A deep Chandra study confirms diffuse nonthermal X-ray emission from the globular cluster Terzan 5

Abstract

Context. Diffuse X-ray emission has been detected from a few Galactic globular clusters (GCs), but its nature 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 nonthermal component described by a power-law model with photon index Γ ~ 1.Aims. With over 16 times the accumulated Chandra exposure time compared to a prior study, we reexamined and verified the diffuse X-ray emission from the field of Terzan 5, which enabled us to place constraints on its nature.Methods. We analyzed all available useful Chandra observations of Terzan 5, including 18 observations over a span of 13 years, with a total exposure time of 641.6 ks. To study the diffuse X-ray emission, we focused on four annular regions with an equal width of 0.72 arcmin centered on Terzan 5 (0.72–3.60 arcmin), from which we extracted and analyzed the X-ray spectra after removing point sources and instrumental backgrounds.Results. We confirm a significant diffuse X-ray excess from the field of Terzan 5 in the band 0.8–3 keV. After constraining the contribution from the local X-ray background, we find a diffuse X-ray component that is genuinely associated with Terzan 5 and can be well described by a power-law model. More interestingly, the fitted photon indices show a significant increase from Γ = 1.96 ± 0.18 in the inner region to Γ = 3.48 ± 0.71 in the outer region. The diffuse X-rays are also well fit by a thermal bremsstrahlung model, with plasma temperatures declining from kT ∼ 3 keV to kT ∼ 1 keV.Conclusions. 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 that the sharp 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.