Near-ultraviolet detections of four dwarf nova candidates in the globular cluster 47 Tucanae (2001.03484v1)

Abstract: We investigate near-ultraviolet (NUV) variability in the Galactic globular cluster (GC) 47 Tucanae (47 Tuc). This work was undertaken within the GC sub-project of the Transient UV Objects project, a programme which aims to find and study transient and strongly variable UV sources. Globular clusters are ideal targets for transient searches because of their high stellar densities and large populations of variable systems. Using all archival observations of 47 Tuc obtained with the UV/optical telescope (UVOT) aboard the Neil Gehrels Swift observatory with the uvm2 filter, we searched for UV variability using a specialised pipeline which utilises difference image analysis. We found four clear transients, hereafter SW1-4, with positions consistent with those of known cataclysmic variables (CVs) or CV candidates identified previously using Hubble Space Telescope observations. All four sources exhibit significant outbursts. Based on the inferred outburst properties and the association with known CVs, we tentatively identify the UV transients as CV-dwarf novae (DNe). Two DNe have been previously observed in 47 Tuc: V2, which has a position consistent with that of SW4; and AKO 9, which was not in outburst during any of the UVOT observations. We thus increase the known number of DNe in 47 Tuc to 5 and the total number of detected DNe in all Galactic GCs combined from 14 to 17. We discuss our results in the context of the apparent scarcity of DNe in GCs. We suggest that the likely cause is observational biases, such as limited sensitivity due to the high background from unresolved stars in the GC and limited angular resolution of the telescopes used. We additionally detected one strongly variable source in 47 Tuc, which could be identified as the known RR Lyrae star HV 810. We found its period to have significantly increased with respect to that measured from data taken in 1988.

• The paper identifies four near-UV transients as candidate dwarf novae in the dense globular cluster 47 Tucanae using Swift UVOT archival data.
• It employs advanced image processing and difference imaging techniques to construct light curves and estimate outburst parameters amid high stellar density.
• Cross-referencing with optical and X-ray catalogs strengthens the classification and highlights challenges in detecting transients in crowded environments.

Analysis of "Near-ultraviolet detections of four dwarf nova candidates in the globular cluster 47 Tucanae"

Globular clusters (GCs) are stellar systems that serve as ideal targets for monitoring transient astrophysical phenomena due to their high stellar densities and varied populations of variable stars. In the paper titled "Near-ultraviolet detections of four dwarf nova candidates in the globular cluster 47 Tucanae," the authors Modiano, Parikh, and Wijnands investigate near-ultraviolet (NUV) variability in 47 Tucanae (47 Tuc). Using archival data from the UV/optical telescope (UVOT) aboard the Swift observatory, they focus on identifying and analyzing transient UV sources within this GC.

Key Findings and Methodology

The research capitalizes on Swift's UVOT instrument, which, despite its relatively small field of view and moderate angular resolution, offers repeated observations allowing effective transient searches. The researchers employed a specialized pipeline leveraging difference image analysis to isolate transients against the backdrop of high stellar density in 47 Tuc. The archival dataset consists of 75 observations performed with a uvm2 filter, where the authors successfully identified four NUV transients labeled SW1--4. These transients are posited as candidate dwarf novae (DNe) based on their variability patterns and spatial congruence with known cataclysmic variables (CVs) or CV candidates identified in prior studies.

A detailed analysis followed, involving:

1. Image Processing and Astrometry: The paper describes the alignment and processing of Swift's UVOT images using advanced astrometric tools. Astrometric precision was a cornerstone to accurately matching the detected transients with known optical sources.
2. Light Curve Construction: By employing UVOT's capabilities, the authors generated light curves for each transient, culminating in the identification of the temporal characteristics of potential outbursts.
3. Cross-referencing with Existing Databases: The positions of these detected sources were compared with known CVs or candidates from optical and X-ray observations, enhancing the likelihood of the transients being actual DNe.
4. Estimation of Outburst Parameters: Properties such as outburst amplitudes and durations were inferred, suggesting consistency with known behaviors of DNe, characterized by brightness increases ranging from several magnitudes.

Implications and Observational Challenges

This research underlines both the potential and limitations of UV studies in GCs. Notably, the paper increases the recorded number of DNe in 47 Tuc and contributes to the understanding of DN occurrences in dense stellar environments. The findings suggest observational biases—such as sensitivity limits due to cluster background and stellar crowding—likely hinder the discovery of more transients in GCs. This implies that a significant fraction of GC DNe may remain undetected.

The research also presents ramifications for future observational techniques and equipment. Enhancements in UV observational capabilities with greater angular resolution would aid in distinguishing transient signals from background interference, potentially reconciling the observed scarcity of GC DNe with theoretical expectations.

Conclusions

The paper contributes to the empirical dataset of transient phenomena in GCs, providing valuable insights into the dynamics of DNe outbursts within these dense environments. While observational constraints are apparent, the results suggest that enhanced methodologies in UV data acquisition could unveil more complete pictures of transient populations in GCs. These findings are instrumental in refining models of binary evolution and understanding the dynamic processes shaping GCs.

By leveraging archival UV data effectively, this work demonstrates the multifaceted use of space-based observatories for probing transient astrophysical phenomena, laying groundwork for both current and future studies in high-density stellar environments.