X-ray observations of parsec-scale tails behind two middle-aged pulsars

Chandra and XMM-Newton resolved extremely long tails behind two middle-aged pulsars, J1509-5850 and J1740+1000. The tail of PSR J1509-5850 is discernible up to 5.6′ from the pulsar, which corresponds to the projected length l⊥ = 6.5d₄ pc, where d = 4d₄ kpc is the distance to the pulsar. The observed tail flux is 2 × 10^-13 ergs s ^-1 cm^-2 in the 0.5-8 keV band. The tail spectrum fits an absorbed power law (PL) with the photon index Γ = 2.3 ± 0.2 and 0.5-8 keV luminosity of 1 × 10³³ d₄² ergs s^_1, for n_H = 2.1 × 10²² cm ^-2. The tail of PSR J1740+1000 is firmly detected up to 5′ (l _ ∼ 2d_1.4 pc),with a flux of 6 × 10^-4 ergs cm^-2 s^-1 in the 0.4-10 keV band.The PLfit yields Γ = 1.4-1.5, n_H 1 × 10²¹ cm^-2, and an 0.4-10 keV luminosity of ∼2 × 10³¹ d_1.4² ergs s^_1. The large extent of the tails suggests that the bulk flow in the tails starts as mildly relativistic downstream of the termination shock and then gradually decelerates. Within the observed extent of the J1509-5850 tail, the average flow speed exceeds 5000 km s^_1, and the equipartition magnetic field is a few x 10^-5 G. For the J1740+1000 tail, the equipartition field is a factor of a few lower. For the high-latitude PSR J1740+1000, the orientation of the tail suggests that the pulsar was born from a halo-star progenitor. The X-ray efficiencies of the ram pressure-confined pulsar wind nebulae (PWNe) correlate poorly with the pulsar spin-down luminosities or ages. The efficiencies are systematically higher than those of PWNe around slowly moving pulsars with similar spin-down parameters.