1. The ultracompact X-ray binary M15 XB-2 The ultracompact X-ray binary M15 XB-2 Andrea 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 (10 10 cm orbit) interacting binaries periods < 80 min might be preferentially formed in dense GC cores M15: only GC with 2 LMXBS: 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 F λ ~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 F λ ~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 2.1x10 10 cm FUV light from accretion disc orbital modulation → inclination angle ≈ 33 ○ L X ≈1.4x10 36 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! M15 XB-2 AC 211