We present a Chandra X-ray Observatory investigation of the millisecond pulsars ( MSPs ) in the globular cluster M28 ( NGC 6626 ) .
In what is one of the deepest X-ray observations of a globular cluster , we firmly detect seven and possibly detect two of the twelve known M28 pulsars .
With the exception of PSRs B1821–24 and J1824–2452H , the detected pulsars have relatively soft spectra , with X-ray luminosities 10 ^ { 30 - 31 } ergs s ^ { -1 } ( 0.3–8 keV ) , similar to most “ recycled ” pulsars in 47 Tucanae and the field of the Galaxy , implying thermal emission from the pulsar magnetic polar caps .
We present the most detailed X-ray spectrum to date of the energetic PSR B1821–24 .
It is well described by a purely non-thermal spectrum with spectral photon index \Gamma = 1.23 and luminosity 1.4 \times 10 ^ { 33 } \Theta ( D / 5.5 ~ { } { kpc } ) ^ { 2 } ergs s ^ { -1 } ( 0.3–8 keV ) , where \Theta is the fraction of the sky covered by the X-ray emission beam ( s ) .
We find no evidence for the previously reported line emission feature around 3.3 keV , most likely as a consequence of improvements in instrument calibration .
The X-ray spectrum and pulse profile of PSR B1821–24 suggest that the bulk of unpulsed emission from this pulsar is not of thermal origin , and is likely due to low-level non-thermal magnetospheric radiation , an unresolved pulsar wind nebula , and/or small-angle scattering of the pulsed X-rays by interstellar dust grains .
The peculiar binary PSR J1824-2452H shows a relatively hard X-ray spectrum and possible variability at the binary period , indicative of an intrabinary shock formed by interaction between the relativistic pulsar wind and matter from its non-degenerate companion star .