow, as indicated by the morphological changes of the VLBA jet.

No abstract

The number of orphan radio afterglows associated with γ-ray bursts (GRBs) that should be detected by a flux limited radio survey, is calculated. It is shown that for jetted GRBs this number is smaller for smaller jet opening angle θ, contrary to naive expectation. For a beaming factor f −1 b ≡ (θ 2 /2) −1 ≃ 500, roughly the value inferred by Frail et al. (2001) from analysis of afterglow light curves, we predict that between several hundreds to several thousands orphan radio afterglows should be detectable (over all sky) above 1 mJy at GHz frequencies at any given time. This orphan population is dominated by sources lying at distances of a few hundred Mpc, and having an age of ∼ 1 yr.A search for point-like radio transients with flux densities greater than 6 mJy was conducted using the FIRST and NVSS surveys, yielding a list of 25 orphan candidates. We argue that most of the candidates are unlikely to be radio supernovae. However, the possibility that they are radio loud AGNs cannot be ruled out without further observations. Our analysis sets an upper limit for the all sky number of radio orphans, which corresponds to a lower limit f −1 b > 10 on the beaming factor. Rejection of all candidates found in our search would imply f −1 b > 100. This, and the fact that some candidates may indeed be radio afterglows, strongly motivate further observations of these transients.

It is shown that particles accelerating near the event horizon of a spinning supermassive black hole that is threaded by externally supported magnetic field lines suffer severe curvature losses that limit the maximum energy they can attain to values well below that imposed by the maximum voltage drop induced by the black hole dynamo. It is further shown that the dominant fraction of the rotational energy extracted from the black hole is radiated in the TeV band. The implications for vacuum breakdown and the observational consequences are discussed.

We present the first full characterization of the transient radio sky via radio and optical follow-up observations of all the possible radio transients we have discovered in a survey covering %1/17 of the sky. The two confirmed radio transients turn out to be an optically obscured radio supernova (SN ) in the nearby galaxy NGC 4216, the first such event to be discovered by a wide-field radio survey, and a source not associated with a bright host galaxy. We speculate that this second source may be a flare from a peculiar radio-loud AGN, or a burst from an unusual Galactic compact object, but its nature merits further study. We place an upper limit of 65 radio transients above 6 mJy over the entire sky (95% confidence level). The implications are as follows. First, we derive a limit on the typical beaming of GRBs; we find f À1 ]. Second, our results impose an upper limit on the rate of events that eject k10 51 ergs in unconfined relativistic ejecta, whether or not accompanied by detectable emission in wavebands other than the radio. Our estimated rate,ṅ 1000 yr À1 Gpc À1, is about 2 orders of magnitude smaller than the rate of core-collapse SNe (and Type Ib/c events in particular), indicating that only a minority of such events eject significant amounts of relativistic material, which are required by fireball models of long-soft GRBs. Finally, we show that wider and /or deeper radio variability surveys are expected to detect numerous orphan radio GRB afterglows and illustrate the great potential of new radio instruments to revolutionize the study of nearby SNe.