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Discovery of Pulsed OH Maser Emission Stimulated by a Pulsar

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Science  01 Jul 2005:
Vol. 309, Issue 5731, pp. 106-110
DOI: 10.1126/science.1112494
  • Fig. 1.

    Stimulated amplification of the PSR B1641–45 signal in an interstellar OH cloud at 1720 MHz. (A) The pulsar-on (top) spectrum, acquired during the pulsar pulse, and (bottom) the pulsar-off spectrum, gathered in the interval between pulses. The two spectra exhibit both emission and absorption against other (nonpulsar) background source(s) lying within the 13–arc min telescope beam, whereas the pulsar-on spectrum additionally contains the pulsar signal. (B) The pulsar spectrum, the difference of pulsar-on and pulsar-off spectra, illustrating the pulsar signal alone as absorbed (or in this case, amplified) by intervening OH. The spike in this spectrum at vLSR ∼–45 km/s results from excess emission in an OH cloud, stimulated by pulsar photons.

  • Fig. 2.

    A schematic model of the ISM toward PSR B1641–45. The ionized region velocities are from (19, 20), the OH velocities are from our current work, and the limiting pulsar HI absorption velocities are from (23). The kinematic velocity-to-distance conversion uses the rotation curve of (13). The lines represent various lines of sight within the 13–arc min telescope beam. The vertical scale has been enlarged for clarity.

  • Fig. 3.

    Spectra of the four 18-cm ground-state rotational transitions of OH toward PSR B1641–45. The left column displays the four pulsar-off spectra, which are sensitive to all emission and absorption in the 13–arc min telescope beam when the pulsar is switched off. The right column shows the four pulsar spectra, which exhibit the absorption or stimulated emission of the pulsar signal alone. The righthand ordinate on each panel is optical depth τ; all eight spectra are plotted with the same optical depth scale. All pulsar-off spectral features are significantly shallower (i.e., have smaller optical depths) than their analogs in the pulsar spectra. In the pulsar-off spectra, the sloping lines of constant optical depth result from changes in the ratio of background to total continuum along the line of sight (see discussion accompanying Eq. 1). Low-order sinusoids were fitted to and removed from the pulsar-off baselines in order to flatten them.

  • Table 1.

    Integration times and pulsar spectrum noise fluctuations.

    PSR J PSR B Freq (MHz) ttot (hour) στ* (pulsar spectrum)
    0742-2822 0740-28 1665/7 3 0.1
    0835-4510 0833-45 1665/7 2 0.1
    0837-4135 0835-41 1665/7 4 0.07
    0908-4913 0906-49 1665/7 4 0.1
    1056-6258 1054-62 1665/7 2 0.1
    1057-5226 1055-52 1665/7 1 0.1
    1157-6224 1154-62 1665/7 2 0.3
    1243-6423 1240-64 1665/7 2 0.1
    1326-5859 1323-58 1665/7 2 0.1
    1327-6222 1323-62 1665/7 2 0.1
    1600-5044 1557-50 1665/7 7 0.1
    1605-5257 1601-52 1665/7 1 0.2
    1644-4559 1641-45 1665/7 5 0.01
    1644-4559 1641-45 1720 5 0.01
    1644-4559 1641-45 1612 4 0.01
    1745-3040 1742-30 1665/7 5 0.2
    1752-2806 1749-28 1665/7 4 0.05
    1803-2137 1800-21 1665/7 5 0.3
    1825-0935 1822-09 1665/7 1 0.3
    1829-1751 1826-17 1665/7 2 0.3
    • * The quantity στ (pulsar spectrum) is the optical depth standard deviation in the Hanning smoothed pulsar spectrum, which includes the effects of radiometer and sky noise, and in some cases, interstellar scintillation.

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    Discovery of Pulsed OH Maser Emission Stimulated by a Pulsar
    Joel M. Weisberg, Simon Johnston, Bärbel Koribalski, Snezana Stanimirovic

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