Free Access
Issue
A&A
Volume 531, July 2011
Article Number A105
Number of page(s) 2
Section Stellar structure and evolution
DOI https://doi.org/10.1051/0004-6361/201117114
Published online 22 June 2011

© ESO, 2011

1. Introduction

The pulsar PSR J0108−1431 (Tauris et al. 1994) is one of the closest to Earth, with a distance  pc, determined from its VLBI radio parallax (Deller et al. 2009). The pulsar period, P = 0.808 s, and period derivative,  = 7.44 × 10-17 s s-1, correspond to spin-down age τ = P / 2 = 166 Myr, rotational energy loss Ė = 5.8 × 1030 erg s-1, and surface magnetic field B = 2.5 × 1011 G. PSR J0108−1431 is one of the oldest non-recycled radio pulsars known to date, and in the P- diagram it falls close to the “death line”. Indeed, PSR J0108−1431 is a very faint radio pulsar, with a 400 MHz luminosity of 0.511 mJy kpc2 (at 240 pc), among the faintest known so far. PSR J0108−1431 has been for a long time undetected outside the radio band until its X-ray counterpart was identified by Chandra (Pavlov et al. 2009), which also yielded its first proper motion measurement (199 ± 65 mas yr-1). By backward-extrapolating the Chandra proper motion, Mignani et al. (2008) identified a candidate counterpart (U = 26.4 ± 0.3; B = 27.9 ± 0.5; V > 27.8), tentatively detected in their previous VLT observations (Mignani et al. 2003). The VLBI proper motion (170 ± 1.7 mas yr-1; Deller et al. 2009), now implies a chance coincidence probability  ~3 × 10-4, i.e. a factor of  ~4 smaller than that derived by Mignani et al. (2008) on the basis of the Chandra proper motion, thus strengthening the association. Thus, PSR J0108−1431 might be the oldest pulsar identified in the optical (e.g., Mignani 2011). The countepart’s fluxes are consistent with a T ~ 3 × 105   (d240 / R13)2 K Rayleigh-Jeans spectrum from the bulk of the neutron star surface (see also Deller et al. 2009), where R13 is the emission radius as seen from infinity in units of 13 km and d240 is the pulsar distance in units of 240 pc, which is above all model predictions for such an old neutron star. Here, we present new VLT observations of PSR J0108−1431 aimed at confirming its identification. Observations and data analysis are described in Sect. 2 while results are presented and discussed in Sect. 3.

2. Observation description and data reduction

We observed PSR J0108−1431 with the VLT on 2009 August 24 and 25 with FOcal Reducer/low dispersion Spectrograph (FORS2; Appenzeller et al. 1998) using the blue-sensitive E2V detector ( field-of-view; 025 per pixel) and the uHIGH (λ = 3610 Å; Δλ = 505.1 Å) and bHIGH (λ = 4400 Å; Δλ = 1035.1 Å) filters, for a total integration time of 18 000 and 14 400 s, respectively. We observed in dark time, with mostly clear sky conditions, airmass of  ~1.05–1.14, and image quality of  ~–1″. We obtained images of the Landolt fields (Landolt 1992), bias, and morning twilight flat-field frames. We reduced the the data using the FORS2 pipeline and we stacked the best image quality science images using the drizzle tool in IRAF. We computed the photometric zero points by fitting the instrumental magnitudes of the Landolt stars to their catalogue values and using the most recent atmospheric extinction coefficients for the E2V detector. We estimate that the zero point error due to the uncertain extinction correction and the unknown colour term is  ~0.1 mag, which we assume as the accuracy of our absolute photometry. We re-computed the astrometry of the FORS2 images by fitting the positions of Guide Star Catalogue 2 (GSC-2; Lasker et al. 2008) objects. This yielded a uncertainty (1σ ) on our astrometry after accounting for the registration of the FORS2 image on the GSC-2 grid and the 015 uncertainty on the link of the GSC-2 to the International Celestial Reference Frame (ICRF). To compute the PSR J0108−1431 position at the epoch of our observations (2009.64) we used its VLBI coordinates (Deller et al. 2009): (epoch 2007.0), with an error of  ≈1 mas per coordinate, and proper motion, μα =  +75.05 ± 2.26 mas yr-1; μδ = −152.54 ± 1.65 mas yr-1.

