|TITLE：||Shedding light on the pulsar wind|
February 9 (Thu.) 15:30 − 16:30
|PLACE:||Kenkyu Honkan 1F, Meeting Room 1 (研究本館１階会議室1)|
Pulsars are believed to eject ultrarelativistic electron-positron winds that propagate to the large distances leading to the formation of the extended non-thermal sources known as plerions. The wind has been proposed to originate close to the light cylinder, and carry the rotational energy losses of the pulsar. At the beginning the wind is dominated by electromagnetic energy (Poynting flux), but at some later stage the wind is accelerated with most of the Poynting flux being converted to the kinetic energy of bulk motion. Such winds have not been yet observed, therefore the proposed scenario has been deemed inconclusive. The `cold' winds (in the sense of the low energy of the electrons in the frame of moving plasma) represent a form of "dark substance" since, despite the very high energy of the wind particles, the wind emission is extremely weak. In fact, the pulsar winds are visible only in inverse Compton gamma-rays. Recent observations in high energy (HE) and very high energy (VHE) domains with Fermi Large Area Telescope (Fermi/LAT) and ground based Cherenkov detectors Magic and Veritas allow the key properties of the pulsar winds to be measured in a few important systems. In particular, the recently observed VHE pulsed gamma-rays emitted by the Crab pulsar are best explained as arising from the X-ray photons scattering off the wind's electrons. This finding supports the presence of an ultrarelativistic wind in the Crab pulsar, and provides unique information about the acceleration site of the wind at R_w ~ 30 light cylinders, and its Lorentz factor, Γ_0 = 5 x 105. In the case of the pulsars located in close binary systems, the dense stellar photon field provided by the companion star can dramatically enhance the interaction rate of wind electrons with target photons, leading to the formation of detectable gamma-ray signal. Interestingly, the bright gamma ray flare of the binary pulsar PSR B1259-63/LS2883 detected by the Fermi/LAT after several weeks of the periastron passage, can be also interpreted as the inverse Compton (IC) emission of the unshocked electron-positron pulsar wind with Lorentz factor Γ_0 ~104.