Characterization of the Inner Knot of the Crab: The Site of the Gamma-Ray Flares?

Abstract

One of the most intriguing results from the γ-ray instruments currently in orbit has been the detection of powerful flares from the Crab Nebula. Such events, with a cadence of about one per year, can be so dramatic as to make the system the brightest source in the γ-ray sky as occurred in 2011 April. These flares challenge our understanding of pulsar wind nebulae and models for particle acceleration. To pinpoint the production site(s) within the Nebula, a multiwavelength campaign has been carried out using Keck, Hubble Space Telescope (HST), and Chandra X-ray Observatory. As the short time scales of the flares ( 1 day) suggest a small emitting region, the Crab's inner knot, located within a fraction of an arcsecond from the pulsar, has been proposed to be just such a site. In this paper, we focus on IR, optical, and X-ray observations of this feature to see if it might be related to the γ-ray flares and to try to understand its nature. We find that the knot's radial size, tangential size, peak flux, and the ratio of the knot flux to that of the pulsar as measured with HST are all correlated with the projected distance of the knot from the pulsar. A new approach, using singular value decomposition for analyzing time series of images, was introduced yielding results consistent with the more traditional methods while some uncertainties were substantially reduced.We exploit the new characterization of the Crab's inner knot to discuss constraints on standard shock-model parameters that may be inferred from our observations assuming the inner knot lies near to the shocked surface. These include inferences as to wind magnetization σ, shock shape parameters such as incident angle δ 1 and poloidal radius of curvature R c , as well as the IR/optical emitting particle enthalpy fraction. We find that while the standard shock model gives good agreement with observation in many respects, there remain two puzzles: (a) the observed angular size of the knot relative to the pulsar-knot separation is measured to be much smaller than expected; (b) the variable, yet high degree of polarization reported is difficult to reconcile with a highly relativistic downstream flow. In contrast, the IR/optical flux of the inner knot is marginally consistent with the scenario that the shock accelerates most of the optical emitting particles in the nebula.Subject headings: neutron stars: general -pulsars: individual -Crab nebula and pulsar (PSR B0531+21) -inner knot γ-ray flares.

Particle acceleration in explosive relativistic reconnection events and Crab Nebula gamma-ray flares

“…As we have discussed, for the parameters of the Crab wind this dissipation leads only to plasma heating (Sec.2.2) and the downstream magnetization is too low to make gamma-ray flares via reconnection of the residual magnetic field. The observations of the inner knot of the Crab nebula, which is best explained as a Doppler-boosted emission of the shocked striped wind (Lyutikov et al 2016), agree with this verdict -its optical and IR emission does not correlate with the gamma-ray flares (Rudy et al 2015).…”

“…This knot is resolved and has the size similar to that of the X-ray knots. The knot is not seen in the radio and X-ray bands (Rudy et al 2015). In addition to these features, there are also fine bright arc-like filaments ("wisps") originating in the vicinity of the inner ring and moving away from it with speeds up to 0.7 c..…”

“…Elsewhere, a relativistic Maxwellian spectrum is expected instead (Sironi & Spitkovsky 2011). This may explain why inner knot is not visible both in the radio and X-ray bands (Weisskopf et al 2013;Rudy et al 2015).…”

Similarity and Difference in Time Structures of Gamma-Ray Flares in the Crab Nebula at Drastically Differing Energies

“…Since then, both the telescopes continue recording such flares approximately once in a year, the strongest of them (super-flare) having been detected in April 2011 [3], [4]. The energy spectra of additional emission during the flares were measured to be different from those of the Nebula, however, "the mechanism driving the flares, their impulsive nature, the 12-month recurrence time, and the location, remain unknown" [5]. For all that, based on multi-wavelength campaign to study the Crab using Keck, Hubble Space Telescope (HST), and Chandra X-ray Observatory, the authors of [5] suggested the so-called "inner knot" to be just the emitting region for the flares.…”

“…The energy spectra of additional emission during the flares were measured to be different from those of the Nebula, however, "the mechanism driving the flares, their impulsive nature, the 12-month recurrence time, and the location, remain unknown" [5]. For all that, based on multi-wavelength campaign to study the Crab using Keck, Hubble Space Telescope (HST), and Chandra X-ray Observatory, the authors of [5] suggested the so-called "inner knot" to be just the emitting region for the flares. In addition, the analysis made by the AGILE collaboration for the September-October 2007 event has found a fine structure in the flare's time behavior [6].…”