Abstract
Dwarf novae in globular clusters seem to be rare with only 13 detections in the 157 known Galactic globular clusters. We report the identification of a new dwarf nova in M13, the 14th dwarf nova identified in a globular cluster to date. Using the 2m Faulkes Telescope North, we conducted a search for stars in M13 that show variability over a year (2005-2006) on timescales of days and months. This led to the detection of one dwarf nova showing several outbursts. A Chandra X-ray source is coincident with this dwarf nova and shows both a spectrum and variability consistent with that expected from a dwarf nova, thus supporting the identification. We searched for a counterpart in Hubble Space Telescope ACS/WFC archived images and found at least 11 candidates, of which we could characterize only the 7 brightest, including one with a 3 sigma H-alpha excess and a faint blue star. The detection of one dwarf nova when more could have been expected likely indicates that our knowledge of the global Galactic population of cataclysmic variables is too limited. The proportion of dwarf novae may be lower than found in catalogs, or they may have a much smaller duty cycle in general as proposed by some population synthesis models and recent observations in the field.
Highlights
Globular clusters (GCs) are old, gravitationally bound stellar systems with extremely high stellar densities in their core which enable cluster members to dynamically interact through regular encounters
The number of neutron star (NS) lowmass X-ray binaries (LMXBs) in GCs as well as part of the cataclysmic variables (CVs) population appears to scale with the stellar encounter rate of their host cluster, indicating that they are produced through dynamical mechanisms (Gendre et al 2003; Pooley et al 2003; Pooley & Hut 2006)
No variability was reported by Kopacki et al (2003, star labeled c), but we show its variability in this study, which seems to occur on timescales of a few days
Summary
Globular clusters (GCs) are old, gravitationally bound stellar systems with extremely high stellar densities in their core which enable cluster members to dynamically interact through regular encounters. Encounters between passing stars and close binaries may harden the latter and accelerate the passing star This leads to the formation of a variety of tight and interacting binaries, and by accelerating stars in the core, close binaries may delay or halt the inevitable gravitational core collapse of GCs. Many of the X-ray sources in GCs discovered in the 1970s have been resolved into a multitude of close binaries and related systems with XMM-Newton and Chandra (see, e.g., Servillat et al 2008a, 2008b). Many of the X-ray sources in GCs discovered in the 1970s have been resolved into a multitude of close binaries and related systems with XMM-Newton and Chandra (see, e.g., Servillat et al 2008a, 2008b) These include active and quiescent lowmass X-ray binaries (LMXBs) with a neutron star (NS) primary, millisecond pulsars (MSPs), cataclysmic variables (CVs), and magnetically active binaries. The same correlation is observed with MSPs in GCs detected in radio (Hui et al 2010), and even in γ -rays (Abdo et al 2010)
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