Abstract

PSR J0205+6449 is an X-ray and radio pulsar in supernova (SN) remnant 3C 58. We report on observations of the central region of 3C 58 using the 4.2-m William Herschel Telescope at the Isaac Newton Group of Telescopes, La Palma, with the intention of identifying the optical counterpart of PSR J0205+6449 and characterizing its pulsar wind nebula (PWN). Around the pulsar position, we identified extended emission with a magnitude of B= 23.97 ± 0.10 mag, V= 22.95 ± 0.05 mag and R= 22.15 ± 0.03 mag consistent with a PWN. From the R-band image, we identified three knots with mR= 24.08 ± 0.07 mag (o1), 24.15 ± 0.07 mag (o2) and 24.24 ± 0.08 mag (o3). We confirm the presence of an optical PWN around PSR J0205+6449 and give an upper limit of mR≈ 24 for the optical magnitude of the pulsar. Furthermore, we make the tentative suggestion that our object o1, with an mR≈ 24.08, is the optical counterpart. If confirmed, the pulsar would have an LR/LX≈ 0.004 and an optical efficiency of about five per cent of the Crab pulsar. Such a low efficiency is more consistent with the characteristic age of the pulsar rather than that of SN 1181.

1 INTRODUCTION

PSR J0205+6449 in supernova (SN) remnant 3C 58 is a recently discovered X-ray and radio pulsar (Camilo et al. 2002; Murray et al. 2002) with a period of 65 ms. 3C 58 was thought to be young due to a possible association with SN 1181 (van den Bergh 1978). Consequently, it should share many of the characteristics of the Crab nebula, including the presence of a young pulsar. However, an embedded pulsar defied detection for over 20 yr. PSR J0205+6449 has a considerably weaker flux than the Crab pulsar. Its X-ray emission is 1000 times lower than the Crab and its radio emission is 120 times lower. Some of this discrepancy can be attributed to the mounting evidence that the characteristic age of the pulsar forumla is near to its true age and hence it is not associated with SN 1181 (Gotthelf, Helfand & Newburgh 2007). Very large Array observations at 1.4 GHz show a nebula expansion rate of 0.014 per cent ± 0.003 yr−1 (Bietenholz 2006), inconsistent with SN 1181 unless substantial deceleration of the remnant has occurred. A comprehensive optical study of 3C 58 showed no evidence of an optical pulsar at mR≈ 22.5 (Fesen et al. 2008). Furthermore, based upon the relatively low proper motion of knots within 3C 58, these authors also cast doubt on the association between 3C 58 and SN 1181. Deeper optical observations of 3C 58 show evidence of an optical nebulosity at the same location as the X-ray counterpart to PSR J0205+6449 (Shibanov et al. 2008). These authors interpret this nebulosity as a pulsar wind nebula (PWN).

PSR J0205+6449 has the third highest spin down energy flux forumla after the Crab and Vela pulsars. The X-ray-determined hydrogen column density (NH≈ 4.1 × 1021cm−2) (Gotthelf et al. 2007) and optical extinction, E(BV) ∼ 0.68 (Fesen et al. 2008), are similar to the Crab pulsar. From H i observations, a kinematic distance of 3C 58 has been established at 3.2 kpc (Roberts et al. 1993), consistent with its dispersion measure (Camilo et al. 2002). These combine to make PSR J0205+6449 a likely candidate for optical emission studies. If we assume that the optical luminosity scales with light cylinder magnetic field, LoptB1.6lc (Shearer & Golden 2001), then we estimate that the pulsar should have a visual magnitude in the range 23–25, depending on interstellar absorption and the effects of beaming geometry (Shearer 2008). This paper reports on a William Herschel Telescope (WHT) service time observation of PSR J0205+6449 to look for evidence of an optical pulsar and its possible PWN.

2 OBSERVATIONS AND DATA REDUCTION

2.1 Observations

Photometric observations of the field surrounding 3C 58 were made on the night of 2007 September 10 using the 4.2-m WHT at the Isaac Newton Group of Telescopes, La Palma. The observations were taken in service mode with the Auxiliary Port Imaging Camera (AUX), using the Harris set of the broad-band BVR filters. It is thought that when the filter response curves are convolved with a typical CCD response, these glass filters provide a closer match to the standard Johnson B and V and the Kron-Cousins R bandpass. AUX, with its 1024 × 1024 TEK CCD, has an unvignetted, circular field diameter of 1.8 arcmin with the pixel size on the sky of 0.108 × 0.108 arcsec2. The integration time was 1000 s in the B and V bands each, and 1400 s in the R band. The Landolt standard star 94 − 242 was observed immediately after the primary target at a similar airmass. The weather conditions were very good and stable during the observations. As a result, the full width at half-maximum of a stellar object in the images is less than 0.75 arcmin in all the bands. Table 1 provides a journal of the observations. The data were reduced using standard techniques within iraf and midas. The frames were debiased and flat-fielded using sky flats from the same night.

Table 1

Summary of observations.

