Astron. Astrophys. 331, 1002-1010 (1998)

5. Discussion

5.1. Are the detected pulsars really associated with their target supernova remnants?

The previously known pulsar PSR B1952+29 was detected during observations of G65.1+0.6. The pulsar lies outside the boundary of the remnant and is clearly not associated with G65.1+0.6 because of its extremely large characteristic age ( yr) and the proper motion measurement (Lyne et al. 1982), which shows that it is moving towards rather than away from the remnant centre as is required for a genuine association.

The evidence that the newly discovered pulsars are associated with their target remnants is not clear. PSR J0215+6218 has a period of 549 ms and a DM of 84 cm^-3 pc. The distance to this pulsar estimated from its DM and Galactic coordinates using the Taylor & Cordes (1993) electron density model is kpc, consistent with the distance to G132.7+1.3 of kpc (Routledge et al. 1991). Timing measurements show that the characteristic age of PSR J0215+6218 is 13 Myr, anomalously large by comparison with the age of G132.7+1.3, estimated to be between 30,000 and 50,000 yr (Leahy et al. 1985).

In the case of the 308 ms pulsar J1957+2831, there is unfortunately no independent distance estimate to the target remnant, G65.1+0.6. However the angular size of the remnant ( degree) indicates that it is likely to be closer than the distance of kpc inferred from the dispersion measure of PSR J1957+2831 (139 cm^-3 pc). Whilst the age of G65.1+0.6 is also not known, it is likely to be much smaller than the characteristic age of PSR J1957+2831 (1.6 Myr).

Thus, in both cases, the pulsar characteristic age is anomalously large by comparison with the expected ages of the supernova remnants. This suggests that they are either not associated with the remnant, or the characteristic ages are anomalously large, due perhaps to the initial spin period of the pulsar being similar to its presently observed value. We note in passing that the spectral index of PSRs J0215+6218 and J1957+2831 are -1.2 and -0.9 respectively, typical of many other young pulsars (Lorimer et al. 1995) and suggesting that these pulsars may indeed be younger than their characteristic ages would suggest. Although this remains a possibility, as we shall show in Sect. 6, on statistical grounds, neither of the newly discovered pulsars are likely to be associated with the target supernova remnants.

5.2. The luminosity of pulsars at birth

In a study of pulsar population statistics Lorimer et al. (1993) suggested that there may be no need for a significant number of pulsars to be born with 400 MHz radio luminosities below 30 mJy kpc². Deep surveys of supernova remnants, like the present one, can in principle be used to test this hypothesis by combining the flux density upper limit () given in Table 1 with the distance to each remnant () to estimate the minimum luminosity () that a young pulsar would need in order to be detectable. In Table 3 we list the 17 supernova remnants in our sample which have reliable distance estimates. Note that we have chosen not to use the surface brightness-angular size () relationship (Clark & Caswell 1976) as a means of estimating since it has subsequently been shown to be unreliable (Berkhuijsen 1987; Green 1991).

Table 3. The supernova remnants targeted by the survey with published distance estimates. From left to right the columns give the remnant name based on its Galactic coordinates, alias(es) by which the remnant may be called, the distance and reference tag, and the corresponding minimum 400 MHz luminosity for a pulsar to be detectable in our survey (see text). The references are: a. Lozinskaya et al. (1993); b. Green & Gull (1989); c. Landecker et al. (1980); d. Green (1989); e. Feldt & Green (1993); f. Tatematsu et al. (1990); g. van den Bergh (1971); h. Reich & Braunsfurth (1981); i. Hailey & Craig (1994); j. Pineault et al. (1993); k. Albinson et al. (1986); l. Green (1996); m. Roberts et al. (1993); n. Routledge et al. (1991); o. Reich et al. (1992); p. Landecker et al. (1989); q. Routledge et al. (1986)

For each supernova remnant in Table 3, we list the minimum detectable luminosity , where the factor of 2 in this expression scales defined for this survey at 606 MHz to 400 MHz assuming a typical pulsar spectral index of -1.6 (Lorimer et al. 1995) . We find the median value of to be 22 mJy kpc², with 12 of these values lying between 1 and 30 mJy kpc². In this sample of 12 supernova remnants with well defined distances, we detected one pulsar (PSR J0215+6218) which has a 400 MHz luminosity mJy kpc². If we assume that each of these remnants contains a radio pulsar born with a luminosity above 30 mJy kpc², then we expect to see pulsars, where f is the mean beaming fraction. A consensus on the beaming fraction of young pulsars has yet to be established. Frail & Moffet (1993) obtained a value based on a deep imaging search at the VLA. In this case we would expect to see pulsars. On the other hand, Tauris & Manchester (1997) recently claim f to be as low as implying a detection of only one pulsar. Thus, although our results are consistent with few pulsars being born with luminosities below 30 mJy kpc², they are still hampered by small number statistics and we conclude that yet deeper searches are required to provide a larger sample with which to test this hypothesis (see also Kaspi et al. 1996).

5.3. Radio-quiet neutron stars

The above discussion does not, of course, exclude the possibility that neutron stars exist in the remnants which do not produce significant radio emission. One classic example is the 6.9 s X-ray pulsar 1E 2259+586 which has long been postulated to be associated with CTB 109 (Fahlman & Gregory 1981). Despite extensive efforts, no radio counterpart to the pulsar has ever been detected. Coe et al. (1994) used the VLA at 1489 MHz to set a 3 upper limit to continuum emission of 50 Jy. Our limit to pulsed emission from CTB 109 2.3 mJy at 600 MHz corresponds to about 500 Jy when scaled to 1489 MHz, again assuming spectral index of -1.6. Another example is 3C 58, a Crab-like plerion containing an X-ray point source which is higly suggestive of a young pulsar (Helfand et al. 1995 and references therein). Our limit of 1.1 mJy to pulsed emission at 600 MHz corresponds, after scaling, to about a factor of two larger than the present best upper limit of 0.15 mJy at 1400 MHz (Frail & Moffet 1993).

One of the supernova remnants observed during this survey, G78.2+2.1, also known as -Cygni, contains the bright -ray source 2EG J2020+4026. Brazier et al. (1996) have recently discovered an X-ray source within the error box of 2EG J2020+4026. Assuming this to be the X-ray counterpart, they propose, on the basis of the -ray to X-ray flux ratio, that this source is most probably a Geminga-like neutron star. No significant radio pulsations were detected during our search of this remnant and our estimated 400 MHz lower luminosity limit of 11 mJy kpc² certainly rules out the presence of a bright, favourably beamed, Crab-like radio pulsar associated with this source of emission.

© European Southern Observatory (ESO) 1998

Online publication: March 3, 1998
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