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 Teff ~ (1-3) × 106 K, emission radius Reff ~ 0.1-3 km, and luminosity LX ~ 1030-1031 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 Γ ~ 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 (LX-Ė relation) and find that for the MSPs with thermal spectra LX ∝ Ėβ, where β ~ 0.2 ± 1.1. Due to the large uncertainties in both parameters, the result is consistent with both the linear LX-Ė relation and the flatter LX ∝ Ė0.5 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.
Highlights
Millisecond pulsars (MSPs) represent a distinct population of rotation-powered pulsars, characterized by short spin periods, P P 25 ms, and small intrinsic spin-down rates, Pi $ 10À20, implying relatively low surface magnetic dipole field strengths Bsurf / (PPi)1=2 $ 108Y1010 G and large characteristic spin-down ages P/2Pi k 1 Gyr
Since the polar cap heating rate has a weaker dependence on the spin-down luminosity (Lh / E 1=2) than magnetospheric emission (Lm / E ), the thermal emission will eventually become dominant over the magnetospheric emission, as is observed in the majority of 47 Tuc MSPs and many field MSPs
We have presented a spectral analysis of the X-ray emission for the complete sample of known MSPs in 47 Tuc
Summary
Millisecond pulsars (MSPs) represent a distinct population of rotation-powered pulsars, characterized by short spin periods, P P 25 ms, and small intrinsic spin-down rates, Pi $ 10À20, implying relatively low surface magnetic dipole field strengths Bsurf / (PPi)1=2 $ 108Y1010 G and large characteristic spin-down ages P/2Pi k 1 Gyr. Fig. 1.—Merged 0.3Y6 keV image of all four Chandra ACIS-S observations of 47 Tucanae showing the positions of the X-ray counterparts of 19 millisecond pulsars. Grindlay et al (2002) have reported in detail on the analysis of the X-ray emission properties of the 47 Tuc MSPs, using the Chandra ACIS-I observations mentioned above. Additional X-ray observations of 47 Tuc have been carried out, allowing further study of the properties of globular cluster MSPs as well as MSPs in general. We present spectral and long-timescale variability analyses of the X-ray counterparts of MSPs in the globular cluster 47 Tuc based on deep Chandra ACIS-S observations. In x 6 we discuss the implications of the results presented, and we offer conclusions in x 7
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