SEARCHING FOR THE PULSAR IN G18.95-1.1: DISCOVERY OF AN X-RAY POINT SOURCE AND ASSOCIATED SYNCHROTRON NEBULA WITH CHANDRA

摘要

Using the Chandra X-ray Observatory, we have pinpointed the location of a faint X-ray point source (CXOU?J182913.1-125113) and an associated diffuse nebula in the composite supernova remnant (SNR) G18.95-1.1. These objects appear to be the long-sought pulsar and its wind nebula. The X-ray spectrum of the point source is best described by an absorbed power-law model with Γ = 1.6 and an NH of ~1 × 1022?cm–2. This model predicts a relatively low unabsorbed X-ray luminosity of about LX (0.5-8.0 keV) 4.1 × 1031?D 2 2?erg s–1, where D 2 is the distance in units of 2?kpc. The best-fit model of the diffuse nebula is a combination of thermal (kT = 0.48?keV) and non-thermal (1.4 ≤ Γ ≤ 1.9) emission. The unabsorbed X-ray luminosity of LX 5.4 × 1033?D 2 2?erg?s–1 in the 0.5-8?keV energy band seems to be largely dominated by the thermal component from the SNR, providing 87% of LX in this band. No radio or X-ray pulsations have been reported for CXOU?J182913.1-125113. If we assume an age of ~5300?yr for G18.95-1.1 and use the X-ray luminosity for the pulsar and the wind nebula together with the relationship between spin-down luminosity (via magnetic dipole radiation) and period, we estimate the pulsar's period to be P 0.4?s. Compared to other rotation-powered pulsars, a magnetic field of 2.2 × 1013?G is implied by its location in the P- diagram, a value which is close to that of the quantum critical field.