Discovery of pulsed OH maser emission stimulated by a pulsar
Pulsars have proved to be outstanding probes of the interstellar medium (ISM). We have recently embarked on a new type of pulsar-ISM study – binned pulsar spectrometry of the OH line. We are using techniques similar to those we developed in an earlier extensive series of HI pulsar spectrometry measurements. Using this procedure, we accumulate spectra toward pulsars separately during the pulsar pulse and in the interval between pulses. In this fashion, we can isolate the effects of the ISM on the pulsar signal alone in order to study the medium along the line of sight.
We observed eighteen low-latitude, inner Galaxy pulsars. We achieved success with PSR B1641-45. Our new observations mark only the second successful detection of interstellar OH lines in a pulsar spectrum, and the first time they have been seen in emission as well as absorption. The emission line is caused by maser processes.
Figure 1: Discovery of pulsed interstellar OH maser emission stimulated by pulsar B1641-45. Top: The "pulsar-on" spectrum, acquired during the pulsar pulse, and the "pulsar-off" spectrum, measured in the interval between pulses. The two spectra exhibit both emission and absorption against other (non-pulsar) background source(s) lying within the Parkes beam, while the pulsar-on spectrum additionally contains the pulsar signal. Bottom: The pulsar spectrum, the difference of pulsar-on and pulsar-off, 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.
Although there is extensive indirect evidence for maser activity in the ISM, stimulated emission of radiation has never been directly observed in astrophysical situations. In this case, the broadband pulsar spectrum exhibits excess line emission at 1720 MHz as the pulsar's photons stimulate the creation of additional photons in an intervening OH cloud. This excess emission switches on and off with the pulsar, clearly demonstrating its stimulated nature. Hence our measurement marks the first explicit detection of stimulated emission in the ISM.
Figure 1 displays the 1720-MHz spectra toward PSR B1641-45 which directly demonstrate the process of stimulated emission. The pulsar-off spectrum, acquired in the interval between pulses, shows both emission and absorption against other background sources lying within the Parkes beam. The pulsar-on spectrum appears at first glance to be merely a copy of the pulsar-off spectrum, shifted upward by a constant equal to the broadband pulsar signal strength. However, when these two spectra are carefully differenced to create the pulsar spectrum, it is clear that there is excess signal at the line frequency during the pulse – the broadband pulse signal has been amplified by stimulated emission at this frequency.
It has long been assumed that stimulated emission plays an important role in astrophysical OH line radiation because it can be shown that the emitting clouds are not in local thermodynamic equilibrium. However, ours are the first measurements directly demonstrating the stimulated amplification of a signal propagating through the interstellar medium, in that the amplification is directly observable as the pulsar cycles on and off. Since this stimulated emission is driven by the pulsar pulse, it varies on a few millisecond timescale, which is orders of magnitude shorter than the quickest maser variations previously detected. The optical depth of the pulsed maser line is 0.05, implying that approximately five excess line photons are stimulated in the cloud for every hundred passing through it. Note that this amplification must occur for any signal propagating through the cloud at this frequency, but it is the pulsed nature of the pulsar signal that enables us to explicitly detect the excess photons generated by the stimulated emission process.
Figure 2: A schematic model of the ISM toward PSR B1641-45. The lines represent various lines of sight within the Parkes beam.
Pulsar B1641-45 lies in the inner Galaxy near the galactic plane, at (l,b) = (339.2, -0.2). There are a variety of complementary measurements of this region which we have combined with our observations to create a map of the ISM along the line of sight (see Figure 2). The distances are derived from kinematic analyses of radial velocity measurements, using a flat galactic rotation model. Note especially the two HII regions lying in this direction, G339.1-0.2, and G339.1-0.4.
Figure 3: Spectra of the four 18-cm transitions of OH toward pulsar B1641-45. The left column displays the four pulsar-off spectra, which are sensitive to all emission and absorption in the Parkes telescope beam when the pulsar is switched off. The right column shows the four pulsar spectra, which exhibit the interstellar absorption or stimulated emission of the pulsar signal alone.
Figure 3 displays spectra toward PSR B1641-45 at the four 18-cm OH lines. Pulsar-off spectra are shown in the left column while pulsar spectra are displayed in the right column. We see a strong line at vLSR ~ 45 km/s in all eight spectra. The line is in absorption at 1612, 1667, and 1665 MHz and in emission at 1720 MHz (the latter being the maser line discussed above). This line must arise in gas lying between the pulsar and Earth, since it is visible in the pulsar spectra. It presumably occurs in OH gas associated with or near G339.1-0.4 since its radial velocity is similar to the HII region's. We see another OH line at vLSR ~ 30 km/s in most of our spectra including the 1665 and 1667 pulsar spectra. This line must therefore also originate in gas nearer than the pulsar to us, also probably associated with or near G339.1-0.4. Note also that all pulsar-off spectra exhibit lines at vLSR ~ (100 to 120) km/s, which are not seen in the pulsar spectra. Therefore they originate in gas beyond the pulsar, which is probably associated with or near G339.1-0.2.
Another interesting phenomenon is also visible in Figure 3. Note that each 1720-MHz spectrum (top row) is an inverted copy of the 1612-MHz spectrum (second row). This phenomenon, dubbed "conjugate" line behavior, is frequently seen in interstellar OH clouds. It results from the initial states of both transitions being overpopulated by an identical physical process, so that the degree of overpopulation is also the same in both. We will be able to derive column densities of the clouds from comparison of the conjugate lines, since the details of the process are density-dependent.
Finally, note that the pulsar spectra (Figure 3, right column) exhibit much stronger absorption and stimulated emission than do the corresponding pulsar-off spectra (Figure 3, left column) at the same velocities. A similar discrepancy between pulsar and pulsar-off spectra was observed in the only other successful pulsar OH absorption experiment. Our new observations strengthen the earlier interpretation that this is a solid-angle effect. Apparently the tiny interstellar column sampled by the pulsar signal possesses significantly different properties from the medium sampled in the pulsar-off spectrum, which averages across the entire Parkes beam.
Presumably the pulsar pulse is encountering a small, dense OH cloudlet whose properties are diluted in the beam-averaged pulsar-off spectrum. This behaviour differs markedly from HI, where absorbing columns of very different solid angles exhibit similar statistics. In retrospect, the result is not too surprising, as molecular gas is known to be more clumped than neutral gas.
Our search for OH absorption and stimulated emission in the spectrum of pulsars yielded a bonanza of interesting results in the direction of one source, B1641-45. We detected the first pulsed interstellar maser line, which provides direct evidence for stimulated emission in the ISM. We observed emission or absorption in the pulsar spectra at all four 18-cm OH lines, with conjugate behaviour observed at 1612/1720 MHz. We mapped the OH and HII concentrations along the line of sight to the pulsar, found that they are associated kinematically and probably spatially as well. The optical depths of pulsar spectral lines are much greater than pulsar-off lines, demonstrating that the molecular medium is significantly clumped at small scales.
References
Stanimirovic, Weisberg, et al., 2003, ApJ, 592, 953
Joel Weisberg (ATNF / U. Sydney / Carleton Coll.), Simon Johnston (ATNF), Baerbel Koribalski (ATNF) andSnezana Stanimirovic (Berkeley)
(Joel.Weisberg@carleton.edu)