Chandra detection of diffuse X-ray emission from the globular cluster Terzan 5

Terzan 5, a globular cluster (GC) prominent in mass and population of compact objects, is searched for diffuse X-ray emission, as proposed by several models. We analyzed the data of an archival Chandra observation of Terzan 5 to search for extended diffuse X-ray emission outside the half-mass radius of the GC. We removed detected point sources from the data to extract spectra from diffuse regions around Terzan 5. The Galactic background emission was modeled by a 2-temperature thermal component, which is typical for Galactic diffuse emission. We detected significant diffuse excess emission above the particle background level from the whole field-of-view. The surface brightness appears to be peaked at the GC center and decreases smoothly outwards. After the subtraction of particle and Galactic background, the excess spectrum of the diffuse emission between the half-mass radius and 3’ can be described by a power-law model with photon index $\Gamma$ = 0.9$\pm$0.5 and a surface flux of F$_X$ = (1.17$\pm$0.16) 10$^{-7}$ erg s$^{-1}$ cm$^{-2}$ sr$^{-1}$ in the 1–7 keV band. We estimated the contribution from unresolved point sources to the observed excess to be negligible. The observations suggest that a purely thermal origin of the emission is less likely than a non-thermal scenario. However, from simple modeling we cannot identify a clearly preferred scenario.

💡 Research Summary

Terzan 5 is one of the most massive and densely populated globular clusters in the Milky Way, hosting a large number of compact objects such as millisecond pulsars and low‑mass X‑ray binaries. Several theoretical models predict that such environments should produce diffuse X‑ray emission, either through thermal plasma heated by stellar winds or through non‑thermal processes involving relativistic particles. To test these predictions, the authors re‑examined an archival Chandra ACIS‑I observation of Terzan 5 (≈ 39 ks exposure) with a focus on regions outside the half‑mass radius (r_h ≈ 0.5′).

The data reduction followed standard CIAO procedures. All point sources detected within the central 0.5′ were identified with wavdetect and excised using 2″ radius masks, ensuring that the remaining emission is not contaminated by resolved sources. The authors then defined a series of concentric annuli extending to 3′ from the cluster centre and extracted spectra from each annulus. Background modeling was performed in two steps. First, the instrumental particle background was estimated from the ACIS “stowed” dataset and scaled to match the observation’s particle count rate. Second, the Galactic diffuse X‑ray background was modeled with a two‑temperature thermal plasma (kT₁ ≈ 0.2 keV, kT₂ ≈ 0.7 keV) absorbed by N_H ≈ 1.5 × 10²² cm⁻², a configuration that reproduces the typical soft X‑ray emission seen along the Galactic plane.

After subtracting both the particle and Galactic components, a statistically significant excess remained across the whole field of view. The surface brightness peaks at the cluster centre and declines smoothly with radius, a morphology inconsistent with residual point‑source leakage. The excess spectrum between the half‑mass radius and 3′ is well described by a simple power‑law model with photon index Γ = 0.9 ± 0.5, indicating a very hard spectrum. The corresponding surface flux in the 1–7 keV band is (1.17 ± 0.16) × 10⁻⁷ erg s⁻¹ cm⁻² sr⁻¹.

To assess whether unresolved point sources could account for the excess, the authors extrapolated the observed X‑ray luminosity function (L ∝ L⁻¹·⁵) below the detection threshold. The integrated contribution from such sources is estimated to be less than 5 % of the measured excess, confirming that the diffuse emission is not dominated by a population of faint, unresolved objects.

The authors discuss possible origins of the diffuse component. A purely thermal explanation, such as hot gas confined by the cluster’s gravitational potential, would require a temperature and emission measure inconsistent with the observed hard spectrum. Non‑thermal scenarios are therefore favored. Potential mechanisms include synchrotron radiation from relativistic electrons accelerated in pulsar wind nebulae (PWNe) associated with the numerous millisecond pulsars, inverse‑Compton scattering of ambient photon fields by the same electron population, or non‑thermal bremsstrahlung from high‑energy electrons interacting with the intra‑cluster medium. Each of these processes can produce a hard power‑law spectrum, but the current data lack the spectral resolution and photon statistics needed to discriminate among them.

In summary, the study provides the first robust detection of diffuse X‑ray emission surrounding Terzan 5, with a surface brightness profile that declines outward from the cluster core. The emission is hard, non‑thermal in nature, and cannot be explained by unresolved point sources or simple thermal plasma models. While the exact mechanism remains ambiguous, the results strongly suggest that relativistic particles, likely supplied by the cluster’s abundant millisecond pulsars, are responsible. The authors recommend deeper Chandra observations and coordinated multi‑wavelength campaigns (radio, γ‑ray) to further constrain the particle population and identify the dominant emission process.