thumbnail Fig. 1

VLT/FORS2 20″ × 20″ B-band image (7800 s) of PSR J0108−1431. The VLBI pulsar positions at the epochs of our FORS2 (2009.64) and FORS1 (2000.6) observations (Mignani et al. 2003) are marked by the black crosses, while that of the candidate counterpart is marked by the white cross. The cross arms correspond to the 3σ error on our absolute FORS2 astrometry. The arrow indicates the pulsar proper motion direction and its length (5″) the expected 30 yr displacement.

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3. Results and conclusions

The PSR J0108−1431 position (epoch 2009.64) is shown in Fig. 1, overlaid on the FORS2 bHIGH-filter image. We could

not detect any object close to the pulsar position in both the uHIGH and bHIGH-filter images. We then determined the detection limit from the standard deviation of the background sampled at the pulsar position in an aperture with diameter equal to 4 pixels, i.e. comparable with the FWHM of the image PSF. After applying the aperture and airmass corrections, we derived 3σ detection limits of U ~ 26.5 and B ~ 27.2. We cross-checked our U and B-band photometry by comparing the fluxes of stars matched with the FORS1 observations of Mignani et al. (2003), and we found a mean difference δU = −0.08 with an rms σU = 0.12, and δB = 0.1 (σB = 0.1). This is consistent with the uncertainty of our absolute photometry calibration (Sect. 2). Thus, our detection limits are compatible with the non-detection of the PSR J0108−1431 candidate counterpart, whose fluxes are U = 26.4 ± 0.3 and B = 27.9 ± 0.5 (Mignani et al. 2008), and do not allow to establish whether this object is real or it was just a background fluctuation, possibly produced by the halo of the close-by ellipitcal galaxy (Fig. 1). Moreover, due to the presence of this galaxy the 3σ detection limits at the epoch 2000.6 pulsar position, i.e. that of the FORS1 observations of Mignani et al. (2003), are even brighter (U ~ 25.7 and B ~ 26.7) that those computed at the 2009.64 position. This means that we cannot determine whether the candidate counterpart (if real) was a back/foreground object or it was the pulsar which now moved away from its discovery position because of its proper motion. Thus, the identification of PSR J0108−1431 is still an open issue. Due to the faintness of the candidate counterpart, and its proximity to the background galaxy, its detection from the ground requires exceptional observing conditions. Thus, more detection chances will come from observations with the Hubble Space Telescope in the near/far-UV where the pulsar is expected to be brighter because of its Rayleigh-Jeans spectrum. Moreover, it is imperative to carry out additional observations as soon as possible because, in a few years from now, the pulsar proper motion direction will bring it closer to the two objects detected south-east of the background galaxy (see Fig. 1), making its faint counterpart much more difficult to resolve.

Acknowledgments

R.P.M. thanks Kieran O’ Brien (UCLA) for support during his last run as Support Astronomer at the Paranal Observatory. The work by G.G.P. and O.K. was partially supported by NASA grant NNX09AC84G.

References

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All Figures

thumbnail Fig. 1

VLT/FORS2 20″ × 20″ B-band image (7800 s) of PSR J0108−1431. The VLBI pulsar positions at the epochs of our FORS2 (2009.64) and FORS1 (2000.6) observations (Mignani et al. 2003) are marked by the black crosses, while that of the candidate counterpart is marked by the white cross. The cross arms correspond to the 3σ error on our absolute FORS2 astrometry. The arrow indicates the pulsar proper motion direction and its length (5″) the expected 30 yr displacement.

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