Upper limit to any 59.35-s periodic radio emission at 18 cm from CG195.5+4.5

Abstract

To understand the true nature of the 100-MeV cosmic γ-ray source CG195.5+4.5 (also known as γ195+5, or Geminga), its identification at other wavelengths seems essential. Unfortunately, its positional uncertainty is large and several attempts to establish a counterpart at radio wavelengths (ref. 1 and H. A. Mayer-Hesselwander, personal communication), optically², in X-rays^3–5, and in very high energy (>10¹¹eV) γ rays⁶, have produced no conclusive identification. In most cases, the reported 59-s γ-ray periodicity^7,8 was used as a basis for a search. We report here an intensive radio search for this object at a wavelength of 18 cm using the 37-m telescope at the Vermilion River Observatory of the University of Illinois, which has a 22-arc min half-power beamwidth at this wavelength. The radiometer consisted of a parametric amplifier with a 10 MHz bandwidth operating in a noise-adding mode to give a system noise temperature of 100 K. The output was sampled and recorded at 184 ms intervals.