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Astron. Astrophys. 335, 991-994 (1998)

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4. Results

Figs. 1 and 2 show the integrated V- and B-band images. The proposed optical counterparts (Blair & Schild 1985; Fesen & Gull 1985) labelled [FORMULA]1' and [FORMULA]2', can clearly be seen in both images. The core region surrounding the two candidates is shown in Fig. 3, which is a sum of all the observations. The positions of the radio pulsar (Foster et al. 1994) and X-ray point source (Wang & Seward 1984) are also marked. Like Fesen & Gull we find no evidence for Blair and Schild's candidate [FORMULA]3' (down to a limiting magnitude of [FORMULA]), consistent with its identification as a H[FORMULA]+[NII] emission knot. The photometry for these candidates, and those discussed later on, is given in Table 3. The magnitudes were calibrated with respect to Blair and Schild's star [FORMULA]A'. The quoted errors were calculated from a combination of counting statistics and flat-fielding errors. Our B1950 source positions of the two pulsar candidates are [FORMULA] (candidate 1) and [FORMULA] (candidate 2).

[FIGURE] Fig. 3. V+B image of the central region of CTB 80. Blair and Schild's candidates 1 and 2 can be seen toward the centre of the image. The radio (white cross) and X-ray (black cross) source positions are marked. (1 pixel = [FORMULA], east is down and north is to the right)


Table 3. Photometry for bright sources and pulsar candidates in the CTB 80 field. (The values for star A are taken from Blair & Schild (1985)). Values for [FORMULA] & [FORMULA] are claculated asuming a distance to the object of 2.5 kpc.

Estimates of the distance to the pulsar vary between about 1.4 kpc (from dispersion measure estimates, Kulkarni et al. 1988) to 3 kpc (studies of the infrared shell, Fesen et al. 1988). Following Safi-Harb et al. (1995) we adopt 2.5 kpc as a compromise between these extremes. Reddening estimates also vary and lie within the range E(B-V) = 0.8-1.4 (Angerhofer et al. 1980; Blair et al. 1984). We use a value E(B-V)= 1.0 which also gives a distance to the pulsar of about 2.5 kpc (Blair & Schild 1985). The measured values are equivalent to a star of absolute magnitude [FORMULA] and intrinsic colour [FORMULA] (candidate 1) and [FORMULA], [FORMULA] (candidate 2).

Photon times were extracted from apertures centred on the two pulsar candidates. No pulsations were seen, to the 1% significance level, in either the B- or V-band data sets. An upper limit for the pulsed fraction was calculated for the B and V emission assuming a duty cycle in the optical of 50%. These upper limits are conservative as pulse profiles in the X-ray (Safi-Harb et al. 1995) suggest a much lower duty cycle. The 3-[FORMULA] upper limits were found to be [FORMULA], [FORMULA], for candidate 1, corresponding to pulsed fractions [FORMULA] in V and [FORMULA] in B. For candidate 2 we found [FORMULA], [FORMULA], corresponding to pulsed fractions [FORMULA] in V and [FORMULA] in B. These limits were calculated from the longest series of data taken with a given filter between resets of the data-collection system (see Sect. 2), a single data file (4,417 seconds) in the case of V and the sum of three files (13,456 seconds) in the case of B.

In Fig. 3 candidate 1 appears to be extended in the direction of the radio pulsar position, a feature that can be seen in both V and B data sets separately.

An accurate PSF for the image was determined using the external IRAF package DAOPHOTX, and used to remove the candidate 1 and 2 stars from the image. We found that the extension was consistent with two point sources. Fig. 4 shows the extension along with another point-like feature nearby (we have labelled these candidates [FORMULA]4',[FORMULA]5' and [FORMULA]6' (Fig. 4)).

[FIGURE] Fig. 4. Contour plot of the region surrounding candidate 1 in the V-band image of Fig. 1. The position of candidates discussed in the text are shown. The position of the radio pulsar (Foster et al. 1994) is close to candidate 4. (1 pixel = [FORMULA], east is down and north is to the right)

We have tried to assign a magnitude and colour to each of these features, although their low signal-to-noise ratio and proximity to a brighter source makes this difficult to do with any accuracy. The photometry for these sources is included in Table 3. Absolute magnitudes and intrinsic colours are calculated assuming a distance of 2.5 kpc to the pulsar.

Times were extracted from all these candidates and analysed using the [FORMULA] statistic and their Fourier power spectrum. Again, no pulsations were seen to the 1% significance level.

Fesen & Gull's (1985) H[FORMULA]+[NII] map of the core shows that some nebulosity is coincident with the candidate positions, although little was detected by Blair & Schild (1985). Spectra of CTB 80 filaments (Blair et al. 1984) show strong [OI], H[FORMULA], [NII] and [SII] emission lines, but these are removed by our filters. Emission lines in the B and V bands are much weaker, although our images do show some evidence of nebulosity. We cannot rule out the possibility that these fainter point-like sources are emission knots in the nebula.

Finally, we extracted photon times from apertures of radius [FORMULA] centred on a two-dimensional grid of points separated by [FORMULA] on the 1997 data. The grid covered an area [FORMULA] centred close to the first candidate position. The photon times were reduced to the solar-system barycentre as before, and the [FORMULA] statistic calculated for each aperture using the ephemeris in Table 2. No evidence for the pulsar was found. The 3-[FORMULA] upper limit corresponds to [FORMULA].

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© European Southern Observatory (ESO) 1998

Online publication: June 26, 1998