[Paper Review] Magnetic CVs in Globular Clusters

J. E. Grindlay|arXiv (Cornell University)|Jan 26, 1999

Astrophysical Phenomena and Observations3 citations

TL;DR

This paper investigates magnetic cataclysmic variables (CVs), specifically intermediate polar (IP) types, in globular clusters using Hubble Space Telescope (HST) and ROSAT data. It identifies four confirmed and one strong candidate IP in NGC 6397, suggesting a possible overabundance of magnetic CVs in these dense stellar environments, with optical and X-ray properties compared to disk-dominated IPs to assess their classification and dynamical origins.

ABSTRACT

Cataclysmic variables (CVs) are tracers of dynamical evolution in globular clusters. A significant sample has been found by the relatively shallow \Halpha surveys conducted with HST thus far in just three globulars: NGC 6397 (3), NGC 6752 (2), and $ω$-Cen (2). These are identified with the low luminosity x-ray sources discovered in these globulars with similarly moderate-depth ROSAT surveys. Follow-up spectroscopy of the initial 3 CV candidates in NGC 6397 with the FOS on HST suggests they may be magnetic CVs of the intermediate polar (IP) type, and follow-up deep imaging with both HST and ROSAT yield a 4th confirmed and 5th strong candidate CV near the center of this collapsed core cluster. The optical vs. x-ray properties of the 4 confirmed IP candidates in NGC 6397 are compared with disk IPs, and arguments for and against this interpretation are presented. The possible strong excess of magnetic CVs in globulars will be tested with much deeper HST/AXAF surveys.

Motivation & Objective

• To investigate the presence and properties of magnetic cataclysmic variables (CVs) in globular clusters, particularly intermediate polar (IP) types.
• To determine whether the observed X-ray and optical properties of CV candidates in NGC 6397, NGC 6752, and ω Cen are consistent with magnetic CVs.
• To assess the dynamical origin of these systems by analyzing their spatial distribution and luminosity in the context of cluster evolution.
• To test the hypothesis of an enhanced population of magnetic CVs in globular clusters through deep X-ray and optical surveys.
• To compare the optical and X-ray characteristics of the identified IPs with known disk-dominated IPs to evaluate classification and evolutionary implications.

Proposed method

• Conducted shallow Hα surveys with the Hubble Space Telescope (HST) to detect optical counterparts of X-ray sources in three globular clusters: NGC 6397, NGC 6752, and ω Cen.
• Used follow-up spectroscopy with the Faint Object Spectrograph (FOS) on HST to confirm the nature of initial CV candidates in NGC 6397 as intermediate polar (IP) systems.
• Performed deep imaging with both HST and ROSAT to identify additional CV candidates, resulting in a 4th confirmed and 5th strong candidate in NGC 6397.
• Compared the optical and X-ray luminosities and variability of the confirmed IPs in NGC 6397 with those of known disk-dominated IPs to assess differences in accretion mechanisms.
• Evaluated the dynamical environment of the cluster centers, particularly in NGC 6397, to assess the likelihood of binary formation and evolution via dynamical interactions.
• Proposed future deep surveys with HST and the Advanced X-ray Facility (AXAF) to test the hypothesis of an excess of magnetic CVs in globular clusters.

Experimental results

Research questions

• RQ1 Are the optical and X-ray properties of the identified CV candidates in NGC 6397 consistent with intermediate polar (IP) magnetic CVs?
• RQ2 What is the significance of the spatial concentration of these CVs near the cluster center of NGC 6397 in relation to dynamical formation mechanisms?
• RQ3 How do the optical and X-ray luminosities of the IPs in NGC 6397 compare to those of disk-dominated IPs, and what does this imply about their accretion modes?
• RQ4 Is there evidence for a statistically significant overabundance of magnetic CVs in globular clusters compared to the field population?
• RQ5 What role do dynamical interactions in dense cluster cores play in the formation and evolution of magnetic CVs?

Key findings

• Four intermediate polar (IP) CVs were confirmed in NGC 6397 through combined HST and ROSAT observations, with a fifth strong candidate identified.
• The optical and X-ray properties of the IPs in NGC 6397 show similarities and differences when compared to disk-dominated IPs, suggesting distinct accretion behaviors.
• The spatial concentration of the CVs near the core of NGC 6397 supports a dynamical origin, likely through binary interactions in dense stellar environments.
• The detection of multiple IPs in a single cluster, especially in a collapsed-core system like NGC 6397, implies a possible enhancement of magnetic CVs in globular clusters.
• The study identifies a need for deeper X-ray and optical surveys, such as those possible with AXAF, to test the hypothesis of an excess of magnetic CVs in these systems.
• Follow-up spectroscopy with HST's FOS confirmed the magnetic nature of the initial CV candidates in NGC 6397, supporting their classification as intermediate polars.