[Paper Review] Chandra X-Ray Observation of the Globular Cluster GLIMPSE-C01

D. Pooley, S. Rappaport+3 more|ArXiv.org|Aug 24, 2007

Astrophysical Phenomena and Observations3 citations

TL;DR

This study presents a 46 ks Chandra X-ray observation of the newly discovered, heavily obscured globular cluster GLIMPSE-C01, identifying 17 X-ray sources with luminosities >0.6×10³¹ erg s⁻¹. The spatial concentration of sources, spectral hardness, and luminosity distribution confirm GLIMPSE-C01 as a rich Galactic globular cluster with a high X-ray source formation rate, estimated at Γ ≈ 400⁺⁶⁰⁰₋₂₀₀, placing it in line with other dense clusters and making it a prime target for millisecond pulsar searches.

ABSTRACT

We have observed the recently discovered rich star cluster GLIMPSE-C01 for 46 ks with the Chandra X-ray Observatory. Seventeen X-ray sources with luminosities greater than 0.6e31 ergs/s were discovered, one of which is likely a quiescent low-mass X-ray binary. The spatial distribution of these X-ray sources with respect to the NIR and IR images of the cluster, combined with the luminosity and spectral hardness distributions of the sources, provide strong evidence that GLIMPSE-C01 is a rich Galactic globular cluster.

Motivation & Objective

• To determine the nature of GLIMPSE-C01, a recently discovered, heavily obscured star cluster in the Galactic plane.
• To assess whether GLIMPSE-C01 is a globular cluster, open cluster, or super star cluster based on X-ray and multiwavelength data.
• To measure the X-ray source population and estimate the cluster's X-ray source formation rate (Γ) using Chandra's high angular resolution.
• To evaluate the cluster's potential for hosting millisecond radio pulsars based on its high X-ray source density and dynamical properties.

Proposed method

• Performed a 46 ks Chandra ACIS-S observation of GLIMPSE-C01 in timed-exposure mode with standard data reduction using CIAO 3.4.1 and CALDB 3.4.0.
• Used wavelet-based source detection (wavdetect) in 0.5–6.0, 0.3–10.0, and 2–8 keV bands to identify 42 X-ray sources on the S3 chip.
• Selected sources within the 36″ half-light radius (12 sources) and applied spectral fitting with absorbed power-law models to assess source hardness and luminosity.
• Constructed a radial light profile from NTT J, H, K-band images and fit it with a Hubble profile to estimate core radius (r_c ≈ 7″ ± 2″) and half-light radius (r_h ≈ 36″).
• Calculated central density (ρ₀ ≈ 10⁵.⁶ M☉ pc⁻³) and dynamical formation rate parameter (Γ ≈ 400⁺⁶⁰⁰₋₂₀₀) using the virial relation and cluster mass estimates (1–3×10⁵ M☉).
• Plotted the number of X-ray sources per unit mass against Γ to compare GLIMPSE-C01 with other Galactic globular clusters, confirming consistency with the established trend.

Experimental results

Research questions

• RQ1 Is GLIMPSE-C01 a globular cluster, given its X-ray source population and spatial distribution?
• RQ2 What is the X-ray source formation rate (Γ) in GLIMPSE-C01, and how does it compare to other Galactic globular clusters?
• RQ3 Does the spatial concentration of X-ray sources and their spectral properties support a globular cluster origin over an open or super star cluster origin?
• RQ4 What is the estimated central density and dynamical state of GLIMPSE-C01 based on its X-ray and infrared morphology?
• RQ5 Is GLIMPSE-C01 a viable candidate for hosting millisecond radio pulsars, given its high X-ray source production rate?

Key findings

• Seventeen X-ray sources with luminosities >0.6×10³¹ erg s⁻¹ were detected, with one likely being a quiescent low-mass X-ray binary.
• The spatial distribution of X-ray sources is concentrated toward the cluster center, matching the NIR and IR light profile, supporting a cluster origin.
• Spectral hardness and luminosity distributions of the sources are consistent with a population of X-ray binaries typical of globular clusters.
• The cluster's estimated formation rate parameter Γ ≈ 400⁺⁶⁰⁰₋₂₀₀ is consistent with the observed trend in other Galactic globular clusters, confirming its high dynamical activity.
• The central density ρ₀ ≈ 10⁵.⁶ M☉ pc⁻³ (assuming 2 M☉/L☉) and core radius r_c ≈ 0.17 pc indicate a very dense, compact cluster.
• GLIMPSE-C01 is confirmed as a rich Galactic globular cluster, making it a prime target for future millisecond pulsar searches.