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Register a new userNew Results from an old Friend: The Crab Nebula and its Pulsar
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Martin C. Weisskopf
Publication date
2011
fields
Physics
and research's language is
English
Authors
Martin C. Weisskopf
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We summarize here the results, most of which are preliminary, of a number of recent observations of the Crab nebula system with the Chandra X-Ray Observatory. We discuss four different topics: (1) The motion on long (> 1yr) time scales of the southern jet. (2) The discovery that pulsar is not at the center of the projected ring on the sky and that the ring may well lie on the axis of symmetry but appears to be displaced at a latitude of about 5 degrees. (Note that this deprojection is by no means unique.) (3) The results and puzzling implications of the Chandra phase-resolved spectroscopy of the pulsar when compared to observations of pulse-phase variations of similar and dissimilar measures in other regions of the spectrum. (4) The search for the X-ray location of the site of the recently-discovered gamma-ray flaring. We also comment briefly on our plan to use the Chandra data we obtained for the previous project to study the nature of the low-energy flux variations recently detected at hard X-ray energies.
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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).
The Crab nebula is one of the most studied cosmic particle accelerators, shining brightly across the entire electromagnetic spectrum up to very high-energy gamma rays. It is known from radio to gamma-ray observations that the nebula is powered by a pulsar, which converts most of its rotational energy losses into a highly relativistic outflow. This outflow powers a pulsar wind nebula (PWN), a region of up to 10~light-years across, filled with relativistic electrons and positrons. These particles emit synchrotron photons in the ambient magnetic field and produce very high-energy gamma rays by Compton up-scattering of ambient low-energy photons. While the synchrotron morphology of the nebula is well established, it was up to now not known in which region the very high-energy gamma rays are emitted. Here we report that the Crab nebula has an angular extension at gamma-ray energies of 52 arcseconds (assuming a Gaussian source width), significantly larger than at X-ray energies. This result closes a gap in the multi-wavelength coverage of the nebula, revealing the emission region of the highest energy gamma rays. These gamma rays are a new probe of a previously inaccessible electron and positron energy range. We find that simulations of the electromagnetic emission reproduce our new measurement, providing a non-trivial test of our understanding of particle acceleration in the Crab nebula.
We observed the young pulsar J1357--6429 with the {it Chandra} and {it XMM-Newton} observatories. The pulsar spectrum fits well a combination of absorbed power-law model ($Gamma=1.7pm0.6$) and blackbody model ($kT=140^{+60}_{-40}$ eV, $Rsim2$ km at the distance of 2.5 kpc). Strong pulsations with pulsed fraction of $42%pm5%$, apparently associated with the thermal component, were detected in 0.3--1.1 keV. Surprisingly, pulsed fraction at higher energies, 1.1--10 keV, appears to be smaller, $23%pm4%$. The small emitting area of the thermal component either corresponds to a hotter fraction of the neutron star (NS) surface or indicates inapplicability of the simplistic blackbody description. The X-ray images also reveal a pulsar-wind nebula (PWN) with complex, asymmetric morphology comprised of a brighter, compact PWN surrounded by the fainter, much more extended PWN whose spectral slopes are $Gamma=1.3pm0.3$ and $Gamma=1.7pm0.2$, respectively. The extended PWN with the observed flux of $sim7.5times10^{-13}$ erg s$^{-1}$ cm$^{-2}$ is a factor of 10 more luminous then the compact PWN. The pulsar and its PWN are located close to the center of the extended TeV source HESS J1356--645, which strongly suggests that the VHE emission is powered by electrons injected by the pulsar long ago. The X-ray to TeV flux ratio, $sim0.1$, is similar to those of other relic PWNe. We found no other viable candidates to power the TeV source. A region of diffuse radio emission, offset from the pulsar toward the center of the TeV source, could be synchrotron emission from the same relic PWN rather than from the supernova remnant.
The Crab nebula pulsar was observed in 2009 January and December with a novel very fast optical photon counter, Iqueye, mounted at the ESO 3.5 m New Technology Telescope. Thanks to the exquisite quality of the Iqueye data, we computed accurate phase coherent timing solutions for the two observing runs and over the entire year 2009. Our statistical uncertainty on the determination of the phase of the main pulse and the rotational period of the pulsar for short (a few days) time intervals are $approx 1 , mu$s and ~0.5 ps, respectively. Comparison with the Jodrell Bank radio ephemerides shows that the optical pulse leads the radio one by ~240 $mu$s in January and ~160 $mu$s in December, in agreement with a number of other measurements performed after 1996. A third-order polynomial fit adequately describes the spin-down for the 2009 January plus December optical observations. The phase noise is consistent with being Gaussian distributed with a dispersion $sigma$ of $approx 15 , mu$s in most observations, in agreement with theoretical expectations for photon noise-induced phase variability.
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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