AN EXCESS OF TeV-ENERGY GAMMA RAYS from galaxy Markarian 421 may oblige astronomers to revise their models of active galactic nuclei (AGN). Many suspect that AGNs, quasars, and indeed all the most violent celestial objects in the universe share a common energy-production architecture---a black hole, supplied by a surrounding accretion disk, broadcasting powerful jets of matter in two polar directions. Mrk421 (400 million light years away) is the closest such object whose jet axis is aimed directly at us. Last year Mrk421 rewarded patient observers with the most explosive gamma display ever, with a flux ten times higher than that of the much closer Crab Nebula, the strongest known steady gamma source in the sky. At last week's APS/AAPT meeting in Washington, DC, Trevor Weekes of the Whipple Observatory presented a detailed spectrum for Mrk421. The flux of gammas falls off at the highest energies (up past 6 TeV), but not nearly as fast as one would have expected. Weekes suggested that the anticipated effect of two sources of attenuation, dust near the AGN and the amorphous population of infrared photons in intergalactic space, may have been overestimated.

A SUPERFLUID ANALOGUE OF A JOSEPHSON JUNCTION has been devised by Richard Packard at UC Berkeley. One of the peculiar properties of superconductors is that the amount of magnetic flux penetrating a sample can only be a multiple of a basic flux unit. At the heart of a superconducting quantum interference device (SQUID) is a pair of insulating barriers which interrupt a ring-shaped superconducting sample. Electron pairs tunneling through the barriers interfere with each other in a way that depends on the amount of flux threading the superconducting circuit; thus the quantization of flux can be exploited to measure tiny magnetic fields. In a superfluid, by contrast, fluid circulation is quantized, and this property can be exploited to measure very tiny rotations. In the Berkeley experiment, the flow of superfluid helium through a ring-shaped vessel is interrupted by a barrier containing a micron-sized pinhole. When the vessel is rotated, the helium must squirt back through the hole to maintain its place in space (like an icecube in your drink wanting to stay where it is when you turn the glass). With this scheme the rotation of the earth can be detected to a precision of 0.5%. (Nature, 10 April 1997.)

FIRST RESULTS FROM JEFFERSON LAB. This new nuclear physics facility in Newport News, Virginia explores the interface between the physics of the nucleus (made of protons and neutrons) and the physics of individual protons and neutrons (made of quarks held together by particles known as gluons). The main machine at Jefferson Lab is the Continuous Electron Beam Accelerator Facility (CEBAF), which accelerates continuous streams of electrons to energies of 4 GeV (with a maximum energy of 8 GeV planned for the future); the electrons are then diverted to one of three experimental halls where they collide with fixed targets containing nuclei. At the APS meeting Rolf Ent of Jefferson Lab described how electron collisions with nuclei are ejecting protons from nuclei at a greater rate than anticipated by the present theories on the subject. Exploring how gamma rays break up deuterons (containing a proton and neutron), Haiyan Gao of Argonne presented measurements showing that the quark substructure inside the deuteron must be taken into account to properly understand the breakup process.(CEBAF illustration at Physics News Graphics)