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Session 107 - The Galactic Center & Bulge.
Display session, Saturday, January 10
Exhibit Hall,
Only 15 radio pulsars of the current catalog of approximately 750 are within 5^\circ of Sgr A and none are within 1^\circ. This deficit is due to interstellar scattering so severe as to smear together individual pulses. Pulse broadening for pulsars in the Galactic center (GC) is 350\,\nu_GHz^-4 seconds\/ for a radio frequency of \nu_GHz GHz. Pulsars beyond---but viewed through---the GC suffer even greater pulse broadening. Periodicity searches at radio frequencies will be successful only at \nu > 5 GHz, and will find only long-period pulsars.
In order to find GC pulsars, we are employing an alternate strategy---an aperture synthesis survey. Our survey is far more sensitive than a periodicity search because angular broadening of compact GC sources does not desensitize the survey. The scattering diameter of Sgr A^* is 1.\!\!^\prime\prime3\,\nu_GHz^-2; OH/IR stars within 25^\prime of Sgr A^* have comparable diameters. The predicted scattering diameter of a compact GC source is 0.\!\!^\prime\prime7 at 1.4 GHz and 12^\prime\prime at 0.33 GHz. These scattering diameters are well-matched to the synthesized beam of the A-configuration VLA: 1.\!\!^\prime\prime2 and 6^\prime\prime, respectively.
Our objective is to find compact, steep-spectrum, polarized sources. We surveyed the inner 1^\circ of the GC at 0.33 and 1.4 GHz with the \hboxVLA. The observation wavelengths are matched to the typical pulsar spectrum which peaks at \nu < 1 GHz. The 1.4 GHz observations were sensitive to cross-polarized emission and are being used to identify linearly-polarized objects. The angular extent of the survey is slightly larger than the extent of the hyperstrong scattering region responsible for the angular broadening of Sgr A^* and the OH/IR stars.
The number of detectable\/ pulsars in the inner 1^\circ (= 150 pc at 8.5 kpc) of the GC may range from 1 to 100, with the larger values resulting from recent, vigorous starbursts. We present our current catalog, which contains 75 objects. We have begun a program of near-IR observations on the Palomar 5 m telescope to aid in the identification of these sources. Our 0.33 GHz observations will also be used in conjunction with a series of 0.33 GHz observations obtained over the past 6 yr to search for radio transients.
Basic research in radio astronomy at the Naval Research Laboratory is supported by the Office of Naval Research. TJWL is supported by an NRC-NRL Research Associateship.
