Chandra X‐Ray Observations of 19 Millisecond Pulsars in the Globular Cluster 47 Tucanae

Abstract

We present spectral and long-timescale variability analyses of Chandra X-Ray Observatory ACIS-S observations of the 19 millisecond pulsars (MSPs) with precisely known positions in the globular cluster 47 Tucanae. The X-ray emission of the majority of these MSPs is well described by a thermal (blackbody or neutron star hydrogen atmosphere) spectrum with a temperature T-eff similar to (1-3) x 10(6) K, emission radius R-eff similar to 0.1-3 km, and luminosity L-X similar to 10(30)-10(31) ergs s(-1). For several MSPs, there are indications that a second thermal component is required, similar to what is seen in some nearby field MSPs. The observed radiation most likely originates from the heated magnetic polar caps of the MSPs. The small apparent scatter in LX is consistent with thermal emission from the polar caps of a global dipole field, although the small emission areas may imply either a more complex small-scale magnetic field configuration near the neutron star surface or nonuniform polar cap heating. The radio eclipsing binary MSPs 47 Tuc J, O, and W show a significant nonthermal (power-law) component, with spectral photon index Gamma similar to 1-1.5, which most likely originates in an intrabinary shock formed due to interaction between the relativistic pulsar wind and matter from the stellar companion. We reexamine the X- ray-spin- down luminosity relation (L-X-E relation) and find that for the MSPs with thermal spectra L-X proportional to E-beta, where beta similar to 0.2 +/- 1.1. Due to the large uncertainties in both parameters, the result is consistent with both the linear L-X proportional to E-0.5. predicted by polar cap heating models. In terms of X-ray properties, we find no clear systematic differences between MSPs in globular clusters and in the field of the Galaxy. We discuss the implications of these results on the present understanding of the X-ray emission properties of MSPs.