Date:
Tue, 01/05/201812:30-13:30
Gamma-ray flares from pulsar wind nebulae
The flares of ∼ 400 MeV gamma-rays observed from the Crab Nebula by the AGILE and Fermi satellites pose a major challenge to theories of particle acceleration, since they (i) substantially exceed the astrophysical “upper limit” of ∼ mec2/αf on synchrotron radiation, (ii) vary on the time scale of hours, although they are likely to occur in the > .1pc-sized Nebula, and (iii) can reach up to 10% of the power of the entire Nebula. Many acceleration scenarios have been proposed to accommodate these properties, generally involving magnetic reconnection, Doppler boosting and/or “magneto-luminescence”. I will describe a new theory that does not straightforwardly fit into any of these categories: inductive acceleration in the pulsar wind. In it, gamma-ray flares result from a sudden drop in the mass-loading of the wind, which causes charge starvation; the current required to maintain wave activity is then carried by very few particles, which are inductively accelerated in the radial direction and reach very high Lorentz factors before encountering the termination shock. On impacting the Nebula, these particles produce a tightly beamed, high luminosity burst of hard gamma-rays, with properties strikingly similar to those observed. I will discuss the predictions of this theory and the prospects for checking them, including the possibility of detecting similar flares from other objects.
The flares of ∼ 400 MeV gamma-rays observed from the Crab Nebula by the AGILE and Fermi satellites pose a major challenge to theories of particle acceleration, since they (i) substantially exceed the astrophysical “upper limit” of ∼ mec2/αf on synchrotron radiation, (ii) vary on the time scale of hours, although they are likely to occur in the > .1pc-sized Nebula, and (iii) can reach up to 10% of the power of the entire Nebula. Many acceleration scenarios have been proposed to accommodate these properties, generally involving magnetic reconnection, Doppler boosting and/or “magneto-luminescence”. I will describe a new theory that does not straightforwardly fit into any of these categories: inductive acceleration in the pulsar wind. In it, gamma-ray flares result from a sudden drop in the mass-loading of the wind, which causes charge starvation; the current required to maintain wave activity is then carried by very few particles, which are inductively accelerated in the radial direction and reach very high Lorentz factors before encountering the termination shock. On impacting the Nebula, these particles produce a tightly beamed, high luminosity burst of hard gamma-rays, with properties strikingly similar to those observed. I will discuss the predictions of this theory and the prospects for checking them, including the possibility of detecting similar flares from other objects.