Discovery of Pulsed OH Maser Emission Stimulated by a Pulsar
Joel M. Weisberg,1,2,3*
Simon Johnston,2,1
Bärbel Koribalski,1
Snezana Stanimirovi
4
Stimulated emission of radiation has not been directly observed
in astrophysical situations up to this time. Here we demonstrate
that photons from pulsar B1641–45 stimulate pulses of
excess 1720-megahertz line emission in an interstellar hydroxyl
(OH) cloud. As this stimulated emission is driven by the pulsar,
it varies on a few-millisecond time scale, which is orders of
magnitude shorter than the quickest OH maser variations previously
detected. Our 1612-megahertz spectra are inverted copies of
the 1720-megahertz spectra. This "conjugate line" phenomenon
enables us to constrain the properties of the interstellar OH
line–producing gas. We also show that pulsar signals undergo
significantly deeper OH absorption than do other background
sources, which confirms earlier tentative findings that OH clouds
are clumpier on small scales than are neutral hydrogen clouds.
2 School of Physics, University of Sydney, Sydney, NSW 2006, Australia.
3 Department of Physics and Astronomy, Carleton College, Northfield, MN 55057, USA.
4 Department of Astronomy, University of California, Berkeley, CA 94720, USA.
* To whom correspondence should be addressed. E-mail: jweisber{at}carleton.edu
