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Register a new userOptical and infrared observations of the Crab Pulsar and its nearby knot
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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.
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We present near-infrared observations obtained with ISAAC on the VLT of the Crab pulsar and its environment. Photometry of the pulsar in Js, H and Ks shows the pulsar spectrum to extend fairly smoothly from the UV/optical regime. PSF subtraction of the pulsar allows us to study its immediate neighborhood in some detail. In particular, the knot positioned just 0.6 arcsec from the pulsar has been revealed in the IR. Using also archival HST data for the knot, we have measured its broad band spectrum to rise steeply into the IR, in contrast to the spectrum of the pulsar itself.
We observed the Crab pulsar in October 2008 at the Copernico Telescope in Asiago - Cima Ekar with the optical photon counter Aqueye (the Asiago Quantum Eye) which has the best temporal resolution and accuracy ever achieved in the optical domain (hundreds of picoseconds). Our goal was to perform a detailed analysis of the optical period and phase drift of the main peak of the Crab pulsar and compare it with the Jodrell Bank ephemerides. We determined the position of the main peak using the steepest zero of the cross-correlation function between the pulsar signal and an accurate optical template. The pulsar rotational period and period derivative have been measured with great accuracy using observations covering only a 2 day time interval. The error on the period is 1.7 ps, limited only by the statistical uncertainty. Both the rotational frequency and its first derivative are in agreement with those from the Jodrell Bank radio ephemerides archive. We also found evidence of the optical peak leading the radio one by ~230 microseconds. The distribution of phase-residuals of the whole dataset is slightly wider than that of a synthetic signal generated as a sequence of pulses distributed in time with the probability proportional to the pulse shape, such as the average count rate and background level are those of the Crab pulsar observed with Aqueye. The counting statistics and quality of the data allowed us to determine the pulsar period and period derivative with great accuracy in 2 days only. The time of arrival of the optical peak of the Crab pulsar leads the radio one in agreement with what recently reported in the literature. The distribution of the phase residuals can be approximated with a Gaussian and is consistent with being completely caused by photon noise (for the best data sets).
Pulsars are well studied all over the electromagnetic spectrum, and the Crab pulsar may be the most studied object in the sky. Nevertheless, a high-quality optical to near-infrared spectrum of the Crab or any other pulsar has not been published to date. Obtaining a properly flux-calibrated spectrum enables us to measure the spectral index of the pulsar emission, without many of the caveats from previous studies. This was the main aim of this project, but we could also detect absorption and emission features from the pulsar and nebula over an unprecedentedly wide wavelength range. A spectrum was obtained with the X-shooter spectrograph on the Very Large Telescope. Particular care was given to the flux-calibration of these data. A high signal-to-noise spectrum of the Crab pulsar was obtained from 300 to 2400nm. The spectral index fitted to this spectrum is flat with alpha_nu=0.16 +- 0.07. For the emission lines we measure a maximum velocity of 1600 km/s, whereas the absorption lines from the material between us and the pulsar is unresolved at the 50 km/s resolution. A number of Diffuse Interstellar Bands and a few near-IR emission lines that have previously not been reported from the Crab are highlighted.
The Fermi space telescope has detected over 100 pulsars. These discoveries have ushered in a new era of pulsar astrophysics at gamma-ray energies. Gamma-ray pulsars, regardless of whether they are young, old, radio-quiet etc, all exhibit a seemingly unifying characteristic: a spectral energy distribution which takes the form of a power law with an exponential cut-off occurring between ~1 and ~10 GeV. The single known exception to this is the Crab pulsar, which was recently discovered to emit pulsed gamma rays at energies exceeding a few hundred GeV. Here we present an update on observations of the Crab pulsar above 100 GeV with VERITAS. We show some new results from a joint gamma-ray/radio observational campaign to search for a correlation between giant radio pulses and pulsed VHE emission from the Crab pulsar. We also present some preliminary results on Lorentz invariance violation tests performed using Fermi and VERITAS observations of the Crab pulsar.
Photometric data of the Crab pulsar, obtained in stroboscopic mode over a period of more than eight years, are presented here. The applied Fourier analysis reveals a faint 60 second modulation of the pulsars signal, which we interpret as a free precession of the pulsar.
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