Date utc Filter Duration (s) sec (zSeeing (arcsec) 
2007 Sep 11 02:34:27 B 1000 1.31 0.75 
2007 Sep 11 02:54:27 V 1000 1.25 0.75 
2007 Sep 11 03:13:50 R 1400 1.25 0.70 
Date utc Filter Duration (s) sec (zSeeing (arcsec) 
2007 Sep 11 02:34:27 B 1000 1.31 0.75 
2007 Sep 11 02:54:27 V 1000 1.25 0.75 
2007 Sep 11 03:13:50 R 1400 1.25 0.70 

2.2 Astrometry

Our astrometry was made using positions of 10 reference stars from the USNO-B1.0 catalogue.1 We used the iraf tasks ccmap/cctran for the astrometric transformation of the images. Formal rms errors of the astrometric fit for the RA and Dec. were 0.066 and 0.059 arcsec for the R, 0.081 and 0.035 arcsec for the V and 0.079 and 0.041 arcsec for the B bands, respectively, which are better than the pixel size of the images.

2.3 Photometry

First of all, the magnitudes of five relatively bright stars visible in the target frame were derived accurately (Fig. 1). This photometric calibration was carried out using the Landolt standard star 94 − 242. Our data do not allow us to determine the atmospheric extinction coefficient. Instead of this, we use the average coefficients kB= 0.21, kV= 0.13 and kR= 0.09, provided for the Roque de Los Muchachos Observatory, La Palma (Kidger et al. 2003). The signal-to-noise ratios (S/N) and the magnitude uncertainties Δm were calculated as  

1
formula
 
2
formula
where fap is the source flux in counts for a given aperture, g the gain, nap the number of pixels in the source aperture, nbg the number of pixels in area used for the background measurement and σbg the standard deviation of the background in counts (Newberry 1991). The stars chosen as secondary standards are marked by numbers in Fig. 1, and their magnitudes with errors are listed in Table 2. The resulting zero-points for the pulsar frames are B= 25.97 mag, V= 26.06 mag and R= 26.20 mag. The formal, statistical zero-point errors are less than 0.01 mag. However, since only an average curve for atmospheric extinction was used and only one standard star was observed and only once, the real errors can be as large as a few × 0.01 mag.

Figure 1

R-band image of the central region of 3C 58. The Chandra position of the pulsar PSR J0205+6449 (Murray et al. 2002) is marked by ×. The numbers mark the secondary photometric standard stars from Table 2.

Figure 1

R-band image of the central region of 3C 58. The Chandra position of the pulsar PSR J0205+6449 (Murray et al. 2002) is marked by ×. The numbers mark the secondary photometric standard stars from Table 2.

Table 2

The secondary standard stars used for photometric referencing.

Star B (mag) V (mag) R (mag) 
19.73 18.77 18.15 
20.60 19.36 18.58 
22.25 20.79 19.86 
21.29 19.74 18.76 
21.96 20.73 19.98 
Star B (mag) V (mag) R (mag) 
19.73 18.77 18.15 
20.60 19.36 18.58 
22.25 20.79 19.86 
21.29 19.74 18.76 
21.96 20.73 19.98 

The formal, statistical errors are less than 0.01 mag for all the stars with the exclusion of the stars 3 and 5 in the B band where they have errors of ∼0.02 mag.

3 MORPHOLOGY OF THE 3C 58 FIELD

Recent observations of 3C 58 (Shibanov et al. 2008) have shown evidence of an optical nebulosity at the same location as the X-ray counterpart of PSR J0205+6449. They described it as a faint, extended, elliptical structure with no resolved point-like object at its centre. Our observations confirm the presence of this nebulosity. However, its brightest area has a non-elliptical shape with an arc-like filamentary structure. Fig. 2 (left-hand panel) shows the R-band image of this region. In fact, such a nebula structure is also clearly seen in the V image of 3C 58 presented by Shibanov et al. (2008) in their fig. 3. For consistency with Shibanov et al., we have performed photometry of this area using the similar elliptical apertures (table 1 in their paper). The aperture that formally encapsulates ≳86 per cent of the total nebula flux is shown in Fig. 2 (left-hand panel) as a white ellipse. The measured integral magnitudes of the nebula are B= 23.97 ± 0.10 mag, V= 22.95 ± 0.05 mag and R= 22.15 ± 0.03 mag. The measured B and V magnitudes agree within errors with those of Shibanov et al. (2008), although our values are a bit brighter. The latter can be explained by non-perfect reduction of the standard stars, as well as by the variable background. We note, however, that in the R band, the nebula is brighter than the upper limit for the pulsar magnitude taken from Fesen et al. (2008).

Figure 2

Fragment of the R image shown with different contrast to emphasize different components of the nebula. The ‘×’ symbol marks the Chandra position of the pulsar PSR J0205+6449. The typical pointing uncertainty, ≲1 arcsec, is shown by the circle. In the left-hand panel, we show the elliptical aperture used for photometry of the nebula. In the right-hand panel, we mark three compact objects inside the nebula.

