On the Parallax of Geminga

Frederick M Walter

Stony Brook University


Geminga is one of the seminal sources for high energy astrophysics. It is a radio-quiet X-ray and gamma-ray pulsar, and one of the strongest sources of pulsed gamma ray emission in the sky. Its spectrum features a soft thermal component and a hard tail. Its characteristic age is 340,000 years, and it may have been born in the Orion OB1 association. Carraveo et al (1996) claimed a distance of 157 (+59, -34) pc using a parallax determination based on 3 HST WFPC2 images. Given the importance of this object for understanding the high energy radiation processes in magnetized neutron stars, we decided to confirm the parallax using an optimized set of HST/ACS images. We obtained deep V-band images in September 2003, March and September 2004, and March 2005, at the extrema of the parallactic shift of Geminga. As of this writing, we are analyzing the images and coming to terms with the instrumental anomalies of the ACS. We expect to report a preliminary parallax of Geminga. Collaborators on this work include Jacqueline Faherty, James Lattimer, and George Pavlov. This work is supported by a grant from the Space Telescope Science Institute.


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