Extended Hard X-Ray Emission from the Vela Pulsar Wind Nebula
The nebula powered by the Vela pulsar is one of the best examples of an evolved pulsar wind nebula, allowing access to the particle injection history and the interaction with the supernova ejecta. We report on the INTEGRAL discovery of extended emission above 18 keV from the Vela nebula. The northern side has no known counterparts and it appears larger and more significant than the southern one, which is in turn partially coincident with the cocoon, the soft X-ray, and TeV filament toward the center of the remnant. We also present the spectrum of the Vela nebula in the 18-400 keV energy range as measured by IBIS/ISGRI and SPI on board the INTEGRAL satellite. The apparent discrepancy between IBIS/ISGRI, SPI, and previous measurements is understood in terms of the point-spread function, supporting the hypothesis of a nebula more diffuse than previously thought. A break at ~25 keV is found in the spectrum within 6' from the pulsar after including the Suzaku XIS data. Interpreted as a cooling break, this points out that the inner nebula is composed of electrons injected in the last ~2000 years. Broadband modeling also implies a magnetic field higher than 10 μG in this region. Finally, we discuss the nature of the northern emission, which might be due to fresh particles injected after the passage of the reverse shock.