Introduction: UCXBs tight (10 10 cm orbit) interacting binaries periods < 80 min

1. The ultracompact X-ray binary M15 XB-2Andrea Dieball (1), Christian Knigge (1), D.R. Zurek (2), M.M. Shara (2) & K.S. Long (3)(1)Univesrity of Southampton, Southampton, UK (2)AMNH, New York, USA (3)STScI, Baltimore, USA • Introduction: • UCXBs • tight (1010 cm orbit) interacting binaries • periods < 80 min • might be preferentially formed in dense GC cores • M15: • only GC with 2LMXBS: AC211 & M15 XB-2 • M15 XB-2 ≈ 2.5x brighter than AC211

2. The period of M15 XB-2: • Relative photometry from 90 images • (Oct. to Dec. 2004) • periodogram peaks at 22.6 min • extensive Monte Carlo simulations showed • that signal is consistent with being coherent • mean subtracted lightcurve • Sine wave with 22.5806 min period • Fit coherent over 3300 cycles

3. The Spectral Energy Distribution: • SED from narrow band photometry • Power law Fλ~λ-2.0 • Excess flux in F150N Colour colour diagram: • Blackbodies • Power law with CIV emission • Power law with HeII emission SED can be described with power law (-2.0) + CIV 1550 + HeII 1640, EW ≈ 30 Å

4. Results: • orbital period 22.6 min • excess emission around 1550 Å • SED can be described by Fλ~λ-2.0+ CIV 1550 Å + HeII 1640 Å emission • Mean density of donor 790 g cm-3 → He White Dwarf • donor mass 0.02 – 0.03 M๏ • donor radius 0.03 – 0.04 R๏ • binary separation 2.1x1010 cm • FUV light from accretion disc • orbital modulation→ inclination angle ≈ 33○ • LX ≈1.4x1036 erg sec-1 → accretion rate 2.4x10-10 M๏ yr • agrees with gravitational-wave-driven mass transfer at this period • M15 XB-2 can be classified as an UCXB, the third confirmed in a GC! AC 211 M15 XB-2