Comet C/1996 B2 (Hyakutake)
Abstract
J. P. Pye and R. G. West, Leicester University; M. Harden and M. Ricketts, Rutherford Appleton Laboratory; K. Dennerl, J. Englhauser, J. Schmitt, and J. Trumper, Max-Planck-Institut fur Extraterrestrische Physik, Garching; and C. Lisse, M. Mumma, and R. Petre, NASA Goddard Space Flight Center, report the discovery of extreme-ultraviolet (EUV) emission from comet C/1996 B2 with the ROSAT Wide Field Camera (WFC), simultaneously with the x-ray measurements reported by Lisse et al. on IAUC 6373: "The WFC measurements were made with the S1A filter, nominal bandpass 90-206 eV (14-6 nm). An image can be found at http://ledas-www.star.le.ac.uk/rosat-goc/comet/. Bright, diffuse emission is seen, roughly coincident with the x-rays and sunward of the nucleus, and strongly time-variable in a similar manner. The peak EUV surface brightness is of order 0.0004 count sE-1 arcminE-2 as measured, or of order 0.003 count sE-1 arcminE-2 when corrected to WFC 'at launch' efficiency. The WFC images also show fainter diffuse emission extending beyond the edge of the HRI field-of-view (i.e., at about 15'-40' from the center of the comet nucleus), forming an arc around the bright central region. The ratio of WFC to HRI countrates shows that the spectrum in the EUV/x-ray band is rather 'soft' and steeply increasing towards low photon energies. This would appear to rule out soft-x-ray line-fluorescence as the dominant emission mechanism. For assumed spectral shapes of a power law or thermal bremsstrahlung emission, the ratio indicates a photon index in the approximate range 2.0-3.0, or a temperature of about 0.1-0.4 keV."
- Publication:
-
International Astronomical Union Circular
- Pub Date:
- May 1996
- Bibcode:
- 1996IAUC.6394....2P