Discovery of Powerful Gamma-Ray Flares from the Crab Nebula

Abstract

The well-known Crab Nebula is at the center of the SN1054 supernova remnant. It consists of a rotationally powered pulsar interacting with a surrounding nebula through a relativistic particle wind. The emissions originating from the pulsar and nebula have been considered to be essentially stable. Here, we report the detection of strong gamma-ray (100 mega-electron volts to 10 giga-electron volts) flares observed by the AGILE satellite in September 2010 and October 2007. In both cases, the total gamma-ray flux increased by a factor of three compared with the non-flaring flux. The flare luminosity and short time scale favor an origin near the pulsar, and we discuss Chandra Observatory x-ray and Hubble Space Telescope optical follow-up observations of the nebula. Our observations challenge standard models of nebular emission and require power-law acceleration by shock-driven plasma wave turbulence within an approximately 1-day time scale.

Pulsar-Wind Nebulae

“…instabilities in the Anvil at the South-East jet base. Variability in this region was detected by optical and X-ray observations both during the September 2010 and April 2011 gamma-ray flares (Tavani et al 2011;. MHD simulations of the Crab South-East jet (Mignone et al 2013) revealed substantial jet deviation and magnetic dissipation.…”

“…The prominent south-eastern (SE) jet (see Figure 1) also shows quite remarkable changes in its shape, based on Xray images (taken over 14-year baseline), which could be explained by either precession of the curved jet or by the motion of kinks along the jet (Weisskopf 2012;Deshpande and Radhakrishnan 2007). Finally, the unexpected detection of γ-ray flares by Fermi LAT and AGILE (Buehler et al 2012;Abdo et al 2011b;Tavani et al 2011) suggests a significant energy release rate (which can reach 4 × 10 36 erg s −1 ; Buehler et al 2012) on timescales of hours ); however, it was not possible so far to pinpoint the location of the flaring region because of the lack of a "smoking gun" at lower frequencies. Consequently, the lack of information about the site of the flare in the PWN has led to a variety of models being suggested (see § 3).…”

“…This region is not quite well resolved in simulations; they just show rapid disruption of the flow by the kink instability. On the other hand, the highly magnetized region is of special interest because the recently discovered strong, short gamma-ray flares from the Crab nebula (Abdo et al 2011b;Tavani et al 2011) are generally attributed to a rapid magnetic energy release via, e.g., reconnection (see Sect. 3), which assumes a magnetically dominated region within the nebula. Lyubarsky (2012) developed a simple model clarifying the structure of the high latitude flow.…”

“…The rapid variability is now a well-established intrinsic property of the Crab Nebula in the GeV gamma-ray band 18 (Tavani et al 2011;Abdo et al 2011b). The flares were not predicted by the models and they generally do not fit in the framework of the classical theory of pulsar wind nebulae and particle acceleration.…”

Relativistic Magnetic Reconnection in Pair Plasmas and Its Astrophysical Applications

“…The surprising discovery of short-lived, bright gamma-ray flares from the Crab Nebula (Tavani et al 2011;Abdo et al 2011) could be the direct evidence of magnetic reconnection in the Nebula Clausen-Brown and Lyutikov 2012;Cerutti et al 2014a). Using 2D and 3D PIC simulations, Cerutti et al (2013Cerutti et al ( , 2014b showed that most of the features of the flares can be explained with relativistic reconnection (timescale, energetics, particle and photon spectra).…”