Variability of 19 Millisecond Pulsars in 47 Tucanae with Chandra HRC-S

Abstract

We present results from our 830 ks observation of the globular cluster 47 Tucanae with the Chandra X-ray Observatory's High Resolution Camera-S. We limit our analysis here to the 19 previously known, localized millisecond pulsars (MSPs) in the cluster. This work more than doubles the sample of X-ray-detected MSPs observed with sensitivity to rotational variability; it is also the first survey of a large group of radio-discovered MSPs for which no previous X-ray pulsations have been detected and is therefore an unbiased survey of the X-ray properties of radio-discovered MSPs. We find that only 47 Tuc D, O, and R show significant pulsations at the ≳ 4 σ level, but there is statistical evidence for rotational variability in five additional MSPs. Furthermore, we constrain the pulsed magnetospheric emission of seven more MSPs using Monte Carlo simulations. The result is that the majority of the 47 Tuc MSPs are characterized by low pulsed fractions, ≾ 50%. In cases where larger pulsed fractions are measured, the folded pulse profiles show relatively large duty cycles. When considered with previous spectroscopic studies, this suggests that the X-ray emission arises from the neutron star's heated polar caps and, in some cases, from intrabinary shocks, but generally not directly from the star's magnetosphere. We discuss the impact of these results on our understanding of high-energy emission from MSPs.

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