NuSTARSTUDY OF HARD X-RAY MORPHOLOGY AND SPECTROSCOPY OF PWN G21.5–0.9

Abstract

We present NuSTAR high-energy X-ray observations of the pulsar wind nebula (PWN)/supernova remnant G21.5−0.9. We detect integrated emission from the nebula up to ∼40 keV, and resolve individual spatial features over a broad X-ray band for the first time. The morphology seen by NuSTAR agrees well with that seen by XMM-Newton and Chandra below 10 keV. At high energies, NuSTAR clearly detects non-thermal emission up to ∼20 keV that extends along the eastern and northern rim of the supernova shell. The broadband images clearly demonstrate that X-ray emission from the North Spur and Eastern Limb results predominantly from non-thermal processes. We detect a break in the spatially integrated X-ray spectrum at ∼9 keV that cannot be reproduced by current spectral energy distribution models, implying either a more complex electron injection spectrum or an additional process such as diffusion compared to what has been considered in previous work. We use spatially resolved maps to derive an energy-dependent cooling length scale, L(E) ∝ E m with m = −0.21 ± 0.01. We find this to be inconsistent with the model for the morphological evolution with energy described by Kennel & Coroniti. This value, along with the observed steepening in power-law index between radio and X-ray, can be quantitatively explained as an energy-loss spectral break in the simple scaling model of Reynolds, assuming particle advection dominates over diffusion. This interpretation requires a substantial departure from spherical magnetohydrodynamic, magnetic-flux-conserving outflow, most plausibly in the form of turbulent magnetic-field amplification.

NuSTAR Observations of G11.2–0.3

“…The spatially integrated spectrum of the pulsar-wind nebula at energies above the Chandra band is well described by a power-law with Γ = 1.71 ± 0.07, consistent with the value of 1.78 ± 0.7 reported by Borkowski et al (2016). That study found no significant steepening in spectrum with distance from the pulsar as is seen in other PWNe such as G21.5−0.9 (Nynka et al 2014); while the nominal best-fit values of Γ did increase, the magnitude of the change was within errors. However, here we find that the PWN extent along the jet direction shrinks with increasing energy, with HWHM ∝ E −γ with γ jet = 0.9 ± 0.3 along the jet axis and γ torus = 0.5 ± 0.4 perpendicular to that direction.…”

“…Most PWNe show, instead, gradual steepening of the spectrum, indicating a mixture of particles of different ages at a given distance from the pulsar, such as might be produced by diffusion (e.g., Reynolds & Jones 1991;Tang & Chevalier 2012) or more complex advective motions (Porth et al 2013). The magnitude of the energy-dependent shrinkage exponent, γ, of about 0.9 is distinct from those measured by NuSTAR in G21.5−0.9 (Nynka et al 2014) and MSH 15−52 , where values of γ of about 0.2 were found. For the Crab, Madsen et al (2015) reported differing shrinkage rates along the jet, counterjet, and transverse (torus) directions of about 0.05, 0.2, and 0.08, respectively.…”

“…There are strong indications, though, that this is not the case. A ∼ 9 keV break was detected in G21.5-0.9 with NuSTAR (Nynka et al 2014), and breaks across the Crab PWN were seen between 8 -12 keV also exclusively with NuS-TAR data (Madsen et al 2015). In MSH 15-52 a break was detected around 6.3 keV , and like for G11.2-0.3, this one was only found when combined with Chandra data.…”

Diffusion in pulsar wind nebulae: an investigation using magnetohydrodynamic and particle transport models

“…The observations therefore represent cumulative data. A 3−45 keV observation for the region r ≤ 30 has also recently been reported by Nynka et al (2014). The authors found statistically significant evidence for a spectral break at ∼ 9 keV, deriving Γ = 1.852 ± 0.0011 for the 3 − 9 keV energy range, and Γ = 2.099 +0.019 −0.017 for the 9 − 45keV energy range.…”

Multiband nonthermal radiative properties of pulsar wind nebulae

“…Here the distance of 4.1 kpc ) is used. Observationally, PWN G21.5-0.9 is detected in radio Wilson & Weiler 1976;Becker & Kundu 1976), IR (Gallant & Tuffs 1998, 1999, X-ray (De Rosa et al 2009;Tsujimoto et al 2011;Nynka et al 2014), and TeV γ-ray bands by HESS . Although Wang et al (1986) suggested that the age of PWN G21.5-0.9 is about 2000 yr because it might be the historical supernova of radius of the PWN is about 3 pc (Bhalerao et al 2015).…”