Figure 2

Fragment of the R image shown with different contrast to emphasize different components of the nebula. The ‘×’ symbol marks the Chandra position of the pulsar PSR J0205+6449. The typical pointing uncertainty, ≲1 arcsec, is shown by the circle. In the left-hand panel, we show the elliptical aperture used for photometry of the nebula. In the right-hand panel, we mark three compact objects inside the nebula.

Furthermore, the R image shows this nebula with some structure significantly above the noise. In the right-hand panel of Fig. 2, we show the same area of the R image, but with different contrast to emphasize different components of the nebula. There are three compact objects in the middle of this nebula, marked in Fig. 2 as o1, o2 and o3 (the last two could be one arc-like object). We have measured the magnitudes of these objects using a 5-pixel radius aperture corresponding to a sky radius of approximately 0.54 arcsec. We used the same background as determined previously for the photometry of the nebula.

The measured magnitudes are 24.08 ± 0.07 mag (o1), 24.15 ± 0.07 mag (o2) and 24.24 ± 0.08 mag (o3). We have also measured the flux from the brightest part of the nebula outside the objects, using the same aperture, and obtained the value ≈ 24.55 ± 0.10 mag, giving us confidence in the reality of the compact objects in the nebula. Unfortunately, the B and V images are too noisy to confirm their existence in these bands, although upper limits can be given. The high contrast B and V and images are shown in Fig. 3. Based on Cardelli, Clayton & Mathis (1989), we expect E(VR) and E(BR) to be ∼0.5 and 1.2, respectively, and this extinction probably accounts for the non-detection in the B and V bands. The results of our photometry, with the measured coordinates of the compact objects inside of the nebula, are presented in Table 3.

Figure 3

High contrast V (left-hand panel) and B (right-hand panel) images of same region shown in Fig. 2.

Figure 3

High contrast V (left-hand panel) and B (right-hand panel) images of same region shown in Fig. 2.

Table 3

Fluxes and positions of filamentary structures.

Knot RA (h m s) Dec. forumla B (mag) V (mag) R (mag) 
o1 02 05 37.90 +64 49 40.6 >26.1 >24.7 24.08 
o2 02 05 37.93 +64 49 41.4 >25.6 >24.3 24.15 
o3 02 05 38.02 +64 49 41.9 >25.5 >24.7 24.24 
Knot RA (h m s) Dec. forumla B (mag) V (mag) R (mag) 
o1 02 05 37.90 +64 49 40.6 >26.1 >24.7 24.08 
o2 02 05 37.93 +64 49 41.4 >25.6 >24.3 24.15 
o3 02 05 38.02 +64 49 41.9 >25.5 >24.7 24.24 

In conclusion, we note that the object o1 is well inside the typical High-Resolution Camera (HRC) pointing uncertainty, ≲1 arcsec, marked in Fig. 2 as a circle around the pulsar position. We propose that this is the best candidate for the optical counterpart of the pulsar.

4 CONCLUSIONS

From these observations, we confirm the presence of an optical PWN around PSR J0205+6449 and identify possible structure within the nebula. Although from these observations we cannot definitively identify the optical counterpart of PSR J0205+6449, we can give a deep upper limit and make a tentative suggestion that our object o1 is the counterpart. If it is shown that o1 is the counterpart, then we can estimate, using Frei & Gunn (1994), the flux from the pulsar to be 0.71 μJy in the range λλ6000–7000 Å, corresponding to a R-band luminosity of 7.3 × 1030 erg s−1 based upon an E(BV) = 0.68 giving AR≈ 1.8. This luminosity gives an efficiency forumla of ≈3 × 10−7 and an LR/LX≈ 0.004. The optical efficiency is about five per cent of the Crab pulsar's optical efficiency and comparable to the efficiency of older optical pulsars such as Vela and PSR B0656+14 (Shearer & Golden 2001). The optical–X-ray luminosity ratio is relatively high at 0.004, but this is more of a reflection of the low X-ray luminosity. From these tentative observations, we can conclude that the optical emission, if confirmed, is consistent with a pulsar with an age similar to its spin-down age rather than that of a very young pulsar. Final optical identification will have to wait for observations sensitive to flux variations on millisecond time-scales using instruments such as Optima (Straubmeier, Kanbach & Schrey 2001) or the Galway Astronomical Stokes Polarimeter (GASP; Collins, Redfern & Sheehan 2008).

1
The stars from the USNO-B1.0 catalogue used for the astrometry: 1548−0060184, 1548−0060191, 1548−0060202, 1548−0060210, 1548−0060227, 1548−0060230, 1548−0060232, 1548−0060256, 1548−0060258, 1548−0060300 and 1548−0060315.

We thank the ING Group of Telescopes for providing these service time observations and, in particular, the support astronomer, Dr Chris Benn, who took the observations. VN acknowledges the support of both Science Foundation Ireland (grant number 02/IN.1/I208 WebCom-G) and the Higher Education Authority, through its CosmoGrid programme. We also thank the anonymous referee for comments that improved an earlier draft of this manuscript.

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