1. An ultra-massive, fast-spinning white dwarf in a peculiar binary system S. Mereghetti (1), A.Tiengo (1), P. Esposito (1), N. La Palombara (1), G.L.Israel (2), L.Stella (2) 1) INAF – IASF MILANO 2) INAF Oss. Astron. ROMA

2. HD 49798 : a single-lined spectroscopic binary Hot subdwarf (sdO type ) B = 8 mag HD 49798 S. MEREGHETTI - Tuebingen - Aug 18, 2010 + ? Companion invisible in optical / UV data

3. The brightest sdO Heber 2009, ARAA HD 49798 T = 47,500 K L ~ 10 4 L sun Log g = 4.25 S. MEREGHETTI - Tuebingen - Aug 18, 2010

4. HD 49798 the brightest sdO  well studied single-lined spectroscopic binary Hydrogen poor, Helium rich P orb = 1.5476632(32) days circular orbit Optical mass function 0.263 +/- 0.004 Msun Thackeray 1970Kudritzky et al. 1978 Hamann et al. 1981Bruhweiler et al. 1981 Stickland & Lloyd 1984Bisscheroux et al. 1997 Hamann 2010Przybilla et al 2010 S. MEREGHETTI - Tuebingen - Aug 18, 2010

5. Optical mass function Invisible companion’s mass HD 49798 mass S. MEREGHETTI - Tuebingen - Aug 18, 2010

6. Discovery of X-ray periodicity ROSAT  Soft X-ray source P = 13.2 s (Israel et al. 1997, ApJ) The companion is either a Neutron Star or a White Dwarf

7. XMM-Newton observation 44 ks observation on May 10, 2008 Soft BB + hard component spectrum L x = 2 10 31 erg/s @ 650 pc (0.2-10 keV) kT=39 eV  ~ 2 N H =1.7 10 19 cm -2 S. MEREGHETTI - Tuebingen - Aug 18, 2010 The companion is a White Dwarf

8. JD - 2440000 Period (s) XMM-Newton results: timing S. MEREGHETTI - Tuebingen - Aug 18, 2010 dP/dt < 4 10 -13 s/s P = 13.18425(4) s 0.1-0.5 keV 0.5-10 keV Pulsed fraction 62%

9. Time delays in X-ray pulses X-ray projected semi major-axis : Ax sini = 9.78 +/- 0.06 light-sec + opt. mass funct.  q = M HD /Mx = 1.17 +/- 0.01 S. MEREGHETTI - Tuebingen - Aug 18, 2010

10. Optical + X-ray mass functions M HD /Mx = 1.17 Mx M HD S. MEREGHETTI - Tuebingen - Aug 18, 2010

11. Discovery of X-ray eclipse Eclipse duration = 1.3 hours Radius of HD 49798 = 1.45+/-0.25 Rsun  inclination 79—84 degrees S. MEREGHETTI - Tuebingen - Aug 18, 2010

12. Opt. and X-ray m.f. + inclination Mx = 1.28 Msun M HD = 1.5 Msun S. MEREGHETTI - Tuebingen - Aug 18, 2010

13. M HD = 1.50 +/- 0.05 M sun M wd = 1.28 +/- 0.05 M sun The companion of HD49798 is a very massive white dwarf S. MEREGHETTI - Tuebingen - Aug 18, 2010

14. Dynamical measurement  Robust, ‘’assumptions-free” result  not dependent on Mass-Radius relation Most WD masses are derived indirectly: spectroscopic and photometric methods  L, T, log g  M/R 2 gravitational redshift  M/R and rely on M-R relation to finally get M S. MEREGHETTI - Tuebingen - Aug 18, 2010 The companion of HD49798 is a very massive white dwarf M wd = 1.28 +/- 0.05 M sun

15. Past evolution S. MEREGHETTI - Tuebingen - Aug 18, 2010

16. Future evolution S. MEREGHETTI - Tuebingen - Aug 18, 2010 The WD is currently accreting in the wind of HD49798 When HD49798 evolves it will fill its Roche-lobe again Stable mass transfer of a few tenths of Msun onto the WD The WD will reach the Chandra limit

17. The fastest spinning white dwarf P=13.2 s (only ~5 times slower than break-up) Fast spin gives a limit on R WD < 7000 km Accretion implies low magnetic field (to avoid the propeller effect)  B < few kG Origin of fast rotation: spun-up by accretion ?? when ? why not to an even shorter P ? rapidly rotating at birth ? S. MEREGHETTI - Tuebingen - Aug 18, 2010

18. Conclusions Robust, dynamical determination of M > 1.2 Msun for a white dwarf Post Common Envelope binary with well determined masses  optimal test-bench for evolutionary models The fastest spinning WD (P=13.2 s)  low B to avoid propeller ( < few kG) Possible future evolution: - SN Ia with short delay time or - non-recycled millisecond pulsar S. MEREGHETTI - Tuebingen - Aug 18, 2010