The emerging population of pulsar wind nebulae in hard X-rays
📝 Abstract
The hard X-ray synchrotron emission from pulsar wind nebulae (PWNe) probes energetic particles, closely related to the pulsar injection power at the present time. INTEGRAL has disclosed the yet poorly known population of hard X-ray pulsar/PWN systems. We summarize the properties of the class, with emphasys on the first hard X-ray bow-shock (CTB 80 powered by PSR B1951+32), and highlight some prospects for the study of Pulsar Wind Nebulae with the Simbol-X mission.
💡 Analysis
The hard X-ray synchrotron emission from pulsar wind nebulae (PWNe) probes energetic particles, closely related to the pulsar injection power at the present time. INTEGRAL has disclosed the yet poorly known population of hard X-ray pulsar/PWN systems. We summarize the properties of the class, with emphasys on the first hard X-ray bow-shock (CTB 80 powered by PSR B1951+32), and highlight some prospects for the study of Pulsar Wind Nebulae with the Simbol-X mission.
📄 Content
The detection of high-energy radiation from rotation powered pulsars shows that particle acceleration takes place close to the pulsars’ magnetospheres as well as in their surroundings. The particle stream initiated by the electrostatic magnetospheric gaps constitutes a magnetized wind, which flows relativistically until it experiences the confinement by the surrounding medium, be it the remnant of the supernova explosion or the interstellar gas. In this interaction a strong shock is produced, which terminates the wind bulk motion and re-accelerates the electrons. The presence of the pulsar wind is so revealed in a Pulsar Wind Nebula (PWN), which shines along all the electromagnetic spectrum: the synchrotron process yields photons from the radio band up to the energies of several MeV, while the inverse Compton scattering of the ambient radiation field yields higher energy photons, up to tens of TeV [1].
As the nebular radiation arises from the integrated history of particle injection by the pulsar convolved with the interaction with the environment, PWNe store, and slowly release, most of the pulsar rotational kinetic power, the so-called spin-down luminosity Ė. However, the synchrotron and inverse Compton radiations can trace particles of different energies and ages [2], hence the combination of X-and γ-ray measurements may allow to disentangle the contribution of the pulsar from the one of its environment. The high energy tail of the synchrotron emission has a special role in this respect, as it is the most closely related to the pulsar: it probes the most energetic particles, up to PeV energies, with short lifetime (for instance, the cooling time for electrons radiating 50 keV photons is ∼ 50 yr -1 kyr). At such energies, deviations from pure power law spectra might be induced by the evolution of the spin-down luminosity, by the radiative cooling, or by the attainment of the maximum acceleration energy.
Although ∼50 PWNe are observed in radio waves, soft X-rays (around 0.5-10 keV), and very high energy (VHE, E > 0.1 TeV) γ-rays, only a few are detected in the hard X-ray band (E 20 keV). The sample of hard X-ray PWNe is increasing thanks to the unprecedented sensitivity and imaging capability (6 ′ angular resolution, HWHM) of the IBIS/ISGRI camera [3] on board the INTEGRAL satellite, which has performed a survey of the Galactic disk from 20 keV up to several hundreds of keV. Taking advantage of the long exposure time accumulated since the launch of the mission (up to 16 Msec in the Galactic Center), the IBIS/ISGRI survey has revealed a dozen of young pulsar/PWN complexes among a few hundreds of compact binary systems and unidentified objects.
Here we summarize their properties, and outline the opportunities for the study of PWNe with the Simbol-X observatory.
Table 1 lists the pulsars/PWNe detected by IBIS/ISGRI. These sources are powered by very young (Crab-like) or youngish (Vela-like) pulsars. For most of them, the hard X-ray flux is dominated by the nebular emission, with the exceptions of PSR B1509-58, PSR J1617-5055, and PSR J1838-0655, whose flux is dominated by the pulsed magnetospheric emission. Two hard and stable unidentified INTEGRAL sources also proved to be linked to young pulsars and PWNe: IGR J18135-1751 [4] and IGR J18490-0000 [5]. Their identification is supported by the detection of a TeV counterpart, like the large majority of identified pulsar/PWN systems. The luminosity in the 20-40 keV energy band of the sources in Table 1 is found to increase with the spin-down luminosities Ė (Figure 1, left panel). This L X -Ė scaling relation is similar to the one known in the soft X-ray band for pulsars as well as for PWNe [e.g., 7,8]. The fact that also the hard X-ray luminosities scale with Ė is not surprising, provided that the spectra in the 20-40 keV energy band smoothly extrapolate the soft X-ray spectra. We also considered the 3σ flux upper-limit at the position of other 1). Dashed line: unweighted least square fit.Two relations for the soft X-ray band are shown for comparison: the relation by [7] rescaled in the 20-40 energy band (dotted line), and the simple scaling L X = 10 -3 Ė by [6] (dot-dashed line). Right panel: IBIS/ISGRI spectrum of CTB 80/PSR B1951+32 [10]. The source is significant up to 70 keV. The Chandra-XRO spectra [9] of the pulsar and of the PWN are also shown. 19 pulsars powering a soft X-ray PWN (assuming a Crab-like spectrum). Some of the upper-limits are well below the unweighted least square fit relation (log 10 L 20-40 keV = 34.5 + 1.24 log 10 Ė37 , where Ė = Ė37 × 10 37 erg s -1 ), but still consistent with the scatter between the measured fluxes and the best fit relation, which can be larger than one order of magnitude. For all the detected pulsars/PWNe Ė > 10 36 erg s -1 ; this apparent spindown threshold can be due to lack of sensitivity.
The hint that also more evolved pulsar/PWN systems may be detected is supported by the recent discovery of CTB 80 power
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