Optical and infrared observations of the Crab Pulsar and its nearby knot

Abstract

We study the spectral energy distribution (SED) of the Crab Pulsar and its nearby knot in the optical and in the infrared (IR) regime. We present high-quality UBVRIz, as well as adaptive optics JHK_sL' photometry, achieved under excellent conditions with the FORS1 and NAOS/CONICA instruments at the VLT. We combine these data with re-analyzed archival Spitzer Space Telescope data to construct a SED for the pulsar, and quantify the contamination from the knot. We have also gathered optical imaging data from 1988 to 2008 from several telescopes in order to examine the predicted secular decrease in luminosity. For the Crab Pulsar SED we find a spectral slope of alpha_nu = 0.27+-0.03 in the optical/near-IR regime, when we exclude the contribution from the knot. For the knot itself, we find a much redder slope of alpha_nu = -1.3 +- 0.1. Our best estimate of the average decrease in luminosity for the pulsar is 2.9+-1.6 mmag per year. We have demonstrated the importance of the nearby knot in precision measurements of the Crab Pulsar SED, in particular in the near-IR. We have scrutinized the evidence for the traditional view of a synchrotron self-absorption roll-over in the infrared, and find that these claims are unfounded. We also find evidence for a secular decrease in the optical light for the Crab Pulsar, in agreement with current pulsar spin-down models. However, although our measurements of the decrease significantly improve on previous investigations, the detection is still tentative. We finally point to future observations that can improve the situation significantly.