We report radio polarization observations of G319.9-0.7 (MSC 319.9-0.7) at 3 and 6 cm obtained with the Australia Telescope Compact Array. The source shows a highly elongated morphology with the energetic pulsar J1509-5850 located at the tip. We found a flat radio spectrum of index \alpha=-0.26 +/- 0.04 and a high degree of linear polarization. These results confirm G319.9-0.7 as a bow-shock pulsar wind nebula. The polarization maps suggest a helical magnetic field trailing the pulsar, with the symmetry axis parallel to the system's inferred direction of motion. This is the first time such a field geometry has been seen in a bow-shock nebula, and it may be the result of an alignment between the pulsar spin axis and its space velocity. Compared to other bow-shock examples, G319.9-0.7 exhibits very different properties in the field structure and surface brightness distribution, illustrating the large diversity of the population.
Deep Dive into Radio Polarization Observations of G319.9-0.7: A Bow-shock Nebula with an Azimuthal Magnetic Field Powered by Pulsar J1509-5850.
We report radio polarization observations of G319.9-0.7 (MSC 319.9-0.7) at 3 and 6 cm obtained with the Australia Telescope Compact Array. The source shows a highly elongated morphology with the energetic pulsar J1509-5850 located at the tip. We found a flat radio spectrum of index \alpha=-0.26 +/- 0.04 and a high degree of linear polarization. These results confirm G319.9-0.7 as a bow-shock pulsar wind nebula. The polarization maps suggest a helical magnetic field trailing the pulsar, with the symmetry axis parallel to the system’s inferred direction of motion. This is the first time such a field geometry has been seen in a bow-shock nebula, and it may be the result of an alignment between the pulsar spin axis and its space velocity. Compared to other bow-shock examples, G319.9-0.7 exhibits very different properties in the field structure and surface brightness distribution, illustrating the large diversity of the population.
Pulsars lose a significant fraction of their rotational energy through their relativistic winds. The consequent interactions with the ambient medium result in broadband synchrotron emission. These structures are collectively referred to as pulsar wind nebulae (PWNe). The properties of a PWN depend strongly on its evolutionary state and environment. Since a pulsar typically has a space velocity of a few hundred kilometers per second, it will eventually escape its natal supernova remnant (SNR) and travel supersonically in the interstellar medium (ISM). In such cases, the pulsar outflow can be confined by ram pressure, resulting in a bowshock nebula. Multiwavelength observations have identified several bow-shock PWN systems (see review by Gaensler & Slane 2006). The best-studied example is 'the Mouse' (G359.23-0.82), in which the X-ray and radio emissions can be well-modeled by a bright head coincident with the pulsar, a 'tongue' region corresponding to the wind termination shock, and an elongated tail associated with the post shock flow material (Gaensler et al. 2004).
Studies of the Mouse and other bow-shock PWNe have set the stage for subsequent theoretical and numerical modeling efforts. Romanova et al. (2005) considered an axisymmetric case of a pulsar traveling along its spin axis direction. They proposed an azimuthal Bfield geometry for bow-shocks, and derived the analytical shape of a pulsar tail. With a similar aligned conncy@physics.usyd.edu.au 1 Current address: Department of Physics, McGill University, Montreal, QC, Canada H3A 2T8
2 ARC Federation Fellow 3 Current address: Department of Astronomy, Cornell University, Ithaca, NY 14853 figuration, Bucciantini et al. (2005) carried out the first relativistic magnetohydrydynamic (MHD) simulations of bow-shocks to study the effects of different wind magnetization. Vigelius et al. (2007) relaxed the assumption of alignment and presented three-dimensional nonrelativistic hydrodynamic simulations to illustrate the dependence of bow-shock morphology on pulsar orientation and ISM gradient.
Observationally, PWNe at radio frequencies are characterized by flat spectra with spectral index4 α ≃ -0.3 to 0.0 and high ( 10%) degrees of linear polarization (Kaspi et al. 2006). Due to the long synchrotron cooling time of the radio-emitting particles, a radio PWN can act as a direct calorimeter to reflect the system’s integrated history over a long period of time. More importantly, radio polarimetry provides a powerful probe of the magnetic field geometry of a PWN, which is relatively less studied. High resolution polarization measurements have only been carried out on a handful of bow-shock systems, e.g. IC 443 and the Mouse (Olbert et al. 2001;Yusef-Zadeh & Gaensler 2005). In particular, VLA observations of the Mouse revealed a highly ordered field wrapping around the bow shock at the apex and extending parallel to the nebular axis downstream (Yusef-Zadeh & Gaensler 2005), in contrast to the theoretical prediction by Romanova et al. (2005).
To generalize our understanding of PWNe, we need to study more examples. In this paper, we report radio observations of a bow-shock nebula powered by the energetic pulsar J1509-5850. This pulsar was discovered in the Parkes Multibeam Pulsar Survey (Kramer et al. 2003). It has a spin period P = 88.9 ms, a high spindown luminosity Ė = 5.1 × 10 35 ergs s -1 and a relatively young characteristic age τ c ≡ P/(2 Ṗ ) = 1.5 × 10 5 yr. As with other energetic pulsars, it has recently been detected in γ-rays by the Fermi Gamma-ray Space Telescope (Weltevrede 2010). The pulsar dispersion measure (DM) of 137.7 pc cm -3 suggests a distance ranging from 2.6 to 3.8 kpc according to different Galactic free electron models (Taylor & Cordes 1993;Cordes & Lazio 2002). Throughout this work, we will adopt a pulsar distance d = 3 d 3 kpc. Some ∼ 4 ′ southwest of the pulsar, there is an elongated radio source MSC 319.9-0.7 (hereafter G319.9-0.7) which was identified by Whiteoak & Green (1996) as a candidate SNR based on observations with the Molonglo Observatory Synthesis Telescope (MOST). The radio source has a size of 7 ′ × 2 ′ , and consists of a bright central bulge and a clump in the south. Hui & Becker (2007) reported an observation of the field of PSR J1509-5850 with the Chandra X-ray Observatory. The X-ray image revealed a tail-like PWN extending from the pulsar in the same orientation as G319.9-0.7, suggesting that it is a bow-shock nebula. At 3 kpc, the tail has a physical scale ∼ 5 pc, one of the longest X-ray tails ever observed. The PWN has an Xray luminosity L X ≈ 10 33 ergs s -1 between 0.5 and 8 keV, corresponding to a high efficiency η ≡ L X / Ė ≈ 2 × 10 -3 . Hui & Becker (2007) argued that G319.9-0.7 is too small to be a SNR associated with the pulsar, and concluded that it is most likely a background object. On the other hand, Kargaltsev et al. (2008, hereafter K08) presented a detailed study using the same Chandra data set, and s
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