2 Observations and data reduction

Our selected sample contains the nearest available pulsars known prior to August 1994: 3 millisecond pulsars and 3 ordinary radio pulsars, whose characteristics are shown in Table 1; note that the nearby pulsar B1257+12 was not available because it was included in a guaranteed time programme with ISOPHOT and in a guest observer programme with ISOCAM; it is added to the table for a later comparison (see Sect. 2.3).

We observed in the mid infra-red (MIR) at 15 $\mu$m with ISOCAM (Cesarsky et al. 1996) and in the far infra-red (FIR) at 90 $\mu$m with ISOPHOT (Lemke et al. 1996); each pulsar being observed for 1223 s and 781 s, respectively, between March 1996, and December 1997. Preliminary results were presented in Koch-Miramond et al. (1999).

 

 

Table 2: 3 $\sigma$ upper limits on flux densities F in mJy from ISO at 15 and 90 $\mu$m and 3 $\sigma$ upper limits on flux densities I in mJy from IRAS/Scanpi at 12, 25, 60 and 100 $\mu$m; when there is a detection the 1 $\sigma$ error is given.

Pulsar	$F_{\rm 15}$	$F_{\rm 90}$	$I_{\rm 12}$	$I_{\rm 25}$	$I_{\rm 60}$	$I_{\rm 100}$
B1534+12	<82.2	<75.0	<90	<90	<140	<390
J2322+2057	<58.8	<72.0	<100	<110	<120	<1200
J2019+2425	<64.5	<130.0	<70	90 $\pm$ 30	140 $\pm$ 60	<2100
B0149-16	<52.8	<75.0	<110	<140	130 $\pm$ 40	<300
B1604-00	<60.0	<90.0	<90	<100	<120	<480
J0108-1431	<66.0	<22.5	170 $\pm$ 40	<110	<90	250 $\pm$ 124
B1257+12			<130	200 $\pm$ 65	<120	<525

2.1 MIR observations and derivation of ISOCAM upper limits

Our additional motivation for the ISOCAM observation was to provide spatial resolution to a possible emission feature. The LW3 filter centered at 15 $\mu$m was used with a spatial resolution of 6 arcsec per pixel. The ISOCAM data were reduced with CIA version 3.0, following the standard processing outlined in Starck et al. (1999). Transient corrections, using the inversion algorithm of Abergel et al. (1996), were applied. No detections were obtained at any of the pulsar positions.

Since the resulting maps gave no indication for infrared sources at the expected source positions, we computed 3$\sigma$ upper limits in the following way: (1) as there were no extended mid-infrared sources, we computed the standard deviations of the noise present in the maps. (2) We then assumed for each source that a point source remained statistically insignificant while its peak had an amplitude less than 3$\sigma$. (3) We then used the known PSF profile to compute the total source flux from the PSF peak value. In that last step, we had to make another assumption, namely the location of the source inside the ISOCAM pixel. Indeed, as ISOCAM generally undersamples the instrumental PSF, the exact position of the source inside the pixel can have a visible impact on the amount of light that falls in the most illuminated pixel of the PSF. We assumed that the source fell at the center of the pixel, which results in maximum light concentration. A point source, brighter than our 3$\sigma$ upper limit, but falling at the edge of a pixel, could still have its most illuminated pixel fainter than 3$\sigma$. However, this configuration would result in typically 2-4 equivalently bright pixels at the source location, which we do not see in the maps. The derived 3$\sigma$ upper limits are between 53 and 82 mJy, (see Table 2).

2.2 FIR observations with ISOPHOT

We obtained ISOPHOT maps at 90 $\mu$m at the positions of the six pulsars using the oversampling mapping mode (AOT P32); the fields were 5 arcmin $\times$ 8 arcmin, with a 46 arcsec square aperture moved in raster steps of 15 arcsec $\times$ 23 arcsec. The data were reduced with version 6.1 of the PHT Interactive Analysis tool (PIA). No flux enhancements were found at the radio positions of the pulsars except for J0108-1431, the nearest known pulsar (Tauris et al. 1994), where a faint enhancement was observed. We therefore reduced the field of J0108-1431 again with version 9.0 of PIA, using several algorithms but no significant flux enhancement was obtained.

To derive upper limit for the 90 $\mu$m emission at the radio position of the pulsars, we measured the $\sigma$ values of the mean flux levels per detector pixel 46 arcsec square in a smooth mapped region around the pulsar position, after correcting for signal losses in the detector due to transients. From these measurements we derived 3 $\sigma$ upper limits between 22.5 and 130 mJy, (see Table 2).

2.3 Upper limits on flux densities at 15 and 90 $\mu$m from ISO and comparison with the IRAS survey results

The 3 $\sigma$ upper limits on the ISO flux densities at 15 and 90 $\mu$m at the radio positions of the six pulsars are shown in Table 2. Lazio et al. (2001) report 60 and 90 $\mu$m observations of 7 millisecond pulsars (including J2322+2057) with ISOPHOT; their typical 3 $\sigma$ upper limits are 150 mJy.

In view of the gain in sensitivity of about a factor 5 of the Scanpi processing of the IRAS survey over the IRAS Point Source Catalog (which has been used by van Buren & Tereby (1993) to search for IRAS sources near the positions of pulsars), we considered the results of the Scanpi processing of all the IRAS scans passing within approximately 1.7 arcmin of the pulsar's positions. We carefully examined the coadded data; in most cases only upper limits can be defined; in a few cases a flux density deduced from the best-fitting point source template was detected at more than 2 $\sigma$ within the 1 arcmin beam of IRAS. Both the 3 $\sigma$ flux limits and flux densities at 12, 25, 60 and 100 $\mu$m are given in Table 2. Although these upper limits are not as stringent as the ISO ones, they put additional constraints on the derivation of upper limits of circumpulsar masses.

The pulsar B1257+12 was added to our sample of six pulsars, not only for its intrinsic interest as the only known pulsar with planets but also because, together with B1534+12, it has published upper limits of fluxes in the mm and sub-mm ranges, which best constrain the upper limits on circumpulsar masses at low temperatures. At 850 $\mu$m using the SCUBA instrument at JCMT, Greaves & Holland (2000) obtained 3 $\sigma$ upper limits on the flux density of respectively 6.5 and 6.8 mJy, for B1534+12 and B1257+12; at 3.03 mm with the Owens Valley array Phillips & Chandler (1994) obtained 3 $\sigma$ flux limits of 21 mJy for both pulsars. These two pulsars have also been observed at 10 $\mu$m with the NASA Infra Red Telescope Facility by Foster & Fischer (1996); they obtained 3 $\sigma$ upper limits on the flux density of respectively 32 and 27 mJy.