Variable Stars in the Globular Cluster NGC 6752

TL;DR

This study presents detailed time-series photometry of 16 variable stars in NGC 6752, revealing new variable objects, candidate binaries, and potential black hole or neutron star systems, with implications for understanding stellar evolution in globular clusters.

Contribution

First detailed photometric survey of variable stars in NGC 6752 identifying 13 new variables and candidate degenerate binaries, including potential black hole or neutron star systems.

Findings

Detected 16 variable stars, including 13 new discoveries.

Identified candidate binaries with degenerate stars and low amplitude variability.

Observed a dwarf nova outburst and periodic modulations in X-ray source counterparts.

Abstract

We report time-series photometry for 16 variable stars located in the central part of the globular cluster NGC 6752. The sample includes 13 newly identified objects. The precision of our differential photometry ranges from 1 mmag at V=14.0 to 10 mmag at V=18.0. We detected 4 low amplitude variables located on the extended horizontal branch (EHB) of the cluster. They are candidate binary stars harboring sdB subdwarfs. A candidate degenerate binary was detected about 2 mag below the faint end of the EHB. The star is blue and its light curve is modulated with a period of 0.47 d. We argue that some of the identified variable red/blue stragglers are ellipsoidal binaries harboring degenerate stars. They have low amplitude sine-like light curves and periods from a few hours to a few days. Spectroscopic observations of such objects may lead to the detection of detached inactive binaries harboring stellar mass black holes or neutron stars. No binaries of this kind are known so far in globular clusters although their existence is expected based on the common occurrence of accreting LMXBs and millisecond pulsars. An eclipsing SB1 type binary was identified on the upper main sequence of the cluster. We detected variability of optical counterparts to two X-ray sources located in the core region of NGC 6752. The already known cataclysmic variable B1=CX4 experienced a dwarf nova type outburst. The light curve of an optical counterpart to the X-ray source CX19 exhibited modulation with a period 0.113 d. The same periodicy was detected in the HST-ACS data. The variable is located on the upper main sequence of the cluster. It is an excellent candidate for a close degenerate binary observed in quiescence.