A deep Chandra study verifies diffuse non-thermal X-ray emission from the globular cluster Terzan 5

Abstract

Diffuse X-ray emission has been detected from a few Galactic globular clusters (GCs), whereas its nature still remains largely unclear. The GC Terzan 5 was previously found to show a significant diffuse thermal X-ray excess from its field, likely contributed by the Galactic background, and a non-thermal component described by a power-law model with photon index $\Gamma \sim 1$. With over 16 times the accumulated Chandra exposure time as in the prior study, we are motivated to reexamine and verify the diffuse X-ray emission from the field of Terzan 5, enabling constraints on its nature. We verify a significant diffuse X-ray excess from the field of Terzan 5 in the band 0.8--3 keV. After constraining the contribution from local X-ray background, we find a diffuse X-ray component that is genuinely associated with Terzan 5, which can be well described by a power-law model. More interestingly, the fitted photon indices show a significant increase from $\Gamma = 1.96 \pm 0.18$ in the inner region to $\Gamma = 3.48 \pm 0.71$ in the outer region. The diffuse X-rays can also be well fitted by a thermal bremsstrahlung model, with plasma temperatures declining from $kT \sim 3$ keV to $kT \sim 1$ keV. We suggest that synchrotron radiation from the combined pulsar winds of Terzan 5's millisecond pulsar population is a possible origin of the observed diffuse X-ray emission, but the the large steepening in the spectra cannot be produced solely by synchrotron cooling. Other radiation processes, like thermal bremsstrahlung, may also contribute to the diffuse X-rays.