The Crab Nebula Jet
M1 (NGC 1952)
RA = 5h 34m 32s Size: 0.5 x 0.9 parsecs (jet)
DEC = +22° 00' 56" Distance = 2.0 kpc (6,500 ly)
Type: Supernova remnant
NGC 1952, the “guest star” reported by Chinese astronomers on July 4, 1054 has developed into the well known Crab Nebula. Known as M1 in Charles Messier’s famous catalogue of non-stellar objects, it is seemingly the most thoroughly studied object in the universe. Over 6,000 research papers, books and symposiums have appeared on this object, most since the discovery of the pulsar at the center of the nebula. Just about every astronomy textbook has photos of the Crab Nebula. Its widespread popularity as an astronomical deep sky object is paramount.
In 1970, Canadian astronomer Sidney van den Bergh detected a faint jet like structure extending away from the north-west part of the Crab’s filaments from a photograph taken with the 48-inch Schmidt telescope on Mt. Palomar. Because of the jet’s low surface brightness, it remained undetected for several decades on photographic plates taken, since most of volume of research on the Crab was in the non-optical region of the spectrum. The jet exhibits the same type of filamentary structure as the rest of the nebula. Figure 1 shows an image of the jet feature taken in 1988. This CCD image was taken on the Kitt Peak 4-meter telescope under reported 0.9 arc second seeing. Measurements of the jets filaments over a 5 year baseline allowed astronomers to determine its motion. Robert Fesen and Bryan Staker reported in Monthly Notices of the Royal Astronomical Society for June 1993 that the jets filaments are moving outward from the nebula at an average speed of 0.26"/yr, or 2,460 km/sec at the presumed distance of M1 of 2 kpc.
Despite the peculiar appearance of the jet with respect to the general appearance of the nebula, the jet takes part in the general expansion of the nebula. This outward expansion velocity is actually consistent with other expansion studies of the outer regions of the nebula. But surprisingly, the symmetry axis of the jet is not aligned with the center of expansion of the remnant, nor that of the current position of the pulsar.
Figure 1. (Left) The Crab Nebula jet photographed by the 4 meter Kitt Peak reflector in November, 1988. (Right) discovery photo of the jet from the 48" Palomar Schmidt telescope from Sidney van den Bergh's discovery announcement in1970. The Jet is the faint nebulosity on the north center of the nebula.
With an assumed distance to the Crab of 2 kpc, the size of the jet measures 0.5 x 0.9 pc. Radial velocity measurements indicate a tube-like structure with walls expanding out at ~ 300 km/sec, or 63 AU/yr. The origin of the jet is controversial. Some of the theories are: 1) a relativistic plasma beam, 2) a pre-supernova red giant mass-loss wake, or 3) propagation of the supernova material along the interior of the mass loss trail.
Several unanswered questions that may help resolve the nature of the jet, 1) is it a hollow structure with few internal knots and filaments?, 2) does the northern tip of the jet show any evidence of closure, as might be expected in a bubble-like structure marked by low velocity filaments along its northern tip? Fesen and Staker measured the radial velocity of a knot near the northern tip of the jet and found it had a high velocity consistent with an open ended form. Their study of 14 knot/filaments suggested a convergence date (outburst date of the supernova) later than the 1054 AD event. This is consistent with the well known acceleration of the expansion of the supernova remnant. 3) Why isn't there a counterjet onthe south side of the nebula?
Although the visible jet is not visible in amateur telescopes, it would be a challenge object for CCD imagers using a red filter.
REFERENCES
Fesen, R., Staker, B., 1993, The Structure and Motion of the Crab Nebula Jet, Monthly Notices of the Royal Astronomical Society, 263, p. 69-74.
van den Bergh, S., 1970, A Jetlike Structure Associated with the Crab Nebula, Astrophysical Journal, 160, L27