July 2007
RCW 86 is a supernova remnant with a barrel-shaped shell, nicely visible in X-rays (top image), in the optical and in radio waves (Fig. 1). Concerning its age, two options are discussed in the literature. Given the modest speeds of the supernova blast wave – 400-900 km/s – determined in some regions of the remnant, the large size of the remnant implies an age of order 10000 years, combined with a distance of about 3 kpc (Rho et al, 2002 and refs. given there). On the other hand, Vink et al. (2007) argue that the spectrum and structures visible in the synchrotron radiation in particular in the high-resolution Chandra images of the north-east corner (middle frame, top figure) require much higher speeds of the blast wave, around 2700 km/s. Acceleration efficiency of synchrotron-emitting electrons in blast waves is closely related to the speed of the blast wave. For modest speeds, the energy losses due to radiation would outweigh the energy gains, preventing electrons from reaching high energies. Vink et al. speculate that the supernova explosion might have taken place in a bubble in the interstellar medium, allowing high blast wave speeds, and that now certain parts of the remnant have reached dense material at the edge of the bubble, slowing down the expansion, whereas other sections still expand at high speed (see also Chandra press release, APOD). In this case, the remnant would be much younger and RCW 86 could be the result of a stellar explosion recorded in 185 A.D. by Chinese astronomers (e.g. Green and Stephenson, 2003)
The relatively large size of the remnant – about 40′ in diameter – and the observation of non-thermal X-rays make it a promising target for gamma-ray observations, aiming at increasing the currently modest number of remnants where the shells are resolved in very high energy gamma rays. Hints for gamma-ray emission from RCW 86 were seen with the CANGAROO-II instrument (S. Watanabe, Proc. 28th ICRC), but no firm detection was claimed. RCW 86 was observed for about 30 h with the H.E.S.S. instrument, and a clear gamma-ray signal with more than 9 standard deviations is detected, using an a priori defined integration region which encompasses the entire remnant (Fig. 2). The exact morphology of the gamma-ray emission is still under study: whereas one type of data analysis shows hints of a 3/4 shell lining up with the X-ray emission (Fig. 2 top, Fig. 3), this morphology is not quite as evident with other analysis techniques (Fig. 2 bottom), and more data may be required to fully settle this issue. The flux from the remnant is 5-10% of the flux from the Crab nebula, with a similar spectral index of 2.5, but the spectrum is also well described by a power law with index 1.5-2 and a cutoff around 5 TeV. The observation of TeV gamma-rays from the remnant favors somewhat the scenario of a young remnant with high expansion speed, easing the acceleration of high-energy particles.
