The Chase, and Consequences

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Session 19 -- Rossi Prize Lecture
Oral presentation, Monday, May 30, 1994, 11:40-12:30

[19.01] The Chase, and Consequences

Giovanni F. Bignami (Univ. of Cassino,Cassino and IFC-CNR,Milano)

The chase after elusive Geminga, covering 20 years from 1973 to 1993, is a usefulexample of the power of multiwavelength astronomy. It was discovered and first studied as a source of high energy ($>$ 100 MeV) gamma-rays, the electro-magneticchannel in which it emits the majority of its energy flux. This is about $10 ^{-9} erg/cm^{2} sec$, i.e. what in the optical would be, roughly, a 8 mag star. But it was identified, and its nature understood as a rotating neutron star, through work in the soft X-ray/optical bands. Historically, it represents also a fine example of the evolution of high-energy space astronomy. Three separate pairs of gamma and X-ray missions were involved: SAS-2/HEAO-1; COS-B/Einstein and CGRO/ROSAT, each giving contributions of increasing accuracy towards the solution. In parallel, ground based observations continued to yield null results at radio frequencies (per se a significant finding), while, with greater luck, an optical counterpart could be found, also thanks to the object's high proper motion. Observationally, the job needs to be finished: foremost is a measurement of the object's distance, but also important would be the detection of optical pulsations, a disentangling of the spectral components in X-rays and a reliable value, from X/gamma-rays, of the second period derivative.This may all be within reach and, in an ideal world, this wouldmake Geminga a perfect test bed for something still lacking: a complete theory of the electro-magnetic emission from a magnetized, rotating neutron star. \\Multiwavelength astronomy has recently increased the observational panorama ona number of other isolated neutron stars, all, unlike Geminga, visible as radio pulsars. The Compton GRO and ROSAT have more than doubled the existing detectionsin gamma and X-rays. Optically, to the classical Crab and Vela pulsar cases, PSR 0540-69 in the LMC and Geminga have been added, and two more objects, PSR 1509-58 and PSR 0656+14, now show probable candidate counterparts. \\Considering the diversity of the objects involved, such upgraded observational panorama should give new impetus to the interpretation work.

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