Hubble Space Telescope observations of the old pulsar PSR J0108-1431

Abstract

We present results of optical-UV observations of the 200 Myr old rotation-powered radio pulsar J0108$-$1431 with the Hubble Space Telescope. We found a putative candidate for the far-UV (FUV) pulsar counterpart, with the flux density $f_{\nu }=9.0\pm 3.2$ nJy at $\lambda =1528$ Å. The pulsar was not detected, however, at longer wavelengths, with $3\sigma$ upper limits of 52, 37, and 87 nJy at $\lambda =$ 4326, 3355, and 2366 Å, respectively. Assuming that the pulsar counterpart was indeed detected in FUV, and the previously reported marginal $U$ and $B$ detections with the Very Large Telescope were real, the optical-UV spectrum of the pulsar can be described by a power-law model with a nearly flat $f_{\nu }$ spectrum. Similar to younger pulsars detected in the optical, the slope of the nonthermal spectrum steepens in the X-ray range. The pulsar's luminosity in the 1500--6000 Åwavelength range, $L\sim 1.2\times 10^{27}(d/210\mathrm{pc}{)}^2$ erg s${}^{-1}$, corresponds to a high efficiency of conversion of pulsar rotation energy loss rate $\dot{E}$ to the optical-UV radiation, $\eta =L/\dot{E}\sim (1$--$6)\times 10^{-4}$, depending on somewhat uncertain values of distance and spectral slope. The brightness temperature of the bulk neutron star surface does not exceed 59,000 K ($3\sigma$ upper bound), as seen by a distant observer. If we assume that the FUV flux is dominated by a thermal component, then the surface temperature can be in the range of 27,000--55,000 K, requiring a heating mechanism to operate in old neutron stars.

14 pages, 7 figures, 7 tables. Accepted to ApJ

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Cited by in corpus (1)

• Timing and Evolution of PSR B0950+08