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Astron. Astrophys. 358, 169-176 (2000)

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5. Follow-up observations

In order to obtain more detailed spin and astrometric parameters of the newly-discovered pulsars, following confirmation, each was included in our monthly [FORMULA] 21-cm timing observations of millisecond pulsars using the Effelsberg-Berkeley-Pulsar-Processor. Full details of the observing procedures are described by Kramer et al. (1999). In brief, during each observing session, a pulse time-of-arrival (TOA) measurement is obtained for each pulsar by cross-correlating the observed pulse profile with a high signal-to-noise "template" profile constructed from the addition of many observations. The template profiles obtained in this way are presented in Fig. 4.

[FIGURE] Fig. 4. Integrated 1400-MHz pulse profiles for the four newly discovered pulsars. The ordinate axis shows intensity, whilst the abscissa shows rotational phase. Each profile displays 360 degrees of rotational phase. These profiles are freely available in digital form at the European Pulsar Network data archive (http://www.mpifr-bonn.mpg.de/div/pulsar/data ).

For each pulsar, the TOAs obtained from all the sessions were referred to the equivalent time at the solar system barycentre and fitted in a bootstrap fashion to a simple spin-down model using the TEMPO software package 1. In Fig. 5, we present the resulting model-observed TOA residuals from this analysis.

[FIGURE] Fig. 5. Timing model residuals for the four newly discovered pulsars. The timing baseline (dotted line) spans 440 days (MJD 51025-51465) in all cases to accommodate early points gathered for PSR J1842-0415. Vertical bars at the end of each plot denote [FORMULA]10 ms.

The phase-coherent timing solutions we obtain for each pulsar indicate that they are all solitary objects. The fitted parameters are summarised in Table 2. A sub-arcsecond position has been determined for PSR J1842-0415, where the baseline of timing observations already spans over a year. The remaining pulsars have timing baselines spanning just over 6 months. This is however, sufficient to decouple the covariant effects of position error and spin down and, as a result, we have determined accurate period derivatives for each pulsar. Table 2 also lists the characteristic ages ([FORMULA]) and surface magnetic field strengths (B) inferred from these measured period and period derivatives (see e.g. Manchester & Taylor 1977 for definitions of these parameters). In addition, we also list the distance (D) to each pulsar inferred from its DM, Galactic coordinates and the Taylor & Cordes (1993) electron density model, as well as the 1400-MHz luminosities inferred from these distances and the observed flux densities as [FORMULA].


Table 2. Parameters for the four newly discovered pulsars. Values in parentheses are uncertainties in the least significant digit.

It is significant that five of the seven pulsars detected in this survey (including all the newly-discovered pulsars) have characteristic ages below 0.5 Myr - over an order of magnitude younger than the median age of the normal pulsars detected by the Parkes 70-cm Southern Sky survey (Manchester et al. 1996; Lyne et al. 1998). This result should not be surprising when it is realised that we have preferentially selected a sample of objects located close to their birth sites along the Galactic plane (Clifton et al. 1992; Johnston et al. 1992a).

By far the youngest of the new discoveries is PSR J1841-0345, which has a characteristic age of only 55 kyr. Since this is within the mean lifetime of supernova remnants (60 kyr - Frail et al. 1994), we checked the position of this and the other newly discovered pulsars with the most recent catalogue of supernova remnants (Green 1998) for spatial coincidences. No supernova remnants in the catalogue lie within 0.3 degrees of J1841-0345, or indeed any of the other new pulsars.

In their study of pulsar-supernova remnant associations Frail et al. (1994) undertook a programme of deep radio imaging to search for previously undetected supernova remnants around several of the young pulsars from the Johnston et al. (1992a) survey. Using the accurate positions we obtained from the timing analysis, we examined the NRAO VLA Sky Survey (NVSS; Condon et al. 1998) images of the fields surrounding each pulsar for evidence of diffuse [FORMULA] 20-cm emission which could be attributed to uncatalogued supernova remnants. The only pulsar for which any diffuse emission is evident in the NVSS survey (down to the 1-mJy sensitivity limit) is J1845-0316, shown in Fig. 6. It is presently not at all obvious whether this emission is attributable to the supernova remnant associated with this pulsar simply because there is such a high density of similar radio sources in this region of the sky. As a result, the by-chance probability of finding unrelated diffuse radio emission, particularly in deeper images of this region, will be rather high, making it difficult to unambiguously identify any associated supernova remnants without additional information (e.g. independent distance estimates to the pulsar and the candidate remnant).

[FIGURE] Fig. 6. NVSS image of the field surrounding PSR J1845-0316. The pulsar position is marked by the + sign at the rim of an extended radio source.

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© European Southern Observatory (ESO) 2000

Online publication: June 26, 2000