The interaction of the pulsar wind with the surrounding environment gives rise to a pulsar wind nebula (PWN) (Gaensler & Slane 2006; Bucciantini 2008; Olmi et al. 2016). A bubble of relativistic pairs and magnetic field, that shines with a non-thermal broad-band spectrum, extending from radio to X-rays and γ-rays, via synchrotron and inverse Compton emission. The Crab nebula is the prototype of PWNe inside the parent supernova remnant (SNR) (Hester 2008). Given that the typical pulsar speed at birth, the so called kick velocity, is usually of the order of 100–400 km s−1 (Cordes & Chernoff 1998; Arzoumanian, Chernoff & Cordes 2002; Sartore et al. 2010; Verbunt, Igoshev & Cator 2017), much smaller than the expansion speed of young SNRs [typically a few thousands km s−1 (Truelove & McKee 1999; Hughes 1999, 2000; DeLaney & Rudnick 2003; Borkowski et al. 2013; Tsuji & Uchiyama 2016)], for the first few thousands of years, the PWN is going to remain confined within the parent SNR (van der Swaluw et al. 2003; van der Swaluw, Downes & Keegan 2004; Temim et al. 2015). However the SNR expansion speed will drop in time as the expansion proceeds (Cioffi, McKee & Bertschinger 1988; Leahy, Green & Tian 2014; Sánchez-Cruces et al. 2018), sweeping up more and more interstellar medium (ISM) material, such that on a typical time-scale of a few tens of thousands of years, the pulsar can escape from the SNR, and begin to interact directly with the ISM. Given the typical sound speed of the ISM, the pulsar motion is highly supersonic, and the ram pressure balance between the pulsar wind and the ISM flow gives rise to a cometary-like nebula (Wilkin 1996) known as bow-shock pulsar wind nebula (BSPWN), of shocked pulsar and ISM material (Bucciantini & Bandiera 2001; Bucciantini 2002b). If the ISM is cold and its neutral fraction is high, these nebulae can be observed in H α emission (Kulkarni & Hester 1988; Cordes, Romani & Lundgren 1993; Bell et al. 1995; van Kerkwijk & Kulkarni 2001; Jones, Stappers & Gaensler 2002; Brownsberger & Romani 2014; Romani, Slane & Green 2017), due to charge exchange and collisional excitation, taking place in the shocked ISM downstream of the forward bow shock (Chevalier, Kirshner & Raymond 1980; Hester, Raymond & Blair 1994; Bucciantini & Bandiera 2001; Ghavamian et al. 2001), or alternatively in the UV (Rangelov et al. 2016), and IR (Wang et al. 2013). On the other hand the shocked pulsar material is expected to emit non-thermal synchrotron radiation, and indeed many such systems have been identified in recent years either in radio or in X-rays (Arzoumanian et al. 2004; Gaensler et al. 2004; Chatterjee et al. 2005; Gaensler 2005; Li, Lu & Li 2005; Yusef-Zadeh & Gaensler 2005; Kargaltsev et al. 2008, 2017; Misanovic, Pavlov & Garmire 2008; Hales et al. 2009; Ng et al. 2009; Ng et al. 2010; De Luca et al. 2011; Ng et al. 2012; Marelli et al. 2013; Jakobsen et al. 2014; Klingler et al. 2016; Posselt et al. 2017). There is also an interesting dichotomy between H α emitting systems and synchrotron emitting ones, suggesting that energetic pulsars, more likely to drive bright synchrotron nebulae, might pre-ionize the incoming ISM suppressing line emission from neutral hydrogen.
Citation: Sánchez-Cruces et al. 2018
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