New Deep Radio Continuum Imaging Still Indicates a Large Reservoir of Undiscovered Millisecond Pulsars in Terzan 5

TL;DR

This study delivers the deepest, highest-resolution 2–4 GHz VLA continuum imaging of Terzan 5 to date, detecting 38 of 49 known timed pulsars and revealing substantial unresolved diffuse flux that implies a large unseen pulsar population. By combining resolved pulsar fluxes, residual core emission, archival data, analytic luminosity-function modeling (power-law and lognormal), and image simulations, the authors infer a minimum pulsar population of about $N\sim250$ with plausible totals extending well into the hundreds or thousands depending on model assumptions. Gamma-ray measurements provide cross-checks but are not definitive constraints on $N$, though the cluster’s high $L_{\gamma}$ is consistent with a large pulsar census. Overall, Terzan 5 remains a keystone target for next-generation radio facilities, which will be essential to characterize its pulsar population and test formation and evolutionary scenarios in dense stellar environments.

Abstract

We present the deepest and highest-resolution radio continuum imaging of the Galactic globular cluster Terzan 5, one of the most crowded locations in the radio sky. In these new 2$-$4 GHz Karl G. Jansky Very Large Array images, we detect 38 of the 49 confirmed pulsars, including extensive multi-frequency eclipse mapping of the luminous redback Ter5A. Nonetheless, there is still a large amount of diffuse residual flux from pulsars that are fainter than our 2.5 GHz continuum detection limit of $\sim 11\,μ$Jy. Using a range of approaches including image-based simulations, we model the fluxes of the detected pulsars together with the residual flux. We find a minimum total population of $N\sim250$ detectable pulsars in Terzan 5 and perhaps substantially more, though the luminosity function remains very uncertain. Consideration of the $γ$-ray properties of the cluster, though also not unambiguous to interpret, leads to consistent conclusions. These pulsar population estimates are larger than inferred from previous work and highlight Terzan 5 as a keystone target for next-generation radio facilities.

Figures (11)

• Figure 1: Top: 2.5 GHz VLA image encompassing the core of Terzan 5. The large red circle denotes the core radius of Terzan 5. Green sources represent the continuum counterparts to timed millisecond pulsars and are labelled with their timing identification. Magenta sources are not associated with known timed pulsars. Red sources are from U20 but not detected in our current data. The FWHM synthesised beam size is indicated by the white ellipse in the bottom left corner. Bottom: same as in top panel, but for 3.5 GHz.
• Figure 2: $50^{\prime\prime}\times50^{\prime\prime}$ residual 2.5 GHz image, with all significant point sources subtracted, of the central region of Terzan 5; note the field of view is larger than in Figure \ref{['fig:image']}. The red circle shows the core radius. The white ellipse in the lower left corner indicates the beam size. Substantial unresolved emission remains within the core even after the subtraction of the significant point sources. The bright source to the north just outside the core is Ter5C, which has not subtracted fully cleanly due to its brightness and scintillation-induced variability.
• Figure 3: Top panel: Flux ratio between the 2.5 GHz 2022 March 14 and the 2.6 GHz 2012 observations for sources detected at both epochs. Unfilled circles are confirmed pulsars. There is some evidence that the 2022 flux densities for central sources are lower, likely due to scintillation. Bottom panel: Ratio of fluxes between March and April epochs. In both panels, red dotted lines represent the core and half-light radii. Sources within the half-light radius that are not confirmed pulsars are identified with their IDs.
• Figure 4: March (top) and April (bottom) flux density and spectral index measurements for the eclipsing redback Ter5A, with filled circles detections and unfilled circles $3\sigma$ upper limits. The dashed lines represent the expected conjunction times for the companion to be in front of the pulsar. The length, depth, and timing of eclipses varies substantially in these data. Data used to created each light curve are reported in Table \ref{['tab:ter5a_flx']}.
• Figure 5: 2.5 GHz VLA flux densities of known Terzan 5 pulsars plotted against their timing fluxes. Timed pulsars are converted to 2.5 GHz using the spectral index from Martsen2022 for the 31 pulsars where this information is available. For the remaining pulsars that do not have timing spectral information, we use an assumed $\alpha=-1.8$. Upper limits are denoted with unfilled circles. Red and black points indicate eclipsing redback (Ter5A, Ter5P, Ter5ad, Ter5ar) and black widow (Ter5O, Ter5aq, Ter5at) sources, respectively. The diagonal dashed line represents unity. The horizontal dotted line indicates our new candidate pulsar Ter5-VLA62. We label notable pulsars (e.g., redbacks, bright sources and outliers).